[ladtx]

Actually I think blade-vortex interaction noise or the pop of the blade occurs when the blade strikes the vortex created by the tip of the preceding blade. It's been a long time since I've done any instructing and I can't remember if the blades have to be supersonic to produce the sound but I don't think so. I would have to dig through my old aerodyamics manual to refresh my memory. FlyLow or leadpenny may be able to correct me if I'm wrong.

I do remember bringing a formation of Chinooks back into Camp Humphreys in Korea one cold clear winter morning. It was a Sunday and we set up a shallow descent to land (the condition that produces the loudest "pop") and knew that "pop" would get everyones attention. When I got to the club later I was told we had the windows vibrating in the place, so much so they were afraid they would crack. I really miss that sound.

[leadpenny]

I really miss that sound.

I do too. It's why I have a Low Rider Harley with straight pipes. As for the sound of the rotor blades - it's caused by magic, blue smoke and mirrors.