Question

pls help i want to know want rpm brushless motor should i use for rc plane ?? r hovercraft ???

Answer 1

there is no answer to the question the RPM of the motor depends on the voltage and required torque. first decide on a prop find the blade dimensions, angle of attack etc then you will need to work out the lift needed for the aeroplane to get airbourne, then calculate the thrust needed to generate that lift. from this you need to calculate the required torque to drive the prop at the speed that will generate that thrust. you will need to find a motor that can produce both the torque and speed (RPM) given your power supply.
for an easier way find a similar sized model aeroplane and scavenge the motor and prop from that
as for a hovercraft the theory is the same however you will also need to work out power loss caused by vectoring some of the thrust downwards

Answer 2

at tha high speed like 24000.rpm

Answer 3

i dont think that would work at that speed