The Vought V-173 "Flying Pancake" was an American experimental test aircraft built as part of the Vought XF5U program during World War II.

Both the V-173 and the XF5U featured an unorthodox "all-wing" design consisting of a flat, somewhat disk-shaped body (like a pancake flying, hence the nickname) serving as the lifting surface.[1] Two piston engines buried in the body drove propellers located on the leading edge, at the wingtips.

The disc wing design featured a low aspect ratio that overcame the built-in disadvantages of induced drag created at the wingtips with the large propellers actively canceling the drag-causing tip vortices. The propellers were arranged to rotate in the opposite direction to the tip vortices, allowing the aircraft to fly with a much smaller wing area. The small wing provided high maneuverability with greater structural strength. The empennage consisted of two vertical fins with rudders, all-moving stabilizers with anti-servo tabs,[5] and two large elevator/trim surfaces on either side of centerline on the trailing edge of the wing planform.

The first flight of the V-173 was on 23 November 1942 with Vought Chief Test Pilot Boone Guyton at the controls. 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. This contributed to the aircraft feeling much too heavy when maneuvering for its light weight. In addition to this on the first few flights, the pilot was never able to achieve enough speed to achieve the correct amount of airflow over the control surfaces to pull the aircraft into level flight. The test pilot Guyton discussed these issues with Zimmerman and they worked to eliminate these issues. In addition to this Guyton commented that the cockpit design was poor. He explained that in addition to the poor comfort the pilot had limited to no use for the clear bottom panels of the cockpit. He explained that the pilot sat too high in the cockpit to effectively use these lower panels for takeoff or landing.[10] Flight testing of the V-173 went on through 1942 and 1943 with 190 flights, resulting in reports of UFOs from surprised Connecticut locals.[11] Charles Lindbergh piloted the V-173 during this time and found it surprisingly easy to handle and exhibiting impressive low-speed capabilities. Both Lindbergh and Guyton found that they were almost unable to stall the aircraft. Guyton was able to keep the aircraft in flight no matter how hard he pulled the stick in low-speed flight ranges at any altitude under 20,000 ft. 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.[12] Despite their inability to stall the aircraft they did find low speed handling to be a persistent issue largely due to the shape of the lifting body. They found that the aircraft acted as an airbrake when it was pulled into a high angle of attack. This meant that the control surfaces, the horizontal stabilizers, in particular, would become very hard to operate at low speeds such as stalls, takeoff, and landing.

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The developmental V-173 made its last flight 31 March 1947. In 131.8 hours of flying over 190 flights, Zimmerman's theory of a near-vertical takeoff- and landing-capable fighter had been proven. This project would be improved upon including the addition of potential armament with the Chance Vought XF5U. This project would improve on many of the weaknesses discovered during the testing of the V-173 prototype.

As of April 2012, after undergoing a long restoration by Vought Aircraft Heritage Foundation volunteers, the V-173 is on loan from the Smithsonian Institution to the Frontiers of Flight Museum in Dallas, Texas.