XV-3 HOVERING ON RAMP. Flight Test of Bell XV-3 Convertiplane. Bell VTOL tilt-rotor aircraft hovering along side Hangar One at Moffett Field. The XV-3 design combined a helicopter rotor and a wing. A 450 horsepower Pratt & Whitney piston engine drove the two rotors. The XV-3, first flown in 1955 , was the first tilt-rotor to achieve 100% tilting of rotors. The vehicle was underpowered, however, and could not hover out of ground effect. Note the large ventral fin, which was added to imrpove directional stability in cruse (Oct 1962) ARC-1959-A-25685-14

The X-planes are a series of experimental United States aircraft and rockets, used to test and evaluate new technologies and aerodynamic concepts. They have an X designator, which indicates the research mission within the US system of aircraft designations. The first, the Bell X-1, became well known in 1947 after it became the first aircraft to break the sound barrier in level flight. Most of the X-planes have been operated by the National Advisory Committee for Aeronautics (NACA) or, later, the National Aeronautics and Space Administration (NASA), often in conjunction with the United States Air Force. The majority of X-plane testing has occurred at Edwards Air Force Base. Some of the X-planes have been well publicized, while others have been developed in secrecy. Most X-planes are not expected to go into full-scale production.