The Sikorsky S-67 Blackhawk was a private-venture, prototype attack helicopter built in 1970 with Sikorsky Aircraft research and development (R&D) funds. A tandem, two-seat aircraft designed around the dynamic drive and rotor systems of the Sikorsky S-61, it was designed to serve as an attack helicopter or to transport up to eight troops into combat. Design and development AAFSS and S-66 bid The United States Army issued a request for proposals (RFP) for its Advanced Aerial Fire Support System (AAFSS) program on 1 August 1964. Lockheed offered its CL-840 design, a rigid-rotor compound helicopter. Sikorsky submitted the S-66, which featured a "Rotorprop" serving as a tail rotor but as speeds increased would rotate 90° to act as pusher prop. The S-66 had short, fixed wings and was powered by a Lycoming T55 turboshaft engine. The design was to have a speed of with the ability for for brief periods. The U.S. Army awarded Lockheed and Sikorsky contracts for further study on 19 February 1965. On 3 November 1965, the Army announced Lockheed as the winner of the AAFSS program selection. The Army perceived Lockheed's design as less expensive, able to be available earlier, and that it would have less technical risk than Sikorsky's Rotorprop. S-67 development Lockheed's design soon ran into development problems and cost and timelines began to grow. Sensing an opportunity, Sikorsky offered an armed SH-3 Sea King (Sikorsky S-61) version. After further AAFSS problems, the company developed an intermediate, high-speed attack aircraft named the Sikorsky S-67 Blackhawk in 1970. Design work on the S-67 began in November 1969 with manufacturing following in February 1970. The Blackhawk first flew on 20 August 1970. The S-67 featured a five-bladed main rotor and tail rotor. The main rotor was taken from the S-61, but was modified to have a hub fairing, swept main rotor blade tips and a special "alpha-1" linkage which was added to the main rotor controls to increase collective pitch sensitivity and so extend the collective pitch range. The 20° swept main rotor blade tips help to overcome a phenomenon called sub-multiple oscillating track (SMOT) that causes variations in tip track at high Mach numbers. These allowed the S-67 to achieve and maintain high cruise speeds. To reduce drag at high speed, the main wheels retracted fully into the stub wing sponsons. It had speed brakes on the wing trailing edges that could be used to decrease