Originally Posted by Tonic Please
Exactly, so how can the tips be going "supersonic"?
They are revolving at 5200rpm, but towards the centre is travelling a smaller circumference (ie distance) than the tips, which are travelling much further in the same amount of time, hence in terms of mph/kph/m/s etc etc, the further from the rotational centre the faster they are travelling. Try it by putting a disc of card on a pencil. Mark a point on the edge and a point near the centre. Turn the disc slowly and see how much faster the mark on the edge has to go to cover one rotation of the disc......
Revolutions per minute may be the speed the shaft is turning at, but it is not the speed through the air that a point on a disc on that shaft is achieving.
PP