Lockheed XH-51

XH-51

XH-51A following its conversion into a compound rotorcraft testbed
Role	Experimental helicopter
Manufacturer	Lockheed Corporation
First flight	2 November 1962
Status	Retired
Primary users	United States Army NASA
Produced	1962-64
Number built	3

The Lockheed XH-51 (Model 186) was an American single-engine experimental helicopter designed by 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.

History

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.^[1]

The first XH-51A landing on USS Ozbourn (DD-846).

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.^[2]

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,^[2] while tests were conducted for balance and handling. The aircraft's first flight as a true compound helicopter took place on 10 April 1965.^[3] and on 29 November 1967 achieved a speed of 263 knots (302.6 mph, 486.9 km/h)^[4] in a shallow descent. The highest level flight speed was 223 kn (413 km/h; 257 mph).^[5]

In June 1964, NASA ordered a five-seat, three-bladed variant, the XH-51N (NASA 531) as a helicopter test vehicle.^[4]

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.^[6]

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.

Variants

The three XH-51s in flight.

XH-51A: 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.
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.

Remaining aircraft

The two XH-51A examples (Serial Numbers 61-51262 and 61-51263) are stored at the United States Army Aviation Museum at Fort Rucker.^[7]

Specifications (XH-51A)

Data from Janes's All The World's Aircraft 1969-70 ^[8]

General characteristics

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)

Performance

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)

References

Notes

↑ Connor, R. Lockheed CL-475. Smithsonian National Air & Space Museum. Revised on 15 August 2002. Accessed on 3 September 2007.
1 2 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.
1 2 Badrocke and Gunston (1998)
↑ Lentine, F. P. ; Groth, W. P. ; Oglesby, T. H. "RESEARCH IN MANEUVERABILITY OF THE XH-51A COMPOUND HELICOPTER" Lockheed, June 1968. Accessed: 27 August 2014.
↑ Landis and Jenkins 2000, pp. 29-31, 34.
↑ "Rotary Wing Collection". United States Army Aviation Museum. Archived from the original on July 13, 2012. External link in |publisher= (help)
↑ Taylor, J.W.R. (ed.) (1969). Jane's All The World's Aircraft 1969-70. London: Jane's. pp. 359–360.

Bibliography

Landis, Tony and Jenkins, Dennis R. Lockheed AH-56A Cheyenne - WarbirdTech Volume 27, Specialty Press, 2000. ISBN 1-58007-027-2.

External links

Wikimedia Commons has media related to Lockheed XH-51.

XH-51A newsreel at archive.org (begins at 3:47 mark)

Lockheed and Lockheed Martin aircraft

Airliners and civil transports	Vega Model 8 Sirius Altair Model 9 Orion Model 10 Electra Model 12 Electra Junior Model 14 Super Electra Model 18 Lodestar Constellation L-049 L-649 L-749 L-1049 Super Constellation L-1249 Super Constellation L-1649 Starliner Saturn L-188 Electra L-402 JetStar L-100 Hercules L-1011 TriStar

Military transports	C-69 Constellation C-121 Constellation C-130 Hercules C-130J Super Hercules HC-130 KC-130 LC-130 MC-130 C-141 Starlifter C-5 Galaxy R6V Constitution Tristar (RAF)

Attack and bombers	YP-24 Hudson XB-30 Ventura AC-130 A-4AR Fightinghawk F-117 Nighthawk Have Blue FB-22

Drones/UAVs	AQM-60 Kingfisher D-21 Desert Hawk Desert Hawk III MQM-105 Aquila Polecat RQ-3 Darkstar RQ-170 Sentinel DC-130

EW and reconnaissance	YO-3 EC-121 Warning Star EC-130 EC-130H Compass Call WC-130 EP-3 U-2 TR-X A-12 SR-71 Blackbird SR-72

Fighters	XFM-2 YP-24 P-38 Lightning XP-49 XP-58 Chain Lightning P-80 Shooting Star XF-90 F-94 Starfire F-104 Starfighter variants YF-12 F-16 Fighting Falcon F-22 Raptor YF-22 F-35 Lightning II

Helicopters	CL-475 XH-51 AH-56 Cheyenne VH-71 Kestrel

Maritime patrol	P-2 Neptune P-3 Orion P-7 S-3 Viking CP-140 Aurora

Trainers	T-33 T2V SeaStar

Experimental	L-133 NF-104A Senior Peg Senior Prom XC-35 XF-104 XV-4 Hummingbird X-7 X-17 X-26B Frigate CL-1200 Lancer X-33 X-35 X-44 X-55 X-56 XFV QueSST

Other	Air Express Big Dipper Model 44 Excalibur Explorer Little Dipper

United States helicopter designations, Army/Air Force and Tri-Service systems

Numerical sequence used by USAAC/USAAF/USAF 1941–present; US Army 1948–1956 and 1962–present; US Navy 1962–present

Main sequence
(1941–1962)

Prefix R-, 1941–1948	R-1 R-2 R-3 R-4 R-5 R-6 R-7 R-8 R-9 R-10 R-11 R-12 R-13 R-14 R-15 R-16

Prefix H-, 1948–1962	H-5 H-6 H-9 H-10 H-11 H-12 H-13/J H-15 H-16 H-17 H-18 H-19 H-20 H-21 H-22 H-23 H-24 H-25 H-26 H-27 H-28 H-29 H-30 H-31 H-32 H-33 H-34 H-35 H-36¹ H-37 H-38¹ H-39 H-40 H-41 H-42 H-43 H-44¹ H-45¹

Main joint sequence
(1962–present)

1962 redesignations	OH-13/UH-13J UH-19 CH-21 OH-23 UH-25 CH-34 CH-37 HH-43

New designations	CH-46/HH-46/UH-46 CH-47 UH-48 XH-49 QH-50 XH-51 HH-52 CH-53/HH-53/MH-53/CH-53E/CH-53K CH-54 TH-55 AH-56 TH-57 OH-58 XH-59 UH-60/SH-60/HH-60/HH-60J YUH-61 XCH-62 YAH-63 AH-64 HH-65 RAH-66 TH-67 MH-68 H-69¹ ARH-70 VH-71 UH-72 H-73 to H-89¹ MH-90 H-91¹ VH-92

1962 redesignations
reusing old numbers

¹ Not assigned

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