Design Analysis of the
Grumman F6F-5 Hellcat


By Moss Ringel

Latest version of the Hellcat has new ailerons. allowing greater maneuverability, improved windshield, strengthened tail and stabilizer structures

Spearheading the Navy's drive against the Japanese is the F6F-5, latest version of the Grumman Hellcat, a class VF, single engine, single place, low wing fighter, designed to take off or to be catapulted from the deck of an aircraft carrier and for landing on an arresting gear or on land. Specifically, the new Hellcats have six principal improvements over their predecessor:

  1. Engine cowling has been further streamlined, allowing more speed.
  2. The paint has been changed to a mirror-smooth finish. The F6F-3 had a rough-surfaced paint. This would not reflect flashes of sunlight, yet its irregular surface caused drag and retarded the plane's speed. Because combat experience proved that the silhouette of a plane was as easy to see as sunlight flashes, the F6F-3's rough paint was abandoned for the waxed, highly polished coating of the F6F-5. This increased the Hellcat's speed, and the color makes an effective camouflage when seen from above against the ocean. Speed of the Hellcat is now in excess of 400 mph.
  3. The F6F-5 has new ailerons, which enable the pilot to maneuver with greater ease, especially at high speeds.
  4. The new Hellcat has an improved windshield. Two metal braces on the old windshield have been removed, providing better visibility.
  5. Tail and stabilizer structures have been strengthened. Thus the F6F-5 is capable of higher diving speeds and more violent pull-outs.
  6. A larger sheet of armor-plate has been placed behind the cockpit, affording the pilot greater protection from Jap bullets and cannon fire.

In addition to its normal armament of six .50-cal machine guns, three in each outer folding panel, the F6F-5 is equipped with rocket racks on the under side of the outer panels. Two 1000-lb bombs can be carried under the wing center section, or one bomb under the belly of the fuselage. A droppable fuel tank may be substituted for the bomb under the fuselage.

Wings

Wing outer panels are folded and spread manually and locked in the spread position by hydraulically operated locking pins controlled from the cockpit. These locking pins are "safetied" by manually controlled safety lockpins.

Wing center section is fabricated around two full cantilever type beams upon which all superstructures are built. The front beam may well be considered the backbone of the plane. This beam is inclined slightly aft to provide the proper axis for manual folding of the wing. The wing folding hinges are bolted to the front beam and the locking unit, which holds the outer panel in the spread position, is secured to the rear beam. Ribs, stringers and bulkheads give body to the structure and its contour is retained by a stressed, flush riveted, Alclad skin. Suitable reinforced walkways of Trimite carborundum covered cloth, molding bound, are provided on either side of the fuselage.

The landing gear shock strut yokes are connected to the center section main beam hinge fitting and the drag strut is supported by a special welded truss installed between the main and rear beams.

Wing center section structure extends outboard on either side of the fuselage at an incidence of 3°. Its construction accommodates landing gear wheel wells and space for fuel cells. The inboard flaps and catapult hook reinforcement brackets on the main beam, likewise, are incorporated in this section. The wing center section is attached to the fuselage structure by two cone shaped main attaching bolts and four nuts, and by an angle which is provided on the wing center section for bolting through the firewall to a reinforcing channel inside the cockpit with 18 hex-head bolts and nuts. Two main wing fittings are secured to the fuselage with nuts and the reinforcing channel which fastens the fuselage to the wing with 25 truss head screws and nuts.

The upper trailing edge of the center section aft of the main beam is attached to the left and right sides of the fuselage by bolting the angles on the wing center section to the fuselage. The lower portion of the center section is attached to the fuselage by 8 hex-head bolts, four on each side, which fasten the bracket part to the fuselage by the use of a spacer bar. The lower trailing edge and the fuselage are attached by an angle which is fastened to the fuselage with head screws and to the wing by rivets. Neoprene strips are cemented to the center section to seal off and make airtight the joint between the outer panel overhang and the center section.

Outer wing panels are constructed with the front beam extending outboard. The D-shaped section formed by the front beam and the leading edge curved plates is designed to withstand bending and torsional forces. The ribs, stamped of aluminum alloy, have vertical stiffeners riveted in place where necessary. Conventional Z stringers run laterally through the wing aft of the front beam, The entire wing structure is covered with flush-riveted skin. The outboard flaps and ailerons, however, are fabric-covered.

The outer panel is hinged outboard the center section at the front beam. The lower hinge of the lower outer panel front beam fitting engages the lower hinge of the center section. The chrome-nickel steel forgings, heat treated to 150,000 psi are held in engagement by special bolts equipped with lubricators. The outer panels are mounted at a 7½° dihedral at the hinge axis. The electrically heated static pitot tube is mounted in the lower surface of the right wing panel leading edge near the wing tip. Red left and green right running lights are installed in the removable wing tip caps. Formation lights are installed topside in the outer panels. An approach light is installed in the leading edge of the left hand wing panel. Disappearing anchor rings are installed on the lower surface of the outer wing panels aft of the main beam. Likewise, a suitable fitting, into which a hoisting ring may be inserted, is installed topside conveniently near the center of gravity of the wing panel.

Three .50-cal M2 Browning machine guns and their attending ammunition boxes mounted outboard of the guns, constitute the armament installation in either outer wing panel of the Hellcat. In addition to this complement of guns, the Hellcat outer panels are equipped with mounting posts to carry rocket projectiles. The mounting blocks for the rocket supporting posts are installed on the front and rear beams.

The all-metal inboard wing flaps occupy the trailing edge position of the wing center section on either side of the fuselage. They are hydraulically operated by cockpit control and although they are not mechanically connected with the outboard flaps, or one another, they function in unison with one another.

Outboard flaps are larger than the inboard flaps and are attached to the wing by means of three lever arms. The space between the inboard and outboard flaps is faired by a triangular shaped pyramidal structure extending from the wing center section. Automatic doors completely close the gaps between the flaps and the wing structure proper.

Ailerons are inherently balanced and are completely differential in operation. They are hinged at three points on the outer panel and are of sufficient area (15.7 sq ft) to insure complete lateral control even at carrier landing speeds. The ailerons are fabric-covered and are easily replaceable.

The left hand aileron is equipped with a movable tab controlled from the cockpit. Spring tab ailerons are provided on the F6F-5 to lighten the control forces. The tabs are connected to the aileron control system so that when the force is applied to the system the tab operates to reduce the force. The tab motion of each aileron is approximately proportional to the hinge moment on the surface. Aileron angle has no effect on the tab except insofar as aileron angle affects hinge moments. The tab on the left hand aileron also operates as a trim tab. The F6F-5 is equipped with fixed aileron tabs left and right as well as the spring tabs. The fixed tab makes it possible to compensate for wing heaviness.

Outer panels of the Hellcat are spread and folded manually from the ground or flight deck and are held in the folded position by automatically engaging locking pins.

Before a panel is spread, this folded position lockpin must be released by actuating the manual release lever located in the wheel well. Panels are locked in the spread position and unlocked before folding by hydraulically operated main locking pins controlled from the cockpit. The two-position hydraulic valve control lever for operating these pins is located on the right hand shelf. The safety lockpins are operated manually by a T handle control located on the lower center control panel. This control "safeties" the main locking pins only after the outer panels are moved to the spread position and locking pins are fully home. These safety lockpins, when engaged, prevent the main locking pins from disengaging regardless of hydraulic pressure. As the safety lockpins are withdrawn during the folding operation, red warning cylinders appear through the upper surface of the wing center section, one on the left and one on the right hand side.

Tail Surfaces

The fixed tail surfaces are of all-metal construction. The statically and dynamically balanced movable tail surfaces are constructed of aluminum alloy and are fabric-covered. The aerodynamically balanced stabilizers are mounted at a 1½° incidence angle. They have a total area of 52.05 sq ft and are built of stamped ribs riveted to a single shear webbed rear beam. The stabilizers are attached to the fuselage structure and on the bulkhead. The juncture of stabilizer and fuselage is sealed with rubber edging.

The fin is a single element formed with stamped aluminum ribs riveted to the rear beam. The entire skin is flush riveted and at the trailing edge is reinforced to form the recess into which the rudder is sealed. The aft tip contour of the fin is designed to accommodate the cap overhang of the balanced rudder. The aft radio mast is secured to the cap. A fairing streamlines the forward fin contour with the fuselage. Left and right elevator units are identical in construction and the individual torque tube flanges are connected at the elevator horn assembly to form an integral unit. The D-shaped nose cover of the landing edge contains the lead balance weights. A simple aft-angled main beam supports the formed nose ribs and terminates in the tip assembly. Each elevator mounts an adjustable all metal trimming tab.

Sheets, angles and extrusions which form the framework of the rudder are made of aluminum alloy. The covering is fabric. The empennage trimming tabs are all similar in construction, but differ only in size.

Fuselage Construction

The fuselage of the Hellcat is of the stressed skin semi-monocoque construction consisting essentially of channel and angle type frames and bulkheads covered with a smooth aluminum alloy skin. The skin is stiffened longitudinally by aluminum alloy angles and channels which extend the entire length of the fuselage. Special stiffeners are used to distribute concentrated stresses at the engine mount fittings, wing center section attachment fittings, cockpit cutout, tail attachment fittings and arresting hook carriage track. The crash bulkhead structure, designed to protect the pilot in case of a nose-over, extends to the top of the cockpit enclosure. Steps and handgrips are provided on both sides of the fuselage. The fuselage is lowered as a unit into position on the wing center section upon final assembly.

The cockpit is enclosed with a Plexiglas hood which can be jettisoned in flight. To open and close the hood, a hand cranking device is provided on the right hand side of the cockpit, consisting of four sprockets, three front and one rear, that drive the chain and flexible cable assembly. These cables are connected to the fittings at the aft end of the hood and travel over a series of pulleys. For emergency exit in flight, the hood can be pushed up into the air stream by unlocking the holding pins.

Access to the fuselage for inspection is by a door on the bottom of the fuselage. It is equipped with quick turn fasteners. The battery and much of the communication and ADI equipment is accessible from this door. Baggage can sometimes be stowed in this compartment.

In the Hellcat, the landing gear is of the retractable type actuated by hydraulic cylinders. It supports the plane during safe landings, parkings and takeoff operation. The shock strut is designed to rotate 90° as it is retracted rearward, enabling the wheels to assume a horizontal position in the wing when fully retracted. A fairing is provided on the main strut which closes over the wheel well opening when the landing gear is retracted so that the under surface of the wing profile is only partially broken by openings.

Hydraulic pressure to operate the double-acting actuating cylinders located in the wing center section is supplied by the engine driven hydraulic pump. A selector valve unit is installed behind the landing gear control located on the lower left hand instrument panel. Multiple locks secure the position of the mechanism and prevent the inadvertent collapse of the landing gear. Wheel shock-loads, not absorbed by the oleo, are thus transmitted directly to the structure of the wing center section. The left hand shock strut is equipped with a control lock actuator located on the upper cylinder just above the torque arm fitting. The tail section is supported upon landing by a swivel type retractable tail wheel. An air system is provided for lowering the gear in the event of failure of the hydraulic system.

The tail wheel assembly is installed as a unit on the bulkhead. The drag link is bolted at two points to the fittings on the rear face of the bulkhead using bolts, bushings and nuts. The compression strut upper terminal is bolted to a fitting at the top rear face of the bulkhead with a special bolt. The fairing, attached to the drag strut, closes the major part of the tail wheel well when the assembly is retracted. Brakes are operated through an independent hydraulic system consisting of two pedals, two master cylinders, two brakes and connecting lines. Goodyear hydraulic disc brakes are of the multiple disc type, specially designed for aircraft. The master cylinder is a Warner brake cylinder No. 3750-1.

The engine is a Pratt & Whitney Double Wasp model R-2800, propeller reducing gear 2:1, 18-cylinder, aircooled radial with 2000 bhp.

The Hellcat is equipped with a constant speed three bladed Hamilton Standard Hydromatic propeller with a blade diameter of 13'1". In the constant speed range, the position of the blades is automatically controlled by an engine operated governor which is adjusted from the cockpit.

The lubricating oil is supplied to the engine by a full pressure system. The system consists of a gear driven pressure pump, scavenge pumps, a supply tank embodying a warmup compartment, oil cooler with automatic bypass valve, oil dilution system, check and drain valves, temperature and pressure gauges and oil lines.

The fuel system consists of two main wing tanks with type D aromatic resistant self-sealing cells located in the wing center section and a reserve tank with type D self-sealing cell located in the fuselage below the cockpit, droppable tank fittings and controls, fuel transfer system, an auxiliary electric fuel pump unit, electric primer, three liquid level transmitters, fuel quantity gauge unit which indicates the quantity of fuel in the three tanks individually on one face, and engine driven fuel pump with integral bypass and relief valve, tank selector valve and control, fuel strainer and drain valves, fuel pressure gauge, Stromberg fuel tank pressurizing control unit and fuel, pressure, drain and vent lines.

A 150-gallon droppable belly type tank is installed to the under side of the fuselage. It is held in place by two straps, which are attached to two bomb racks, located in the forward portion of the fuselage immediately aft of the firewall. Turnbuckles and a terminal eye are provided on one end of the straps, thereby making it possible to take up any slack. Two sway-braces, which are expendable, are installed on the forward portion of the tank and a sway-brace pin is built in the fuselage structure near the aft portion of the tank.

The flight and engine instruments are located on the main and lower right instrument panels. These panels are cushioned against shock by lord mounts and the lines leading to the instruments are flexible to eliminate vibration. The instruments on the Hellcat are indirectly lighted by red coated lamps inserted in a reflector panel. The main panel is directly in line with the pilot's vision immediately below the gunsight, and the lower panel is angularly mounted and entirely within the field of vision at all times.

This Design Analysis article was originally published in the June, 1945, issue of Industrial Aviation magazine, vol 3, no 1, pp 7-8, 10, 12-14, 16, 18-23.
The original article includes 5 photos, a three-view and 11 detail drawings and diagrams, and 2 data tables.
Photos are not credited.
A PDF of this article [PDF, 16.2MiB] is available on the site.