Answer
Suppose the pilot wants to change the main rotor’s angular speed; then a torque must be applied to it. That torque is applied by the helicopter body. An application of Newton’s third law shows that an equal and opposite torque is applied by the main rotor to the helicopter body. Some helicopters have a sideways-directed tail rotor that thrusts on the tail of the helicopter to create a net cancellation of torque, preventing the body of the helicopter from spinning.
Work Step by Step
Without the second rotor, every time the main rotor changed speed, the helicopter body would begin to spin around a vertical axis. This is not a good design!
Another way to solve the problem: Chinook helicopters have two matched rotors that spin in opposite directions. Any change in the angular speed requires a net torque of zero, so the helicopter body does not tend to spin.
