The experimental winged rotor-wing Sikorsky S-72 made its first flight on this day in 1976.
The Sikorsky S-72 in its X-wing configuration
The Sikorsky S-72 was a so-called compound helicopter that was built by Sikorsky for the US Army and NASA. It was also known as the RSRA (Rotor Systems Research Aircraft. The S-72 was built using an Sikorsky s-67 airframe and Sikorsky S-61 rotor and gearbox. To be able to operate as a compound helicopter it could fitted with TF34 turbofans and wings. In this compound mode it could fly up to speeds of 300 knots (560 kph).
A unique feature of the S-72 was the emergency escape system for the crew. In case of an emergency, the system would be activated which would fire explosive shear bolts on the rotor blades to jettison them. The crew could then choose to fly the aircraft as a fixed-wing aircraft. Or if needed they could subsequently jettison escape panels from the roof of the aircraft and the crew would vacate the aircraft using ejection seats,
The S-72 in its compound configuration
The first aircraft was completed as a pure helicopter, the second production was in the compound configuration. During the program, NASA started to investigate the "stopped rotor system. In the configuration, the main rotor could be halted in flight and still be used as a lift generating device, similar to fixed wings. This would allow the aircraft to lift off vertically and transition to horizontal flight like a conventional aircraft. A configuration in which the aircraft was flown although the rotor was removed before flight.
The S-72 during high-speed taxi tests and in flight, in its conventional aircraft configuration
The X-wing system used a complex, computer-controlled, compressed air system to control the lift generated by the blades. The system altered the edge of the blade from which air was ejected depending on the position of the blade in the rotor disc (forward travelling or rearward travelling) A system that was comparable to the blown flaps principle. Although any hours went into development and ground testing with an X-wing rotor configuration the aircraft never flew in this configuration. The project was halted in 1988. Some specifications for the Sikorsky S-72: General characteristics
Crew: 3
Length: 70 ft 7 in (21.51 m)
Wingspan: 45 ft 1 in (13.74 m)
Height: 14 ft 6 in (4.42 m) to top of rotor hub
Wing area: 370 sq ft (34 m2)
Empty weight: 21,022 lb (9,535 kg) (compound helicopter configuration)
Gross weight: 26,200 lb (11,884 kg) (compound helicopter configuration)
Powerplant for rotor system: 2 × General Electric T58-GE-5 turboshaft, 1,400 shp (1,000 kW) each
Powerplant for propulsion: 2 × General Electric TF34-GE-400A turbofan, 9,275 lbf (41.26 kN) thrust each
Main rotor diameter: 62 ft 0 in (18.9 m)
Main rotor area: 3,019 sq ft (280.5 m2)
Maximum speed: 300 kn (350 mph, 560 km/h)
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