Sikorsky, who may rightly be considered one of the giants of the helicopter industry, has taken part in all the design competitions for combat helicopters held by the American armed forces. In 1964, it submitted the S-66 project to the US Army for the AAFSS specification, calling for an aircraft with a maximum speed of approximately 418km/h and ten minutes' hovering capability.
The S-66 looked very much like the Lockheed AH-56A Cheyenne (which won the contest), but had a Rotorprop tail rotor which could rotate its axis through 90° to act both as a conventional anti-torque rotor in horizontal flight and as a pusher propeller, thereby transforming the S-66 into a compound aircraft in cruising flight.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984
Technical data for Sikorsky S-66
Crew: 2, engine: 1 x Lycoming T55 turboshaft, rated at 2530kW, max speed: 450km/h, cruising speed: 370km/h