Why doesnt helicopter turn in opposite direction of rotation of propeller?
For single main rotor helicopters, there's a small rotor or exhaust port at the tail for most helicopters. This equalizes the torque from the main rotor, but then you end up with a net side force that requires the helicopter to lean a bit to one side to compensate for the side force.
Great question. The answer is that without extra features, the helicopter does turn in the opposite direction. As rcgldr pointed out, a tail rotor pushing in the opposite direction can cancel out the torque. Another interesting approach is to have two main propellers rotating in opposite directions so that the torque cancels, and then you don't need a tail rotor. The two propellers can be side by side, inter-meshed, in tandem, or coaxial (notice no tail rotors):
if the engine is just at cg then no torque will be produced. then why are extra blades necessary?
Incorrect. Why do you think that it is?
Without any air the rotor blades could spin without the body having a torque, I believe. What causes the body to spin is either a change in the speed of rotation (conservation of angular momentum / newton's 3rd law means the body must rotate the opposite way) or air resistance, whose opposition to the fast motion of the blades imparts on the aircraft a torque in the opposite direction of the blade's rotation. This is transferred through the mast to the fuselage, which begins to spin.
I'm not entirely sure but I think from that, the reason the body would rotate without a tail rotor is because the air resistance opposing rotation of the blades > the air resistance opposing rotation of the body.
Yes, there is an aerodynamic torque on the main rotor, opposite to it's rotation. This external torque needs to be balanced by some other external torque on the system.
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