NASA / US Army / Sikorsky S-72 Rotor Systems Research Aircraft (RSRA)
In 1971, NASA and the US Army released a request for proposal (RFP) for a “Rotor Test Vehicle Predesign Study” for 9-month study contracts for a platform to test advanced helicopter rotor configurations.
Bell and Sikorsky both won study contracts by presenting their respective proposals to fulfill the requirements. Sikorsky won the contract in 1974 to build two demonstrators with the rotor and gearbox of a Sikorsky S-61/SH-3 Sea King. Thus, the S-72 Rotor Systems Research Aircraft (RSRA) was born.
These very unusual aircraft were able to be modified from a rotary-wing to a fixed-wing configuration. They were powered by General Electric engines to adapt to the testing needs. The helicopter configuration used two T58-GE-5 turboshafts of 1,400 shp (1,000 kW) each, while the rotorless configuration used two TF34-GE-400A turbofans producing 9,275 lb (41.26 kN) of thrust each (plus some shaft power for the tail rotor); the thrust-compound helicopter configuration used all four engines.
The aircraft was 70 ft 7 in (21.51 m) long and had a removable wing with a span of 45 ft 1 in (13.74 m). In compound helicopter configuration, the RSRA had an empty weight of 21,022 lb (9.5 tonnes) and a gross weight: 26,200 lb (11.9 tonnes). The baseline (S-61) main rotor diameter was 62 ft (18.9 m).
The RSRA, which had a crew of three, was also the first helicopter equipped with a crew emergency extraction system — ejection seats and a rotor blade severance system.
Although the aircraft flew in all 3 configurations, it was never flown for its intended purpose as a rotor testbed. One of the two prototypes was modified into a demonstration testbed for the Sikorsky X-Wing Circulation Control Rotor (CCR) concept, with the rollout in August 1986. The aircraft was transported to Edwards Air Force Base for flight testing, but the X-Wing program was canceled in mid-1987 and, unfortunately, the RSRA testbed was mothballed.
— Text by Thomas Ernst and Mike Hirschberg
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