The Bell P-39 Aracobra was very much like any fighter of the time except that it's engine was mounted amidships, behind the pilot. The tricycle type landing gear was also somewhat new at the time.

First flown in 1938 and becoming operational in 1941, over 9,500 were built but over half of them were sent to Russia during the lend-lease program. The airplane was not terribly popular with US pilots and was quickly surpassed by aircraft such as the P-47 Thunderbolt and the P-51 Mustang.

Chuck Yeager flew a P-39 during his early training years.

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Bell P-39 Airacobra

By Joe Baugher

The Bell P-39 Airacobra had its origin in June 1936 when the Buffalo, New York-based Bell Aircraft Corporation's design team, headed by Robert J. Woods and Harland M. Poyer, began the design of a single seat fighter. They conceived the idea of mounting the engine in mid-fuselage, driving the propeller via a ten-foot extension shaft. Such an arrangement was not exactly new, having been tried earlier by the experimental Westland F.7/30 biplane and by the Dutch Koolhoven F.K.55 monoplane. Among the potential advantages offered by such an arrangement was the possibility of superior maneuverability, since the weight of the plane would be more nearly concentrated at the center of gravity. In addition, it would facilitate the installation of a heavy nose armament, since the armament could be mounted near the centerline, minimizing the effects of recoil forces. It would also offer good visibility for the pilot, and would permit the installation of a tricycle undercarriage.

Bell's original proposal was to place the pilot behind the engine, forcing the cockpit very far to the rear and making the proposed aircraft look a lot like the Curtiss XP-37. A mockup with this configuration was built with this configuration and was given the company designation Bell Model 3. However, the problem of visibility over the engine eventually forced Bell engineers to move the pilot ahead of the engine, and a revised mockup, given the company designation Bell Model 4, was used as the basis of a formal submission to the USAAC on May 18, 1937. The Bell submission promised a top speed of 400 mph at 20,000 feet and a gross weight of only 5500 pounds.

The USAAC was sufficiently intrigued by the proposal that they ordered one prototype on October 7, 1937 under the designation XP-39. The serial number was 38-326. The powerplant of the XP-39 was the 1150 hp Allison V-1710-17 l2-cylinder liquid-cooled Vee which was fitted with a B-5 supercharger on the portside of the central fuselage. A radiator/oil cooler scoop was fitted on the other side of the fuselage. Provision was made for two 0.50-inch machine guns in the forward fuselage and one 25 mm cannon firing through the propeller shaft. In December 1938, it was decided to replace the 25-mm cannon by an even heavier 37-mm Oldsmobile cannon. In the event, no armament was actually fitted to the first prototype.

The fuselage was of all-metal construction and had an oval cross section. It was built up in two main sections: the forward section that included the engine mounts and wing center sections which was built around two longitudinal strength member and the cockpit deck plate and the semi-monocoque rear section built up of bulkheads and stringers. The tail had all-metal fixed surfaces and fabric-covered control surfaces. The wing panels joined at a point 22 inches out from the centerline. The wing structure had three spars, and the Frise-type ailerons were fabric covered. The inboard wing trailing edge carried split flaps.

The cockpit canopy had six transparent panels, and offered exceptional all- round visibility. An unusual feature of the Airacobra was the automobile-type door on each side of the cockpit, which allowed easy access by the pilot to the cockpit from either side. The doors even had roll-down windows! The engine behind the pilot's seat drove the propeller by means of a driveshaft mounted under the pilot's seat. Early Airacobra pilots feared what might happen if the driveshaft were to break loose or were to start whipping around inside its mount. However, in practice there were no more problems encountered with this driveshaft than with more conventional arrangements. However, there were problems with the complex nose-mounted reduction gear, which caused reliability problems and resulted in fairly low serviceability rates as compared with other fighters. The exhaust system lead to six ejector stubs on each side of the fuselage (changed to 12 stubs on later models).

The fuel was carried in tanks totaling 60 gallons in capacity in the wing outer panels. There was a reserve tank of 30 gallons in the left wing.

The XP-39 was completed at Bell's Buffalo plant and shipped by truck to Wright Field in Ohio. It was reassembled there and flown for the first time on April 6, 1938. The performance was excellent, the prototype reaching a speed of 390 mph at 20,000 feet. An altitude of 20,000 feet could be attained in 5 minutes, quite impressive climbing performance for the time. Service ceiling was 32,000 feet. Weights were 3995 pounds empty, 5550 pounds gross, and 6304 pounds maximum takeoff. The USAAC was quite impressed with the performance, perhaps ignoring the fact that the XP-39 carried no military equipment or armament and was thus much lighter that that which could be anticipated for production models.

The initial XP-39 tests went quite well, and the only problem that was encountered being some engine overheating difficulties. At first, it was thought that the overheating problems might be due to bad ventilation, and the left-hand supercharger and the right-hand radiator intakes and exhausts were both enlarged. However, this did not cure the problem, and it was found later that the problem was easily cured by a simple change in the structure of the oil system. With this change, the XP-39 was accepted for production with an initial order for twelve service-test YP-39s (Bell Model 12) and one YP-39A in April 1939. Serials were 40-027/039. The YP-39A (40-039) was to have been powered by a high-altitude V-1710-31 engine of 1150 hp.

In the meantime, the XP-39 underwent a series of full-scale wind-tunnel tests in NACA's wind tunnel at Langley Field, Virginia. After the tests, the XP-39 was returned to Buffalo for revisions. The rebuilt XP-39 emerged as the XP-39B. Most of the changes were improvements in the streamlining of the airframe. The cockpit canopy was changed to a longer and lower shape. Changes were made to the wheel doors. The oil cooler and radiator intakes were moved from the fuselage right side to the wing roots. The wing span was decreased from 35 feet 10 inches to 34 feet, and length was increased from 28 feet 8 inches to 29 feet 9 inches.

The most serious change, however, was the elimination of the supercharger. This change was a result of a shift in philosophy on the part of the USAAC. The USAAC believed that the widths of the Atlantic and Pacific Oceans made the USA virtually immune from high-altitude attack by enemy bombers. Therefore, the development of high-altitude interceptors was curtailed in favor of strike fighters optimized for low-level close support. The 1150 hp V-1710-17 of the XP-39 was replaced by a non-supercharged V-1710-39 engine rated at an altitude of 13,300 feet. The carburetor air intake was mounted in a dorsal position just behind the cockpit, where it was to remain throughout the Airacobra production run.

The XP-39B resumed flight trials on November 25, 1939. Empty weight had grown from from 3995 lbs to 4530 lbs, and normal gross weight was up to 5834 pounds from 5550 pounds, and the aircraft STILL didn't have any armament. The removal of the supercharger was to have fateful consequences for the future of the Airacobra. Although the Allison engine was more reliable and more easily service when the supercharger was eliminated, the engine only performed well at low and medium altitudes and lost power quite rapidly at altitudes over 15,000 feet. Even in spite of the improved streamlining, the XP-39B suffered a severe degradation in high-altitude performance. Maximum speed fell from 390 mph at 20,000 feet to 375 mph at 15,000 feet, and it now took 7.5 minutes to reach 20,000 feet rather than five minutes. However, there was an increase in low-altitude maneuverability because of the reduced wing span, and the decrease in low-altitude performance was only marginal.

The USAAC was satisfied with the low-altitude performance of the Airacobra, and directed that the twelve YP-39s be completed without superchargers. It was decided to embody the changes made in the XP-39B in the service test machines. The YP-39A was originally to have been delivered with the high-altitude V-1710-31 engine, but the change in USAAC philosophy caused this plan to be abandoned and this aircraft was delivered as a standard YP-39.

The first YP-39 (40-027) was flown on September 13, 1940 with the 1090 hp V-1710-37 engine. It differed from the XP-39B primarily in having a larger vertical tail. Full armament was fitted, comprising the 37 mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 300 rounds per gun, and two 0.30-inch machine guns with 500 rounds per gun. All of these guns were mounted in the nose. Some armor protection was provided for the pilot. Empty and normal loaded weights rose to 5042 pounds and 7000 pounds, respectively. In comparison, the XP-39 prototype had a normal loaded weight of only 5550 pounds. Consequently, the performance of the YP-39 dropped to a maximum speed of 368 mph at 15,000 feet. An altitude of 20,000 feet could be attained in 7.3 minutes. Service ceiling was 33,300 feet.

In spite of the degradation in performance, the USAAC was generally pleased with the Airacobra, and an initial order for 80 production examples (Bell Model 13) was issued on August 10, 1939. Serials were 40-2971/3050. For some reason, the USAAC allocated the new designation of P-45 to these machines, even though they were almost identical to the YP-39 service test aircraft. However, in the political climate of 1940, it was virtually impossible for the USAAC to acquire any new aircraft. But it could order more examples of an already-existing model. Consequently, the designation of the Airacobra was changed to P-39C prior to the delivery of the first aircraft.

The first P-39C (Ser No 40-2971) flew in January 1941. In the event, only twenty Airacobras were actually completed to C-standards--serial numbers 40-2971/2990. The rest, serial numbers 40-2991/3050, were completed as P-39Ds.

The P-39C was powered by a 1150 hp Allison V-1710-35 engine. Weights were 5070 pounds empty, 7075 pounds gross (combat weight), and 7300 pounds maximum takeoff. Maximum speed was 379 mph at 13,000 feet. An altitude of 12,000 feet could be reached in 3.9 minutes. Service ceiling was 33,200 feet. Armament was one 37-mm cannon, two 0.50-inch and two 0.30-inch machine guns, all in the nose.

Combat reports coming back from Europe led the USAAC to conclude that the P-39C was inadequately armed and armored for its intended ground-support role. On September 13, 1940, 344 P-39Ds were ordered. The serials were 41-6722/7115. These differed from the P-39C in having four wing-mounted 0.30-inch machine guns with 1000 rounds per gun, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun, plus the 37-mm cannon with 30 rounds. Bulletproof windshield panels were added, and some armor protection for the pilot was provided. Self-sealing fuel tanks were introduced, which reduced internal fuel capacity from 141.5 Imp. gal. to 100 Imp. gal. This internal fuel could be supplemented by a 72.4 Imp. gal drop tank. In addition, a very small dorsal fin was added just ahead of the rudder. The bulletproof windshield and armor protection added 245 pounds to the weight of the aircraft, causing the climb and altitude performance to suffer.

The engine of the P-39D was the same as that of the P-39C, a 1150 hp Allison V-1710-35. Maximum speed of the P-39D was 309 mph at sea level, 335 mph at 5000 feet, 355 mph at 10, 000 feet, 368 mph at 12,000 feet, and 360 mph at 15,000 feet. An altitude of 5000 feet could be reached in 1.9 minutes. It took 5.7 minutes to reach an altitude of 15,000 feet and 9.1 minutes to reach 20,000 feet. Service ceiling was 32,100 feet. Maximum range (clean) was 600 miles at 10,000 feet at 231 mph. Range with one 145.7 Imp gal drop tank was 1100 miles at 196 mph. Weights were 5462 pounds empty, 7500 pounds gross, and 8200 pounds maximum takeoff. Dimensions were wingspan 34 feet 0 inches, length 30 feet 2 inches, height 11 feet 10 inches, and wing area 213 square feet.

On the day after the P-39D order, the initial contract for the P-39C was modified, and the 21st and subsequent machines on the P-39C order (serials 40-2991/3050) were ordered completed as P-39Ds.

The first P-39D Airacobras entered service with the USAAC in February 1941. The P-39D (along with the P-400) was the first to see combat in US service. There were a great many weaknesses in the Airacobra, apart from the general problem of poor high-altitude performance. Among these were the lack of gun heaters which caused the guns to freeze up and jam at altitudes over 25,000 feet, the lack of hydraulic chargers which made it difficult to charge the guns in the air, and the forward gear box just behind the propeller which had a tendency to throw oil.

All of the media hype surrounding the spectacular performance of the XP-39 prototype had caught the attention of the British Direct Purchase Commission which had visited the USA in 1940 in search of combat aircraft. Seduced by promises of 400 mph top speed, a tricycle undercarriage, heavy cannon armament, and high climb rates, the British ordered 675 examples of the Airacobra. Unfortunately, Bell's glossy advertising brochures did not distinguish between the performance of a lightly-loaded, unarmed, highly-polished experimental prototype and a production fighter heavily-loaded with military equipment and armament, and the British were to rue the day that they ever looked at an Airacobra.

In 1940, the British were desperate for combat aircraft and were willing to consider just about anything that had wings, irrespective of how poor its performance might be. Consequently, when Bell submitted specifications to the British Direct Purchase Commission for a fighter with a top speed of 400 mph, a ceiling of 36,000 feet, and a range of 1000 miles, the Commission literally salivated on the spot and ordered 675 Bell Model 14s sight unseen on April 13, 1940.

The RAF model was at first named Caribou, but the American name of Airacobra was adopted in July 1941. The British Airacobra was virtually identical to the American P-39D, but the 37-mm cannon was replaced with a Hispano 20-mm cannon with 60 rounds. Two 0.50-inch machine guns were mounted in the fuselage, and four 0.30-inch machine guns were mounted in the wings. The engine of the Model 14 was the 1150 hp Allison V-1710-E4 (-35). The British serials of the Airacobras were AH570/AH739, AP264/AP384, BW100/BW183, and BX135/BX434.

Bell began test flying the first Model 14 Airacobra I in April 1941. It carried the British serial number AH570. Tested on the second British Airacobra (AH571) was a revised rudder of more angular shape and less area. Although the aircraft was delivered to England in this form, this rudder was not adopted as standard. A very small dorsal fin just ahead of the rudder became a standard feature of the RAF Airacobras and was also a distinguishing feature of the American P-39D and subsequent versions.

President Roosevelt signed the Lend-Lease Act on March 11, 1941, permitting large quantities of war supplies to be transferred to the Allies. Among the early requisitions under Lend-Lease were three P-39Cs intended for "war tests" plus a batch of 150 Airacobra IAs--the A suffix being used to distinguish between Lend-Lease and Direct Purchase machines, which were otherwise identical. In the event, only the three P-39Cs were ever delivered as British machines, and were assigned the serials DS173/DS175. The P-39Cs could be distinguished from the British Airacobras by the four machine guns in the nose and the lack of wing guns.

The first of these P-39Cs actually arrived at RAF Colerne on July 3, 1941, followed by the other two the next day. It made its first test flight in England on July 6. However, during trials at Duxford, the performance proved disappointing. Although the test pilots praised the general ease of handling of the aircraft, the maximum speed was a shocking 33 mph lower than that anticipated. The fighter proved to be definitely inferior to the Hurricane and Spitfire in climb rate and ceiling, and the 750-yard takeoff run of the Airacobra excluded its operation from some smaller fighter airfields. There was universal shock and dismay among the RAF personnel. What had gone wrong? Bell Aircraft executives later sheepishly admitted that their performance figures had been based on the unarmed and unequipped XP-39 prototype, not on the armed and equipped P-39C.

The first British-purchased Airacobras began arriving at Colerne before the end of July, joining the three P-39Cs already there. Deliveries of the Airacobra to Britain had to be made by sea, since the Airacobra lacked the range to make the Atlantic crossing. By the end of September, eleven machines had been received. No. 601 "County of London" Squadron was selected to be the first Fighter Command squadron to equip with the Airacobra.

No. 601 Squadron pilots found numerous flaws and weaknesses during their initial work-up with the the Airacobra. Some of them were a question of improving operational efficiency and pilot comfort, but others were considered essential to make the aircraft operational. Numerous modifications were made in the field in an attempt to make the aircraft suitable for combat. A master valve was introduced to allow oxygen to be turned on from the cockpit. The gunsight mas modified to improve forward visibility. Changes to the ammunition tanks for the wing guns were made. Modifications were made to the cockpit harness release in order to simplify the operation. The IFF (Identification, Friend or Foe) set was removed from behind the pilot, where it obstructed aft view. A throttle control quadrant friction damper was introduced.

The Air Fighting Development Unit received a British Airacobra I on July 30. They subjected it to tests and completed their report on September 22. They found the aircraft to be pleasant to fly and easy to takeoff and land. Controls were well balanced and although heavier than those of the Spitfire at normal speeds, did not increase appreciably in weight at high speeds as they did in the Spitfire. It was difficult to hold the aircraft in a dive at high speeds unless the aircraft was trimmed nose-heavy. During a turn, the Airacobra would give ample warning of a high-speed stall by severe vibration of the whole airframe. Handling in formation and formation attacks was good, although deceleration was poor because of the plane's aerodynamic cleanliness. Take-offs and landings in close formation were not considered safe, since there was considerable difficulty in bringing the aircraft back to its original path after a swing.

The Airacobra I was powered by an Allison V-1710-E4 twelve-cylinder V in-line engine rated at 1150 hp for takeoff. Weights were 5462 pounds empty and 7845 pounds normal gross. Maximum speeds were 326 mph at 6000 feet, 343 mph at 10,000 feet, 355 mph at 13, 000 feet, 341 mph at 20,000 feet. Initial climb rate was 2040 feet per minute. With an internal fuel capacity of 100 Imp gal the Airacobra had an endurance of 1 hour 20 minutes at maximum continuous cruising speed at 6000 feet, 1 hour 5 minutes at 12,000 feet, and 1 hour 35 minutes at 20,000 feet. The true airspeeds at these altitudes were 287 mph, 327 mph, and 308 mph, respectively. Under most economical cruise conditions, the endurance increased to 3 hours 20 minutes, the relevant speeds being 183 mph at 6000 feet, 217 mph at 12,000 feet, and 215 mph at 20,000 feet.

Under maximum continuous climb conditions, it took 15 minutes to reach 20,000 feet. The operational ceiling was considered to be about 24,000 feet, although there was a marked decrease in performance above 20,000 feet. At the Airacobra's rated altitude of 13,000 feet, it was 18 mph faster than the Spitfire VB. However, the speed fell off rapidly above that height, and the two planes were almost exactly matched at 15,000 feet. At 20,000 feet, the Spitfire VB was 35 mph faster and at 24,000 feet it was 55 mph faster. The ground run of the Airacobra during takeoff was 2250 feet, as compared with 1470 feet for the Hurrican II and 1590 feet for the Spitfire V.

The AFDU also did some comparative dog-fighting tests with the Airacobra against a Spitfire VB and a Messerschmitt BF 109E.

The Airacobra and the Bf 109E carried out mock dog-fighting at 6000 feet and 15,000 feet. The Bf 109E had a height advantage of 1000 feet in each case. The Bf 109, using the normal German fighter tactics of diving and zooming, could usually only get in a fleeting shot. The Bf 109 could not compete with the Airacobra in a turn, and if the Bf 109 were behind the Airacobra at the start, the latter could usually shake him off and get in a burst before two complete turns were completed. If the Bf 109 were to dive on the Airacobra from above and continue the dive down to ground level after a short burst of fire, it was found that the Airacobra could follow and catch up to the Bf 109 after a dive of over 4000 feet. When fighting the Bf 109E below 20,000 feet, the Airacobra was superior on the same level and in a dive.

A similar trial was carried out against a Spitfire V. Although the Airacobra was faster than the Spitfire up to 15,000 feet, it was outclimbed and out-turned by the Spitfire. Unless it had a height advantage, the Airacobra could not compete with the Spitfire. If on the same level or below, at heights up to about 15,000 feet, the Airacobra would have to rely on its superior level and diving speeds and its ability to take negative "G" without the engine cutting out. Above 15,000 feet, the Airacobra lost its advantage in level speed.

The Airacobra was considered to be very suitable for low altitude operations because of the excellent view and controllability, and it was fully maneuverable at speeds above 160 mph. It was not difficult to fly at night, but the exhaust flames could be seen by another aircraft flying three miles to the rear. The flash from the nose guns was blinding, and could cause the pilot to lose not only his target but also his night vision. Firing of the nose guns caused the buildup of carbon monoxide contamination in the cockpit, and this could reach a lethal level very quickly. The guns were fairly inaccessible, and maintenance was troublesome.

By the end of September, No. 601 Squadron had received permission to take its Airacobras into action. On October 9, two Airacobras took off from RAF Manston and flew across the Channel on a "rhubarb"--a code name for a small-scale raid by fighters against targets of opportunity. On this raid, they shot up an enemy trawler near Gravelines. The next day two Airacobras visited the same area, but found no targets. On October 11, two aircraft flew to Gravelines and Calais and hit some enemy barges and then three Airacobras flew to Ostend, but no targets were found.

After these four missions, the RAF Airacobras were taken off operations because of difficulties encountered with the compass. The compass was too close to the guns in the nose, and when the guns were fired, the compass got thrown out of alignment. Deviations of anything from 7 degrees to 165 degrees were recorded. Without a reliable compass, pilots tend to get themselves lost. In December of 1941, the Airacobra was officially withdrawn from operational service with the RAF.

In spite of the problems with the compass and the need for flame dampers for the exhaust and flash suppressors for the nose guns, the RAF concluded that the Airacobra would make an excellent day fighter at altitudes below 20,000 feet and was well suited for the ground-attack role. However, before these plans could be implemented, a decision was made to divert the bulk of the British Airacobra contract to Russia.

By the time this decision was made, production of British-contract Airacobras had reached four a day at Bell's Buffalo plant. The initial contract for 170 planes (RAF serials AH570 thru AH739) had been completed before the end of September, and all but six of these planes had actually been shipped to Britain. However, many of them remained in their crates and were shipped directly to the Soviet Union without being opened. Somewhere between 80 and 100 Airacobras were assembled and flown in Britain by the end of 1941. They were gathered at maintenance units for final modification before being re-crated and shipped to the Soviet Union during 1942. Some In all, the Soviet Union received 212 of the British Airacobras (some of them shipped direct from the USA), but 49 more were lost at sea en route.

No 601 Squadron relinquished its 13 Airacobras in March of 1942 in favor of Spitfires. One Airacobra was fitted with an arrester hook and was used for deck landing trials at the RAE at Farnborough.

After Pearl Harbor, the USA found itself in desperate need of aircraft to stem the Japanese onslaught in the Pacific. Consequently, nearly 200 of the British direct-purchase Airacobras still in the USA were promptly requisitioned by the USAAC. Although they were similar to the USAAC's P-39Ds, they were not identical and were known by the USAAC under the non-standard designation of P-400. The P-400 designation had, in fact, been associated with the British Airacobras for contractual purposes as early as August 1941. The USAAC P-400s retained their original British serial numbers. Most of these planes were used for training stateside, but some of them were rushed to the Southwest Pacific in an attempt to stem the onrushing Japanese advance.

The P-400s also saw some use closer to Britain. 179 of the Airacobras sent to Britain were re-acquired by the USAAF and were sent to North Africa to join the Twelfth Air Force.

Sources:

War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.

The American Fighter, Enzo Anguluci and Peter Bowers, Orion Books, 1987.

United States Military Aircraft since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian Institution Press, 1989.

P-39 Airacobra in Action, Ernie MCDowell, Squadron/Signal Publications, 1980

The Calamitous 'Cobra, Air Enthusiast, August 1971.

Joe Baugher

AT&T Bell Laboratories

2000 North Naperville Road

Naperville, Illinois 60566-7033