https://en.wikipedia.org/wiki/Lockheed_XH-51

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.

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.


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

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 2,900 lbf (12.9 kN) 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 a shallow
descent. The highest level flight speed was 223 kn (413 km/h; 257 mph).

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 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.

Specifications (XH-51A)

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)




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