The anti-torque system of a helicopter is used to counter the torque reaction from the main rotor. The control system used in the majority of helicopters is a tail rotor or counter-rotating main rotor mounted either coaxially or longitudinally on the helicopter.
In a full size helicopter, the anti-torque system is controlled by the pilot via foot pedals; in a model helicopter the transmitter's tail (or rudder) control, along with help from the gyro has the same effect. In a conventional helicopter, the inputs change the pitch (or sometimes the speed) of the tail rotor blades. In a coaxial helicopter, torque control is achieved by changing the drag of the two rotors, either by using equal and opposite collective pitch or changing the RPM of both rotors. In a tandem helicopter, anti-torque control is primarily achieved by the two contra-rotating main rotors; the balance is then fine tuned by applying opposite amounts of cyclic to each rotor as needed.
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