A compound helicopter project aircraft contracted to Sikorsky by NASA and the US Army, known as the Rotor Systems Research Aircraft. The first prototype was completed on June 7, 1976 as a pure helicopter; the second production was the compound configuration. Lift and torque of the rotor system were able to be measured directly due to a main transmission mounted on a balance; the wings were fitted with speed brakes for precise control. An Emergency Escape System allowed the jettison of the crew after severing the main rotor blades overhead with explosive charges; the crew could choose to continue flight of the aircraft in the fixed-wing airplane configuration. NASA and DARPA began work towards a "stopped rotor" system, in which the 4-bladed main rotor could be halted during flight and still produce lift as fixed wings; the project was known as the X-Wing system. Alas, despite many hours of ground testing with an operational X-Wing rotor, the aircraft never flew. There have been no plans to continue the project since it was halted in 1988.

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