Revo Mixing

Revo mix takes a percentage of the pitch output and feeds it into the rudder output.

In control system terms, this is a feed forward input which increases the anti-torque force of the tail rotor as the torque from the main rotor increases as you pull more pitch on the blades.

With fast, accurate, modern gyros and digital servos we can get away without worrying about revo mixing for most belt or torque tube driven helicopters. The gyro senses the movement of the tail in response to the changing torque of the main rotor and changes the tail rotor pitch to compensate.

Before gyros though, the only chance to fly a helicopter was to have the main rotor pitch tied to the tail rotor pitch thus reducing the amount of compensation required from the pilot. 

These days revo mixing is all done inside the computerised radio and I needed it for the Honey Bee brushless tail rotor conversion. I tried flying the heli without it and the gyro did a pretty good job but with fast application of the throttle the tail would kick at least a quarter of a turn before the gyro could spin up the tail rotor to compensate. Exactly the problem revo mixing is designed to help with.