Bell P-39 Airacobra
Placing the engine behind the pilot has become common since jet propulsion, but this arrangement was first
introduced to American fighters by Bell’s Airacobra. Previously, it had been seen abroad in Westland and Koolhoven designs, but the P-39 was the first such design in mass production.
Several advantages became apparent when the engine was moved back to the center of gravity. A slimmer nose allowed better streamlining and visibility, and made room for heavier armament and the retracted nose wheel of a tricycle landing gear. This undercarriage offered better ground handling and permitted higher landing speeds, which in turn led to smaller wings -not an unmixed blessing since the higher wing loading handicapped climb and maneuverability at high altitudes.
Another disadvantage was that the crowded interior limited the space available for fuel, but that did not seem serious since the Air Corps specification X-609, dated March 19, 1937, called for an single-engine interceptor pursuit with cannon and turbosupercharger, designed to meet attacking enemy bombers. Designer Robert Wood had experience with the new Allison engine in Bell Model 2, the XA-11A modification that first tested that power plant. The Buffalo company offered designs planned with a 25-mm cannon in the propeller shaft. Bell Model 3 had the cockpit behind the engine as usual, but Model 4, dated June 3, 1937, placed the cockpit ahead of the Allison.
Rival Curtiss Model 80 and Seversky AP-3 high-altitude interceptor proposals were also submitted. Model 80 was based on the XP-37 and offered to put a pair of 23-mm Madsen cannon under the wings, while Model 80A proposed a single 23-mm Madsen in the nose. Seversky’s AP-3 offering had two .50-caliber guns in the nose and two in the wings, but AP-3A had just the nose guns, but added a 25-mm cannon two feet below the centerline.
The Army chose Model 4 over the more conventional layouts, and a prototype, designated XP-39, was ordered October 7, 1937. An Allison V-1710-17 with a ten-foot extension drive shaft to the reduction gear box behind the propeller and a B-5 turbosupercharger under the engine delivered 1,150 hp.
Provision was made for two synchronized .50-caliber guns above a 25-mm Hotchkiss gun with its barrel pointed through the propeller hub. Since that weapon never appeared, by December 31, 1937, it was decided to use the new 37-mm Colt T-9 cannon. (Actually, no weapons were ever installed on the prototype during flight tests.)
The original X-609 specification called for a minimum top speed of 290 mph at sea level and 360 mph at 20,000 feet, but 400 mph was the desired goal. Bell guaranteed a top speed of 330 mph at sea level and 400 mph at 20,000 feet, climbing to that altitude in five minutes, but this was for a weight of 3,995 pounds empty and 5,550 pounds gross.
After the XP-39 was moved to Wright Field in December 1938, Army inspectors listed the actual prototype weight as 4,545 pounds empty and 6,104 pounds gross. Air intakes protruded behind the engine, the left side to cool the oil radiator, the right side for the carburetor and for the turbosupercharger intercooler, The first test flight was made at Wright Field by James Taylor on April 6, 1939. Drive shaft vibration curtailed tests and excessive airframe drag limited speed to nearer the minimum than the desired level. While detailed reports are not available, one Army chart stated top speed was 365 mph at 20,000 feet.
On April 27, 1939, without waiting for complete XP-39 tests, a service test contract was approved for 12 YP-39s and one YP-39A, the first to be delivered in ten months. Originally, the YP-39 (Bell Model 12) was to have the same turbosupercharged engine as the prototype, do 375 mph at 20,000 feet, climb to that altitude in six minutes, and have a 41,300-foot service ceiling. The proposed YP-39A’s V-1710-31 would utilize internal geared supercharging instead of a turbosupercharger. While top speed at 20,000 feet would be reduced to 360 mph, the lighter weight and reduced drag was expected to improve low-altitude performance.
After unsatisfactory trials, the XP-39 prototype went to NACA’s full-scale wind tunnel on June 6, 1939. That study suggested several modifications: the front wheel should no longer protrude below the nose, smaller main wheels could be enclosed in the wings, the cockpit hood lowered, oil cooling duct moved to the wings, and the turbosupercharger replaced by an internal geared system. If these changes were made, Larry Bell wrote the Material Division on August 30, the P-39 could attain 400 mph at 15,000 feet with 1,150 hp.
By this time the Air Corps no longer saw the Bell as a high-altitude partner of the P-38, so this proposal was accepted and the XP-39 was shipped back to the factory for modifications. A carburetor air scoop was placed behind the cockpit, the turbosupercharger and side intakes removed, and the original engine replaced by an interim V-1710-37 rated at 1,090 hp at 13,200 feet with a single-stage geared supercharger.
First flown November 25, 1939, the revised prototype was designated XP-39B and proved satisfactory enough for its features to be incorporated into all 13 service-test aircraft. A guaranteed top speed of 375 mph was promised at 13,200 feet.
“We have eliminated a million and one problems by the removal of the supercharger...” rejoiced Larry Bell on January 17, 1940. This crucial decision to omit the turbosupercharger improved low-altitude performance, but seriously crippled high-altitude performance, a fault shared by earlier P-40s.
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