Lockheed CL-595 / Model 186 Aerogyro / Model 286 / XH-51A Aerogyro
Lockheed began developing its rigid rotor concept with the CL-475 helicopter design in 1959 and the performance of the CL-475 encouraged Lockheed to continue 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, eventually sold them to a collector where they were destroyed by fire in 1988.
The two XH-51A examples (Serial Numbers 61-51262 and 61-51263) are stored at the United States Army Aviation Museum at Fort Rucker.
Civil version of 286/XH-51
Rotor dia: 35 ft
Length: 32 ft
286 / XH-51A
four place, three-bladed rotor
Engine: 1 × Pratt & Whitney Canada PT6B-9 turboshaft, 550 hp (410 kW)
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)
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/sq.ft (20.9 kg/sq.m)
Power/mass: 0.27 hp/lb (0.44 kW/kg)
modified with a four-bladed rotor and stub wings and an auxiliary 2,900 hp Pratt & Whitney J60-2 engine.
Rotor diameter: 31 ft 7 in
Length: 31 ft 7 in
five place civilian or military light helicopter offered for sale, none were sold.
Model 286 / XH-51N
five place, three-bladed rotor modified for NASA test purposes.