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Development In February 1938 the U.S. Navy Bureau of Aeronautics published two requests for proposal for twin-engined and single-engined fighters. For the single-engined fighter the Navy requested the maximum obtainable speed, and a stalling speed not higher than 70 miles per hour (110 km/h). A range of 1,000 miles (1,600 km) was specified.[8] The fighter had to carry four guns, or three with increased ammunition. Provision had to be made for anti-aircraft bombs to be carried in the wing. These small bombs would, according to thinking in the 1930s, be dropped on enemy aircraft formations.

The XF4U-1 prototype in 1940/41, showing its more forward cockpit locationIn June 1938, the U.S. Navy signed a contract with Vought for a prototype, the XF4U-1, BuNo 1443. The Corsair design team was headed up by Rex Beisel. After mock-up inspection in February 1939, construction of the XF4U-1 powered by an XR-2800-4 prototype of the Pratt & Whitney Double Wasp twin-row, 18-cylinder radial engine, rated at 1,805 hp (1,346 kW) went ahead quickly. When the prototype was built it had the biggest and most powerful engine, largest propeller and probably the largest wing on any fighter to date.[9] The first flight of the XF4U-1 was made on 29 May 1940, with Lyman A. Bullard, Jr. at the controls. The maiden flight proceeded normally until a hurried landing was made when the elevator trim tabs failed because of flutter.[10][11]

On 1 October, the XF4U-1 became the first single-engine U.S. fighter to fly faster than 400 mph (640 km/h) by setting an average ground speed of 405 miles per hour (652 km/h) during a flight from Stratford to Hartford.[12] The XF4U-1 also had an excellent rate of climb but testing revealed that some requirements would have to be rewritten. In full-power dive tests, speeds of up to 550 miles per hour (890 km/h) were achieved but not without damage to the control surfaces and access panels and in one case, an engine failure.[13] The spin recovery standards also had to be relaxed as recovery from the required two-turn spin proved impossible without resorting to an anti-spin chute.[12] The problems clearly meant delays in getting the type into production.

Reports coming back from the war in Europe indicated that an armament of two .30 in (7.62 mm) (mounted in engine cowling) and two .50 in (12.7 mm) machine guns (one in each outer wing panel) was insufficient. The U.S. Navy's November 1940 production proposals specified heavier armament.[14] The increased armament consisted of three .50 caliber machine guns mounted in each wing. This improvement greatly increased the ability of the Corsair to effectively shoot down enemy aircraft.

Formal US Navy acceptance trials for the XF4U-1 began in February 1941. The Navy entered into a letter of intent on 3 March 1941, received Vought's production proposal on 2 April and awarded Vought a contract for 584 F4U-1 fighters, which were given the name "Corsair", on 30 June of the same year. The first production F4U-1 performed its initial flight a year later, on 24 June 1942.[15][16] It was a remarkable achievement for Vought; compared to land-based counterparts, carrier aircraft are "overbuilt" and heavier, to withstand the extreme stress of deck landings.

[edit] Design 2,000 hp (1,500 kW) Pratt & Whitney R-2800-8 in a Goodyear FG-1 Corsair[edit] Engine restrictionsThe F4U incorporated the largest engine available at the time: the 2,000 hp (1,500 kW) 18-cylinder Pratt & Whitney R-2800 Double Wasp radial. To extract as much power as possible, a relatively large Hamilton Standard Hydromatic three-blade propeller of 13 feet 4 inches (4.06 m) was used. To accommodate a folding wing, the designers considered retracting the main landing gear rearward, but for the chord of wing that was chosen, it was difficult to make the landing gear struts long enough to provide sufficient clearance for the large propeller. Their solution was an inverted gull wing, which considerably shortened the required length of the main gear legs.[17] The anhedral of the wing's center-section also permitted the wing and fuselage to meet at the optimum angle for minimizing drag, without using wing root fairings.[17] Offsetting these benefits, the bent wing was heavier and more difficult to construct.

[edit] Landing gear and wingsThe Corsair's aerodynamics were an advance over those of contemporary naval fighters. The F4U was the first U.S. Navy aircraft to feature landing gear that retracted into a fully enclosed wheel well. In a similar manner to that of the Curtiss P-40 the landing gear oleo struts rotated through 90° during retraction, with the wheel atop the lower end of the strut; a pair of rectangular doors completely enclosed the wheel wells, leaving a completely streamlined wing.[18] This swiveling, aft-retracting landing gear design was common to the Curtiss P-40 (and its predecessor, the Curtiss P-36), as well as the F4U Corsair and its erstwhile Pacific War rival, the Grumman F6F Hellcat. The oil coolers were mounted in the center-section of the wings, alongside of the supercharger air intakes, and used openings in the leading edges of the wings, rather than protruding scoops. The large fuselage panels were made of aluminum[19] and were attached to the frames with the newly-developed technique of spot welding, thus mostly eliminating the use of rivets. While employing this new technology, the Corsair was also the last American-produced fighter aircraft to feature fabric as the skinning for the top and bottom of each outer wing, aft of the main spar and armament bays, and for the ailerons, elevators and rudder. In addition, the elevators were constructed from plywood.[20] Even with its streamlining and high speed abilities, with full flap deployment of 60°, the Corsair could fly slowly enough for carrier landings.

[edit] Technical problemsIn part because of its advances in technology and a top speed greater than existing Navy aircraft, numerous technical problems had to be solved before the Corsair would enter service. Carrier suitability was a major development issue, prompting changes to the main landing gear, tail wheel and tailhook. Early F4U-1s had difficulty recovering from developed spins, since the inverted gull wing's shape interfered with elevator authority. It was also found that the Corsair's starboard wing could stall and drop rapidly and without warning during slow carrier landings.[21] In addition, if the throttle were suddenly advanced (for example, during an aborted landing) the port wing could stall and drop so quickly that the fighter could flip over with the rapid increase in power.[22] These potentially lethal characteristics were later solved through the addition of a small, 6 in (150 mm)-long stall strip to the leading edge of the outer starboard wing, just inboard of the gun ports. This allowed the starboard wing to stall at the same time as the port.[23]

An early F4U-1 showing the "birdcage" canopy with rearwards production cockpit location. Compare with the XF4U-1.Other problems were encountered during early carrier trials. The combination of an aft cockpit and the Corsair's long nose made landings hazardous for newly-trained pilots. During landing approaches it was found that oil from the hydraulic cowl flaps could spatter onto the windscreen, badly reducing visibility, and the undercarriage oleo struts had bad rebound characteristics on landing, allowing the aircraft to bounce out of control down the carrier deck.[23] The first problem was solved by locking the top cowl flap down permanently, then replacing it with a fixed panel. The undercarriage bounce took more time to solve but eventually a "bleed valve" incorporated in the legs allowed the hydraulic pressure to be released gradually as the aircraft landed. The Corsair was not considered fit for carrier use until the wing stall problems and the deck bounce could be solved. In the event, because the more docile, and simpler to build F6F Hellcat had begun entering service, Corsair deployment aboard U.S. carriers was to be delayed until late 1944.[N 1]

[edit] Design modificationsProduction F4U-1s featured several major modifications compared with the XF4U-1. A change of armament to six wing mounted .50 in (12.7 mm) M2 Browning machine guns (three in each outer wing panel) and their ammunition (400 rpg for the inner pair, 375 rpg for the outer)[25] meant that the location of the wing fuel tanks had to be changed. In order to keep the fuel tank close to the center of gravity, the only available position was in the forward fuselage, ahead of the cockpit. Accordingly a 237 gal (897 l) self-sealing fuel tank replaced the fuselage mounted armament, the cockpit had to be moved back by 32 in (810 mm) and the fuselage lengthened.[17] In addition, 150 lb of armor plate was installed, along with an 1.5 in (38 mm) bullet-proof windscreen which was set internally, behind the curved Plexiglas windscreen. The canopy could be jettisoned in an emergency and curved transparent panels, providing the pilot with a limited rear view over his shoulders, were inset into the fuselage, behind the pilot's headrest. A rectangular Plexiglas panel was inset into the lower center-section to allow the pilot to see directly beneath the aircraft and assist with deck landings.[N 2] The engine used was the more powerful R-2800-8 (B series) Double Wasp which produced 2,000 hp (1,491 kW). On the wings the flaps were changed to a NACA slotted type and the ailerons were increased in span to increase the roll rate, with a consequent reduction in flap span. IFF transponder equipment was fitted in the rear fuselage. All in all these changes increased the Corsair's weight by several hundred pounds.[26]