On the Canadian Harvard Aircraft Association website it says "The 9 foot propeller is powered by a massive radial engine causing the propeller tips to go supersonic causing an unforgettable roar."

CHAA

The third entry in the second section states how propellers go supersonic - Aerospaceweb.org

Last edited by Powertothepeople; 05/28/12 08:37 PM.
The only reasonable argument for owning a gun is to protect yourself from the police.