Unveiling The Airacobra

Robert J Woods
Chief Design Engineer Bell Aircraft Corporation
Chief Engineer describes details of Bell mystery ship

During the past few months increasing emphasis has been placed on improvement and development of military aircraft, to obtain the maximum speed possible, with improvement in fire power and increase in utility of the aircraft as a weapon a close second in consideration. The designer of the P-39 recognized these trends more than two years ago and realized that if the single-engine pursuit airplane was to keep pace with advances in foreign light bombers radical departure from the conventional engineering practices were essential. The result is an airplane designed for quantity production whose performance puts it well in the lead of its foreign competitors. New and novel from nose to tail, the Bell P-39 single-place, single-engine pursuit, has been built with the aim of carrying the high-speed laurels to around the 400 mph mark. The potency of this airplane as a weapon is attested by the 37-mm aircraft cannon and the four synchronized machine guns carried in the nose of the fuselage. The utility of the machine is established by the tricycle-type landing gear, the excellent vision provided for the pilot in all directions and the facility for inspection, maintenance and repair made possible by the novel arrangement of the engine amidships, which, together with the relative low wing loading for this type of airplane, produces a degree of maneuverability not possible with the conventional arrangement.

Incorporating aerodynamic and design features heretofore generally reserved for the laboratory, the P-39 is a powerful, complete, fully-developed tactical weapon. Completely new in so many respects, the decentralized arrangement of the component essential installations of the P-39 airplane is a most striking feature. Now that it has been done, it seems most logical, and offers many advantages.

The installation of the engine near the middle of the fuselage and the use of the extension drive shaft to drive the propeller permit realization of aerodynamic improvements not heretofore practical. The nose shape of the fuselage is optimum for low drag. The location of the heavy weight of the engine near the center of gravity of the airplane provides for economy in structural weight and reduces the polar moment of inertia of the mass of the airplane around the center of gravity to a marked degree which, in turn, provides a degree of maneuverability not e in an airplane of this size and weight with the conventional arrangement with the engine in the nose of the fuselage. The location of the pilot, approximately on the center of gravity, also increases his ability to stand the physical strain caused by abrupt maneuvers. The vision provided for the pilot is better than that provided in most multi-engine airplanes. It is completely unrestricted forward and down and is not blanketed by nacelles to the side. The projection of the cabin above the fuselage top line and the transparent "turtle back" permit vision directly aft and up. The blind spot to the pilot in all single-place pursuit airplanes aft and down is protected from hostile fire by the mass of the engine. In foreign types this protection is achieved by the dead weight of armor plate with the resultant sacrifice of useful load.

The windshield, windows and transparent panels of the cabin are Plexiglas. Two outward-swinging doors are provided; one on each side of the cabin, the one on the right side for normal entrance and exit. However, both doors come free for emergency exit by means of quickly releasable hinge pins that can be operated from inside or outside the cabin. Both doors have roll-down windows. Large baggage space is provided behind the pilot inside the cabin on the top deck over the engine compartment, which is accessible to the pilot from within the cabin. The pilot's cockpit is neatly, but compactly arranged and provides exceptional room for so small an airplane.

Some of the modern European military airplanes in use now carry automatic cannons which fire explosive shells of great destructive power. In general these guns are 20-mm and 23-mm bore. The Bell P-39 carries a 37-mm cannon capable of firing explosive shells. It is believed that this airplane is the only single-engine pursuit airplane in the world thus armed. In addition to the 37-mm cannon, the P-39 mounts four machine guns for synchronized fire through the propeller disc. A fume-proof bulkhead at the rear of the gun compartment keeps gun gas from the cockpit.

The power plant is manufactured by the Allison Engineering Company, Indianapolis, IN. The engine is an Allison, direct drive, Vee, 12-cylinder, liquid-cooled. Power is transmitted to the propeller through an extension shaft assembly that is provided with suitable universal-joint action and intermediate bearing supports. The extension shaft was designed with full consideration of the factors of fuselage deflection for all conditions of flight and power, and has shown no defect or fault in over 500 hours of full power testing on the test stand and in flight. The independent reduction-gear box is attached to the front end of the fuselage longitudinal beams and drives the hollow-hub propeller. The offset of the reduction gears permits the installation of the 37-mm cannon on the propeller shaft centerline. The gear box also mounts the propeller governor, propeller bush housing and gun synchronizers. It has a completely independent lubricating system and oil supply. The weight of the power plant is surprisingly low, being only a little over the weight of the standard V-1710 Allison engine. The oil radiator is located in the righthand wing root and the Prestone radiator in the left hand wing root. Shutters are provided to control the engine temperatures.

The three elements of the tricycle landing gear are completely retractable in flight. The retraction mechanism is mechanical, operated by an electric motor, or by hand from the pilot's cockpit. Hydraulically-operated Goodyear, 11-plate multiple-disc brakes are used in the rear wheels. Of interest, from a design standpoint, is the hydraulic stabilizer built integrally into the fork and shock absorber of the nose wheel to prevent the occurrence of "shimmy" of that element and at the same time provide suitable caster action to permit the P-39 airplane to be steered readily on the ground. The landing gear is arranged so that the thrust axis of the fuselage sets +5° to the ground when at rest, and the length of stroke in the front and rear elements of the landing gear are adjusted so that at 90 mph takeoff speed the angle of attack is increased to provide lift equal to the gross weight of the airplane. When landing at 75 mph along the path of glide, the front and rear wheels contact the ground at the same instant.

The general construction of the P-39 airplane follows the present practice of all-metal, stressed-skin design, but it is noteworthy to point out a few of the detail refinements that have been incorporated in this airplane. The fuselage structure is comprised of two sections:

  1. a forward section extending from the nose to the bulkhead aft of the engine, mounting the reduction gear box and extension drive shaft, landing gear nose wheel, armament, cockpit and engine and incorporating the wing center section and
  2. an aft section extending from behind the engine to the tail.
The two sections of the fuselage are bolted together in a manner to permit rapid disassembly for shipment or repair. The forward fuselage section consists principally of two main longitudinal beams, with a horizontal top deck between, upon which the installed equipment is mounted, much in the manner it would set on a platform. The fuselage covering above the longitudinal beams is almost entirely cowling, which when removed, provides an unprecedented degree of accessibility that is further emphasized by the decentralization of the power plant, armament, cockpit and radio equipment. The wing affords an excellent platform for working on the engine. The structure of the P-39 airplane has been completely static tested to 100% of the design loadings, which are the latest requirements of the US Army Air Corps. The wings of the P-39 airplane are of the cantilever type of all-metal construction throughout. The internal ribs and bulkheads are stamped or pressed metal. Fuel tanks providing for storage of more than 150 gallons of fuel are built integrally into the wings. Ducts of the type developed by the NACA, extending from the leading edge of the wing to the upper surface of the wing near the trailing edge are built into the root of the outer wing panels, providing cooling air flow for the Prestone and oil radiators. The rear elements of the tricycle landing gear retract, completely flush, into the outer wing panels, aft of the main spar structure. The airfoil sections are: at the root the NACA 0015 section and at the tip the NACA 23009 section modified. A very successful feature of design of the wing is the alignment of the axis of symmetry of the symmetrical after body of the NACA 23009 airfoil with the chord line of the NACA 0015 section used at the root to produce a simplified symmetrical wing structure for manufacturing purposes, with an effective degree of aerodynamic twist, or "wash in" at the tip which avoids the common fault of early "tip-stalling" so prevalent in many low-wing monoplanes.

Split trailing-edge wing flaps are provided on each outer wing panel, extending from the inboard end of the ailerons to the fuselage. A feature of the flap design is the proportioning of the span, chord and angular deflection of the flaps, so that the effective downwash angle of air flow on the tail surfaces is increased when the flaps are deflected to a degree where the change in trim of the airplane is negligible from stabilized level flight, at cruising power, flaps up, to a stabilized glide, power off, at approximately 100 mph providing a perfectly coordinated trimming condition for the landing approach. The relatively low wing loading and the relatively large and effective wing flaps make the stalling speed less than 70 mph which, together with the tricycle landing gear and the delayed stalling character of the wing tips and lateral control system, provides maximum safety and facility for landing, even under adverse conditions or by pilots of limited experience, an important feature in military aircraft.

The ailerons are constructed of metal with a doped fabric covering, and are of the balanced slotted type. A trim tab is provided on each aileron, controlled from the cockpit. A feature of the aileron design and aileron control system is the combination of a venturi-shaped slot with a rounded modified Friese-type nose balance actuated by a differential link motion in the control system. This provides slotted aileron function to maintain lateral control at the stalling point of the wing. It also provides a variation in the slot, from wide open at zero angular deflection to full closed in the extreme "up" and "down" positions, to provide sensitive, effective control and feel for small displacements of the ailerons and reduced operating loads for large angular deflections of the ailerons. The arrangement of the aileron slot and slot entrance radii is also such that the effectiveness of the slot varies with the angle of attack to permit relatively large airflow through the slot at high angles of attack and almost no airflow at low angles of attack, a circumstance that gives a more uniform control force throughout the speed range with minimum drag at high speed. The lateral control system of the P-39 airplane provides a maximum of good handling characteristics and safe lateral control at low speeds. The tail surfaces are of the full cantilever type, with the fixed control surfaces of all-metal construction and the movable surfaces consisting of a metal frame with fabric covering. Trim tabs are provided on each movable surface, controlled from the cockpit. The radio antenna mast is enclosed in a transparent plastic leading edge on the vertical fin, which eliminates the drag or the hazard of ice formation on the conventional external radio mast. Anti-icing boots can be installed over the transparent leading edge without affecting the radio function.

The two-way radio and storage battery are located in the aft section of the fuselage. The radio is operated by remote control from the pilot's cockpit. The external surfaces of the P-39 airplane have been specially treated to reduce skin friction drag to minimum and act as camouflage. Flush rivets are used all over and thin coating of special filler followed by a coat of primer and two coats of special aluminized lacquer are applied as a finish. The finished surface has a dull texture that is aerodynamically smooth, but practically non-reflecting. The dull finish and clouded grey color form effective camouflage. Specifications and performance supplied by the manufacturer follow:

Wing span  …34'
Overall length  …29'9"
Overall height  …9'3"
Wing area  …213 sq ft
Wing loading  …28.3 lb/sq ft
Power loading  …5.6 lb/bhp
Empty weightapproximately5,000 lb
Useful loadapproximately1,550 lb
Gross weightapproximately6,000 lb
Maximum speed  around400 mph
Cruising speedapproximately325 mph
Service ceiling  above36,000 ft
This article was originally published in the March, 1940, issue of Aviation magazine, vol 39, no 3, pp 40-41, 104.
The original article includes a photo of the author and 4 photos of the plane.
Photos are not credited, but are probably from Bell Aircraft.

The plane in the photo has "meatball" insignia and striped rudder markings