In the early 1970s Hughes helicopter was given a contract by DARPA (Defense Research Agency). They were to develop a silent helicopter. They started out by designing a dynamometer that would be capable of simulating the loads generated by the main rotor, the tail rotor and the engine. They placed a model 500 on the dynamometer and ran it as a normal helicopter. They took sound readings on several hundred points to see how the mechanical sounds were transmitted through the structure and broadcast to the surrounding environment. They also took sound readings of the engine, the main and tail rotors and the respective elements of the drive train. They then hooked the 500 up to the dynamometer and removed the main rotor. This was followed by the tail rotor being removed and the main rotor being reinstalled. This test was followed up by the engine being disconnected from the drive train and connected to the dynamometer. Once all of the sound generating points and the broadcast points (vibrating skins) were identified and quantified they went about installing insulation, a muffler on the engine, a five blade main rotor and an X tail rotor and placed the helicopter on the dynamometer and re-ran the tests modifying as they went along to decrease sound generation. When they were satisfied they flew the helicopter and it could fly 4-500 feet overhead and all you could hear was a muffled whoosh. Most of us at Hughes felt that this for some special ops in Vietnam. The helicopter was shipped out and we never saw it again.