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XF5U "Flying Pancake"
Vought-Sikorsky V-173 "Zimmer Skimmer"

While continuing production of the F4U Corsair, Vought developed the SV5U "Flying Pancake" of "Flying Flapjack" which gained its nickname because of its flattened, rounded shape, and the F6U Pirate, one of three first-generation naval jets along with the North American FJ-1 Fury and McDonnell FH-1 Phantom.

This multi-million dollar project, the most radical conventionally-engined aircraft ever built, nearly became the first V/STOL (vertical takeoff and landing) fighter. It was a high-performance long range propeller fighter that had a stall speed of about 40mph, which means very short take off and landing. It could thus launch and land on short aircraft carrier decks, which meant possibly smaller aircraft carriers, and even fly off of jury-rigged flight decks on Merchant ships or cow pastures or beaches. Some commentators claim that the aircraft's large propellers served as lifting rotors in hovering flight, but this is counter-factual, as they were only used for forward flight.

Throughout its history, the "Vought" name was synonymous with aviation firsts, including many one-of-a-kind and experimental aircraft. One of the more unusual was the XF5U-1, an early vertical takeoff and landing (VTOL) design built in 1948. Only two of these aircraft were built by Chance Vought Aircraft. Although the XF5U-1 were never flown, the smaller V-173 prototype did fly. It gained its nickname, the Flying Pancake, because of its flattened, rounded shape.

One of the most unusual aircraft ever designed for the U.S. Navy was the Chance Vought V-173, also known as the Zimmerman Skimmer. The most radical conventionally-engined aircraft ever built, it was a prototype "proof of concept" aircraft that lacked wings, instead relying on its flat circular body to provide the lifting surface. This multi-million dollar "Flying Flapjack" project nearly became the first V/STOL (vertical takeoff and landing) fighter. While continuing production of the F4U Corsair, Vought developed the SV5U "Flying Pancake," which gained its nickname because of its flattened, rounded shape with an almost circular wing.

Charles Zimmerman, famous for the XF5U "flying flapjack," which he designed for Vought during the 1940s, had been busy for a number of years trying to make the airplane into a VTOL machine. He had started his career at NACA Langley in 1929, only two years after Lindbergh's transatlantic flight. Zimmerman was a long-time employee of the NACA, a member of the Space Task Group (the Project Mercury management team), an aircraft industry designer, an Army aviation chief engineer, and a NACA Headquarters manager, among other accomplishments.

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, efficiently to provide lift at high speeds. Another feature of novelty which increasesd 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 and their rotation in such directions that most of the energy which would otherwise be lost in twist of the slip-stream is returned to the machine in the form of diminished induced drag.

Essentially, the invention consisted of a stream-line body, of which fore-and-aft sections are airfoil profiles and transverse sections are approximately ellipses, housing the load and accessories and mounting the control surfaces and en- 15 gines; two engines of light weight per horsepower; and controllable pitch propellers mounted at the wing tips and rotating so that the blade tips move downwardly when farthest from the plane of symmetry of the machine. Two engines were used so that torque and gyroscopic couples will be neutralized, to provide structural and aerodynamic efficiency and to make possible the continuation of flight on one engine in case of the failure of the other. While the maximum aerodynamic efficiency was realized by the sectional contour, it was understood that the enclosure for the pilot and passengers may be made deeper and be suitably faired into the adjacent part of the surface of the craft.

The stream-lined body of the craft formed a low-aspect-ratio airfoil. The shape chosen was such as to create a minimum of profile drag and has sufficient span to keep the induced drag to a small value at cruising speeds. The entire surface of the illustrated embodiment of the present invention was stream-lined in all dimensions. The substantially elliptical cross-sectional shape provided an effective dihedral angle on the lower surface, and the fore-and-aft contour is such as to secure a smooth, substantially undisturbed flow of air over the craft.

The most efficient streamlining requires that the depth of the body in smaller machines, be such that the pilot and passengers, if any, occupy a prone position. This position is not objectionable and is by far the most comfortable in case of air-sickness. As above stated, the 50 depth may be made greater to permit the pilot to sit as in conventional aircraft, but except for machines of large size this involves departures from the almost perfect stream-lining attained in the craft The propellers are mounted at the extreme tips of the entering edge of the craft and rotate in such directions that the tips of the blades move downwardly when farthest from the plane of symmetry. It is apparent that the described positions and directions of rotation of the propellers result in an increase of the effective span of the structure, since the upwardly acting component of reaction to the downwardly moving propeller blade beyond the wing tip is transmitted to the craft as lift. In addition, the energy normally wasted in the twist of the slipstream is to a great extent recovered in that it acts upwardly upon the lower surface of the craft and thereby increases the lift, and on the upper surface thereof it sweeps outwardly toward the lateral edge and so prevents formation of wing-tip vortices and eliminates the very considerable induced drag due to such vortices.

The propellers are of the controllable-pitch type so that they may be given a low pitch when taking off or hovering and a higher pitch for high speed, substantially horizontal flight. The engines are connected to 30 the propellers by means of conventional reduction gearing and clutches whereby, if one engine stops it may be disconnected from its propeller and the two propellers will be driven by the other engine at sufficient speed to enable the pilot to select a landing place and bring the machine down safely. Other advantages are derived from the described arrangement and operation of the propellers in that torque and gyroscopic couples are neutralized and there are no turning moments impressed upon the craft due to the rotation of one propeller at a speed different from that of the other.

The propellers are so placed relative to the remainder of the craft that the 'propeller slip-streams "are at all times directed over the lifting and control surfaces. With the fairly high loading of the propellers employed the slip-stream velocity will always be high, 60 to 100 miles per hour, so that the control surfaces will be very effective in hovering and low-speed flight. This is a definite advantage over helicopter types where a relatively low loading of the sustaining rotating member is used, for in the latter case the speed 10 of the slip-stream or down-wash is low and cannot provide good control. The use of fairly heavily loaded propellers for lifting surfaces in hovering flight has an additional advantage over the use of comparatively lightly loaded sustaining rotating members as in the case of helicopters and other rotating wing devices in that gusts and changes in wind direction will produce much less violent changes in attitude or velocity of the craft.

With the concurrence of NACA, Zimmerman approached United Aircraft Corporation with his novel design in 1937 and joined United's Chance Vought Aircraft Division in that year as project engineer. 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.

The V-173 was flying wing, to minimize wetted area and parasite drag, with the propellers at the wing tips, rotating so as to oppose induced drag. Twin fins and rudders were always part of the design for directional stability and control. The Vought-Sikorsky V-173 "Zimmer Skimmer" was the airplane prototype on which the XF5U was based. The military version of this aircraft, the XF5U-1 - a fighter aircraft with an almost circular wing, was constructed later but never flown.

The Navy initiated development of the Flying Flapjack with award of contract to Vought-Sikorsky for design of the VS-173 on 27 February 1940. The Vought-Sikorsky V-173 was set up in the NACA Langley full-scale wind tunnel in 1941. For many years the NACA had the best aeronautical research facilities in the world, and in many ways these facilities determined what the NACA would choose to do and be required to do. Having the world's only full-scale wind tunnel enabled the Committee to perform unique experiments, but it also dictated that the research program make full use of the full-scale tunnel.

On November 23, 1942 the "Flying Flapjack," made its 1st flight when Chance Vought test pilot, Boone T. Guyton, took the V-173 into the air. Flight testing of the V-173 went on through 1942 and 1943, resulting in reports of "flying saucers" from surprised Connecticut locals. The V-173, flown many times, was capable of very short takeoffs and landings, and it was flown by Charles Lindbergh.

In January 1942 BuAer requested the proposal for two 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 planform (less ailevators 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 XF8U-1.

The Navy's Bureau of Aeronautics established the Navy Liaison Office at Edwards AFB on Aug. 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.

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Page last modified: 07-07-2011 02:32:00 ZULU