Design Analysis of the
Bell Airacobra

From Cannon to Tail

By E Eugene Miller
Managing Editor, Aviation

Here is the first part-by-part study of a leading American warplane. revealing hitherto undisclosed technical details of the famous P-39 lighter and arranged for engineering reference.

We went back to fundamentals!" said Larry Bell in his article in the July, 1941, Aviation, describing the design philosophy of the Bell P-39 Airacobra.

The fundamentals were first, firepower; second, good pilot visibility; and third, good landing and ground characteristics. And these engineering objectives were to be over and beyond ordinary fighter characteristics such as speed, maneuverability, and pilot protection — built into the Airacobra as a matter of fundamental "fundamentals".

The "over and beyond" were achieved in a single engineering stroke, ie, locating the engine behind the pilot and transmitting its power. through a drive shaft to u propeller gear box in the nose. Result: Firepower to spare! A 37-mm cannon installation and two .50-cal machine guns in the nose in addition to multiple wing gun installations. Anti-tank, anti-bomber, anti-ground transport, anti-shipping armament! Anti-fighter? — the record says it's tops!

Visibility is excellent. With engine behind and below his head level, the pilot can see in all directions with a minimum of obstruction.

The answer to safe high-speed landings and improved ground handling characteristics is a tricycle landing gear. To date, no one has engineered a retracting nose wheel into an engine compartment. This may not be impossible, but it was not necessary in the Airacobra, with the engine out of the way.

The remote drive installation is the most obvious design characteristic of the Airacobra. But Larry Bell, Bob Woods, chief design engineer, and Harland M Poyer, director of engineering, have not mustered theirs' and their staffs' brilliant talents to rest on this achievement alone. The Airacobra is an outstanding design and engineering achievement from stem to stern. One way to demonstrate this is to take it apart and look it over.

Fuselage

The fuselage is comprised of two spliced sections known as the "forward" and "aft" fuselages.

The forward section is the structural focal point of the Airacobra and where its design philosophy takes concrete shape.

Integrally it comprises the major portion of the whole fuselage and contains the center wing section, engine bed, extension shaft and propeller gear reduction assembly mounts, principal armament mounts, nose wheel attachment fittings, and supports and brackets for myriad engine accessories — fuel, lubricating, oxygen, ventilating, control, electrical and hydraulic assemblies. To it are fastened the outer wings, pilot's cabin, engine and accessories, extension shaft and reduction gear box, heavier armament and ammunition magazines, oil and Prestone cooling systems, nose wheel assembly, and aft fuselage.

A section of such impressive function must be of rugged, multi-purpose construction. It consists principally of two built-up longitudinal beams which are cradle shape in profile. Each beam is made of extruded aluminum alloy angle sections tied with virtually solid, heavily reinforced aluminum webbing. Outboard shape is imparted through a series of formed aluminum bulkheads. However, two of these bulkheads, located where the center wing section is tied in, are steel castings and become an integral part of this wing section.

A heavy gauge aluminum, stamped deck plate is riveted to the tops of these bulkheads and extends the full length of the beam. To the rear of the beam a sturdy forged angle member is mounted to form the engine bed.

The outer skin of the forward fuselage consists of formed aluminum sheet riveted to the bulkheads. Skin panels in the nose section and beneath the pilot's cabin are .051 gauge, and the rear panels are .032-gauge sheet.

The two main longitudinal beams are maintained rigidly parallel as a self-contained unit, principally by tubular spreader bars, a forward bulkhead and former member, and the aft splicing bulkhead. However, the center wing section, coolant radiator supports, and pilot cabin which is joined later, also act as tying members.

The pilot cabin is superimposed on the forward fuselage but designed and attached as an integral part of the fuselage just forward of the engine compartment. Fume-tight bulkheads are provided between the engine compartment and pilot cabin and between the cabin and armament compartment forward.

Aft Fuselage

The aft fuselage section supports the complete empennage assembly and contains the radio installation. It is of ordinary semi-monocoque construction with eight principal forming bulkheads. The splicing bulkhead has a beaded sheet. web and a number of drilled stiffeners for bolts joining the aft and forward fuselage. The two forward bulkheads are tied outboard by two longitudinal bulkheads to form a compartment for the engine oil tank. The skin is .032-gauge formed aluminum sheet which is flush-riveted throughout.

Cowling

Cowling is attached to the fuselage at four main points: the gun compartment forward of the- pilot cabin, engine compartment, the Prestone and oil radiator compartments on the under side of the wing center section, and oil tank compartment in the aft fuselage behind the engine accessories bay.

Cowlings are formed aluminum sheet ranging from .051 gauge at the gun, Prestone and oil radiator compartments to .032 gauge at the engine and oil tank compartments. Access doors, an integral part of the cowling, are located in the engine compartment for access to the Prestone expansion tank, and in the radiator compartments for access to the Prestone drain. A section of cowling directly over the rear of the engine contains the carburetor air intake scoop. Another section, directly over the engine, houses the aft cabin Plexiglas enclosure.

Gun compartment cowl sections are attached to removable channel-shaped formers and bulkheads with flush-type fasteners. Engine compartment sections are strengthened with channel stiffeners and the radiator compartment cowls by several permanently riveted interlocking stiffeners.

Wing Center Section

The wing center section is in two parts, forward and trailing edges, both of which are designed as integral parts of the forward fuselage and wing outer panels.

The forward structure is built up of six principal parts: a forward and a rear beam corresponding precisely to the main beams in the outer wing panels, two inner bulkheads, and two outer bulkheads for splicing to outer wing panels.

The forward beam consists of top and bottom capstrips of milled extruded aluminum shapes, tied by heavily reinforced aluminum webs in four principal panels. The two inside panels are solid single layer pieces. Outside web panels are made up of three layers of heavy gauge aluminum, the center layer acting as a reinforcing member. The outside layers have formed 90° flanges at the edge. These web panels have a dual function and must be so rigidly constructed. Large oval-shaped holes are cut through their centers to accommodate oil cooler ducts extending from the wing leading edge to oil coolers installed in the aft portion of the center wing section. In addition, four forged steel wing bolt fittings are fastened on each side, top and bottom of these web panels. In the main, they are much more than web panels, but vitally important structural members.

The rear beam is all formed steel except for the aluminum capstrips superimposed upon and riveted to the steel flanged web. The web is in two layers or sections riveted back to back. Strength considerations made it necessary to use steel for this member because four large holes are cut in the web to accommodate air ducts for the oil coolers (two outside circular holes), and air ducts for the Prestone radiator (two square inboard holes), which is also located in the aft portion of the wing center section. As in the front beam, eight forged steel fittings for wing bolts are riveted to the steel web, lying snugly against the capstrip and side flanges.

The two internal bulkheads consist of reinforced aluminum web sections riveted to extruded aluminum forming members. The outboard bulkheads, too, are reinforced webs with extruded angle forming members. Web sections are heavily reinforced where the wing bolts run through from the outer panel beams to fittings on the center section beams.

Top and bottom skin sections are formed aluminum sheet, reinforced with angle stringers and heavy plate where holes are cut for wing bolt access doors.

Trailing Edge Section

The wing center section trailing edge consists of a single beam which acts as a carry-through member for the auxiliary beam of the outer wing panels. This beam is built up of formed sheet. Its sections are tied across by a heavy T-section extrusion further reinforced by extruded angle sections. Each side is cut through to accommodate air ducts to the Prestone radiator, which is mounted just aft of these beams and extends up through the longitudinal beams of the forward fuselage. Extending forward from the beam are parallel extruded channel sections to which outboard splicing bulkheads are riveted and upon which the partial skin covering is placed and riveted. Forged fittings are riveted to the auxiliary beam for additional wing splice bolts, and the bulkhead is strongly reinforced at these points.

The oil radiators are strapped into compartments formed by the bulkheads ahead of the auxiliary beam.

Outer Wing Panels

Outer wing panels are joined to the center wing section on each side at approximately 22 in outboard of the P-39's center line of symmetry. Thirty bolts are used to make each splice, twelve are cold forged with recessed heads. There are also four large bolts at each front and rear beam connection. and four smaller ones at the auxiliary beam. In addition, six regular hex- head bolts are used at each of the three beam splices. The three spars of the outer wing panel are called (from LE to TE) "front beam", "rear beam", and "auxiliary beam". Front and rear beams consist of flanged capstrips of extruded aluminum shapes milled to distinctive and exacting dimensions with the flange thickness tapering from a maximum of 0.865 in at the inboard end to 0.154 in at the outer tip. Solid heavy gauge aluminum sheet webs with occasional stringers complete the beam construction. The auxiliary beam is made up of formed aluminum capstrips with a solid aluminum web. The top capstrip is in dual sections running from the inboard splice bulkhead to just beyond the fifth bulkhead. In the trailing edge assembly, an intercostal beam is installed for strength and also forms a compartment with the auxiliary beam for the flap actuating push-pull rod assembly. Running between the first and third trailing edge bulkheads are short beams to support the wing catwalk. The trailing edge strip is a drawn aluminum piece which clamps around the skin and is held together by rivets.

There are 13 principal wing panel bulkheads of pressed and beaded aluminum, ranging in gauge from .072 for the inboard splice bulkhead to .064 for the wing tip splice bulkhead.

In the Bell production set-up, the bulkheads are numbered in stations from one to ten. Counting inboard, the first seven bulkheads are whole-numbered stations. The eighth bulkhead is Sta 7.5, and each represents a half station from there to the thirteenth bulkhead at Sta 10.

Compartments formed by bulkhead Sta 2 to Sta 7.5 between front and rear beams accommodate the self-sealing fuel cells. A longitudinal bulkhead between Stas 4 and 5, about one-third of the distance forward from the rear to front beam, forms a compartment for the main landing gear retracting spindle assembly. Twin .30-cal machine guns are installed between Stas 7.5 and 8, with a false rib bulkhead between them.

An intercostal beam of formed, blanked, and beaded aluminum sheet webbing between drawn aluminum capstrips runs between Stas 8 and 10, about midway between front and rear beams, forming a compartment with the front beam to accommodate wing ammunition boxes.

The landing gear wheel wells between Stas 1 and 2 are shaped of formed and drawn aluminum capstrips with formed aluminum sheet webbing. Two angle-shaped stringers extend from Sta 1 bulkhead over the top of the wheel well to the outer periphery to reinforce the wing catwalk.

Wing Skin

The lower surface wing skin is in three principal sections, forward and aft of the rear beam, and the leading edge, Each section consists of two panels running inboard to approximately Sta 7.5 and from there to Sta 10.

The two aft panels are .025-gauge formed sheet with numerous reinforced access door cutouts. The inboard panel between rear and forward beams is .051-gauge formed sheet, largely built up with reinforcing panels. The outboard front panel is .032-gauge sheet with a large access door of the same gauge aluminum for the wing ammunition cases. The leading edge is .032-gauge formed sheet

The skin from top to lower surface consists of panels similar in arrangement and gauge, The inboard forward panel has a number of reinforced access holes for fuel tanks; the outboard forward panel has a large door for installation or removal of wing guns. Skin stringers are, for the most part, Z-section, rolled or drawn aluminum. Flush riveting is used throughout.

Wing Tips

The wing tip comprises two bulkheads and three tapering beams. The wing splice bulkhead is built up of rolled capstrip and sheet webbing of .064-gauge aluminum. The internal bulkhead is in three sections, each of formed beaded sheet, with a series of beaded lightening holes. These bulk-head sections are riveted to the beam. Beams are also formed and blanked sections; the trailing edge beam is riveted to the rear section internal bulkhead in two sections. Skin stringers are drawn hat sections riveted across the beams. Tip edge is a formed aluminum strip enclosing the skin, which is .032 gauge, entirely flush riveted.

Ailerons

Ailerons are fabric covered monospar structures of the "Frise" type with built-up ribs of extruded shapes in some cases and formed ribs in others. The built-up ribs, tied together with gusset plates, are capstripped with metal channel assemblies used in conjunction with thin metal retainer strips to tie down fabric skin covering, giving a flush surface. Leading edge ribs are formed and blanked sheet riveted to the .040-gauge Alclad beam. Trailing edges are formed strips enclosing the skin fabric.

Trim and Servo Tabs

Controllable trim tabs of laminated phenolic plastic are located at the trailing edge of each inboard aileron. These trim tabs also act as a servo control through a mechanical linkage which automatically rotates the tab to an angle opposite to the movement of the aileron, creating a force which reduces the energy required to actuate the aileron. An additional servo tab of laminated plastic, not controllable by the pilot, is located just outboard of the trim tabs. The servo tabs are actuated by a mechanical linkage attached to the aileron hinge bracket maintaining the servo tab at a neutral setting 2° above the thrust line of the airplane with the ailerons positioned at any angle in flight.

Wing Flaps

The wing flaps are split trailing edge type and form the lower rear surface of the outer wing panels. They extend from Sta 1 to the inboard end of the aileron. Attached to the wing by a piano-type hinge extending the full flap length, they are connected to the auxiliary beam lower flange. Flaps are operated by a push-pull tube and an electrically driven connecting link mechanism. The operating member consists of five turnbuckle connecting links between the flap beam and the push-pull tube. The single beam is a formed channel section of .040-gauge aluminum. Ribs are formed solid sheet sections in two parts, riveted fore and aft of the beam. Skin stringers are also formed channel flanged sections. Skin is a solid panel of .025-gauge sheet. A doubler sheet with a number of blanked lightening holes is riveted to the ribs forward of the beam. The flaps are stressed so that full extension (43°) may be used at 150 mph maximum speed.

Empennage

The horizontal stabilizer is constructed as a single unit with sections cut away in the center to permit access of the vertical stabilizer to the fuselage. It is a twin-spar structure of conventional stressed skin design. The forĀ· ward beam is constructed in three flanged sections formed of heavy gauge sheet. The center section is a short member, perpendicular to the airplane center line, to which are attached two outer beam sections with a pronounced sweepback, approximately parallel to the leading edge. A series of false ribs of formed and blanked aluminum sections form the leading edge.

The rear beam, formed from .072-gauge sheet, is in two half sections spliced with a riveted aluminum plate. Each half section has a solid formed sheet tip attached with rivets. About half way from center on each side, forged aluminum alloy steel hinge fittings for the elevators are fastened to the rear beam. The front and rear beam are tied together with a series of blanked and formed bulkheads. Z-shaped stringers support the flush-riveted skin. The leading edge skin is a single formed section on each side, extending from the top flange of the front beam around the forming ribs to the bottom flange.

From the front beam to the rear edge, the skin is in four formed aluminum sheet sections, two on each side, top and bottom. Stabilizer tips are single formed aluminum sections riveted to the single rib of the stabilizer proper. Four threaded studs, two on each beam, provide for attaching the stabilizer to the aft fuselage with lock nuts.

Elevators

Fabric-covered elevators are similar in construction to the ailerons. Right and left hand elements are joined at the control quadrant by tubular steel members spliced by forged, flanged steel collars. The left elevator has a trim tab fastened to an auxiliary beam just forward and inboard of the trailing edge section. Two hinge settings are installed on the main beam and attached to the horizontal stabilizer. A mass balance, tubular-shaped weight, used for dynamic and static balancing, is located in the foremost section of the leading edge.

Vertical Stabilizer

The vertical stabilizer is similar in structure to the horizontal stabilizer, except that beams are one piece construction. A hole is cut in the skin on both sides for the navigation light installation. Cast fittings are installed on the projecting ends of the main and rear beams for attachment by nuts and bolts to the aft fuselage. Two hinge fittings are riveted to the aft flange of the rear beam for the rudder attachment.

Rudder

The fabric covered rudder is similar in structural arrangement to the elevators and ailerons. However, the top and bottom portions are covered by formed and beaded aluminum sheet. Two hinge fittings for fastening to the vertical stabilizer and rudder control quadrant are installed on the main beam. The auxiliary beam supports a plastic trim tab, and a round mass-balanced weight is mounted in the foremost section of the leading edge.

Nine pieces of formed aluminum sheet and formers comprise the empennage fillet assembly, which is attached by flush screws and channel nuts.

Pilot Cabin

The cabin with its six transparent panels, arranged for maximum visibility, is in two sections — the forward cabin for the pilot, a permanent built-up structure superimposed upon and forming an integral part of the forward fuselage ahead of the engine; and the removable aft cabin enclosure directly over the engine and joining the forward section at the turnover beam.

The forward section consists of drawn aluminum frames for the glass and Plexiglas enclosures as well as the formed aluminum skin sides forward of the door. Directly ahead of the pilot is a windshield panel of ¼-in laminated shatterproof glass; two side windshield panels are ¼-in. convex Plexiglas and directly overhead is a panel of 3/32-in Plexiglas formed to the cabin contour. All panels are set in rubber channel retainer strips.

Aluminum automobile type doors are located on both sides of the cabin. Both have a 21/64-in roll down laminated glass panel operable on the ground or in the air at any speed up to design high speed in level flight. Emergency door release handles, painted red, are just forward of each door frame in the cockpit. When the handle is pulled out and turned 90°, the hinge pin is parallel with the door release slot and the door is free to be pushed outward away from the airplane. Both cabin doors are held tightly closed by a door latch at the top to prevent the door from opening at high speeds. This fastening is broken when the emergency handle is operated.

Instrument Panel

The main instrument panel, constructed of one piece of aluminum alloy, is flexibly mounted on three Lord mounts just below the center windshield. An auxiliary instrument panel is set to the immediate right of the main panel and attached directly to the fuselage. A left hand panel consists principally of switches controlling the electrical installations. There are some 36 instruments and sets of switches.

Pilot Seat

The pilot's seat is bucket-type, metal construction, and non-adjustable. A Type B-11 Sutton safety harness is attached to the seat with a roller mechanism and is released or locked by a. control under the seat. The cabin is designed to accommodate a pilot 5 ft 8 in high, weighing 200 lb with parachute.

Turnover Bulkhead

The turnover bulkhead behind the pilot is of extremely rigid construction capable of withstanding loads considerably in excess of the airplane weight. It consists of two main beams of very heavy gauge aluminum tied together by a number of formed and blanked bulkhead sections and heavy gauge formed aluminum skin flush riveted to the beams and bulkheads. Additional strength is provided by crossed streamline wire bracing tightened by turn-buckles running from the top of each side of the assembly to the bottom of the other side.

Aft Cabin

The aft cabin is a shallow streamline structure conforming to fuselage contours. It consists of channel formers and a beaded aluminum deckplate for housing a portion of the radio installation. It is enclosed by two convex panels of 5/32-in Plexiglas.

Differing from the forward pilot cabin, which is permanent, the aft cabin section is removable. It joins the forward section at the turnover beam.

Landing Gear

The Airacobra is equipped with a fully retractable tricycle landing gear. Nose wheel is self-castering, non-steerable type and retracts up and aft into the forward fuselage; the main wheels retract up and inboard into the outer wing panels. Retracting mechanism is operated by an electric motor through a system of torque tubes, universal joints, gear boxes, and splined connections. Operation of the landing gear motor is governed by a toggle switch in the pilot cabin. In the event of power failure, the wheels can be operated by an emergency ratchet crank at the right of the pilot's seat.

Shock Struts

Cleveland Pneumatic Tool air-oil shock struts are used on both nose and main gear. Main gear struts are attached to a fitting of the spindle assembly in the wing panel by four nuts and bolts. The forged steel landing gear fork is attached to the piston tube of each strut which operates with the wheel. Conventional torque scissors are employed on the main and nose wheel installations.

The nose wheel shock strut is hinged to the forward fuselage inside the nose wheel well and is retracted by a linkage assembly. A centering cam is installed in the oleo strut keeping the wheel in a central position and preventing it from fouling against the airplane during re- traction. An adjustable Houdaille Engineering shimmy absorber is installed in the lower end of the strut piston tube.

The nose wheel fork is a steel forging connected to the lower end of the strut assembly by four bolts, located in pairs at the front and rear of the fork, and lockwired in pairs, Holes are tapped in the lower end of the piston tube and lined up with the fork bolt holes.

The nose wheel is mounted on a chrome molybdenum cadmium-plated, steel tube axle, which slides through the fork ends from left to right and is held in place by a large nut lockwired to the fork. The tire is 19 in in diameter with a high pressure dual seal safety tube.

Oil and gas resistant, pliable rubberized boots are fastened onto each wheel strut assembly with a criss-cross lace. At the top they are held by fairing clamps and at bottom by two slots into which the two fork-to-scissors brackets are inserted.

Wheels and Brakes

The main wheels are magnesium alloy equipped with 10.OH x 5 disk type hydraulic brakes. Main wheel tire casings are 26:16 in, 6-ply rayon, with high pressure, puncture proof tubes. Three types of interchangeable tires are used — treaded for ice or snow-packed fields, flat contour for desert or sandy fields, and smooth contour for normal operations.

Each main wheel brake assembly has eleven plates, six stationary steel disks and five movable brass disks. The stationary disks are held in place by six slots on the inner side which slide over six tongues on the axle hub. The movable disks have six tongues on the outer edge which slide into six corresponding slots in the brake drum. A Warner hydraulic master brake cylinder and brake pedal assembly are mounted in a vertical position and pivoted on each end of the rudder control pedal cross bar in the cockpit. Master brake cylinder units are connected to the brakes by a flexible leather-covered rubber hose and 5/16-OD aluminum alloy tubing. A control on the main instrument panel operates a parking brake in conjunction with the main wheel brakes.

Retracting Mechanism

The landing gear is operated by a reversible, 24-volt motor giving ¾ hp. at 3,800 rpm. Incorporated in the assembly are a clutch and reduction gear drive with a 40 to 1 ratio. The clutch is designed to slip at an output torque (at slow speed shaft) of 700 in-lb. The motor is mounted on the forward fuselage deck at the right side of the engine. An operating control switch is mounted just forward of the lefthand cabin door in the cockpit.

Main wheels retract into the wing by means of a worm sector gear which is installed with a spindle assembly on the aft face of the rear main beam by twelve nuts and bolts lockwired in pairs. Each strut is attached to a fitting of the spindle assembly by four nuts and bolts. A worm gear installed in the spindle assembly is actuated by the landing gear motor by means of torque tubes. The worm gear rotates the sector gear, which in turn retracts or extends the wheels.

Nose wheel retraction mechanism consists of a retracting screw installed in the forward fuselage and driven by the landing gear motor through torque tubes and a 90° gear drive. This actuating screw is attached to the main leg of the nose wheel linkage assembly. A coupling shaft links the retracting screw and gear drive.

The retracting linkage consists of a short link, attached to the oleo strut, to which is hinged a large or main link, hinged in turn to two A-frame fittings in the fuselage.

The emergency handcrank is equipped with a ratchet and can be reversed by a switch at the top outboard face of the crank. A clutch handle is located just aft and outboard of the handcrank on the cabin floor, to change from electric to manual operation or vice versa.

Landing Gear Fairings

The main wheel fairing is made up of three sections. Two are attached to the main wheel strut at two points. The lower section of fairing laps over the upper section, but the two are not connected in any way, the overlap permitting motion of the oleo strut without damage or buckling. Lower section is attached at the main wheel axle by a washer and castellated nut and to the upper portion of the fork by a link assembly. The upper section of the fairing is attached to the strut in two places by clamps. Five bolts hold the lower clamp and four hold the upper clamp to the fairing. Each clamp is held to the strut by a bolt and locknut. The third section of fairing, known as the "flipper door," is hinged to the lower surface of the wing center section near the wing splice bulkhead. It opens toward the airplane centerline and is actuated by the main wheels during extension and retraction. The wheel extends allowing the spring-loaded arm of the door to straighten, forcing the door open. Upon retraction, the tire comes in contact with the spring-loaded arm causing it to fold upward, drawing up the door.

The nose wheel fairing is in two sections. One is bolted to the 'top of the nose wheel strut and lies flush with the undersurface of the fuselage when the wheel is retracted. The other section consists of right- and left-hand doors hinged to the forward fuselage at four hinge points. These doors are actuated by a spring loaded arm in a manner similar to the main wheel fairing doors.

Engine and Accessories

The Allison Type V1710 engine is mounted on four "Fabreeka" pads inserted between the engine mounting points and airplane beam fittings. It is secured by eight bolts and nuts arranged in four pairs, through fittings riveted to the inboard side web beneath the top longitudinal beam flange. A propeller reduction gear box is located in the fuselage nose and is bolted to the forward bulkhead. The gear box is connected to the engine by a 10-ft drive shaft operating at crankcase speed. This shaft runs through the beam fuselage under the pilot's seat and consists of a flanged coupling and center bearing. The reduction gear box has a separate oil system.

The exhaust system is comprised of dual stacks, six on each side, of formed seam-welded stainless steel with sand blast finish. Stacks are welded to mounting flanges for attachment to engine.

Fuel System

The fuel system consists of two 60-gal tanks integrally built into the outer wing panels. The left wing tank includes a reserve area of 20 gal. A droppable auxiliary fuel tank of either 75- or 150-gal capacity may be carried in the bomb rack suspended from the wing center section.

Fuel is supplied to the carburetor by a pressure pump, located on the rear of the engine accessory housing, augmented by either two booster fuel pumps in the wing, or by a single booster pump in the wing center section. Booster pumps are electrically driven and are used for starting, warm weather takeoff and high altitude flying to prevent vapor lock.

Between the booster pump and engine driven fuel pump is a Lunkenheimer fuel strainer, an air vapor eliminator, and air vapor control valve. The latter two are installed to provide a steady flow of fuel when the fuel selector valve is actuated to switch fuel intake from an empty to full tank. A vent line from the carburetor to the left wing tank is routed through the air vapor control valve which prevents vapor in the left wing tank from backing up into the carburetor.

A fuel pressure warning switch, connected to the carburetor and carburetor air intake balance line, is provided along with a light indicator to warn the pilot of fuel pressure conditions.

The carburetor air intake system is comprised of a ramming air scoop, located behind the aft cabin on the airplane vertical centerline, in a straight duct into the carburetor. The forward face of the duct has a door controlled from the cockpit, which closes off cold air and admits warm air from the engine compartment.

A primer pump is installed at the lower right-hand side of the radio control panel. It is hand-operated and draws fuel from the booster pump and injects it into the engine intake manifold system.

Fuel Tanks

Each fuel tank consists of six leakproof fuel bags. The left wing tank is equipped with two finger-type fuel strainers, one providing for normal fuel consumption, and the other for reserve fuel consumption. The right-hand tank includes only one finger strainer providing for consumption of the entire tank content.

Each tank is equipped with a liquidometer unit with a resistance strip and movable contact arm, at the end of which is installed a pivoting cork float which actuates the contact arm in accordance with the tank fuel level. The contact arm position is transmitted electrically to an indicator graduated in gallons and located on the main instrument panel. Electrical connections are installed within shielded conduits between each tank unit and the instrument panel indicator.

Fuel Controls

The engine throttle and carburetor mixture control levers are assembled into a single unit (propeller pitch control is also in this unit), mounted on the left cabin wall at the forward door frame. In order to prevent creeping due to vibration, the control levers are governed by a bronze friction adjusting nut which is attached to the quadrant bolt and may be tightened or loosened to permit easy sliding of stiff levers or to allow tightened action of loose levers.

The throttle control lever is located most outboard on the quadrant and governs speed and manifold pressure. Lever knob is equipped with a spring-loaded push button which operates the throat microphone, allowing pilot to operate the radio-transmitter without removing his hand from the throttle.

The mixture control which is incorporated with the injection type carburetor has four main control settings, namely, Full Rich, Automatic Rich, Automatic Lean and Idle Cutoff, in the order mentioned. Carburetor mixture can be adjusted by motion of the lever between Automatic Rich and Automatic Loan by "click and feel". Automatic altitude mixture control is also maintained for any fixed position of the control lever between Automatic Rich and Automatic Lean.

Oil System

Engine lubricating oil is circulated by a main pressure pump having a built-in check valve, and a scavenger pump located at the lower right-hand side of the engine accessory housing. Oil is supplied to the pressure pump "In" from the bottom of the tank and circulates through the engine. Oil leaves the engine from the main scavenger pump "Out" and is delivered equally to each of the oil coolers and returns through parallel lines connecting to a single line attached to the top of the oil tank. The propeller reduction gear box is lubricated by a separate oil circulating system with its own tank and pump.

Engine Oil Tank

The engine main oil tank, of 13.8-gal capacity, is constructed of seam welded magnesium alloy sheet located in the aft fuselage behind the engine accessory bay on the airplane centerline. Tank oil level is measured by a flexible bayonet-type sound rod located in the tank top casting. It is impossible to fill the expansion space provided within the tank with the airplane in its normal rest position.

Oil Cooling System

The oil cooling system consists of two oil coolers with separate air ducts located within each outer wing panel and connecting to each cooler in the wing center section. The coolers incorporate an independent, fully automatic, thermostatically controlled by-pass valve which circulates oil along the cooler coil until the oil is properly cooled.

Cooling air enters each duct through an individual opening on either side of the fuselage at the wing leading edge. Each duct leaves the outer wing panel between the wing front and rear beams and joins the section of duct connected to the coolers. Air exhausts through flap-type shutters located aft of the coolers on the underside of the wing center section. Shutters operate in unison by a lever control located on the right forward face of the turnover beam near the cabin floor and can completely restrict airflow through the coolers.

Oil Dilution System

A solenoid operated oil dilution valve is maintained in the system for cold weather starting. Dilution of oil is accomplished by controlled addition of engine fuel to the oil inlet line by operation of a toggle type switch on the left hand auxiliary switch panel.

An oil thermometer well is installed in the oil line from the main engine oil tank to the oil drain "Y" fitting. A thermometer bulb connects to the well and runs to the oil temperature gauge on the instrument panel and registers oil "In" temperature.

The oil drain Y fitting is located in the right hand side of the airplane in the oil line which runs from the botĀ· tom of the main oil tank to the oil pressure pump. The drain Y fitting is in the lowest part of the oil system which can be drained from this point with the airplane at its normal rest position on the ground.

Surge Valve

A by-pass surge valve is incorporated in the oil system. When the airplane has been idle for any length of time in cold weather, the oil in the coolers become congealed and may cause damage from oil pump pressure when the engine is started. If oil pressure exceeds 60 psi, the surge valve opens, releasing pressure in the coolers and sending the majority of oil flow directly back to the main oil tank. When seepage from the surge valve loosens the congealed oil, the valve closes, allowing oil to flow normally to the coolers.

Two engine breather pipes are connected to the engine, one to the top forward end, and one to the top of the accessory housing. Both breather pipes are routed downward to the bottom of the airplane fuselage and exhaust into the slip stream.

All oil lines are of aluminum alloy tubing with hose connections, nipples and elbows of standard Army Air Force type. A Pesco oil separator is incorporated in the oil system and is mounted on the left-hand deck of the fuselage. The unit contains no working parts and is sturdily constructed of welded sheet aluminum. Pressure gauge connections and tubular bracket mountings are used throughout.

Reduction Gear Box Oil Tank

The reduction gear box oil tank is constructed of magnesium alloy sheet with welded seams and is located aft of the gear box. Oil pressure lines for this system are aluminum alloy tubing with brass liners at each connection. The line from the gear box to its pressure gauge on the main instrument panel is composed of aluminum tubing, copper tubing and a flexible connection from the line to gauge.

Cooling System

Engine is cooled by a high-temperature, liquid Prestone cooling system. The Prestone radiator is of the cartridge core type, constructed in two sections and assembled as a single unit in a radiator mounting cradle. The assembly is mounted on four flexible vibration insulating units below the engine, between the longitudinal beams aft of the wing carry-thru rear beam. Coolant is carried in lines from the outlet on the top forward end of each cylinder head to the radiator. Coolant enters the radiator through a compound inlet on each side of the radiator top and flows downward and rearward returning to the coolant circulating pump inlet collector.

The Prestone expansion tank is so arranged that when filled to the level of the filler neck, when the airplane is in its normal rest position, it will contain the proper amount of coolant, and the proper amount of expansion space. The system is vented to automatically prevent air locks, allow the expansion of coolant, relieve coolant pressure beyond 3 lb, and permit re-entrance of air after coolant temperature drops. The automatic relief and sniffle valves are included in the cooling system filler unit located on top of the tank.

Coolant circulation pump is of centrifugal type and forms a part of the engine. It is mounted on the bottom of the engine accessory housing on the rear left side of the engine.

Two separate air ducts are located on either side of the fuselage at the wing center section leading edge. The ducts flow rearward and converge directly in front of. and connect to the coolant radiator with an airtight seal. Radiator shutter control is located on the cockpit floor just below and forward of the oil shutter control consisting of a gear box with a small handcrank which turns clockwise to close, and counterclockwise to open. A flexible shaft extends aft, inside the fuselage beam to a screw actuating a trunnion on the shutter operating arm. The shutter is located at the air exit of the radiator and can completely restrict airflow through the radiator.

All coolant lines are of aluminum alloy tubing. Coolant temperature thermometer well is permanently installed in the right-hand coolant line from the engine top to the radiator to measure the coolant "Out" temperature. Temperature indicator is on the main instrument panel.

Electrical System

The electrical system is of the single conductor type, shielded and protected by rigid and flexible conduits. A main electrical control switch panel is rigidly mounted to a metal box for shielding and is located to the left of the cock- pit main instrument panel.

A standard ac-type battery is mounted in the forward fuselage between the .50-cal fuselage guns and beneath the gun compartment cowling, aft of the reduction gear box. It is provided with one drain and one vent to which acid- proof rubber tubing is attached by hose clamps. Battery fumes exhaust into a glass bottle containing a pad saturated with a solution of bicarbonate of soda to counteract the acid. The bottle is vented to the airplane slipstream.

Conduit is provided for the dual purpose of protecting the electrical system and providing electrostatic shielding. Rigid conduit is supported and grounded to the airplane, and flexible conduit is used where flexibility is required. Standard type plugs, connectors, connector plugs, socket assemblies and fittings are used for connecting conduit where disconnection of the electrical system is necessary to remove assemblies. A ground wire is attached to the landing gear nose wheel fork on some models, but on others a special static-conducting tire is installed for grounding purposes. The main fuse panel is located on the left hand side of the landing gear nose wheel well within the forward fuselage. Fuse capacity and identification are marked inside the box lid and spare fuses are carried in the fuse box.

Engine Starting System

Electrically operated, inertia type engine starters are used on Airacobras, mounted near the bottom right hand side of the engine accessory housing. A starter pedal is located on the right hand side of the cockpit floor. To energize the starter, the pedal is pressed rearward with the heel. It is held in this position until starter is energized sufficiently to rotate the engine and then the pedal is depressed forward with the toe to engage the starter with the engine. A handcrank is provided and installed in a compartment within the right wing trailing edge. The handcrank extension shaft and sleeve is permanently installed and has a bronze bearing support within the fuselage. It is accessible through a hinged door located on the right hand side of the fuselage near the wing trailing edge.

On some models a mechanism is included for lifting the starter brushes to prevent generation of a charge while the starter is being energized manually. A lever, located in an access door next to the starter door, is operated when the handcrank is being used. When the starter is engaged and the engine started, the lever is operated to return the brushes to their normal position. There is an electric plug located in the left-hand wing trailing edge fillet which may be used in conjunction with an outside battery for cold weather starting. The plug is connected to the electric inertia starter mechanism.

Generator and Booster Coil

An engine-driven generator is mounted on the engine accessory housing to the rear. The generator control panel is mounted on vibration absorbing units and is installed in a structural enclosure within the left side of the fuselage. A generator relay is operated by a toggle switch on the main electrical control panel.

A battery-operated booster coil unit is mounted in a shielded box on the right-hand inclined deck aft of the engine within the fuselage. The circuit is connected so that the coil is energized when the foot pedal starter switch is depressed forward to the engaged position, bringing the solenoid operated starter meshing mechanism into operation.

Current is carried from the booster coil to the right hand distributor by a high tension lead. The solenoid-operated starter meshing mechanism is mounted on the starter.

Landing Light

A three-focus electric retractable landing light assembly is installed flush with the lower surface of the left-hand outer wing panel. It can be lowered and stopped at any intermediate position or retracted by means of an electric motor controlled from a switch on the main control panel. A cutoff switch is provided on the unit which automatically stops the motor when the light reaches its maximum extended or retracted position.

Heating and Ventilating System

The cabin is heated by two 3-in metal tubes which take hot air from directly behind the Prestone radiator, and expel it to two ducts on the cabin floor beneath the pilot seat. The hot. air duct extends between the engine and fuselage longitudinal beams. Cool air is supplied to the cabin by two ducts which take air from the Prestone cooling duct just forward of the radiator. Cool air is emitted to the cabin through the same ducts as the hot air. Selection of hot and cold or mixed air is controlled by two butterfly flaps, one of which is located on each duct Y at the union of each hot and cold air duct. Two L-shaped handles, located adjacent to each other on the floor to the right of the pilot seat operate the butterfly flaps.

Air is exhausted from the cabin through ducts over the rudder pedal well. These ducts lead to the cannon and .50-cal machine guns. The air supply to the cabin is constant and only temperature may be regulated. Because of this, there is greater air pressure in the cabin than in the gun compartment, thus preventing fumes from the gun compartment entering the cabin.

The two flexible heater tubes emerging into the gun compartment extend upward and join together at a duct attached to the pan and feed chute assembly of the .50-cal guns. This duct has two flanged outlets which heat the guns. The forward end of the duct extends downward and to the left and tapers out fan shaped for heating the cannon installation.

Vacuum System

A vacuum pump is mounted on the rear of the engine and is used for forming a vacuum essential for operation of certain instruments. Pump includes a suction line from the instrument panel and an exhaust line to the Pesco oil separator. A relief valve is mounted on top of the vacuum pump to prevent over exertion of suction on the instruments.

A suction gauge mounted on the instrument panel records vacuum suction in the lines. An air filter is included with the vacuum system and is mounted on the left hand forward inclined deck beneath the gun compartment cowling. This unit is maintained to remove any minute particles from the air that might become injurious to the mechanism of the instrument.

De-icing Equipment

De-icing equipment includes a glycol spray to prevent formation of ice on the windshield and a propeller blade de-icing installation. Both units are supplied from a single glycol tank mounted in the gun compartment of the forward fuselage. The windshield de-icing, equipment consists of a hand pump located on the right-hand instrument panel, a nozzle assembly on the windshield, and a de-icing rheostat, also on the right-hand instrument panel. When the pump is operated, a glycol fluid is drawn from the bottom of the glycol tank and is pumped to the windshield nozzle assembly located just above the windshield armorplate. The nozzle is curved to the windshield contour and is profusely punctured to facilitate several streams of fluid to be ejected upward on the glass.

Propeller de-icing equipment consists of an electric-driven Adel pump with a filter, a nozzle assembly, a propeller slinger ring (included with the propeller assembly), and a control switch on the right-hand instrument panel. When the control switch is turned on, the electric motor pumps fluid from the glycol tank. The fluid passes through a filter before entering the pump and is sent forward to the slinger ring which splashes it onto the blades.

Surface Control System

The control system of the Bell P-39 Airacobra is of the stick type, the control column attached to a yoke at the cockpit floor through which the engine drive shaft runs from behind the pilot to the propeller reduction gear box. Control column travel is regulated by four stop bolts, two of which are located so that they contact the aileron lever (regulating lateral movement), and two located to contact the yoke bottom (regulating fore and aft movement). Rudder pedals are adjustable in five positions ranging from 3¼ in forward to 3¼ in aft of the 90° setting. They are adjusted by two levers, one located outboard of each pedal. These foot-operated levers are spring-loaded and release a pin from a stop when depressed. The main control cables are constructed of extra flexible corrosion-resistant steel and the control cable pulleys are of the anti-friction, ball bearing type.

Elevator and Aileron Controls

Elevators are controlled through a push-pull tube assembly attached to the bottom of the control column yoke and running back through the fuselage to a quadrant in the aft fuselage section. From this point, a duplicate cable system (four cables in all) extend along the lower aft fuselage, around a set of pulleys at the extreme rear, and upward to the elevator control quadrant at the elevator splice bar.

The aileron lever on the control column actuates an aileron link rod connected to a quadrant in the fuselage. Control cables run outboard from this quadrant along the leading edge of each outer wing panel. At wing Sta 7.5, the cables make a 90° turn aft and run to a quadrant located on the forward side of the wing auxiliary beam. The quadrant actuates a linkage assembly that connects to a fitting on the aileron leading edge.

Rudder control consists of two link rods connecting the rudder pedals to two quadrants in the center section of the fuselage. Control cables are connected to these quadrants and extend aft through the fuselage, guided and supported by pulleys, to the rudder control quadrant on the rudder main beam.

Flap Control

Flaps are actuated by a ½-hp electric motor, located on the fuselage deck underneath the right wing fillet, and controlled by a toggle switch in the cockpit. The motor operates two sprocket housings, one located immediately inboard of each wing splice. The right hand sprocket housing is connected directly to the motor assembly by means of a drive shaft running through the fuselage. Sprocket housing is attached on the outboard side to the flap mechanism consisting of a universal joint, a housing, a screw, a push-pull tube, and five turnbuckles. The universal joint attaches to a splined shaft of the sprocket housing and connects to the screw assembly which operates the push-pull tube. This tube rolls along on five roller assemblies attached to the wing auxiliary beam. Turnbuckles form the connecting links between the push-pull tube and flap. Limit switches stop the flap motor when the flaps are completely up or down. If limit switches fail to work, a friction clutch, which is part of the motor and adjusted to slip at an output torque, at slow speed shaft, of 110 in-lb, will stop the motor.

Trim Tab Controls

Trim tabs are located on each aileron, the rudder, and left-hand elevator. Separate control knobs for all three are located on the cockpit floor to the left of the pilot seat and each is marked in degrees.

A sprocket-driven chain from the elevator trim control unit is connected by turnbuckles to control cables which are supported by pulleys and run aft through the fuselage. The cables are connected to a straight drive unit. A flexible drive shaft runs from the straight drive unit to the trim tab actuator installed on the left-hand elevator beam.

To the aileron trim control is connected a sprocket-driven chain connecting to control cables at the wing leading edge near the wing splice. Cables run outboard in the leading edge of each outer wing panel to wing Sta 7.5 where they make a 90° turn aft and continue to the wing auxiliary beam. At this point cables attach to a chain which operates a sprocket drive unit mounted on the forward side of the auxiliary beam. A flexible drive shaft connects the sprocket drive unit to the actuator mounted on the aileron main beam.

A sprocket-driven chain runs from the rudder trim control to a point just aft of the fuselage turnover beam where it is connected to control cables which run aft through the fuselage and connect to a chain in the aft fuselage. This chain actuates a 90° drive unit which connects to the trim tab actuator on the aft face of the rudder beam through a flexible drive shaft.

Armament

The Bell P-39 Airacobra armament consists of a 37-mm cannon located on the fuselage centerline just above the extension drive shaft with the gun barrel projecting through the reduction gear box and propeller hub; two .50-cal Type M2 machine guns installed in the forward fuselage just ahead of the pilot and synchronized to fire through the propeller are; four .30-cal free firing machine guns installed in pairs in each outer wing panel.

Cannon and machine guns are manually charged and electrically fired by solenoid units actuated by two firing switches on the control stick, one for the cannon and the other for machine guns. The .50-cal guns are equipped with flame suppressor blast tubes. An impulse tube type synchronizer is used on these guns, the tubes fabricated from chrome moly steel and clamped to the gun barrels at approximately 12-in intervals. Exhaust louvers are installed in the gun compartment cowling to evacuate fumes.

Each .50-cal gun has a separate ammunition case containing 200 rounds. Ejection chutes are provided to carry off links and cases. Both chutes deposit their contents into a triangular space between the longitudinal beams and outer skin.

Each set of .30-cal guns has an ammunition box located outboard of the guns between the front and rear beams. Boxes contain 1,000 rounds, 700 of which are considered alternate load. Ejection chutes are installed for wing guns, the links dropping inboard from their respective guns and the cases dropping straight down. Four small doors on each lower wing surface eject the contents into the slipstream.

The 37-mm magazine is a circular endless belt type installed around the cannon and containing 30 rounds. An ejection compartment for links and eases is located between the longitudinal beams' web and outer skin.

Armor

A considerable amount of armor is installed at vital points on these airplanes. Homogenous steel plate, face-hardened steel plate, and bullet-proof glass are used. .

Armor plate at the propeller reduction gear box consists of 5 pieces of 5/8-in homogenous steel and completely protect the gear box from frontal fire. The fume-tight bulkhead armor plate between the pilot and gun compartment is fastened to the forward side and is a. single piece of 5/8-in face-hardened steel plate. A 7-mm nonmagnetic homogenous armor plate is installed on the fuselage outer surface just forward of and overlapping the lower part of the windshield. The turnover bulkhead armor plate consists of two pieces of ¼-in face-hardened plate fastened to the aft side of the turnover beam immediately forward of the engine. Both sections extend inboard from the beam and overlap each other one inch. They are bolted together at the overlap. A body turnover armor plate consists of two pieces of ¼-in face-hardened plate bolted to the aft face of the turnover bulkhead. The plate is of a general horseshoe shape and fits around the aft armor glass, giving pilot full rear protection. A ¼-in face-hardened plate is bolted to the bulkhead just aft of the oil tank and is designed to protect this tank.

The forward armor glass is 1½-in thick and is located above and forward of the gunsight in the cabin. It is mounted at the bottom of the gunsight support casting and at the top of the transverse windshield frame. In conjunction with the armor plate at the lower outside windshield, it protects the pilot from frontal fire.

Aft armor glass is 2½-in thick and is located in the curve of the fuselage turnover beam aft of the pilot. It is held securely in place by a jam nut and dovetailed screw which fastens to the top inside curve of the turnover beam.

Acknowledgment

The foregoing description of the Bell Airacobra was made possible with the generous assistance and cooperation of the Bell engineering department, particularly Mr Harland M Poyer, Martin W Haney, and Marvin M McEuen.

This design analysis article was originally published in the May, 1943, issue of Aviation magazine, vol 42, no 5, pp 126-155.
The original article includes 24 photos, a three-view and 35 drawings and diagrams, a portrait of Lawrence Bell, and 8 data tables. Photos are not credited.