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” WWII Navy fighter program. Both aircraft featured an unorthodox all-wing design consisting of a flat, disk-shaped body 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 XF5U had a remarkable stall speed of just 20 knots, making it ideal for carrier operations.

The V-173 blueprints were shown to the Navy in 1939. On 27 February 1940 the Navy initiated development with award of a contract to Vought-Sikorsky. Wind tunnel tests on full-scale models were conducted in 1940–41. On 23 November 1942 Vought test pilot Boone T. Guyton took the V-173 into the air for its first flight.

Charles H. Zimmerman promoted his “Flying Pancake” design from 1933 to 1937 while working for NACA at Langley Field, Virginia. He filed for a design patent on April 30, 1935 (granted February 14, 1938). The invention approximated the ultimate ideal for high-speed aircraft: a streamlined body enclosing the load and power plants, flattened to an elliptical cross section for efficient lift at high speeds. The propeller location at the wingtips recovered energy that would otherwise be lost in slipstream twist, reducing induced drag.

In January 1942, proposals were requested for a prototype known as the VS-135, given the military designation XF5U-1. It had two Pratt & Whitney Twin Wasp 1,350 hp engines driving propellers via geared shafts. The XF5U-1 was taxi tested on February 3, 1947 at Stratford, Connecticut, but excessive vibration between the engine-propeller shafting and airframe was never fully resolved. The arrival of the jet age led to cancellation of the contract in March 1947. The XF5U-1 prototype was scrapped; the V-173 prototype was saved and given to the Smithsonian.