RAF Bell P-39 Caribou fighters on the two main assembly lines at the Buffalo works. At each station is a stack of shelves carrying all materials needed at that point.
The Bell P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War Two. It was the first fighter in history with a tricycle undercarriage and the first to have the engine installed in the centre fuselage, behind the pilot. Although its mid-engine placement was innovative, the P-39 design was handicapped by the absence of an efficient turbo-supercharger, limiting it to low-altitude work. The P-39 was used with great success by the Soviet Air Force, who scored the highest number of individual kills attributed to any U.S. fighter type. Together with the derivative P-63 Kingcobra, these aircraft became the most successful mass-produced fixed-wing aircraft manufactured by Bell.
In 1940, the British Direct Purchase Commission in the US was looking for combat aircraft; they ordered 675 of the export version Bell Model 14 as the ‘Caribou’ on the strength of the company’s representations on 13 April 1940. The British armament was two nose mounted 0.50 in machine guns, and four 0.303 inch Browning machine guns in the wings; the 37 mm gun was replaced by a 20 mm Hispano-Suiza.
British expectations had been set by performance figures established by the unarmed and unarmoured XP-39 prototype. The British production contract stated that a maximum speed of 394 mph (+/- 4%) was required at rated altitude. In acceptance testing, actual production aircraft were found to be capable of only 371 mph at 14,090 ft. To enable the aircraft to make the guarantee speed, a variety of drag reduction modifications were developed by Bell, after which the aircraft was about 200 pounds lighter. The second production aircraft (AH 571) reached 391 mph at 14,400 ft, in flight test. As this speed was within 1% of the guarantee, the aircraft was declared to have satisfied the contractual obligations, but none of the modifcations were applied to other production P-39s. Later testing of a standard production aircraft at the Aeroplane and Armament Experimental Establishment in Great Britain revealed a top speed of only 359 mph.
The British export models were renamed ‘Airacobra’ in 1941. A further 150 were specified for delivery under Lend-lease in 1941 but these were not supplied. The RAF took delivery in mid 1941 and found that performance of the non-turbo-supercharged production aircraft differed markedly from what they were expecting. Tests by the RAE at Boscombe Down showed the Airacobra reached 355 mph at 13,000 ft. The cockpit layout was criticised, and it was noted that the pilot would have difficulty in baling out in an emergency because the cockpit roof could not be jettisoned. The lack of a clear vision panel on the windscreen assembly meant that in the event of heavy rain the pilot’s forward view would be completely obliterated; the pilot’s notes advised that in this case the door windows would have to be lowered and the speed reduced to 150 mph.
A RAF P-39 fuselage with cabin attached nearing completion on the assembly line.
The fuselage ‘chassis’ for the P-39 was built in a different manner to all other aircraft of the day, with a strong lower half that carried all the main components.
Despite the obvious problems, the Airacobra was considered effective for low level fighter and ground attack work. The problems with gun and exhaust flash suppression and the compass could be fixed.
601 Squadron RAF was the only British unit to use the Airacobra operationally, receiving their first two examples on 6 August 1941. On 9 October, four Airacobras attacked enemy barges near Dunkirk, in the type’s only operational action with the RAF. The squadron continued to train with the Airacobra during the winter, but a combination of poor serviceability and deep distrust of this unfamiliar fighter resulted in the RAF rejecting the type after just one combat mission. In March 1942, the unit re-equipped with Spitfires.
The Airacobras already in the UK, along with the remainder of the first batch being built in the US, were sent to the Soviet Air Force, the sole exception being AH574, which was passed to the Royal Navy and used for experimental work, including the first carrier landing by a tricycle undercarriage aircraft on 4 April 1945 on HMS Pretoria Castle, until it was scrapped on the recommendation of a visiting Bell test pilot in March 1946.
A Bell Airacobra in RAF markings.
Lines of cabin units on wheeled trolleys in the sub-assembly shop.
The Airacobra was one of the first production fighters to be conceived as a ‘weapons system’; in this case the aircraft was designed around the 37mm T9 cannon. The 200 lb, 90 inch long weapon had to be rigidly mounted and fire parallel to and close to the centreline of the new fighter. It would be impossible to mount the weapon in the fuselage, firing through the propeller shaft as could be done with smaller 20mm cannon. Weight, balance and visibility problems meant that the cockpit could not be placed farther back in the fuselage, behind the engine and cannon. The solution adopted was to mount the cannon in the forward fuselage and the engine in the centre fuselage, directly behind the pilot’s seat. The tractor propeller was driven via a 10-foot-long drive shaft which was made in two sections, incorporating a self-aligning bearing to accommodate fuselage deflection during violent manoeuvres. This shaft ran through a tunnel in the cockpit floor and was connected to a gearbox in the nose of the fuselage which, in turn, drove the three- or (later) four-bladed propeller via a short central shaft. The gearbox was provided with its own lubrication system, separate from the engine; in later versions of the Airacobra the gearbox was provided with some armour protection. The glycol-cooled radiator was fitted in the wing centre section, immediately beneath the engine; this was flanked on either side by a single drum shaped oil cooler. Air for the radiator and oil coolers was drawn in through intakes in both wing-root leading edges and was directed via four ducts to the radiator faces. The air was then exhausted through three controllable hinged flaps near the trailing edge of the centre section. Air for the carburettor was drawn in via a raised oval intake immediately aft of the rear canopy.