I think you should check your math because i got that a 14" prop would only have to go 18,000 rpm which is easily doable and at 250,000 rpm the whole blade would be super sonic so i don't know where you got the 700,000 from. Still, my plane is not getting those speeds and even if it were, there would be a pop from the tips breaking the sound barrier and from them ripping to shreds and no more noise from then on except for the thump of your plane hitting the ground or a scream from shrapnal hitting someone. You're right about it not going supersonic but not completely, unless my math is also wrong.anyone else want to check??