gc textbook chapter 8 Rotational Motion (id: e186f7457)

Consider a helicopter in the air with the rotor spinning.  To change the rotor’s angular speed, a torque must be applied to the rotor.  That torque has to come from the helicopter, and so by Newton’s 3rd law, and equal and opposite torque will be applied by the rotor to the helicopter.  Any change in rotor speed would therefore cause the body of the helicopter to spin in a direction opposite to the change in the rotor’s angular velocity.  

Some large helicopters have two rotor systems, spinning in opposite directions.  That makes any change in the speed of the rotor pair require a net torque of 0, and so the helicopter body would not tend to spin.  Smaller helicopters have a tail rotor which rotates in a vertical plane, causing a force on the tail of the helicopter in the opposite direction of the tendency of the tail to spin.