Bell P-39 Airacobra
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Introduction

The P-39 Airacobra was not exactly one of the best aircraft of the Second World War. It was criticized for its low service ceiling, its slow rate of climb, and its generally poor high-altitude performance vis-a-vis the German Me-109 fighter. However, along with the Curtiss P-40, the P-39 was the only fighter available in quantity to fight against the Germans on the Russian Front during the first six months of the war of the U.S. commitment to Russia. Nevertheless, the Airacobra did give a fairly good account of itself. It was a well-built and reliable aircraft capable of absorbing quite a bit of battle damage and still returning to base.

Early Development

Work on the Bell P-39 Airacobra started in June 1936 when the Bell Aircraft Corporation responded to an Army Air Corps request for a new single-seat fighter design that would be equal to the new European fighters just then beginning to undergo flight test. The Bell design was based around mounting the engine in mid-fuselage, driving the propeller via a ten-foot extension shaft. 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. This design 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 ordered one prototype on October 7, 1937 under the designation XP-39. 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 37-mm T9 cannon designed by the American Armament Corporation. 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. 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. 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 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 first YP-39 (40-027) was flown on September 13, 1940 with the 1090 hp V-1710-37 (E5) engine driving a Curtiss Electric propeller. It differed externally from the XP-39B primarily in having a wider-chord vertical tail. The first few YP-39s were initially flown without armament, but subsequent machines were fitted with a 37 mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 200 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.

Variants

Bell P-39C Airacobra

The first production version of the Airacobra was the P-39C, reflecting the number of changes that had been introduced since the prototype first flew. The first P-39C flew in January of 1941. The P-39C was almost identical to the YP-39, with the exception of the engine, which was a 1150 hp Allison V-1710-35 (E4). The production of the P-39C began in 1940. The Army discovered almost immediately that the P-39C was not combat ready, since it lacked armor and self-sealing tanks. In the event, only twenty Airacobras were actually completed to C-standards. On September 14, 1940 the initial order for 80 P-39Cs was amended to provide for self-sealing fuel tanks. The remaining 60 planes of the order were completed to this standard and were redesignated as P-39Ds.

Specification of Bell P-39C Airacobra

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.

Bell P-39D Airacobra

On September 13, 1940, 394 P-39Ds were ordered. The P-39D differed from the P-39C primarily in having four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun, plus the 37-mm cannon (with increased ammunition capacity of 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. gall. to 100 Imp. gall. This internal fuel could be supplemented by a 72.4 Imp. gall drop tank carried on a strongpoint fitted underneath the fuselage. In place of the drop tank, a 300 lb or 600 pound bomb could be carried. The bulletproof windshield and armor protection added 245 pounds to the weight of the aircraft, causing the climb and altitude performance to suffer. The maximum speed at 15,000 feet dropped to 360 mph.

The first P-39D Airacobras entered service with the USAAC in February 1941 with the 31st Pursuit Group based at Selfridge Field, Michigan. The P-39D was the first to see combat on the Russian Front while 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.

Specification of Bell P-39D Airacobra

Engine: One 1150 hp Allison V-1710-35 twelve-cylinder liquid cooled engine. Performance: Maximum speed 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: 5462 pounds empty, 7500 pounds gross, and 8200 pounds maximum takeoff. Dimensions: Wingspan 34 feet 0 inches, length 30 feet 2 inches, height 11 feet 10 inches, and wing area 213 square feet. Armament: One 37-mm cannon in the nose with 30 rounds. Four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun. One 250 lb, 325-lb, or 500-lb bomb could be carried underneath the fuselage.

Bell P-39E Airacobra

The P-39E was an effort to correct the faults of the P-39D, effectively by going back to the XP-39 prototype and working from there. This aircraft became the P-45 Airacobra.

Bell P-39F Airacobra

The first Airacobra model to be produced in really large numbers was the P-39F, 2095 examples being built. All P-39Fs were powered by the V-1710-85 (E19) engine rated at 1200 hp for takeoff and 1115 hp at 15,500 feet. After completion of the first 166 P-39Ns, the USAAF requested that four fuel cells be removed in order to reduce the internal fuel capacity from 120 to 87 US gallons, and so to reduce the maximum permissible gross weight from 9100 lbs to 8750 lbs. This kept weight down, but unfortunately it also restricted range. Therefore, kits were provided that allowed the four fuel cells to be refitted in the field. The 500 P-39Fs were followed by 900 P-39F-1s. These differed only in some minor internal changes which altered the location of the center of gravity. The last Fs were the 695 P-39F-5s. They differed from earlier Fs in having the total weight of armor reduced from 231 to 193 pounds. A curved armor head plate supplanted the bulletproof glass behind the pilot.

Bell P-39G Airacobra

The P-39G was the last version of the Airacobra to roll off the production lines at Bell. It was also the version which was built in the largest numbers, 4905 P-39Gs being built before production finally ended. The principal difference between the P-39G and earlier version was in the fighter's armament—the four wing-mounted 0.30-inch machine guns were replaced by a single 0.50-inch machine gun mounted in a fairing underneath each wing. The ammunition capacity of the underwing guns was 300 rounds per gun. The two fuselage-mounted 0.50-inch machine guns with 200 rpg, plus the hub-mounted 37-mm cannon with 30 rounds, were retained. The Russians usually had the underwing gun pods removed.

A few P-39Gs were modified into two seaters with dual controls for use as advanced trainers under the designation RP-39G (redesignated TP-39G after 1944). All armament was removed. The second cockpit was placed in front of the original cockpit, and the pilot in this extra cockpit sat under a hinged canopy. The extra cockpit was fitted with only rudimentary controls. The original cockpit retained the same controls and instruments as the standard P-39G. Production of the P-39G finally terminated in August of 1944. Most of the P-39Gs were delivered to Russia. Only a few ended up serving with American units. One of these was the 332nd Fighter Group which took on 75 P-39Gs in Russia during February 1944. After only two months, these Airacobras were replaced by P-47s, finally retiring the type from US service.

Specifications of the P-39G

One Allison V-1710-85 engine rated at 1200 hp at sea level and 1125 hp at 15,500 feet. Maximum speed 330 mph at 5000 feet, 357 mph at 10,000 feet, 376 mph at 15,000 feet. Climb to 5000 feet in 2.0 minutes. Climb to 20,000 feet in 8.5 minutes. Maximum range (clean) was 525 miles at 20,000 feet at 250 mph. With one 145.7 Imp gal drop tank, range was 1075 miles at 196 mph. Service ceiling was 35,000 feet. Weights were 5645 pounds empty, 7600 pounds normal loaded, 8300 pounds maximum loaded. Dimensions: Wingspan 34 feet 0 inches, length 30 feet 2 inches, height 12 feet 5 inches, wing area 213 square feet.

Bell P-400 Airacobra

The P-400 was originally intended for the Royal Air Force that ordered no less than 675 of them. The original P-400 had the slower-firing 37-mm cannon replaced with the faster-firing and more reliable 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). Following the Halifax-Butler coup, this orderw as cancelled and the aircraft reordered as P-45s. However, 170 had been completed and these lacked an owner. Unlike the Curtiss Hawk 75s and Hawk 81s, there was no real demand for these aircraft. Eventually, they were disarmed and used by the USAAF as advanced trainers.

P-39 Combat Career

Satirists have suggested that the P-39 was fortunate in that it fought over the Russian Steppes that were flat so it didn’t have to climb anywhere. In fact, there is some truth to that; the P-39 was at its best at low altitude which suited the conditions of the Russian Front down to the ground. Few there flew much above 10,000 feet, 15,000 was considered high altitude. At those altitudes, the P-39 was at its best. Russian and American pilots considered their P-39Fs and Gs to be equal to the Me-109F and superior to the Me-109E.

The Russians modified their P-39s by removing the wing guns completely, believing this significantly improved the agility of the aircraft. Many American pilots followed this example. The armament of the P-39 was controversial; Russian pilots regarded the combination of two 0.5 inch machine guns and a 37mm cannon as being very effective while Americans regarded it as being almost useless due to the difference in trajectory of the two weapons. The reason for this difference of opinion was interesting; Russian-built fighters had very poor gunsights so their pilots got in very close, often within a few tens of yards, before firing. Then, they would fire a burst from the .50s to get on target followed by one or two rounds from the 37mm. American pilots were used to the high-quality gunsights on their fighters and fired from longer ranges where all 30 of their 37mm rounds would be expended to get a single hit. Of course, the big shell of the 37mm meant that the one hit was decisive. Once American pilots got into the habit of firing from extremely close range, their kill rate – and their opinion of the P-39s armament - soared.

Contrary to many statements in reference sources, the P-39 was never used by either the Russians or the Americans as a primary ground attack aircraft. It always served as a fighter, covering other ground attack aircraft. One P-39G, flown by Lieutenant Paul Lazaruski achieved the notable feat of shooting down an Me-262 in air combat.


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