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The Vought XF5U-1 Flying Flapjack was an experimental U.S. Navy fighter aircraft designed by Charles H. Zimmerman during World War II. This unorthodox design consisted of a flat, somewhat disk-shaped body (hence its name) serving as the lifting surface.[1] Two piston engines buried in the body drove propellers located on the leading edge at the wingtips.

The original prototype, designated the V-173 (Flying Pancake), was built of wood and canvas and featured a conventional, fully symmetrical airfoil section (NACA 0015). Designed more as a "proof-of-concept" prototype, the initial configuration utilized F4U Corsair propellers but these were replaced by a specially modified "flapping" blade arrangement with "one pair in each set staggered ahead of the other."

The development version, the XF5U-1, was a larger aircraft with all-metal construction, and was almost five times heavier, with two 1,600 hp Pratt and Whitney R-2000 radial engines. The configuration was designed to create a low aspect ratio aircraft with low takeoff and landing speeds but high top speed.

The XF5U looks like it should not be able to fly, as its wing area looks so small. Normally, a wing with such a low aspect ratio will suffer from very poor performance due to the degree of induced drag created at the wingtips, as the higher pressure air below spills around the wingtip to the lower-pressure region above. In a conventional aircraft, these wingtip vortices carry a lot of energy with them and hence create drag. The usual approach to reducing these vortices is to build a wing with a high aspect ratio, i.e. one that is long and narrow. However, such wings compromise the maneuverability and roll rate of the aircraft, or present a structural challenge in building them stiff enough. The XF5U overcomes the tip vortex problem using the propellers to actively cancel the drag-causing tip vortices. The propellers are arranged to rotate in the opposite direction to the tip vortices, which retains the higher-pressure air below the wing. Since this source of drag is eliminated, the aircraft will fly with a much smaller wing area, and the small wing yields high maneuverability with greater structural strength.

The propellers envisioned for the completed fighter were to have a built-in cyclic movement like a helicopter's main rotor, with a very limited ability to tilt up and down to aid the aircraft in maneuvering. An obvious problem with this arrangement on the XF5U-1 was that the propeller's radius covered nearly the entire frontal area of the aircraft. A typical wing installation of any forward-firing weapons such as machine guns, cannon, or missiles would be virtually impossible. Although the prototype was unarmed, a combination of machine guns and cannons would have been installed under the nose. [3]Also, the radar would have to be mounted forward of the propellers to prevent interference. An ejection seat was fitted to allow the pilot to clear the massive propellers in the event of an in-flight emergency.

The first flight of the V-173 was on 23 November 1942. The aircraft's most significant problem concerned its complicated gearbox that routed power from the engines to its two long propeller shafts. The gearbox produced unacceptable amounts of vibration in ground testing, delaying the aircraft's first test flight for months. Flight testing would continue for over a year. Charles Lindbergh piloted the V-173 during this time and found it surprisingly easy to handle. On one occasion, the V-173 was forced to make an emergency landing on a beach. As the pilot made his final approach he noticed two bathers directly in his path. The pilot locked the aircraft's brakes on landing causing the aircraft to flip over onto its back. Remarkably, the airframe proved so strong that neither the plane nor the pilot sustained any significant damage.

Though the follow-up XF5U design was promising, it came at the time where the United States Navy was switching from propeller driven to jet propelled aircraft. By 1946 the XF5U-1 project was already long over its expected development time, and well over budget. With jet aircraft coming into service the Navy finally canceled the project on 17 March 1947 and the prototype aircraft (V-173) was transferred to the Smithsonian Museum for display. Although two aircraft were constructed, a lone XF5U-1 underwent ground runs but never overcame vibration problems. Taxi trials at Vought's Connecticut factory culminated in short "hops" that were not considered true flights.  The only completed XF5U-1 proved to be so structurally solid that it had to be destroyed by a wrecking ball.

General characteristics

    * Crew: One, pilot
    * Length: 28 ft 7 in (8.73 m)
    * Wingspan: 32 ft 6 in (9.91 m)
    * Height: 14 ft 9 in (4.50 m)
    * Wing area: 475 ft² (44.2 m²)
    * Empty weight: 13,107 lb (5,958 kg)
    * Loaded weight: 16,722 lb (7,600 kg)
    * Max takeoff weight: 18,772 lb (8,533 kg)
    * Powerplant: 2× Pratt & Whitney R-2000-7 radial engine, 1,350 hp (1,007 kW each) each

Performance

    * Maximum speed: 413 knots / 475 mph at 28,000 feet (estimated)[8] (765 km/h at 8,534 meters)
    * Range: 1,064 miles (1,703 km)
    * Service ceiling: 34,492 ft (10,516 m)
    * Rate of climb: 718 ft/min (219 m/min)
    * Wing loading: 35 lb/ft² (172 kg/m²)
    * Power/mass: 0.16 hp/lb (0.27 kW/kg)

Armament

    * 6 × .50 machine guns or
    * 4 × 20 mm machine guns or
    * 2 × 1000 lb. bombs

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