Bell XP-39 Airacobra

Last revised June 19, 1999

The Bell P-39 Airacobra was not exactly one of the best aircraft of the Second World War. Many historians list the P-39 as being a failure. It was criticized for its low service ceiling, its low service ceiling, its slow rate of climb, and its generally poor high-altitude performance vis-a-vis the Japanese Zero fighter. However, along with the Curtiss P-40, the P-39 was the only fighter available in quantity to fight against the Japanese advance during the first six months of the war.

Its poor high-altitude performance was a result of a critical decision made early in the Airacobra's design process, namely, the decision to remove the turbosupercharger. Nevertheless, the Airacobra did give a fairly good account of itself, all things considering. It was a well-built and reliable aircraft capable of absorbing quite a bit of battle damage and still returning to base. It excelled in the low-altitude ground support, its heavy armament making it a good tank buster, especially in service with the Soviet Air Force.

A total of 9589 Airacobras were built. This number is based on a count of serial numbers. Some other sources list 9558 being built.

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. The Bell corporation was responding to a 1936 Army Air Corps request for a new single-seat fighter design, one which would be equal to the new European fighters just then beginning to undergo flight test.

Woods and Poyer 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 company designation was Model 12. The serial number was 38-326. The powerplant of the XP-39 was the 1150 hp Allison V-1710-17 (E2) l2-cylinder liquid-cooled Vee which was fitted with a B-5 two-stage turbosupercharger on the portside of the central fuselage. A somewhat smaller 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 T9 cannon designed by the American Armament Corporation, a subsidiary of the Oldsmobile automobile manufacturer. 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 cockpit was fairly easy to enter and exit, but the doors had a tendency to fly open in midair at high speed if improperly secured.

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 Army dictated that a tricycle landing gear be fitted. The gear incorporated a non-steering, self-castoring nosewheel that retracted upwards and back. The main wheels retracted inward to wells underneath the wing. All wheels were constructed of magnesium alloy and used tubed tires.

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, 1939, Bell test pilot James Taylor being at the controls. 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 of the YP-39s were 40-027/038. 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 turbosupercharger, and its replacement by a single-stage geared 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 (E2) of the XP-39 was replaced by a V-1710-37 (E5) engine rated 1090 hp 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 turbosupercharger was to have fateful consequences for the future of the Airacobra. Although the Allison engine was more reliable and more easily service when the turbosupercharger 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 XP-39B was damaged in a belly-landing at Wright Field, Ohio on January 6, 1940. It was repaired and resumed flying, later to be demonstrated at Bolling Field in Washington, DC. Unfortunately, the XP-39B was destroyed in an accident after only 28 flying hours.


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