Few categories of aircraft have been so controversial as the propeller-driven, twin-engined fighter aircraft of the 1930s and 1940s. Some designs in this category, such as the Lockheed P-38 Lightning, still have many enthusiastic admirers, as well as a few convinced detractors. Other types, such as the Messerschmitt Bf 110, are almost routinely dismissed as near-failures, despite their historically very important role. On the whole, the judgement of most aviation enthusiasts about the twin-engined fighters has been markedly negative. Historians tend to be more moderate, but most of them still tend to regard the category as relatively unimportant, compared to the more performant single-seat fighters, or the heavy bombers on which the USAAF and RAF centered their strategy.
The controversy has never died, because the facts allow everyone to have a little bit of truth. Despite the basic, overall similarity in concept, the aircraft in this category were very diverse, with a wide range in capabilities corresponding surprisingly different intended roles. Still, most people tend to judge the twin-engined fighter on its ability to match the more glamorous single-engined fighter in the very roles in which the latter excelled: Daylight air superiority, interception and escort. This, inevitably, puts the heavier and less manoeuverable twin-engined types in an unfavourable light. A few could indeed compete with single-engined fighters in these roles, but even in these cases arguments of economy and efficiency usually favoured the single-engined fighter. By these standards, the “twin” appears to have been a pointless aberration, but is that a fair judgment?
Many thousands of these aircraft were built, to a substantial number of different designs. And while some were clear failures, others served with great distinction, as long-range fighters, ground attack aircraft, nightfighters, bomber interceptors, anti-shipping aircraft and reconnaissance aircraft. When given missions that corresponded to the strengths of the twin-engined designs, rather than their weaknesses, they proved highly useful and successful.
Ancestry: The Great War
The first moderately successful use of large, twin-engined fighter aircraft can be attributed to the French during the First World War. The British conducted numerous experiments with multi-engined fighters, with a distinct lack of success. The German and Austro-Hungarian air forces showed relatively little enthusiasm for the concept. The German production of large long-range bombers was not paralled by a similar development of large long-range fighters to escort them.
The more economical solution, which usually prevailed during the First World War and remained popular after it, was the single-engined two-seat fighter. This could fly many of the missions of the larger multi-engined type, demonstrated great operational flexibility, and was considerably less expensive. Such aircraft had fixed forward-firing guns and one or two flexible weapons for the rear gunner. Their manoeuverability was reasonably close to that of the single-seaters, and their performance, in an era of drag-inducing struts and wires, was not much inferior. The crews of the Bristol F.2 ‘Brisfit’ demonstrated that the best way to operate a good two-seater was to fly it in combat as a single-seater, using the fixed gun as primary armament.
France
In March 1915 Caudron derived the twin-engined G.4 from the single-engined G.3, in response to a requirement for an Army co-operation aircraft armed with a machine gun. Instead of one 80 hp Le Rhône engine in the nose, two such engines were installed between the wings. A gunner was seated in the nose of the fuselage, with a single 7.7 mm Lewis or Hotchkiss machine gun. The G.4 was larger and heavier than the G.3, with a wing area of 36.8 square meter (vs. 27 square meter); top speed was increased from about 110 km/h to 130 km/h. The G.4 was used mostly for army co-operation or bombing missions, but escadrille C 66 flew long-range escort missions for the Voisin bombers of GB 2, and some other units followed this example.
By early 1916 the G.4 was already obsolescent. Initial attempts to find a replacement focused on the A3 category, defined as a twin-engined three-seat aircraft that would be able to fly long-range reconnaissance, bombing, and escort fighter missions. A number of aircraft were built to meet this specification, but none of them was suitable as a fighter. In November 1916 a C3 requirement for a specialized three-seat escort fighter emerged. The specifications called for an aircraft with a top speed of 170 km/h at 2000 m, able to climb to 3000 m in less than 15 minutes. It had to be able to carry two machine guns, with 500 rounds each, and light armour.
Whether the Caudron R.11 was designed to meet the C3 or A3 specification is unclear, but eventually this was the type that was employed as a long-range escort fighter. With two 200 hp Hispano-Suiza 8Ba engines, the R.11 met the C3 specifications, flying at 183 km/h at 2000 m and climbing to 3000 m in 14 minutes and 30 seconds. It carried five 7.7 mm Lewis guns, in two flexible twin mounts on the nose and aft fuselage, plus a single flexible gun mount in the lower nose, to be used by the nose gunner. However, only 54 are reported to have been in service at the end of the war, because the type arrived late and was further delayed by engine problems. Of the original 1000 ordered, only 370 were completed.
These performance figures deserve some comment. Operationally, the bomber the R.11s usually escorted was the excellent Breguet 14, which in its two-seat bomber B2 version with a single 300 hp Renault 12Fcx engine had a top speed of 195 km/h, but it needed 16 minutes and 30 seconds to climb to 3000 m. Top cover was often provided by the SPAD 13 single-seat fighter, also powered by the Hispano-Suiza 8B, with a top speed of the 211 km/h at 1000 m and 208.5 km/h at 2000 m. The SPAD 13 could climb to 3000 m in 8 minutes 45 seconds. Thus the escorted bomber formation was reasonably cohesive in performance, but the R.11 could well be the slowest aircraft in it, especially after the Breguet 14s had dropped their bombs.
And their success was debatable. Initially, the R.11s flew ahead of the bomber formations to take on the German fighters. Later tactics saw the R.11s flying close escort among the bombers, contributing to the firepower of the formation with their armament, while single-seat fighters provided top cover. The crews of the R.11s had a strong bond with the bomber crews they protected, because every R.11 escadrille was assigned to provide cover for a specific bombardement escadre. (This escadre typically contained two or three groupes de bombardement, each containing three escadrilles.) But when the top cover of SPAD 13s or other fighters was not present, the combined formation of bombers and R.11 escorts still suffered heavy losses to enemy fighter attacks.
Two variations had entered tests at the end of the war, but were abandoned after the Armistice. The R.12 was an R.11 with 300 hp Hispano-Suiza 8Fb engines. The R.14 was a larger development of the R.11 designed to meet the Ca3 requirement for a three-seat fighter armed with a 37-mm Hotchkiss cannon. It was powered by the same 8Fb engines as the R.12, and retained the five machine guns in addition to the cannon.
There were other French experiments with multi-engined fighters during WWI, with besides the escort mission also a putative “anti-airship fighter” role driving such projects. But they remained far removed from the level of performance and success achieved by the R.11. The Letord 6, for example, was a development of the Letord 3 night bomber with a 37-mm cannon in the nose, to meet a Ca3 requirement for a cannon-armed escort fighter. Considered inferior to the Caudron R.14, it was abandoned.
| Caudron G.5 | Caudron R.11 | |
|---|---|---|
| Engines | Le Rhône | Hispano-Suiza 8Bda |
| Power | 2 x 80 hp | 2 x 215 hp |
| Wing Span (m) | 16.89 | 17.92 |
| Length (m) | 7.19 | 11.22 |
| Wing Area (m2) | 36.83 | 54.25 |
| Empty Weight (kg) | 733 | 1,422 |
| Loaded Weight (kg) | 1,232 | 2,165 |
| Max Speed | 130 km/h at sea level | 183 km/h at 2,000 m |
| Climb | 2,000 m in 15 min | 2,000 m in 8 min 10 sec |
| Ceiling (m) | 4,300 | 5,950 |
| Range (km) | (5 hrs) | 600 |
| Guns, flexible mounts | 1 x 7.7 mm Hotchkiss or Lewis | 5 x 7.7 mm Lewis |
Great Britain
In Britain too interest in the twin-engined fighter concept arose early. In 1915 Vickers flew the F.B.7 and F.B.8. The F.B.7 was a twin-engined biplane designed to accomodate a nose gunner with a 1-pounder cannon in the nose. (The Vickers 1-pounder was a fully automatic weapon of 37-mm calibre, capable of firing 450 gram shells at the very modest muzzle velocity of 365 m/s.) This was an ambitious idea, but with two 80 hp Renault engines the top speed was a disappointing 121 km/h. Vickers convinced the RFC that the smaller F.B.8, powered by 100 hp Gnome radials and armed only with a Lewis machine gun, was a better concept; but its performance was still judged insufficient.
Of course, these aircraft were designed at a time when there was little knowledge about the best configuration for a fighter aircraft, and for a time the F.B.7 and F.B.8 must have seemed to have as much potential as the F.B.5 single-engined pusher biplane and its developments. Experimentation continued and a number of twin-engined fighter prototypes were built and flown, only to be rejected for service. Even with the benefit of hindsight, one can hardly dispute the wisdom of those decisions.
The Avro 523 Pike, first flown in March 1916, was designed for the Admiralty as a long-range escort and anti-airship fighter; it had two 150 hp pusher engines and single Lewis guns fore and aft. It did not enter production, despite attempts to develop it as a night bomber.
The Bristol T.T.A (Twin Tractor Model A), following one month later, was intended as a defensive fighter, with two flexibly mounted Lewis guns in the nose. (A cannon was considered as alternative armament.) The pilot, seated well aft, was supposed to use a flexible Lewis gun for the rear defense, as well as fly the aircraft. Powered by two 120 hp Beardmore engines, the T.T.A made an unfavourable impression on test pilots. Only the two prototypes were completed.
The Pemberton-Billing P.B.29E and P.B.31E were dedicated anti-airship fighters, designed to mount standing patrols at night and engage the enemy with their flexible armament. These were large, lumbering aircraft: The P.B.29E was a quadruplane powered by two 90 hp Austro-Daimler engines, with a gunner in a nacelle mounted between the upper pair of wings, while the fuselage was between the lower pair of wings. On the improved, more streamlined P.B.31E, the gunner’s pulpit was faired into a repositioned fuselage. The P.B.31E could boast an endurance of 18 hours on patrol and an armament of one 1 ½-pounder recoilless cannon and two machine guns, but was too slow and underpowered to intercept an enemy airship. By 1917, when the P.B.31E flew, the threat was already shifting to enemy bombers, and these outperformed the underpowered quadruplane by a considerable margin.
| Vickers F.B.8 | Pemberton-Billing P.B.31E | |
|---|---|---|
| Engines | Gnome Monosoupape | Anzani 9-cylinder radial |
| Power | 2 x 100 hp | 2 x 100 hp |
| Wing Span (m) | 11.68 | 18.29 |
| Length (m) | 8.58 | 11.24 |
| Height (m) | 3.00 | 5.40 |
| Wing Area (m2) | 43.48 | 89.37 |
| Empty Weight (kg) | 835 | 1,668 |
| Loaded Weight (kg) | 1,225 | 2,778 |
| Max Speed (km/h) | 158 km/h at 1,525 m | 121 km/h |
| Climb | 1,525 m in 10 min | 3050 m in 1 hr |
| Ceiling (m) | 42170 | |
| Endurance | 3 hrs | 9 hrs |
| Guns, fixed | – | – |
| Guns, flexible | 1 x 7.7 mm Lewis | 1 x 1 1/2 pdr Davis 1 x 7.7 mm Lewis |
US Navy
At the end of the war, the Allies formulated a requirement for an aircraft that could protect long-range flying boats, such as the Curtiss H-16 and Felixstowe F.5. The U-boat patrols flown by these aircraft were essential, but over the North Sea they risked interception by German fighters. Although the specification for a long-range escort fighter was prepared by the British government, the task of actually designing and building it was handed to the Naval Aircraft Factory, an aircraft design and production facility that was controlled by the US Navy.
The war was over before construction work could begin. The TF or Tandem Fighter made its first flight in October 1920, powered by two 300-hp Hispano-Suiza engines, installed in tandem between the biplane wings. The configuration of the TF was that of a flying boat with a short hull, its tail surfaces lifted high above the water by an arrangement of struts. The engine installation caused no end of trouble, as the rear engine could not be satisfactorily cooled. Four aircraft were completed and work continued until January 1923, when it was finally realized that any usefulness the TF might once have possessed, had evaporated long ago.
The Central Powers
One German venture in the field of the twin-engined fighter is worth mentioning because it pioneered the concept of the ‘centreline thrust’, in which the fuselage contains a tractor and a pusher engine. This avoids power asymmetry in the event of an engine failure, and allows two engines to be installed in a smaller airframe than would be necessary if the engines were on (or between) the wings.
The Siemens-Schuckert DDr.I was a triplane with relatively large gaps between the wings. The nacelle was attached to the central wing had a 120 hp Siemens-Halske Sh.I engine fore and aft. The DDr.I was a single-seat fighter armed with two synchronized LMG 08/15 machine guns. The tail was carried on a cage of struts around the rear propeller, as on most WWI pusher types; to accomodate this the pusher propeller was four-bladed and of smaller diameter than the two-blade tractor propeller. In November 1917 a single test flight was made, which ended in a crash. The plan to develop a more powerful DDr.II was abandoned.
In Austria-Hungary, a ‘battleplane’ program was launched in August 1914. The specification called for a twin-engined aircraft armed with two machine guns, and protected by 120 kg of armour plate. In November formal orders were placed with Aviatik, Albatros and Lloyd to develop their designs further. But by December 1916, officers had come to the inevitable conclusion that the ‘battleplane’ concept did not correspond to valid a military role, and so all production orders were cancelled. Only a few prototypes were flown.
Of these the Lohner 10.21, flown in April 1916, was a fairly conventional biplane with two 150 hp Daimler tractor engines, strongly swept back wings, and unsatisfactory performance. It had a front gun position with room for two gunners, and a single gunner in a rear position. The Phönix 20.10 was flown in September 1916, also a conventional biplane with two 200 hp Hiero engines. After cancellation of the battleplane program it was suggested that this type might be developed as a trainer, but no more 20.10s were built. The innovative Lloyd 40.06 tried to achieve streamlining by burying the two 160 hp Daimler engines in the front fuselage, driving a common gearbox, from which two tractor propellers were driven. Development of the gearbox was identified as a problem early on, and in fact this was never completed, preventing assembly of the 40.06. Engineering work was nevertheless allowed to continue until June 1917, more out of interest in the gearbox than in the aircraft.
Interbellum: The battleplane
After the end of the Great War, several influential theorists formulated their vision on the use of combat aircraft. They based their opinions both on their experience in the previous conflict and on their imaginative view of the future, with perhaps an overly strong bias towards the latter. The most famous was the Italian Giulio Douhet, who advocated an unrestrained air offensive as the decisive factor in the wars of the future. In his theories there was little room for fighter aircraft, because these were seen as elements of a defensive strategy that he rejected. Instead, the domination of the air was to be ensured by offensive aircraft, i.e. bombers, whose performance and heavy armament would make any attempt at defense futile. Adherents of this theory were induced to design “battleplanes”, well-armed aircraft that would be able to bomb as well as fight for air superiority.
For many years, this remained little more than theory. Stocks of war materiel were plentiful and defence spending limited. During the 1920s, design teams remained active, and some quite innovative aircraft were produced; but they met little interest from their potential customers. For the most part, air forces were content with designs that were fundamentally similar to those of 1918, although they replaced wood by metal, which was lighter and easier to maintain, and installed more powerful and reliable engines. In the 1930s, under the strong influence of innovations in civilian aviation and a worsening of the political situation, change could no longer be avoided, and then it became both sudden and radical.
France
In March 1933 the French government decided to entirely renew the organisation and equipment of its air force. An independent air force, L’Armée de l’Air, was created on the first of April. Officially, an offensive air strategy was adopted, but actually 56% of the available aircraft were equipped for army co-operation or reconnaissance, while a mere 16% were bombers and 28% were fighters. To increase the offensive power of the air force, it was decided to both reduce the share of reconnaissance squadrons and to give them aircraft with more offensive ability, so-called BCR types. The acronym stood for Bombardement – Chasse – Reseignment, or Bombing – Pursuit – Reconnaissance.
This, of course, was only a few months after Hitler came to power in Germany. Contrary to what is often assumed, the French government was well aware of the need to build a modern and powerful air force to counter the growing threat at the other side of the Rhine, but there was a deep gap between the series of increasingly ambitious re-equipment plans (calling for first for 1023, and then 1554, 2617, 2938 and finally over 9000 aircraft in the first line) and the meagre capability of the industry to develop new aircraft and mass-produce them. Frustrated, Pierre Cot, Minister of Air between 1936 and 1938, nationalized most of the aircraft manufacturers and regrouped them into larger entities. After much initial confusion this did some good, but it was only a half-measure – the production of airframes indeed increased but the production of engines and instruments (which had not been reorganized, because of budgettary restrictions) failed to follow.
In July 1934 the French parliament approved the so-called Plan I, also called the Plan of 1023, because it called for 1023 combat aircraft in the first line and 342 in reserve. This ambitious plan had two major flaws: The aviation industry was incapable of building the required aircraft (by the end of 1935 it would have delivered much less than half the aircraft desired) and it relied far too much on the success of the BCR category. This became a serious disappointment indeed and only one real BCR entered service, the Potez 540. The BCR requirement of October 1933 called for a top speed of 350 km/h and a cruising speed of 250 km/h. This was modest enough, but unfortunately most of the design teams appeared to be participating in a dubious contest for aeronautical ugliness, to a degree that was highly incompatible with efficient aerodynamics. In fact the armament that was supposed to allow these aircraft to win a clash with enemy fighters only amounted to a few machine guns extra, yet the desire to create gun positions with a good field of view and fire resulted in some very awkward constructions. In 1934 L’Armée de l’Air finally decided that nose gun turrets should be omitted from the BCR designs, in an attempt to reduce drag.
Of course, even in 1933, the idea was far from new; specifications for a multiplace de combat or multi-seat combat aircraft had been released regularly since the early 1920s. A radically innovative effort in this direction started as early as 1922, when a team at Blériot conceived a monoplane successor to the Caudron R.11. In 1923 it offered the Blériot 117, a wooden monoplane with gunnery positions in the nose and at the rear end of both engine nacelles, which had been deepened and elongated for the purpose. Two years later this had evolved in the Blériot 127, a good performer for the time with a top speed of 221 km/h at 4000 m, equal to that of fighters. It was declared the winner of the 1926 multi-seat combat aircraft competition, and 34 were ordered to equip two “protection squadrons” within the 11th Escadre de Bombardement. But the success story ended abruptly after a series of fatal accidents. An investigation revealed that the turbulence generated by the deep wing root, the large engine nacelle and the gunner’s position caused the small tailplane to lose effectiveness, with fatal results. The aircraft was grounded.
Complete redesign to solve the aerodynamic problems of the 127 resulted in the Blériot 137, flown in 1930. This aircraft was of all-metal construction. The new wing and tailplane were located higher, and the rear gunners were no longer at the rear of the engine nacelles, but in cylindrical bulges attached to the side of the fuselage. Unfortunately, official tests revealed that this disturbed the flow over the tail even more than the original solution: The 137 entered a steep vertical dive if the engines were throttled back and remained uncontrollable until power was restored! It has been claimed that Blériot’s test pilots had discovered this before the 137 was submitted for official testing (as indeed they should have), and the company had chosen to keep this secret and simply restricted the throttle travel! Whether this story is true or not, the firm might as well not have bothered, because eight of these totally useless aircraft were ordered anyway. Officially this was a compensation for the replacement of the 1928 multiplace de combat programme by the new BCR programme, which would have ruled out an order for the 137 regardless of the results of the test programme.
The SPCA 30 M of 1931 was a fairly elegant cantilever low-wing monoplane, of all-metal construction with corrugated metal skinning. Gunnery positions were located in the nose and in each of the twin tail booms. The 30 M was big, with a wing span of 25.60 m and a maximum take-off weight of 7200 kg; even the wheels of the fixed landing gear stood as tall as a man. With two 650 hp engines its top speed was 255 km/h, and its flying qualities reportedly left much to be desired.
The Blériot and SPCA designs look awkward today, but they were models of streamlining in comparison to the extraordinary Breguet 413. This was a twin-engined biplane with a short, deep and angular front fuselage, and a slender tail boom to create a good field of fire for the rear gunner. The 413 looked as if it had come from the drawing board of a designer of toys instead of real aircraft. It was armed with six machine guns: Two each in a nose turret, dorsal gun mount and ventral turret. First flown in February 1933, and powered by two 650 hp Hispano-Suiza 12Ybrs engines, it was capable of 310 km/h. (The winner of a contemporary competition for a single-seat fighter, the monoplane Dewoitine D.500, reached 367 km/h at 5000 m.) This bizarre aircraft was about to enter production, for 50 had been ordered under the Plan I, when an accident with the prototype led to cancellation.
The Amiot 140, also designed for the 1928 M4 requirement, was at least superficially a much more impressive design, because it was a cantilever monoplane with smooth stressed-skin construction and, in prototype form, fairly clean lines. But the streamlined appearance was deceptive; it had a very thick wing with an enormous area (adopted to keep the landing speed low), and the front fuselage had been designed as a two-decked structure of which only the upper deck faired in with the tail. The lower deck provided a location for the lower rear gunner, ample viewing windows for the observation mission, and even back-up flying controls. Top speed was only 242 km/h at sea level. The open cockpit and ditto gunnery position in the nose were already very uncomfortable at such speeds, and the production Amiot 143 had an enclosed cockpit and nose turret; the dorsal and ventral positions remained open but were better faired in. With 800 hp radial engines, the aircraft then reached 293 km/h at 4000 m. By the standards of 1933, the Amiot 143 could not hide its obsolescence, but nothing better was available, so the aircraft was ordered as a bomber.
To create a competitor for the 1933 BCR requirement, Amiot suggested a cleaned-up 143 with a smaller wing and retractable landing gear. On paper the Amiot 144 met the BCR specification, but when it the prototype was tested in 1936 the results were disappointing. Amiot persisted in calculating the hypothetical performance with equally hypothetical engines of 1200 hp or even 1500 hp, but the air force abandoned the 144 and ordered a few more 143s instead.
The Potez 540 and its derivatives finally emerged as the winners of the BCR orders. It was a high-wing strut-braced monoplane with engines carried in nacelles below the wings, a concept that was rather old-fashioned. It did allow the main landing gear to retract in the rear of the engine nacelles. The 540 had nose, dorsal and ventral turrets, each with a single 7.5 mm Darne machine gun. The top speed of 310 km/h already fell considerably below the mark for a good multi-role aircraft. When the Spanish Civil War broke out, a number of Potez 540s were delivered to the Republicans, but the combat experience with these obsolescent aircraft was poor.
Marcel Bloch, later to become more famous as Marcel Dassault, designed the Bloch M.B.130. This was a streamlined, elegant twin-engined monoplane (not unlike the later Martin Maryland), but it did not have a performance commensurate with its slender lines. It was first flown in June 1834. The further developed M.B.131 was, with two 880 hp Gnome-Rhône 14No radials, capable of 400 km/h. It was ordered by the air force, but as light bomber and reconnaissance aircraft. The fighter role was abandoned, and armament restricted to three 7.5 mm MAC 1934 machine guns and a small bomb load. Pictures do exist, however, of a M.B.131 with a much more substantial weapon, difficult to identify, in the nose.
| Bleriot 127 | Potez 540 | |
|---|---|---|
| Engines | Hispano-Suiza 12H | Hispano-Suiza 12Xirs |
| Power | 2 x 550 hp | 2 x 690 hp |
| Wing Span (m) | 23.20 | 22.10 |
| Length (m) | 14.50 | 16.20 |
| Height (m) | 3.40 | 3.88 |
| Wing Area (m2) | 88 | 76 |
| Empty Weight (kg) | 3682 | 3785 |
| Loaded Weight (kg) | 5200 | 5950 |
| Max. Speed (km/h) | 221 | 310 |
| Ceiling (m) | 8100 | 5182 |
| Guns, fixed | – | – |
| Guns, flexible | 6 x 7.7 mm Lewis | 3 x 7.5 mm Darne |
Britain
During WWI, Britain had been the target of German bombing attacks, first by airships, and then by German long-range bombers. After attacks in daylight proved too costly for the unescorted bombers, the offensive switched to night bombing. The best night fighters of the period were slightly modified day fighters, and their effectiveness was quite low. It was only logical that after the war a specification 4/24 was formulated for a twin-engined home defence fighter, intended to defend the country against night bombers. A few months after its publication, in September 1925, the Air Ministry informed the contenders of a requirement to install two 1½ pdr COW guns, as well as supercharged engines. A top speed of 200 km/h (125 mph) was required, and a landing speed of no more than 80 km/h (50 mph). Although the 37-mm COW cannon was light for its calibre, it was bulky. Therefore such an aircraft would necessarily be rather large and could be expected to have a less than brilliant performance.
Two designs were offered. Bristol’s Type 95 Bludgeon, later renamed Bagshot, was superficially the more advanced of the two, because it was an all-metal, high-wing, semi-cantilever monoplane. Or perhaps it ought to be called a sesquiplane, because there was an aerofoil of considerable size between the fuselage and the main wheels. It had gunner’s positions in the nose and behind the wings, while the pilot sat close to the leading edge of the wing; all the cockpits were open. In addition to the heavy mounting points for the COW gun, there was a Scarff ring for a machine gun in the rear cockpit. Bristol calculated that the Bagshot would not meet the requirements, but the Air Ministry insisted on ordering a prototype. This revealed structural problems on its first flight, and was not flown again.
The competing Westland Westbury was a traditional biplane design, and as an aircraft it was considerably more successful, although it too failed to meet the performance targets. As the RAF had quickly abandoned the concept of the large, slow nightfighter, only the two prototypes were completed. They were used extensively for armament trials at Martlesham Heath, and fired large numbers of rounds at air and ground targets, reportedly not without some structural damage due to the recoil of the cannon. Besides the COW gun positions fore and aft, the Westbury had a ventral Lewis gun and a fixed Vickers. As on the Bagshot, the rear cannon had limited mobility on a simple trunnion mounting, but a complex mounting allowed a wide field of fire for the nose gun.
A much more interesting experiment was the Boulton & Paul Bittern, built to specification 27/24 for a twin-engined, single-seat nightfighter. The Bittern was a semi-cantilever, shoulder wing-monoplane. The choice for a twin-engined monoplane nightfighter was definitely a radical one, but logical enough, for a nightfighter needed the best possible view for the pilot, so it made sense not to obstruct it with an engine in front of him or an upper wing above him. Unfortunately, problems with wing and aileron flutter delayed the Bittern, and a comparison of early and later photographs shows that the problem was solved by the addition of extensive struts under the wing, which marred the clean lines of the aircraft and can have done little for its performance. With two small 230 hp Armstrong-Siddeley Lynx radials, the Bittern was underpowered. The second and last prototype incorporated an interesting experimental armament: Vertically swivelling Lewis guns in barbettes at the sides of the fuselage, coupled to a frame around the cockpit that carried the gunsight and moved synchronously.
Another twin-engined fighter design resulted from specification F.7/30 for a new RAF day fighter. Most designs to this specification were single-engined, but uncoventional designs were being encouraged by the RAF, which stressed the need for higher performance and a better field of view than offered by the old biplanes. Boulton-Paul offered the P67 design, an interesting mix of the old and the new. It was a low-wing, braced monoplane with an open cockpit and retractable landing gear, powered by two 395 hp Napier Rapier air-cooled, 16-cylinder in-line engines. A top speed of 365 km/h was calculated for this design. The RAF rejected this proposal, and the P67 was never built.
| Westland Westbury | Boulton & Paul Bittern | |
|---|---|---|
| Engines | Bristol Jupiter VI | Armstrong Siddeley Lynx |
| Power | 2 x 430 hp | 2 x 230 hp |
| Wing Span (m) | 20.73 | 12.50 |
| Length (m) | 13.23 | 9.75 |
| Height (m) | 4.19 | |
| Wing Area (m2) | 85.19 | |
| Empty Weight (kg) | 2370 | |
| Loaded Weight (kg) | 3770 | |
| Max Speed (km/h) | 190 km/h at 2000 m | 233 km/h |
| Climb | 2000 m in 7 min 6 sec | |
| Ceiling (m) | 6400 | |
| Guns, fixed | 1 x 7.7 mm Vickers | |
| Guns, flexible | 2 x 37 mm COW 1 1/2 pdr 1 x 7.7 mm Lewis | 2 x 7.7 mm Lewis |
Zerstörer
In 1932 specifications were prepared for the next generation of aircraft for the secret German airforce. They included a single-seat and a two-seat fighter, both single-engined. The two-seat fighter would be a multi-role type, suitable for air combat, ground attack, day and night reconnaissance, light bombing, and the release of chemical weapons. However, in January 1933 Hitler came to power, and subsequently the Reichs Aviation Ministry (RLM) was created, with Hermann Göring at its head. Before the end of the year the re-armament programme was revised.
The single-engined, two-seat fighter of 1932 was replaced by a twin-engined ‘Zerstörer’, with a crew of 3 or 4. The actual role of this substiantially larger aircraft was only loosely defined, but it was primarily a fighter; long-range reconnaissance was to be a secondary task. A maximum speed of 400 km/h at 6,000 m was called for, and the ability to reach this altitude in 15 minutes. Range was to be 2000 km at the 330 km/h cruising speed. Landing speed was restricted to no more than 100 km/h, which had important implications for the wing area and loading. Crucially, there was no detailed specification of the armament, so the designers were left to choose for themselves how to install the two 20-mm cannon and two machineguns required. Presumably, one or more turrets were expected.
The largest of the contenders for the order was the Focke-Wulf Fw 57. With an empty weight of 6,800 kg and a wing span of 25 m, this was close in size to a medium bomber. It was intended to be powered by two DB 600 V-12 engines, but Rolls-Royce Buzzards were installed instead for the first flights, because the DB 600 was not yet ready. The flat nose of the Fw 57 was intended to carry a pair of 20-mm MG-FF cannon on semi-flexible mounts, i.e. with a restricted range of movement, while a Mauser-developed dorsal turret would contain a third cannon. It flew in early 1936, without the armament (which was never installed) but the performance of this overweight and underpowered aircraft fell well below the requirements. Only the three prototypes were completed.
The Henschel Hs 124 was lighter and smaller, with an empty weight of 4,250 kg and a wing span of 18.20 m. The three prototypes were completed to different configurations: The first one carried a mock-up of a planned nose turret with a Rheinmetall-Borsig cannon, and was powered by Jumo 210C engines. The second example had BWM 132Dc radial engines, and the nose turret was replaced by a glazed nose with a vertical slot, intended for the installation of two 20-mm cannon. The third prototype was completed as a two-seat aircraft with two 20-mm and two 7.9-mm guns fixed in the nose; the intended DB 600 V-12 engines had to be replaced by BWM 132 radials. The Hs 124 left a rather better impression than the Fw 57, but any series production was prevented by the success of the Bf 110.
| Focke-Wulf Fw 57 | Henschel Hs 124 V2 | |
|---|---|---|
| Engines | Daimler-Benz DB 600 | BMW 132c |
| Power | 2 x 960 hp | 2 x 850 hp |
| Wing Span (m) | 25.00 | 18.20 |
| Length (m) | 16.57 | 14.50 |
| Height (m) | 4.10 | 3.75 |
| Wing Area (m2) | 73.50 | 54.60 |
| Empty Weight (kg) | 6800 | 4250 |
| Loaded Weight (kg) | 8300 | 7320 |
| Max. Speed (km/h) | 404 km/h at 3000 m | 410 km/h at 3000 m |
| Climb | 2000 m in 4.4 min | |
| Range (km) | 2450 | |
| Guns, fixed | ||
| Guns, flexible | 2 x 20 mm Ikaria MG-FF | 2 x 20 mm |
As in the case of the Bf 109, Willy Messerschmitt ignored large parts of the official specification with the Bf 110. The Bf 110 was the smallest of the contenders, with a wing span of 12.6 m. It was basically a two-seat design, although a third crewmember could be seated if required. There was a single defensive gun at the rear of the cockpit canopy, but all other armament was to be fixed. This helped to reduce the size and weight of the aircraft and, given the limited power of the available engines, increased its performance.
The third prototype was the first to carry any armament, four fixed 7.92-mm MG 17 machine guns in the upper nose. On production aircraft, two MG-FF cannon were installed in the lower fuselage, firing through long blast tubes that ended at the lower side of the nose. This put the breeches of the cannon and the ammunition magazines behind the pilot’s seat. The second crewmember operated a single MG 15 machine gun to defend the rear.
The prototype had DB 600 engines, but these were not available for the production B-0 and B-1 series. With Jumo 210D engines, performance of the B-series fell well short of that of the prototypes. But the Daimler-Benz DB 601A was installed in the Bf 110C, D and E series from late 1938 onwards, and with these engines, the Bf 110 could fly at 510 km/h at 5,000 m altitude.
There was, however, a competing design that promised to achieve higher performance. At Focke-Wulf, the disappointing results of the twin-engined Fw 57 and the single-engined Fw 159 (a parasol-wing aircraft that competed with the Bf 109) inspired Kurt Tank to design a radical, small twin-engined fighter. There was no official requirement for such an aircraft, but the RLM was sufficiently impressed to write a contract around Focke-Wulfs specifications in late 1935. The first flight of the new Fw 187 was made in April 1937, and it proved to be faster than the contemporary Bf 109B.
However, this Fw 187 did not qualify as a Zerstörer. Officially, it was classified as a light fighter, being single-seat and armed only with two MG 17 machine guns. As such it competed with the Bf 109 rather than the Bf 110, and the small speed advantage over the Bf 109, and the larger advantage in range, hardly appeared to justify the cost of an additional engines. The RLM desired a second crew member, and the addition of two 20-mm MG-FF cannon to the fixed armament. Two prototypes were completed in the two-seat configuration with Jumo 210 engines, and two more with DB 600 engines. The inevitable downside of these modifications was that the loaded weight of the small fighter rose rapidly, while range decreased.
Only three Fw 187A-0 pre-series aircraft were completed. By all accounts, the pilots who flew them on operations in Norway during the winter of 1940 were greatly impressed by these aircraft. Armament consisted of four fixed MG 17s, installed in the sides of the fuselage just below the sill of the cockpit canopy, and two MG-FF cannon in the lower fuselage. But the Luftwaffe still preferred the Bf 110, which was larger and roomier, and therefore more adaptable to other tasks.
| Messerschmitt Bf 110C-1 | Focke-Wulf Fw 187A-0 | |
|---|---|---|
| Engines | Daimler-Benz DB 601A-1 | Junkers Jumo 210Ga |
| Rating | 2 × 1100 hp | 2 × 730 hp |
| Wing Span (m) | 16.25 | 15.30 |
| Length (m) | 12.07 | 11.12 |
| Height (m) | 4.13 | 3.90 |
| Wing Area (m2) | 38.40 | 30.40 |
| Empty Weight (kg) | 5,600 | 3,700 |
| Loaded Weight (kg) | 7,200 | 5,000 |
| Max. Speed (km/h) | 540 km/h at 6,000 m | 529 km/h at 4,200 m |
| Climb | 660 m/min | 1050 m/min |
| Ceiling (m) | 10,000 | |
| Range (km) | ||
| Fixed Guns | 2 × 20 mm Ikaria MG-FF 4 × 7.92 mm MG 17 | 2 × 20 mm Ikaria MG-FF 4 × 7.92 mm MG 17 |
| Flexible Guns | 1 × 7.92 mm MG 15 |
Even more radical concepts were being developed at Arado. From 1935 onwards, designer Walter Blume worked on a twin-engined, twin-boom aircraft with large, streamlined gun turrets on the upper and lower side of the central nacelle. These would have a free field of fire towards the front and rear, because of the absence of a central tailplane between the booms: The horizontal tails were to extend to the outside. By 1937 this had evolved in a project E 500, which was to have two powered gun turrets with two 20-mm Rheinmetall-Borsig Lb 202 cannon each. The RLM initially showed some interest, but soon lost it.
Arado then started work on a Zerstörer with fixed armament. Project E 651 of 1937 envisaged a streamlined four-seat aircraft, with two Daimler-Benz engines buried in the wing/fuselage blending, driving tractor propellors on the wings through a gearbox and shafts. This was rejected by the RLM as too complicated, but it must have left a mark. When in 1938 work was started on a successor for the Bf 110, Arado was given the opportunity to compete with Messerschmitt, and encouraged to be radically innovative.
Meanwhile, the Bf 110 did become the standard twin-engined fighter of the Luftwaffe. The Zerstörer were soon regarded as the elite of the German air force, perhaps because they embodied the agressive spirit of the new German regime better than the short-ranged Bf 109, which primarily was a defensive fighter. Accordingly, the aircraft was given much publicity by the Nazi propaganda ministry, and the sleek Bf 110 became the inspiration for a series of foreign designs. After a decade in which twin-engined fighters had almost invariably been disappointments, the category suddenly became fashionable again.
To be fair to the air forces and design teams, both the advance of aeronautical technology and the tactical thinking of the time favoured the twin-engined concept. The single-engined fighter was evolving towards the combination of the smallest and sleekest possible airframe with the largest possible engine. This combination resulted in fast, agile fighters with a minimal range, such as the Bf 109 and Spitfire. These were highly optimized for air defense operations (directed by radar, which was secretly being developed in several countries simultaneously) against the new fast bombers, but handicapped in supporting offensive operations. This created a niche for a larger aircraft, and most assumed that it would have to be twin-engined to achieve anything like the performance of the smaller type. Advances in technology eliminated the numerous struts and wires that had handicapped the performance of older designs, and made it possible to design large fighters that were aerodynamically efficient.
It remained to be seen how the new twin-engined fighters would perform in actual combat. At the outbreak of the Second World War, the Bf 110 was the equal or the better in performance of most fighters in service; only the Bf 109 and Spitfire were clearly superior. Over Poland, Norway, and France, against weak opposition in the air, the Bf 110 proved quite effective on offensive missions, including interception, air superiority and ground attack. But even over Poland, where the opposition consisted mainly of the antiquated PZL P.11 fighter, crews flying escort missions for bombers felt vulnerable, because they were forced to await being attacked. It would soon get worse.
Jachtkruiser
On 16 March 1937 an unusual shape made its first flight in the Dutch skies: The Fokker G.1, a twin-engined, twin-tailboom multi-role aircraft. The two-seat G.1 was classified by the Dutch as a “jachtkruiser” (cruiser pursuit) but could also serve as reconnaissance aircraft and light bomber, having a small internal bomb bay. The prototype had French engines, 690 hp Hispano-Suiza 14AB radials, probably because Fokker was hoping to get orders from France: There was no official Dutch requirement for the G.1, which was a private venture. The unreliable 14AB engines were later replaced by American 825 hp R-1535-SB4-G Twin Wasp Junior radials, and when the Dutch government placed an order, it chose the 830 hp Bristol Mercury VIII engine, which already powered other Dutch aircraft.
The G.1 was a clean cantilever monoplane, but under its skin the structure largely followed the traditional Fokker pattern: A wooden wing, with plywood skinning, and a fuselage that combined welded steel tube and wood. Only the tailbooms were light alloy semi-monococque structures. The short fuselage ended in a “Fokker conical turret”, a grand name for a cone that rotated around its horizontal axis, with a slot for a machine gun. The performance was not particularly exciting by the standards of the time, and the French decided that the G.1 wasn’t better than their own Potez 63. However, it was slightly faster than the Fokker D.XXI, a single-engined aircraft with fixed landing gear, which was the Dutch standard fighter of the period. Fokker found a number of export customers for the two-seat G.1B, while the Dutch military ordered the larger, three-seat G.1A. (But most would be delivered with just two seats installed.)
The armament varied with the customer. The original proposal was to have two 23-mm Madsen cannon in the nose, plus two 7.9-mm machine guns, also from Madsen, and one similar weapon in the turret. A proposal to install 20-mm Hispano cannon had to be abandoned because of an excessive shift of the center of gravity. The Dutch G.1A had eight 7.9-mm FN-Browning M.36 guns in the nose instead, and another one in the tail cone. In May 1940 only 23 were available for service, and one more G.1A and three G.1Bs were rapidly added. The latter, built for export, received an interim armament of four FN-Browning guns in the nose.
After brief and unequal combat, the Luftwaffe overwhelmed the opposition. Production of the G.1 continued for the Luftwaffe, which used it as advanced trainer. This brought the total number of G.1s produced to 62.
German bombs destroyed the prototype of the Fokker D.XXIII, an ingenious twin-engined, single-seat fighter. The D.XXIII represented an attempt to achieve high performance by using two engines on a relatively small airframe: It featured two slender tail booms, and a short fuselage with an engine in the nose and at the rear. The prototype had relatively low-powered, air-cooled in-line engines, 540 hp Walter Sagitta I-SR inverted V-12s. First flown on 30 May 1939, it suffered from the predictable problems with the cooling of the rear engine, and only short flights were accomplished. Armament was to be two 13.2-mm and two 7.9-mm machine guns.
The concept was to be tried again in another country. In late 1940 the Soviet design team of Aleksandr Moskalyev flew an aircraft that was clearly inspired by the D.XXIII, the SAM-13. It was smaller and lighter, with a loaded weight of only 1183 kg against 2950 kg for the D.XXIII, and powered by 236 hp MV-6 (licensed Renault) engines. Speeds up to 560 km/h are reported to have been achieved during the brief test programme, before this prototype too was destroyed during the German attack.
| Fokker G.1 | ||
|---|---|---|
| Engines | Mercury VIII | |
| Power | 2 x 830 hp | |
| Wing Span (m) | 17.16 | |
| Length (m) | 10.87 | |
| Height (m) | 3.80 | |
| Wing Area (m2) | 38.30 | |
| Empty Weight (kg) | ||
| Loaded Weight (kg) | 4790 | |
| Max. Speed (km/h) | 475 km/h at 4100 m | |
| Climb | ||
| Ceiling (m) | 9300 | |
| Range (km) | 1520 | |
| Guns, fixed | 8 x 7.9 mm FN-Browning | |
| Guns, flexible | 1 x 7.9 mm FN-Browning | |
| Bombs, internal | 400 kg |
Polish hopes
In the middle of the 1930s, future planning for the Polish air force included the concept of a twin-engined multi-role aircraft, to serve as interceptor fighter, attack aircraft, and dive bomber. A mixed armament of cannon and machine guns was specified, while the recommended powerplant was the 8-cylinder, inverted, air-cooled Foka, which was expected to deliver around 600 hp. By 1936 the interceptor fighter role had been dropped, and the requirement refined to primarily one for a dive bomber and attack aircraft, with a secondary fighter role.
The P.Z.L. P.38 Wilk was structurally similar to the P.37 Los bomber, but smaller. It was intended to have two 20-mm Wz 38 cannon and two 7.7-mm machine guns in the nose, while a pair of 7.7-mm flexible guns was installed to defend the rear. Maximum speed was expected to be 520 km/h, but development of the Foka engine ran into severe difficulties, and by the end of 1937 it still was not ready for flight. In May 1938 the second prototype flew with American Ranger SGV-770B engines instead. These delivered only 450 hp, so could be regarded only as a stopgap. The first prototype made its first flight in January 1939 with improved Foka II engines. However, these too were not yet ready for production.
An alternative engine had been identified in the French Gnome-Rhône 14M Mars, a small-diameter radial. This was selected for the P.48 Lampart, a substantially redesigned aircraft, expected to be built in a version with two Wz 38 cannon and four Wz 36 machine guns in the nose, and one with eight Wz 36 machine guns. Top speed would was predicted to be 560 km/h. In September 1939, the prototypes were still being assembled, and both are believed to have fallen into German hands.
French rearmament
On 31 October 1934, the French air ministry issued a specification for a light multi-seat fighter. One of the roles envisaged for this aircraft was that of a three-seat command aircraft, flying in combat with an unit of single-seat fighters, carrying the latter’s commander. More conventional roles were that of two-seat escort fighter and two-seat nightfighter. The aircraft was to weigh less than 3000 kg (a limit later increased to 3500 kg) and have a maximum speed of 400 km/h (soon raised to 450 km/h) at 4000 m. The fixed armament was specified as two 20-mm cannon.
France maintained a substantial number of aircraft manufacturers, and they replied with offers that included the Breguet 690, Hanriot H.220, Loire-Nieuport 20, Potez 63, and Romano R.110. Potez offered the 63 with alternative Hispano-Suiza or Gnome-Rhône engines, the corresponding aircraft becoming the models 630 and 631, respectively. The Loire-Nieuport design never materialized, but prototypes of all the others were built.
The Breguet 690 was a neat mid-wing monoplane with a dolphin-shaped fuselage and twin tail fins. With 680 hp Hispano-Suiza 14Ab radials, it reached a speed of 490 km/h at 4000 m. Armament consisted of two fixed 20-mm cannon and a 7.5-mm machine gun in the rear cockpit. This aircraft held promise, but it made its first flight in March 1938, when the Potez 63 had already been selected to fill the fighter requirement. The 690 was further developed as attack aircraft.
The Hanriot H.220 looked unusual but sleek, with a short fuselage and, in its initial form, long nacelles for the Renault 12Roi in-line air-cooled engines, so that the propellers were well ahead of the nose. With these 450 hp engines the design was underpowered, so 680 hp Gnome-Rhône 14M radials were installed. Serious handling problems (and major structural damage following a forced landing) indicated a need for major redesign. For the time being, it was out of the running.
The Romano R-110 was remarkably awkard. Conservative in structure, with wooden wings and a steel-tube fuselage, it featured a second cockpit, stacked above and behind the normal position of the pilot. This was intended for use as aerial command post, the commander thus being provided with an excellent outside view. Although only powered by 450 hp Renault 12Ro in-line engines, the R-110 was credited with a top speed of 470 km/h, but like the Breguet 690 it flew in March 1938, too late to compete with the Potez 63. Development of this design, which appears to have had little enough potential, was discontinued.
The Potez 630 made its first flight in April 1936, powered by 580 hp Hispano-Suiza 14Hb engines. It was a very elegant, streamlined aircraft: A fuselage of oval cross-section, a low-set wing with straight taper outboard of the engines, close-cowled radial engines, and a tailplane with twin fins. During flight tests a tailplane with dihedral was installed to improve handling, and the engines were replaced by more powerful 14Ab of the same manufacturer. The Potez 630 reached 460 km/h at 5000 m. The second prototype was completed as a 631, with Gnome-Rhône 14M Mars engines, whose smaller diameter resulted in more streamlined engine cowlings, compensating for their lower power, 660 hp at rated altitude against 725 hp for the 14Ab.
The Potez 631 was selected and ordered into production, but because of a shortage of 14M engines, a larger number of Potez 630s were also ordered. The first production aircraft, a 630, was accepted in May 1938. The basic airframe proved robust, easy to fly, and adaptable. Orders also followed for derivatives, such as the 633 light bomber, the 637 observation aircraft, and the 63.11 reconnaissance aircraft. The observer in the 637 had to be content with a small ventral gondola with windows, while the 63.11 had a redesigned nose with large transparant panels.
As usual, the production aircraft were slower than the prototypes. Measured top speed at 4,000 m was 448 km/h for the 630 and 437 km/h for the 631. On the other hand, the lighter 631 reached this altitude in 5 minutes and 6 seconds, while the 630 needed 7 minutes. For their size, these aircraft were too underpowered to show excellent performance. They fell short not only of international standards, but also of the requirements of the current French re-armament plan, the Plan V. But nothing better was available, so relatively large orders were placed for the 631. It was decided to withdraw the 630 from first-line service, because of its unreliable engines.
Production remained disappointingly slow, hampered by shortages of engines, propellers, and guns. On 1 January 1939 the air force should have had a total of 201 model 630 and 631 fighters in service; it actually had 45 and 27, respectively. In the factories, 74 airframes were still awaiting engines, while many of the aircraft delivered had temporary two-bladed wooden propellers, instead of the metal three-bladed service unit. The planned armament of two 20-mm cannon was not available for the first 630s, so they were armed with four 7.5-mm machine guns instead. Many 631s had just one 20-mm cannon fitted, the other being temporarily replaced by a 7.5-mm gun.
At the outbreak of war the deliveries totalled 206 Potez 631s and 85 Potez 630s, but many had not yet reached combat units. In early 1940, the idea of using the aircraft as a flying command post was abandoned, and the last 630s were withdrawn from operational service. To improve the firepower of the 631s, the second 20-mm cannon would be installed during overhaul. It was also intended that four additional machine guns would be added under the wings, but only a handful of aircraft were so modified. Six night fighter groups (GCN, Groupe de Chasse de Nuit) were equipped with Potez 631s in May 1940; the type was no longer present in day fighter units. The derivative Potez 63.11, a tactical reconnaissance aircraft, was in service in significant numbers.
| Potez 630 | Potez 631 | |
|---|---|---|
| Engines | Hispano-Suiza 14Ab | Gnome-Rhône 14M4/5 |
| Power | 2 x 700 hp | 2 x 700 hp |
| Wing Span (m) | 16.00 | 16.00 |
| Length (m) | 11.07 | 11.07 |
| Height (m) | 3.62 | 3.62 |
| Wing Area (m2) | 32.70 | 32.70 |
| Empty Weight (kg) | 2450 | 2450 |
| Loaded Weight (kg) | 3850 | 3760 |
| Max. Speed (km/h) | 460 km/h at 4500 m | 442 km/h at 4500 m |
| Climb | 4000 m in 5 min 56 sec | |
| Ceiling (m) | 6000 | 6000 |
| Range (km) | ||
| Guns, fixed | 2 x 20 mm | 2 x 20 mm |
| Guns, flexible | 1 x 7.5 mm MAC 34 |
Wisely, a programme to find a successor to the Potez 630 had been formally launched in 1937, even before the aircraft entered service. The new A22 requirement specified a top speed of 550 km/h and heavy armament. The aircraft also had to have an autonomy of three hours at an elevated cruising speed, 90% of the maximum speed. The crew, for the moment, remained at three. Because of fears of a shortage of light alloys during wartime, the use of “non-strategic” materials such as wood and steel was recommended.
Further development of the 630 series produced the Potez 670, initially intended as a three-seat long-range fighter, but modified to have a crew of two before the first flight. It was a bit smaller than the 63, and the wing had an elliptical instead of a straight-tapered plan. The 670 was powered by 700 hp 14M radials, but the prototype was subsequently modified to represent the 671 with 800 hp 14AB 12/13 engines. Having made its first flight in March 1939, the type was still undergoing tests at the time of the French defeat, although work on a small production batch was already ongoing. With a top speed of 500 km/h at 6000 m, the 671 failed to meet the A22 specifications.
Hanriot offered the latest developments of the H.220. At first, the wing and engines were retained, and combined with a new fuselage and tail, this becoming the H.220-2. It made its first flight in March 1939 and showed a decent level speed performance, with a top speed of 532 km/h at 5,000 m. However, aerodynamic problems prompted further redesign, including the replacement of the wing. Because Hanriot had been absorbed in the SNCA du Centre, the aircraft was renamed the Centre NC.600. The first flight was made on 15 May 1940, after the German attack, precluding any possibility of series production. With two 710 hp 14M0/01 engines, the top speed was raised to 542 km/h. Armament was two 20-mm HS.404 cannon and two 7.5-mm MAC34 machine guns in the nose, and a flexible 20-mm cannon for the defense of the rear. The latter was stowed in a slot in the rear decking when not in use.
The most radically innovative of the A22 fighters was known at first as the Lioré et Olivier LeO 50. But before the prototypes were ordered Lioré et Olivier was absorbed in the SNCASE, the Societé Nationale de Constructions Aéronautiques du Sud-Est, also simply known as Sud-Est, and the aircraft became the SE.100.
The SE.100 was designed around a wooden wing, of small area but with extensive flaps to keep the landing speed down. Instead of normal ailerons, a large part of the wingtip was made movable, a design that was thought to provide more room for flaps on the trailing edge of the wing. The fuselage was constructed from steel tube and wood, was well streamlined, and unusually short; it ended in a large tailplane with twin fins. The landing gear consisted of a large retractable nosewheel and smaller wheels retracting into the lower part of the tail fins. Armament initially included two fixed 20-mm HS.404 cannon, and a single HS.404 on a hydraulically-powered mount to defend the rear; the latter could be neatly stowed in the rear decking of the fuselage when not in use.
The SE.100 looked futuristic, but below the sleek skin its wood and steel structure was heavy. The prototype weighed 5,732 kg empty and 7,679 kg loaded. With two 900 hp Gnome-Rhône 14N-0 engines, the SE.100 needed 8 minutes and 20 seconds to climb to 4,000 m, and its top speed of 547 km/h at 4,400 m was well below the 600 km/h expectation of its designers. Hope was not abandoned; the design team considered production versions of the SE.100 with a lighter structure, more powerful engines, and much more powerful armament: One three-seat attack-bomber version was to have six forward-firing HS.404 cannon, a single HS.404 and two 7.5-mm machine guns in a ventral gondola, and a HS.404 at the tail. The defeat of May 1940 meant that all such plans were to remain dreams.
The Bf 110 in the Battle of Britain
Events were now leading to a confrontation between the Luftwaffe and the RAF, in which the Bf 110 would suffer heavily. Consequently, sharp criticism of this aircraft, and by extension the entire class of twin-engined fighters followed, well beyond what is justified by the actual events.
It is as well to start by looking at the performance parameters. The Bf 110C-4, a common model at the time, was powered by two 1,100 hp Daimler-Benz DB 601A engines, and had a normal loaded weight of 6,940 kg; so its power loading was 3.15 kg/hp. The Hurricane Mk.I weighed 2,924 kg loaded and was powered by a 1,030 hp Merlin Mk.III engine, giving a better power loading, of 2.84 kg/hp. The Spitfire Mk.I, powered by the same engine, was lighter because of its more advanced structure, and the normal loaded weight of 2,812 kg resulted in a power loading of only 2.73 kg/hp.
The Bf 110C-4 was claimed to be capable of a maximum level speed of 562 km/h at 7,000 m altitude, and 473 km/h at sea level. British test pilots, flying a captured C-5 reconnaissance version at lower weights, measured only 547 km/h at 6,700 m, so perhaps the German figures were too optimistic, but such variations between individual aircraft were common and the test aircraft had been assembled from two wrecks. This was still considerably faster than the 508 km/h at 5,400 m and 409 km/h at sea level of the Hurricane Mk.I, and competitive with the 557 km/h at 4,570 m of the Spitfire Mk.IA. The German fighter also performed reasonably well in the climb, with an initial rate of climb of 670 m/min, somewhat inferior to the 770 and 740 m/min attributed to the Hurricane and Spitfire. Nevertheless it could climb to 5,500 m in 7 minutes, while the Spitfire Mk.IA needed 6.85 minutes to 4,570 m (and the Hurricane 6.3 minutes).
During the battle of Britain, 100-octane fuel was used by RAF fighters. By allowing the Merlin to run at higher pressures and generate 1,200 hp, this considerably improved their performance, especially in climb. That came as an unpleasant surprise for the Luftwaffe. The German synthetic fuel industry would produce limited amounts of high-octane C3 fuel, and the Bf 110 would get priority for the DB 601N engine that was tuned for better performance on C3, but a series of technical issues would develop and this engine was not a success. Nevertheless, in terms of performance the Bf 110 appeared competitive enough. Its weakness in combat against the RAF fighters was its manoeuverability, which was good for an aircraft of its size, but no match for the opposition it faced. The wing loading of the C-4 model, 181 kg/m2, was considerably higher than the 123 kg/m2 of the Hurricane and the 125 kg/m2 of the Spitfire, and this resulted in a larger turning circle. The handling of the Bf 110 was good at normal speeds, with effective controls and generally pleasant flying characteristics. The controls heavied up considerably at higher speeds, and the ailerons were almost immovable in fast dives. But this, it must be said, was a weakness of all fighters at the time, German and British alike.
Overall, the characteristics of the Bf 110 implied that the Zerstörer were at a serious disadvantage whenever they were forced into a defensive posture. Their speed and firepower served them well when they made hit-and-run attacks, exploiting their good climb and dive characteristics, while declining to engage in turning combat. Most fighter-versus-fighter combat during battle was of short duration anyway, typically less than half a minute; long dogfights were very rare because they left those who engaged in them in a very vulnerable position, low and slow, and too preoccupied with their target to watch their rear.
When flying Freie Jagd fighter sweeps at high altitude (above 6,700 m) over Southern England, the Zerstörer units demonstrated that the Bf 110 was indeed a dangerous opponent, and could be more than a match for the Hurricane. Unfortunately for the Germans, this was not the pattern of the battle to come. Fighter Command soon decided to avoid contact with the fighter sweeps, and concentrate its attacks on the bomber formations. Then, flying at low and medium altitude, and robbed of the advantage of surprise by radar and the advanced command-and-control system of Fighter Command, the German crews often found themselves at a serious tactical disadvantage. This, of course, also handicapped the Bf 109, but the nimbler single-engined fighter could extract itself from an unfavourable situation with much more ease. A frequent reaction of Bf 110 crews was to seek safety in a Lufbery circle, a defensive formation in which fighters fly a ring pattern, each covering the rear of the one next ahead in the formation. While an effective tactic, this could only be maintained for as long as the fuel reserve allowed, and the RAF fighters waited above for an opportunity to strike.
Indicative of how just badly things could go wrong was an attack on Croydon on 15 August 1940. This attack was executed by bomb-carrying Bf 110s and Bf 109s of Erprobungsgruppe 210. Serious damage was done to airfield, but while still in its attack run at low level the Germans were bounced by a superior number of Hurricanes. They found temporary protection in a defensive circle, but finally had to make a break for home; the RAF fighters immediately fell onto their targets, shooting down seven out of the twenty-two strong enemy force.
The Bf 110 also had to take on the role of escort fighter for the bombers, because the fuel capacity of the Bf 109 allowed it only a short stay over England. But the German escort tactics were flawed. (Arguably, they were not much more flawed than some other attempts to provide bomber escort, such as the early efforts of the USAAF.) Too many fighters were ordered to fly close escort, in formation with the bombers; in that position they could do little to defend the bombers and became vulnerable targets themselves. The error was made worse in early September: Against the strong opinion of the fighter leaders, they were ordered to stay closer to the bombers, and even to break off combat with enemy fighters if the bombers were threatened. The Bf 110 turned out to be unsuitable for close escort missions, and losses were heavy.
That said, the often-repeated story that the Bf 110s themselves had to be escorted by Bf 109s is a myth. Tactically, this would have made no sense. The only Bf 110s that were really escorted by single-engined fighters were the fighter-bombers of Erprobungsgruppe 210, which were otherwise too vulnerable with their combat loads, or when making their attacks at low level. Perhaps this, or maybe the complex organisation of the escort fighters around a bomber formation (including fighters flying distant cover, close cover, and top cover) gave raise to the story.
The failure of the Bf 110 as an escort fighter was just one more nail in the coffin of a doomed operation, but as they had been a much-propagandized part of the German air force, it became symbolic. Its importance should not be exaggerated, for at the height of the battle, Bf 110s were less than 12% of the Luftwaffe’s available force, and only just over a quarter of the fighter force. There were many more important factors that doomed the German attempt to subdue Britain to failure: The German command had seriously underestimated RAF strength, and ignored the importance of the radar and command-and-control network. The available bomber strength was insufficient, and in turn the fighter strength fell short of the requirement to provide escorts in a 2:1 ratio. Aircraft production and crew training lagged behind the RAF. Intelligence and target identification were very poor, so that many bombs were wasted on targets of little importance, and enemy losses strongly overestimated. The range of the Bf 109E was too short, the use of drop tanks was neglected, and the available bombers had small bomb loads and weak defensive armament. Last but not least, the operation itself was handicapped by two conflicting goals, preparing for an invasion by destroying the RAF and forcing Britain to come to a negotiated peace by bombing, and neither was a realistic war plan.
The catalogue of failure should not obscure that the Bf 110 had also found a role in which it was very effective, that of a fighter-bomber. This was especially so in the hands of Erprobungsgruppe 210, a unit that specialized in precision attacks on targets such as airfields, radar stations, and shipping. At the beginning of the battle it operated a number of Bf 110C-6 with a powerful MK 101 30-mm cannon in a fairing under the fuselage, but operational and technical problems lead to the removal of this weapon. The occasional heavy losses demonstrated the need for air superiority on such operations, but a series of successful attacks also confirmed the effectiveness of the aircraft in this role. The large Messerschmitt fighter still had a long and generally successful career ahead of it.
RAF
As we have seen above, none of the British twin-engined fighter projects during the interbellum produced an useful combat type. Instead, single-engined two-seat fighters were in service, the last biplane representative being the Hawker Demon, a development of the Hart light Bomber. Its direct technical successor was arguably the Boulton-Paul Defiant, but there also appeared to be an operational requirement for a twin-engined, long-range fighter. It so happened that a convenient solution seemed to be around in the form of the Bristol Blenheim light bomber, an aircraft with a fairly high performance. Blenheim Mk.I bombers were already being replaced by later versions in Bomber Command, and after some debate a very simple conversion of these aircraft allowed them to be used as heavy fighters. The changes consisted of four .303 machine guns in a ventral gun pack, light armour plate for the pilot, and a gunsight (or more precisely two, a reflector sight and a ring-and-bead sight as backup). This version was known as the Blenheim Mk.IF. It also retained the simple and rather ineffective Bristol dorsal turret with a single Vickers Class K gun, and a fixed gun in the left wing.
Introduced in late 1938, this amounted to a conversion of a faintly obsolescent light bomber into an even more obsolescent heavy fighter, underpowered and undergunned. With 840 hp Mercury VIII engines and a top speed of 423 km/h at 3,050 m, the Mk.IF was no match for the Bf 110, and combat against enemy single-seat fighters clearly had to be avoided. As a day fighter, the Blenheim was best used only as a trainer, although some went to Coastal Command. (In vain, this Command pressed for the replacement of the .303 guns by two 20-mm cannon.) It was the best available candidate for conversion into a radar-equipped nightfighter, however, despite the disadvantage of reflections in the complex cockpit glazing.
The installation of radar was done in great haste. The first demonstration of the prototype AI radar set (installed in a Fairy Battle) was done in June 1939. It was quickly decided to equip 30 Blenheims, and 15 sets were delivered at the beginning of the war, to No 25 Squadron. Results were not forthcoming, because the ground control, while good enough to intercept enemy formations in daylight, was incapable of putting a nightfighter within radar range of an enemy target. Not until 1941 was an effective “Ground Controlled Interception” (GCI) radar available.
Meanwhile, work proceeded on a number of other designs, in fact quite a large number, and designed to a series of different specifications: F.37/35, F.18/36, F.9/37, F.11/37, F.18/37, F.22/39, F.18/40, and others. A few designs were built, and the Westland Whirlwind even briefly entered service. But the outcome of all this work was that the RAF fought the war with two twin-engined fighters, the Beaufighter and the Mosquito, which were fortunate by-products of other programmes.
There was no single cause for this state of affairs, but several important ones can be identified. Many of the first generation of twin-engined designs were intentionally kept small, to stay close to the weight and size of single-engined fighters. But this not only limited their usefulness and development potential, it also tied them to lighter engines of lower power, while wartime efficiency dictated that only a few types of the most powerful engines were mass-produced. Other designs were prepared to carry powerful cannon armament in flexible mounts, up to quadruple 20-mm cannon or a single 40-mm cannon. This resulted in aircraft that were far too big and heavy, and was tactical nonsense. Finally, first the need for high production of defensive fighters and later the excellence of the Mosquito blocked the way for a number of designs with high potential, of which the production could not be justified on grounds of industrial policy.
Sad Whirlwind
The unfortunate story of the Westland Whirlwind illustrates the point. It originated from specification F.37/35 for a fighter armed with four 20-mm or 23-mm cannon, a weapon that had been selected to arm the next generation of RAF fighters. (The Hispano had been selected, but apparently the choice between the 20-mm HS.404 or a 23-mm development of this weapon had not yet been made.) The specification left open whether the aircraft would have one or two engines, but design teams apparently felt that they needed 1600 to 1800 hp, an amount of power that could not be delivered by a single one of the engines in production at that time. The most promising of the single-engined designs was judged to be the Boulton-Paul P.88, offered in versions with a Bristol Hercules or Rolls-Royce Vulture engine, neither of which was immediately available. A (rather more elegant) offering by Bristol, the Type 153 design, also featured a Hercules radial, but Bristol also created the Type 153A, a small fighter powered by two Bristol Aquila radials. The 153A had some features in common with the Grumman XF5F-1 Skyrocket, including a fuselage nose that did not extend beyond the leading edge of the wing, radial engines in short stubby nacelles spaced closely together, and two small tailfins. The four cannon were in the bottom of the fuselage. It looked neat, but with hindsight the aerodynamics of it are rather suspect, and perhaps it is fortunate that the Type 153A was not built.
At Westland, W.E.W. Petter solved the problem by designing a small, trim fighter with two Rolls-Royce Peregrine engines, rated for 885 hp at 4,600 m. It was a highly streamlined design, and not much bigger than a Hurricane or a Spitfire. The Westland P 9 design also featured a magnesium alloy monocoque fuselage, a relatively small wing with large Fowler flaps, integral fuel tankage, radiators buried in the wing roots, and an all-round vision cockpit. The four 20-mm cannon Hispano were closely grouped together in the nose. It really was an innovative and attractive design, but in September 1936 the RAF also chose Westland because it had a less busy schedule than other manufacturers, its main other task so far being the development and production of the Lysander close-support aircraft. The order for the P 9 was confirmed in February of the following year.
There was a risk in choosing a small and relatively inexperienced company without large production facilities. The prototype flew in October 1938, but production was not expected before June 1940; the schedule continued to slip and the first operational squadron was only equipped in December 1940. Even in the first production aircraft many modifications were made, which confirmed the RAF’s fears that Westland was not capable of mass-producing a modern fighter. The company defended itself by arguing that the Whirlwind’s engine and handling faults were not worse than those encountered by most fighter prototypes, which was probably true. But Westland was too small to solve them quickly, the RAF was losing confidence, and Rolls-Royce gave the Peregrine a low priority. In August 1939 staff officers were already considering the replacement of the F.37/35 by a new design. There was nothing wrong with the performance of the aircraft, which at low altitude was faster than a Hurricane or Spitfire, and highly manoeuverable. Its misfortune was that because of all the delay, it entered service at a time when other single and twin-engined fighters were already being adapted to the same missions, and in many ways showing themselves more suitable.
And the choice for the Peregrine engine had been a fundamental error. About 20% less powerful than the Merlin, this engine was used by no other aircraft, and it was soon obvious that the war required Rolls-Royce to concentrate its effort on a few much-needed engines, each the most powerful in their class. Continued Peregrine production was only possible by building fewer Merlins, and because of reduced efficiency the exchange would be at a rate of perhaps two to one, meaning that one Whirlwind would cost the engines for four Spitfires or Hurricanes. Or, alternatively, by postponing the production of the much-needed Vulture or Griffon, which was also out of the question. Therefore, when Rolls-Royce stated that these were the options from which a choice could be made, the fate of the Whirlwind was sealed. To be fair to Westland, Bristol’s choice for the Aquila radial, another small engine, was not any better, and Supermarine suggested to use the doomed Rolls-Royce Goshawk or even the Hispano-Suiza 12Y in its Type 313. All twin-engined offerings to F.37/35 suffered from the same problem.
Anyway, in May 1940, months before the fighter even entered service, it was decided that Peregrine production would end in December. Therefore production of the Whirlwind would have to be stopped after only 114 had been delivered. The type continued in service until December 1943, mainly in the fighter-bomber role with a 500 lb bomb under each wing. With this relatively modest bombload the Whirlwind handled well, and the cannon were useful for strafing.
The installation of the Merlin might have salvaged the concept, but at the time Westland rejected this. The airframe was too small to take a bigger engine. (Later, Petter admitted that it would have been better to design the aircraft around two Merlins from the start.) Westland proposed to cure the main faults of the Whirlwind in a Mk.II version, which would have a larger internal fuel capacity, 120 instead of 60 rounds per cannon, and further developed Peregrine engines – but Rolls-Royce poured scorn on that idea. Another alternative suggested by Westland was the installation of American radial engines, perhaps the R-1830 or R-1820. The RAF had already lost interest in the Whirlwind, so these were dead ends.
Size matters
The twin-engined fighter that would do most to replace the Whirlwind was born out of a very different project. In October 1938, the first flight was recorded of the Beaufort, a twin-engined torpedo-bomber that would have a useful but troubled service life. Within the next few days, Bristol proposed the Type 156, a fairly straightforward development that would retain the wings and tail of the Beaufort, with a new front fuselage, and more powerful Hercules radials replacing the Taurus engines of the Beaufort. Four cannon would be installed in the bottom of the new front fuselage. (After the first 50 aircraft, six .303 wing guns were added.) Bristol predicted a top speed of 580 km/h at 4,600 m. And crucially, it indicated that this aircraft could be available early in 1940, even calculating that it would be able to produce 100 of them before the end of 1939. The idea was accepted, and specification F.17/39 was written around Bristol’s design.
The story is told that W.E.W. Petter was invited to have a look at the mock-up of the competing design, and commented that it was “far too large, especially the fuselage.” To which Bristol’s designer Frise replied that in two years time, such size would be necessary – and he was right. Size saved the Beaufighter, as it would be called, when the Whirlwind was cancelled. Because of its size, it had much more development potential, and unlike the Whirlwind, could do things no single-engined fighter was capable of. The pugnaciously ugly Beaufighter was an aircraft the RAF refused to do without, even when concerns about delivery and performance reached the alarm level.
It soon became clear that the Hercules VISM engine, for which the original performance estimates had been made, would not be ready for some time, and that the first production aircraft would have the less powerful Hercules II or III. The optimism of early production schedules could not be maintained either; the first prototype flew in July 1939 and the first production examples reached operation squadron in September 1940. Three squadrons were fully equipped with Beaufighters before the first Whirlwind squadron became operational, but this had been achieved by skipping final testing stages, and as consequence the serviceability of these aircraft was low. Especially the cannon caused a lot of problems, because the Hispano demanded a very stiff mounting. Early Beaufighters carried their total of 240 rounds per cannon in smaller drums, and the hapless observer had to exchange these heavy objects, groping around in a cluttered and dark fuselage. After much delay, a belt feed mechanism was developed and installed.
Tests by the A&AEE of the first four prototypes, with different engines, revealed top speeds between 539 km/h and 483 km/h, considerably inferior to the Whirlwind and a cause for serious concern. However, the Beaufighter was ideally suited as a nightfighter, because there was room in the fuselage for radar and a radar operator, and was also eagerly awaited by Coastal Command. The Mk.IC version for Coastal Command had extra fuel tankage instead of the 7.7-mm wing guns, giving it a range of 2900 km.
But performance really had to be improved. With Hercules III engines, the top speed of a production Mk.IF was 520 km/h at 4,400 m; with Hercules X or XI engines this was raised to 539 km/h at 4,700 m. Concern about Hercules engine production lead to the Beaufighter Mk.IIF version powered by Merlin XX engines, which during tests gave this model a better high altitude performance, 534 km/h being reached at 9,150 m. But these good results were not reproduced on production aircraft, and in addition the different shape of the engine nacelles created aerodynamic problems. The 1,670 hp Hercules VI or XVI, when finally available, was installed in the Beaufighter Mk.VI, but the performance improvement with the new engines was disappointing.
More hopes were raised by the prospect of installing Griffon engines, or entirely redesigning the fuselage to reduce drag. Because of wartime production pressures these projects were not pursued, and the introduction of the superior nightfighter Mosquitoes in mid-1942 removed much of the need for a faster Beaufighter. From then on the Beaufighter was gradually phased out as a nightfighter, although Fighter Command did not release the last ones until June 1944. Four USAAF squadrons, operating in the Mediterranean, were also equipped with nightfighter “Beaus”.
The Beaufighter then primarily evolved into a highly successful strike fighter. It had generally good handling characteristics, but early “Beaus” suffered from longitudinal instability, and the Mk.VI would introduce a tailplane with strong dihedral to improve stability at the cost of some loss in manoeuvrability. The heavier TF Mk.X also needed a large addition to the dorsal fin. As a daylight fighter the aircraft was not a great success, as it was inferior in speed and manoeuverability to the Bf 110. But it was an immensely strong, rugged aircraft, capable of flying long patrol missions and delivering a heavy war load. It was an ideal aircraft for interdiction and anti-shipping missions.
Coastal Command took almost all of the Beaufighter Mk.X production. The Beaufighter was perfectly suited to attack the German convoys or U-boats in the North Sea and the Bay of Biscay, with their heavily armed escorts. While some operated as fighters, others carried bombs or eight rockets under the outer wings. The TF models, also known as “Torbeau”, carried a torpedo, fitted with a “Mono Air Tail” to stabilize it in the air and extend its range of allowable launching conditions. Thus the development of the aircraft had gone full circle: Originally derived from a torpedo-bomber, the Beaufighter had now itself become a very successful torpedo launcher.
| Westland Whirlwind Mk.I | Bristol Beaufighter TF Mk.X | |
|---|---|---|
| Engines | Rolls-Royce Peregrine | Bristol Hercules XVII |
| Power | 2 x 885 hp | 2 x 1735 hp |
| Wing Span (m) | 13.72 | 17.64 |
| Length (m) | 9.83 | 12.71 |
| Height (m) | 4.83 | |
| Wing Area (m2) | 23.22 | 47.13 |
| Empty Weight (kg) | 3768 | 7082 |
| Loaded Weight (kg) | 5175 | 11441 |
| Max. Speed (km/h) | 570 km/h at 4817 m 488 km/h at sea level | 499 km/h at sea level |
| Climb | 4570 m in 5 min 42 sec | 1525 m in 3 min 30 sec |
| Ceiling (m) | 9240 | 4575 |
| Range (km) | 2367 | |
| Guns, fixed | 4 x 20 mm Hispano | 4 x 20 mm Hispano |
| Guns, flexible | 1 x 7.7 mm Vickers K, often replaced by a 7.7 mm Browning | |
| External load | Two 250 lb or 500lb bombs | One torpedo, or eight rockets with 25 lb AP or 60 lb HE warheads, or two 250 lb bombs |
F.9/37
One of the neatest of the twin-engined fighters was built to Specification F.9/37, which in turn was inspired on work by Gloster to earlier specifications for a two-seat fighter, armed with a dorsal turret. Originally this required a twin-engined fighter with a fixed nose armament of two 20-mm cannon, installed to fire slightly upwards at the so-called ‘no-allowance angle’, as well as a retractable four-gun dorsal turret. But before Gloster’s G.39 took the air in April 1939, the dorsal turret had been abandoned. Instead, three extra cannon were to be installed in the aft fuselage; because of their no-allowance trajectory they were aimed a few degrees upwards, firing over the head of the pilot. With the original 1,050 hp Taurus T-S(a) radials the G.39 demonstrated a speed of 579 km/h at 4,600 m, but these engines were troublesome and still about a year away from reaching maturity. With 900 hp Taurus T-S(a)3 engines the speed dropped to 534 km/h, and with 880 hp Peregrine in-line engines the second prototype attained 531 km/h.
The G.39 was a neat and manoeuverable aircraft, but it was considered too small for development, and the late change in armament had left it with a center of gravity problem. For some time the delay in the development of the Beaufighter, and the downward revisions of its performance figures, appeared to create an opportunity for the G.39. However, the Taurus and Peregrine engines were non-starters. In the spring of 1940 Gloster began work, at a suggestion from the Ministry of Aircraft Production, on a development with two Merlin XX engines. Called the ‘Reaper’, this was to carry four 20-mm cannon and eight machineguns in its single-seat version, and four cannon in its two-seat version. At the end of 1940 it was judged to be the best night fighter design that could be expected. However, Gloster was already engaged in the development of jet fighters, and chief designer George Carter warned that he did not have sufficient design staff to develop both the Reaper and a jet fighter. Furthermore, it would take at least 18 months before the aircraft could enter service. With the Mosquito available to take on the night fighter role, the Reaper design never progressed beyond the drawing board.
Wooden Wonder
The best of the British wartime twin-engined fighters was the de Havilland Mosquito. While this originated as a fast unarmed bomber, its potential as a fighter was obvious. It was, in fact, hard to decide whether the Mosquito was most desirable as a reconnaissance aircraft, a bomber, or a fighter; an uncertainity that created some problems with the organisation of the early production runs.
This is not the place to repeat the history of the development of the Mosquito bomber. When the first order for fifty aircraft was placed on March 1940, this included only bomber and reconnaissance models. The project was shelved a few months later when Lord Beaverbrook reorganized aircraft production to concentrate it on the most urgently needed types only. Work on the Mosquito, never really halted, was officially restarted in July; and in that month de Havilland also received instructions to complete one of the prototypes as a fighter, with four 20-mm Hispano cannon in the belly and four 7.7-mm Browning machine guns in the nose. A contract for fighters, to specification F.21/40, followed in November, also the month in which the first prototype made its first flight. At that time, there was still discussion about the final role of the type, and a long-range day fighter version to protect convoys against German bombers was considered.
The Mosquito was, famously, built from wood. In a way, the Mosquito preceded modern trends in aircraft design, where increasing use is made of composite materials, often sandwiched around a foam core. Wood is a natural composite, consisting of cellulose fiber in a lignin matrix. For the fuselage, de Havilland opted for a sandwich consisting of skins of birch plywood with a balsa filler, this structure being three times as a strong as a plywood panel without a balsa core. The fuselage halves where glued together over a concrete mould, each plywood skin consisting of three layers 1.5 to 2 mm thick, arranged with the grain of the layers of wood at 45 degrees to each other. The wings had wooden box spars (consisting of laminated spruce flanges and plywood webs), spruce stringers, and plywood skins.
Important advantages of such a structure were that it needed a relatively small amount of light alloys (although the required high-quality wood was not that easy to find) and that it allowed woodworking shops to take part in the war effort. Another important advantage of plywood was that it could be given a very smooth finish, reducing drag. Close-cowled Merlin engines, radiators buried in the wing roots, careful design of the propellor hubs, and great attention to the details of streamlining resulted in an excellent performance: The first prototype recorded a top speed of 624 km/h at 6,700 m. For about two and a half years the Mosquito could be said to be the fastest aircraft in operational service, also thanks to new versions of the Merlin engine.
This level of performance, in combination with initial doubts about the concept of an unarmed bomber, resulted in a decision in the summer of 1940 to give the fighter version a higher priority than the bomber. Official vacillation about the role of the first Mosquito production aircraft made de Havilland’s work more difficult. While there was much commonality between fighter and bomber models in the fuselage structure (of course a nosecone with gun mounts replaced the transparent nose cone of the bomber version), the fighter model had a different, stronger wing, to take the higher loads of combat manoeuvering. It also had an armoured glass windscreen. The Mk.II fighter version finally flew its operational mission at the end of April 1942 — about a month ahead of the first operational use of the bomber model.
These fighters were F Mk.II models, most of them operating as NF Mk.II nightfighters with AI Mk.IV or Mk.V radar. They were much faster than the Beaufighters they replaced, and had a higher service ceiling, better handling and greater manoeuverability. Armament consisted of four 20-mm cannon in the belly and and four 7.7-mm machine guns in the nose. The aircraft was soon sent on offensive missions as well, stripped of its radar and equipped with extra fuel tanks, that allowed it to range deep over Germany. Later this “Intruder” role passed to the FB.VI, which could carry 1,000 lb of bombs in an internal bomb bay.
The number of roles of the fighter Mosquito soon expanded. Nightfighter Mosquitoes escorted RAF bombers into Germany, taking an increasingly heavy toll of the German Nachtjagd. Conventional escort operations were accompanied by Flower intruder operations that targeted German fighter bases, while on Mahmoud missions the British nightfighters deliberately offered themselves as bait, in the expectation of turning the tables on any attacker. Ranger missions were flown over German-held territory in day and night, looking for targets of opportunity, and Instep patrols over the Bay of Biscay and the Western Approaches targeted German long-range fighters and other aircraft over the travelling routes of the German submarines. In the 2nd Tactical Air Force, fighter-bomber Mosquitoes replaced light and medium bombers, making pin-point raids on selected targets and providing air support for the invasion forces after D-Day. In Coastal Command, Mosquitos flew anti-shipping missions over the Atlantic ocean and the Norwegian coast. And in June 1944, two Mosquito nightfighter squadrons were diverted to the interception of V-1s at night.
To cope with a wide range of tasks and increasing demands on the aircraft, the Mosquito was repeatedly upgraded, but due to the high demand for the type, older models had to serve until worn out, or were modified to bring them up to the most recent standards. In the nightfighter line, the NF Mk.II with Merlin 21 engines and AI Mk.IV or Mk.V radar (466 completed) was followed, in January 1943, by the NF Mk.XII with AI Mk.VIII radar. The latter was a centimetric radar set, giving much better results than the older radars; a number of Mk.II models were converted to Mk.XIIs. As the new radar was in a radome on the nose, the 7.7-mm machine guns were removed. The similar NF.XVII was equipped with the AI Mk.X radar, alias SCR 720, a development of the British centrimetric radar produced in the USA. The NF.XIII of February 1944 was a new-built model that combined the AI Mk.VIII with the airframe of the FB.VI, which incorporated outer wing fuel tanks, and an internal bomb bay for intruder missions. The engine remained the Merlin 21 or 23, but fifty NF.XIII were equipped with nitrous oxide injection to boost their high-altitude performance for short periods — a speed gain of 75 km/h was measured at 6100 m.
The NF.XV, which turned out to be superfluous, was a highly specialised high-altitude model with extended wing tips, only four 7.7-mm Browning machine guns as armament, and Merlin engines with two-stage superchargers; only four Mk.IIs were converted to this standard, which boasted a service ceiling of 13,595 m. The NF.XIX of April 1944 had the airframe of the Mk.XIII, more powerful Merlin 25 engines, and could carry either AI MK.VIII or Mk.X in an “universal” nose radome. The final wartime nightfighter model, the NF.30, was a derivative of the NF.XIX with 70-series Merlin engines with two-stage superchargers; this gave a much better high-altitude performance with top speed raised from 595 km/h at 4270 m to 682 km/h at 8,075 m. It entered service in the spring of 1944.
The fighter-bomber models were an offshoot from the early success of the fighter models. The FB Mk.VI was essentially a derivative of the NF.II with the ability to carry bombs, both internally (in a small bomb bay aft of the cannon) and externally (under the wings). From October 1943 onwards, the aircraft could also carry eight rocket projectiles under the wings. The success was so great that ultimately, one third of the Mosquito production run consisted of fighter-bomber versions. This was helped because the fighter-bomber model was also built in Canada (the main production model being the Mk.26, with Packard Merlin engines) and in Australia as the Mk.40. For the anti-tank role, a small number of FB.XVIII ‘Tsetse’ were produced, armed with the powerful 57-mm Molins cannon. But ultimately the RAF had misgivings about the usefulness of this type and gave the aircraft to Coastal Command for anti-shipping missions.
The Mosquito was a great success as a nightfighter and a fighter-bomber, clearly superior to the Ju 88, Bf 110 or Me 410. But it was unwise for Mosquito crews to engage in combat with enemy single-engined day fighters, if they could avoid it. The Mosquito was fast but it could not match a Fw 190 or Bf 109 in manoeuverability or rate of climb. While realizing the maximum of the potential of the twin-engined type for the period, the Mosquito had sacrificed the ability to fly the missions of a single-engine fighter. That probably was an unavoidable choice, and the war potential of the Mosquito proved that it was a good one.
| Mosquito FB.VI | Mosquito NF Mk.XIII | Mosquito NF Mk.XXX | |
|---|---|---|---|
| Engines | Rolls-Royce Merlin 21 | Rolls-Royce Merlin 21 | Rolls-Royce Merlin 76 |
| Power | 2 x 1300 hp | 2 x 1300 hp | 2 x 1711 hp |
| Wing Span (m) | 16.51 | 16.51 | 16.51 |
| Length (m) | 12.34 | 12.47 | 12.64 |
| Height (m) | 4.65 | 4.65 | 4.65 |
| Wing Area (m2) | 41.81 | 41.81 | 41.81 |
| Empty Weight (kg) | 6227 | 6489 | 6875 |
| Loaded Weight (kg) | 9843 | 9072 | 9798 |
| Max. Speed (km/h) | 608 km/h at 4025 m | 595 km/h at 4270 m | 682 km/h at 8075 m 544 km/h at sea level |
| Climb | 9.50 m/sec | 4570 m in 6 min 45 sec | 11.4 m/sec |
| Ceiling (m) | |||
| Range (km) | 1803 | 2993 | 1900 |
| Guns, fixed | 4 x 20 mm Hispano 4 x 7.7 mm Browning | 4 x 20 mm Hispano | 4 x 20 mm Hispano |
| Bomb load | 2 x 500 lb bombs (series 1 aircraft, 250 lb) |
Failed Efforts
In the Beaufighter and Mosquito, the RAF had two of the most successful twin-engined fighters of the period. Curiously enough, both were a side product of bomber development, although before and during the war there were many other efforts to develop twin-engined fighters. Of the resulting designs some had a high potential but were abandoned because the types in service already met the operational need, while others were embarrasingly misguided.
Specification F.18/37 (actually issued in March 1938) called for a high-speed single-seat fighter, with a top speed not less than 400 mph at 15,000 ft, and an armament of twelve Browning .303 machine guns or more. The Bristol, Gloster and Hawker submissions were single-engined, but Supermarine offered twin-engine designs. It argued that for this category of aircraft, a twin-engined type could actually be smaller than a single-engined type, and it offered a better forward view and (with airscrews rotating in opposite directions) better handling. The Type 324 and 325 both featured a wing of elliptical platform, only 20% larger than that of a Spitfire, and a slender fuselage with a short nose. Guns were installed in the outer wing panels. The Type 324 had its engines installed as tractor engines, while the 325 had pusher engines; in both case there was a choice between Merlin V-12s or Taurus radials. The Supermarine offerings lost out to the Hawker design (which became the Vulture-powered Tornado and the Sabre-powered Typhoon) because the Air Staff saw little advantage in the twin-engined layout.
But Supermarine’s design was given a second chance, although not without doubts, when the Air Staff decided to switch to six-cannon armament. The resulting Supermarine Type 327 was a tractor design with six cannon installed very closely together in the wing roots, and a preference for the Merlin engine. A mock-up of the 327 was built, but there was a good deal of skepticism on the practicality of this design. Supermarine could not be expected to develop the 327 quickly, and there were already three twin-engined fighters under development. It was decided to discontinue this project.
Other efforts to develop a twin-engined fighter followed a rather more complex storyline, with rapidly evolving specifications reflecting that the ideas on this subject were far from frozen. Specification F.6/39, issued in April 1939, called for a powerful “fixed gun fighter”, mounting four 20-mm or two 40-mm cannon. But F.22/39 of September imagined an aircraft with a 40-mm cannon in a nose turret, powered by two Griffon engines, and capable of at least 400 mph at 20,000 ft. The latter specification morphed into F.16/40 for a “fixed cannon gun fighter”, basically the F.22/39 modified to carry eight 20-mm cannon fixed to fire forward, with 200 rounds per gun. The final iteration of the specification was F.7/41, which converted the aircraft into a high-altitude fighter with a pressure cabin, reaching its top speed of at least 415 mph at 33,000 ft, powered by two Merlin 61 engines, and with six cannon with 120 rounds each.
The unfortunate recipient of this bewildering series of specifications was Vickers, which mounted a six-year effort that resulted in one completed prototype and very little flying. However, it at least partially had itself to blame. When F.6/39 was issued, the design team of Rex Pierson drew the RAF’s attention to its Type 414, designed to carry a Vickers S 40-mm cannon in the nose, and a predictor fire control system operated by a gunner who sat next to the pilot. (A concept similar to the Bell FM-1 Airacuda.) The gunner would aim through a sight, and his input would be fed into a predictor unit, through flexible shafts and photo-electric units that would make corrections for the own speed and height. The predictor unit would aim the nose turret, which allowed for 45 degrees of elevation, 5 degrees of depression, and 20 degrees of aim to the side. It was this Vickers design that triggered the issue of F.22/39, while F.6/39 was cancelled.
In April 1940, following an inquiry on the possible installation of fixed guns, Vickers offered the Type 420, with eight 20-mm or two 40-mm cannon in its nose. The fuselage was redesigned to reduce the cross section, and the second crew member now set behind the pilot, near the trailing edge of the wing. The elliptical wings were swept slightly forward, giving the design an unusual shape. The RAF issued F.16/40 to cover the Type 420, and ordered both the F.22/39 and F.16/40 designs to be built. Early in 1941, the RAF had apparently lost interest in both aircraft, which were rather too large and heavy to fill the twin-engined fighter role effectively. Vickers countered by offering a high-altitude version with Merlin engines and a pressurised fuselage, claiming that it would be easy to seal and pressurise the circular-cross section, stressed-skin fuselage. The RAF duly cancelled the F.22/39 aircraft, and at the end of the year issued F.7/41 to cover the new Type 432. This was to carry a pilot only, confined in a primitive pressure cabin, 5 m long and 1.4 m in diameter. This could be entered through a heavy circular door installed in the fuselage; but the transparent canopy, a round blister, could also be opened. The six cannon were to be installed in a ventral pack under the whale-shaped fuselage.
Nicknamed “Tin Mossie” because of its superficial similarity with the Mosquito, the aircraft finally made its first flight on 24 December 1942. Unfortunately, the type 432 was beset with aerodynamic and handling problems, engine cooling problems, and unsatisfactory performance. By the autumn of 1943, the 432 was due to be cancelled, but in a last confusing iteration, Vickers proposed to use the aircraft to test an installation of a 40-mm cannon with a predictor sight. It is possible that some work on this was done before the aircraft was finally scrapped, but it was not flown in this configuration.
The conversion of the Vickers project to a high-altitude fighter had been prompted by fears that German bombers would switch to operations at very high altitude. The development of pressure cabins and turbochargers made such operations technically feasible, justifying the development of a fighter to counter this treat, which in the end did not materialize (except for a small number of high-altitude reconnaissance aircraft). In the summer of 1941, Specifation F.4/40 for a single-seat high altitude fighter was released, stipulating a design service ceiling of no less than 45,000 ft. The bulk and weight of the pressurised cabin, and the reduced power of the suggested Merlin RM.6SM engines at such high altitudes, made a twin-engined aircraft the most practical solution, but a single-engined design was also possible.
General Aircraft, the only manufacturer with practical experience in the design of pressure cabins, suggested the GAL.46, an elegant design with twin tail fins and large wing root fairings, blending into a slim fuselage. However, General Aircraft lacked any experience in fighter design. Westland, on the other hand, suggested an larger extrapolation of the Whirlwind with Griffon or Merlin engines and a long-span wing with a high aspect ratio. A single-engined offering by Hawker, based on a Typhoon with a Sabre engine and two-stage supercharging, was actually liked best; but Westland won the contract. The Westland F.4/40 was ordered as the Welkin, in Mk.I version as a single-seat day fighter and Mk.II as two-seat nightfighter, both with Merlin engines, with Griffon engines planned for a later version. When F.7/41 was issued, the Welkin design was modified to meet this specification as well.
The Welkin was the biggest single-seat fighter of its time. It looked good, with the angular contours of the wing and tail surfaces complemented by a streamlined fuselage and closely-cowled Merlin 61 engines. The pilot, seated well forward, enjoyed an excellent forward view. The first flight was made on 1 November 1942, after a creditably short development period. To avoid leakage problems with pushrods or cables moving through the pressure wall of the cabin, many of the systems were electrically controlled. This turned out to be something of a maintenance nightmare, while serious problems with the engines and propellers would be a recurrent theme during testing. The Welkin also needed some aerodynamic modifications need cure handling deficiencies. However, what really handicapped the Welkin was the relatively thick section of its wing, a 21% thickness/chord ratio at the root tapering down to 15% at the tip, which gave it a critical Mach number of 0.74, with severe pitch oscillation already occurring at Mach 0.68. This was not acceptable for a good high-altitude fighter, but fortunately the operational needs could be met by other aircraft. Although 103 Welkins were completed, most were put in storage, and the Welkin never equipped a squadron.
Italy
The twin-engined fighter also attracted the attention of the authorities and manufacturers of Fascist Italy. They were in a poor position to take up the concept, because in the late 1930s the Italian aviation industry and air force were conservative, with entrenched defenders of the agile but slow biplane fighter — they fielded the last of its kind, the Fiat C.R.42. Equally harmful was a lack of modern and powerful engines. Old-fashioned structures and low engine power were probably more detrimental for heavy fighters than for any other kind of aircraft. Besides, the available production capacity was small and poorly managed, and it was impossible to build a sufficient number of these aircraft to make their development worth while.
Much of this shows in the Fiat CR.25, flown in 1937 as a twin-engined long-range fighter. Roughly the size of a Mosquito and somewhat lighter, the CR.25 had a crew of three and was powered by two Fiat A 74 RC 38 radial engines of 840 hp. With these engines and a clean but conventional design, the CR.25 had a maximum speed of 460 km/h at 5,500 m, creditable but far from brilliant. The production CR.25bis version had a modest armament of just two fixed 12.7-mm Breda-SAFAT machine guns and one flexible gun of the same type in a dorsal turret. Just 10 served operationally, and in defiance of their apparent vulnerability they soldiered on until 1943 without operational losses, despite clashes with Hurricanes, Blenheims, Beaufighters and even P-38s.
A compact aircraft produced by IMAM took the other path in the development of twin-engined fighter. The Ro 57 was a bit smaller even than the Whirlwind, and powered by the same A 74 RC 38 radials as the CR.25. The Ro 57 was considerably slower than the Whirlwind, however, with a top speed of 500 km/h, and carried much weaker armament, just two 12.7-mm machine guns. A less advanced structural and aerodynamic design appears to have been the cause, although the Italian fighter took to the air a bit later, making its first flight in early 1939. Official doubts about the usefulness of this type of aircraft and its ability to survive combat with single-engined opponent took its toll, and the Ro 57 finally entered limited service as a fighter-bomber in early 1943. It is useful to compare this with the development time of the Whirlwind, which was judged by the RAF to have been far too long: That of the Ro 57 took about two years more!
The Caproni firm, founded by Gianni Caproni, had made its reputation during the First World War as the manufacturer of large bombers. The typical Caproni aircraft of the immediate post-WWI period were sesquiplanes, but in the early 1920s drawings were made for a twin-engined monoplane fighter of all-metal construction, with a blunt-nosed fuselage, a cantilever wing, corrugated metal skinning and fixed landing gear. A quite radical design for the time, this Ca.65 was not built.
The firm had another opportunity in 1940 when Caproni built the elegantly streamlined Ca.331. This aircraft possesed a modern all-metal monocoque structure, highly streamlined lines, two 750 hp air-cooled in-line Delta RC.35 engines, and in its initial reconaissance version, a smooth transparent nose. In May 1942 the air force ordered the conversion of the second prototype in a nightfighter version, with a stepped cockpit and a solid nose, four 20-mm MG 151/20 cannon, two 12.7-mm Breda-SAFAT machine guns, and a flexible Breda-SAFAT in a dorsal and ventral defensive position. While the aircraft looked good, it was underpowered for a fighter, even after the installation of 800 hp engines or with the planned 850 hp engines. Top speed of the production version would have been 505 km/h at 5300 m, but Italy surrendered before production could be seriously contemplated.
SIAI-Marchetti contributed one of the more radical concepts. The S.M.91 had the same general configuration as the Lockheed P-38 Lightning, twin tails booms connected by a tailplane and a central nacelle for the crew. The S.M.91 was rather bigger and heavier than the P-38, however, with a crew of two and about a third more wing area. As flown in March 1943 with 1,290 hp Daimler-Benz DB 605A-1 engines, the top speed was a modest 585 km/h at 7,000 m. In the hope of reducing drag and increasing performance, the team developed the S.M.92 in parallel; this had no central nacelle, and the crew was accommodated in a cockpit in the left-hand tail boom. Armament had to be relocated as well, and while the the S.M.91 had three MG 151/20 cannon in the nacelle and two more close to it in the wing roots, the S.M.92 had two cannon in the centre wing section, one cannon installed to fire through the hub of the right-hand engine, and two synchronized machine guns on each engine. One more gun, firing aft, was installed on the tailplane. The S.M.92 was briefly flown by the Germans after the Italian surrender.
Probably the most promising of the Italian designs was the IMAM Ro 58, a clean all-metal design with twin tail fins, 1160 hp Daimler-Benz DB 601A-1 engines, and an armament of five 20-mm MG 151/20 cannon and a flexible 12.7-mm. The prototype made its first flight in May 1942 and was highly praised, but again the surrender of Italy in August 1943 prevented any production from taking place. It fate was thus the same as that of the last generation of Italian single-engined fighters, which were also compared favourably with German aircraft powered by the same engines, but were built in too small numbers to have any impact.
The Japanese Army
The concept of the twin-engined fighter held considerable appeal the staff of the Air Headquarters of the Japanese Imperial Army. For an organisation that was usually conservative in its approach to fighter design, it represented a radical step. However, Japan sought to pursue its interests over a vast geographical area, and the potential of a long-range fighter was evident to Japanese strategists. Such an aircraft would require the latest in aviation technology to be a success, a factor that stimulated the interest of a service that was still catching up with western developments, but also suggested a cautious approach. Hence, in March 1937 the Koku Hombu contacted the three major manufacturers, Nakajima, Kawasaki and Mitsubishi, and assigned them the experimental airframe designations Ki-37, Ki-38 and Ki-39. The specification that they were given was only for an interim twin-engined fighter design, intended primarily as an experimental aircraft, and it left much to be decided by the designer. It was challenging nevertheless, and Nakajima and Mitsubishi declined the submit a design.
The Kawaski Twin-Engined Fighters
Kawasaki undertook the project with more enthusiasm, and a mock-up of the Ki-38 had reached an advanced stage when in October 1937, the Army decided to stop work on the project and replace it with one around a more accurately defined specification. The primary task of the aircraft, it was now agreed, would be that of a long-range escort fighter. This resulted in a fairly light and maneuverable aircraft, with the relatively modest fixed armament of two 7.7-mm Type 89 machine guns and one 20-mm Ho-3 cannon, but fuel for 4 hours and 40 minutes at economic cruising speed. Top speed was to be 540 km/h at 3,500 m, in itself not particularly challenging in comparison to the speed of other twin-engined fighter projects initiated in this period, but difficult enough to achieve in combination with the range requirement. In December Kawasaki was instructed to update the project, which was now given the designation Ki-45, around the new specification. Kawasaki’s new chief designer, Takeo Doi, put considerable pace behind the project and a prototype was ready in January 1939.
The Ki-45 was a mid-wing monoplane with pleasant lines and a crew of two, pilot and rear gunner. It had an elliptical wing platform, and rounded horizontal and vertical fins. Two 820 hp Ha-20-Otsu radial engines were installed in fairly short and stubby nacelles, which also housed the semi-retractable landing gear. The engines and nacelles turned out to be the weak point of the design. The Ha-20-Otsu was highly unreliable and the nacelles were a source of aerodynamic problems. Both performance and manoeuverability of the Ki-45 fell well short of requirements. Not without hesitation did the Army order an improved version with 1050 hp Ha-25 engines, which flew in the late summer of 1940. This at least raised performance, and hopes.
Meanwhile, Takeo Doi and his team had started on an extensive reworking of the design. Although the aircraft they were now preparing would enter service as the Ki-45-KAI, the abbreviation KAI meaning Kaizo, modified, it was in reality an almost entirely new design, with only the most superficial similarity to the original Ki-45. The cross-section of the fuselage was reduced. The wing abandoned the elliptical planform in favour of straight taper with rounded tips, and the engine nacelles were moved lower in relation to the wing and aerodunamically improved. The changes made the aircraft slightly larger, but more significantly, improved performance and made it easier to build. The Ha-25 engines were retained for the very first Ki-45-KAI aircraft, but were replaced by the more reliable Ha-102 later.
Where the Ki-45 had possesed pleasing lines conveying the general impression of a unfinished 1930s design, the Ki-45-KAI looked neat, crisp, and modern. Performance and handling were judged entirely satisfactory, and in early 1942 the Ki-45-KAI-Ko started to come off the production line, under the official designation of Type 2 Two-Seat Fighter. Armament was now two 12.7-mm Ho-103 machine guns in the nose, a 20-mm Ho-3 cannon in a ventral tunnel, and a flexible 7.92-mm Type 98 in the rear cockpit. This fell short of the firepower carried by competing twin-engined fighters, especially considering the low rate of fire of the Ho-3, but on the positive side the Ki-45 was the most manoeuverable twin-engined fighter of the war.
The operational roles of the Ki-45-KAI Toryu, which was given the recognition name “Nick” by Allied Intelligence, soon diverged from the intended bomber escort role. The type was too much at a disadvantage in combat with Allied single-engined fighters. It was, however, an effective ground attack and anti-shipping aircraft and a good interceptor. When the need for a nightfighter arose, the Toryu was the logical choice, equipped with a pair of obliquely upward firing Ho-103 machine guns or 20-mm Ho-5 cannon instead of the upper fuselage fuel tank. To save weight, the Ho-3 cannon in the ventral tunnel was often removed. By day and night, the upward-firing guns made it possible to attack from below, where a bomber’s defences were weakest.
This combat experience uses resulted in the development of adapted versions. Great confusion exists over the armament and specifications of the Ki-45 developments, but it appears that the Ki-45-KAI-Otsu was a specialized attack version, with the Ho-3 cannon replaced by a manually loaded 37-mm cannon, the Type 94. The Ki-45-KAI-Hei retained the Ho-3 cannon but omitted the fixed machine guns, to carry instead a 37-mm Ho-203 cannon in the nose. The Ho-203 fired a lighter projectile at a slightly lower muzzle velocity than the Type 94, and although automatic it fired at only 120 rpm. Such armament was unsuitable for combat against enemy fighters, but effective against ground targets or bombers. The Ki-45-KAI-Tei on the other hand adopted the 20-mm Ho-5 upward-firing guns of earlier improvised nightfighters as standard, in combination with the Ho-203 in the nose, but it still lacked radar. (A version with radar in the nose appeared near the end of the war, but probably did not see combat.) Another armament decision made in the final production runs was no longer to install the Type 98 machine gun in the aft cockpit, because it was ineffective.
While the Ki-45-KAI had been an indisputable succes, it had only been really sufficient for a short period. It had entered service in the middle of 1942, but from the second half of 1944 the Japanese needed an interceptor capable enough to provided a defence against the large, high-flying B-29. For this, the Toryu lacked the performance, although the heavy cannon of the later versions gave them more firepower than other Japanese interceptors. As it was one of the best aircraft Japan possessed for this task, an increasing number of Ki-45-KAI units was employed for home defence. A better aircraft was urgently needed.
Kawasaki had not neglected to develop such an aircraft, but it had been delayed by doctrinal doubts on the success and correct employment of a twin-engined fighter. At first, Kawasaki started (on its own initiative and without army support) on a Ki-45-II development with a better streamlined fuselage and 1,500 hp Ha-112-II radial engines. At the suggestion of the Koku Hombu, the prototypes were completed as single-seat fighters, the first with the rear cockpit simply faired over, the second and third more thoroughly redesigned and fitted with an all-round vision canopy. The Ki-96 was an excellent aircraft, with a maximum speed of 630 km/h at 9,500 m and better handling than the Ki-45-KAI. However, when it flew in September 1943, the Army had already reversed its decision on the crew arrangement, and the goal of the Ki-96 programme was reduced to providing data for the two-seat Ki-102.
The development of the Ki-102 was given a high priority. Based on the Ki-96, this was an aerodynamically highly refined aircraft, planned from the start in two basic versions. The Ki-102-Ko would be a high-altitude interceptor with turbocharged engines and a 37-mm Ho-203 cannon and two 20-mm Ho-5; this was the aircraft that was needed to combat the B-29. And the Ki-102-Otsu was to be a ground attack model, lacking the turbochargers, and with a 57-mm Ho-401 instead of the 37-mm weapon. In parallel, Kawasaki was to work on the Ki-108, which reverted to the single-seat configuration, and would be equipped with a pressure cabin for high-altitude operations.
The first Ki-102-Ko was flown in June 1944, and following successful tests it was decided to phase out Ki-45-KAI production in favour of the Ki-102. However, while the aircraft was ready, the turbochargers were not, and therefore production started in October 1944 with the Ki-102-Otsu, officially designated Army Type 4 Assault Aircraft. Although a handful of interceptor models were completed before the end of the war, none of them was ever handed over to a combat unit. The bombing of the factories reduced even the output of the Ki-102-Otsu line to only 215. The prototypes of a Ki-102-Hei nightfighter version, with radar based on the German FuG 240 and two 30-mm Ho-155 cannon, never flew; both prototypes being seriously damaged by bombing before their first flight. The Ki-108 on the other hand did fly, but flight testing was also affected by bombing, and had not been completed at the end of the war.
Rob
One of the most daring attempts to build a twin-engined fighter was initiated by Kawasaki’s Takeo Doi in 1939. Inspired by the engine installations of high-speed racing aircraft and developments in France, he projected a fighter that would be powered by two engines, one in the nose and one in the aft fuselage, each driving its own propeller on the nose — the aft engine driving the front propeller, but means of an extension shaft passing through the hollow gearing of the front engine. By combining two liquid-cooled Ha-40 engines (the Japanese licensed version of the Daimler-Benz DB 601) and using surface cooling to reduce drag, a speed of 700 km/h was within reach.
The technical problems and complications were obvious, and the Army did not give permission to start development work until late in 1940. The combination powerplant, designated Ha-201, was tested on the bench from the end of 1942 until the middle of 1943, and the prototype fighter was finally completed in November 1943. It was given the designation Ki-64, and the allied reporting name “Rob”. On the fifth flight a fire broke out in the rear engine bay, where cooling problems had been persistent, and although the pilot made a successful emergency landing, the sole Ki-64 never flew again.
Dinah
Mitsubishi had declined to submit the Ki-39 design as a heavy fighter, but it had served as the basis for a reconnaissance aircraft, the Ki-40. However, because this did not meet the army specifications for a long-range reconnaissance aircraft, it was abandoned and replaced by the Ki-46, which entered production just before the war in the Pacific broke out. As a reconaissance aircraft, the Ki-46 was a brilliant design, an elegant, streamlined aircraft with a high performance. The Ki-46-III model of late 1942 had a maximum speed of 630 km/h at 5,800 m, powered by 1250 hp Ha-112-II radials. Allied intelligence gave this excellent aircraft, which was able to outrun fighters, the reporting name “Dinah.”
This was well above the performance of the Ki-45-KAI and stimulated interest in a fighter version. The Ki-46-III-KAI, also known as the Army Type 100 Interceptor, featured a stepped cockpit window instead of the streamlined unstepped nose of the reconnaissance version. Armament consisted of two 20-mm Ho-5 cannon and a 37-mm Ho-204, mounted between the pilot’s and observer’s cabins, and angled 70 degrees up. A few of the older Ki-46-II were converted to incerceptors in the field, with the 37-mm Type 94 cannon. However, the Ki-46 had not been designed as a fighter. It was a lightly-built aircraft optimized to cruise at high speed and altitude. As fighter, it lacked the rate of climb, manoeuverability or sturdiness for its task. A version for ground attack, the Ki-46-III Otsu, presumably would have been even more vulnerable, but only a few were completed.
An even more unwise improvisation delivered the Ki-109, a derivative of the Ki-67 Hiryu bomber. The Ki-67 was an excellent bomber, fast and very agile for its size. The Ki-109 was in the weight and performance class of a Bristol Beaufighter, although it was a considerably bigger aircraft. The intended use of turbo-supercharged engines promised to make the Ki-109 capable of intercepting the B-29. But the unsatisfactory state of development of the turbines forced the type into service without them, as the Ki-109-Ko, in November 1944. Without the turbochargers the aircraft was unable to get near enough to a B-29 to use its 75-mm Type 88 anti-aircraft cannon. Besides, this manually-loaded cannon and a 12.7-mm Ho-103 tail gun were its only armament, so that the Ki-109-Ko would have been woefully vulnerable to US escort fighters if it had encountered them. The Ki-109-Ko can only be considered a complete failure.
A much more promising Mitsubishi effort was the Ki-83, designed to meet a May 1942 requirement for a two-seat long-range fighter. The Ki-83 had simple, clean lines, with a streamlined fuselage and powerful Ha-211 turbo-supercharged engines. Armament would consist of two 20-mm and two 30-mm cannon in the nose. The aircraft was highly agile and the fastest Japanes fighter built during the war. Official performance figures included a top speed of 704 km/h at 9,000 m, but during post-war tests in the USA, using (much better quality) American fuel, it was good for 756 km/h. It hardly needs to be said that the development of the turbocharged engines was slow and plagued by problems. The fuselage and tail of the Ki-83 needed modifications too. The first prototype flew on 18 November 1944, but no production aircraft were delivered; in fact by the end of the war the emphasis had shifted to a specialized interceptor derivative, the Ki-103.
Another design that did not reach the frontline was the Ki-93. This aircraft was designed, not by a private manufacturer, but by the Army Aerotechnical Research Institute (Rikugun Koku Gijutso Kenkyujo, also known as Giken). Work on the drawings started already before the Pacific War, but in part due to changes in the tasks assigned to the aircraft, and in part to the inexperience of the Giken in aircraft design, a prototype was not flown until 8 April 1945. Like most Japanese fighters of its generation, the Ki-93 was elegantly streamlined. The six-bladed propellers of the two 2,700 hp Ha-214 radials emphasised the promise of high performance. In fact its predicted performance was good but not impressive, and the real strong points of the Ki-93 were ruggedness, extensive armour protection, and heavy armament: a 57-mm Ho-402 cannon in the nose, a 20-mm Ho-5 in each wing root, and a flexible Ho-103 in the rear cockpit. In other times it could have been a successful fighter-bomber and close-support aircraft, but in reality its first flight was also its last one, for the prototype was damaged during the landing and subsequently destroyed by bombing.
In summary, the experience of the Japanese Army with its twin-engined fighters was not a very happy one. It managed to enter the war with the Ki-45-KAI, which was a sound, versatile design, but like many Japanese designs it aged quickly and in the second half of the war it was no longer a match for the enemy. Attempts to replace it by a more modern aircraft all foundered, both for technical and for institutional reasons. The Army changed its mind on the features such an aircraft should have a few times too often, and the Japanese industry never brought to an acceptable standard of reliability the powerful, high-altitude rated engines that were so much needed.
The Japanese Navy
Lack of cooperation between the Imperial Japanese Army and Navy was one of the most serious weaknesses of the Japanese war effort. In line with the same tortuous logic that caused the Army to operate its own submarines, the Navy developed several categories of land-based aircraft, distinct from army types. For the history of the twin-engined fighter this is perhaps fortunate, as there was little room for twin-engined types on an aircraft carrier.
In June 1983, the Navy formulated a requirement for a twin-engined long-range escort fighter. At that time, of course, work on the Ki-45 was already in progress, and the Navy had kept itself informed on this project. But the specification for the “13-Shi twin-engined land-based fighter” was somewhat different, in that the Navy wanted a three-seat rather than a two-seat design, and the range requirement of 2400 km (3700 km with auxiliary fuel tanks) was well above what the Ki-45 was capable of. On the other hand, the top speed of 520 km/h was below that specified for the Ki-45, and armament requirements were almost identical. Perhaps the Navy really wanted to avoid purchasing a version of the Ki-45, for it submitted its request only to Nakajima and Mitsubishi. (Both these large aircraft manufacturers supplied aircraft to both Army and Navy, although different ones. Other manufacturers produced for only one of the services; and Kawasaki was an Army supplier.) Again, Mitsubishi declined to participate.
The Nakajima J1N became a bigger and somewhat more complex aircraft than the Ki-45, but despite Nakajima’s best efforts, it was not a very successful one. To meet the range requirement and have a sufficiently low wing loading for high agility, the type was given a wing span of 17 m, and equipped with automatic “combat flaps”. But on test flights, the maneuverability of the J1N was found to be insufficient for a fighter. The engines were to be Nakajima Sakae 14-cylinder radials, rotating in opposite directions, the upper blades moving inwards. But the modified gearing in the right-hand Sakae 22 engine reduced its power, and finally the production aircraft were equipped with two Sakae 21 engines, both turning in the same direction. The sophisticated rear defence system of the J1N was one of its most innovative elements. The cockpit fairing ended in two small remote-controlled turrets, each with a pair of 7.7-mm Type 92 Model 3 machine guns. The turrets were staggered, the second one behind and below the first one, and a sliding cover reduced their drag when not in use. Unfortunately, the complexity of this system delayed the delivery of the aircraft, and ultimately it was never perfected; the aiming system remained too slow and inaccurate. Only a small number of J1Ns were equipped with it, but meanwhile, its weight helped to reduce the performance of the J1N, which was found to be below specifications.
Tests following the first flight on 26 March 1941 revealed these and other defects, and as a result the future of the J1N became uncertain. The aircraft was considered unable to operate as a day fighter, and Navy’s carrier-based fighter, the famous A6M “Zero”, possessed an impressive range that removed the need for a dedicated escort fighter. The story of the J1N would probably have ended with its condemnation in the summer of 1942, if the Navy had not realized at that time that its land-based reconnaissance aircraft, the Mitsubishi C5M, was obsolete, and that the J1N might replace it. The result was the “Type 2 land-based reconnaissance aircraft”, or J1N1-C.
The J1N1-C lacked the fixed 20-mm cannon of the fighter prototype, but intially the pair of 7.7-mm Type 97 machine guns in the nose was retained, although it was removed from later models. The rear armament was also progressively modified, first by replacing the troublesome remote-controlled turrets by a simple flexible gun, and at the end of 1942 by installing an enormous and highly drag-inducing manually-operated turret with a 20-mm cannon, taken from the cancelled G5N heavy bomber. In the middle of 1943 a somewhat more streamlined turret was introduced, but the J1N1-C was hardly a success. It was clearly inferior to the Army’s Ki-46 reconnaissance aircraft, which began to be used also by the Navy.
The J1N was rescued from oblivion by the great enthusiasm of a Navy officer, Commander Kozono, for obliquely-firing guns. Cdr. Kozono Yasuna appears to have been rather eccentric and some of the gun installations proposed by him were useless, but his installation of oblique-firing cannon in the J1N made a qualified success out of an indifferent aircraft. The first two modifications consisted of two 20-mm Type 99-2 cannon aimed 30 degrees up, and two more 30 degrees down, all installed in the space once taken by the remote-controlled gun turrets. Matching sights were installed in the cockpit. Trials showed the value of the installation, especially for the interception of bombers at night, and the modified aircraft went into production as the J1N1-S Gekko (Moonlight). From the 301st aircraft onward, the complicated fuselage decking for the gun turrets was finally omitted, and the cannon installed in a simpler and better streamlined fuselage structure.
Armament installations varied locally, as older aircraft were modified to J1N1-S configuration, or J1N-1S were adapted by installing or removing cannon. A few came out of the factory as J1N1-Sa models with three upward-firing guns instead of two. The downward-firing cannon were often removed, as they were not very useful and the Gekko needed to be light to stand a chance of intercepting a B-29. A major step foward could have been the first installations of radar on a Japanese fighter, inspired by captured American radars, but by 1945 the Japanese industry was seriously troubled by lack of resources and the manufacturing quality of these radars was apparently poor enough to render them useless.
Overall, the J1N1-S proved to be a fairly effective nightfighter, and its victims included two dozen B-29s. Whether this really justified the existence of a separate fighter type in parallel with the Ki-45, can be seriously doubted.
| Kawasaki Ki-45-KAI-Hei | Nakajima J1N1-S | |
|---|---|---|
| Engines | Mitsubishi Ha-102 | Nakajima NK1F Sakae 21 |
| Rating | 1080 hp | 1130 hp |
| Wing Span (m) | 15.00 | 16.98 |
| Length (m) | 11.00 | 12.77 |
| Height (m) | 3.70 | 4.56 |
| Wing Area (m2) | 32.00 | 40.00 |
| Empty Weight (kg) | 3695 | 4840 |
| Loaded Weight (kg) | 5276 | 8184 |
| Max. Speed (km/h) | 547 km/h at 6000 m | 507 km/h at 5840 m |
| Climb | 5000 m in 6.12 min | 5000 m in 9.58 min |
| Ceiling (m) | ||
| Range (km) | 2000 | 3780 |
| Guns, fixed | One 37-mm Ho-203 cannon in the nose One 20-mm Ho-3 cannon in a ventral tunnel | Two 20-mm Type 99-2 firing obliquely upward Two 20-mm Type 99-2 firing obliquely downward |
| Guns, flexible | One 7.92-mm Type 98 in the rear cockpit |
The Navy imitated the Army in more than one way, for it too failed in its effort to find a modern replacement for its twin-engined fighter. The Nakajima J5N Tenrai, first flown in July 1944, had poor handling qualities and, despite the use of 1990 hp Homare 21 engines, disappointing performance. It achieved a top speed of only 597 kmh at 6,000 m. Four prototypes were completed as single-seat fighters and two as two-seaters before the inevitable cancellation. Aichi contribution a dedicated nightfighter design, the heavily armed S1A Denko, but at the end of war its prototypes had not yet been completed.
The Navy did have a bomber that appeared to be highly suited for a nightfighter conversion. The P1Y had been designed by the Yokosuka naval arsenal as a fast medium bomber for use against naval targets, capable of level attacks from altitude, dive bombing, and torpedo bombing. The P1Y was compact for a bomber, with a wing span of 20 m. It carried only only a moderate bomb load or a single torpedo, and was fast enough to outrun fighters at low level. When first reported by Allied intelligence, the type was assumed to be a fighter and accordingly given a boy’s first name as reporting name: Francis. When the aircraft was discovered to be a bomber, this could be changed to Frances without creating confusion. The potential of the type was, however, balanced by its complexity and difficult maintenance.
A number of P1Y1 bombers, powered by 1825 hp Nakajima NK9C Homare 11 radials, were modified to P1Y1-S nightfighters, equipped with a pair of obliquely upward firing 20-mm cannon in the front fuselage and a second pair in the aft fuselage. They were given the popular name Byakko (White Light). The P1Y2-S version, powered by more reliable 1850 hp Mitsubishi Kasei 25 engines, was known as Kyokko (Aurora), and was factory-produced by Kawanishi, rather than a bomber conversion. The P1Y2-S possessed an AI radar, of which the antenna array was installed on the nose. Besides two upward-firing 20-mm cannon in the aft fuselage, the P1Y2-S had a single flexible 20-mm or 13-mm weapon to defend the rear, and it retained the internal bomb bay for intruder missions.
But neither version turned out to be very useful for the Japanese. The Kyokko made its first flight in June 1944, and when it entered service the biggest threat was the B-29 bomber. Like most Japanese fighters of the period, the P1Y2-S lacked the performance to be an effective B-29 interceptor. The original bomber model had been fast, but it had been fast at low altitude, where its missions and its targets were to be found. The nightfighter conversion failed because the performance at high altitude was disappointing. In addition, we can consider that the armament of the P1Y-2S was too light and its radar probably insufficiently reliable. It is reported that most of the small number (96) of P1Y2-S nightfighters built by Kawanishi were converted back to bombers, with removal of their radar and fixed guns. The limited number of P1Y1-S conversions also failed to make an impression.
Thus the IJN’s efforts to develop twin-engined fighters produced one qualified success, the J1N1-S, and a series of failures. At the end of the war, the Japanese army could at least show a series of impressive prototypes, although to little benefit for themselves. The Navy did not even have that much to show for its efforts.
Perhaps the Japanese experiences demonstrate how much the success of the twin-engined fighter concept depended on technical advances, and especially on powerful engines. Japanese aircraft designers had to work with engines of generally lower power and inferior reliability, compared to those fielded by Japan’s enemies. As the war progressed and took a turn for the worse, the ability of the national industry to manufacture armaments of advanced technology and high quality entered a terminal decline, as it ran out of the necessary raw materials and skilled manpower. The result was that impressive prototypes were doomed to remain impressive prototypes, for Japan was unable put them into service.
Soviet Luxuries
The Japanese experience would have been understandable to Soviet policy makers and designers. For most of World War II, the USSR regarded twin-engined fighters as a luxury it could not afford. Given the focus of the authorities on tactical air warfare, this might be considered a bit surprising. One could very well imagine a useful role for a heavily-armed twin-engined fighter-bomber over and behind the battefield, especially over a battle area that was as vast as the Eastern Front. And the USSR’s only twin-engined fighter in service, the Pe-3, actually found a lot of use in this type of mission, but only a few hundred of these aircraft were completed. A combination of small single-engined fighters and single-engined attack aircraft was preferred, with a modest investment in twin-engined light and medium bombers. And this regardless of a long-running effort to design heavy fighters, that achieved some excellent results.
Tupolev
Despite the isolation of their country in the late 1920s, Soviet policy makers appeared to have been as much influenced by the international fashion for twin-engined fighters as anyone else. In these years, the USSR was building a strategic, long-range air force, and it possessed a fleet of ANT-6 (military designation TB-1) heavy bombers that, by the standards of the time, was very impressive indeed. One of the roles envisaged for a twin-engined fighter was long-range bomber escort, but it was also expected to fly independent “cruiser fighter” missions deep into enemy territory, and to perform reconnaissance, bombing, and torpedo-bombing.
Andrei N. Tupolev suggested that the requirement could be met by a scaled-down version of the ANT-6. This made more sense than one would think, for in an age of biplane fighters with mixed structures, the huge monoplane ANT-6 had been a technological leap forward. His proposal was accepted, and in September 1929 the ANT-7, designated R-6 by the military, made its first flight. It had the familiar Tupolev trademarks: Thick monoplane wing and rectangular fuselage, corrugated metal skinning, fixed landing gear, and open cockpits. It was powered by liquid-cooled M-17 engines, a license-built version of the BMW VI. Production began in 1930 and ran for four years in a number of different versions, but at the end of that period the aircraft was already obsolete, overtaken by the rapid technological development of the 1930s. The KR-6 “cruiser recconaissance” version had a top speed of 226 km/h at sea level, needed no less than 13 minutes to climb to 2000 m, and had a range of 1480 km. The armament consisted of open nose and dorsal gunnery positions with two 7.62-mm DA guns each. Other versions (but not the KR-6) also had a retractable ‘dustbin’ below the fuselage with a single DA. This was nothing more than the standard bomber armament of the period.
In 1932 the prototype of a replacement for the ANT-7 had been ordered, the ANT-21. The ANT-21 initially re-used some parts of the ANT-7, including the engines and some structural components in the wing, but it featured a new semi-monocoque fuselage, smooth instead of corrugated metal skinning, and retractable landing gear. These refinements were amply rewarded, for the top speed at sea level rose to 351 km/h. An accident during flight testing resulted in the construction of a much-modified ANT-21bis, also known as the MI-3D, with M-34N engines. However, the type was not accepted for production.
| Tupolev KR-6 | Tupolev MI-3D | |
|---|---|---|
| Engines | M-17 | M-34N |
| Rating | 680 hp | 820 hp |
| Wing Span (m) | 23.20 | 20.76 |
| Length (m) | 15.06 | 11.57 |
| Height (m) | ||
| Wing Area (m2) | 80.0 | 59.2 |
| Empty Weight (kg) | 3,870 | 4,058 |
| Loaded Weight (kg) | 5,992 | 5,463 |
| Max. Speed (km/h) | 226 km/h at sea level | 350 km/h at 5,000 m |
| Climb | 2,000 m in 13 min | |
| Ceiling (m) | 8,300 | |
| Range (km) | 1480 | |
| Fixed Guns | 2 × 7.7 mm PV-1 | |
| Flexible Guns | 2 × 7.7 mm DA in nose Tur-6 2 × 7.7 mm DA in dorsal Tur-5 | 1 × 20 mm ShVAK in nose mount 2 × 7.7 mm DA in dorsal mount |
At this point in time, Soviet development of twin-engined fighter was influenced by a new armament option. In the early 1930s Leonid V. Kurchyevskii developed a family of recoilless guns, building on the earlier designs by the American Cleland Davis. The Davis and Kurchyevskii designs solved the recoil problem by firing two projectiles: A shell forward, and a counterweight to the rear. The recoil forces generated by the firing of the two projectiles compensated each other. The concept produced in a long gun, with muzzles at both ends and a complicated breech mechanism in the middle; all very inconvenient for a traditional fighter. However, the elimination of recoil made it possible to install 37-mm, 76-mm, and even 120-mm guns in a relatively small aircraft. A specially designed twin-engined fighter was thought to be the best candidate to carry such a weapon.
One of the possibilities was demonstrated by the ANT-23 fighter, a small twin-boom design with a tractor and a pusher engine in its short fuselage. The unique feature of the ANT-23 was that the tail booms were actually formed by the barrels of the 76.2-mm APK-4 cannon. Despite its unusual configuration the aircraft flew well, but it was overtaken by other developments.
The ANT-29, also known as DIP, appears to have been broadly based on the ANT-21bis, but had a short, deep and narrow fuselage designed to house two large APK-8 recoilless guns in its bottom, with the muzzle of the barrels protruding from the nose and a counterweight fired from the tail. In relation to the compactness of its fuselage the ANT-29 had a huge 56.8 square meter wing, and the engines were imported French Hispano-Suiza 12Ybrs V-12s of 760 hp. The ANT-29 flew late in 1935, but was not accepted for service. Nevertheless, the work on the ANT-29 was not entirely wasted: The aircraft was closely related to the ANT-40 or SB, which was the most important bomber of the late 1930s and represented a quantum leap forward in the design of combat aircraft when it entered service.
And fighter development in this family had not yet been abandoned. The development of the ANT-46 or DI-8 was allowed to continue in parallel with that of the ANT-49, and it was flown in 1936. The DI-8 was armed with a 37-mm APK-11 cannon buried in each wing outside the propeller arcs, plus four ShKAS machine guns in the nose. However, 1936 was also the year in which the great Stalinist purges began. And the victims of the purges were found not only among political and military leaders, but also included factory managers, scientists and engineers. Leonid Kurchyevskii disappeared. Andrei Tupolev and many of the members of his team were arrested. As a sign of the times, the accusations against him included the absurd statement that Tupolev had supplied the Germans with the plans for the Bf 110.
Petlyakov
One result of Stalin’s reign of terror was that many Soviet aircraft designers found themselves imprisoned in design and study offices controlled by the NKVD. Many Soviet combat aircraft were designed by what was in effect slave labour, engineering offices filled with a captive staff that in some cases had a suspended death sentence looming over them. Vladimir Petlyakov, formerly one of Tupolev’s closest associates, thus became the engineering lead of an imprisoned group of fifty people tasked with the development a twin-engined fighter. (The formal head of the design bureau was an NKVD officer.) They were ordered to design a highly ambitious high-altitude, long-range fighter, intended both as an interceptor and as an escort fighter for the new ANT-42 bomber. The specifications called for a top speed of 630 km/h at 10,000 m, with an operational ceiling of 12,500 m.
The VI-100, which made its first flight on 22 December 1939, certainly looked ready to meet this demanding specification. It had an elegantly streamlined fuselage, almost a monococque structure with its strength derived from a relatively thick skin attached to the fuselage frames, without supporting stringers. The wing carried the two M-105 engines in front of the leading edge, in neatly streamlined nacelles that also contained the TK-2 turbosuperchargers and the wells for the main landing gear. The superchargers also provided the compressed air for the crew of three in two pressure cabins, fore and aft. The fixed forward-firing armament consisted of two 20-mm ShVAK cannon (300 rpg) and two 7.62 mm ShKAS (900 rpg). The planned use of a pressure cabin made it hard to provide flexible guns for the defense of the rear, so the installation of a single fixed rearward-firing ShKAS was contemplated, although not proceeded with. The first prototype could also carry bombs under the wings, while the second prototype even possessed an internal bomb-bay.
The VI-100 had important teething troubles, and was slightly slower than has been hoped, but nevertheless the design was regarded a success. However, its intended purpose was to be changed abruptly. In 1940, it became apparent that the potential enemies of the USSR would not be putting into service high-altitude bombers at any time soon. In that context, the production of the complex and expensive VI-100 was not justified. However, the VVS urgently needed a new tactical low-altitude bomber to replace the obsolescent SB. Hence it was decided to use the VI-100 as the basis for the development of a twin-engined dive-bomber, the PB-100.
The VI-100 was not cancelled outright, but the relatively low priority accorded to it inevitably resulted in its neglect. For the changes required to create the bomber version were very substantial, and only by great exertion was it possible to fly the first PB-100 on 15 December 1940. It entered service as the Petlyakov Pe-2, which soon proved itself an excellent light bomber. A total of 10,574 were delivered to the VVS during the war.
This left the USSR without a twin-engined fighter, and soon after the opening of the war with Germany a need was felt for just such an aircraft, mostly as a nighfighter. As the Pe-2 was the most suitable of all available aircraft, the conversion of one to a fighter prototype was ordered — and reportedly only seven days passed between the formulation proposal and the completion of its state flight tests. The new fighter was known as the Pe-3.
Of course, development in such a short time was only possible because very limited modifications were allowed. The fuel tankage was increased, the crew reduced to two, and the armament slightly increased: The production Pe-3 initially had two fixed UBK 12.7-mm machine guns with 250 rounds each, a ShKAS in the turret at the rear of the cockpit, and (because the ventral gunner had been omitted) a fixed ShKAS on a ventral mount to deter attackers. This was clearly too light, and fixed armament was soon increased by installing a 20-mm ShVAK cannon, while the flexible ShKAS was replaced by a 12.7-mm UBT. With some other changes, this resulted in the updated Pe-3bis. A later change relocated the machine guns from the nose to the former bomb bay. As could be expected for a hastily improvised fighter, the armament installation of the Pe-3 was never entirely satisfactory. In the summer of 1941 a more through fighter modification, the Pe-2I, was ready for testing, but despite better performance and a better armament installation, it was not accepted.
Because the Pe-2 had possessed excellent performance, the Pe-3 could match a Bf 110C in speed, though not in manoeuverability, and it was lightly armed for a heavy fighter. It also lacked basic equipment for the nightfighter mission; the initial production runs had poor radio and not even a radio compass. Only 360 Pe-3s were completed, and they were more useful as long-range reconnaissance aircraft and ground attack aircraft than as fighters.
Yakovlev I-29
There was another light bomber that held, at least on paper, some potential as a twin-engined fighter. Yakovlev’s BB-22, later called the Yak-2, was a fast light bomber. The prototype made its first flight in early 1939, and the BB-22 short-range, fast bomber version entered production in 1940. A highly streamlined design, it was greeted as something of a miracle by the Soviet leadership, but it turned out to be a very disappointing combat aircraft. The high speed of the prototype, 567 km/h at 9,900 m, had been achieved by sacrificing operational equipment and armament. Adding these reduced speed by over 50 km/h, and the handling characteristics left something to be desired, too. Worse, the production aircraft was barely able to carry 400 kg of bombs.
From the start, the aircraft had also been intended as a fighter, but characteristic for the initial concept was that the proposed fixed armament was just a single 20-mm ShVAK, replacing the fixed 7.62-mm ShKAS of the bomber version. This was later revised to a more realistic pair of ShVAK, a single ShKAS in the lower nose, and two ShKAS more installed between the cylinder banks of the engines, firing through the propeller hubs. The compensate for the weight increase, the crew of the fighter version was reduced to one.
The highest priority was given to the bomber version, and then to the R-12 reconnaissance model; so an actual fighter prototype was not flown until December 1940. This I-29 was powered by M-105 engines and was armed with two ShVAK cannon in the lower fuselage. However, with the evacuation of the aviation industry to the East, beyond the reach of the German forces, work on the I-29 was halted.
Polikarpov
In 1935, the Nikolai Polikarpov started work on the development of a twin-engined multi-role aircraft. After what appears to have been considerable debate on the intended role of such an aircraft, with a related series of designation changes, a prototype finally emerged in 1937 as the VIT-1, a designation indicating that it was a Vozdushny Istrebitel Tankov, or anti-tank fighter. It was a three-seater powered by two 960 hp M-103 engines, and its most striking features were the two long barrels of the powerful ShK-37 cannon installed in the wing roots. A 20-mm ShVAK in the nose, with a limited range of movement, and a 7.62-mm ShKAS in the rear cockpit completed the gun armament. An internal bomb bay and external bomb racks were the other part of it, for the VIT-1 was also expected to act as a dive bomber.
The VIT-1 was a good basis for further development, but more powerful engines were required to improve the top speed (450 km/h) and take-off and landing characteristics. Hence the development of the VIT-2, powered by 1050 hp M-105 engines and distinguished most easily by its twin tail fins, but in fact extensively redesigned and provided with heavier armament. The aircraft was ready in the spring of 1938 and with M-105 engines installed, the VIT-2 prototype reached a speed of 513 km/h at 4500 m. However, the type still suffered from unreliable engines and numerous aerodynamic problems, and needed considerable modifications to rectify the worst of them. It was early 1939 before the VIT-2 finally passed its state testing and was recommended for production. But it was not to be. Limited industrial resources, political disagreements, and a preference for the SPB dive-bomber version of the same airframe, all led to the abandonment of the project.
Of course, even if the projected VIT-2S production version had been built, it would have been a ground attack aircraft with perhaps a limited air combat capability. Before the type fell by the wayside, Polikarpov was asked to design a proper twin-engined fighter, intended as a heavy escort fighter, the TIS. Work on the VIT and SPB delayed progress on the TIS, and the first flight of the TIS(A) prototype was made in September 1941. It was smaller than the VIT, but nevertheless considerably heavier, despite the use of all-metal instead of mixed construction. This was a characteristic of several Soviet twin-engined fighter designs, and the result of building them to very high stress factors, up to loads of 12 g. Of course, the additional weight hampered performance, and especially take-off and landing from rough fields.
But the TIS(A) had very clean, elegant lines, with closely cowled 1400 hp Mikulin AM-37 in-line engines and twin tailfins. The wing had a broad chord at the root and tapered sharply towards the tips, with most of the taper on the trailing edge. Top speed achieved during tests was 555 km/h at 5800 m, but clearly the AM-37 engines were far from ready for service use. The German attack, the evacuation of the design offices to the East and the urgency of other work caused work on the TIS to be temporarily set aside, until the second half of 1943. In June 1944 flight testing began of an improved TIS(MA), intended to have AM-39 engines, but in fact equipped with much more readily available AM-38F power plants. With these low-altitude rated engines it was capable of 535 km/h at 1650 m. As impressive the aircraft could have been in 1941, by late 1944 there was little future and no operational need for the type; and after the death of Polikarpov in July 1944 his design bureau was closed down.
The original TIS(A) had been armed with four fixed 7.62-mm ShKAS machineguns in the upper nose, two ShVAK cannon in the wing roots, and two more ShKAS guns on a dorsal TSS-1 and a ventral mount. The latter two were both controlled by the observer, for the TIS was a two-seat aircraft. The ventral gun was rather impractical, for its use required the observer not only to leave his seat, but to squeeze his way to under the armour plate that protected his station. It was abandoned on the TIS(MA); the dorsal mount was replaced by a more powerful VUB-1 with a 12.7-mm UBT but the biggest change was made to the nose armament. Two 12.7-mm UBS were installed in the nose instead of the four rifle-calibre machine guns, and two Sh-37 37-mm cannon were placed in the wing roots. A ventral pack could accomodate a NS-45.
| Polikarpov VIT-2 | Polikarpov TIS(A) | |
|---|---|---|
| Engines | Klimov M-105 | Mikulin AM-37 |
| Rating | 2 x 1050 hp at 4000 m | 2 x 1400 hp |
| Wing Span (m) | 16.50 | 15.5 |
| Length (m) | 12.25 | 11.7 |
| Height (m) | ||
| Wing Area (m2) | 40.76 | 34.8 |
| Empty Weight (kg) | 4332 | 5800 |
| Loaded Weight (kg) | 6302 | 7840 |
| Max. Speed (km/h) | 513 km/h at 4500 m | 535 km/h at 7000 m |
| Climb | 5000 m in 7.2 min | 5000 m in 7.3 min |
| Ceiling (m) | 8200 | 10250 |
| Range (km) | 2900 | 1720 |
| Fixed Guns | 2 × 37-mm ShK-37 and 2 × 20-mm ShVAK in wing roots | 2 × 20-mm ShVAK in wing roots and 4 × 7.62-mm ShKAS in nose |
| Flexible Guns | 1 × 20-mm ShVAK in nose, 1 × 20-mm ShVAK dorsal, 2 × 7.62-mm ShKAS ventral. | 1 × 7.62-mm ShKAS dorsal, 1 × 7.62-mm ShKAS ventral |
Mikoyan-Gurevich
Of the several Soviet twin-engined fighters that got close to production, one of the most interesting ones was the DIS-200, designed by the new OKB or design bureau constituted by Artyom Mikoyan and Mikhail Gurevich. (Both, incidentally, managed to stay of Stalin’s jails.) Their initial concept included the use of the new powerful diesel engines then being developed Charomskyi. Diesel engines may sound like an unlikely choice for a fighter, but the advantage of such power plants was that they were fuel-efficient, and on paper allowed the aircraft to have a long range. However, the intended engines never matured sufficiently, and the first prototype had conventional liquid-cooled Mikulin AM-37 V-12 engines, while the second switched to M-82F radials.
The DIS-200 was a compact single-seat fighter, with a wing span and length close to that of the Fw 187. It was considerably heavier, however, and wing loading was high. The wing had a very deep chord at the wing root, and tapered sharply towards the tips. To reduce take-off and landing speeds the aircraft was given generous flaps on the trailing edge, as well as ailerons that could be lowered together with the flaps. The engine nacelles were slung below the wing, which in turn was attached to the bottom of the slender, short-nosed fuselage. The pointed engine cowlings were reminiscent of that of the MiG-3, with prominent oil coolers installed at each side. The radiators for the engine coolant were narrow slots, also on each side of the engine cowling, with an exhaust at the rear of the cowling near the wing trailing edge.
The bubble cockpit and inverted gull shape of the wing gave the pilot a good view foward and in the upper hemisphere, but he could look downwards only through glass panels in the bottom of the nose. The wing roots carried a fixed armament of two 12.7-mm UBS and four 7.62-mm ShKAS guns. Options for armament fitted under the fuselage included a pack with a powerful 23-mm VYa-23 cannon for the fighter role, a torpedo for anti-shipping missions, or a bomb. (The second prototype, with more powerful engines, could reportedly carry a pack with two cannon.)
In weight and performance, the DIS-200 was close to the Mosquito. It was, incidentally, also built largely of wood, to save on scarce light alloys. High performance and multi-role versatility could have given it a formidable potential. However, the first prototype had the misfortune to fly at the end of May 1941; the German attack in the next month upset all planning, and the notion of producing the aircraft in series as the MiG-5 had to be abandoned. As in the case of the TIS, the type was also handicapped by its underdeveloped and unreliable AM-37 engines.
The DIS was not abandoned. In late 1941 the design team was reinforced with Petr Grushin, whose own Gr-1 twin-engined fighter had been destroyed by German ttack before its first flight. (Very little is known about the Gr-1, but sketches show an aircraft remarkably similar to the Bf 110.) The second DIS-200 prototype, completed after the OKB had been evacuated to the East, flew in January 1942, with better developed 1,700 hp M-82F radials, in close-cowled nacelles and driving four-bladed propellers. This was well ahead of the TIS(MA), but nevertheless too late; the VVS had already settled on a policy of tactical air support and no longer needed a long-range fighter.
The Unfortunate
The Polikarpov and Mikoyan-Gurevich bureaus found a competitor in the OKO team, lead by Vsevolod Tairov. His OKO-6 design was ambitious and radical. Of mixed construction, with a relatively generous use of duralumin and magnesium alloy, the OKO-6 was very compact, with a wing span of 12.65 m and only 25.4 square meter wing area. It had a deep but narrow and short fuselage, ending in a single tail fin; the rather plump engine nacelles housed M-88 radials, cooled through narrow slots behind large spinners. The propellers rotated in opposite directions. Armament consisted of four ShVAK cannon in the fuselage bottom and two ShKAS in the upper nose decking.
The OKO-6 looked like a recipe for aerodynamic problems, and indeed tests, following the first flight on the last day of 1939, revealed a need for substantial changes. A year passed before a modified OKO-6bis flew, also known as Ta-1 in honor of its designer, with a longer fuselage ending in twin tail fins, besides a series of other improvements. But an engine failure resulted in a crash, and again several months were lost before the first prototype had been rebuilt. This was brought to OKO-6bis configuration but with M-89 engines, and renamed Ta-3. In this form, the aircraft was said to posses better handling than the MiG-3. This was very faint praise indeed, but the type’s high performance and heavy armament still resulted in a recommendation for production. However, a requirement for a considerable range increase triggered yet more delays, and it was May 1942 before the Ta-3bis (with fuel tanks in the magnesium alloy outer wing panels) entered official tests.
By August 1942, two and a half years had passed since the first flight of the OKO-6, but aerodynamic problems persisted and the M-89 was still too unreliable for production. Designer Tairov himself had been killed in an air accident in December 1941, and the interest in twin-engined fighters had waned. The aircraft was abandoned and forgotten.
Loss
And so the Soviet effort to develop twin-engined fighters turned out to be a waste of time. Despite a surprisingly large number of prototypes: The VI-100, I-29, VIT-2S, TIS, DIS-200, Gr-1 and Ta-3, the only twin-engined fighter to see service was the improvised and rather unsatisfactory Pe-3. This was operated on a limited scale and mostly in a light bomber role. After the war some nightfighter developments of the Tu-2 bomber appeared.
Some technical reasons can be pointed out. In the story of the aircraft above, engine troubles are a constant. A good twin-engined fighter needed the most powerful available engines, but Soviet engine development during the war years largely failed to make new designs sufficiently reliable. The M-105 was too small for a heavy fighter, and the M-82 reached a sufficient state of development in late 1942, when it was already too late. The AM-38 was a good engine, but heavy and rated only for low altitudes. Without a good turn of speed, a twin-engined fighter flying at low level was very vulnerable to being bounced.
Other factors helped to eliminate the operational need. As Soviet night-fighter radars were not developed until after the war, there was evidently no need for an aircraft able to carry one and its operator. Operations over sea were mostly limited to areas where the enemy had no fighter bases, so there was no requirement for a long-range fighter.
In the end, however, it was a doctrinal choice. The VVS chose not to deploy such aircraft, and as long as it did not want to operate deep over enemy territory, in did not need to. There was always the possibility of constructing airfields close to the front. One can speculate that tactical air operations over the vast area of the Eastern Front would be an excellent environment for a fast, long-range fighter and fighter-bomber, striking at gathering areas and transport infrastructure behind the enemy front lines. However, for most of the war the USSR could rely on partizan forces to make the hinterland unsafe for the enemy.
The American Effort
Whatever limitations the Soviet twin-engined fighters had, from a tactical point of view all of them made some sense, with perhaps the exception of the Yakovlev I-29. The same cannot be said of the US efforts in the same direction. Perhaps the operational experience of Russian participants in the Spanish Civil War played a role in keeping the wilder ambitions of Soviet designers in check. However, one gets the feeling that many more outlandish concepts were studied in the USA for the simple reason that with all the technology available there, much more was possible. Yet these white elephants would not have been created without the blessing of the air force.
Bell FM-1 Airacuda
In the early 1930s, the Army Air Corps was highly conservative. In 1934 it adopted its first monoplane fighter, the Boeing P-26. This not only had wire-braced wings and fixed landing gear, it had the dimensions and the weight of a traditional biplane fighter. Even by the standards of the mid-1930s it was small, light, and powered by a light engine. The P-26 was too slow to catch modern bombers and too weakly armed to shoot them down.
Nevertheless, in 1934 the Materiel Division initiated a study of a heavy “multiplace fighter” design, which centered around the concept of a Martin B-10 bomber modified to operate as escort fighter, or as the documents quaintly put it, “a bombardment accompanying weapon”. This study mainly demonstrated that this was a poor concept. The Martin B-10 would only be slowed down by adding extra armament and reinforcing it for combat manoeuvering, making it slower than the bombers it would escort. Besides, the basic design was already ageing rapidly.
This could have been the end of it, but the designer Bob Woods had been considering a much more advanced multiplace fighter, and Larry Bell successfully promoted it to the air corps. They key elements of the Bell Aircraft Corporation offer were the new Browning-designed 37-mm cannon and the Allison V-1710 V-12 engine. The Allison engine, in its turbosupercharged form, offered high performance at the expected cruising altitudes of long-range bombers. The cannon would allow enemy bombers to be destroyed from a safe distance, well out of reach of their defensive armament. For this purpose, they would be installed on powered mounts, and aimed by an automatic fire control system. A specification X-604 was written around this concept, and Lockheed was invited to prepare a competing design. But it was the Bell aircraft that won the order, by a narrow margin.
The aircraft was given the designation FM-1 for Fighter, Multiplace, at a time when single-seat fighters were still given P designations for Pursuit. Several layouts had been studied, with up to four cannon, but the Bell FM-1 Airacuda emerged with two V-1710 engines driving pusher propellers, in large nacelles that each also contained the powered gun mounts and gunner’s stations in front. The main role of the gunners was to feed the cannon, for the weapon that was used did not have the belt feed that characterised the M4 cannon as later used in the P-39. A .30 Browning machine gun was installed co-axially with the cannon to aid in aiming, but visual gunlaying from the nacelle would still have been poor, also because the view from them was rather restriced.
Instead, the guns were aimed from a station in the main fuselage. The fire control system was based on systems developed for laying anti-aircraft guns, and was quite sophisticated for the time. Inputs from an optical sight and a rangefinder were processed by a computing unit, and then the aiming parameters were electrically signalled to the nacelles. There, hydraulic servo controls aimed the guns as desired.The movement was restricted to a 30 degree cone in front of the aircraft, so the Airacuda was to fly behind the enemy bomber formation, supposedly picking off its targets one by one.
All this came at a cost, and the XFM-1 prototype, flown in July 1937, possessed a 21.33 m wing span, an empty weight of 6,200 kg, and room for a crew of five. With V-1710-13 engines that delivered only 1,090 hp each, considerable less than had been hoped for, the top speed was only 434 km/h at 6,100 m, well below the original specification. Nevertheless, the prototype was followed by 13 service test aircraft under the designation YFM-1 — actually eight YFM-1 aircraft with turbosupercharged V-1710-23 engines, three YFM-1A with turbocharged engines and nosewheel landing gear, and two YFM-1B with tail-dragger landing gear and V-1710-41 engines without the turbo. These aircraft had, besides the cannon and their co-axial .30 machine guns, two hatches in the sides of the aft fuselage mounting .50 Browning machine guns, a retractable dorsal turret mounting a .30, and a belly fairing with a .30 at the end.
The YFM-1s were very little flown, totalling less than 500 flying hours between them before the survivors were disposed of. They were highly specialized aircraft, with a dubious operational role against a threat that turned out to be non-existent; the continental USA was well beyond the range of enemy bomber fleets. Their real value was that of an exercise in design and engineering of advanced combat aircraft.
Skyrocket
Not only in the army, but also in the navy, brains were kept busy considering the defense of the USA against enemy bombers. The naval mind was of course much concerned with coastline targets, naval bases and ships, which could be attacked with little warning. (Such concerns were of course very prescient.) To effectively defend such targets against attack by modern high-altitude bombers, a fighter needed heavy armament and a very high climb rate. In 1935, it appeared to the Bureau of Aeronautics (BuAer) that the power of the available engines would not allow single-engined fighters to meet the requirements of this role, even allowing for engines then under development. And carrier fighters, of course, would be handicapped by their need for extra structural strength and arrester gear.
The result of this line of thought was, in 1935, BuAer Proposal SD-24D for a fast-climbing naval interceptor, followed in 1938 by Specification SD112-14. The Navy’s emphasis on climb and the requirement to operate the aircraft from carriers of course ruled out elephantine concepts such as the FM-1. What it wanted was a small, twin-engined, single-seat interceptor with excellent performance. The design that the Navy picked to be built and flown was the Grumman G-34, and the prototype was given the official designation XF5F-1.
This was the smallest aircraft that could be built around two powerful radial engines. To be able to install the engines as close together as possible, the fuselage of the XF5F-1 was given a small and short nose, that allowed for closely spaced propellers; and after wind tunnel trials it was cut back further, so that it did not even extend as far as the leading edge of the wing. The engine installation not only reduced wing span, it reduced asymmetric effects when flying on one engine, and allowed for smaller tail fins. The downside of this configuration was that much of the aircraft was now behind the center of lift of the wing; the engines themselves contributed only about a third of the total empty mass. This made it difficult to get the center of gravity of the XF5F-1 right. It also gave the aircraft unique looks, with a noseless fuselage, radial engines in stubby nacelles, and angular twin tail fins.
The engines were a problem. The two options were the Wright Cyclone, which had its cylinders arranged in single row, and the Pratt & Whitney Twin Wasp, a two-row design. Their respective designations R-1830 and R-1820 indicate that the engines were close in internal volume, as they were in weight and power; but the Pratt & Whitney design had a smaller diameter and offered less drag. The Navy would have preferred it, but the Wright engine was more readily available. The Navy instructed Grumman to build a full-scale wind tunnel model with two different engine nacelles, and finally ordered the prototype with the R-1820 Cyclone. The XF5F-1 had counter-rotating propellers, which would especially improve handling during the take-off and landing phases that are so critical for a carrier-borne aircraft.
Armament choices also created difficulties. The Navy wanted a more effective weapon than the .50 machine gun, but no suitable gun was being manufactured in the USA. A test batch of four Danish 23-mm Madsen cannon had been ordered in 1937, and it was planned to install two of these weapons in the F5F; but this was never done. Ground tests with the Madsen cannon revealed a low rate of fire and poor reliability, and the Navy lost interest. Instead two .50 and two .30 guns were installed in the nose; the centreline installation still offered clear benefits for gun aiming but the aircraft no longer had a firepower advantage over modern single-engined fighters. Perhaps in compensation, the Navy asked Grumman to provide for the carriage of 20 anti-aircraft bombs, internally in the wing. These 2.4 kg weapons were to be dropped on top of an enemy formation, a concept that was widespread in the late 1930s but was sadly deficient in operational value.
The XF5F-1 took to the air on the first of April 1940. Tests were relatively trouble-free, although the unusual configuration caused some aerodynamic problems. These resulted in repeated modifications during a long test programme. The new fighter was about 80 km/h faster than the single-engined F4F Wildcat and possessed an impressive rate of climb, which resulted in the type being dubbed the Skyrocket. Nevertheless, the F5F would never enter production, and this was quite obvious by the end of 1940. Hence, in January 1941, Grumman asked for the termination of the program.
The reality was that, like other small twin-engined fighters, the Skyrocket lacked development potential and a future. The weight growth of the prototype was already a concern before it first flew, and there was little room for the additional equipment needed to upgrade it to a combat type. Contrary to earlier expectations, engines were appearing that had twice the power of the R-1820. The Navy was already awaiting Voughts F4U, which was single-engined, but about as big and heavier than the XF5F-1, and carried heavier armament. Similar to the contemporary Westland Whirlwind, the Skyrocket was a technological dead end, overtaken by the rapid development of the 1940s.
Nevertheless the aircraft kept flying and being modified. As late as November 1942, the XF5F-1 reached its final form, with a longer nose, which again extended over the wing’s leading edge to reduce drag and make gun installation easier. It then also featured lengthened engine nacelles, wing root fillets, and propeller spinners. This development work was useful to support the later Grumman F7F.
Lockheed P-38 Lightning
In the 1920s and early 1930s, the US Army Air Corps had conformed to the norms set by other air forces worldwide. It was very conservative in its technical requirements and had no money for new developments. In 1934 the Army did order a monoplane fighter, the Boeing P-26, but this retained as much of the concept of the biplane fighter as a monoplane possibly could: It was small and light, powered by a relatively light engine, and weakly armed. It had fixed landing gear and wire-braced wings. The P-26 was a good dogfighter, but the latest bombers outpaced it with ease.
To the more ambitious minds in the Air Corps, the lesson was that the force needed an interceptor, a fast-climbing, high-speed fighter with heavy armament, designed specifically to destroy enemy bombers. Such design goals were common enough in Europe, where most nations had the enemy on their doorstep, but the need for them was less obvious in the USA, if only because nobody could really explain where these enemy bombers were likely to be based. However, to the visionaries in the Air Corps, the interceptor specification represented more a way to break with tradition and demand a serious increase in performance, than an actual operational task.
Because both single-engined or twin-engined interceptor designs had their advocates, and general Westover admitted that theoretical studies could not really determine which option was better, specifications were finally issued for both. Specification X-608, issued in February 1937, was for the twin-engined fighter. X-609 was its single-engined equivalent, and would lead to the Bell P-39 Airacobra. The key to the required dramatic performance increase over fighters already in service was the combination of the new Allison V-1710 engine with General Electric turbochargers, as already pioneered by the FM-1. During World War 2, only the USA fielded operational aircraft with turbochargers: The idea was not new, but turning it into a practical device was remarkably difficult, and few succeeded. The turbocharger could give the aircraft equipped with it matchless performance at very high altitude. However, to reach the required 360 mph (580 km/h) at 20,000 ft (6100 m), a much sleeker design than the FM-1 had to be prepared.
Vultee’s design XP 1015 showed what a conventional layout for such an aircraft would be. It featured a long, very slender fuselage with a bubble canopy for the pilot, and a tapering wing. The engine nacelles were highly streamlined, thanks to coolant radiators buried in the outer wing panels, and the turbochargers were located at their rear. With a wing span of 16.46 m, the XP 1015 was a fairly big aircraft. By 1937 standards it was sleek enough, but today, the design looks uninspired, an awkward hybrid of the Bf 110 fuselage with Mosquito wings. (Later, during the Battle of Britain, a Bf 110 fuselage was shipped to Vultee for structural analysis. It would be interesting to know what the conclusions of Vultee’s engineers were.)
Not so the Lockheed offering. Model 22 was the result of a thorough evaluation of a range of radical options by Hall Hibbard and Clarence “Kelly” Johnson. The complex engine installation, comprising the the engine itself, the turbocharger, and the cooling system, required considerable volume in the engine nacelle, which was going to be long if it had to be streamlined. An advantageous way to incorporate such a nacelle into the aircraft was to extend it into a long tail boom. If these tail booms carried the tail surfaces, the fuselage could be reduced to an abbreviated nacelle in the center of the wing. (The configuration of the Italian S.M.92, with the pilot seated in one of the booms and no central nacelle, was also evaluated, but not adopted.) The design of Lockheed’s model 22 was sleek and elegant, and the predicted performance included a phenomenal 417 mph (670 km/h) at 20,000 ft. In June 1937, Lockheed was chosen as the winner of the contract.
On January 27, 1939, the prototype made its first flight. Lockheed’s new fighter possessed the grace of a large wasp, with sleek pointed lines enhanced by polished flush-riveted metal. The C-series V-1710 engines used on the XP-38 had a low trust line and an elongated housing for the gearbox, which resulted in more elegant engine cowlings than most production Lightnings would have. As a portent of problems that would follow later, the XP-38 possessed tiny coolant radiators in the aft tail boom, a small oil cooler scoop, and intercoolers buried in the wing leading edge: Designed to cool the V-1710-11/15 engines at their 1000 hp continuous power, and actually not even sufficient for that purpose, this installation was only expected to permit the engines to run at their full 1150 hp for five minutes. The General Electric B-2 superchargers were located overwing, on top of the tail boom, under a streamlined fairing with a nice inlet scoop — that fairing was not to be retained on production models. The prototype carried no armament or armour. Its life was short: On February 11, after crossing most of the continental USA at an average airspeed of 350 mph, Lt. Kelsey crash-landed the aircraft on a golf course after the engines lost power on landing approach. The cause was never conclusively clarified, but later attributed to carburettor icing. The XP-38 was beyond repair.
There had not been time to seriously test the XP-38, but it had demonstrated the great promise of the concept, and the Air Corps ordered 13 service test aircraft, designated Lockheed model 122, and officially YP-38. However, serious redesign was necessary to turn the XP-38 into a viable combat aircraft, and the YP-38s had few parts in common with their precursor. They featured F-series V-1710 engines with a higher trustline and different gearbox, an entirely redesigned engine installation, a redesigned structure that was a bit lighter and more suitable for mass production, and provisions for armament (which was not installed). This delayed the first flight of a YP-38 to 17 September 1940. The YP-38, a fast aircraft that easily picked up speed in a dive, soon confronted its test crews with a dangerous phenomenon, known at the time as compressibility. Despite its looks the fighter was in some ways aerodynamically conservative, with a traditional wing profile. Its relatively low critical Mach number of 0.68 was easily exceeded in a power dive. The accelerated air flow over parts of the aircraft became supersonic, and the change in its properties made the control surfaces ineffective. The turbulent wake of the center section also made the elevators ineffective and generated strong vibration in the tail section. Recovery from such a dive was beyond the pilot’s control. On 5 November 1941, test pilot Ralph Virden lost his life. Wind tunnel research led to the introduction of a small fillet at wing root, which at least removed the extreme tail buffeting that had plagued the YP-38s in a high-speed dive, but did not restore controllability at high Mach numbers. For much of its operational career, P-38 pilots had to refrain from following enemy fighters in a dive.
The YP-38s were not combat-ready fighters, nor were the first P-38s off the production line: All lacked armour and self-sealing fuel tanks. Their elegantly curved, unarmoured windscreens were a clear hint. In August 1941, delivery of the P-38D began, incorporating some of the lessons of combat in Europe, but still with an interim armament installation. The army (somewhat optimistically) had decided to give ‘D’ designations to all fighters that had been brought up to the latest combat standard. The 1st Pursuit Group, based on Selfridge Field in Michigan, was the first to be equipped with these fighters, and in the autumn of 1941 it participated in exercises. At the time of Pearl Harbour, it would be the only fighter group fully equipped with the P-38.
Finding an optimal armament installation was difficult. The original concept combined the 37-mm T9 (or in its developed form, M4) cannon, two .50 machine guns, and two .30 machine guns. This was less than ideal. In theory, an excellent armament installation was possible on a twin-engined fighter, because the trajectory of nose guns did not need to converge to the center. But these guns were a poor match for trajectory. The M4 was a slow-firing weapon with modest ballistic performance, and was in short supply. (Contrary to some reports, there is no indication that the more performant but far heavier M9 cannon was ever installed in a P-38.) In August 1940, the Army decided to install a single 20-mm Hispano cannon and four .50 Browning M2 machine guns, a much more harmonious combination. And while the American-built version of the French Hispano HS.404 cannon had an indifferent reputation at best, it seems to have been relatively trouble-free in the P-38, perhaps because it was firmly supported by its fuselage mounting, but also because a mechanism was provided to clear jams.
With this armament installation, the P-38E was the first major production version, although the Army still considered it an interim type. Deliveries began in November 1941, but most of the 210 produced were used for training purposes, designed RP-38E with the R of ‘restricted’, because they were still not considered entirely combat-ready. A few were sent north in 1942, in response to Japanese attacks on the Aleutian islands. The P-38F model, first delivered in March 1942, had 1325 hp V-1710-49/53 engines and the capability to carry external drop tanks or bombs. The P-38F was the first Lightning to be built in large quantities and sent out to combat theatres. It was followed by the P-38G, with V-1710-51/55 engines and numerous small modifications.
From January 1939 to March 1942, more than three years had passed from the first flight of a prototype to the introduction of a model that was considered entirely fit for combat. (And which, incidentally, was still having some problems with its engine installation, specifically the intercoolers.) This gestation period was considerably longer, for example, than the development time and production run of the Westland Whirlwind. The USAAF’s perseverance with the P-38 was a sign of their great confidence in the future of the type, and perhaps also of their lack of good alternatives. At the same time it was a strong indication of the risks that Lockheed had taken with the ground-breaking design of the P-38.
Of course, meanwhile there had been another model of the fighter Lightning: The Lightning Mk.I ordered by and built for the RAF — a type not unfrequently referred to as the castrated Lightning. This because it was powered by C-series Allison engines, without the turbochargers and handed propellers. It has to be taken into account, however, that the British had taken the bold step to order a large number of these Lockheed model 322-B Lightnings in March 1940, well before the first YP-38 had flown. (And the French ordered even more.) If foregoing the most advanced features of the new fighter was an error, it was at least erring on the side of caution, and not unreasonable considering that the buyers were allocating virtually unprecedented amounts of cash in the hope of taking delivery of advanced fighters within one year. And an even larger order for Lightning Mk.II fighters with turbosuperchargers followed right on the heels of that contract, showing that the RAF was well aware of the value of his equipment. But when the first Lockheed model 322-61 arrived in England in December 1941, both the strategic and the tactical situation had changed in the wake of the Battle of Britain. The RAF could now afford to choose, and it chose not to have the Lightning, which in its Mk.I form and at the time of testing was handicapped by poor high-altitude performance, speed restrictions because of the unresolved tail vibration problems, and relatively poor manoeuverability for a fighter. Perhaps crucially, Britain had already received its first Mustang Mk.I in October. The Mustang Mk.I had the same restrictions on high-altitude performance as the Lightning Mk.I, but it was better in most other respects.
In July 1942, the first P-38F fighters of the Eight Air Force arrived in Britain, having flown across the Atlantic as part of operation Bolero. Ready for operations in August, they stayed in Britain only a short time before being moved to the Meditteranean Theatre of Operations (MTO) in support of operation Torch. In the MTO the Lightning proved valuable because of its range and firepower, but combat experience was not entirely satisfactory. The P-38 proved vulnerable in combat with German fighters, at least in part because of tactical errors and the tendency of pilots to engage in turning combat instead of exploiting the excellent zoom climb capability of their aircraft. The P-38 could make very tight turns, but its size and mass distribution reduced its in roll rate, and thus it could not enter turns quickly. Because of its size and unique configuration, the P-38 was also easily seen and identified in the air, which put it at a tactical disadvantage. In late 1942 the P-38s also appeared in the South-West Pacific Area (SWPA) as part of the 5th Air Force, flying its first combat missions there in December. Here too the excellent range of the fighter was of great value, and tactical problems were smaller. It had already been recognied that it was foolish to attempt to ‘mix it’ with the nimble Japanese fighters, but on the other hand the A6M or Ki-43 could not match the zoom climb of the Lightning. Previously P-39 pilots had found themselves at a disadvantage against the Japanese because their aircraft was inferior in climb and operational ceiling, but the high-altitude performance of the P-38 was excellent.
The scale of P-38 operations soon proved difficult to maintain, with losses exceeding the production rate, and commanders in the South-West Pacific were told that the MTO had priority. By September 1943, the Pacific had 212 operational P-38s and the Mediterranean fewer than 200. New aircraft were destined for the European Theatre of Operations (ETO), were a long-range bomber escort force was urgently needed. In November 1943, P-38s started flying operations against targets in Germany.
By that time, a new Lightning was making its debut. Allison had created the V-1710-F17 engine — left and right hand turning engines as installed in the P-38 were known as the V-1710-89/91 by the military — which thanks to higher supercharger gearing and 100/130 octane fuel, delivered more power. They were rated for 1425 hp with a War Emergency Rating (WER) of 1600 hp. However, the existing engine installation of the P-38 did not permit such power to be actually used, because the intercooler capacity was too small. So although the P-38H had the -89/91 engines, it could not use their full power. For this more redesign was necessary, including the introduction of a core-type intercooler located in a chin intake under the propeller hubs. The result was the P-38J, which entered production in August 1943. At the same time production finally came up to speed, to that in the end three-quarters of all P-38s featured the new engine nacelle.
But operating at high altitudes over Europe to escort heavy bombers, the new engines failed. Often catastrophically so, with bearings failing and connecting rods being thrown. The spate of failures and crashes seriously undermined confidence in the P-38J. In early 1944, attempts to operate in cold weather resulted in a quarter or even half of the fighters turning back with engine problems. Flying at lower altitude in warmer air reduced the number of failures, but at the cost of putting the P-38 at a tactical disadvantage.
Being a twin-engined fighter, the P-38 did offer the additional comfort of being able to return home on one engine. Unfortunately, the damaged engine frequently caught fire. In one respect the streamlined engine installation of the P-38 was a serious disadvantage, in that the engine installation was a dense but long package, presenting a fairly large vulnerable area to the enemy. And of course an aircraft struggling home on one engine was easy to prey to enemy fighters, if spotted. Aircraft that straggled behind the formation, for whatever reason, were highly likely to be shot down.
There may not have been a single cause for the technical problems. Probably poor pilot training, too much use of the 1600 hp WER without corresponding maintenance, and fuel quality problems all played a role. The management of these turbocharged engines was complex, perhaps too complex in a combat situation. A weakness was found in the engine installation: The new intercoolers were a bit too effective, and reduced temperature in the carburetor inlets too much, causing condensation of the fuel and the tetra-ethyl-lead additive. Modifications by Lockheed and Allison cured that problem, but that took months, and for the P-38 the time lost was crucial. From June 1944 onwards, P-38 Fighter Groups in the 8th AF converted to the P-51.
Ironically, this decision was made when the P-38 finally reached its fully mature form. Dive flaps, installed from the P-38-J-25 onwards, finally made high-speed dives safe. Power-boosted ailerons reduced control forces and transformed the combat handling of the P-38, giving it a roll rate better than that of many single-engined fighters. The V-1710-111/113 engine featured many improvements, and could deliver 1725 hp WER on 150 octane fuel. The latest P-38s could be said to the equal or the better of the best single-engined fighters in air combat; a great achievement for a large twin-engined fighter. This was something the RAF pilots who flew the Lightning Mk.I in December 1941 and rejected it, probably could not have guessed. Of course, June 1944 was quite late in the war. This permits some of the controversy surrounding the Lightning to persist, forever. Its defenders can point to the P-38L of 1944, definitely one of the best fighters of the war. Its detractors can refer to the P-38F of 1942, struggling with the Bf 109 and Fw 190 and generally emerging second best.
While the P-38 was no longer in view as the 8th AF’s long-range escort fighter, it was doing equally valuable and rather more dangerous work in the 9th AF. Focusing on tactical bombing and close-air support, this force profited from the P-38’s ability to lift a substantial bomb load. The first P-38s joined the 9th in April 1944. Because of the vulnerability of its liquid-cooled engines, pilots did not regard the P-38 as an ideal fighter-bomber, and low-level strafing missions were highly dangerous. On the other hand, the inner wing pylons originally fitted to carry drop tanks could carry bombs up to a weight of 2000 lb, making the type a useful light bomber. The P-38L was also fitted to carry ten 2.75 inch HVAR rockets under the outer wing panels. One advantage enjoyed by P-38 pilots was that the unique configuration could easily be recognized, reducing the risk of “friendly fire”. P-38s were chosen to fly air cover for the D-day invasion convoys for this reason, as naval gunners had a reputation for firing indiscriminately at any aircraft. (Events, and bitter complaints addressed to the naval commanders, showed that this precaution was amply justified.)
Bombing missions inspired an interesting modification: The Droop Snoot variant of the P-38J. The Lightning could carry a bomb load similar to that of a medium bomber, but not drop it with the same accuracy because it lacked a bombardier and bomb sight. The solution was to modify P-38s to act as lead bombardier aircraft, with a bomb sighting position equipped with a Norden bomb sight. The bombardier and his equipment displaced the armament from the nose of the P-38, which was extended forward and capped with a plexiglass bubble, incorporating a flat sighting window at its bottom. Even after the addition of armor plate, some lead ballast needed to be added to the nose to restore the balance of the aircraft, but the modification was found very satisfactory. Small numbers P-38s were modified to Droop Snoot configuration, acting as leaders for formations that dropped their bombs together on a signal from the bombardier. An even more drastic modification, never given an official designation, involved the fitting of a AN/APS-15 radar in a bulbuous nose to permit bombing through overcast; its operator sat behind it.
The most useful “special” versions of the Lightning were the photo-reconnaissance variants. It was a logical choice for this role because it was fast, had a decent range, and could operate at high altitude. And the front of the nacelle was highly suited to the installation of cameras. The USAAF came to operate over 1400 of these fast, usually unarmed aircraft: Some were purpose-built, more were modified from aircraft completed as fighters. The initial conversions, based on the P-38E airframe, were designated F-4, while modifications of the P-38G were known as the F-5A. Yet the first operational deployments to the ETO, in the summer of 1942, could have led to the end of this type’s career: USAAF officers were so impressed by the excellent performance of the RAF’s Mosquito that they asked to equip all their PR units with it. However, the Mosquito was in short supply. The USAAF received a small number (designated F-8 in USAAF service) but the most realistic option was to standardize the PR units on the F-5, a decision taken in the spring of 1944. Because it lacked some of the performance characteristics that made the Mosquito an ideal reconaissance platform, the Lightning did suffer higher losses, but it nevertheless was a fine PR platform that delivered a vital service. The reconaissance model was of course upgraded together with the fighter model, and conversions of the P-38L were known as F-5F and F-5G.
A less significant variant of the P-38 was the nightfighter version. The first modifications were single-seat P-38s equipped in the field with a small radar set, in an attempt to overcome the performance shortcomings of the P-61. Operating the Phillipines in late 1944, they proved useful at dawn and dusk, but the workload for the pilot was too high for successful night interceptions. The P-38M, modified from the P-38L, therefore included a radar operator’s position behind the pilot. Inserted behind the pilot’s seat and covered by a bulged canopy, this position was extremely cramped, and it was recommended that operators be no taller than 5 ft 6 in (1.67 m). The small AN/APS-4 radar set was installed in a pod that was fitted below the nose. Although 75 aircraft were ordered to be modified from P-38L airframes, the P-38M was too late to see service in WWII. With exposed glowing red-hot turbochargers and gun muzzle flash within the field of view of the pilot, the P-38M was not an ideal nightfighter, and the air force preferred the P-61 despite its lower performance.
It is difficult to make a final balance of success for the P-38. The design was an inspired solution to a highly demanding specification, but that specification was oriented towards defense against enemy bombers, not an offensive against enemy fighters. When a P-38F was tested against a captured Fw 190A-3, the German fighter was found to be superior in all aspects except climb above 20,000 ft (6100 m) and speed above 22,000 ft (6700 m). Fortunately, the opposition in the Pacific was not of the same quality. Later developments improved the performance of the P-38 to make it highly competitive, but this lengthy development negated one of the biggest potential advantages of the P-38: Flown first in January 1939, it could have given the USAAF a modern fighter when it needed one most.
And perhaps it was a missed opportunity. As a single-seat day fighter, the P-38 could do little a P-51 could not do at lower cost. In all its other roles, the P-38 was too heavily compromised to really excel. Maybe a more radical design change would have produced an even more useful aircraft. And as we shall see next, attempts at this were made, but with very little success.
XP-49 and XP-58
The first of these efforts was prompted by Circular Proposal 39-775. The logic behind this was simple enough: An even better fighter might be produced by using the latest engines, and perhaps at low cost if they could be installed in existing airframes. The Army ordered prototypes of two proposed aircraft, the Grumman XP-50 (to be discussed later) and the Lockheed XP-49, its preferred choice. The XP-49, alias Lockheed model 522, used the general layout of the P-38 and had many parts in common. With a pressurised cockpit and heavier armament, the aircraft was hoped to achieve excellent performance by using Pratt & Whitney H-2600 or Wright R-2160 engines, both expected to deliver over 2000 hp. Although both manufacturers were almost exclusively associated with air-cooled radials, these engines were the product of a development program for advanced liquid-cooled engines, which was supported by the Army in the 1930s. The research aimed to produce 1 hp from a cylinder volume of 1 cubic inch and for a weight of not more than one pound, and it resulted in the development of a number of “Hyper” engines.
The H-2600, known to Pratt & Whitney as the X-1800, was designed as an engine of H layout, with 24 cylinders and sleeve valves. Inspired by the Napier Sabre, it promised to offer over 2000 hp in a compact, streamlined package, and was designed to be installed with its cylinder banks oriented horizontally or vertically depending on what the aerodynamics of the aircraft required. However, when George Mead retired as vice-president of Pratt & Whitney in 1939 and was succeeded by Leonard Hobbs, the latter questioned the wisdom of investing more effort in liquid-cooled engines, because the company’s area of expertise was in air-cooled radials such as the new R-2800 and the promising R-4360. The Army agreed to cancel the H-2600.
The Wright R-2160 Tornado fared worse. It was a radial of most unusual design, with 42 cylinders in six banks of seven, making it a long and streamlined package. Such a configuration was not compatible with air cooling, and the R-2160 was liquid cooled. Unfortunately the targeted delivery date of December 1941 was missed by several years. In 1943, with no flight-cleared engines available yet, Wright and the Army finally agreed to cancel the R-2160.
The alternative engine for the XP-49 was the Continental I-1430. The development path that had produced this inverted V-12 had already been long and complex when it was first tested in 1939. But when the I-1430 was chosen for the P-49 in March 1940, the I-1430 still was an engine under development, and it was not yet flight-cleared when Lockheed finally received a pair of engines in April 1942. The XP-49 finally made its first flight in November. The I-1430 had a lower frontal area than the V-1710, and the engine installation was beautifully streamlined. But it was considerably smaller and less powerful than the engines that originally had been intended, and speed estimates dropped from 475 mph (765 km/h) to about 400 mph (644 km/h). The P-49 would no longer be faster than the P-38. It did benefit from having a pressure cabin and slightly more powerful armament, two 20-mm cannon and four .50 machine guns. (The armament was never installed.)
When the XP-49 finally reached Wright Field to be tested by the Army, in the spring of 1943, it was obviously too late. The I-1430-13/15 engines delivered 1600 hp at 25,000 ft (7620 m) and 1350 hp for take-off: Not enough to give the aircraft a performance advantage over the P-38J. The I-1430 program would continue into 1944, but finally it was accepted that the production of a new engine that did not produce an increase in power, even if it was a bit more efficient, was not feasible at this time. The XP-49 lived on for a short time as an engine testbed before it was used for destructive tests of landing gear strength.
If the XP-49 turned out to be a regrettable waste of time, this was nothing like the painful experience of the P-58 project. Although it started out as an “improved Lightning” for the bomber escort role and was given the name of Chain Lightning, the XP-58 could hardly have more dissimilar from the graceful P-38. Conceptually, it came closer to the battleplane dream of Douhet, than to an interceptor: A big, powerful aircraft with turreted guns. Instead of being a better P-38, the P-58 evolved into a more advanced replacement for the Boeing YB-40, the escort gunship version of the Boeing B-17. The seeds of misfortune were sown in April 1940, when it was agreed that Lockheed would develop the P-58 at nominal $1 cost to the army, in return for permission to sell the P-38 to the British. (But that did not include changes to the design or the second prototype, and the US government finally spent over $2.3 million on the P-58.)
The original design was offered in two models, one a straightforward development of the P-38 with I-1340 engines, the other a two-seater with a remote-controlled, aft-firing gun in each tailboom. The two-seat proposal was accepted, but the customer insisted on using H-2600 engines, relocating the guns to a remote-controlled dorsal turret, and increasing the fixed armament by adding an additional 20-mm cannon. In this form, the aircraft was ordered in October 1940. The modifications had greatly increased the weight and size of the aircraft, and when the H-2600 was cancelled only a little later, the I-1430 was no longer a realistic option for the XP-58. Unfortunately, the resulting design review and study of alternative engines did not reduce the size and weight of the XP-58. Instead, when the mock-up was finally presented in August 1941, the addition of a ventral turret and cabin pressurization had contributed to making the XP-58 a 16-ton airplane, compared to the 9 tons of the original concept. The rear gunner would sit in the aft end of the central nacelle, aiming the two gun turrets, which would each have two .50 guns with 300 rounds per gun. Four underwing hardpoints, stressed for 1000 lb, would carry external fuel tanks or bombs. To give this monster the required 445 mph (716 km/h) performance, the Wright R-2160 engine had been selected.
After the entry of the USA into the war, the timeline inevitably slipped because other projects had a higher priority. A new armament option appeared in the form of the 75-mm M10 cannon, an enormous weapon with an autofeed mechanism for 20 rounds. With this weapon, the XP-58 might be a potent bomber destroyer, or an effective ground-attack and anti-tank aircraft. After toying with the ground attack concept, and briefly accepting it in September 1942, the Army finally settled on ordering the XP-58 as a high-altitude bomber destroyer with the 75-mm cannon, a pressure cabin, remote-controlled gun turrets, and turbosupercharged R-2160 engines. Because the 75-mm cannon was not yet available, the first prototype would have four 37-mm cannon instead, and the second would have the 75-mm M10. That second prototype was expected to weigh 38,725 lb (17,565 kg) loaded.
At this stages the design still featured the R-2160 engine, but while the XP-58 prototypes were being constructed in early 1943 it became clear that the “Tornado” would not be available. The only available engine in the same class was the Allison V-3420. This consisted of two V-1710 V-12s, assembled on a single case with a common gearbox. The V-3420 was of similar weight and power to the R-2160, despite having a nearly 60% larger swept volume. It was conceptually similar to the DB606 and DB610 units that powered the German He 177 heavy bomber (which were pairs of coupled DB601 or DB605 engines, respectively), but reportedly more reliable. With V-3420-11/13 engines, the weight of the XP-58 rose to 39,149 lb (17,758 kg) loaded. After yet more delays, this time because of engine cooling problems, the first XP-58 finally made its first flight on 6 June 1944.
The second prototype had already been cancelled in April, when 65% complete, to allow Lockheed to devote more efforts on the P-80 jet fighter. The first prototype made only about 25 flights, and its armament and pressure cabin were never installed. With hindsight it is obvious that even if the H-2600 or the R-2160 would have been available in time and successful, the P-58 would have been an aircraft without a role. The enemies of the USA simply did not have the large bombers the Chain Lightning was designed to destroy. (They could not have afforded to build and fuel them even if they had wanted to.) Perhaps the P-58 could have been an effective attack aircraft or light bomber, but for that role it was too complex and its liquid-cooled engines were a handicap.
| Lockheed XP-49 | Lockheed XP-58 | |
|---|---|---|
| Engines | Continental XI-1430-13/15 | Allison V-3420-11/13 |
| Rating | 2 x 1,600 hp at 7,620 m | 2 x 3,000 hp at 8,500 m |
| Wing Span (m) | 15.85 | 21.34 |
| Length (m) | 12.22 | 15.06 |
| Height (m) | 3.17 | 4.18 |
| Wing Area (m2) | 30.43 | 55.74 |
| Empty Weight (kg) | 6,990 | 14,344 |
| Loaded Weight (kg) | 8,504 | 17,777 |
| Max. Speed (km/h) | 653 km/h at 4575 m | 702 km/h at 7620 m |
| Climb | 16.7 m/s initial climb | 11.6 m/s initial climb |
| Ceiling (m) | 11,430 | 11,700 |
| Range (km) | 2,900 | 1,865 |
| Fixed Guns | 2 × 20-mm Hispano M2 4 × .50 Browning M2 (never fitted) | 1 × 75-mm cannon with 20 rounds 2 × .50 Browning M2 |
| Flexible Guns | Dorsal and ventral turrets with 2 × .50 Browning M2 each. |
Skyrocket Again
The Army had made a second choice when it placed an order for the XP-49: Grumman had received an order for an updated version of the F5F Skyrocket, the Grumman G-45, which received the designation XP-50. The most obvious difference with the XF5F-1 was a much longer nose, now extending well in front of the propellers. It served a purpose, because the XP-50 featured tricycle landing gear, and the nosewheel leg was stowed in the long slim nose. A less obvious difference were R-1820-67/69 radial engines fitted with turbosuperchargers, in nacelles that now extended aft to the trailing edge of the wing. Armament was to be two 20-mm cannon and two .50 machine guns, plus a small bomb load.
The choice of the proven R-1820 engine allowed the XP-50 to make its first flight on 18 Feburary 1941. However, the aircraft was lost on 14 May when one of the turbosuperchargers exploded in flight. Grumman was asked to build a second prototype to replace the lost aircraft, but refused to do so unless it also received a production order. That may seem an unusual attitude, but Grumman had already made considerable progress on the design of more advanced twin-engined fighters. In March, it had submitted to the Navy its G-51 design, which was to emerge as the F7F Tigercat, to be discussed below. In June, the USAAF also accepted the G-51 as the XP-65 project, and devoted the remaining funds from the XP-50 project to its development. One consequence of this was, of course, that eventual delivery of a modern twin-engined fighter was pushed into the future.
Havoc
This series of ambitious projects, most of them ultimately unsuccessful, left a gap in the American arsenal when an aircraft was needed to fill the roles for which a twin-engined fighter was most suitable. Especially for the nightfighter role, the US did not produce a suitable airframe, although thanks to the British transfer of the technology of the cavity magnetron, it produced excellent radars. Naturally enough the attention was drawn towards an excellent light bomber that, much like the Blenheim or the Ju 88, might be pressed in service as a stopgap heavy fighter.
The history of the Douglas A-20 began in 1936, when Jack Northrop made the first sketches of a relatively light three-seat, twin-engined attack and reconnaissance aircraft, known (perhaps retrospectively) as the 7A. Lessons learnt from the fighting in Spain and China contributed to design changes which resulted in the 7B, a considerably more powerful and heavier aircraft powered by two R-1830 radials. A clean mid-wing design with tricycle landing gear, the aircraft was to have two nose options: A bomber nose with large transparancies, and a metal nose with six .30 and two .50 machine guns for the attack mission. The 7B made its first flight on 26 October 1938, just after the Munich crisis.
It was lost on 22 January of the next year, in circumstances that created a scandal: It was being secretly demonstrated to French officers, and one of them died in the crash. Neither the incident nor the vociferous protests of American isolationists stopped the French from ordering 100, as the DB-7. This version was much redesigned in detail, with the same wing but a deeper fuselage. It was equipped it as a three-seat light bomber, armed with 640 kg of bombs, four fixed FN-Browning machine guns in the nose, and MAC 34 machine guns in dorsal and ventral defensive positions. The first were delivered in October 1939. The French would go on to order more, and are thought to have received 126 before the defeat of June 1940.
At that time, the Americans, French and British agreed that the British would take over the French orders, including aircraft already paid for by the French. Thus the British received 144 DB-7s with R-1830 engines, designated Boston Mk.I or Boston Mk.II depending on the installed engine version. Because of their good performance and the urgent need for nightfighting aircraft, The RAF modified most of these into nightfighters or intruder aircraft. They were given the name Havoc, which was also adopted by the USAAF when it started to purchase the DB-7 as the A-20 attack aircraft.
The basic Havoc Mk.I nightfighter was a fairly straightforward modification, featuring eight .303 Browning machine guns in a metal nose and the AI Mk.IV or Mk.V radar with external dipole antennas. A Vickers K gun in an open dorsal position offered a very modest defense of the rear. With a top speed of 475 km/h at 4000 m the Havoc Mk.I was faster than the Blenheim Mk.IF, but its performance was clearly inferior to that of the Beaufighter, making it only a temporary expedient. The type was more valuable in the “Intruder” role, attacking enemy bombers over the own bases during take-off and landing, as well as the bases themselves. Because it operated over enemy territory, the Havoc Mk.I (Intruder) was denied the use of the secret radar equipment. Instead it had the glazed bomber nose, four fixed machine guns, and the ability to carry 2400 lb of bombs.
The Havoc also became a platform for some of the weirder experiments conducted in the search for an effective weapon against night bombers. The Havoc Mk.I (LAM) was modified to carry the Long Aerial Mine, an explosive charge attached to a long cable and a parachute, designed to be dropped in the path of enemy bomber formations. The Havoc Mk.I (Turbinlite) had its guns replaced by a powerful searchlight, which was supposed to illuminate targets for Hurricane fighters. Both concepts were impractical.
The airframe clearly had a potential that could be realized by more powerful engines. The installation of a another twin-row, 14-cylinder radial, the 1600 hp Wright R-2600 Cyclone 14, produced the DB-7A. From the DB-7A the British derived the Havoc Mk.II, most of them fitted with a longer nose with twelve machine guns and no dorsal gun. Further redesign resulted in the DB-7B, a version that was built for the US Air Army Corps as the A-20. This additional redesign more than doubled the fuel capacity, considerably increased the weight of the aircraft, and slightly increased its length. The engine of the first Air Corps model, the A-20, was intended to be the R-2600-7 with General Electric turbosuperchargers, which would have given the aircraft an exceptionally good high-altitude performance. However, because of the underdeveloped state of the turbocharger the aircraft instead entered service with R-2600-11 engines, as a fast low-altitude attack aircraft.
The first aircraft of the production line was promptly converted into the first P-70 nightfighter. To improve performance and in line with the type’s offensive role, some armor plate was removed. As as a necessary adaptation to the nocturnal role, flame dampers were fitted to the engine exhausts. The British AI Mk.IV radar would be produced in the USA as the SCR-540 and equipped the P-70A. The main armament consisted of four 20-mm Hispano cannon in a ventral pack. A total of 60 aircraft of this version were delivered between April and September 1942. They were followed by 39 P-70A-1s modified from A-20C bombers and armed with six .50 machine guns (early US-produced Hispano cannon were unreliable). Another 65 P-70A-2s were converted from A-20Gs, retaining the six nose-mounted machine guns of this attack version. The availability of the superior SCR-720 centimetric radar set resulted in the production of the P-70B, with radar in the nose and gun packs installed on the sides of the fuselage. The single P-70B-1 was followed by 105 P-70B-2s, all converted from A-20G or A-20J airframes, that were mostly used as trainers for nightfighter crews. The gun packs, which caused aerodynamic problems, were omitted from these training aircraft.
The P-70A did see operational service with four US squadrons in the Pacific. Unfortunately for their crews, this improvised night fighter had a top speed of 529 km/h at 4270 m but needed 8 minutes to climb to 3660 m. This made it only a little slower than the Japanese Ki-45-KAI-Hei, but much inferior in climb. The P-70 had inherited the engines of the A-20 attack aircraft, which were optimized for low-altitude operations, and detachments operating in the Solomons and New Guinea found that their aircraft needed over over 40 minutes to climb to 6100 m, which enormously restricted their effectiveness against Japanese bombers. Even after the aircraft was lightened as much as possible, enemy aircraft flying at 8500 m were immune from interception, as the P-70 could only reach 8000 m and was barely controllable at that altitude. A range of field modifications to improve the performance of the P-70 brought no solution. Out of necessity, night interception was soon entrusted to P-38s, while the P-70s were switched to intruder operations.
Deadly Spider
The American search for a better nightfighter results in a big twin-engined nightfighter that was also the only wartime aircraft entirely designed for the role: The iconic Northrop P-61 Black Widow. Unfortunately, it appeared to prove that purposeful design was not necessarily superior to fortunate improvisation.
John K. Northrop was already one of the most influential American aircraft designers of the 20th century when in March 1939 he founded — for the third time — a new company that bore his name. His financial backers might have sympathy for Northrop’s desire to build highly efficient aircraft, but they also saw that the imminent war in Europe was creating a huge need for military aircraft. And in 1940 Northrop was contacted by the British Purchasing Commission to design and build a new night fighter. What the RAF had in mind was an aircraft with modern radar, heavy armament and the endurance to fly long standing patrols.
However, in the summer of 1940 an American mission, headed by Lt. Gen. Emmons, visited Britain and returned with a recommendation that the USAAC develop a modern night fighter. An informal requirement was presented to Northrop in October 1940, closely following the lines of the British specification. Discussions resulted in the formal presentation of Northrop Specification NS-8A, for which a contract was approved on 30 January 1941. The aircraft was given the designation P-61, and two XP-61 prototypes were ordered. It is as well to remember that few World War II combat aircraft were actually designed during the war, because the time from drawing board to production was usually four years or more, and that the development of the XP-61 put a major challenge to a small company, although one with a highly experienced designer.
And the P-61 was an unusually complex design. The first drawings showed a twin-boom configuration with a central nacelle for a crew of three and the radar in the nose. Besides four 20-mm cannon in the wings, the P-61 was intended to be armed with two remote-controlled barbettes, the dorsal one with four .50 guns, the ventral one with two. The aircraft was as big as a medium bomber and powered by two Pratt & Whitney R-2800-A5G radials. Cooling, at this time, was expected to be through ducted spinners, to reduce drag. Gross weight was estimated to be 24,000 lb (10,900 kg), a figure that turned out to be too optimistic by almost three tons when the aircraft finally entered service. The crew of three included the pilot and gunner sitting in tandem in a nose with two stepped cockpits, not unlike a modern attack helicopter, while the radar operator sat in the rear of the nacelle behind the turret.
The complexity of the armament installation was apparently the result of British advice; if so it is hard to say on what this advice was based, except perhaps the brief and only modestly successful use of the Boulton-Paul Defiant as a night fighter. The barbettes, for which General Electric was the contractor, clearly contributed to the weight and size of the P-61. After a mock-up inspection in April it was decided to delete the ventral barbette and move the cannon from the wings to the bottom of the fuselage nacelle. The dorsal barbette was retained, although it became clear that General Electric would not be able to provide one in time for the first flight.
An order for 13 pre-production aircraft, YP-61s, followed in March. In September 1941, well before the first XP-61 had flown, Northrop received an order for 150, increased soon after Pearl Harbour with an additional 410. The first XP-61 finally made its first flight on 26 May 1942. The aircraft behaved well but testing soon revealed the need for a series of changes that would have to be made in production aircraft. Those included changes to the wing to replace the “Zap” flaps of the XP-61 with easier to manufacture slotted flaps, which finally resulted in a complete redesign that introduced spoilers as the primary means of lateral control, with only small ailerons retained. The full-span flaps helped to keep landing speed down, a desirable characteristic in an aircraft operating at night. Between the YP-61 and the production P-61, it was also decided to replace the welded magnesium tail booms with more conventional aluminium alloy structures: In some ways the XP-61 had been a bit too innovative for its own good. It was also necessary to reinforce the rear spar, redesign the cannon installation, and reinforce parts of the structure against blast and recoil damage.
The prototypes were flown with a dummy barbette as this item was not yet ready, and SCR-540 radar pending availability of the definitive SCR-720. Unfortunately, when the YP-61s came off the assembly line in the autumn of 1943, testing revealed that elevating or rotating the guns caused serious tail buffet. Hence the first 37 P-61A-1 production aircraft were delivered with the barbette fixed forward, while on the next 163 it was omitted altogether. This reduced gross weight by 744 kg, or slightly over 5%. While the speed gain was a modest 4.8 km/h, indicating that the barbette did not cause much drag by itself, there can be no doubt that the P-61 would have been lighter, smaller, and faster if it had been designed with only fixed guns from the outset. The dorsal barbette was also in short supply as they were also wanted by the B-29, but it was reinstated on the production line from the P-61B-5 model onwards, after the aerodynamic problems were solved. More than half the P-61s were delivered without it, but it was retrofitted to some aircraft.
For its size, the P-61 possessed excellent manoeuverability. Roll rate was usually a weakness of a twin-engined fighter because of its mass distribution. The P-61 relied on spoilers for lateral control, with only small ailerons, and it combined a decent roll rate (though inferior to a P-38) with excellent lateral control. The rate of turn was very good, as good or even better than single-seat fighters. And the P-61 could perform most aerobatic manoeuvre with ease and precision. Nevertheless, its inertia and power-to-weight ratio did not allow it dogfight with a day fighter, making the aircraft vulnerable to such encounters at dawn or dusk.
The crew further benefitted from a good radar and powerful armament. The SCR-720 was an excellent radar which would remain in service into the 1950s, and P-61s were generally well-equipped. The P-61B featured an improved SCR-720C in a slightly longer nose, and night binoculars for the pilot to complement the radar, besides a number of other improvements. The view from the cockpit came in for some criticism as it was by 1944 standards rather heavily framed; at this time bubble canopies were usually found on fighters. But the Black Widow was well liked by those who flew it in combat, both in the Pacific and in Europe. Its first recorded aerial victory was in October 1944, over Saipan.
The major flaw of the big P-61 was that it never lived up to the 375 mph (604 km/h) of the original specification, and did not come close to the 450 mph (724 km/h) that had been the wilder hope of project officers. The final model to see wartime service, the P-61B-20, could reach 366 mph (589 km/h) at 20,000 ft (6,100 m) with the engines running at war emergency power. It needed 12 minutes to reach that altitude. Admittedly this was vastly better than the P-70A, but combat experience in the Pacific demonstrated that the P-61 fell short of the needed speed and altitude performance.
However, desirable performance characteristics also depends on the assigned role. When the P-61A entered combat in October 1944 the RAF had already introduced the Mosquito NF.30, which was 40 mph (65 km/h) faster. But this was high-altitude performance achieved thanks to two-stage supercharged engines, which also gave the NF.30 an excellent operational ceiling. For the intruder missions at lower altitude the P-61A and P-61B had good characteristics. Nevertheless the deficiencies of the Black Widow in fighter performance led to the development of improvised night fighter versions of the P-38 in the Pacific.
Northrop’s response was the development of the P-61C with turbocharged R-2800-73 engines. While this version did show significantly improved performance, the handling suffered from the substantial weight increase. Production of this version was started, but quickly ended at the end of the war with only 41 aircraft delivered, none of which saw combat.
The Black Widow, as the P-61 was named appropriately for its role and spider-like looks, did deliver excellent service as in the intruder role. The P-61A-11 had been provided with two hardpoints for the carriage of bombs or extra fuel tanks, and on the P-61B-10 this was increased to four hardpoints plus eight stub launchers for 5-inch rockets. When the P-61 entered service, enemy aircraft were becoming scarce, so the switch in role to night attack was a logical one, made first in Europe and then in the Pacific.
Northrop also produced two prototypes of an XP-61E long-range escort fighter, without radar, without the barbettes, but with a crew of two under a bubble canopy in a streamlined nacelles. Unfortunately performance wasn’t much better than that of the P-61B, but a conversion of one of these aircraft to XF-15 reconnaissance aircraft succeeded in prolonging the production run of the type with 36 production F-15 Reporters, which saw service in the Korean War. The P-61 night fighter itself would continue in service until 1950, with growing concerns about its inadequacy as the all-weather air defence of the USA in the nuclear age.
Assessing the P-61 fairly is difficult. Northrop managed, despite several important design changes, to deliver the Black Widow to the front line in less than four years, an major achievement for such a complicated aircraft. Besides being the first purpose-designed night fighter ever built, the P-61 contained a number of technical innovations. It was, however, the product of a too complicated tactical concept, which had made it big, heavy, and too sluggish to be a really good night fighter. Ironically the dorsal barbette was omitted from most production aircraft, and combat units discovered the advantage of flying it with a crew of two, relocating the radar operator to the nose, immediately behind the pilot. It is tempting to think that if the turrets had never been part of the design, Northrop could have built the best night fighter of the war.
Bat
America’s most remarkable twin-engined fighter prototype of the time did not start out as a twin-engined design, although it did start as a twin-propeller design. The drawings of McDonnell’s Type 1, the first design of a company founded in 1939, featured an engine buried in the fuselage, and pusher screws behind the wing’s trailing edge driven by an arrangement of shafts and gears. An idea that harked back to WWI, when some large aircraft had such an arrangement to make the unreliable engines accessible in flight. In this case the goal was streamlining and freeing up the nose of the aircraft for a cockpit with good forward view. The engine would be the Pratt & Whitney H-3130 or the Allison V-3420. The latter engine consisted of two V-1710 V-12 engines joined together, so that with it the Type 1 could have been considered a twin-engined fighter if the concept was loosely defined.
The Type 1 was not accepted by the Air Corps, perhaps fortunately as the complex drive train would likely have been troublesome. But the aircraft was also aerodynamically innovative, with careful blending together of wings and fuselage in an attempt to reduce drag, and the USAAC was sufficiently impressed to encourage McDonnell to design a twin-engined successor. By April 1941 this had resulted in the Type 2A, an aircraft profiled entirely to a laminar wing profile of 13% thickness: Not just the wings, but the fuselage and engine nacelles too, and the deep fairing that blended them together. The design looks futuristic even today. The engines were Continental I-1340 inverted V-12 engines in large nacelles with turbosuperchargers and trust-generating exhaust pipes at the rear. This went against the wisdom of the time, which recommended not to install an untried engine in an untried aircraft. The XI-1340-17 and -19 engines (with a different direction of rotation) of the prototype were as experimental as the aircraft itself. They were rated for 1350 hp each, although Continental projected a 2100 hp war emergency rating using 150-octane fuel.
Conceptually, the P-67 was a single-seat high-altitude fighter, with a pressurised cabin and turbocharged engines. The armament was planned to be six 37 mm cannon of the Browning M4 type, a flawed choice because of the poor ballistic characteristic of this weapon and its limited ammunition supply. The envisaged role was the defence of the USA against high-altitude bombers, even in 1941 a somewhat unlikely contingency. But the armament was never installed in the single completed prototype. Because of its unusual looks, the unofficial nickname ‘Bat’ stuck to it. It was pained olive green with grey undersurfaces, but on most pictures it looks almost black.
Two XP-67 prototypes were ordered in September 1941. That late date would have a detrimental impact on the project, because soon the highest priority for McDonnell would be subcontractor work to turn out proven combat aircraft, and P-67 project was delayed. Development work continued, but it was 6 January 1944 before the first XP-67 made its first flight. The construction of the second prototype had already been suspended because of uncertainty about the best radiator inlet configuration, and besides the USAAC was already looking for alternative engine installations. This wasn’t just because of problems with the XI-1340 engines. In early 1944, McDonnell was already working on a jet fighter for the Navy, and it was increasingly obvious that the P-67 concept was outdated. The air force was toying with the idea of installing more reliable reciprocating engine and jet engines in the after section of the engine nacelles, thus converting the P-67 into a mixed-power fighter.
With XI-1430 engines, the performance of the XP-67 failed to match its futuristic looks. The fighter was overweight and underpowered, resulting in a top speed of only 652 km/h instead of the hoped for 760 km/h. The dangers of engine overheating, also because of a less than satisfactory cooling inlet design, contributed to a 300 hp shortfall in output power. An engine fire occurred during taxi trials before the first flight; on the fourth flight, the engines were destroyed by overspeeding. Finally, in September an in-flight engine fire resulted in an emergency landing and the destruction of the aircraft. After only 43 hours of flight, there was a consensus to abandon the program. The second prototype was never completed.
One More Failure
Some aircraft manufacturers emerged from the war with a great reputation. And some did not: Curtiss, unfortunately, was one of the losers. Its P-40 fighter, developed before the war, gave good service; but most of its wartime development projects were failures. The XP-71 project took its rightful place in a dismal series.
The concept was awkward enough: A twenty-ton high-altitude fighter armed with one 75-mm cannon and two 37-mm cannon. The 75-mm weapon would have 20 rounds in a closed-loop belt feed. Twin turbosupercharged R-4360 radials, the most powerful engines available, would power the aircraft. They were to be installed with eight-bladed pusher propellers in streamlined nacelles. Originally the aircraft was to have a crew of two, but after the mock-up inspection this was changed to a single pilot, perching high in a bubble canopy between the engine nacelles.
Even when the contract was signed in October 1941, the long timelines for the development of aircraft and engines raised concerns, which were dealt with not very effectively by delaying contract signature until March. While the big gun could in theory achieve a kill at a long range, developing an aiming and rangefinder system that would enable such hits was a complex task. And ground tests of the cannon installation in February 1943 resulted in back-to-the-drawing-board damage. The project managed to stretch its life until October 1943, when it was cancelled, before a prototype was completed. It was never a very good idea.
Nachtjagd
Even during the Battle of Britain, RAF Bomber Command had gone on the offensive. Painful and bloody experience earlier in the war had demonstrated that unescorted bombers could operate over enemy territory only at night, and so they did, at first with very limited efficacy but in increasing numbers. For the Luftwaffe this created similar problems as earlier in the war for the RAF. At this time, its ground-based radars were more technologically advanced than the British radars, but the problem of directing a fighter close enough to the enemy bombers to initiate attack had not been solved. The air defence of the Reich now needed a fighter with good handling characteristics for night operations, the endurance to loiter in the battle area and pursue fleeting contacts, heavy armament, and the ability to carry of variety of more or less experimental gear.
Kauz
One option was to convert a light, fast bomber, as the British had done with the Blenheim. The closest German equivalent was the Dornier Do 17, a streamlined twin-engined bomber which had first seen combat during the Spanish Civil War and was at the end of its production run in mid-1940. Do 17Z bombers already operated in the intruder role, with extra fuel cells in the bomb bay and sometimes additional guns fitted in a modified nose. Dornier was instructed to create dedicated nightfighter and intruder versions, which they reportedly did by adopting the nose cone of the Ju 88C-2 to their own aircraft. The new model appeared as the Do 17Z-7 Kauz, with a “solid” nose design carrying one 20-mm cannon (first the MG FF and later the MG-151/20) and three (MG-17) rifle-calibre machine guns, modest enough armament by the standards of the time. Ventral and dorsal installations of MG 15 machine guns were retained. The crew of three was given some extra armour protection, and fuel tankage redesigned and increased. The engines remained 1000 hp Bramo 323P-1 radials, doing little to give the aircraft the required performance: It was also primarily intended, it appears, to operate as a long-range intruder aircraft. An interesting concept tested on one of the only two aircraft completed was an infrared detection device known as Spanner. This combined an infrared searchlight with a viewer in the cockpit, allowing the aircraft, at least in theory, to illuminate targets without being seen. (The device would also be installed on some Bf 110s, with little success.)
Remarkably enough, it was decided to convert an additional nine Do 17Z-3 bombers to the Do 17Z-10 Kauz II configuration, which featured a redesigned nose with an extra machine gun. These aircraft were successful enough to serve until 1943. A Do 17Z made the first successful night interception with radar guidance from the ground, in October 1940. More nightfighter conversions were produced of the Do 215, itself a variant of the Do 17 powered by slightly more powerful Daimler-Benz DB601 V-12 engines. The Do 215B-5 Kauz III nightfighter version of this aircraft thus was faster, at 465 km/h instead of 420 km/h. In August 1941 a Do 215B-5 was equipped with the very first Telefunken FuG 202 Lichtenstein BC airborne radar. Its array of external antennas cannot have helped performance but the radar was effective, reportedly allowing its pilot to shoot down six aircraft before the set broke down. About 20 aircraft were converted to nightfighter configuration and they stayed in service until May 1944, with the addition of a ventral tray with an extra pair of cannon.
| Dornier Do 215B-5 | Dornier Do 217N-2 | |
|---|---|---|
| Engines | Daimler-Benz DB601Aa | Daimler-Benz DB603A |
| Rating | 2 x 1100 hp | 2 x 1750 hp |
| Wing Span (m) | 18.00 | 19.00 |
| Length (m) | 16.66 | 18.00 |
| Height (m) | 4.97 | 4.60 |
| Wing Area (m2) | 55.0 | 57.0 |
| Empty Weight (kg) | 10,270 | |
| Loaded Weight (kg) | 13,200 | |
| Max. Speed (km/h) | 465 km/h at 4,800 m | 515 km/h at 6,000 m |
| Climb | ||
| Ceiling (m) | 9000 | |
| Range (km) | 1,800 | 1,755 |
| Guns, fixed | 1 × 20 mm MG FF in nose 4 × 7.92 mm MG 17 in nose 2 × 20 mm MG FF in ventral tray | 4 × 20 mm MG 151/20 in nose 4 × 7.92 mm MG 17 in nose |
| Guns, flexible | 2 × 7.92 mm MG 15, ventral and dorsal |
The logical successor to the Do 17 and Do 215 was Dornier’s Do 217. It is often dismissed as a derivative of these aircraft but was in fact new design, substantially larger and more powerful, a heavy medium bomber in the class of the American B-26. Although some early production aircraft had DB 601 engines, most production aircraft had more powerful BMW 801 radials or DB 603 in-line engines. The BMW-engined Do E-2 was the first to be converted into an improvised night fighter, the Do 217J, which featured a new “solid” nose with four MG 17 machine guns and four MG FF/M cannon, while retaining its dorsal and ventral MG 131 defensive guns. The J-1 did not have radar, but was equipped as an intruder aircraft with full bombing equipment. The J-2 was a true nightfighter with FuG 202 Lichtenstein BC radar. The Do 217J was not very successful but soldiered on from the spring of 1942 until 1944, with a number being transferred to Italy’s Regia Aeronautica. A similar conversion of the faster DB 603-engined Do 217M entered production in late 1942 as the Do 217N. To increase performance, the Do 217N-1 quickly saw the deletion of defensive armament as the N-1/U-1, a choice that became standard on the N-2 model. But although the Do 217 was a good aircraft for night flying and possessed a decent performance, it lacked the agility and rate of climb to make a good fighter. Most of the 364 fighter versions of the Do 217 ended at operational training units.
Besides their pioneering role in the introduction of radar and the less successful infrared devices, the Do 17 and Do 217 family also featured the first installations of a new armament variation that would become notorious as Schräge Musik (slanted music). Rudolf Schönert is credited with the installation of a upward-firing machine guns in a Do 17Z-10 and later in a Do 217. The idea was not truly successful until late 1942, when twin MG FF cannon, angled 70 degrees up, were installed in the rear cockpit of a Bf 110 of II/NJG5, then commanded by Schönert. Production installations later appeared on all German nightfighter types, including a heavy installation of four MG 151/20 cannon on the Do 217N.
Old Soldier
The development of aircraft such as the Do 17Z-7 in 1940 indicates that the Luftwaffe was not totally neglecting night combat. Nevertheless in the first months of the war the home defence consisted of obsolescent models of the Bf 109. These would perform night interceptions with the guidance of searchlights on the ground, a dubious concept as the little fighter was likely to run out of fuel long before it had managed to stalk a bomber this way. The first dedicated Nachtjagdgeschwader was created on 26 June 1940. While at first these units had both the Bf 110 and the Bf 109 on the strength, it was not long before conversion to the Bf 110 was recognised as the best solution. The twin-engined fighter had the endurance and flight characteristics for safe night operations, good enough performance to intercept bombers, and extra crew members to help look for the enemy, the human eye being at this stage the only detection device carried on these aircraft.
The first Bf 110 units added to the growing night defences of the Reich flew day fighter models with minor modifications, such as flame dampers added to the exhaust and changes to cockpit lighting for night operations. Indeed the first nightfighter units were created by renaming Zerstörer units. A layer of matte black paint initially identified the nightfighter aircraft. The night interception technique relied mostly on the development of better ground radars: The Freya warning radar was combined with two short-range Würzburg radar sets, which allowed a ground intercept officer to plot the location of both the target and the intercepting fighter. Reliance on an accurate but short-range ground radar set did tie the nightfighter to the small sector, named Himmelbett, that could be controlled by the radar stations. The “Kammhuber Line”, named after the commander of the Nightfighter Division, gradually expanded to stretch multiple lines of such stations as a barrier between the German cities and the RAF.
More flexible tactics became possible in February 1942 when the first airborne radars were installed. Development had begun in 1940, but was hampered because until late 1941 the Luftwaffe opposed the installation of drag-inducing external antennas, and at the radar wavelengths then in use, internal antenna arrays performed poorly. Thus it was February 1942 before early production models of the FuG 202 Lichtenstein BC radar were installed. Operating at 490 MHz, this radar set required a complex “mattress” of sixteen pairs of small dipoles antennas on the nose. It had a range of about 4 km and a minimum range of about 175 m, enough to help crews to intercept a target and bring it within sight. The drag penalty was substantial, reducing speed by 40 km/h, and crews were distrustful at first. Telefunken’s Dr. Kümmritz complained that some aircrew resorted to a hacksaw to remove the offending antennas. Over time, the radar proved its value.
The Bf 110F-4 was the first dedicated night fighter model, with a third crew member, FuG 202 radar (not installed on the first production aircraft) and the ability to carry ventral tray with two 30-mm MK 108 cannon. It had a top speed of 500 km/h at 4,500 m, or 40 km/h slower than the Bf 110C-1 despite the installation of more powerful 1350 hp DB 601E engines. The further addition of two Schräge Musik cannon (denoted by the suffix /R8), and a growing collection of electronic devices to help in the interception of the RAF bombers, made the aircraft more effective at the cost of performance and handling.
Some relief was expected from the Bf 110G series, which featured the more powerful DB 605 engine. This proposal was accepted in January 1942 when it was clear that the replacement of the Bf 110 by the Me 210 would have to be delayed. Initial production plans envisaged a G-1 fighter-bomber, G-2 heavy fighter and a G-3 reconnaissance machine; the G-1 was soon cancelled and the first production model was the G-2, which entered service in January of 1943. The nightfighter model became the G-4. Improvements during the production run of the G-series included the replacement of the ventral MG FF cannon by the MG 151/20, the replacement of the defensive MG 15 by the MG 81Z, and on the G-4 series, the introduction of larger tail fins to improve handling. During 1943 the G-series and the F-series were produced in parallel, with both models incorporating the same airframe changes, but F series aircraft built to compensate for the limited supply of DB 605 engines and their initial unreliability. As supply and reliability problems eased late in 1943, also F-4 airframes were completed with the new engines.
As daylight attacks of US bombers intensified during 1943, the G-2 Zerstörer version was found effective against them because of its heavy armament, which various modifications sought to enhance. Experiments with a 37-mm BK 37 cannon in a belly pack, which replaced the ventral 20-mm cannon in the /R1 modification, were not very effective, but the weapon saw some combat against US bombers in 1944. More successful, from the autumn of 1943, was the optional installation of four tubular launchers for the WGr 21 rocket, two under each wing. A barrage of such rockets could disrupt a bomber formation when their 40.8 kg warheads exploded at the preset distance, between 600 and 1200 m. However, the tubular launched induced a lot of drag and each unit weighed 111 kg. Even nightfighters, despite the performance penalty imposed by their extra equipment and the lack of training of their crews for such missions, were deployed to oppose the USAAF bomber raids. But when P-51B Mustang escort fighters appeared over Germany in late 1943, the Bf 110 soon ceased to be a viable daylight interceptor, and the use of nightfighters during daylight was finally understood to be a tactical error that resulted in prohibitively heavy losses.
Hence the G-4 nightfighter model became the most built version of the Bf 110, with a production run of about 1850 aircraft, from January 1943 to early 1945. Increasingly, the age of the design was hard to compensate for. Perhaps nothing indicated this as much as the /R3 modification, which entailed the replacement of the four MG 17 machine guns in the upper nose of the fighter by a pair of 30-mm MK 108 cannon. These weapons were far more destructive than the rifle-calibre machine guns that they replaced, but the installation of short-barrelled cannon in the upper nose of a nightfighter was nevertheless a flawed and unpopular concept. Complaints about the blinding muzzle flash were predictable but flame dampers were still under development at the end of the war. To make it worse, soot was deposited on the windscreen, the blast could damage the radar antennas, and crews complained that the powerful but short-range cannon made the aircraft fly through the debris of the enemy bombers.
From June 1943 onwards, the Bf 110G-4 began to receive the FuG 220 Lichtenstein SN-2 radar, which operated at a different (longer) wavelength than the FuG 202 and was thus more resistant to British countermeasures such as Window, at least initially. FuG 220 came with a simpler, but larger set of dipole antennas on the nose of the aircraft. But the Model A radar had to be combined with a FuG 212 Lichtenstein C-1 radar because its minimal range of 500 m was too long to bring the fighter into visual contact with its target, so a smaller set of four pairs of dipoles appeared in the center of the FuG 220 antennas. Later models of the FuG 220 had a shorter minimal range and allowed the FuG 212 to be removed.
Until late 1944 the Bf 110 was an effective nightfighter. It was a clear example where “size matters”: If the Luftwaffe had chosen the little Fw 187 over the Bf 110 in 1940, it would have had a serious problem in 1943, for the sleek Focke-Wulf fighter would not have been able to carry the extra load for this mission. As it was, even the Bf 110 airframe was reaching its limits with the addition of radar, electronics gear, additional armament packs, and usually external fuel tanks to increase the aircraft’s endurance. But the Luftwaffe had not had much success in its search for an alternative, and it was more luck than planning that gave it a better nightfighter in the final years of the war.
| Bf 110G-2 | Bf 110G-4 with FuG 202 | |
|---|---|---|
| Engines | 2 × DB 605B-1 | 2 × DB 605B-1 |
| Rating | 1475 hp | 1475 hp |
| Wing Span (m) | 16.28 | 16.28 |
| Length (m) | 12.07 | 12.91 |
| Height (m) | 4.16 | 4.16 |
| Wing Area (m2) | 38.37 | 38.37 |
| Empty Weight (kg) | 5959 | 6088 |
| Loaded Weight (kg) | 7788 | 7917 |
| Max. Speed (km/h) | 561 km/h at 5800 m | 510 km/h at 5800 m |
| Climb | ||
| Ceiling (m) | ||
| Range (km) | 900 km, 1300 km with external fuel | 880 km, 1270 km with external fuel |
| Fixed Guns | 4 × 7.92 mm MG 17 in nose 2 × 20 mm MG 151/20 in nose 2 × 20 mm MG 151/20 in ventral pack | 4 × 7.92 mm MG 17 in nose 2 × 20 mm MG 151/20 in nose 2 × 20 mm MG 151/20 in ventral pack |
| Flexible Guns | 1 × MG 81Z | 1 × MG 81Z |
Innovative Failure
In May 1940, a possible successor to the Bf 110 had made its first flight: The Arado Ar 240. This first flight occurred with some delay, as after the outbreak of the war development work had been put on hold to give priority to the production of types already in service, in the expectation that the war would soon be won. Arado had proposed a fast three-seat fighter in 1937, soon after the first flight of the Bf 110. This E 651 design shared features with McDonnell’s Type 1 of two years later: Two Daimler-Benz engines would be buried in a wide but well streamlined fuselage, and connected to a complex gearbox. From there a shaft would drive a propeller on each wing, installed at the front of a small nacelle, which apparently also contained the cooling radiator. The E 651 would be armed with four cannon in the lower fuselage, while the gunner would be able to operate a dorsal or a ventral MG 81Z. The RLM was not enthusiastic about this complicated concept, and requested a conventional twin-engined design.
The resulting Ar 240 blended features of the E 651 with the E 310 design for a multi-engined, multi-role carrier aircraft. It had a superficially conventional layout, but nevertheless some radical design characteristics. To reduce drag, the engine nacelles and fuselage were slender and long. The fuselage cross-section was barely larger than that of the Bf 109. Annular radiators for the 1100 hp DB 601A engines were installed in front of the engine nacelles, with large hollow propeller spinners to provide airflow. And although the aircraft would be about a ton heavier than the Bf 110, the wing area was 30% smaller. This resulted in a wing loading of 300 kg per square meter, very high for the period. Large Fowler flaps and automatic leading-edge slats were provided to get acceptable landing speeds.
Calculations suggested that it would have a top speed of 674 km/h when equipped with the smaller of two wing designs, of 12.1 m span, and a pressure cabin. But by being a small but heavy aircraft, the Ar 240 would inevitably incur some penalties in handling, worsening as more war equipment was added to the airframe. Indeed the development story of the Ar 240 reveals a constant need for growth, with changes involving bigger wings and tail, a longer fuselage, and more powerful engines. Making things more complicated without much benefit was a dive-bombing requirement. The first prototype was equipped with dive brakes in a tail cone, apparently found unsatisfactory and omitted from the later aircraft.
The first flights revealed that the Ar 240 was indeed fast, but had unsatisfactory handling characteristics, being unstable around all axes. The wing profile was thought to be the cause of the ineffectiveness of the ailerons, on which some development work was done. As work was already proceeding on a much-modified third prototype, the test programmes of the Ar 240V1 and V2 were cut short and ended obscurely. The most obvious change in the Ar 240V3 was the location of the cockpit, which Arado had been instructed to move to the extreme nose, instead of over the wing centre section as on the first aircraft. The new cockpit provided not only a good view of the upper hemisphere, but provided the pilot with a transparent panel in the nose, down to his feet. (The better forward view must have been offset by a much greater vulnerability, and Arado proposed an armour plate in front of the pilot that could be rotated to either provide better protection or the optimal view.) As a consequence of this design change, the engine nacelles were extended forwards to avoid putting the pilot in the plane of the turning propellers, which would have been noisy as well as dangerous. An aft fuselage extension was designed to restore stability. Wing span was increased to 14.3 m.
The Ar 240V3 was still unstable, although the test center in Rechlin found handling in August 1941 to be generally acceptable. Development continued with the V5 and V6 which were more representative of the planned production version, including the installation of the intended defensive armament. Improved rear defence was to be provided by remotely controlled gun barbettes, an upper and a lower FA 13, each containing a MG 81Z twin machine gun. The gunner would aim these from his seat, with remote periscopic sighting heads. The development and test programme stretched deeply into 1942, but already in February 1941 the RLM had accepted a proposal to build a small pre-series of six aircraft, and the first Ar 240A-0 pre-series aircraft flew in June 1942.
Plans to put the type into production evaporated. As a replacement of the Bf 110, the Ar 240 competed directly with the Me 210. The latter had more than its fair share of aerodynamic problems, but in late 1942 solutions for those were being developed. The Ar 240 might be faster but its handling characteristics of the were far removed from those of a good fighter. Its stability problems, bad stalling characteristics, and poor harmony of control made it a bad weapons platform and restricted its maneuverability too much. And there was no spare production capacity to support an additional type. It is not quite clear why the team of chief designer Blume did not take more radical steps to address the aerodynamic problems. Instead the team appears to have produced a steady stream of proposals to fit more powerful engines.
The Ar 240 did see limited service as a reconnaissance aircraft, a role for which its potential had been recognised early in 1941. Prototypes and A-series aircraft were converted for this role, in which its high speed was used to advantage. Forward firing armament was restricted to an MG 17 in each wing root. The remote-controlled gun turrets were retained and functioned well. The Ar 240A was credited as the only German reconnaissance aircraft able to complete missions over Britain.
Plans were still made for an Ar 240B powered by DB 605 engines, envisaged by the Luftwaffe as a fast bomber. This was a somewhat odd concept as the type could carry bomb loads only externally. Arado preferred an Ar 240C powered by the bigger DB 603, and converted two A-0 pre-series aircraft to have these engines. The production C-series would be bigger, with a wing span of 16.6 m and a length of 13.35 m. But in December 1942 Arado was told to stop all work on the Ar 240. An attempt to revive the type as the Ar 440 — what’s in a name? — was not successful.
Embarrassment
Messerschmitt’s concept of a successor for the Bf 110 was in some ways close to Arado’s: A smaller, heavier, and faster aircraft with improved defensive armament. But it was a less radical and more elegant design, and sufficiently convincing that in the autumn of 1938 the RLM ordered 2000 aircraft off the drawing board.
The Me 210V1 prototype made its first flight on 2 September 1939. It was a slightly smaller aircraft than the Bf 110, with conventional but pleasing lines. The cockpit was well forward in the nose, which featured a transparent panel to give the pilot a good forward and downward view. Below the cockpit floor, in an unusual location forward of the wing’s leading edge, was a small internal bomb bay. As so many German aircraft of the period, the new fighter was required to have a dive-bombing capability. The 1300 hp DB 601E engines were installed in neat cowlings. The V1 had twin tail fins, also installed at first on the second prototype, Me 210V2. But this was subsequently modified to the single tail fin that had been designed as an alternative, and which was used on all other Me 210s. The V2 also featured the type’s innovative defensive armament: Remote-controlled barbettes with a 13-mm MG 131 each, installed on the side of the fuselage. The rear of the cockpit canopy was modified to provide flat clear-vision panels.
The Me 210 looked good, although its fuselage was tadpole-shaped, with a round, wide nose and a short, sharply tapered aft fuselage. Test flights revealed marginal stability around all axes, poor harmony of controls, and disappointing performance. In an echo from the Ar 240, the wing profile was blamed for longitudinal stability problems. Handling was improved with the single tail, but not sufficiently so. Although a series of prototypes was built incorporating various changes, Messerschmitt failed to make sufficient modifications to cure the problems. In June 1941 it was concluded that after a long development period, the Me 210 was still unsatisfactory; it was too unstable to be a good gunnery platform, unsatisfactory as a dive-bomber, was too difficult to control during take-off, and suffered reliability problems including engine fires. The cause of this failure is unclear but it is likely that Willy Messerschmitt’s strong inclination to build aircraft as light as possible got in the way of some necessary changes, such as an extension of the aft fuselage to make the tail surfaces more effective. Only in February 1942 did the Me 210V17 fly with a longer, deeper fuselage.
The original plan had been to start production in late 1941, and in expectation of this the production of older aircraft had been allowed to taper off. Despite the enduring problems with the prototypes, these plans maintained their momentum. Erprobungsgruppe 210 began operational testing in September 1941, and other units began to receive the new fighter in early 1942. With two 1350 hp DB 601F engines, a fully loaded weight of more than ten tons, a wing loading of 285 kg per square meter, and enduring handling problems, these aircraft were widely perceived to be failures. They were faster than the aircraft they were to replace, but they were also plainly dangerous, with a high accident rate. Especially landing accidents were frequent.
On 12 March 1942, the production of the Me 210 was suspended. The RLM decided to increase production of older types to fill the gap and launch production of the Bf 110G while attempting the repair the defects of the Me 210. Willy Messerschmitt lost his directorship of the company that he had founded, retaining only the leadership of the design office for new projects. Airframes under construction and parts were stored while awaiting the results of the planned modifications, which included a longer fuselage and wing slats.
The project was saved from complete failure by the effect of these modifications, which greatly improved handling. The extension of the fuselage from 11.18 m on the Me 210A-0 to 12.13 m on the A-1 greatly improved controllability during landing and take-off. Slats were added to the wing’s leading edge to enhance controllability at low speeds, the dive brakes were moved to a position in which they no longer disturbed the airflow over the tail, and measures taken to restore the position of the center of gravity. Even so, the flying characteristics of the Me 210 were not such that the type could be operated safely at night, and thus the Bf 110 would remain in production as a nightfighter. The good news for the RLM was that the short-fuselage aircraft still on the production lines could be converted, and about 200 probably were; a roughly equal number were built from the start with the new fuselage.
These changes made the Me 210A a useful combat aircraft again, but it was underpowered. A possible solution was the installation of the DB 605, which resulted in the production of 272 Me 210Ca-1 in Hungary, on a production line set up to produce aircraft but for the Luftwaffe and for Hungary’s own MKL. German production focused on a version the new, bigger DB 603 engine, first flown in August 1942. The additional forward weight of the engines had to be compensated for the by a reduction of the backward sweep of the outer wing panels, but the change could be made on existing airframes. Nevertheless, instead of being called Me 210E, the aircraft would go in production as the Me 410. It would receive priority for the DB 603, while the Bf 109 and Bf 110 would receive the DB 605. The Me 410 had excellent low-altitude performance and satisfactory handling, and was produced in parallel as a fast low-level bomber and attack aircraft, as well as a fighter.
However, the installation of more powerful engines predictably resulted in cooling problems, compounded by reliability issues with the new engine. Because there were also problems to get mass production organised again, the Me 410 did not come off the production lines in substantial numbers until the middle of 1943. The A-1 was a fast bomber version and the A-2 a heavy fighter, both powered by DB 603A engines. They would be followed by the B-1 and B-2, powered by the 1900 hp DB 603G, from April 1944 onwards. The normal fixed armament consisted of a pair of MG 151/20 and a pair of MG 17s, while the flexible barbettes with a MG 131 each provided a rear defence.
The type’s usefulness as a fast bomber was quickly cast in doubt, because of its short range and small bomb bay: With two SC 500, i.e. 500 kg high-explosive bombs, the doors would not even close; leaving a gap that had to be filled by an expendable wooden panel. (The high-explosive bombs were bigger than other bombs of the same weight, as explosives are less dense than steel.) The bomber models also had provision for external bomb racks under the wing roots. But these could not carry extra fuel tanks to extend the type’s range. Bomber versions could be converted to the Zerstörer role by installing a cannon pack in the bay under the cockpit instead, as the Me 410A-1/U2. In May 1944, the conversion of all bomber models was ordered.
The concept of a cannon pack that could be installed as an exchangeable unit was innovative as well as practical, but as was common for German fighters of the period, a bewildering set of armament options was developed. These included the WB 151A with two MG 151/20 cannon, which seems to have been used most, but there were also alternative packs with two 30 mm MK 103, four 30 mm MK 108, or one 50 mm BK 5. Of these the BK 5 was probably the least useful. Derived from a tank gun, this weighed 540 kg and had an ammunition supply of only 21 rounds, fired at 50 rpm. The goal was to shoot US bombers down from a safe distance with a weapon that could destroy them with a single hit, but accuracy and reliability problems made it ineffective. From early 1944 onwards, the presence of US escort fighters put and end to the effectiveness of the Me 410 as a daylight interceptor.
It remained a troubled aircraft in search of a role. A lighter single-seat version without defensive armament was abandoned as it would not be able to fight on equal terms with allied fighters anyway. Finally, in June 1944, in order to rationalise German war production, the Me 410 was abandoned to free raw materials and manpower for other aircraft. The production run of the Me 410 had produced only 1160 aircraft, so that the total production of the Me 210 and Me 410 finally added up to less than the original off-the-drawing-board order.
Heavyweight
The aircraft that filled the gap in the German nightfighter arsenal was the Junkers Ju 88. It was an unlikely success, because the Ju 88 was a medium bomber, and most fighter conversions of such aircraft were seriously disappointing. And the Ju 88A bomber was a substantial aircraft too, with a wing span of 20.08 m and maximum take-off weight of 14 tons, substantially bigger than a Blenheim or an A-20. On the other hand, it was relatively fast, with a top speed of 510 km/h in the Ju 88A-4 bomber version, and remarkably manoeuverable for its size, with light, effective controls. As a bomber, the Ju 88 had its weaknesses, especially its light defensive armament, but it was a true pilot’s aircraft.
Early in the war, the Ju 88 was expected to be the backbone of the Luftwaffe, with a planned strength of 7000 aircraft backed by a complex of factories that would turn out 250 aircraft per month. When shortages forced a reduction of German rearmament plans in the summer of 1939, this was the one programme that the Luftwaffe sought to preserve at all cost. Heinrich Koppenberg was made “special plenipotentiary” for the production of the Ju 88, and he hoped to use the techniques of mass production — often named ‘Fordism’ after the man who had become the symbol of them.
In that context, a fighter version of this versatile aircraft was useful to explore, and in September 1938 the Ju V7 prototype flew with a simple modification, three fixed MG 17 machine guns and one MG FF cannon installed in the right side of the nose. Two further prototypes were more thoroughly modified with a sheet metal nose, and the option to carry the MG 151/20 instead of the MG FF. Experiments were also conducted with a modification of the ventral fairing under the nose of the Ju 88 to install a MG 151/20 or MG 151. The results were promising enough to put a Zerstörer version into production as the Ju 88C, although at first on a very limited scale: Out of 2208 Ju 88s produced in 1940, only 62 would be fighter variants. In 1941 the number would be 66, while 1942 saw a modest increase to 257. In 1943 the production of Ju 88 fighter versions would be above 700, as the German air force was pushed into a defensive role. In 1944, 2518 out of the 3234 Ju 88s produced would be fighter models!
The first Ju 88C-2 model was a straightforward conversion of the A-1 bomber model, with the metal sheet nose containing three machine guns and one 20-mm cannon. The elimination of external bomb racks, dive brakes, and the defensive machinegun installed in the windscreen of the Ju 88A did reduce drag. But the Ju 88C-2 did retain its internal bomb bays, although it often carried an extra fuel tank in the forward bay. And the engines were the same 1200 hp Jumo 211B or 211G used by the bombers, as the desired BMW 801 radials were not available. The Ju 88C-4 was a similar direct conversion of the Ju A-5 bomber, which featured a wing of increased span. It’s forward-firing armament could be boosted by installing two MG FF cannon in the ventral gondola. The Ju 88C-6 had more powerful Jumo 211J engines with paddle-blade propellers, improved defensive armament, and some additional armour plate in the cockpit.
The missions flow by these aircraft were quite diverse. The first deployment, in the spring of 1940, was to bases in Norway to fly anti-shipping missions. The aircraft proved very effective in this role. But later in the year, the units were incorporated in the nightfighter forces, operating from Holland and flying mostly intruder missions against British bases. After intruder missions were cancelled in October 1941 (a serious tactical error on the Luftwaffe’s part) the aircraft were shifted to the Mediterranean instead, as they were thought more suitable for offensive missions than for interceptions at night. In June 1942 the Junkers began to operate against allied anti-submarine aircraft in the Bay of Biscay, a crucial transit area for submarines based in ports on the French coast.
The opponent encountered over the Bay and in the Mediterranean, but also at night over Britain, was frequently the Bristol Beaufighter. The Beaufighter Mk.VI was considerably smaller and lighter (9,798 kg loaded versus 12,350 kg loaded) than the Ju 88C-6, and had more powerful engines (1600 hp Hercules VI radials versus 1400 hp Jumo&211J V-12 engines), It is no surprise that it was faster, with 536 km/h at 4,755 m versus 494 km/h at 5,300 m. However, the Junkers was the more manoeuverable aircraft. The Beaufighter had superior forward firepower, with four 20-mm cannon and six rifle-calibre machine guns against three and three, although only the Junkers also had defensive guns.
Late in 1942, the Ju 88C-6 also began to receive FuG 202 Lichtenstein BC radar sets with their characteristic antenna array on the nose of the aircraft, replaced later by the FuG 212 and in late 1943 the FuG 220. While the Bf 110 was the most numerous aircraft in the Nachtjagd, the bigger Junkers had a greater ability to carry additional equipment, such as the FuG 227 Flensburg homing device, which picked up the emissions of the Monica tail-warning radar fitted to RAF bombers. It was also fitted with Schräge Musik installations, with the guns installed in the bomb bay and firing upward through the fuselage aft of the cockpit. The MG 151/20, heavier but more powerful than the MG FF usually found on the Bf 110, was most often found in such installations.
A big aircraft such as the Ju 88 could always benefit from more engine power if it had to operate in the fighter role. The BMW 801 radial was installed on the Ju 88C-5, but only ten aircraft of this version were built because the engine was much in demand for the Fw 190. This model also saw deletion of the ventral gondola and the installation of a streamlined ventral weapons pack, albeit with just two rifle-calibre machine guns. Similar changes appear to have been made to a handful of Ju C-7 machines, powered either by the Jumo 211J or the BMW 801. In early 1943 the decision was at last made to produce the Ju 88R-1, a straightforward modification of the C-6 with 1600 hp BMW 801A engines, while the R-2 was powered by the 1700 hp BMW 801D. Both models were built as nightfighters, the R-1 with FuG 202 or FuG 212 and the R-2 with the FuG 220. Armament remained unchanged except for the deletion of the ventral defensive gun.
But the R series, of which the production ended in early 1944, was an interim version awaiting the production of the Ju 88G. As it came off the production line in late 1943, the Ju 88G-1 could easily be recognised by its larger, squared-off tail surfaces, adopted from the Ju 188 bomber, which gave more stability. The larger ailerons, fitted to improve control response, were less obvious. Both changes had been required to cope with the increasing weight and power of the nightfighter models. The armament configuration was redesigned, with deletion of the gondola under the cockpit and the installation of four MG 151/20 cannon in a ventral pack. Two more cannon were installed in the nose, but in service these were frequently deleted because of their muzzle flash. A dorsal MG 131 was retained as defensive gun. The engines were BMW 801D. FuG 220 Lichtenstein SN-2 was standard, with masts and antennas reshaped to reduce drag. The changes gave the Ju 88G a different allure than earlier models, pugnacious and sleek.
The final production fighter was the Ju 88G-6, with 1750 hp Jumo 213A inverted V-12 engines, installed in round cowlings with annular radiators in front, so that both versions looked superficially similar. The G-6 was 40 km/h faster than the G-1, despite an increase in weight. Usually FuG 220 radar was fitted, with the antennas installed at 45 degrees from the vertical. Some late-war aircraft had alternative installations, including some that carried FuG 240 Berlin, a copy of the allied magnetron-based, centimetric radar sets, installed with a dish antenna under a wooden nose cap. The G-7, once thought to have been a late-war production model, probably did not get beyond the prototype stage, while the long-range G-10 was produced but did not reach combat units.
The Ju 88G became the Luftwaffe’s main nightfighter in the final year of war. Although it was still fundamentally an improvised fighter, it had good handling and high performance. The large airframe allowed the aircraft to carry heavy armament and a high standard of equipment with a relatively modest performance penalty. Top speed of the model with Jumo 213 engines is usually given as 626 km/h at 9100 m. British test pilot Eric Brown claimed to have reached 644 km/h in one!
Last of the Line
Junkers planned nightfighter derivatives of its later bombers, including a Ju 188R and finally the Ju 388J. The Ju 188 was originally developed as a stop-gap design to counter delays in the Ju 288 project, and was evolved from the Ju 88 with various improvements to the aerodynamics and the cockpit layout. Although the bomber version entered production on a modest scale, the Ju 188R nightfighter model did not. It probably would have offered few advantages over the Ju 88G.
The failure of the Ju 288 and the entire Bomber-B project lead to a further revision of the Ju 88 and Ju 188 line, to meet a requirement for a fast high-altitude bomber under the Hubertus Programm. The Ju 388 thus became a straightforward development of the Ju 188 with high-altitude engines and inheriting some of the equipment developed for the Ju 288.
Accordingly, the first prototype of the Ju 388J was flown in January 1944 with turbocharged BMW 801TJ radials. It carried FuG 220 radar, a belly pack with two 20-mm MG 151/20 and two 30-mm MK 108 cannon, and a remotely controlled tail turret with two 13-mm MG 131 machine guns. The next two prototypes omitted the tail turret and carried FuG 218 Neptun radar, with a streamlined nose cone fitted to reduce the drag of the antenna mast.
The Ju 388J was intended to be a specialised high-altitude nightfighter with a service ceiling of 13,000 m. But it never entered service, as in early 1945 the worsening German war situation resulted in the cancellation of the Ju 388 programme.
Unglued
The Junkers Ju 88 was the closest German equivalent to the British de Havilland Mosquito: A versatile, high-performance twin-engined combat aircraft. But there was also a German aircraft that emulated to Mosquito in another important aspect: Wooden construction. The Ju 88 was of course an all-metal aircraft, but like many other participants in the war, the German government was worried about shortages of aluminium alloys. Late in the war it would achieve some savings by substituting wood for metal parts, for example by fitting a wooden tail to the Bf 109.
The Focke-Wulf Ta 154 was a more radical attempt to develop a multi-role combat aircraft of wooden construction. Proposed by Focke-Wulf’s chief designer, Kurt Tank, it was clearly inspired by the British example. It would make use of advanced woodworking techniques and be glued together with the new Tego-Film adhesive. The initial requirement was for an aircraft powered by two Jumo 211 engines, which were readily available. Discussions during most of 1942 envisaged the new aircraft as a fast bomber, with some concerns raised about the risks of a wooden design. But due to the growing need to defend Germany against bomber attack, the design was recast as a nightfighter in October 1942. The RLM wanted the new aircraft to fly in July 1943, less than one year after signature of the contract. Initially it was called the Ta 211, with the “Ta” code assigned in honor of Dr. Tank, but this was changed into Ta 154.
The Ta 154V1 prototype emerged in June 1943 as a sleek aircraft with a wing at the shoulder position, a tricycle undercarriage, tandem seating for the crew of two, and 1340 hp Jumo 211F engines in streamlined nacelles with circular radiators in front. Both the fuselage, which had an oval cross-section little larger than that of the engine nacelles, and the wing were made of wood. However, metal panels were used to cover the engine nacelles, and the control surfaces were also of metal construction. The wooden construction was potentially heavier than the equivalent all-metal design. With a maximum take-off weight of more than eight tons, the Ta 154 was heavier than the Bf 110, though lighter than the Me 210.
The prototype made its first flight on the first of July. The aircraft flew well, with light, effective controls and a high rate of roll. Nevertheless its flying characteristics came in for some criticism, addressed during development by enlarging the tail to counter the destabilising effect of the radar antennas and to improve controllability with one engine out. And very late in the type’s development, upturned wingtips would be added to improve lateral stability. But the Ta 154 did not have the kind of handling problems that had ruined the Me 210 and Ar 240.
The armament of four cannon (in production aircraft two MG 151/20 and two MK 108) was to be installed in the sides of the fuselage, aft of the cockpit. The gun muzzles were as far forward as the pilot’s seat, well below the rim of the cockpit, but perhaps not ideal when considering the effect of muzzle flash on the crew’s night vision. The crew had a benefit of a cockpit well protected with armour plate and an armour glass windscreen, but the view for the radar operator in the rear seat, below the leading edge of the wing and in between the engine nacelles, was very restricted. The pilot had good forward and upward view, but a poor view to the sides and the rear. This was judged to be acceptable for night operations but considered a serious weakness in daylight.
The first prototype lacked most military equipment; the second had FuG 212 radar and the third carried armament, although only the fourth had the definitive armament installed. Vibration and local damage occurred when firing the guns, and reinforcements were needed. Following the initial successful flights, the RLM had already ordered 250 aircraft, and assigned factories to build the Ta 154 at (as usual) an unrealistically high rate, not only as nightfighters, but also as heavy day fighters and (again) fast bombers. These production schedules soon came apart, not in the least because several factories were destroyed by allied bombing. But the pre-series of 22 A-0 aircraft was followed by the first production aircraft in May 1944. These were A-1 and A-4 models, powered by the Jumo 211N or Jumo 211R and distinguished mostly by their radar, FuG 212 for the A-1 and FuG 218 or FuG 220 for the A-4. They had a wing span of 16.00 m and a maximum take-off weight of 8250 kg.
The top speed of 615 km/h at 5790 m was respectable, but fell short of the initial expectations. The aircraft was underpowered with these engines, and the addition of flame dampers and antenna arrays for night combat badly eroded performance. The evaluation center (Erprobungsstelle) of Rechlin calculated that the top speed of a Ta 154A fully equipped for combat would be only 580 km/h. Combined with the restricted view from the cockpit this made an unfavourable impression on some Luftwaffe pilots, crucially including Adolf Galland and the highly experienced nightfighter pilot Werner Streib. In May 1943, Galland ordered that until a better cockpit could be provided, other development work on the Ta 154 would be stopped. Aircraft built with the old cockpit would serve only as nightfighters or fast bombers. On the positive side, the Ta 154 was easy to fly, which given the poor standard of training of most Luftwaffe pilots in 1944 was very important, and very agile for an aircraft of this size. But the question evidently arose whether this was enough to justify its existence. The programme was being challenged by those within the Luftwaffe who expected more from alternative designs, such as the Ju 388J and the He 219.
The need for more powerful engines had been accepted already in January 1943. The Jumo 213 was the obvious candidate, but its development (and that of the competing DB 603) had been delayed by the low attention given to new projects earlier in the war: It would not be available for production aircraft before March 1944. The version powered by the 1775 Jumo 213A was to be the Ta 154C. To address the criticism of the crew positions, this version was to have a new all-metal forward fuselage, with a bubble canopy to improve rearward view, while the crew was given ejection seats. An extension of the aft fuselage would compensate for the additional engine weight forward. Total take-off weight would climb to 9920 kg. Both a single-seat day fighter version and a two-seat version were being proposed. But in the end, only prototypes were completed with with Jumo 213 engines, the first being the Ta 154V8.
Industrial, more than technical issues, sealed the fate of the Ta 154. The success of the test programme had already been seriously undermined when inspection of the fifth, six and seventh prototypes revealed major defects in construction. This confirmed what opponents in the RLM had already feared, i.e. that wooden construction posed new challenges to an industry used to working in metal, and the need for skilled labour had been underestimated. This seriously affected the ambitious production plans. There were also concerns about field repairs, which would require a new set of skills. In addition reliability issues with the landing gear and hydraulics continued to plague the Ta 154. In June 1944 a production stop had to be imposed by Dr. Tank, while it was investigated why the wing of a Ta 154A-1 had disintegrated in flight. The cause of the problem, rather ironic for a fighter that had been designed as a wooden aircraft to avoid supply problems, was the destruction of the single factory that produced the Tego-Film adhesive. A substitute glue, Kaurit, had been selected, but its application resulted in unsatisfactory quality of construction, as the glue damaged the wood instead of correctly bonding it.
Kurt Tank was forced to defend himself against accusations that he was sabotaging the war effort. In August 1944 he was cleared, but meanwhile the program had been cancelled. As the regime tried to rationalise war production by focusing on a limited number of types, the Ta 154 fell into disfavour, perhaps also because it was regarded as a too specialised nightfighter, despite proposals to build also day fighter versions. About 50 aircraft may have reached combat units. Both the Ta 154C and the further developed Ta 254 remained on the drawing board.
The Ta 154 demonstrated the ability of German engineering to build a high-performance twin-engined fighter out of wood. While the Messerschmit and Arado teams had failed because of unsolved aerodynamic problems, the Focke-Wulf project failed mostly for industrial reasons. The design was not without its flaws, especially the original cockpit design, which was unsuitable for a fighter presumably because the aircraft had originally been intended as a fast bomber. These faults could all have been resolved. But given the choice between the wooden Ta 154 and the all-metal Junkers and Heinkel designs, the more conventional construction technique had substantial advantages.
Overrated
One of the most famous of the German nightfighter designs originated in 1940 as a fast bomber and reconnaissance aircraft. The initial proposal was for a fast bomber and reconnaissance aircraft derived from the Heinkel He 119. That was a very unusual research aircraft which featured a DB 606 engine buried in the mid fuselage, driving the propeller on the nose by means of long extension shafts. The DB 606 consisted of two coupled DB 601 units, and for its new aircraft Heinkel proposed to use the DB 613 (created by coupling together two of the new DB 603 engines) or the DB 615 (two coupled DB 614 engines, themselves planned developments of the DB 603). The DB 613 was expected to deliver 3,800 hp for take-off. At least this avoided the complex angled drives proposed earlier by Arado and McDonnell, but in August 1941 the RLM nevertheless decided that it wanted a heavy fighter and nightfighter of more conventional layout, scrapping the design work done so far. The origins of the project remained visible only in the designation given to it: He 219.
It was intended to be powered by DB 603G high-altitude engines, which delivered 1900 hp for take-off and 1560 hp at 7200 m. But its slow development initially restricted the He 219 to the DB 603A, which delivered only 1750 hp for take-off. It was in this form that the prototype was flown on 6 November 1942. With a wing span of 18.5 m, a wing area of 44.5 square meters and a take-off weight of 11,750 kg, the He 219 was a substantial aircraft for a fighter. It had plain, angular lines: The fuselage had a rectangular cross-section and little taper until half way to the tail. The wing, with a plain and a straight leading edge, passed through the fuselage without any blending or fairing. The tail was a simple structure with twin tail fins. As common on German aircraft if this time, the engines were installed in round nacelles with an annular radiator in front. The design of the He 219 had been optimised for ease of production, allowing Heinkel to claim that the He 219 could be built for a third of the cost of the competing Junkers Ju 188.
Following the pattern now favoured by the Luftwaffe, the crew of two sat in the extreme front of the fuselage, well ahead of the propellers. They sat on ejection seats powered by compressed air, which would lift the seats about four meters, high enough to clear the propellers in an emergency. These seats were back-to-back under a bubble canopy, giving an excellent view forward and all-around the upper hemisphere. The view for landing and take-off was also very good, for the Heinkel had tricycle landing gear. Bays for fixed guns were provided in the wing roots and in a ventral pack, putting the muzzle flash well outside the field of view of the pilot. Earlier plans to install defensive guns were dropped before the aircraft entered production, though they would be reappear in later design sketches.
Early test flights revealed some instability in jaw, as well as vibration of the tail section. These were cured by lengthening the tail and enlarging the tail fins. Otherwise the handling of the prototypes was good. The DB 603A engines left the big aircraft somewhat underpowered and it was slower than expected. But results were encouraging enough to proceed with a series of twelve prototypes.
Its good characteristics would save the He 219 from oblivion, because the production of a specialised nightfighter was repeatedly challenged by the champions of multi-role designs, such as the Junkers Ju 88. After the first comparative flight tests in January 1943, the RLM established a production plan that would see the Bf 110 succeeded as a nightfighter by the Ju 88 and Ju 188, excluding the He 219. But the positive evaluation of the prototype by Major Werner Streib, further comparative tests in March, and the front-line testing of V7, V8 and V9 prototypes from June 1943 onwards, changed these plans. The nightfighter force wanted the He 219, and when offered the Ta 154 instead, turned down the latter because of its inferior cockpit design. General Kammhuber and his staff in the Nachtjagd wanted to have specialised nightfighter, even if the He 219A was a bit too slow to be a good Mosquito interceptor. But these conflicts would have a disruptive influence on the development and production of the He 219. Meanwhile, Heinkel projected multi-role versions, especially in the planned He 219C series, and even proposed a He 419 high-altitude bomber with turbocharged engines.
The production of the He 219 was also beset by the inefficiencies and vacillations that plagued the German armaments industry. In April 1943 the future production rate was set at fifty aircraft per month, far more reasonable than those given for some other projects, although it was increased in June to 150. However, amidst conflicts about the allocation of factories and workers, the initial production rate was only 14 per month. And not only the quantity, but also the quality of the produced aircraft was deficient. At this stage of the war, the German skilled workforce was diluted more and more by foreign workers and slave labour, while German workers were drafted by the army. This was combined with the disruptions caused by allied bombing and the dispersion of the He 219 production over Germany, Austria and Poland. Production finally continued until March 1945, despite a decision in May 1944 to cancel the Heinkel nightfighter, which was soon reversed. Production rates finally stabilised at about 20 aircraft per month, and total production is thought to have been about 330 over a period of nearly two years, a far cry from the force of 2000 aircraft that the Nachtjagd had hoped for.
The type entered combat in June 1943 with NJG 1 at Venlo in the Netherlands, which would for some time be the only base able to service it. The He 219 proved to be a popular aircraft. It was not particularly fast, as top speed with DB 603A engines, flame dampers and the Lichtenstein SN-2 antenna array was only 585 km/h. But it did offer better endurance than other nightfighters, which made the force more effective. The first pre-production A-0 was flown in combat by Major Wener Streib on 11 June 1943, and he claimed five RAF bombers in a mission of 144 minutes. Although the aircraft was lost during landing because of defective flaps, the He 219 had made a successful entry into combat.
The A-0 preproduction run of more than hundred aircraft with FuG 212 radar was followed by the A-2, which had FuG 220 radar and longer range. The later versions are poorly documented, and it is likely that only the A-7 was produced in any number, with DB 603E engines, although the first ones were delivered with the DB 603A. Several of the many prototypes were reconditioned and delivered to combat units, thus creating sparse deliveries of models such as the A-5.
Armament installations were equally confusing. Depending on the model and one of many different variant installations, the wing roots were fitted with the 20-mm MG 151/20 or the 30-mm MK 108, while the ventral pack held four MG 151/20, two or four MK 108, or two high-velocity 30-mm MK 103 cannon. In practice it appears that the reliable and readily available MG 151/20 was often retrofitted in the field. Schräge Musik, when installed, consisted of two MK 108 cannon, but it appears that many crews preferred to have this removed.
To give a much-needed performance boost, versions with more powerful engines were explored. Six A-series aircraft were converted to the Jumo 213E with methanol-water injection (MW 50) to boost performance, but cooling and other installation problems were not yet solved at the end of the war. The engine mounts of the He 219 deviated from the normal German practice and did not permit the exchange of the engines as “power eggs”. A simpler alternative was to install a jet engine to boost performance when needed, and two prototypes flew with a BMW 003 jet engine under the fuselage, the first in August 1943. This gave a boost of 45 km/h when the engine was running, and a penalty of only 10 km/h when it was not.
Rather more far-fetched were plans drafted around the Jumo 222, an unusual engine with six banks of four cylinders in a star configuration, making it conceptually a liquid-cooled radial. Offering 2500 hp at take-off, it was never fully developed. Nevertheless two aircraft became the V16 and V23 prototypes when fitted with these engines, which would have powered the projected H 219B. As the Jumo 222 weighed about 20% more than the earlier engines, the wing span was increased to 22 m and the landing gear wheels enlarged. Without the ventral armament tray and with a redesigned cockpit canopy to reduce drag, the prototype was capable of 700 km/h. The B-2 was planned to be a three-seat aircraft. Also the projected C-series would have been three-seat aircraft, with the addition of a remote-controlled defensive turret, the HL 131V with four MG 131 machine guns.
It is often claimed that the Northrop P-61 was the war’s only purpose-designed nightfighter, but the He 219, despite its complex origins, could be said to be another one. As such it clearly was a competent aircraft, well liked by those who flew it. A less complex design than the P-61, it had similar performance characteristics — agile for its size but handicapped by shortfall in engine output. It also acquired a similar reputation. But in the end it is difficult to escape the conclusion that those in the RLM who preferred the Ju 88 were right. Developments of the Junkers bomber were more than adequate nightfighters and they could be produced in substantial numbers. While the He 219, although cheaper to build in theory, could not. This does not reflect on the quality of the aircraft design, but the state of the German industry in 1944 and 1945.
Push-Pull
Near the end of World War II, a new implementation entered service of an old concept, the twin-engined fighter with a push-pull configuration, with both engines buried in the fuselage. The evident advantage of this concept was that it eliminated engine nacelles on the wing, which otherwise increase drag and result in an unfavourable mass distribution, at least for an aircraft that needs to be agile. Also, in a tandem design there is no asymmetry when one of the engines is shut down. On the other hand, much of the volume of the fuselage is taken up by the two engines, and power transmission from the rear engine is a challenge, as it needs to be in the rear fuselage reasonable close to the centre of gravity.
Several earlier designs, such as the Fokker D.XXIII, solved the latter problem by adopting twin tail booms, which allowed a short and simple transmission to a pusher propeller at a penalty in structural complexity and weight. Dr. Claudius Dornier instead favoured a long drive-shaft to drive a propeller at the extreme end of the aircraft, behind the tailfins. He patented the concept in 1937 and in 1940-1941, proved the feasibility with the small Göttingen Gö 9, which featured a pusher propeller behind a cruciform tail. This tail, of course, necessitated tricycle landing gear.
Work on what would become the Do 335 started in 1942 to meet a request for a fast twin-engined fighter-bomber, and three prototypes were ordered in January 1943. The Do 335V1 made its first flight on 26 October of that year, a very respectable pace of development for a large and complex new aircraft, which broke new ground in many ways. The Do 335 handled well, was fast at sea level, and flew well with one engine shut down. A slightly higher speed could be reached flying with only the rear engine than with only the front engine. The throttle settings of the engines could affect longitudinal stability, but otherwise handling was conventional, with some complaints about heavy aileron forces. During development, leading edge sweep was added to the wing root to cure a tendency towards asymmetric stall.
A dual control trainer was developed in 1944, featuring a second cockpit raised above the first one, over the wing, offered a good forward view for the instructor. A small number of aircraft was converted to this configuration as Do 335A-11 and 1-12, which were strictly intended as a trainer and not as a nightfighter.
The Achilles heel of the design was the cooling of the rear engine. The Do 335 was powered by large liquid-cooled V-12 engines, the DB 603 in the first prototypes and most production aircraft, and the alternative Jumo 213 in the V6 and V7 prototypes. The front engine had a frontal ring cooler, much favoured on late-war German aircraft and quite efficient, but the rear engine was provided by a cooler to which air was fed by a large ventral scoop. This worked very well on other aircraft (notably the P-51 Mustang) but its implementation on the Do 335 was problematic. As the control cables ran close to the rear engine, fires in this area would be fatal. The rear engine drove the pusher propeller via a 3.5m long, hollow extension shaft. Reversible pitch on one of the propellers helped to shorten the landing distance.
Inevitably, the Do 335 was big. The first A-0 pre-production series had an empty weight of 6530 kg and a loaded weight of 9510 kg, with a wing span of 13.80 m. But it is fair to compare this with the contemporary Me 410, which also featured two DB 603 engines: The Me 410B-2 had an empty weight of 7982 kg (22% more) and a loaded weight of 11,236 kg (18% more), while its wing span was 16.35 m (18% larger). The Do 335A-0 was a single-seater with a top speed of 732 km/h at 7100 m, enough to make interception by Allied fighters quite difficult, while the Me 410B-2 could do no better than 630 km/h at 8100 m and was quite vulnerable to escort fighters when deployed on bomber interception missions. The Do 335 had excellent speed and acceleration, and was very agile for an aircraft of its size.
The pilot of a Do 335 was not to be envied in all respects. The cockpit was rather heavily framed and offered little or no rearward vision, though rear-view mirrors were fitted and forward view was good. Should the pilot need to leave the aircraft, then he had a press three buttons, one to blow off the rear propeller, a second to remove the upper fin, and a third to arm his ejector seats, then grip two large levers to jettison the canopy, and finally squeeze a trigger to activate the ejector seat, which was then expelled by compressed air.
As a heavy fighter-bomber, the initial Do 335A-0 was fitted with a small bomb-bay, for a 500 kg bomb or an extra fuel tank. External bomb racks could be installed under the wings. A powerful high-velocity 30-mm MK 103 cannon was installed on the centreline, firing through the propeller hub of the front engine, and two 20-mm MG 151/20 in the upper engine cowling.
Considering the size of the aircraft, that armament was modest enough. A planned B-2 series, never produced, would have increased armament by installing two big MK 103 on the wings, in front of the main wing spar, under large fairings — a somewhat awkward-looking arrangement but probably optimal in terms of drag and recoil transfer.
As was commonplace in the Reich, the management of the Do 335 program was inefficient, with only small-scale production even after the He 219 was cancelled in favor of the Do 335, and an excessive number of different prototypes to keep the project team busy: Heavy Zerstörer, trainers, nightfighters, and reconnaissance aircraft with different versions of the DB 603 or Jumo 213 engines. A series of paper designs explored the possibilities of extended wing span, jet engines, and swept wings. Dornier even explored the “twinning” of the Do 335 fuselage to produce a four-engined Do 335Z, which then resulted in a Junkers project to develop this as a strategic reconnaissance aircraft under the name Ju 635, which apparently got as far as the mock-up stage before the end of the war.
Most significant among the prototypes that were actually flown were the nightfighter versions. Two aircraft were completed as the Do 335M10 and M17, prototypes for the A-6 and B-6, respectively. These featured a cabin for a radar operator behind the pilot’s cockpit, with flush-fitting window offering almost no outside view (much in contrast to the two-seat trainer). To compensate for the reduction in size of the main fuel tank, the tankage in the wings was increased. The antennas of the FuG 218 Neptun radar were installed on the wings. Only the M10 was completed during the war, while the M17 was completed after the wa by the French. The M10 may have seen some action in the form of operational testing, but by 1945 it was already thought that the advent of jet aircraft would cut short the operational life of this type.
The British test pilot Eric “Winkle” Brown, who had the opportunity to the Do 335 after the war, praised the aircraft’s combination of speed, stability, and endurance, and believed that it would have been effective as a nightfighter, though it clearly was not agile enough to be a good dogfighter. He also observed, however, that the complex and not fully developed Do 335 was the mechanically most troublesome aircraft of all the German types tested at Farnborough.
The Last Twins
As the end of the second World War came in sight, the attitude of air forces towards the development of new twin-engined fighters was showed an uncomfortable split. Jet fighters would soon be displacing the single-engined piston-engined fighters because of their superior performance, but they appeared to leave open a niche for piston-engined fighters as nightfighters and attack aircraft. Not in the least because the first jet engines had a very high fuel consumption and limited thrust, and were a far from evident choice for a nightfighter, a heavy fighter-bomber, or a carrier aircraft. Nevertheless the writing was clearly on the wall for the heavier piston-engined fighters too.
The swan song would consist of a number of types that were only partially successful, as they were plagued by technical and operational problems. Technological overstretch explains part of these issues. But no doubt it was also a factor that they were operating in peacetime, and risks that would have been accepted as necessary in wartime, were no longer tolerated.
Hornet
Perhaps the type with the greatest legacy to live up to was the de Havilland Hornet, designed during the war as a smaller development of the Mosquito. It was considered that by using the layout and aerodynamics of the Mosquito as basis, the risks of development would be kept low, while the adoption of a smaller airframe, improved streamlining, and the latest Merlin engines would provide high performance. Nevertheless the risk of built-in obsolescence was clear, as de Havilland was already working on the D.H.100 Vampire jet fighter. What finally convinced the Air Staff was the potential of the D.H.103 as a twin-engined long-range fighter for use in the final phase of the war against Japan.
To make this feasible, the Air Staff decided to order it even before the first prototype flew, in the hope of receiving the first production aircraft in December 1944. Specification F.12/43 envisaged it as a long-range day fighter with a top speed of no less than 772 km/h. The Merlin 130/131 series of engines was employed, which was carefully redesigned to permit an aerodynamically very clean installation, and “handed” to have the propellers rotate in opposite directions. As the radiators were in the wing leading edge, the Merlin 131 did not only have a different direction of rotation for its propeller; it had a relocated coolant pump and a reverse coolant flow. Later the Merlin 134/135 would be fitted, with a different throttle.
The DH.103 was the smallest possible package that could be designed around two powerful engines, four 20-mm cannon, a pilot and enough fuel for a 1600 km endurance (2000 km with external tanks). The specification emphasized the highest possible speed, rate of climb and manoeuverability, and in these regards the Hornet did not disappoint. Contrary to expectations, the aerodynamics were not trouble-free, but nevertheless development was rapid. The first prototype flew in July 1944 and demonstrated a top speed of 790 km/h at 7,500 m, which was certainly impressive, though production aircraft, delivered from February 1945 onwards, were slower. If the war in the Pacific had continued until 1946, as anticipated, the Hornet could have played a vital role. Instead the early end of the war curtailed the production run for the RAF at 193. They would serve until 1955: Their usefulness as fighters was short-lived but they were effective strike aircraft, and saw action in Malaya.
The story did not end there, as the Royal Navy showed interest in the type. During the war it had evaluated the Mosquito for shipboard use, and even ordered the production of a small series of Sea Mosquito TR.33 torpedo-bombers, which had a short service life. It did see something in the Sea Hornet as long-range fighter, and wrote specification N.5/44 around a carrier version with folding wings, modified landing gear, and an arrester hook. It has to be remarked that technologically, the Hornet was different from the Mosquito in construction, featuring a mixture of metal and wood with Redux bonding, and was expected to survive better in a damp environment than the wooden Mosquito. The first Sea Hornet F Mk.20 flew in April 1945. It remained a big aircraft for the RN’s wartime carriers, especially as the wings folds were well outboard of the engines, giving it a large footprint when parked. The Fleet Air Arm would take only 27, equipping one squadron.
The next Navy version, the Mk.21, was a nightfighter. However, this revealed the limitations of the Hornet’s design, as the airframe did not have room for a large state-of-the-art radar set. The Mk.21 had to make do with a small ASH set in a thimble radome, in an elongated nose that put it just in front of the large four-bladed propellers. And a decidedly cramped and uncomfortable compartment for the the radar operator was arranged in the aft fuselage behind the wing trailing edge, under a small dome. Because the Hornet was so small, the additional weight of the radar installation seems to have had a serious performance impact. The type had obvious limitations, not in the least the very limited range of its radar, but as it was the best available option (the RN studied the acquisition of larger aircraft but had to conclude that development times would be long), production began in 1948. Eighty were delivered. Again there was only one front-line squadron to operate it.
The Hornet was a great technical achievement but obsolescent by design. It was optimised for high performance at a time when it was obvious enough that jet fighters would soon leave it far behind. Both customer and manufacturer knew this, and they accepted it because the Hornet was only wanted for a final wartime campaign, which never happened. Its life did not stretch much beyond that because it was too small to have much development potential. If the RAF commented that the cockpit of the single-seat fighter was too small and uncomfortable for long missions (though pilots seems to have like the inclined seat which increase their G tolerance), the two-seat nightfighter was even worse with conditions for the radar operator that hampered his effectiveness.
FMA I.Aé.30 Ñancú
One of the last twin-engined piston fighters emerged in Argentina, as late as 1948. Designed by an Italian engineer who had emigrated to Argentina, Pallavicino, the Ñancú was conceived to be an interceptor and long-range escort fighter. Conceptually, it was superficially similar to the de Havilland Hornet, but entirely of metal construction. The three prototypes had similar clean lines with a sleek and streamlined fuselage, 2035 hp Rolls-Royce Merlin 134/135 engines, and radiators buried in the leading edges of the wing inboard of the engines. Much of its wing planform and engine nacelle design had a strong de Havilland flavour to it. The new aircraft made its first flight on 17 June 1948.
The performance of the new fighter exceeded the expectations of its designers, a rare enough occurence. Performance was indeed impressive, with a top speed of 740 km/h, even a bit better than that of Argentina’s first jet fighter prototype, the disappointing I.Ae.27 Pulqui. The Ñancú was not a small aircraft, with a wing span of 15 m, an empty weight of 6208 kg, and a maximum take-off weight of 8755 kg: It carried enough fuel for a 1500 km radius of action and could fly 5000 km with external fuel tanks, though Pallavicino vetoed a plan to fly the Ñancú prototype across the South Atlantic and show it at Farnborough.
The production aircraft were to be powerfully armed with four or six 20-mm HS-804 cannon in the lower fuselage below the cockpit, and the ability to carry rockets and light bombs. The first prototype was unarmed, with a plexiglas nose cone to improve downward view. And although the first prototype apparently handled well, modifications of the tail surfaces were planned for the second prototype, which was never completed. It also was to carry armament and have a redesigned cockpit canopy.
As impressive as the Ñancú was, it nevertheless represented old technology. Argentina was taking delivery of Gloster Meteor jet fighters, and Kurt Tank was already working on the advanced Pulqui II jet fighter. Thus plans to order 210 were cancelled in 1949, also for cost reasons. Pallavicino proposed jet-powered developments, but these remained dead letter.
Twin Mustang
There was another attempt to create a heavy fighter quickly by taking advantage of the aerodynamics of an existing type: The North American P-82, later F-82, Twin Mustang. The desire was the same: to have a long-range fighter for the final phase of the Pacific War. When the F-82 was sketched in the middle of the war, the long-range versions of the P-51 were not yet in service, and besides, long escort missions put a lot of strain on the pilot of a single-seater. Compared to the Hornet, the American fighter had a different set of design priorities, and the result was an aircraft of which the empty weight was a bit heavier than the loaded weight of the Hornet. Or, the weight difference was some two tonnes empty, and four tons loaded, as the result of the US fighter being designed for a crew of two, more range, and a heavier war load.
Superficially, the P-82 looked like two P-51 Mustang fighters connected to each other by a new central wing section and a new horizontal tail. But there were quite a few design changes under the skin, and apart from the engine installation, the XP-82 prototype had no parts in common with the P-51 production line. Designer Edgar Schmued had initially studied a number of different configurations for a twin-engined long-range fighter, before settling on a twin-boom design without central nacelle as the most efficient. The XP-82 copied the proven aerodynamics of the P-51, specifically the refined XP-51F, but it was a new design in detail.
The twin-boom configuration dictated a number of features, such as longer fuselages and very different landing gear. Less visible was that the wing also had a longer chord, so that the outer wing panels were not those of the P-51. All gun armament was removed from the outer wing panels and moved to the wing center section. The wings were strengthened and provided with hardpoints for the carriage of external fuel tanks or bombs. The gun armament was six Browning .50 machine guns, albeit soon upgraded the faster-firing M3 version, but a pack with eight more could be carried under the center section. And under the skin, the P-82 was given the latest in electronics and electrical engineering, which unfortunately turned out to be a maintenance nightmare. The aircraft was flown primarily from the left cockpit, with more limited equipment in the right cockpit. The off-centre position of the cockpits turned out to be no disadvantage for aerobatics.
The USAAF ordered prototypes powered both by the Merlin (in its Packard V-1650-23/25 form) and by the equivalent Allison V-12 (the V-1710-119). It also ordered 500 Merlin-engined P-82B production aircraft “off the drawing board” well before the first prototype flew, in another similarity with the Hornet. That first flight happened in June 1945, with considerable difficulty: The original engine installation, with the “handed” engines turning so that the blades rotated upward over the center section, caused the center section to stall, and the engines had to be swapped. (Something similar had happened to the Hornet.) Other than that, the P-82 behaved well.
But it was too late for WW2, and that had consequences for the small post-war production run, although it allowed for some further refinement of the design. Because Rolls-Royce now wanted to a charge a $6000 license fee per engine for the US production of the Merlin, this was ended, and P-82B production was abruptly curtailed after 20 deliveries. The air force now opted to have V-1710 engines in the production P-82 series. Allison had developed the V-1710 to the equivalent of the Merlin 61 series, i.e. with a two-stage supercharger and an aftercooler, to achieve better high-altitude performance without having to resort to turbo-charging. The F-32R (the military -119) series and its developments had several very advanced features and impressive performance on paper. The final version chosen for the P-82 production was the V-1710-143/145, in theory good for 1600 hp on take-off and a 2250 hp War Emergency Rating at 3200 rpm, but suffering from backfires and reliability issues that caused its performance to be downgraded in service, running on 60 inches of manifold pressure instead of 90. The delivery of the production aircraft was delayed by about a year while Allison struggled to resolve the problems. North American was convinced that it had solved the problems when it proposed to install a backfire screen (a raster to prevent ignition of the gas in the intake manifold) but Allison did not accept this.
Fundamentally, the -143/145 stretched the engine technology of the time a bit too far, and the hurdles were never satisfactorily overcome. Performance limitations were put in place and P-82 pilots gained a lot of single-engine flying time. The notional top speed of 742 km/h at 6400 m (for the P-82G nightfighter) was probably fraught with risk, thought one assumes that it it ever had come to intercepting nuclear-armed enemy bombers, crews would have taken their chances.
Apart from some prototypes and small series, the USAF took 100 P-82E (later redesignated F-82E) day fighters and 150 P-82F, P-82G and P-82H all-weather fighters equipped with radar in a long pod, slung below the wing center section and extended well ahead to put the radar in front of the propeller blades. The pod was given the nickname “pickle” or less gentlemanly “dong” and contained various types of radar, such as the SCR-720, the more compact but less powerful AN/APS-4, or the APG-28. The second pilot, in the right-hand cockpit, became the radar operator. With the large radar pod installed, gun armament was restricted to the six .50 machine guns, which left the P-82 seriously under-gunned for night interception of heavy bombers. It also lacked airbrakes and was considered not maneuvrable enough for the job, with a rather poor view directly forward due to the long engine cowlings. However, while the Air Force awaited delivery of the interim F-94 Starfire and final F-89 Scorpion jet all-weather fighters, the P-82 was urgently needed as stop-gap solution to replace the war-weary P-61. As it was a fairly big and powerful aircraft, performance didn’t suffer much by the addition of the radar.
Operational aircraft were delivered between 1946 and 1949, with obsolescence setting in during the production run. They nevertheless gave essential service providing air defense for the continental USA. The F-82H model was winterised for operations in Alaska, as this was the shortest route for Soviet bombers, and Twin Mustang operations there continued until 1953. The type also saw brief service in the Korean war, where their range made them effective on intruder and interdiction missions, but a lack of spare parts cut their combat career short. Operations were limited in scope and directed against fixed targets only, to economize on scarce flying hours. Nobody had expected the F-82 to last very long.
The official USAF history would attribute the limitations of the Twin Mustang to the failure of the V-1710-143/145 engine to live up to the (sky-high) expectations, and that was fair enough. With a more reliable engine, the F-82 could have had a longer career, judging by the longevity of the ancestral P-51, and its service record in Korea. But it was fundamentally a transitional type, as the last USAF piston-engined fighter to enter service.
Tigercat
The other American twin-engined fighter very much fit the same pattern, but with an interesting twist as it was a naval aircraft. When the war ended in 1945, the US Navy was awaiting completing of the carriers of the Midway class. At 45,000 tons water displacement (59,900 tons fully loaded), USS Midway was over 60% bigger than the wartime Essex class. She had a longer flight deck, about 300 m against about 260 m for the Essex class, but this did not account for all the growth: The class needed to have a bigger hull because they had an armoured flight deck. Nevertheless, its availability induced the Navy to embrace bigger and more complex aircraft, including twin-engined types. For the most part this prompted the construction of a number of exceedingly complex and unsatisfactory prototypes, before the USN relented and opted for simpler types.
The somewhat more fortunate exception was the Grumman F7F, ordered as early as June 1941, thus approving the G-51 proposal of Grumman, which incorporated the lessons learnt from the F5F. At first the type was also planned to be produced for the USAAF as the XP-65, but soon it was concluded that the requirements of the two services were too different: The USAAF wanted a high-altitude interceptor with turbocharged engines, a pressurised cockpit, and a pair of 37-mm cannon instead of the four 20-mm cannon of the Navy type. Thus the XP-65 was abandoned to avoid compromising the F7F too much.
Development would be slow, as the F7F had lower priority than the F6F, which was ordered on the same day. The F6F was wanted in combat as soon as possible, so its made its first flight in June 1942 and still entered production in the same year. Development of the F7F was considerably slower: Its first flight was made in November 1943, and the first batch of 34 F7F-1 fighters was delivered to the US Marine Corps between April and October 1944. (USS Midway would be commissioned after the end of the war.) There seems to have been no urgency to deploy the type, and although USMC-operated Tigercats arrived at the front in the last days of the war, the F7F did not see combat in WWII. Afterwards, it had a dithering development history, suggesting that the USN and USMC did not quite know what to do with it.
It looked attractive enough. The F7F had a well-streamlined fuselage with, in stark contrast to the F5F, a long tapering nose. The wing carried two big R-2800 radials in substantial nacelles. The prototypes featured big propeller spinners in an attempt to reduce drag, but these were soon omitted. The F7F had tricycle landing gear, which was a first for a carrier aircraft. Similar in size and weight to the P-82, the F7F was rather more heavily armed, with four 20-mm Hispano cannon in the wing roots, four .50 Browning machine guns in the nose, and the ability to carry an array of bombs and rockets up to a total of 2087 kg. Grumman had developed a heavy fighter-bomber, whose direct rival was the F4U Corsair, and this in part would be the Tigercat’s undoing as the Corsair was very successful in that role.
The F7F could also serve as a nightfighter, so the F7F-2N featured a compartment for a radar operator behind the pilot and AN/APS-6 radar in the nose, while the machine guns were removed. A production run of 65 was delivered. The fully developed form of the Tigercat was the F7F-3, which featured a larger vertical fin and rudder to resolve stability issues, stronger wings, R-2800-34W engines, and changes to the hardpoints under the wings and the fuselage to enable it to carry a heavier war load and even a Mk.13 torpedo. Of an order for of 250, at least 60 were completed as F7F-3N nightfighters with SCR-720 radar. Some 60 F7F-3 and -3N were modified into F7F-3P reconnaissance aircraft. The production run closed with a dozen F7F-4N models that featured additional inner wing strengthening and modified landing gear, and finally qualified for carrier operations. Production was ended in November 1946 with the 364th Tigercat.
The career of the F7F was thus rather short and not altogether satisfactory. Part of the problem was the introduction on carriers of a big, fast aircraft with tricycle landing gear. While its ample engine power and quick acceleration ensured a good take-off, it proved difficult to land, with occurrences of structural damage as the result of catching the arrester wires off-centre. This was enough for the USN to condemn the type as not satisfactory for carrier landings and hand it over to the USMC. The Marines welcomed the opportunity to acquire a nightfighter, but even they found it a handful. The final development of the F7F-4N addressed the issues, but this was a qualified success as the type was finally accepted for carrier use, too late for it to become operational.
The F7F was fast despite its size, only a little bit slower than the Sea Hornet, with a good rate of climb. Pilots found it agreeable to fly, but also observed poor harmony of control with a very light power-boosted rudder but heavy ailerons, which solidified in fast dives and provided ineffective control in the landing configuration. A small twin-engined aircraft with very powerful engines needs careful aerodynamic design to ensure that it remains controllable in all conditions and also with one engine out, and in this respect the F7F was not entirely successful, though presumably the bigger tail of the F7F-3 helped. In particular carrier landings on a single engine were not feasible. Fortunately the R-2800 was a very reliable engine.
For land-based operations the F7F was a good aircraft, as it was powerful, rugged, and well armed. The Tigercat did see combat in the Korean War, between 1950 and 1953, with Marine Corps nightfighter squadrons, flying intruder missions against ground targets, flying ahead of B-29 formations to suppress enemy defenses, and intercepting Po-2 biplanes that were on night harassment missions. They were useful not in the last place because F-82 operations were restricted by a lack of spare parts and low serviceability, so the USMC flew the bulk of night missions using its F7F-3N and F4U-5NL fighters. However, the appearance of the MiG-15 made the obsolescent fighter vulnerable, and its career came to its end. The F7F was replaced by the F3D jet fighter, which was not entirely satisfactory either.
In the final accounting, one is tempted to speculate that if the F7F would have been a successful carrier type, the USN would have embraced the opportunity to have a good carrier-based nightfighter, as the single-seat nightfighter models of the F4U had significant limitations. But when the Tigercat was restricted to land-based operations, the future of the F7F was severely affected. On paper it could have had a second career as heavy fighter-bomber, but in that role it faced stiff competition from the F4U and AD.
| Sea Hornet Mk.21 | F-82G | F7F-2N | |
|---|---|---|---|
| Engines | Rolls-Royce Merlin 133/134 | Allison V-1710-143/145 | Pratt&Whitney R-2800-22 |
| Rating | 2030 hp | 2250 hp | 2100 hp |
| Wing Span (m) | 13.72 | 15.73 | 15.70 |
| Length (m) | 11.28 | 12.86 | 13.88 |
| Height (m) | 4.31 | 4.22 | 4.19 |
| Wing Area (m2) | 33.54 | 37.90 | |
| Empty Weight (kg) | 6,455 | 7,256 | 7,270 |
| Loaded Weight (kg) | 8,856 | 11,744 | 9,838 |
| Max. Speed (km/h) | 692 km/h at 6700 m | 734 km/h at 6400 m | 724 km/h |
| Climb | |||
| Ceiling (m) | 11,125 | 8,630 | 12,375 |
| Range (km) | 2,980 | ||
| Fixed Guns | 4 × 20 mm Hispano HS.404 | 6 × 12.7 mm Browning .50 M3 | 4 × 20 mm Hispano M2 |
Tupolev Tu-1
The Soviet Union also took part in the WW2 concept of converting a fast bomber into a fighter, selecting to implement it based on the excellent Tu-2 bomber family, of which the Tu-2S had entered service in late 1942. But they waited until 1947 to fly the Tu-1, alias 63P. That designation indicated that it was a fighter development of the 63, a fast day bomber extrapolation of the family with in-line instead of radial engines.
The Tu-1 was envisaged as a day and night interceptor and a long range escort fighter, to be equipped with the PNB-1 (alias Gneiss-7) radar, which was a development of the German FuG-220 set. It was to have AM-43V engines driving four-bladed propellers, for a performance gain over the basic Tu-2S, but because of the slow development of these engines the Tu-1 would only fly in 1947. It was powerfully armed, with two NS-45 in the lower nose, two NS-23 in the wing roots, and 12.7 mm UBT guns in dorsal and ventral defensive positions. Its performance, including a top speed of 641 km/h at 8600 m, was considered satisfactory, but the engines were unreliable.
And in 1947 the sleek, menacing lines of the 63P could not hide its fundamental obsolescence; its performance was no longer good enough. And the AM-43V engine would never enter production. The 63P prototype nevertheless served a useful purpose as the USSR’s first radar-equipped fighter.
Postscript
It is a truism that a successful aircraft type needed to achieve a balance of features, the result of the right set of design priorities: All engineering involves compromises. The difficulty for the designers of the 1930s and 1940s was to figure out which compromises to make. Generally, a too conservative approach did not pay off, as designers needed to take some risks if they were to design aircraft with sufficient longevity.
What was the best compromise for a twin-engined fighter in the 1935-1945 period? Looking back at the history of the types, the main factors were size and complexity. A too small aircraft might be able to compete in speed and agility with the single-engined fighters, but only at the price of not having any benefits that justified the cost of two engines: The Focke-Wulf Fw 187, Westland Whirlwind, and Lockheed P-38 were ultimately uneconomic. The USA could afford the P-38 but still recognized that the P-51D could do the same job at lower cost.
At the other end of the scale, featuritis drove the development of aircraft that were far too complex, too heavy and too big to be competitive; weird wonders such as the Lockheed XP-58 and Vickers 432. Arguably the Northrop P-61 was too complex too, but it got away with it thanks to powerful engines and, again, an economy that could bear the cost.
The ideal WWII compromise turned out to be a two-seat aircraft with a loaded weight of about ten tons, powerful engines, and a fairly lean airframe design. Desirable complexity turned out to be a matter of good equipment, such as powerful armament, radar, navigation instruments, and electronic warfare equipment. Gun turrets, additional crew members, and fancy airframe layouts were undesirable complexity. There could be advantages to a bigger airframe (the Ju 88G weighed 13 tons loaded) as long as the aircraft operated in an environment away from enemy fighters, but otherwise it was better to accept a size restriction and the inconveniences that came with it, to achieve high speed.
Flexibility was key because, in large part, the pre-war assumptions of how such aircraft would operate turned out to be flawed. Thus the successful types were the ones that could be easily adapted for new tasks, such as the Bf 110. Versatility was the more important because under the wartime economic strains, the ability to use the same airframe in multiple roles simplified production and even justified some extra cost. Thus, the fact that almost none of the wartime nightfighters had been designed for that role was more an advantage than a disadvantage.
Several air forces had expected the twin-engined fighters to operate as long-range bomber escorts: This turned out to be unworkable. They had also expected them to be bomber interceptors, and this was a partially successful prediction as they were vitally important as nightfighters, but they proved to be too vulnerable by day. Long-range fighter-bomber operations and anti-shipping strikes were experimental at the start of the war but grew enormously in importance and required aircraft that could carry much heavier armament. The importance of strategic reconnaissance, a task for which a number of twin-engined types were successfully adapted, seems to have been woefully underestimated when the war began: It required aircraft with a high speed and no or little armament.
On the whole, the British were most successful in this class, with the Mosquito and the much less famous but nevertheless very important Beaufighter. The Germans managed to be second best with the Ju 88 and Bf 110, but struggled to deploy more modern types, and were technologically backward in radar development. The USA developed a useful but expensive fighter in the P-38 Lightning. Japan fielded a very attractive design in the Ki-45 Toryu, but could not maintain the technological pace, mainly due to its lack of powerful engines. France was thrown out of the war very quickly. The USSR regarded twin-engined fighters as a luxury that it could not afford, though it produced a substantial number of impressive prototypes. Italy’s wartime economy really made them unaffordable.
A metric of the success of the heavy twin-engined fighters is the number of types they displaced: Light bombers, single-engined night fighters (except on carriers), and dedicated anti-shipping torpedo-bombers (again, with the exception of carrier use). These versatile aircraft were the forerunners of the modern multi-role fighter, much more than the single-engined fighters of WWII were.
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