History
Background
In 1937, Charles H. Zimmerman, the aeronautical engineer celebrity who had been working on a variety of projects, joined the Vought company. One of his projects he worked on was the Vought V-162. It featured an unconventional all-wing design consisting of a flat, disk-shaped body serving as the lifting surface, powered by two buried piston engines. The design presented a greater interest for the US Navy, and Zimmerman was offered funding for his further development of the V-162.
The Navy obtained data and concept documentation in 1939, with wind tunnel tests on full-scale models being built and completed in 1940-1941. Subsequently, the proof-of-concept prototype V-173 was constructed of wood and canvas as a lightweight test model powered by two 80-hp Continental A-80 engines turning F4U Corsair propellers, later fitted with a pair of modified 16-foot and 6-inch three-bladed propellers. It had a tall and fixed undercarriage combined with a small tailwheel that gave the aircraft a 22-degree nose-high angle. Its disc wing design was to specifically overcome the disadvantages of induced drag created at the wingtips, with the large propellers actively canceling the drag-causing tip vortices.
On November 23, 1942, the V-173 flew for the first time. After eliminating the issues with the flight, the V-173 aircraft went on to do more flight testing through 1942 and 1943 with 190 true-controlled flights, causing reports of UFOs from the local civilians. Remarkably, the aircraft was so durable that it was not severely damaged when flipping over and crashing on a beach during an emergency landing. The pilot did not sustain any significant injuries either. The V-173 made its last flight on March 31, 1947, with 131.8 hours and 200 flights.
Design and Development
Flashbacking to January 19, 1942, the US Navy’s Bureau of Aeronautics requested Vought and Zimmerman to propose a fighter built similar to the V-173 design, and on September 17, 1942, the Navy issued a letter of intent for two VS-315 fighters, designated XF5U-1.
The Vought XF5U-1 would be a rigid aluminum airframe covered with lighter and stronger Metalite. Overall, it had the same basic configuration as the V-173 but was much heavier and more complex. The XF5U’s entire disk-shaped fuselage was intended to provide lift along with a short wingspan and large counter-rotating propellers placed at the wingtips. The aircraft was fitted with two vertical tails, and between them were two stabilizing flaps. The stabilizing flaps were there to be deflected up by air loads flying near the ground. It allowed air to escape from under the aircraft. The stabilizing flaps improved aircraft control during landing.
On the sides of the XF5U were hydraulically boosted, all-moving ailavators, combining both ailerons and elevators. The ailavators had a straight leading edge rather than the swept leading edge used on the V-173’s ailavators. Two large balance weights projected forward of each ailavator’s leading edge. A conventional seating position was chosen for the pilot instead of a prone position. A catapult bridle could be attached to the aircraft’s main gear to facilitate catapult-assisted launches from aircraft carriers. For carrier landings, the XF5U had an arresting hook that could be deployed from the aircraft’s upper surface and hung over the rear of the aircraft.
As designed, the XF5U was to be powered by two 1,600-hp Pratt & Whitney R-2000-2 engines, later substituted by two 1,350-hp R-2000-7 due to development changes. The engines were buried in the fuselage, and engine-driven cooling fans drove air through intakes in the aircraft’s leading edge. Cooling air exit flaps were located on the engine nacelles on the upper and lower fuselage. An exit intercooler air was located farther back on the top side of each nacelle. The engines were controlled by a single throttle lever and could not be operated independently except at startup.
The propellers were to have a built-in cyclic movement similar to a helicopter’s main rotor. The aircraft initially used propellers similar to those on the V-173 prototype, but they were later replaced with propellers taken from the Vought F4U-4 Corsair, with the lessons learned from the ongoing flight tests of the V-173.
The aircraft had provisions for the armament consisting of six M2 Browning machine guns, three stacked on each side of the cockpit, with 400 rounds per gun. Two hardpoints under the aircraft could each accommodate a 1,000-lb bomb. However, the armament was not installed on the prototype.
A wooden mockup of the XF5U was built and evaluated by the Navy in June 1943. On July 15, 1944, the Navy ordered two fully-powered prototypes. Because of the complex systems and unconventional layout, the construction of XF5U was delayed and further stagnated by higher priority work during World War II.
“Flight” Trials
On August 20, 1945, the XF5U with serial 33958 was finally built and completed at the Vought factory in Connecticut. It was rolled out with the F4U-4 propellers installed, and some ground runs were conducted. However, it was not ready for more tests until late 1946, when the articulating propellers arrived for replacement. On February 3, 1947, the XF5U started taxi tests from the factory, but concerns over the propeller drive quickly surfaced. The aircraft made only a few hops in the flight. No true-controlled test flights were attempted. Because the rigid metallic construction made it difficult to disassemble, further trials of XF5U were to be taken at Edwards Air Force Base, where facilities could handle any complications, especially with untested engine-propeller transmission systems, that might arise. It had to be transported on the ship from Connecticut to California via the Panama Canal since the aircraft was too large to be transported over roads. It was an expensive transportation request for the Navy.
However, the Navy had been switching its focus from piston-engined aircraft to all-jet aircraft. Furthermore, the delay and overruns of the XF5U project failed to meet the original development time and budget. With jet aircraft coming into service, the Navy ultimately terminated the project on March 17, 1947. Orders were issued to scrap the XF5U before it made its first flight. With the engines and instruments salvaged first, it was found difficult to scrap the XF5U’s entire airframe due to its rigid metallic construction. It was to be smashed by a giant wrecking steel ball, but the first drops failed to destroy the strong airframe. After adjusting to determine where the ball must be dropped, the aircraft was finally destroyed and became a pile of metal scraps after a few successful controlled drops. The remains of scraps were sold, removing any traces of them. The second XF5U airframe with serial 33959 was assembled and destroyed during static tests. Only the prototype V-173, the father of these XF5Us, was spared from fate and transferred to the Smithsonian Museum for display.
It was a tragic and unfortunate event for the prototype that came close to flight testing and never flew. Zimmerman had moved on to develop VTOL aircraft for the rest of his career.
