Lockheed began developing its rigid rotor concept with the CL-475helicopter design in 1959. The choice of a rigid rotor meant that the helicopter was more agile than it would have been with a flapping rotor. The performance of the CL-475 encouraged Lockheed to seek further development. Lockheed submitted the CL-475 to the Army as a candidate to replace the Bell OH-13 Sioux and Hiller OH-23 Raven observation helicopters. Lockheed also tested the commercial market waters without success. However, in February 1962, Lockheed's Model 186, a new design based on the CL-475 rigid rotor, was selected as the winner for a joint Army-Navy program to evaluate the rigid rotor for high-speed flight capability.
Operational history
Two four-seat, three-bladed XH-51As were ordered and built for the program. Powered by the 550 shp Pratt & Whitney Canada PT6B-9 turboshaft engine, XH-51A first flew on 2 November 1962. As flight testing progressed, the original three-bladed, rigid rotor system demonstrated instability at higher speed ranges. Lockheed engineers solved the problem by modifying the aircraft with a four-bladed rotor system. In 1963, the Army's Technology Research and Evaluation Command contracted with Lockheed to modify one of the XH-51 aircraft into a compound helicopter. The second XH-51A was subsequently converted by adding wings with a span of 16.1 ft, and a 2,900 lbf Pratt & Whitney J60-2 turbojet engine mounted on the left wing to increase performance. The XH-51A Compound first flew without powering up the turbojet on 21 September 1964, while tests were conducted for balance and handling. The aircraft's first flight as a true compound helicopter took place on 10 April 1965. and on 29 November 1967 achieved a speed of 263 knots in a shallow descent. The highest level flight speed was. In June 1964, NASA ordered a five-seat, three-bladed variant, the XH-51N as a helicopter test vehicle. Lockheed built two demonstrator aircraft, designated the Lockheed Model 286, to market to the public. These aircraft had the five-seat configuration of the XH-51N with the four-bladed rotor system of the XH-51A. The Model 286 was certificated for civil operation by the FAA on 30 June 1966, but Lockheed never sold any aircraft. Lockheed used the aircraft for several years as executive transports. The aircraft were eventually sold to a collector and later destroyed by fire in 1988. To meet the US Army's "Advanced Aerial Fire Support System" programme for an attack helicopter, Lockheed designed a rigid rotor compound helicopter with a pusher tail-mounted propellor which was ordered into production as the Lockheed AH-56 Cheyenne attack helicopter. However technical problems led first to delays then to a suspension of production. Compounded by inter-service rivalry and political issues the Cheyenne was cancelled completely in 1972 and it was Lockheed's last helicopter.
