X-18

In February 1957, the USAF ordered ten models of the Hiller X-18, a stocky tilt-wing convertiplane powered by two 6000shp Allison turboshafts. This aircraft, weighing nearly 15 tonnes, had the fuselage of the Chase YC-122C twin-engine tactical transport aircraft. The two wing-mounted 7,100 eshp Allison T40-A-14 turboshafts came from the XFY-1/XFV-1 tailsitter (#23 and #24) vertical tail landing fighter aircraft initiated by the US Navy. It had three engines, the two turboprops drove the 16 ft diameter counter-rotating three-bladed propellers and a 3,400 lb thrust Westinghouse J34 turbojet which produced pitch control thrust. Compressed air ejected from a large pipe at the tail controlled longitudinal trim during flight as a helicopter.

The X-18 was tested in November 1959 and made a total of about 20 flights, but the system proved unsatisfactory. The X-18 made its first conventional flight on 24 November 1959. On the 20th flight it had a propeller pitch control problem at 10,000 ft and went into a spin. It was recovered before impact, but was grounded, having never achieved hover. It continued to test ground effects before it was damaged by a test stand failure.