P-39 Airacobra

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Image:P-39.jpg The Bell P-39 Airacobra was one of the principal fighter aircraft in service with American forces at the start of World War II. (The P-39 was at first for a short time designated XP-45.)

1 History
2 Variants
3 Operators
4 Specifications (P-39Q Airacobra)
5 Reference
6 External links
7 Related content

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History

The aircraft's unusual design featured its Allison engine mounted in the middle of the fuselage, just behind the pilot, driving the propeller through a driveshaft passing between the pilot's feet. The purpose of this was to free up space for the heavy main armament, a 37 mm T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. The weight distribution necessitated a tricycle undercarriage, a first among American fighters. Entry to the cockpit was through a side door rather than a moving canopy. The weight distribution of the P-39 supposedly is the reason for its tendency to enter a dangerous flat spin — a characteristic Soviet tests proved to the then-skeptical manufacturer who had been unable to reproduce them.

The P-39's Allison V-1710 engine had a single-speed, single-stage supercharger, which brought about a decrease of performance compared to the promising prototype which had been fitted with an exhaust-driven turbo-supercharger. Due to the high weight of the P-39 and the poor high-altitude power of the mechanically supercharged Allison, the P-39's performance was markedly inferior to the contemporary European fighters, and as a result the first USAAF fighter units in the European Theater were equipped with the Spitfire V (which ironically featured a single-speed, single-stage supercharger, too).

Above the V-1710's full throttle height of about 17,000 ft (5,000 m), the P-39's performance dropped off rapidly. This limited its usefulness in traditional fighter missions, in Europe as well as in the Pacific where it was not uncommon for Japanese bombers to attack at altitudes above the P-39's operational ceiling (which in the tropical hot air inevitably was lower than in moderate climates).

The British Direct Purchase Commission in the US in 1940 was looking for combat aircraft, they ordered 675 of the export version Bell Model 14 on the strength of the company's representations on April 13 1940 as the Caribou. These would be renamed Airacobra in 1941. A further 150 were specified for delivery under Lend-lease in 1941 but these were not supplied. The Royal Air Force (RAF) took delivery in mid 1941 and found that actual performance differed markedly from what they were expecting. In some areas the Airacobra was inferior to existing aircraft such as the Hurricane and Spitfire and its performance at altitude was less. On the other hand it was considered effective for low level fighter and ground attack work. Problems with gun and exhaust flash suppression and compass were fixable. The need of the USSR was determined to be greater however and 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 US then requisitioned some 200 of the next part of the order as the P-400.

It is mistakenly believed that the Airacobra was used as a ground-attack airplane by the Soviet Union; the lack of a turbo-supercharger restricting it to low-altitude combat. In fact, the Soviets considered it to be a high-altitude fighter and used it to provide top cover. The tactical environment of the Eastern Front did not demand the high-altitude operations that the RAF and USAAF used with their big bombers. In the relatively low-altitude operations in the East the lack of a turbocharger was not as bad a handicap. The low-speed, low-altitude turning nature of most air combat on the Russian Front suited the P-39's strengths rather than its inherent weaknesses. The second-highest scoring Allied ace, Pokryshkin, flew the P-39 from late 1942 until the end of the war; his unofficial score in the Airacobra stands at nearly 60 Luftwaffe aircraft.

9,584 were produced, with over half being sent to the Soviet Union under the Lend-Lease program.

A number are still in existence of which three are still flying.

The mid-engine, gun-through-hub concept was developed further in the Bell P-63 Kingcobra.

A naval version with taildragger landing gear, the XFL-1 Airabonita, was ordered as a competitor to the F4U Corsair and XF5F Skyrocket, but was rejected by the Navy.

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Variants

XP-39 - first prototype, unarmed
YP-39 - service test version, V-1710-37 (E5) 1,090 hp engine, 12 built
YP-39A - intended to have a high-altitude V-1710-31 engine (1,150 hp) but delivered as a regular YP-39, one built
XP-39B - streamlined XP-39 based on NACA wind tunnel testing resulting in revised canopy and wheel door shape, oil and radiator intakes moved from right fuselage to wing roots, increased length (by 1 ft 1 in to 29 ft 9 in) and decreased wingspan (by 1 ft 10 in to 34 ft). Turbosupercharger replaced with single-stage geard supercharger, Allison V-1710-37 (E5) engine rated to 13,300 ft.
P-39C - first production version, identical to YP-39 except for V-1710-35 1,150 hp engine. Armed with 1x 37 mm cannon, 2x .50 cal and 2x .30 cal machineguns. First aircraft lacked armor and self-sealing fuel tanks.
P-39D - 245 lb of additional armor, self-sealing fuel tanks. Armament increased to 1x 37 mm cannon (30 rounds), 2x .50 cal (200 rounds/gun) and 4x .30 cal (1,000 rounds/gun) machineguns. Provisions for a single 250-lb, 325-lb, or 500-lb bomb under the fuselage.
P-39D-1 - Lend-Lease version, Hispano 20 mm cannon instead of the 37 mm cannon
P-39D-2 - Lend-Lease version, upgraded V-1710-63 (E6) engine with 1,325 hp; restored the 37 mm cannon; provisions for a single 145 US gallon drop tank under the fuselage.
Bell Model 14 - export version, ordered by France but not delivered.
P-400 Airacobra I - P-39D for Royal Air Force, briefly called Caribou; Hispano 20 mm cannon (60 rounds) instead of the 37 mm cannon. The P-400 designation came from advertised top speed of 400 mph.
XP-39E - intended for Continental I-1430-1 engine with 2,100 hp; see Bell XP-76
P-39F-1 - Aeroproducts constant speed propeller
P-39F-2 - field conversion of P-39F-1 with additional belly armor and cameras in rear fuselage
TP-39F - Two-seat training version, built in small numbers.
P-39G - intended to be a P-39D-2 with an Aeroproducts propeller. Due to modifications during production no P-39G were actually delivered. Instead, these aircraft were designated P-39K, L, M, and N.
P-39J - P-39F with V-1710-59 1,100 hp engine with automatic boost control
P-39K - P-39D-2 with Aeroproducts propeller and V-1710-63 (E6) 1,325 hp engine; one aircraft designated P-39K-5 and fitted with a V-1710-85 (E19) engine to serve as a P-39N prototype
P-39L - P-39K with Curtiss Electric propeller, revised nose gear for reduced drag, provision for underwing rockets.
P-39M - 11 ft 1 in Aeroproducts propeller, V-1710-67 (E8) 1,200 hp engine with improved high-altitude performance at the expense of low-altitude performance. 10 mph faster than P-39L at 15,000 ft.
P-39N - V-1710-85 (E19) 1,200 hp engine; Aeroproducts propeller enlarged from 10 ft 4 in to 11 ft 7 in starting with 167th airplane. P-39N-5 had reduced armor.
P-39Q - wing-mounted 0.30 cal machineguns replaced with a single 0.50 cal with 300 rounds of ammunition in a pod under each wing. These wing guns were often removed on Soviet aicraft. P-39Q-21 had a four-bladed Aeroproducts propeller. P-39Q-30 reverted to a three-bladed propeller because the four-bladed unit worsened directional stability.
RP-39Q - Two-seat training version, built in small numbers.
P-45 - The P-45 was the initial designation of the P-39C or Model 13.
F2L - Seven P-39s were supplied to the US Navy to be used as target drones.
XFL-1 - One prototype for the US Navy.
A-7 - Proposed radio-controlled target drone, never built.