[QUOTE=slawecki] there are convoys of heli running low(under DCA flightpath) up to the federal area from Quantico. they frequently are in a hurry, so run at close to max speed. the rotor tips approach/exceed the speed of sound, [QUOTE]
FYI, a limiting factor in helicopter airspeed is that the blade tips cannot go supersonic. The aerodynamics and shock wave effect associated with supersonic flight would cause the rotor blade to go badly out of balance and self-destruct. You are correct, however, that at high speed the tip of the advancing rotor blades are nearing supersonic speed.