Air Tech presents the Bell P-39 Airacobra

by Martin Caidin

From the biting cold and howling winds of Russia's frigid Northern battlefront to the incessant heat and blanketing humidity of sand and desert, Bell Airacobras have roared into the whirling cataclysm of aerial combat to sound a devastating call of battle from their booming nose cannon. Whether defending Allied forces from fierce enemy attacks as at Stalingrad, or sweeping over the shattered remnants of blasted Panzer divisions, the Airacobra has left its fatal mark. Axis airmen have paid the cocky little fighter its highest compliment in their advice to green combat pilots — "the 'Cobra has poison in its nose — stay away from the front end!"

In its eventful period of combat the Airacobra has blasted everything from locomotives on the ground and destroyers on the sea to fighters and bombers in the air with as heavy an array of armament as designers ever packed into a fighter of comparable size. Unorthodox in the fact that the Airacobra, type P-39 to the USAAF and P-400 Airacobra to our Allies, was conceived around an aircraft cannon, the basic design of this ship developed to the point where it has been acclaimed as one of the finest of this war. Its most striking feature is the position of the Allison engine behind the pilot, necessary because the heaviest piece of ordnance ever mounted on a single-engine plane, the 37-mm cannon, filled all the space in the nose of the aircraft. This unusual arrangement provides many other advantages: the mass of the engine, being closer to the center of gravity, gives unexcelled performance in turning radius; the design allows visibility almost as great as the pusher-type aircraft without incurring the aerodynamic disadvantages of having the propeller close to the tail.

Excellent streamlining is accomplished, for the nose is no larger than the diameter of the moderate propeller spinner. The cockpit canopy flows so neatly into the sweeping contours of the fuselage that designers have acclaimed the Airacobra as one of the best-designed airplanes in the world, Even Britain's famed Spitfire series is no cleaner in design.

Latest in the Airacobra series, the model P-39Q has an armament of either four or six .50-cal machine guns and one 37-mm nose cannon. Initial version of the P-39Q has two .50-cal machine guns mounted on the nose, synchronized to fire through the propeller arc, which supports the heavy cannon firing thirty high explosive or armor-piercing shells through the propeller hub. Two .50-cal guns are located in each wing in buried positions with blast tubes of the muzzles protruding, fire incendiary or armor-piercing bullets. Second version of the Q mounts one .50-cal gun underslung in a basket-like position firing freely outside the propeller arc. The wing armament arrangement closely resembles the wing cannon armament of the Messerschmitt Me-109G4 fighter. However, as armament varies widely and continuously according to specific demands and sectors of combat, there is no standard arrangement of armament in the Airacobra.

Range of the small fighter varies continuously, depending upon the load carried, altitude and speed of the aircraft on mission, weather conditions, and whether or not the aircraft will have to enter combat during flight. But Airacobras have been seen escorting Fortresses over Europe, evidence that a minimum combat range of several hundred miles is present.

While the maximum speed of the Airacobra is 377 mph at 15,500 feet in older versions, it has recently been boosted to 394 mph with full combat equipment, a worthy accomplishment when one considers that its inline Allison engine develops only 1,000 hp at 14,000 feet altitude. The Airacobra has been recognized as the fastest fighter plane in the world at sea-level, even eclipsing the zero-altitude performance of the special-wing Mustang.

Up to altitudes of 16,000 feet, where it was designed to fight, and not above, the 'Cobra's performance surpasses that of the magnificent Spitfire and the tough Luftwaffe Messerschmitt Me-109G4 or Focke-Wulf FW-190A4. Above 16,000 feet, performance of the American fighter drops like a plummeting stone, even though the service ceiling of 33,000 feet can be attained with comparative ease.

The Airacobra can turn within a shorter radius than the Mitsubishi or Nakajima Zero fighter series, but is outclimbed and outdived by its nimble Oriental adversary. The P-39Q will soon be giving way to its more powerful and deadlier successor, the Bell P-63 Kingcobra, which resembles its smaller cousin in great detail.

Squatting in its familiar tail-high, nose-down position, the Airacobra is a happy sight to any crew chief. The design of the fighter permits easy, quick access to the armament and engine, also provides immediate overhauling. The engine can be dismantled in record time, or overhauled easily while still mounted.

The 1,150-hp in-line Allison V-1710-E4 direct-drive twelve cylinder Vee Prestone-engine has been replaced by the Allison V-12 1,200-hp V-1710-85 engine. The following engine information pertains to the V-1710-E4 type.

Mounted behind the pilot within the fuselage, the Allison drives a tractor airscrew through an extension shaft. An independent reduction gear box attached to the front end of the fuselage longitudinal beams drives the hollow airscrew hub with its three-bladed prop. Although normal engine design provides for the power to be fed by a 2:1 reduction gear hookup in which the propeller shaft is supported by thrust bearings at the front end, at the rear by large roller bearings, here the gear housing is bolted directly to the front of the crankcase. The eight-foot shaft through which power is transmitted to spin the prop consists of two flanged sections, 2.5" diameter and 48" long, is supported at the center by a self aligning ball-bearing mount. Two aluminum alloy castings of the reduction gear support the propeller shaft, thrust bearings, reduction and pinion gear. Extension shaft drive is transmitted through a flexible splined coupling which is bolted to a supporting flange on the front end of the crankcase. The Prestone radiator is located in the port wing root, the oil radiator directly opposite in the starboard wing root, and both are provided with temperature-control shutters. Built integrally into the wings, one on each side of the fuselage, the two fuel cells are bullet-resisting self-sealing tanks.

The Airacobra is a low-wing cantilever-type monoplane. The wing section is NACA 0015 at the root and modified NACA 23009 at the tip. The whole all-metal structure with stamped or pressed metal ribs and bulkheads is covered with a flush-riveted stressed skin. Ailerons are metal frames and fabric covered, are differentially controlled and have modified "Frise" type nose-balanced and venturi-shaped slots. The arrangement provides a variation in the slot from wide open at zero angular deflection to a full closed in the up or down positions and gives effective control and feel for small displacements of the ailerons and reduced operating loads for large angular deflections. The split-trailing edge flaps are centered between the ailerons and fuselage.

Tail unit is of a cantilever monoplane type. All trim tabs are movable and fixable directly from the cockpit while in flight or on the ground. Fixed surfaces are all-metal and the movable control units have metal frames and fabric covering. A single fin and rudder plus the stabilizers and elevators comprise the complete empennage.

The landing gear is of a fully-retractable tricycle type. Retraction is electrical with an emergency hand gear for lowering or raising all three landing units. Main wheels are raised inwardly into wells in underside of wings aft of the main spar structure. The castoring nose wheel folds up and rearwards and sets below the nose cannon when in retracted position. Hydraulic suspension extends to all wheels, hydraulic multiple disc brakes to main landing gear.

The fuselage is an all-metal oval structure which divides into two sections. Forward section consists primarily of two main longitudinal beams with a horizontal upper deck between and extends to the bulkhead aft of the Allison. Detachable cowlings in the fuselage above the main beams give easy access to engine, radio compartment, armament and cockpit. The aft section of the fuselage is a metal monocoque. Both sections are bolted and may easily be detached for shipment or repair.

Equipment

The fully-enclosed cockpit canopy is located ahead of the leading edge of the wing which accounts for the excellent visibility. Two outward-swing auto-like doors are located one on each side of the cabin. The starboard side door is used for normal entrance or emergency exit. Both doors have immediate release hinge-pins which can be operated from either the interior or outside the aircraft for emergency release. Both doors are equipped with roll-down windows. There is a large baggage space behind the pilot over the engine compartment and accessible from either inside or outside the aircraft. Armor plating covers all vital spots and provides maximum protection for pilot and plane. The remotely-controlled two-way radio is located in the aft section of the fuselage with the storage battery, while the radio antenna mast is enclosed in a transparent plastic leading edge of the vertical fin. Cabin is heated and provisions are made for full oxygen and electrical equipment.

Flight Characteristics

All pilots receive special flight instructions so as to derive maximum performance from their aircraft and also to preserve the combat life of the machine.

For takeoff, a mechanical takeoff is recommended. Because of the tricycle landing gear, it is good practice to ease the ship from the ground when an indicated airspeed of 100 mph is attained. If the engine fails on take-off, the pilot is warned to land the ship on its belly. Emphasis is placed on the fact that the belly tank should be dropped immediately before attempting a forced landing so as to cut danger of an explosion.

The Airacobra has good stalling characteristics with flaps up at speeds up to 105 mph, 90 mph with flaps down. The aircraft mushes considerably in the vicinity of stalling speeds, and at least 130 mph is recommended before attempting stall recovery.

It is necessary to trim the nose "heavy" before diving, or else the aircraft will effect a severe, sharp pullout imposing unnecessary strain upon the pilot. Pullouts from a maximum speed dive must be started at or above the minimum safety level of 9,750 feet or else there is immediate danger that the aircraft will crash.

In specifying details and performance characteristics of the Airacobra, it must be remembered that there is no definite arrangement for any one model. Whereas the P-39Q develops 1,200 hp for takeoff, the P-39K and L models each develop 1,325 hp at takeoff, and 1,150 hp at 12,000 feet compared to the P-39Q's 1,000 hp at 14,000 feet. All specifications are met upon the demands of missions and equipment required. A minimum cruising speed horsepower rating of the P-39L is 370 hp at sea level; it may be higher or lower in other fighter or bomber models. Gross weight of the bomber Airacobra does not usually exceed the safety maximum of 8,400 lbs, while weight of the fighter is about 7,400 lbs. The bomber may carry either one 500-lb or 600-lb bomb and can alternate as a fighter with the releasable belly tank.

Wingspan of the Airacobra is exactly 34', length 29' 9", height 9' 3".

Wing area is 214 sq ft, empty weight 5,400 lbs, wing loading 34.6 lbs/sq ft; power loading of the fighter 6.42 lbs/hp. The 'Cobra is the smallest of any American operational type plane to fight in this war under the American insignia.

So when you again read reports of this "long-nose fighter," as it is called by the harassed Nipponese pilots all over the Pacific, you will know why the Airacobra is now ringing up such an enviable record, while early reports immediately after Pearl Harbor condemned the fighter. Since high-altitude fighters have come into being, the P-39 fighter series can now be employed with the maximum effectiveness for which it was designed. The Airacobra is known from Leningrad to Stalingrad, from Kiska to Australia, all through the African campaign to the fighting in Italy. The opening of the second front in France saw swarms of the little cannon-bearing fighters screaming down to blast and rip armored vehicles, infantry, motorized columns, tanks and enemy fighter cover. The bombs carried by the 'Cobras have knocked out pillboxes and enemy shipping. One Airacobra in Russia has thirty-one little swastikas painted on its gleaming silver fuselage, ample testimony of the capabilities of this buried-engine warplane, forerunner of many more symbols to be added.

This article was originally published in the August, 1944, issue of Air Tech magazine, vol 5, no 2, pp 17-23, 68.
The original article includes 12 photos, a three-view drawing and 5 schematic diagrams.
Photos credited to Bell Aircraft Co, Wide World, Rudy Arnold, USAAF, International News Photos, European.

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