First flown tethered on 15 February 1954, the XV-1 'gyrodyne' or 'convertiplane' had a piston engine that could be coupled to the propeller in flight, but powered compressors that fed air to burners on rotor tips for VTOL. Two small tail rotors were added to improve yaw authority during hovering flight. The second of two XV-1s, 53-4017, was the first rotorcraft to exceed 200 mph (320 km/h) but the propulsion system was found to be too complex. Photo from: Vertical Flight Society