Before Roswell, there was…

One of the most intriguing aircraft ever designed was Charles Zimmerman’s Chance Vought XF5U-1 fighter-bomber – variously called the “Flying Pancake” or “Zimmer’s Skimmer” or, more generically, a “flying saucer.”

***image1***Zimmerman’s basic concept was a disk-shaped airplane that used its flat, circular body to provide the lifting surface that would be capable of flying at high speeds and near-hovering for landing. The disc-shaped concept won a 1933 National Advisory Committee for Aeronautics design competition and the U.S. Navy, seeing the potential for operations from small deck spaces, asked for a proof-of-concept test model, the V-173. It first flew Nov. 23, 1942, and the Navy then asked for two prototypes of a military version, the XF5U.

The XF5U was a disk, except for the pilot’s cockpit and the horizontal and vertical tail surfaces. Most of the aircraft was Metalite – a “sandwich” material with a balsa wood core and thin sheets of aluminum bonded on both sides, which was strong and light.
It was powered by two Pratt and Whitney R-2000-7 radial engines buried in the wing and driving 16.5-foot diameter, four-bladed cross-shafted, counter-rotating
propellers at the wing tips.

The massive propellers required a very complicated linking system to guarantee that if one engine failed, it could be instantly de-clutched from the system so the remaining engine would drive both propellers. The propellers also had a built-in cyclic movement like a helicopter’s main rotor and a low-disk loading, which allowed them to generate high thrust at low or zero speed and “hang on the props” for landing.

Two vertical tails provided directional control and “elevons” – a combination of elevator and aileron – provided lateral and longitudinal control. For low speed control, the XF5U-1 had a 15-square-foot stability flap located at the wing trailing edge that, when the landing gear was down, automatically provided for changes in airplane trim with changes in attitude.

Aerodynamically, Zimmerman’s aircraft had a low aspect ratio wing which provided high stall resistance and the ability to fly at a high angle of attack, slowly with the nose high in the air, under full control – ideal for carrier landings. Normally, a low-aspect wing has poor performance because of the induced drag created at the wingtips from wingtip vortices. Zimmerman overcame this by mounting the huge propellers on the wing tips and making them counter-rotating – turning in the opposite direction to the tip vortices and blowing the vortices away from the wing tips.

This gave his aircraft roughly the same induced drag as a conventional aircraft, but with a much smaller, stall-proof wing area. The problem with this arrangement was that the propeller’s radius covered nearly the entire front area of the aircraft, eliminating the possibility of forward-firing rockets on the “wing,” and would have required any radar to be mounted forward of the propellers to prevent interference.

Because Vought was fully occupied producing the F-4U Corsair fighter, the V-173’s flight tests progressed slowly. But in several hundred flights, it proved it could fly as slow as 15 mph and had a top speed of 138 mph, despite its low-powered engines.
It was flown by Charles Lindbergh and a number of Navy pilots, among others. The success of the V-173 made the XF5U design look promising, but it was not until after the war that the program got serious.

At that point, taxi tests of the XF5U showed extreme mechanical vibration between the complicated engine-propeller shafting, gear boxes and airframe structure. The XF5U occasionally “hopped” off the ground, but it was determined that flight tests would require the open spaces of Edwards Air Force Base, Calif., which would mean shipping the aircraft to Edwards through the Panama Canal on a barge.

This was the last straw for the Navy, who preferred to spend its Research and Development money on the new jet aircraft and canceled the program in late 1947.
The prototype V-173 was transferred to the Smithsonian Museum and is
currently being refurbished for display.