Sometimes the only way one will learn is for them to be hit by a flying prop, I've been lucky enough to see it come off and fly into the side of a vehicle, If you saw the mark it left on the panel door of a full size van, you wouldn't never even have to think about being in front of the motor, EVER,,,,,, at any speed.... and that was just a 10 x 6, but words can not express to some just how powerful a flying prop can be until they experience it first hand, another 2 cents,, for what its worth.