To: rotorcraig
A silent helicopter is not as impossible as you might think. During the Vietnam War (or was it a police action?) Hughes Helicopters under contract from the Defense Advanced Research Projects Agency (DARPA) modified an OH-6 so that it was almost silent during a fly over. Hughes using a special test stand was able to run the helicopter with no rotor, with no transmission, with no tail rotor gearbox and without a tail rotor. All of the dynamic loads were absorbed by a waterbrake dynamometer. They also monitored the amount of sonic energy broadcast by the aluminum skin and the primary structure. Once the noise characteristics were established Hughes did the following. The structure and skins were covered on the interior with insulation to minimize noise transmittal. The gearboxes were covered with insulation. The engine was fitted with a muffler. A new five-blade rotor system was fitted and it ran at a lower speed and an X type tail rotor was fitted which also turned at a lower speed.
I saw this helicopter fly and it went overhead at about 500 feet and all you heard was a Whooshing sound. After the first few flights this helicopter disappeared and I assume it was sent to Vietnam.
