Designing the Bent-Wing Bird

By Alfred I. Sibila

The design team had to resolve many conflicting requirements before arriving at the final configuration for an aircraft that promised speeds of 400 miles per hour, and one of the key constraints was the large-diameter propeller required to convert the high engine power to the thrust necessary to achieve maximum performance. The aerodynamicists calculated the static thrust necessary to accelerate the aircraft to takeoff speed and tried to minimize propeller tip-speed compressibility losses in order to achieve maximum performance; selection was based on efficiency throughout all flight regimes-takeoff, climb, cruise, and top speed. Using a new Hamilton Standard propeller design handbook, they determined that the aircraft required a 13-foot, 3-inch three-bladed propeller. By 1938 standards, it was a very big prop.

Excellent slow-speed characteristics in the landing configuration and good stall indications were required for carrier compatibility, but Chief Engineer Rex B. Beisel made it clear to his 90- man engineering staff on many occasions that "speed is king" and there would be no compromises on that point. We already had the biggest engine available, so it was obvious that the only way to increase performance was to decrease drag. The National Advisory Committee for Aeronautics (NACA-later NASA) had sent us a number of technical reports relating to the optimum position of the wing relative to the fuselage, i.e., mid-wing versus low wing.

The reports showed that the mid-wing configuration, with the wing meeting the fuselage at an angle of 90°, was superior in achieving low drag. The low-wing position resulted in flow separation at the fuselage junction, especially at the wing's trailing edge where a cusp formed with the convex underside of the fuselage. While this could be ameliorated somewhat by using a fillet or fairing along the trailing edge of the wing-fuselage intersection, it was evident that the wing must join the fuselage at right angles similar to a mid-wing configuration if the fighter were to have any chance of achieving the desired speeds.

Still another important design constraint was the length of the landing gear that would retract into the wing. The structures and weights people pointed out that the large-diameter propeller and the mid-wing configuration would dictate a long and heavy landing gear and a complex and costly folding mechanism.

The Corsair's distinctive gull-wing design actually emerged from a small engineering meeting in Rex's office, which had assembled to resolve the conflicting design requirements. The writer recalls that among those present were: Beisel; James M. Shoemaker, Assistant Chief Engineer; Paul S. Baker, Chief of Aerodynamics and Flight Test; William C. Schoolfield, Head of Aerodynamics; Earnest Mailloux, Chief of Structures; George F. Darracott, Head of Weights; Fred N. Dickerman, Design; and yours truly, Aerodynamics. The group reviewed a number of aircraft configurations that had been generated in preliminary design studies.

Assessing the options while sketching on a desk pad, Rex conjectured that if the wing in front view had to be normal to the fuselage at its intersection for low drag, and the landing gear had to be short, why not just leave the wing low, put a little bend in it at the landing gear station, and let it rise to meet the fuselage at right angles?

Then, going outboard, the curved section could rise, meeting the outer panel at the wing-fold station. He also pointed out that such an arrangement would permit an even shorter landing gear than a low-wing. The aerodynamicists confirmed that this configuration would provide essentially the same low drag as a mid-wing. It became clear that the inverted gull wing was the best approach, and Beisel directed that it be incorporated as the basic element of the XF4U-1. The bent-wing bird had been born—and the rest is history.

The detail design work began with a contract go-ahead for the prototype XF4U-1 in June 1938. The Chance Vought Air-U.S. Navycraft plant was moved from East Hartford to Stratford, Connecticut, in 1939 where the design continued as engineers and manufacturing experts worked out the details. They solved the problem of incorporating slotted flaps in the curved gull-wing section and, taking a tip from the SB2U-1 Vindicator, developed a landing-gear retraction mechanism that folded back and rotated 90°, permitting the wheel to be housed flat within the thickness of the wing. To reduce drag, the cockpit and canopy were streamlined into the fuselage; in later models, however, the cockpit was raised and the canopy redesigned to improve visibility.

The Corsair's horizontal and vertical tail configuration was similar to that of the Vindicator and the OS2U-1 Kingfisher. In an effort to get the last knot of speed, the engine exhaust stacks were designed to provide some jet thrust. Speed was indeed king.

Fabricating the curved wing center section was not as difficult as first thought-but only because Rex Beisel recognized the complexity of the task and assigned the best designers to tackle the problem. The centerpiece of the effort was a massive jig fixture assembly built by the artisans in the tool shop. The key to assembling the wing main spar, firewall bulkhead, and other structural components, it was more comprehensive and intricate than any fixture previously used in aircraft manufacturing, and it stood tall in the shop as a tribute to Vought's manufacturing ingenuity.

The designers knew that fighters needed a high roll rate with light aileron forces and the Corsair design emphasized maneuverability. To achieve it, they incorporated a variable lateral control linkage in the experimental XF4U-1 and used wooden ailerons to facilitate modifications to the aerodynamic nose balance. As the flight tests proceeded, the aerodynamicists and engineers varied the control linkage and whittled the aileron nose shape until they had what they wanted: a highly maneuverable production F4U-1 fighter that proved vastly superior to the Japanese Zero. Early production Corsairs also had wooden ailerons; one of the main concerns for production versions was whether manufacturing could fabricate metal ailerons as accurately as the wooden ones that performed so well. They did—and Vought took its final step in the conversion from wooden to metal aircraft construction.

Other wing features included a leading-edge air intake at the fuselage intersection that provided cooling air for the oil system and the engine supercharger inter-stage. Under certain flight conditions, particularly high-speed dives, the intakes created a distinct screaming or high-pitched whistling sound. Japanese ground troops under attack by Corsairs soon linked the sound with the fearsome ordnance capabilities of the bent-wing bird. Because of this, and the Corsair's superior air combat capabilities, the Japanese referred to the F4U as "Whistling Death."

The distinctive aircraft acquired many nicknames. Navy and Marine Corps pilots called it "Hose Nose" and "Hog" because of the long fuselage and large engine cowling, especially as viewed from the cockpit; "U-Bird" and "Bent-Wing" followed. New Zealanders dubbed their F4Us the "Kiwi Corsair" and U.S. Marine Corps infantrymen called it "The Sweetheart of Okinawa" for its air-support role in the hard-fought campaign to capture the island.

Design of the production F4U-1 began in December 1940 and Navy officials signed an initial production contract for 584 aircraft in June 1941. The first production Corsair flew in June 1942. There were many models of the Corsair, from the XF4U-1 through the F4U-7, spanning the years from 1940 through 1953, and including the AU-1 (F4U-6), built specifically for ground-attack. Each version was either an improvement or a modification, such as the F4U-5N night fighter that carried a radar mounted in a fairing on the right wing. The F4U-5NL was a winterized version of the -5N that had been modified for operations in the Korean War. The F4U-5P was a photographic reconnaissance version.

More than 11,000 Corsairs were built during the war approximately 6,600 at the Vought plant in Stratford; 4,000 at the Goodyear plant in Akron, Ohio; and 735 by Brewster on Long Island. During 1944, when the plants worked around the clock, the production rate exceeded 550 aircraft per month-18 aircraft per day-several times. This was a remarkable achievement in those days but not without its problems. A few days of bad weather or a lack of ferry pilots resulted in Corsairs lining both sides of all available runways.

Early in the flight test program, the XF4U-1 became the first single-engine fighter to break the 400 mile-per-hour barrier in level flight. The Corsair's speed, maneuverability, and climb rate enabled it to achieve an outstanding combat record in the Pacific: 2,140 enemy planes destroyed against 189 losses—better than an 11:1 ratio.

The Corsair's up-rated power plants gave it improved performance throughout the war . The XF4U-1 was built around the P& W R-2800-4 engine rated at 1,850 horse power. The F4U-5 used an R-2800-32W that produced 2,200 horsepower and enabled the aircraft to reach 465 miles-per-hour in level flight.

Armament also increased:

  • The XF4U-1 had two .30-caliber machine guns synchronized to fire through the propeller and two wing-mounted .50-caliber machine.
  • The F4U-1 A carried six.50-caliber guns—three per wing.
  • The F4U-1C carried four 20-mm cannon—two per wing.
  • The F4U-1 D fighter-bomber (the production version of the fighters that Pacific Marines and Chance Vought field representatives had jury-rigged with bomb rack s) eventually carried a 2,000-pound bomb load. In 1944, Colonel Charles Lindbergh, then serving as a consultant to United Aircraft Corporation, flew a Corsair in the Pacific with a 4,000-pound bomb load—about what the B-25 twin enginebomber carried.

Corsair production continued at Vought after the war, and the company delivered 467 planes prior to moving the plant from Connecticut to Dallas, Texas, in 1948. Vought produced 306 Corsairs in Texas; the last 94 F4U-7s went to the French Navy in 1953. Overall production totaled more than 12,000 aircraft, as Corsairs enjoyed a longer production run than any other U.S. propeller-driven fighter.

Corsairs served in many foreign countries after World War II. U.S. Marines flew Corsairs in peace-keeping efforts in China at the close of the war and the French and British continued to fly their Corsairs, using them in Indochina, Algeria, and during the Suez Crisis. Navy and Marine Corps Corsairs flew throughout the Korean War in the early 1950s, and a Corsair pilot shot down a MiG-15. Many South and Central American countries bought Corsairs, including Argentina, Honduras, and El Salvador.

In December 1955, the U.S. Navy decommissioned its last active-duty Corsair squadron, VC-4, at Naval Air Station Moffett Field, California, but Navy and Marine Corps reserves flew the aircraft until the Marines retired the last of their AU-1s at Quantico, Virginia, in August 1957. French Corsairs flew last in 1964, but the bent-wing aircraft flew until 1978 in Central America—38 years after the prototype's first flight.

Of the more than 60 F4Us that remain, most are in museums. Surprisingly, about 20 are still flying. What an aircraft.

Mr. S ibila was Vought's C hief of Aerodynamics and Man age r of Spa ce Sc i e nce s when he retir ed in 1 9 79 af t e r more than 42 years with the co mpany. He joined C han c e Vought Aircraft in 1937 as a n Aerodynamics and Fli g ht Test Engineer an d worked on a ircraft , mi ss ile s, and space programs throughout a di s tinguished c areer. We regr et to report, he di ed in May 1994.

 

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