The Flying Cobra
Bell's P-39 Airacobra
by Randy Wilson
Copyright © 1997 by the Confederate Air Force and Randy Wilson. All rights reserved.
In 1936, the US Army Air Corps invited a number of aircraft manufacturers to submit proposals for a new single-seat fighter, with greater speed, maneuverability and heavier armament than current models. The Bell Aircraft Corporation had been founded just the year before, on June 20, 1935, but its president, Larry Bell and chief designer, Bob Woods, felt that a combination of novel design features could be combined to create a very advanced fighter.
While Larry Bell and other members of the new company had many years of experience in the aircraft industry with Consolidated and other companies, in 1936 Bell Aircraft had won only a single production order from the Air Corps for a design designated the FM-1 (for Fighter Multiplace) Airacuda. Powered by two turbo-supercharged Allison liquid-cooled engines driving pusher propellers, the Airacuda was intended as a long-range escort fighter for the new bombers, including the Boeing B-17. Armed with two 37mm Madsen cannons in the forward part of the engine nacelles, the Airacuda reflected Bell's innovative engineering, however, its poor performance combined with a complex and costly design resulted in only 13 YFM-1s being built before development was stopped in 1940.
To provide both high performance and greater firepower in the new single-seat fighter, Bell and Woods decided to design the aircraft around the new T-9 (later M-9) 37mm cannon, produced by the American Armament Corp (a subsidiary of Oldsmobile), and power the design with the easily streamlined Allison engine. Most contemporary fighters had only two or three rifle-caliber machine guns with the heaviest armament being a 0.50 caliber gun, and many had bulky radial engines. Bell's design also had a novel feature for a fighter, a tricycle landing gear for improved ground handling and take-off and landing characteristics.
To make room for the more than 8 foot long 37mm cannon, the Allison V-1710 engine had to be moved rearward in the fuselage, driving the propeller through a long extension shaft. In the first mock-ups, designated the Model 3 by Bell, the cockpit was located behind the engine, but as this placed the pilot behind the trailing edge of the wing, visibility forward and down was very poor. By early 1937, a second design, the Model 4, had the cockpit in front of the engine, and in May of that year, both versions underwent study by the Air Corps. The result was a contract awarded on Oct. 7, 1937 for a single prototype with the designation XP-39, based on the design of the Model 4.
While the prototype was being built, Larry Bell made a tour of European aircraft plants, at the request of the American government. Bell was most impressed with the manufacturing capabilities and techniques in Germany, and he especially noted the lightweight structures of the Bf 109 fighter. Work continued on the prototype after his return to the US, and in late December 1938 the XP-39 was disassembled and trucked to Dayton, Ohio, for its first taxi and flight tests at Wright Field. The prototype was powered by a 1,150 hp Allison V-1710-17 engine equipped with a General Electric B-5 turbo-supercharger. An extension shaft ran forward through the cockpit to a reduction gearbox in the nose, which drove a three-bladed Curtiss Electric propeller.
More than three months were spent at Dayton readying the XP-39 for its first flight, which took place on April 6, 1939*. Within its first few flights, weighing only about 6,200 pounds without its heavy cannon or other armament, the XP-39 climbed to 20,000 feet in only five minutes and hit a maximum speed of 390 mph. This was a sensational performance compared to existing US fighters, but fell just short of Bell's promised 400 mph mark. However, it was good enough for the USAAC to order thirteen YP-39s from Bell for service evaluation on April 13, 1939.
[*Many references give the date of the XP-39's first flight as April 6, 1938, however, this seems to be an error which was repeated a number of times. A close study of various sources, plus an examination of the time sequence of the Airacobra's design, all support the 1939 date.]
In June, following extended tests at Wright Field, the prototype was sent to Langley Field, Virginia for testing in the National Advisory Committee for Aeronautics' (NACA) wind tunnel. The NACA recommended a number of changes after studying the XP-39, including repositioning of the engine coolant radiator and air intake, a lower-profile cockpit canopy and the addition of fairing doors for the main wheels. In addition, the removal of the turbo-supercharger and its air intake were recommended. The XP-39 returned to the Buffalo plant for modifications, emerging as the XP-39B powered by a 1,090 hp Allison V-1710-39 engine without turbocharging and incorporating most of the recommendations made by the NACA.
The modified prototype first flew on Nov. 25, 1939 and although low-altitude performance and maneuverability were improved, top speed was only 375 mph at 15,000 feet, and a climb to 20,000 feet took 7 ½ minutes. It was obvious that the deletion of the turbo-supercharger had significantly degraded high-altitude performance.
Many writers have noted that the deletion of the turbo-supercharger from production P-39s kept them from ever performing up to their true potential in aerial combat, and often point out that this decision was not Bell's but the Army Air Corps'. It seems that the Army saw the P-39 primarily as a low-altitude fighter and ground attack aircraft, which simply did not need the added complexity of a turbocharger installation. While the XP-39's engine could produce all 1,150 hp up to 25,000 feet, the early production P-39s' non-turbocharged engines began to loose power as low as 11,800 feet.
A factor in this decision may have been that three months before the XP-39 first flew, Lockheed's XP-38 Lightning was test flown in California, powered by two turbo-supercharged Allison engines. With a top speed of 413 mph at 20,000 feet and a planned armament which also included a 37mm cannon and four 0.50 caliber machine guns, the Lightning may have simply seemed the better bet for a high-altitude interceptor.
The first YP-39 was delivered in September 1940 and was based on the NACA-modified prototype. Fully equipped with the M-9 37mm cannon with 15 rounds, two 0.50 caliber and one 0.30 caliber machine guns, and some cockpit armor, the YP-39's weight had risen to 7,235 pounds. Even before all thirteen YP-39s had been delivered, the Air Corps ordered 80 production aircraft, initially designated P-45 Airacobras, but soon changed to back to P-39. The first twenty aircraft were P-39Cs, similar to the YP-39s but fitted with a 1,150hp V-1710-35 engine, deliveries beginning in January 1941. The next sixty, designated P-39Ds had four 0.30 caliber guns in the wings in place of the two originally in the nose, and provisions to carry an external fuel tank or 600 pound bomb under the fuselage. A small fillet added to the base of the vertical stabilizer, visually differentiated the P-39D from earlier Airacobras.
The P-39 design took an odd detour in November 1938, when the US Navy ordered one prototype XFL-1 Airabonita, which was essentially a navalized Airacobra fitted with conventional tail-wheel landing gear and other modifications for operation from aircraft carriers. The Airabonita first flew on May 13, 1940 but failed its carrier qualification tests. A design from Vought, which eventually emerged as the F4U Corsair, was ordered instead, and the sole XFL-1 was eventually reduced to scrap in anti-aircraft gun tests.
From late 1940 through mid-1941, a total of 923 P-39Ds were built, some with a 20mm AN-M2 cannon in place of the 37mm, and most of these were delivered to Great Britain or the Soviet Union, under the Lend-Lease program. The few dozen operated by the USAAC were often modified for ground attack and other low-level missions. Airacobras armed with the 20mm cannon can be identified by the protrusion of the thinner 20mm gun barrel well in front of the propeller spinner, as the lighter cannon had to be mounted farther forward than the nearly 400 pound 37mm gun.
Even before the beginning of the Second World War, France and Great Britain had been negotiating for modern combat aircraft from the United States. In March of 1940, Bell received a $9,000,000 contract for 200 P-39s for France, however, the collapse of that country in June prevented delivery of any of the aircraft. Equally desperate, Great Britain ordered 675 fighters under the name Caribou, which differed from the P-39D mostly in armament. Shortly after the first Caribous arrived in Great Britain in July 1941, they were renamed Airacoba Is by the Royal Air Force.
However, the RAF pilots who first flew the Airacobras were disappointed with their top speed of only 355 mph and service ceiling of 29,000 feet. Even the older Hawker Hurricane out climbed the new American fighter. After a brief period of operation, most of the British Airacobras were shipped to the Soviet Union, and after the Japanese attack on Pearl Harbor, 179 remaining in the US were taken over by the USAAC with the designation P-400 and sent to Pacific bases, including Australia.
American pilots found that the Airacobra's 37mm cannon was a potent aerial weapon, easily destroying most Japanese aircraft with one or two hits. However, the Allison engine's single-stage, single-speed mechanical supercharger could not produce enough power above 15,000 feet to allow the fighter to intercept Japanese bombers. The Airacobras did function well in the ground attack role, and they soldiered on in the Pacific until replaced by Lockheed P-38s and Republic P-47s early in 1944.
In February 1942, responding to P-39's lackluster performance above 15,000 feet and other shortcomings, Bell proposed an improved version, and in April the Air Corps ordered two XP-39Es as prototypes. Fitted with an improved tail unit, laminar flow wings with squared tips and greater span, and powered by a turbo-supercharged Allison engine, the XP-39Es eventually became the prototypes of an entirely new design, the Bell P-63 Kingcobra. The development and history of the P-63 will be featured in a future issue of the Dispatch.
By late 1941, Bell was producing P-39Fs fitted with Aeroproducts Hydromatic propellers, due to a shortage of Curtiss Electric props. A few of these were converted into two-seat trainers, under the designation TP-39F, with dual-controls but no armament. A total of 229 P-39F and similar J models were produced.
Equipped with a 1,325 hp V-1710-63 engine and Aeroproducts propeller, 210 P-39Ks were delivered in 1942. A few were converted to ground attack versions, and one became the prototype for the first model to see large-scale production, the P-39N. Performance was improved below 15,000 feet with the more powerful engine, and in the 250 P-39Ls, which were similar except for a Curtiss propeller, top speed at 15,000 feet was 360 mph. Even faster were the 240 P-39Ms with a top speed of 370 mph. More than 330 of the K, L and M versions were delivered to the Soviet Union.
The P-39N was the first large-scale production version of the Airacobra, 2,095 being delivered, powered by a 1,200 hp V-1710-85 engine. Late production P-39Ns were fitted with a larger diameter Aeroproducts propeller, modified cockpit armor and improved radios, but to reduce weight, internal fuel capacity was reduced to 87 gallons from the earlier 120 gallons. Armament consisted of the 37mm cannon, plus two 0.50 and two 0.30 caliber machine guns and one 500-pound bomb. Performance was increased to a top speed of 399 mph at 9,700 feet and a service ceiling of 38,500 feet.
Delivery of the first P-39Q in May of 1943 marked the last and largest production batch of Airacobras, totaling 4,905 aircraft. Basically similar to the P-39N, the major difference was the replacement of the two 0.30 caliber guns with two of 0.50 caliber mounted in pods under each wing. The Soviet Union received the bulk of the P-39Q production, totaling 3,291 planes, most ferried over the Alaska-Siberia route. Soviet pilots liked the Airacobra and enjoyed considerable success with a design deemed ill suited to aerial combat by most other countries. Several of the highest-ranking Soviet aces flew P-39s.
Airacobras also equipped Free French squadrons from 1943 and Polish Air Force units from late 1944. After Italy's surrender in 1944, 149 P-39N and Qs saw action with the Italian air forces in the Mediterranean and elsewhere. Portugal received eighteen P-39Ds by accident, when their American pilots landed in that country by mistake while enroute to North Africa.
Nearly 9,600 P-39s were built from 1941 to 1944 and the last USAAF unit to be initially equipped with P-39s traded them in for Republic P-47s in the spring of 1944. A little over half of all P-39s were shipped or ferried to the Soviet Union, including most of the last and most improved version, the P-39Q. Called by proponents the "Flying Cannon" and by some others the "Art Deco Failure", Bell's innovative Airacobra has written a significant chapter to our American air power heritage.
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