Cool stuff. Incidentally, I was told by a fantastic docent at the Air and Space Museum in Dulles that the Huey's unique sound is also because the ends of the blades go supersonic.
I don't believe that's entirely true Ken, although aircraft and helicopter propellers can go supersonic they aren't designed do so and become very inefficient at supersonic speeds. We'll have to ask Tom (exlabdriver) but it is my understanding that the advancing blade (the upwind blade of a forward moving helicopter) is limited in speed so that it will not go supersonic.
I believe the unique sound from the Huey is due to the width and length of the blades rather than them going supersonic.
Ken, here is a simple explanation...
'Because the blade is rotating, and not flying straight into the air, then the outer tip will be moving through the air faster than the base. In fact, the airspeed of the blade will increase as you move out. So what will happen is that the tips of the blades will be the first to reach mach 1. A shock wave will form at the tip of the blade. As the blade increases rotational speed, the shockwave will move along the blade as more of the blade goes supersonic.
The big problem with this is that the blades really aren't designed to withstand the stresses of supersonic travel. They will end up disintegrating. It also means that a portion of the blade will be 'transonic' (at or near the speed of sound). In this region there are problems with airflow and controllability, which will severely hamper the performance of the blade. Loss of lift and poor control will be major symptoms. Noise is the other issue.'