Refer to figures.
To turn the main rotor the engine must exert a turning force or “torque” on the main rotor shaft.

According to the Newton’s 3rd Law: Every action must have an equal and opposite reaction. So, the effect of applying torque to the main rotor shaft, is an equal and opposite rotation of the fuselage.

Conventional helicopters use a tail rotor and some others, the NOTARs, a variable pitch fan at the front of the tail boom, to counterbalance the torque reaction through the anti-torque or yaw pedals, producing a horizontal tail rotor thrust.

Because of the tail rotor drift (or translational tendency), the helicopter will always drift towards the side of the advancing blade. This can be corrected by producing a horizontal component of total rotor thrust to the opposite direction (retreating side).

Having corrected for the tail rotor drift, there is now a total rotor thrust that is not vertical and together with any moment from the tail rotor will create a rolling moment towards the retreating side.

The amount of roll depends on the amount of thrust produced by the tail rotor and the positions of the tail rotor and main rotor hub compared to the longitudinal Centre of Gravity.

The Tail Rotor Roll can be corrected with following methods:

• The rotor mast can be offset.
• The tail rotor can be designed to be level with the main rotor hub.
• The helicopter can be loaded to remove the roll.