The Vought XF5U based on the V-173 aerodynamic research aeroplane. The XF5U had a basically circular wing with twin vertical surfaces at its rear ‘corners’ outside two stability flaps and inside two projecting ‘ailavators’ for pitch and roll control. The primary structure was of Metalite, a material of bonded aluminium and balsa that offered exceptional strength with great lightness. The powerplant comprised two 1600-hp (1193-kW) Pratt & Whitney R-2000-7 radials buried in the thick inner portions of the wings and driving, via a complex double right angle transmission system of shafts and gearboxes, two 4.88-m (16-ft) four-blade propellers located on the forward ‘corners’ of the wing. The propeller blades were articulated and could be moved fore and aft on their shafts in the manner of a helicopter's rotors, permitting the craft to 'hang' on its propellers in a semi-hover at low airspeed.
Vought's specification for the Flapjack called for a maximum speed of 811 kph (504 mph) at 6100m (20,000 ft), while the landing speed was to be as low as 32 kph (20 mph). A proposed turbine-engined variant would have been even faster. The aircraft was to have been armed with six 12.7-mm (03-in) machine-guns or four 20-mm cannon, or two 454-kg (1000-lb) bombs.
The prototype XF5U-1 first began engine tests in August 1945, but it was not until 1947 that the articulating propellers became available and the Flying Flapjack began taxying trials at Vought's Stratford, Connecticut plant.
On 17 March 1947 the US Navy had cancelled the programme in favour of jet-powered fighters, and the prototype, the sole XF5U destroyed. Flight trials were scheduled for 1948.
Engines: 2 x Pratt & Whitney R-2000-7 radials, 1600-hp (1193-kW)