US Navy Flying Pancake

The Vought V-173 "Flying Pancake" or "Zimmer Skimmer", designed by Charles H. Zimmerman, was an American experimental test aircraft built after Pearl Harbor as part of the Vought XF5U "Flying Flapjack" World War II United States Navy fighter aircraft program. Both aircraft featured an unorthodox "all-wing" design consisting of a flat, somewhat disk-shaped body (hence its name) serving as the lifting surface. Two piston engines buried in the body drove propellers located on the leading edge at the wingtips.  The V-173 stalled at 40 knots, but the XF-5U had the remarkable stall speed of 20 knots, making it ideal for use on carriers.

The V-173 blueprints were shown to the Navy in 1939. Based upon research of Charles H. Zimmerman, on 27 February 1940 the Navy initiated development of the Flying Flapjack with award of contract to Vought-Sikorsky for design of the VS-173. The design promised high speed with low takeoff speed. Wind tunnel tests on full scale models being done in 1940-41. On 23 November 1942 the "Flying Flapjack," made its 1st flight when Chance Vought test pilot, Boone T. Guyton, took the V-173 “Flying Flapjack” research aircraft into the air.

Charles H. Zimmerman promoted his “Flying Pancake” design from 1933 to 1937 while working for the National Advisory Committee for Aeronautics (NACA) at Langley Field, Virginia. He filed for a design patent on April 30, 1935 and was granted patent #2,108,093 on February 14, 1938. The invention was claimed to approximate the ultimate ideal for high speed aircraft, which is a stream-line body enclosing the load and the power plants with other accessories, and flattened so that its cross section is elliptical rather than round to provide lift efficiently at high speeds.  Another feature which increased the over-all efficiency above that of other aircraft of the same span loading at the same speed, is the location of the propellers.  At the wing tips they rotated in such directions that most of the energy which would otherwise be lost in twist of the slip-stream was returned to the machine in the form of diminished induced drag.

Essentially, the invention consisted of a stream-lined body, of which fore-and-aft sections are airfoil profiles and transverse sections are approximately ellipses.  The two  engines were of light weight per horsepower and controllable pitch propellers mounted at the wing tips rotated so the blade tips moved downwardly when farthest from the plane's centerline. Two engines were used so torque and gyroscopic couples would be neutralized, providing structural and aerodynamic efficiency.  The design made possible the continuation of flight on one engine in case the other failed.

In January 1942 proposals were requested for prototype airplanes of an experimental version of the V-173, known as the VS-135. This version had more powerful engines and was given the military designation XF5U-1. It had two Pratt & Whitney twin WASPs 1350 hp reciprocating engines buried in wing driving two propellers out at wing tips by geared shafts. The basic wing area (427 sq ft.) and plan form (less elevators and propeller nacelles) of the V-173 and XF5U-1 were identical. Mock-ups of the XF5U-1 were done in the summer of 1943, but due to Vought's preoccupation with the Corsair and Kingfisher, the program proceeded slowly during the war. Testing included the 1/25-Scale Model of the Chance-Vought XF5U-1.

The Navy's Bureau of Aeronautics established the Navy Liaison Office at Edwards AFB on August 28, 1946, to oversee development of the Vought XF5U-1 Skimmer. The disk-shaped fighter underwent many hours of engine run-up, which showed excessive mechanical vibration between the engine-propeller shafting, gear boxes, and airframe structure. The airplane was taxi tested on February 3, 1947 at Stratford, Connecticut, but, again, vibration levels were considered excessive.

It was very revolutionary and high performance, with a 425 mph top speed and 20 mph landing speed. It was scheduled to fly at Edwards AFB in 1947. The arrival of the jet age saw the cancellation of the XF5U-1 contract by the Navy in March 1947, despite the fact that the aircraft was due to take its first test flight later that year. The contract was canceled on 17 March 1947 because of still unsolved technical problems and the lack of Navy R&D money. The XF5U-1 prototype was scrapped, though the V-173 prototype was saved and was given to the Smithsonian.

More info on these sites:

http://www.globalsecurity.org/military/systems/aircraft/f5u.htm

http://daveswarbirds.com/usplanes/aircraft/flapjack.htm

http://www.fiddlersgreen.net/models/aircraft/Vought-XF5U.html