More on supersonic props, just because I find them fascinating.

Look [link=http://en.wikipedia.org/wiki/XF-84H_Thunderscreech]HERE[/link] for a brief history of the brief but exciting career of the Republic XF-84H "Thunderscreech" which was AFAIK the only purpose-built aircraft ever with a supersonic propeller.

Also, consider the 1:1 scale Unlimiteds at Reno, in particular those with big round engines, e.g., Rare Bear. They are up against the 'sound barrier' and it's the prop rather than the engine or airframe that can't breach that barrier. As appears obvious, some of the prop blade must extend past the cowl to egest (spallchucker doesn't like that word....) air into the slipstream and so produce thrust. That means the prop tips have a limited minimum distance from the hub, and so travel faster than shorter blades used on planes with inline engines. The engines are more than capable of driving prop tip speed supersonic. When that happens prop efficiency goes to hell in a hand basket and the airplane slows down. As if the race pilot didn't have enough to do, he's got to have his mind and his hand on the throttle constantly to keep it on the edge between all that physics will allow and a tiny bit too much.