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The Lockheed XH-51 (Model 186) was an American single-engine experimental
Lockheed CL-475
'. Smithsonian National Air & Space Museum. Revised on 15 August 2002. Accessed on 3 September 2007.
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
"Hybrid helicopters: Compounding the quest for speed"
, ''Vertiflite''. Summer 2006. American Helicopter Society. Accessed on 8 November 2008. 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 2,900 lbf (12.9 kN) Pratt & Whitney J60-2
US and Russian Helicopter Development In the 20th Century
''. American Helicopter Society, International. Accessed on 16 November 2007. and on 29 November 1967 achieved a speed of 263 knots (302.6 mph, 486.9 km/h)Badrocke and Gunston (1998) in a shallow descent. The highest level flight speed was . 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" program for an attack helicopter, Lockheed designed a rigid rotor compound helicopter with a pusher tail-mounted propeller which was ordered into production as the
; XH-51A
: four place, three-bladed rotor
; XH-51A Compound
: modified with a four-bladed rotor and stub wings and an auxiliary 2,900 lbf (12.9 kN) Pratt & Whitney J60-2 engine.
; XH-51N
: five place, three-bladed rotor for NASA test purposes.
; Model 286
: five place civilian or military light helicopter offered for sale, none were sold.
XH-51A newsreel at archive.org
(begins at 3:47 mark) {{USAF helicopters 1960s United States helicopters Aircraft first flown in 1962 Compound helicopters Jet-propelled helicopters H-051 Rigid rotor helicopters Single-turbine helicopters United States military helicopters
helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribu ...
designed by Lockheed Aircraft Lockheed (originally spelled Loughead) may refer to:
Brands and enterprises
* Lockheed Corporation, a former American aircraft manufacturer
* Lockheed Martin, formed in 1995 by the merger of Lockheed Corporation and Martin Marietta
** Lockheed M ...
, 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
The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, ...
and United States Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
to explore rigid rotor technology.
Design and development
Lockheed began developing itsrigid rotor
In rotordynamics, the rigid rotor is a mechanical model of rotating systems. An arbitrary rigid rotor is a 3-dimensional rigid object, such as a top. To orient such an object in space requires three angles, known as Euler angles. A special ri ...
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
The Bell H-13 Sioux is an American single-engine helicopter, light helicopter built by Bell Helicopter and manufactured by Westland Aircraft under license for the British military as the Sioux AH.1 and HT.2.
Development
In 1947, the United S ...
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.Connor, R. Lockheed CL-475
'. Smithsonian National Air & Space Museum. Revised on 15 August 2002. Accessed on 3 September 2007.
Operational history
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
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
.Robb, Raymond L"Hybrid helicopters: Compounding the quest for speed"
, ''Vertiflite''. Summer 2006. American Helicopter Society. Accessed on 8 November 2008. 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 2,900 lbf (12.9 kN) Pratt & Whitney J60-2
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, a ...
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.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. and on 29 November 1967 achieved a speed of 263 knots (302.6 mph, 486.9 km/h)Badrocke and Gunston (1998) in a shallow descent. The highest level flight speed was . 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" program for an attack helicopter, Lockheed designed a rigid rotor compound helicopter with a pusher tail-mounted propeller which was ordered into production as the
Lockheed AH-56 Cheyenne
The Lockheed AH-56 Cheyenne was an attack helicopter developed by Lockheed for the United States Army. It rose from the Army's Advanced Aerial Fire Support System (AAFSS) program to field the service's first dedicated attack helicopter. Lock ...
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.
Variants
; XH-51A
: four place, three-bladed rotor
; XH-51A Compound
: modified with a four-bladed rotor and stub wings and an auxiliary 2,900 lbf (12.9 kN) Pratt & Whitney J60-2 engine.
; XH-51N
: five place, three-bladed rotor for NASA test purposes.
; Model 286
: five place civilian or military light helicopter offered for sale, none were sold.
Surviving aircraft
The two XH-51A examples (Serial Numbers 61-51262 and 61-51263) are stored at theUnited States Army Aviation Museum
The United States Army Aviation Museum is an aviation museum located on Fort Rucker near Daleville, Alabama. It has the largest collection of helicopters held by a museum in the world.Phillips 1992, p. 37.Purner 2004, p. 204. The museum feature ...
at Fort Rucker
Fort Rucker is a U.S. Army post located primarily in Dale County, Alabama, United States. It was named for a Civil War officer, Confederate General Edmund Rucker. The post is the primary flight training installation for U.S. Army Aviators and ...
.
Specifications (XH-51A)
See also
References
; Notes ; Bibliography *External links
XH-51A newsreel at archive.org
(begins at 3:47 mark) {{USAF helicopters 1960s United States helicopters Aircraft first flown in 1962 Compound helicopters Jet-propelled helicopters H-051 Rigid rotor helicopters Single-turbine helicopters United States military helicopters