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 old 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 with a concurrent worsening of the political situation, change could no longer be avoided.

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 – Renseignment, 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 French industry to develop new aircraft and mass-produce them. France had too many small manufacturers without up-to-date industrial plant. Pierre Cot, Minister of Air between 1936 and 1938, nationalised 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 reorganised, because of budgetary 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. 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.

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 sesquiplane 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

Next: Chapter III