Bell P-39 Airacobra: Introduction and Development

The Bell P-39 Airacobra is still one of the most controversial fighter aircraft of the Second World War. With it’s sleek racing lines, apparent high speed and powerful cannon it received a great deal of positive publicity before the United States entered the war, but it was rejected as a front line fighter by the RAF and was generally unpopular with the American pilots who flew it in combat. Despite this the P-39 served with great distinction on the Eastern Front in the hands of Soviet pilots, and was used by four of the top six Soviet Aces of the war.

The unusual layout of the Airacobra was dictated by the choice of the American Armament Corporation T-9 37mm cannon as its main weapon, firing through the propeller spinner. This large gun left no room in the nose for the engine, and so the Allison V-1710 engine was placed behind the pilot. This then required the use of a long geared propeller shaft running under the pilot’s seat. This caused a great deal of nervousness amongst Airacobra pilots, worried about the damage this shaft could do in a crash, but in fact this arrangement was perfectly safe, if somewhat awkward to maintain. The distribution of weight in the Airacobra also forced the adoption of tricycle landing gear, making it the first AAF fighter to be so equipped. The tracked landing gear would later help the Airacobra operate from rough airstrips, on snow and even from German autobahns.

The main problem with the P-39 in British and American service was the lack of any second stage supercharger. This meant that the aircraft could not compete with German or Japanese fighters above 15,000 feet and was at its best below 10,000 feet. At that altitude it could just about hold its own with the Bf 109 and even the Fw 190, and could out-manoeuvre the German fighters. This helps to explain the great success of Soviet pilots when flying the P-39 – on the Eastern Front there was very little high altitude flying, for both sides concentrated on tactical operations to support their armies, which in turn forced the German fighter pilots to come down to the altitudes where the P-39 was at its best. 

XP-39

In May 1937 Bell responded to a USAAF call for a new pursuit aircraft with both its Model 3 and Model 4 designs. The Model 4 was selected for further development as the P-39, and on 7 October 1937 Bell received an order to produce the XP-39 prototype.

The very first Airacobra, the single XP-39 experimental prototype, was also the most promising version of the aircraft. It was powered by the Allison V-1710-17(E2), a liquid cooled twelve cylinder inline engine that came with a built-in single-stage single-speed mechanical super-charger, similar to the “blower” found in British engines. This was supported by a B-5 turbosupercharger. It would be the only Airacobra to have any form of second stage supercharging, and as a result had much better high altitude performance than the production aircraft.

The XP-39 made its maiden flight on 6 April 1939, staying in the air for 20 minutes. Boosted by the turbosupercharger, it achieved a top speed of 390mph, but suffered from some serious aerodynamic problems

XP-39B

Bell P-39 Airacobra from the right

After undergoing initial tests the XP-39 was extensively rebuilt, emerging as the XP-39B. The most significant change was the removal of the turbosupercharger, which was causing aerodynamic and mechanical difficulties.

The removal of the turbosupercharger reflected the Army Air Corps’s view that the pursuit aircraft would operate as a low-level army cooperation aircraft, while the heavily army high altitude bombers (specifically the B-17 Flying Fortress) would be able to defend themselves without fighter escort. The P-39 would therefore not need to have good high-altitude performance, and so the supercharger could safely be removed.

This decision relegated the P-39 to the second division of fighter aircraft. In Germany the Junkers Jumo 210 engine used in the earlier models of the Bf 109 came with a built in mechanical supercharger, as did the engines used in the early versions of the Mitsubishi Zero (under development at roughly the same time as the P-39). When the first P-39C reached the United Kingdom tests revealed that it had a top speed of 359mph – only ten mph slower than the 369mph top speed of the Spitfire Mk.V, but while the P-39 produced its top speed at low altitude, the Spitfire was at its best at 19,500ft.

The removal of the turbosupercharger from the XP-39B and all future models meant that the Airacobra would play a limited role with the British and American air forces operating against both the Germans and the Japanese. As a result large numbers of P-39s would be available to go to the Soviet Union.

The aerial fighting on the Eastern Front was very different to that anywhere else. Soviet aircraft rarely flew at higher altitudes, instead concentrating on lower level ground attack and army-cooperation duties (much to the annoyance of many German fighter pilots who fought their aircraft’s excellent high level performance entirely negated). In these circumstances the P-39 Airacobra was better able to hold its own against the Bf 109.

YP-39

The final pre-service version of the Airacobra was the service-test YP-39. Thirteen of these aircraft were ordered on 27 April 1939, and the first aircraft made its maiden flight on 13 September 1940. All thirteen had been completed by the end of the year. Although none reached active service, they were used extensively for test flights. The YP-39 was powered by the V-1710-37 (E5) engine, capable of producing 1,090hp. It was armed with the standard 37mm cannon as well as two .50in and two .30in machine guns.

Bell P-39 Airacobra, Robert F. Dorr with Jerry C. Scutts (Crowood Aviation). A detailed looked at the development and service history of this controversial American fighter aircraft. The P-39 had a poor reputation amongst British and American pilots, and Dorr examines the reasons why, as well as looking at why the same aircraft was so much more popular in Soviet Service. Scutts provides a chapter on the P-63 Kingcobra, and the book also covers the numerous Bell fighter projects that failed to enter production.

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