The title given to this text is slightly misleading. It is not intended to be a discussion of all lightweight fighters of WWII. It is focused on those fighters aircraft that were powered by air-cooled, in-line engines, as opposed to the air-cooled radial engines or liquid-cooled in-line engines that powered most combat aircraft of the war. It is only slightly misleading, though, because with the technology available in the late 1930s and early 1940s the air-cooled in-line engine could deliver considerably less power than the alternatives: For most practical purposes, only about half the power of a contemporary radial engine or liquid-cooled engine. The limits of the cooling, especially of the rear cylinders, was a brake on the development of higher-powered engines. Therefore, designers who opted for the air-cooled in-line engine were automatically forced to design a comparatively light and small aircraft.
On the other hand, many designers who decided to try their hand at a lightweight fighter also chose, for economic or other reasons, to use an air-cooled in-line engine. The benefits of this engine layout were simplicity and relatively low weight, because of the absence of a liquid cooling system, and lower drag than an air-cooled radial engine. Although as the war progressed, the cowling of radial engines rapidly became more efficient, reducing this advantage of the in-line design.
If we page through the engine lists in section D of the 1938 edition of Jane's All the World's Aircraft, we find a fair number of air-cooled in-line designs, but most of them are small, intended for light aircraft. Only a few were sufficiently powerful to be considered for fighter aircraft.
The De Havilland Gipsy Twelve was an inverted, air-cooled V-12, which delivered maximum 525 hp for a dry weight of 481 kg, to which one needs to add about 40 of necessary accessories. It was known as the Gipsy King in RAF service. As far as I know it was never considered for a fighter, and the only production aircraft that used it where the beautiful DH.91 Albatross airliner and the unsuccessful DH.93 Don trainer.
Two odd but potential useful engines were the Napier Rapier and Napier Dagger, the first of a 16-cylinder vertical H configuration, the second similar in configuration but with 24 cylinders. Unlike the later liquid-cooled sleeve-valve Sabre, these engines used traditional poppet valves. They were remarkable for their high rotational speed, with the Dagger VIII having a max rpm of 4200! It is not surprising, perhaps, that the Dagger acquired a reputation for high maintenance requirements, but on the other hand it delivered 1000 hp at 2700 m, making it a viable alternative for the most powerful engines in use in 1938. For this reason, the Air Ministry promoted it for a while, even suggesting it to Supermarine as a possible choice for their Type 300, which later evolved in the Spitfire. However, the arrival on the scene of the Rolls-Royce PV-12, later known as the Merlin, put an end to that.
The Rapier saw use in the DH.77 lightweight monoplane fighter of 1929, which was not accepted by the RAF, and a small number of Fairey Seafox shipboard seaplanes. The Dagger had a checkered career. It was used in the Hawker Hector army co-operation biplane, but did not perform well in tropical conditions, restricting the operational usefulness of the Hector. It was installed in the Handley Page Hereford, which was a re-engined Hampden, but in this application it proved noisy and sufficiently unreliable to make the Hereford unsuitable for combat. The Dagger also saw use in a lightweight fighter prototype, the Martin Baker M.B.2.
In Czechoslovakia, one of the most promising engines in this class was produced, the Walter Sagitta. This was an inverted V-12. The Sagitta powered a small Latvian fighter, the Irbitis I-16, and two were installed in a tandem configuration in the Dutch Fokker D.23 fighter. Both were doomed to remain prototypes by the occupation of their home countries. The same fate seems to have been suffered by the single Czech aircraft to use the engine, the Praga E51, a small twin-engined reconnaissance aircraft. Therefore the Sagitta never saw any operational use.
The Polish engineer Stanislaw Nowkunski contributed the concept of an inverted V-8 called the Foka, which aimed for 600 hp. Unfortunately Nowkunski died in an accident before the Foka was completed, and the development of the Foka encountered serious problems. By 1937 the Foka I engine developed 420 hp but was too unreliable to be flight-tested. The Foka II of late 1938 could be run up to 620 hp on the test bench, but the Polish government decided against production and only eight prototype engines were completed. The P.38 Wilk fighter, which was intended to have two Foka engines, only flew with 450 hp American Ranger SGV-770B engines. A development with radial engines, the P.48 Lampart, was preferred. The Foka and the V-770 were also the alternative engines considered for the P.45 Sokol fighter, but here too attention switched to a small radial. The P.45 was destined never to fly, development work being interrupted by the German occupation in 1939.
In France, the Renault 12R had a inverted V-12 configuration, and work was being done on a 16-cylinder air-cooled engine. The 12R powered the Caudron C.714 light fighter, one of the few of the kind to see combat. Two were installed in the prototype of the awkward Romano R.110 three-seat heavy fighter, and it also powered the Dewoitine D.720 light reconnaissance-bomber, types that were not accepted by the French air force. The 16-cylinder Renault 626 engine was installed in the Caudron CR.770 light fighter, a further development of the C.714, but this was flown only once before the armistice — and that flight ended when the crankshaft broke.
The USSR acquired a license from Renault in 1936 to produce three air-cooled in-line engines: The four-cylinder 4Pe, the six-cylinder 6Q, and the 12R. The 12R was never actually produced in the USSR, the designation MV-12 was used for imported examples only. Modified versions of the four and six cylinder engines did enter production as the MV-4 and MV-6, but these were unsuccessful. They were not very reliable and difficult to start in the Russian winter, and only the UT-3 trainer entered production with an MV-4. But remarkably, two MV-6 powered a fighter prototype, the small Moskalev SAM-13.
France also produced the Salmson 12Vars, another inverted V-12, rated at 450 hp. It powered the little ANF-Mureaux 190 fighter of 1936. Little is known about this engine, which seems to have been abandoned quickly because of its unreliability, and the 190 with it.
In Germany, Argus first developed the Argus As 10, an inverted air-cooled V-8, and later the Argus As 410, an inverted V-12. The As 10 was also known in a developed version as the As 401, which can create some confusion. The V-8 engine powered a number of successful light aircraft, such as the Fieseler Fi 156 Storch, the Arado Ar 66 trainer, and the Messerschmitt Bf 108. It was also the powerplant of choice for a 1933 programme to develop an advanced fighter trainer, to double as home defence fighter. The winners were the Arado Ar 76 and Focke-Wulf Fw 56.
The As 410 became available in 1938. It was by far the most successful engine in its class, with a production run that continued after WWII in France as the SNECMA 12S and ran to over 20000 engines. A later development, the As 411, delivered up to 600 hp at 3300 rpm, making it also one of the more powerful engines in this class. The As 410A-1 saw service in the Fw 189 twin-engined reconaissance aircraft, the Siebel Si 204 liaison aircraft, and the Arado Ar 96 fighter trainer. Production of the Ar 96 alone ran to about 11500, including a small A-series powered by the As 10 engine. The As 410 was also considered for the Hs 129 attack aircraft, but flight testing proved that the heavily armoured Hs 129 needed considerably more power. There were no fighter applications of this engine. In 1938 Gotha did propose several designes for a twin-engined fighter with As 410 engines for home defense purposes, but the RLM was not interested.
Also in Germany, Hirth created the Hirth HM.512, another inverted V-12 rated for 450 hp at take-off in its B version. Apparently it saw little or no use. A single example of the Bf 108 light aircraft, the Bf 108C, was flown with a HM.512 engine. It also equipped the prototype of the small Do 212 flying boat, which proved unable to fly when tested in 1942. The reasons for that failure, it should be added, concerned the engine and propeller installation, not the engine itself.
In Italy, Isotta-Fraschini developed an inverted V-12 that was initially known as the A.120 and later as the Isotta-Fraschini Delta. The Delta was bigger and heavier than the other engines in this class, with a 132 mm bore and 160 mm stroke, giving it a displacement only slightly smaller than a Rolls-Royce Merlin. The Delta was a reasonably successful engine that saw a moderate amount of use, most widely in the Caproni Ca.313 and Ca.314 twin-engined light bombers and reconnaissance aircraft, of which 640 were completed in total. These attracted foreign orders as well, but the Ca.313 is reported to have had its share of engine problems. The famous light fighter designs powered by the Delta included the SAI.207 and the SAI.403 Dardo. Of these light fighters the Italian air force wanted to acquired thousands, but only a dozen SAI.207s were completed before attention shifted to the more advanced SAI.403, and development of the SAI.403 ended at the armistice in 1943 with only the prototype completed. The Delta also powered the prototype of the French Caudron CR.760, a development of the C.714. And Caproni contributed the Ca.331, of which prototypes were completed as reconnaissance aircraft and nightfighter, and which was (under)powered by two Delta IV engines.
The United States competed with the Ranger V-770, also an inverted V-12. This had the merit of being fairly light for its power, resulting in a power-to-weight ratio of slightly better than 1.5 hp/kg. Its only production applications were in the Fairchild AT-21 Gunner trainer and the Curtiss SO3C Seamew shipboard seaplane. The AT-21 was a somewhat underpowered design that had a small production run of 175 aircraft, while the SO3C proved unfit for service and a number of the 794 completed ended their life as target drones. The V-770 was also installed in prototypes of the Bellance O-50 STOL aircraft and Edo OSE floatplane, but it most well-known use was in the little Bell XP-77 light fighter. There was also a Douglas XP-48 proposal around the same engine, which remained a paper design. After the war, Ranger created a D-series development of the Ranger engine for commercial use, rated for 700 hp, but this was not taken up.
In summary, the only one of these air-cooled in-line engines that was a real success was the Argus As 410, with a production run of approximately 20900. The Isotta-Fraschini Delta also had modest success, although it was installed in less than a thousand aircraft. The V-770 powered a larger number of aircraft than the Delta, but its service life was not a happy one. The most successful applications for any of these engines were in light twin-engined aircraft: The Focke-Wulf Fw 189, Siebel Si 204, Dassault MD.311 Flamant, Caproni Ca.313, and Caproni Ca.314. A dozen light fighters were proposed or built for these engines, but only one saw combat, the C.714, and that with little success.
|Configuration||Swept Volume||Compression||Dry Weight||Max Power||Cruise Power|
|Gipsy Twelve||inverted V-12||18.37 l||6||481 kg||525 hp at 2600 rpm||330 hp at 2200 rpm|
|Rapier||16-cylinder upright H||8.86 l||7||324 kg||395 hp at 4000 rpm||310 hp at 3500 rpm|
|Dagger VIII||24-cylinder upright H||16.85 l||1000 hp at 4200 rpm||770 hp at 3600 rpm|
|Sagitta I-MR||inverted V-12||18.4 l||6.25||385 kg||550 hp at 2600 rpm||500 hp at 2500 rpm|
|Renault 12R||inverted V-12||19 l||6.3||426 kg||450 hp at 2500 rpm|
|Argus As 410||inverted V-12||12 l||6.4||315 kg||485 hp at 3100 rpm||340 hp at 2820 rpm|
|Delta RC.40||inverted V-12||26.69 l||6.4||510 kg||730 hp at 2400 rpm|
|V-770B-5||inverted V-12||12.66 l||6||291 kg||450 hp at 2900 rpm||420 hp at 2800 rpm|
Chronologically the first in our story, the Fw 56 was the product of a specification issued in 1933 for an advanced fighter trainer that could also serve as home defence fighter. This aircraft would share a fighter's armament, flying characteristics, and take-off and landing speeds, but the specified engine was the 240 hp Argus As 10. With a third of the power of a contemporary fighter, performance could not be comparable. The preference of the Air Ministry was for a high-wing monoplane, because this would allow the trainer to have similar dimensions and wing loading as a biplane fighter, and because of the success of the gull-winged Polish P.Z.L. P.24.
Five diferent prototypes were constructed. Henschel, a new company in aircraft manufacturing, produced both the Hs 121 which closely imitated the P.24 in layout, and the mid-wing Hs 125. The reason appears to have been that the Hs 121 suffered from some serious defect, so Henschel quietly abandoned it and developed the Hs 125 instead. Arado and Focke-Wulf contributed high-wing parasol monoplanes, the Ar 76 and Fw 56. Heinkel first built a mock-up of an advanced monoplane with a cantilever wing, but finally offered the He 74, a biplane which arrived too late and had some handling problems. Contracts, therefore, were issued to Arado and Focke-Wulf.
The Ar 76 had outstanding handling characteristics, but perhaps its stability was considered a disadvantage for advanced training. It was also slightly heavier and slower than the Fw 56. Whatever the reason, Arado completed only 182 machines, and a fair number of these went to civilian users and export. About 1000 of the Fw 56 were produced until 1940. By that time, their usefulness as a fighter was clearly non-existent.
The de Havilland DH.77 was designed to RAF specification F.20/27, issued in 1927 to select a new single-seat interceptor fighter. In these days of relatively rapid and cheap aircraft development, twelve different designs were offered by nine different manufacturers. Three of them were low-wing monoplanes: The Vickers Jockey, and Westland F.20/27, both powered by the new Bristol Mercury radial, and the de Havilland DH.77, with its novel Napier Rapier engine. The DH.77 looks more modern than most of its competitors, with its closely cowled engine, with a strut-braced wing and wide-track, fixed landing gear. As was conventional at the time, the construction was mixed, with a wing built around two steel spars and wooden ribs, and a fuselage structure made up by a steel tube girder and wooden frames. Wing and fuselage were fabric covered. Armament consisted of the contemporary standard of two 7.7-mm Vickers machine guns.
Frank Halford, the engine designer of Napier, was at the time also working for de Havilland Engines Ltd., and the DH.77 was the result of a close cooperation between Halford and aircraft designer W.G. Carter. This resulted in an aircraft design tuned to its engine, which was installed in a very neat cowling that blended smoothly into the fuselage. At a time when radial engines were uncowled or at most had a simple Townend ring, the in-line configuration of the Rapier had clear aerodynamic advantages. The Rapier was a tall engine but narrow, and the nose of the DH.77 sloped down, so that the pilot presumably had a reasonably good view directly forward and an excellent view slightly towards the side, which was more than could be claimed for most radial-engined fighters. Unfortunately, the Rapier also suffered from persistent reliability problems, which eliminated the DH.77 from consideration in the final choice. In the end, after it lost the F.20/27 competition, the prototype served to flight test the Rapier engine. The Rapier I of 301 hp was later exchanged for a Rapier II of 295 hp, which presumably was more reliable.
The low weight of the DH.77 gave it a performance nearly matching that of the final winner, the more powerful Hawker Fury: The Fury had a gross weight of 1500 kg and was powered by a liquid-cooled Rolls-Royce Kestrel IIS, which gave 480 hp at 2250 rpm. The DH.77 had a gross weight of 1035 kg and its Rapier engine delivered 300 hp at 3500 rpm. The DH.77 was good for a top speed of 328 km/h, the Fury was a mere 5 km/h faster. The Fury could reach 4575 m in 7 minutes and 12 seconds, the DH.77 took twelve seconds more. On the other hand, the handling of the DH.77 had some flaws, which presumably could have been eliminated by further development.
The Martin-Baker M.B.2 was a private venture design, built to meet the requirements of specification F.5/34, which called for a fast-climbing interceptor fighter armed with at least eight 7.7-mm machine guns provided with 300 rounds each. Despite the type's private venture status, the Air Ministry supported it by supplying an engine, propeller and machine guns. However, there was a downside to the Ministry's willingness to supply an engine: It was an air-cooled, in-line Napier Dagger III, instead of the Rolls-Royce Merlin that James Martin really wanted to have. The M.B.2 might be designed as a simple, lightweight fighter but it was not intended to have a light engine. From the Ministry's point of view, however, Merlin engines were in short supply and alternatives were worth considering.
The M.B.2 was carefully designed to be produced at low cost by semi-skilled workers. It was characterized by a steel-tube structure, numerous removable panels for easy maintenance, and fabric covering of the aft fuselage and wing. The ease with wich the engine could be replaced or the guns reloaded, and great attention to detail, evoked the enthusiasm of the RAF test establishment at Martlesham Heath. The fixed landing gear with wide "trouser" fairings around the main legs was was somewhat old-fashioned, did not meet the stipulations of F.5/34, and was regarded as an undesirable feature although it no doubt saved on weight and complexity.
Unfortunately, the M.B.2's behaviour in the air failed to generate an equally favourable opinion. As initially delivered in late 1938, the aircraft possesed a long and slender fuselage behind its Napier Dagger III engine, with the cockpit set well back, no fixed vertical fin and a small rudder. In this form the M.B.2 demonstrated serious directional instability. A larger fin and rudder improved handling but the aircraft remained unpleasant to fly and unsuitable for aerobatic maneuvers. The RAF was sufficiently impressed by Martin-Baker's engineering to buy the M.B.2 and continue testing it, but it was clearly unfit to be a production fighter. The M.B.2 was clocked at 515 km/h with full armament, which compares favourably to the 508 km/h of a Hurricane Mk.I with a 1030 hp Rolls-Royce Merlin III engine. The M.B.2 was a lighter, with a loaded weight of 2512 kg against up to 3021 kg for a Hurricane, but the wing area was also considerably smaller at 19.79 square meters vs. 23.97, so that both types had about equal wing loading.
The VEF I-16, designed by Karlis Irbitis and built by a state factory in Latvia before the Soviet occupation of the country in 1940, was a true lightweight fighter. At 1540 kg loaded, it weighed about a ton less than even the M.B.2. Owing much to a series of lightweight monoplanes, it was a compact aircraft of wooden construction with plywood skinning. It had generally pleasing lines, although the position of the cockpit behind the trailing edge of the wing was inconvenient for a fighter, because this restricted view over the nose. Wide-track fixed landing gear was installed on the prototype, but retractable landing gear intended for any production aircraft. The engine was the 454 hp Walter Sagitta I-SR, produced in Czechslovakia, an inverted air-cooled V-12. Basic armament was restricted to a pair of synchronized rifle-calibre machine guns (not installed in the prototypes). This was too light to be effective in 1940, but it could be augmented by a pair of 20-mm cannon installed on the wings.
Assembly of the I-16 was delayed by the slow delivery of imported parts, including the engine. This is not surprising, considering events in Czechoslovakia. In the spring of 1940 the aircraft was ready, but the first flight was ended after 20 minutes by an engine failure. Support from Walter was not forthcoming because of the German occupation of the remainder of Czechoslovakia, but a few flights could be made. Top speed was determined to be 460 km/h, below the manufacturer's estimate, but probably in sub-optimal conditions. Flight testing of the I-16 was sufficiently encouraging to result in an order for a small production run of 12 aircraft and plans for the development of a more powerful and heavier I-19. However, the Soviet occupation ended these plans. The I-16 was subsequently test flown by both Soviet and German pilots.
The performance handicap of the I-16 may appear acceptable when compared with contemporary fighters, but it was only a prototype. Production aircraft could hardly have entered service before 1941, which would have made its potential opponents in combat fighters such as the Bf 109F and MiG-1, which outperformed the I-16 by a wide margin. The VEF I-16 was roughly the equal of the Polikarpov I-16 Typ 24, a derivative of an early 1930s design which was 300 kg heavier, possessed a more powerful engine, and had heavier armament. In summary, while Irbitis' design was an interesting engineering exercise, it was hardly a viable fighter.
The Fokker D.23 was not designed as a light fighter but as the fighter that would make the best use of light engines. In 1937 it was clear that the next generation of fighters would be powered by engines of over 1000 hp, but no such engine was available in the Netherlands. Fokker therefore designed a fighter around two 528 hp Walter Sagitta I-SR engines. To keep the aircraft small and manoeuverable, they were installed in a push-pull configuration at the front and rear of a short fuselage, while the tail was carried by two booms. Compared with a conventional twin-engined layout, this reduced the frontal area and avoided problems of asymmetric power during flight on one engine. However, performance was not quite at the same level as that of fighters powered by a single 1000 hp engine, and Fokker considered the installation of more powerful liquid-cooled V-12 engines in the D.23.
The configuration of the D.23 required tricycle landing gear to create enough clearance for the rear propeller, which gave the D.23 a modern look. In fact the construction of the prototype was traditional Fokker, with a steel tube fuselage and a wooden wing, although a metal wing was proposed for production aircraft. Its armament included two synchronized 7.9-mm machine guns in the cowling, and two 13.2-mm machine guns in the front ends of the tail booms, probably both FN-Brownings. The ammunition storage of the wing boom guns extended through the wing leading edge to the fuselage near the wing root. A concern not addressed was the fate of the pilot in case of an emergency, the bad choices on offer being sliced by the rear propeller when attempting to abandon the aircraft in the air, or crushed between the two heavy engines in an crash landing.
The ccooling of the rear engine was was problematic, because the air scoop that extended below the fuselage was insufficient. (Probably because it did not extend far enough beyond the stagnant boundary layer under the fuselage.) In a little less than a year before the aircraft was destroyed by German bombs on 10 May 1940, if flew only 11 times for a total of only about 4 hours.
Production aircraft with a thinner metal wing were expected to reach 525 km/h, which was a good performance by the standards of 1940 but would have probably have been too inferior by the time the aircraft reached service. Installing more powerful liquid-cooled engines would have helped, but not indefinitely because those engines that were small enough to fit the D.23 were being abandoned in the countries of origin in favour of bigger and more powerful ones. The D.23 was probably doomed to suffer the fate of the Westland Whirlwind or Focke-Wulf Fw 187, which were also relatively small twin-engined fighters. If the Netherlands had been able to stay neutral and out of the fighting, the D.23 might have been a valuable addition to its air defenses.
A fighter similar to the D.XIII, and indeed inspired by it, was built in Russia and completed in late 1940. Aleksandr Moskalev had started to develop aircraft around the MV-6, a Russian version of a Renault six-cylindre in-line engine, around 1936. Apparently this was less out of choice than because he was denied more powerful engines, but the SAM-13 made the best of it. It had a short, flat-sided fuselage with MV-6 engines at front and rear, and twin tail boom carrying an elliptical horizontal tail with, very unusually, a single tail fin in the middle. The sharply tapering wing was small, a mere 9 square meters in area. The compactness was strictly necessary because the MV-6 developed only 270 hp. The all-wooden (except for the engine bearers) SAM-13 was very light, 754 kg empty. The armament included two synchronized ShKAS machine guns and two more in the wings.
During test flights, the SAM-13 demonstrated a top speed of 560 km/h at 4000 m, which was no small achievement and only slightly slower than the prototype of the Yak-1. Development was abandoned after the German invasion in the summer of 1941, and the prototype was destroyed before the design bureau was evacuated to the east. Whether the SAM-13 would have been able to achieve the 680 km/h at 4000 m claimed for the design, remains forever an open question. It was lightly armed and powered by engines that were not known for their reliability. For the USSR it made more industrial sense to develop the new Yakovlev and Lavochkin fighters, which were also small and mostly of wooden construction, but powered by the more successful M-105 engine which had far more development potential.
Of all these light fighters, the Caudron C.714 is probably the one that saw most combat, although only between 50 and 100 were completed. It could trace its conceptual origins back to the Caudron C.460, a small racing aircraft which set a world speed record of 505.8 km/h in December 1934. The C.460 was powered by a Renault Bengali 6-cylinder in-line engine of 370 hp, highly streamlined, and very small, with a mere 7 square meters wing area and an empty weight of 590 kg. It was built for the Coupe Deutsch de la Meurthe race, and this was restricted to aircraft with engines of less than eight liters swept volume, although a bigger engine was installed for the attempt on the world speed record. Designed with a retractable landing gear, it often flew with fixed mainwheels because the original retraction mechanism was too unreliable. The C.460 went on to win the Coupe Deutsch in 1935, and flown in the USA by Michel Detroyat, the Greve Trophy and Thompson Trophy races of 1936. It was faster than the Gee Bee Super Sportster R-1 and the Wedell-Williams 44 which had held the world speed record previously, although these were also small aircraft and powered by 730 hp and 800 hp radial engines, respectively.
Such performance made it a logical step for Caudron to develop the C.690 fighter trainer, powered by the 240 hp Renault 6Q, and the C.710 Cyclone fighter, powered by the 450 hp Renault 12R. Developed in response to an official requirement for a light fighter, the C.710 was a simple but streamlined, wooden aircraft with fixed, spatted landing gear. The original elliptical tail fin was rather odd and ineffective, and replaced with a more conventional tail on the second C.710 to fly. A 20-mm Hispano-Suiza HS.9 cannon was installed in each wing just inboard of the landing gear legs. The C.710 was considerably bigger and heavier than the C.460, with a 12.5 square meter wing and an empty weight of 1243 kg. Despite the more powerful engine, the maximum speed fell to 455 km/h at 4000 m. Two prototypes were built, of which the first flew in July 1936.
A third (actually the second on the production line) C.710 airframe was completed as the sole C.713, fitted with retractable landing gear. The twin cannon armament was initially retained, but later removed. The speed gain from the retraction of the landing gear was modest, to 470 km/h at 4000 m. But this was promising enough to lead to an order for a production derivative, the C.714, which was armed with four 7.5-mm MAC 1934 machine guns in underwing trays. The C.714 featured a redesigned wing and structural reinforcements, and was again a bit slower than the C.713. It reached 455 km/h at 4000 m, which when the first aircraft were delivered in June 1939, was slow for a fighter. Its rate of climb was criticised, as it needed 9 minutes and 40 seconds to reach 4000 m &ndash in that time, a Bf 109 or a Spitfire could climb to above 6000 m. This was crucial, for the small C.714 could have been most useful as a point defense interceptor, a role that required a high rate of climb because the French had no effective early warning system.
Development potential of the C.714 was seriously limited by the small airframe, which could not take a more powerful engine. But in an attempt to generate publicity for its new fighter, Caudron gave the designation C.714R to a specialized racing aircraft, which actually had only the most superficial resemblance in common with the C.714. Never flight tested because of the outbreak of the war, the C.714R was predicted to reach 730 km/h! However, the C.714R was totally unsuited for the fighter role.
Overall, the C.714 was inferior as a fighter to the Morane-Saulnier MS.406 which the L'Armee de l'Air was trying to phase out in favour of the modern D.520 and Hawk 75A. Hence it was considered unsuitable for operational use, and orders were slashed to 90 of which 50 were made available for donation to Finland (which was fighting the Winter War with the USSR). The six C.714s actually sent to in Finland arrived there too late to see combat. But another purpose was found for the C.714: A number of Polish pilots who had escaped the occupation of their country in September 1939, had formed a fighter squadron in French service, and were equipped with the C.714. The first 35 were assigned to I/145 on 18 May.
Combat experience with the Caudron was discouraging. The little fighter had not enough power to compete with the Bf 109 in speed or climb. It had good handling and a lower wing loading than the German fighter, which was of some benefit in maneuverability, but overall the C.714 was felt to have little operational value. It was formally withdrawn from service already on 25 May, but this order was simply ignored by the Poles, who continued to fight and claimed 12 kills for a loss of 9 of their own – which, allowing for the vagaries of combat claims, probably means that they actually suffered worse than the enemy. Production of the C.714 was halted at the armistice.
The story of the Caudron fighter line is not complete with this, because two more prototypes had flown, the CR.760 and CR.770. The CR.760 had a 730 hp Isotta-Fraschini Delta R.C.40 engine, which may seem an odd choice, but in the late 1930s it was not clear which side Italy would choose in the coming war. Flown in May 1940, the CR.760 was destroyed a month later to keep it out of German hands. It was expected to be slightly faster than the C.714 at 555 km/h and perhaps more importantly, to climb to 4000 m in under six minutes. The CR.770 was similar but had a unique engine, a Renault 16-cylinder air-cooled in-line inverted-V. This was intended to give 800 hp at 4000 m, but the first and only flight of the CR.770 ended when its crankshaft broke.
The ANF-Mureaux 190 owed its existence to the same light fighter requirement as the Caudron C.710. But while the C.710 was largely built of wood, the 190 was of all-metal construction; and it was powered by a Salmson inverted V-12 instead of by a Renault. This turned out to have been a poor choice, for the 450 hp Salmson 12Vars was so unreliable that it caused the design to be abandoned in 1937.
The Salmson engine apparently allowed the installation of a 20-mm cannon firing through the propeller hub, but this was apparently not installed on the prototype, nor were the two rifle-calibre machine guns in the wings. A better performance was claimed for the 190 than for the C.710, 500 km/h at 4000 m. It had less engine power but was also considerably smaller and lighter, with a wing area of 10 square meters and an empty weight of only 850 kg, so this does appear a reasonable expectation.
These three 1938 Gotha projects deserve mention, although they remained strictly paper projects, because they were fighter designs around the Argus air-cooled engines. A series of lightweight Zerstörer projects were proposed by Albert Kalkert and Egwin Leiber. The P8-01 was a two-seat fighter with twin tails to create a free field of fire for the rear gunner, and two As 10 engines. The P14-02 was similar but used the more powerful As 410, while the P20 could have both engines but was to be flown by a pilot only and had a single tail. Kalkert's argumentation is said to have been that these aircraft would be suitable as home defense fighters, considering that French and Polish long-range fighters and bombers also had modest performance and armament. It is rather to the credit of the RLM that it refused to entertain this notion, for these aircraft would have been of very limited usefulness in later war years.
The Italian air force had two good reasons to consider light fighters: First, it lacked high-powered engines of indigeneous design, and in the end was forced to rely on copies of the Daimler Benz DB601 and 605. So desperate was the situation, that one prototype was built of a Reggiane Re 2001, lightened and powered by an 840 hp Isotta-Fraschini Delta RC.16/48 – according to chief designer Roberto Longhi, the sorriest thing he had ever seen. The other reason that Italy was dependent on Germany for fuel supplies as well, at a time when Germany had little enough to spare. Smaller engines burnt less fuel.
In these conditions, the Ambrosini SAI.207 and SAI.403 became the object of grandiose plans to build thousands. In reality, a handful SAI.207s briefly served with the Regia Aeronautica. This contrast was only too typical of the chaotic state of the Italian aircraft industry. In 1943, the Italian air force had aircraft of 30 different types in service, with an average number of 43 of each type!
The ancestry of these light fighters can be traced back to the SAI.7 high-performance touring aircraft, which later was developed into a fighter trainer. This was a beautiful and well-balanced design, constructed entirely from wood, with elegant lines and retractable landing gear. It was first flown in the summer of 1939. It says something about the qualities of this aircraft that about ten years later, production of the SAI.7 was restarted with a 225 hp Alfa-Romeo 115 engine, and the S.7 set speed records for its class over 100 and 1000 km as late as 1951!
Figher development started in 1941 with the one-off SAI.107, powered by a 504 hp Isotta-Fraschini Gamma RC.35. The SAI.107 was a proof of concept aircraft lacking military equipment, with a maximum weight of 1000 kg. The further developed SAI.207 was much heavier and had the 750 hp Delta RC.40 engine. The armament of two 12.7-mm machine guns was normal enough by Italian standards, but insufficient for the realities of combat in 1943. One S.207 was fitted with two 20-mm MG 151/20 cannon in the wings as well, although by some accounts the cannon had to be removed because of their weight and recoil. The claimed performance of 640 km/h at 4500 m was impressive for a low-powered fighter. The air force placed first an order for a small pre-series of 12 aircraft for combat evaluation, and then enthusiastically issued a production order for 2000. By Italian standards, 2000 aircraft was a large number indeed, in fact larger than the production of any Italian WWII fighter.
But before production could start, the order was cancelled and replaced by one for 3000 of the SAI.403 Dardo. This had a Delta RC.21/60 engine, modified wings and tail surfaces, a more robust structure, a retractable tail wheel, and increased ammunition capacity: Desirable improvements, if perhaps a bit academic in the light of Italy's war situation, which lead to its surrender a few months later, before a single production Dardo was delivered. Armament installation was made variable in three versions, with only the 12.7-mm guns, only the 20-mm guns, or both, depending on the role. Only the prototype of the SAI.403 was built and flown.
This left the pre-series of twelve S.207 as the only ones to see combat. A mere handful, probably not more than six, SAI.207 fighters served with 18 Gruppo in July 1943, where these aircraft were found suitable for bomber intercepts at low and medium altitudes, with apparently good handling characteristics. In August these were passed to 161 Gruppo, which was intended to re-equip with this fighter, but never had enough available.
The Douglas model 312 project for a lightweight fighter was submitted in early 1939. It envisaged a small aircraft with a very thin wing with an unusually high aspect ratio, 9.75 m span and an area of 8.55 square meters. Because this wing provided no room to stow the landing gear, the tricycle landing gear would fold in the fuselage. The engine was to be a supercharged 525 hp Ranger SGV-770, and the armament one .30 and one .50 gun in the fuselage decking, synchronized to fire through the propeller. It would weigh 1213 kg empty and 1542 kg gross, but thes numbers would probably have been difficult to maintain because they did not include the armour plate or self-sealing fuel tanks that combat experience in Europe would highlight as essential for a modern fighter.
The Air Corps was sufficiently interested in the Douglas offer to give it the designation XP-48 and sign a contract. However, after evaluation the staff at Wright Field concluded that the XP-48 would not meet current performance standards, and the contract was cancelled again. No XP-48 was completed. But this did not stop air corps officers from re-inventing the concept two years later.
In 1941, a concept was developed in air corps circles that was denoted Tri-4. This advocated a fighter that would weigh 4000 pounds (1800 kg), have a 400 horsepower engine, and reach 400 mph (644 km/h). As a comparison with the specifications of contemporary lightweight fighters (table below) shows, this was far from realistic. A realistic top speed from a 4000&nsp;lb fighter with a 400 hp engine was more likely to be 300 mph (483 km/h) than 400 mph. This was the top speed of the VEF I-16, which was lighter and had a more powerful engine than the Tri-4.
Nevertheless, in October 1941 discussions were opened with Bell. The stated goal of the project was to build a lightweight fighter around the Ranger V-770 air-cooled engine, using wood rather than aluminum for most of the construction. Shortages of aluminum were feared due to the rapid expansion of the US aviation industry, and it was imagined that a wooden aircraft could be built by workers in the furniture industry instead of scarce skilled metalworkers. The aircraft designed by Bob Woods and Sam Treman would weight 3600 lb (1630 kg) gross, and it aimed for 410 mph (660 km/h) at 27000 ft (8200 m) using a supercharged V-770-9 engine. Armament would be one 20-mm cannon firing through the propeller hub and two .50 machine guns. The model 32, designated XP-77 by the air force, was of course quite small and highly streamlined. Because the fuel tank was over the wing in the centre of gravity, the cockpit was quite far back, close the tail. The pilot's view on take-off and landing was less apalling than it could have been, because the XP-77 had tricycle landing gear, but it can't have been good. The cockpit hood did not slide back, probably to avoid interference with the tail, but rotated rearwards and downwards into the aft fuselage.
Development of the XP-77 quickly ran into difficulties. The wooden construction had been expected to be cheap and convenient, but the XP-77 seems to have had the same problem as the German Ta 154: Because the aviation industry had been quick to convert to metal, few people had experience with wooden aircraft construction, and certainly few at Bell. The bonded plywood construction in fact demanded considerable effort to ensure a good quality of glued joints. Woodwork factories were already busy turning out assault gliders, and the subcontractor that had been selected for the wings of the XP-77 had to be replaced in late 1943 by another one, whose work was still felt to be barely acceptable. One advantage of wood was that it permitted a high quality exterior finish, which was important for the laminar flow wing of the XP-77.
The engine was problematic too. The 575 hp V-770-9 engine was not available, so the project was forced to use the 520 hp V-770-6 that was in production for the Navy, rated at lower altitude. This received the Air Force designation V-770-7, but the Navy was only willing to part with six engines. The V-770-7 did not have a war emergency power rating, and would restrict the performance of the XP-77. Bell also experienced “unexpected and serious” cooling problems with the Ranger engine.
By early 1943, the project was in deep trouble. It was suffering from increasing delays, the projected cost for the construction of six prototypes had rocketed from $698,762 to over $2.5 million, and Wright Field's performance analysis was pessimistic. The XP-77 had a high wing loading and a low power loading. One recommendation was to try to reduce to 3000 lb (1360 kg), which was quite unrealistic as the fighter's empty weight was only 2800 lb. A much more powerful engine would obviously have been useful as well, but the the V-770-9 promised only a marginal improvement. Cancellation was considered, but the argument that prevailed was that the XP-77 could produce valuable engeering and flight data about a small wooden aircraft. In August 1943, the contract was reduced to one for two prototypes, and engine development plans were abandoned.
Finally, the two XP-77s were delivered in early 1944, and the first flight made on 1 April 1944. Flight testing was restricted because the quality of the construction of the first aircraft was judged barely adequate, and the second was still to be handled with caution. Performance fell rather widely short of the initial hopes of the program, with a top speed of 330 mph (531 km/h) at 4000 ft (1220 m). Normal gross weight was 3671 lb (1665 kg), including military equipment but not the 20-mm cannon, which was either omitted to save weight or could not be combined with the V-770-7 engine. The official evaluation found the XP-77 to be operationally unsuited, with considerably lower performance than initially predicted.
An interesting aspect of the XP-77 is its weight breakdown, as compared in the table below with that of another Bell product, the P-39D. The P-39 was one of the smaller and lighter of US production fighters, though powered by a liquid-cooled Allison V-1710 which was more than twice as heavy as the Ranger. The ratio of empty weight vs. useful load was 3.5 for the XP-77 and 2.8 for the P-39D-2, in line with the normal expectation that in a smaller aircraft, a greater weight fraction needs to be allocated to structures. The bigger aircraft carried proportionally heavier armament and more armour plate, which translated in a greater effectiveness in combat.
|Empty Weight||Total||1295 kg||2566 kg|
|Armour||35 kg||112 kg|
|Engine||330 kg||650 kg|
|Useful Load||Total||370 kg||925 kg|
|Pilot||82 kg||73 kg|
|Fuel||142 kg||283 kg|
|Oil||25 kg||32 kg|
|Guns and Ammunition||117 kg||357 kg|
|Gun sight and camera||3 kg||2 kg|
Most of these aircraft were remarkably similar. With a few exceptions, such as the SAM-13, they were streamlined low-wing designs, with wooden constructions and a cockpit set well back into the fuselage. And without exception, they failed to contribute anything to the war effort of their producers.
The commonality of both design features and failure may have the same reason. A lightweight fighter with an in-line air-cooled engine might have appeared a technical possibility in the early 1940s, but it was a narrow one, with very little room for variation and development. This constrained all their designers to produce quite similar aircraft. It also contributed to the failure of these fighters, for flexibility and development potential were essential for a World War II fighter aircraft to remain competitive. The best of the light fighters possessed, in prototype form, a performance that was on a par with contemporary production fighters. This sounded impressive but it was not good enough, for when the light fighter entered service two or three years later the competition would have moved on, and the light fighter then lacked the development potential to catch up.
Added to these woes were the engine problems. Of all the inverted V-8 and V-12 engines that were considered for fighters, only the Argus As 410 was a reliable and really successful product, and it found its only fighter application in the marginal and outdated Fw 56. The other engines were notoriously troublesome, suffered from cooling problems in a fighter application, or saw their development cut short by the occupation of their home countries. Designers who chose these motors for their fighter designs combined a new engine with a new airframe, a practice that was usually discouraged as asking for trouble, especially when both operated at the margins of the available technology.
In the end, the most valuable component in an operational fighter was the pilot. The feared aluminium shortages that justified the designs of wooden lightweight fighters, turned out to be insignificant compared to the very real shortages of trained pilots. It made little sense to dedicate valuable resources to train a pilot, and then put him in an aircraft that was deficient in performance, armament and protection. The concept of the light fighter was technically attractive, but operationally it was badly flawed.
A considerable number of the aircraft listed below never reached production, and for several development and performance testing were not completed. In such cases, quoted performance figures are often manufacturer's estimates, which need to be treated with some reservations. Performance of combat aircraft generally drops during development as more equipment is added and weight increases, and these small aircraft would have suffered more from it than standard fighters.
|Engine||Power||Wing Span||Length||Empty Weight||Gross Weight||Max. Speed||Ceiling||Armament|
|Arado Ar 76A||Argus As 10C||240 hp||9.50 m||7.20 m||750 kg||1070 kg||267 km/h at m||6400 m||2 × 7.92-mm MG 17|
|Focke-Wulf Fw 56||Argus As 10C||240 hp||10.50 m||7.55 m||670 kg||1040 kg||270 km/h at sea level||6200 m||2 × 7.92-mm MG 17|
|de Havilland DH.77||Napier Rapier I||301 hp||9.80 m||7.44 m||751 kg||1034 kg||327 km/h at 3050 m||m||2 × Vickers 303|
|Martin-Baker M.B.2||Napier Dagger III||798 hp||10.36 m||10.51 m||kg||2512 kg||515 km/h at m||m||8 × Browning .303|
|VEF I-16||Walter Sagitta I-SR||454 hp||8.23 m||7.30 m||1100 kg||1540 kg||483 km/h at 7900 m||m||2 × 7.9-mm
[2 × 20-mm]
|Fokker D.XXIII||2 × Walter Sagitta I-SR||2 × 528 hp||11.50 m||10.10 m||2180 kg||2950 kg||521 km/h at m||9000 m||2 × 7.9-mm and 2 × 13.2-mm FN-Browning|
|Moskalev SAM-13||2 × MV-6||2 × 270 hp||7.30 m||7.85 m||754 kg||1183 kg||680 km/h at 4000 m||10000 m||4 × 7.62-mm ShKAS|
|Caudron C.714||Renault 12R03||500 hp||8.97 m||8.63 m||1395 kg||1880 kg||460 km/h at 5000 m||9100 m||4 × 7.5-mm MAC 1934|
|Ambrosini SAI.207||Isotta-Fraschini Delta RC.40||750 hp||9.99 m||8.02 m||1750 kg||2415 kg||625 km/h at 5000 m||12000 m||2 × 12.7-mm Breda-SAFAT|
|Ambrosini SAI.403||Isotta-Fraschini Delta RC.21/60||750 hp||9.80 m||8.20 m||1983 kg||2640 kg||648 km/h at 7200 m||10000 m||2 × 20-mm Mauser MG151/20 and/or 2 × 12.7-mm Breda-SAFAT|
|Bell XP-77||Ranger V-770-7||520 hp||6.96 m||8.38 m||1295 kg||1665 kg||531 km/h at 1220 m||9150 m||2 × Browning .50|