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 flexibly mounted weapons for the rear gunner. Their manoeuvrability 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-seat fighter was to fly it in combat with the same tactics 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 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. (As the picture above illustrates, the improved aerodynamics were still offensive.) 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 the big flying boats 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 synchronised 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 accommodate this the pusher propeller was four-bladed and of smaller diameter than the two-bladed 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, mounted on small streamlined nacelles between the wings. 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.
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