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The 'Lockheed XH-51' was an experimental helicopter design by Lockheed utilizing a rigid rotor. Lockheed's design, ''Model 186'', was selected for a US military requirement for a high-speed, research helicopter.

Contents

History

Survivors

References

External links

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History

The rigid rotor concept had been in use by Lockheed in an earlier helicopter, the CL-475, since 1949.^[1] The choice of a rigid rotor meant that the helicopter was more agile than it would have been with a flapping rotor. The CL-475 competed against the Hughes 269 in the Army's early quest for a Light Observation Helicopter and Lockheed tested the commercial market waters without success. However, in February of 1962, Lockheed's Model 186, a new design based on the CL-475's successful rigid rotor, was selected as the winner for a joint Army-Navy program to evaluate the rigid rotor for high-speed flight capability.
Two four-seat, three-bladed XH-51As were ordered and built for the program. Powered by the Pratt & Whitney Canada PT6B-9 (550-shp) turboshaft engine, the first helicopter first flew on 2 November 1962. As the aircraft began testing, the original three-bladed, rigid rotor system developed instability at higher speed ranges. Lockheed solved the problem by modifying the aircraft with a four-bladed rotor system. The second XH-51A was subsequently converted into a compound helicopter using stub wings (16.1 ft span), a 2,900 hp Pratt & Whitney J60-2 turbojet engine mounted on the left wing, and the same four-bladed rotor system to increase performance. On 29 November 1967 this XH-51A Compound helicopter achieved a speed of 302.6 mph (486.9 km/h).
In June 1964, NASA ordered a five-seat, three-bladed variant, the 'XH-51N' (NASA 531) as a helicopter test vehicle.
Lockheed built two demonstrator aircraft, designated the Lockheed Model 286, to market to the public (registration numbers N286L and N265LC). 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.

Survivors

The two XH-51A examples (Serial Numbers 61-51262 and 61-51263) are stored at Fort Rucker's Army Aviation Museum.^[2]

References

Uncited references listed below as: ^[3]
1. Lockheed CL-475
2. Rotary Wing Collection
3. Lockheed aircraft cutaways: the history of Lockheed Martin, , Michael, Badrocke, Osprey Aviation, 1998,

External links

★ XH51

★ XH51 Compound

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