Charles Zimmerman (1908-1996) served as an aeronautical engineer through most of his life and operated under the Langley Memorial Aeronautical Laboratory in the 1930s during the world lead-up to World War 2. One of his more radical concepts became the V-173 "Flying Pancake" of the early 1940s which actually led to serious US Navy interest in such a fighter as the XF5U-1. Zimmerman developed a unique design centered around a wing-less frame which was rounded in its general appearance (hence the nickname of "Flying Pancake" assigned to the type). The low-drag design was intended to harness the natural occurring forces of propwash along the aircraft's surface area and promote much improved lift principles. It was believed that such a design would lead to aircraft exhibiting shorter take-offs and landings - critical components of at-sea carrier fighters. In many ways, this could lead to realizing the concept of Vertical Take-Off and Landing (VTOL), harnessing the vertical hover-power of a helicopter with the on-call forward movement of a conventional fixed-wing aircraft.
Zimmerman's original vision included forward-mounted engines driving propellers. To this was added a prone cockpit position to advocate the slimmest forward profile possible, retaining maximum aerodynamic efficiency. With his team, Zimmerman produced scale models of the concept for testing. In 1937, Zimmerman went to work for Vought as a consultant and brought with him the circular low-drag concept to which he was granted a construction of a larger, electrically-driven, remote-controlled testing platform designated as the "V-162".
By this time, the United States Navy was looking to the emerging events of Europe primarily driven by German aggression. As such, many programs were beginning to garner attention of authorities for a full-scale war in Europe was only avoidable for so long to the United States. In 1939, the V-162 concept garnered enough interest from the Navy to warrant a limited developmental contract for a full-scale technology demonstrator from Vought. This product then became the V-173 and was known to the Navy's Bureau of Aeronautics (BuAer) as "VS-135".
The V-173 took the form established by the preceding V-162 and expanded upon its dimensions and end-goals. The prone position for the pilot was never a viable one in military fighter terms and dropped for a more conventional upright seat position. Construction of the airframe was primarily wooden with a fabric covering and power was served through a pair of Continental A-80 horizontally-opposed, 4-cylinder engines of 80 horsepower each driving three-bladed propellers (the original design held two-bladed propellers). The engines were buried within the fuselage which required use of a networked shaft system to drive the blades. The blades were held along structural protrusions at the fuselage leading edges and within clear view of the cockpit. To manage the required clearance of the 16.5-foot spinning blades, the airframe was given a fixed "tail dragger" undercarriage with two single-wheeled main legs. A pair of vertical tail fins were fitted aft as were traditional horizontal planes. However, the V-173 lacked true main wings, making her a highly unorthodox fighter approach.
Development of the V-173 proved slower than expected and the program only received small funding from the US Navy. In its full-scale form, the airframe was entered into a wind tunnel for testing in late 1941 which required some additions - ailevators as it were, these combining the function of both ailerons and elevators. The engines received their due attention before the middle of 1942. On November 23rd, 1942, the aircraft - engines and all - achieved its first flight.
In testing, the prototype exhibited excellent short field qualities and handling but lacked the required fighter performance - it was only able to reach speeds nearing 140 miles per hour and this in a dive. The program suffered at least two forced landings though with no deaths. The prototype was also able to demonstrate a rate-of-climb of 5,000 feet reached in 7 minutes. More design changes followed and the US Navy was sufficiently interested that a prototype for a military version-minded version was agreed upon. This became the XF5U-1 detailed elsewhere on this site.
The V-173 continued in its developmental endeavors for a period longer, achieving a total of 190 to 200 test flights (sources vary) into 1947 despite the war being over by September of 1945. It was formally retired from its testing service on March 15th, 1947. The singular V-173 prototype was saved from the post-war scrap heap and is currently in line for full restoration under the ownership of the Smithsonian group.