|XH-51A following its conversion into a compound rotorcraft testbed|
|Role||Helicopter, compound helicopter|
|First flight||2 November 1962|
|Primary users||United States Army
|Developed from||Lockheed Model 86|
The Lockheed XH-51 (Model 186) was a three-bladed, single-engine experimental helicopter designed by Lockheed-California, a division of Lockheed Aircraft, utilizing a rigid rotor and retractable skid landing gear . The XH-51 was selected as the test vehicle for a joint research program conducted by the United States Army and United States Navy to explore rigid rotor technology.
Lockheed began developing its rigid rotor concept with the CL-475 helicopter 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.
Two four-seat, three-bladed XH-51As were ordered and built for the program. Powered by the 550 shp (410 kW) Pratt & Whitney Canada PT6B-9 turboshaft engine, XH-51A (serial number 61-51262) 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 (TRECOM) contracted with Lockheed to modify one of the XH-51 aircraft into a compound helicopter.
The second XH-51A (serial number 61-51263) was subsequently converted by adding wings with a span of 16.1 ft (4.9 m), and a 2,500 hp (1,864 kW) 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 (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. 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 coumpound 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.
- four place, three-bladed rotor
- XH-51A Compound
- modified with a four-bladed rotor and stub wings and an auxiliary 2,900 hp Pratt & Whitney J60-2 engine.
- five place, three-bladed rotor for NASA test purposes.
- Model 286
- five place civilian or military light helicopter offered for sale, none were sold.
Specifications (XH-51A) 
Data from Janes's All The World's Aircraft 1969-70 
- Length: 40 ft 9 in (12.40 m)
- Rotor diameter: 35 ft 0 in (10.67 m)
- Height: 8 ft 2½ in (2.50 m)
- Disc area: 962 ft² (89.4 m²)
- Empty weight: 2,790 lb (1,265 kg)
- Max. takeoff weight: 4,100 lb (1,864 kg)
- Powerplant: 1 × Pratt & Whitney Canada PT6B-9 turboshaft, 550 hp (410 kW)
- Maximum speed: 151 knots (174 mph, 280 km/h)
- Cruise speed: 139 knots (160mph, 257 km/h)
- Range: 226 NM (260 mi, 418 km)
- Service ceiling: 16,000 ft (4,876 m) (hover ceiling (in ground effect))
- Rate of climb: 2,000 ft/min (10 m/s)
- Disc loading: 4.26 lb/ft² (20.9 kg/m²)
- Power/mass: 0.27 hp/lb (0.44 kW/kg)
See also 
- Related development
- Related lists
- Connor, R. Lockheed CL-475. Smithsonian National Air & Space Museum. Revised on 15 August 2002. Accessed on 3 September 2007.
- Robb, Raymond L. "Hybrid helicopters: Compounding the quest for speed", Vertiflite. Summer 2006. American Helicopter Society. Accessed on 8 November 2008.
- Hirschberg, Michael J. and David K. Daley. "Lockheed", US and Russian Helicopter Development In the 20th Century. American Helicopter Society, International. Accessed on 16 November 2007.
- Badrocke and Gunston (1998)
- Landis and Jenkins 2000, pp. 29-31, 34.
- "Rotary Wing Collection". United States Army Aviation Museum.
- Taylor, J.W.R. (ed.) (1969). Jane's All The World's Aircraft 1969-70. London: Jane's. pp. 359–360.
- Landis, Tony and Jenkins, Dennis R. Lockheed AH-56A Cheyenne - WarbirdTech Volume 27, Specialty Press, 2000. ISBN 1-58007-027-2.
|Wikimedia Commons has media related to: Lockheed XH-51|
- XH-51A newsreel at archive.org (begins at 3:47 mark)
- Lockheed Model 186 / CL-575 / XH-51 - All the World's Rotorcraft
- Lockheed XH-51A Compound - All the World's Rotorcraft