Originally Posted by DuckieHo
What is required for autorotation?
I mean during the power loss or whatever in a standard helicopter, the rotors spin as air passes through them and generates some lift. Are electric helicopters different? If you dis-engage the motor-rotor interfaces, would it be the same thing?
for autorotation you're correct you use the air to spin the blades up but you need to be able to pitch the blades to control them. You pitch them negatively to spin up the blades quickly as you descend and then use the momentum of the blades to ramp up the pitch just before touchdown to give you the lift to cushion your landing. Done correctly an auto rotation landing can be as gentle as a powered landing... you just get one chance!
with a fixed pitch rotor the best you could achieve was a slower descent relying on the drag of the blades unless you were high enough up to use them as a generator for your descent and power your motor just before touchdown but this would only work in the event of power loss.
Quad and more rotor vehicles are really only useful for UAVs and other lightweight craft as the power density of motors drops as you get large due to requiring cooling of windings and so forth.
You could also achieve yaw control if you had the rotors inclined relative to the horizontal plane of the vehicle if they were equal and opposite. When spinning at the same rate your forces would be balanced but vary them and you could yaw.
I was a PhD postgrad about 8 years ago working with helicopters so was looking quite extensively at them for a while!Edited by pharcycle - 11/22/13 at 11:04am