91Ó°ÊÓ

A helicopter has a large main rotor that rotates in a horizontal plane and provides lift. There is also a small rotor on the tail that rotates in a vertical plane.

When the helicopter is in air, and is moving with a constant velocity, no external forces or torques act on it thus conserving its angular momentum.

If the main rotor changes speed, then to balance the changing angular momentum, the passenger cabin would have to rotate in the opposite sense as the main rotor.

Some helicopters have two main rotors that rotate in the horizontal plane as the two rotors rotate in the opposite sense, one clockwise and the other anti-clockwise. Thus net angular momentum of these rotors is zero. This prevents the main cabin from rotating, and no tail rotor is necessary.