Conceived as revolutionary and advanced naval fighter, its design fueled by the rapid advancement in aviation technology at the time, the Vought XF5U project promised a highly efficient and tough aircraft with amazing capabilities and flying characteristics.
The V-173 Pancake Prototype
Charles H. Zimmerman dreamt of novel aircraft designs, including those with flat circular bodies that acted as the lifting surface and would be capable of VSTOL (vertical and/or short take-off and landing). Born in Olathe Kansas in 1908, he attended the University of Kansas, receiving a degree in electrical engineering and he also received a master’s degree in aeronautical engineering from the University of Virginia. He did work for the NACA (National Advisory Committee for Aeronautics), later to be known as NASA (National Aeronautics and Space Administration), designing a wind tunnel for the testing of model aircraft. He also worked on a myriad of aircraft design topics and theory.
Working in the 1930’s with the Chance Vought Division of the United Aircraft Division, Zimmerman built a scale remote controlled model of a rather flat disc-shaped aircraft. The model was around 2 feet long and was electric-powered. The model airplane was a success.
Success of a large-scale flying model designated the V-162, and Navy funding led to the creation of a full-scale model being produced for wind tunnel testing at Langley, Virginia. These tests were conducted in 1940-1941. Results of the wind tunnel testing led to building a prototype to test the concept, and would be designated the V-173 and took on the names “Zimmer Skimmer”, “Flying Pancake” and “Flying Flapjack”.
The V-173 was constructed mostly of wood and covered in fabric, with the forward cockpit structure and propeller nacelles made of aluminum. The aircraft was lightweight and powered by two Continental piston engines turning specially designed three-bladed propellers measuring 16 feet 6 in. The flat four-cylinder engines only generated 80 horsepower each, but speed was not the goal at the time.
The two Continental engines were placed deep in the fuselage/wing, one on either side of the cockpit, and the propellers placed at the wingtips at the leading edge, driven by a complex series of gears and shafts, and rotated opposite one another. The power shafts were designed so that if one engine failed, the other engine would drive both propellers, however vibration issues with the system were never fully resolved. The air-cooled engines had a cooling fan attached and inlets in the leading edge of the aircraft allowed for cooling air, being exited by flaps in the upper fuselage.
The aircraft had lengthy fixed main landing gear and a tiny tailwheel. With the aircraft sitting with the nose angled up at 22 degrees, a hatch for access to the cockpit was placed on the underside of the aircraft, and windows below the cockpit allowed the pilot to see the ground during take-offs. Two vertical fins with rudders were at the tail of the aircraft, and after difficulty landing the aircraft, caused by air becoming trapped below the aircraft and force the tail upwards, two stabilizing flaps were added between the tails separated by the tailwheel. This allowed the trapped air to escape.
A horizontal stabilizer was attached to each side with a surface that acted as both an elevator and aileron (ailavator). Wingspan was 23 ft 4 in, length was 34 ft 9 in, and it was almost 13 ft tall. Top speed was only 138 mph, and it carried 20 gallons of fuel.
With a loaded weight of just over 3,000 pounds, the V-173 first flew on Nov. 23, 1942 following wind tunnel testing. Several Navy pilots flew the aircraft, as well as Charles A. Lindbergh. A total of 131 hours of test flights proved successful with the aircraft being quite maneuverable, the only issue being the aircraft was somewhat underpowered. It was discovered the aircraft could take-off within 200 feet though and, if facing at least a 25-knot headwind, coupled with its high angle of 22 degrees to the ground, the V-173 could achieve vertical take-off. The aircraft had remarkable low-speed capabilities and was almost impossible to stall.
Flipped Flapjack
One incident of note during test flight occurred on Jun. 3, 1943, when pilot Richard Burroughs had to make an emergency landing on a beach near the Vought plant in Connecticut. An engine vapor lock problem caused the emergency, and Burroughs attempting to land on the beach, noticed sunbathers in his path, and in order to avoid them he locked the brakes and flipped the “Flying Pancake” onto its top.
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The only damage was to the propellers, with both the pilot and sunbathers walking (if not running) away from the scene unharmed. The strength of the design was evidenced by the aircraft landing upside down on the cockpit without the cockpit being crushed. It is reported a beach towel was recovered from under the aircraft. The propellers were repaired and the flights continued. This incident encouraged Lindbergh to fly the aircraft later in the year.
Naval Interest Leads to the XF5U
The successful tests of the V-173 and its remarkably short take-off and landing abilities along with the aircraft’s tight turning radius interested the United States Navy. The aircraft would be ideal for carrier operations or operations from any large ship. Zimmerman’s concept was proven, now they needed one that was larger, faster, and armed. Zimmerman and Vought worked to finalize a design for a fighter version, and designated it the VS-315. The Navy requested two prototypes, naming them the XF5U-1.
Layout was similar to the V-173, but instead of using fabric covering, a material known as “Metalite” was used to cover the airframe. Metalite was made by a layer of balsa wood bonded between layers of thin aluminum. The material was very strong. Zimmerman had originally envisioned the pilot in the prone positon, but conventional seating was used instead. Access to the cockpit was via a series of recessed steps up the back of the aircraft instead of the lower trap door design used on the V-173.
While maintaining the same basic layout as the V-173, the XF5U was larger and heavier. The aircraft’s normal loaded weight was 16,764 lb, up to a maximum of 18,772 lb. A pair of Pratt and Whitney Twin Wasp radial engines was placed deep in the fuselage, again with one on either side of the cockpit. Generating 1,350 horsepower each, they delivered power using a similar complex configuration of shafts and right angle gear boxes and drives. Again large propellers were placed on the leading edge at the wingtips. Engine cooling was performed in a similar fashion as the V-173.
Retractable landing gear was also a part of the new design, the long main gear giving the aircraft an 18.7 degree ground angle. For carrier operations, an arresting hook deployed from the aircraft’s upper surface and hung over the rear of the plane. A catapult bridle could be attached to the main gear to allow for catapult assisted launching.
An acrylic nose allowed for a gun camera and, although the prototype was not armed, plans for armament included six .50 caliber Browning machine guns, three on each side of the cockpit with 400 rounds per gun. The four lower guns could be removed and replaced with two 20 mm cannons. A pair of hardpoints under the aircraft could each carry up to a 1,000 lb bomb.
Delayed by the prioritizations of war and the complexity and unconventional design, the first XF5U prototype began ground tests in Aug. 1945. Specially designed propellers had been ordered for the aircraft, but delays necessitated propellers from the F4U-4 Corsair to be installed until the proper ones arrived. With the proper propellers installed, taxi tests began in Feb. 1947. Concerns arose again over the complex system of drive shafts and gear boxes bringing power from the engines to the propellers, and test pilot Boone Guyton only made small hops in the aircraft in Connecticut, not enough to be considered actual flight time. It was decided the aircraft should be tested in California at Edwards Air Force Base.
The problem was the aircraft could not be disassembled for transport, and was too large to be shipped over the road. The aircraft would have to be placed on a ship and taken through the Panama Canal to California. With the expenses adding up, and the onset of jet fighter technologies, the Navy soon lost interest in the project and it was quickly cancelled on Mar. 17, 1947. Two prototypes were built.
The Pancake is one Tough Cookie
Unfortunately, the one complete ready-to-fly example of the XF5U would be slated for destruction once the project was cancelled. However, it is reported that during dismantling of the aircraft, a wrecking ball had to be employed due to the aircraft’s extreme toughness. The other airframe was destroyed during ground tests.
The single example of the V-173 made its last flight on Mar. 31, 1947. It was placed in storage at Norfolk Virginia for years, fell into decay and was forgotten. Then in 1960 the aircraft was donated to the Smithsonian National Air and Space Museum and placed in storage until 2003. The aircraft was then moved to Texas and restored by the Vought Heritage Foundation in Grand Prairie. The V-173 was beautifully restored by volunteers and placed on display at the Frontiers of Flight Museum in Dallas, TX.
