Bell Airacobra

In terms of its design, the Bell Airacobra broke with convention in several respects. It was powered by an Allison V-1710-E4 liquid-cooled engine of 1150 hp, but this was mounted in the centre of the aircraft behind the pilot, the power being transmitted to the nose-mounted propeller via a long extension shaft. It was hoped that positioning the engine over the aircraft’s CG would aid manoeuvrability, but a further advantage of this arrangement was that it allowed the fitment of a large-calibre cannon, firing through the propeller spinner. Although it was capable of accommodating a gun of 37 mm, most Airacobras flown by the RAF were fitted with a 20-mm Hispano cannon in addition to two nose-mounted 0.50-in and four wing-mounted 0.303-in machine-guns. The Airacobra also featured a tricycle undercarriage in place of the more normal tailwheel undercarriage fitted to most other fighters of the time.

The prototype Bell XP-39 was flown for the first time on 6 April 1938. Initial flight testing was encouraging and very soon a speed of 390 mph had been recorded, together with a time of five minutes to reach 20,000 ft. However, these figures were achieved before the fitment of guns, protective armour or other operational equipment. Encouraged by the performance figures being quoted in the US, 200 examples of the Airacobra were ordered for the French Air Force in March 1940 and 675 were ordered by the Air Ministry the following month. After the French collapse, interest in the Airacobra (known for a time in Britain as the Caribou) was taken over by the British Direct Purchase Commission and the first ‘Caribous’ were flown in April 1941. Deliveries to the UK commenced with three P-39Cs, which became DS173, DS174 and DS175, the first two being tested at A&AEE and AFDU respectively, before the delivery of the first batch of Airacobra Is.

One of the first Airacobras to be assessed by Boscombe Down was AH573 in August 1941. Even before the pilot had settled into the cockpit, there was plenty for him to consider for his subsequent report. Entry was extremely difficult, especially with the engine running (ground running of the Allison engine had to be at 1000–1200 rpm to minimise vibration). A handhold had been provided for the pilot to pull himself onto the mainplane, but there was then no further assistance until reaching the cockpit door. A further handhold would have made entry much easier. Getting into the cockpit itself was not an easy task as the roofline was so low and the roof itself did not open. Three hands would also have been useful, two to gain access to the cockpit and one to hold the door open.

Once inside, unlike many other American aircraft, the cockpit was found to be small and only suitable for pilots of small or medium frame, as there was no headroom for taller pilots. However, it was understood at the time that this problem was under investigation with a view to lowering the seat. Exiting the aircraft when on the ground was not particularly difficult, although it was considered that this would be an entirely different matter in the air in an emergency situation, and a much more difficult operation compared with an aircraft with a jettisonable hood. There was also a good chance of the pilot hitting the tail as he baled out, as he was forced to exit from the side door. Two sliding windows were provided for ventilation and with both shut, no carbon monoxide fumes were detected and no stuffiness was apparent in flight. The cockpit was rated as being slightly less noisy than that of a Spitfire. The view out of the Airacobra was generally good in all directions except straight ahead, where it was spoilt by the metal top of the bullet-proof windscreen and the metal frame of the cabin top. This was again due to the pilot sitting too high and would have been improved by lowering the seat.

The elevator and rudder controls could be moved without undue friction being felt. However, there was excessive friction in the aileron control, which increased with the angle of movement. The elevator trim wheel was located to the left of the pilot and operated satisfactorily, although the associated indicator was not easily seen as it was positioned too close to the seat. To the rear of this was the rudder bias gear, which was operated by a large knob. This performed in an acceptable manner, but due to lack of space, it was difficult to operate. The aileron bias gear was situated forward of the elevator trim wheel, low down on the left-hand side of the cockpit, where it could not be reached by the pilot when he was strapped in tightly. To operate this control, the pilot was thus forced to slacken his straps and lean forward.

The throttle, mixture and propeller controls were mounted on a quadrant in the conventional manner on the pilot’s left and were satisfactory in operation. However, there was no adjustable friction grip for the throttle, which tended to slip a little. The fuel cock was considered to be badly positioned as it was forward and to the right of the aileron bias and was extremely difficult to reach, even when the pilot released his straps. The radiator shutter control was operated by a crank handle on the right-hand side of the pilot’s seat and was easy to operate.

The flaps were operated electrically and were controlled by a three-position tumbler switch situated on the front left corner of the cockpit. An indicator was provided, consisting of a pointer moving over a scale graduated in quarters. Although this system worked well enough, A&AEE pilots criticised the lack of any alternative method of lowering the flaps in the event of electrical failure. The undercarriage was operated in similar fashion to the flaps, but in this case there was a manual back-up system. The pilot could select either ‘electric’ or ‘manual’ and there was the usual pictorial type of indicator, together with a visual indicator to show when the wheels were locked down, although there was nothing to confirm that they were locked up. The brakes were operated by pedals above the rudder bar and were easy to operate. A locking device was provided for parking. No standard blind flying panel was fitted, but all instruments were clearly visible and the layout was considered to be adequate.

During the assessment of the Airacobra’s flying qualities, AH573 was flown at an all-up weight of 7850 lb and with the CG 20.6 in aft of datum (normal setting). Taxying was very easy and even over rough ground was comparatively smooth and comfortable. The undercarriage performed adequately and exhibited good shock-absorbing qualities. It was felt, however, that the aircraft’s ground handling was over-dependent on the efficiency of the brakes, and in the event of them failing it was thought that the aircraft might be difficult to control. The take-off was straightforward with a slight tendency to swing to the left as the engine was opened up but this was easily checked by the application of right rudder. The take-off speed was approximately 100 mph IAS, whilst the initial climb was made at around 140 mph IAS. The undercarriage could be raised as soon as the aircraft had left the ground and the retraction sequence took around fifteen seconds. There was no noticeable change of trim as the undercarriage was raised. Owing to the limited amount of time available for the tests on the Airacobra, the optimum flap setting for take-off was not determined, although 15 degrees was used quite successfully. Raising the flaps caused the aircraft to sink slightly.

Once in the air, the rudder and elevators were found to be light, quick in response and effective. The fabric-covered ailerons were light in normal flight, but tended to become much heavier during dives at speeds above 300 mph IAS. Lateral control was considered to be comparable with the early Spitfire Is with fabric-covered ailerons, but by mid 1941 more was required. Much effort had been expended to improve the Spitfire’s handling in this respect by developing metal-covered ailerons and these had first appeared on the Spitfire V. Although there was no time for full stability tests, early indications showed the Airacobra to be stable throughout the full speed range. Aerobatics could be flown with ease and there were no undesirable handling qualities.

The stall speed with the flaps and undercarriage up was 105 mph IAS and 88 mph IAS with the flaps and undercarriage down. Compared with most other fighters of the early war period, these speeds were quite high, being approximately 25–30 mph faster than the Spitfire, and consequently led to higher approach speeds. There was no real stall warning, only a slight wallowing, followed by a gentle nose drop at the point of stall. With the flaps and undercarriage down, a gentle drop of the left wing was noted before the nose went down. The best glide speed when coming in to land was about 110 mph IAS with a little power and the approach and landing was simple and easy in execution. It was recommended that the nose be held up as long as possible on landing, to achieve the lowest touchdown speed and shortest run. However, it appeared that old habits were slow to change, as it was noted that the aircraft could also be landed by flying it onto the ground as for a three-point touchdown – a recipe for disaster in a tricycle-undercarriage aircraft if attempted by low-time pilots.

Climb and level speed performance tests were carried out, commencing on 29 July 1941. The aircraft used for climbing tests was AH573 once again, but following a crash, it was replaced by AH589. This machine, however, suffered from constant ignition trouble and the trials were eventually carried out by AH701, which was already at A&AEE for an investigation into gun heating. Both AH573 and AH701 were powered by an Allison V-1710-E4 engine driving a Curtiss Electric constant-speed propeller. The aircraft had similar exhausts, which consisted of two backward-facing open-ended stub pipes per cylinder. Full armament was carried, but all the gun muzzles were sealed. A small bead sight was fitted to both aircraft in front of the windscreen. Aerial masts were fitted (but no aerial) and only AH573 had IFF aerials fitted. AH573 was flown at a take-off weight of 7830 lb and with CG at 20.3 in aft of datum (CG range was established as being between 18.8 in and 22.3 in aft of datum). Climbs were made with the oil cooler and radiator shutters fully open. For the level speed tests using AH701, the shutters were flush with the surface of the fuselage. The limitations of the Allison engine were as follows:

Take-off (5 min)

3000 rpm

44½ in of Hg pressure

Max on climb (30 min)

2600 rpm

37 in

Max for rich level flight (5 min)

3000 rpm

42 in

Max for rich mixture cruise

2600 rpm

37 in

Max for weak mixture cruise

2300 rpm

30½ in

During the climbing tests, it was established that the maximum rate of climb was 2040 ft/min at 10,300 ft (full throttle height) and the service ceiling was calculated as being 29,000 ft, with an absolute ceiling of 30,200 ft. The time to 10,000 ft was 5.1 minutes and it took 11.7 minutes to reach 20,000 ft. The best climbing speed was found to be 150 mph IAS below full throttle height, decreasing by 3 mph per 2000 ft above this height. The full results were as follows:



The Maximum True Air Speed (TAS) in level flight was 355 mph at 13,000 ft, which represented the aircraft’s full throttle height. Measured readings up to 24,000 ft were:



Trials were later carried out using AH574 fitted with an Allison V-1710-E12 engine in place of the normal -E4. The -E12 had a revised supercharger impeller gear ratio of 9.6 to 1 and a propeller reduction gearbox with a gear ratio of 2 to 1. Corresponding figures for the -E4 engine were 8.8 to 1 and 1.8 to 1 respectively. The aircraft was flown at similar weights and CG to that of the previous tests. It was discovered that the full throttle height on the climb was raised by about 2000 ft, but the maximum rate of climb was rather less. The full throttle height in level flight was also raised by about 2000 ft and the maximum speed increased by 10 mph. In the climb, the full throttle height occurred at 12,500 ft (6.85 minutes from start, rate of climb 1845 ft/min) and during level speed runs, the full throttle height was achieved at 15,600 ft (365 mph TAS, 283 mph IAS).

Although it was the fastest of the early American fighters, the Airacobra I was handicapped by its Allison engine, which had a full throttle height of only 13,000 ft. As a result, the Airacobra was hopelessly out-performed at higher altitudes. The Allison had also developed a reputation of being somewhat fragile and a number of aircraft were lost as a result of engine failure. Difficulties were also experienced with servicing and excessively long rearming times. It was apparent at an early stage that the Airacobra was unlikely to make it as a fighter, nor was it suitable for any other role within the RAF. Despite this, No. 601 Squadron was earmarked as the first Airacobra unit and moved to Duxford in August 1941 to begin its work-up period. By the time that it converted to Spitfire VBs in March 1942 , it had suffered eight accidents with the Airacobra, causing the death of three of its pilots. Three of the crashes were as a result of engine failure, two were due to fuel problems and three to pilot error – all this for a few desultory Rhubarb operations over northern France the previous year when a small detachment operated from Manston. [Rhubarb operations were normally carried out by two aircraft at low level looking for targets of opportunity].

The Airacobra was still to be seen in the skies over Duxford until the end of 1942. Len Thorne of AFDU was tasked with carrying out an air test in one on 29 August. He recalls:

It was a little strange because it was the first time that I had flown an aeroplane with a tricycle undercarriage, but it was quite nice to fly. It had the engine mounted at the rear of the cockpit and I approached the flight with a certain amount of trepidation as I had an uncomfortable feeling about what was in that high revving propeller shaft between my legs. I had visions of it breaking with lots of sharp ends flying about but, to the best of my knowledge, that never happened. The Russians, of course, used them extensively and rated them quite highly but most of their combats were at low altitude which suited the Airacobra and its Allison engine. I only got to fly it the once, which, I think, was enough!

The aircraft that remained with the RAF were eventually flown to Maintenance Units by ATA pilots, the majority being prepared for packing and delivery to Russia, where they were used in the ground-attack role. Further deliveries heading for Britain were diverted to the USAAF, many being taken on by the 350th Fighter Group, which formed at Bushey Hall in October 1942, before moving to Duxford prior to being transferred to the Twelfth Air Force in the Middle East.

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