In 1964 Lockheed put forward a design in the US Army’s Advanced Aerial Fire Support System (AAFSS) program that would eventually win and become the AH-56 Cheyenne. But Lockheed was not alone in tendering a proposal: Sikorsky entered their S-66 design.
Both helicopters would have been advanced and fast. Lockheed made the AH-56 fast by adding a pusher prop to the tail, just aft of the conventional anti-torque rotor. But Sikorsky went a slightly different route: the S-66 used the “rotaprop” tail. This combined anti-torque duties with forward thrust in a single mechanism… a single somewhat complicated mechanism that could turn the sideways-thrusting anti-torque rotor 90 degrees to point it aft, providing forward thrust. This would have decreased drag and potentially decreased weight, but at added technical risk compared to the relatively simple solution of just adding a dedicated pusher prop. In late 1965, the Army decided that the risk was too high, and went with Lockheeds design.
The S-66 design got a fair amount of publicity during the contest, but I’ve never actually run across the proposal documentation or design diagrams. A few photos of a display model and a single cutaway drawing are presented HERE. Below are some magazine illustrations of the S-66. It would have been an impressive helicopter had it been built and if it had been successful, looking not unlike a sleeker version of the Mil 24 Hind attack helicopter.
Sorry to be the bearer of bad new. AH-56 Cheyenne was never accepted to active duty. It was classified as to technical and cost to high, they went with AH-1 Cobra instead.