The scissors tail rotor design was created to minimize the acoustic signature. The design concept was proven on an OH-6 that was modified to the acoustic requirements of a DARPA contract. The design concept included mounting the OH-6 airframe on a waterbrake dynamometer in order to isolate the different generators of noise. In the end the helicopter included a five-blade rotor that turned at a lower speed, the scissors tail rotor, a muffler on the engine exhaust and every part of the airframe was insulated to minimize the broadcasting of noise. The rotor systems were eventually incorporated on later models of the H-500.
When this helicopter flew over at 500 feet all you could hear was a “whoosh”. I think it ended up in Vietnam.
