Lockheed XH-51

The Lockheed XH-51, company Model 186, is an American single-engine experimental helicopter designed by Lockheed Aircraft, using a rigid rotor and retractable skid landing gear. It was the test vehicle of a joint United States Army and Navy research program on rigid rotor technology, selected in February 1962 after Lockheed's CL-475 rigid-rotor design of 1959 had shown the agility of the concept. Two four-seat XH-51A aircraft were built with the 550 shaft horsepower Pratt and Whitney Canada PT6B-9 turboshaft engine, and the first flew on 2 November 1962. Instability at higher speeds led Lockheed to fit a four-bladed rotor. In 1964 the second XH-51A was converted into a compound helicopter with a 16.1 foot wing and a Pratt and Whitney J60 turbojet on the left wing, and on 29 November 1967 it reached 263 knots in a shallow descent. NASA ordered the XH-51N, and Lockheed built two Model 286 demonstrators, certificated in June 1966 but never sold. The XH-51 led to the AH-56 Cheyenne, Lockheed's last helicopter.