RC Gyro is an electronic device that is connected between the receiver and the tail rotor control (either servo or small motor).
The gyro, technically called an accelerometer, senses any rotational movement of the helicopter that isn’t a result of a signal to the receiver, and it makes fine and rapid adjustments to the tail rotor speed or blade pitch to suit the torque force at that precise moment, hence dampening out any unwanted yaw. Gyros make these calculations and corrections at lightning speed, so much so that the pilot doesn’t notice anything other than a stable helicopter!
The gyro sensitivity (‘gain’) can be adjusted by the pilot either directly on the gyro itself or remotely from the transmitter.
Heading Hold Gyros go one step further than a standard gyro by performing more complex calculations to keep the helicopter pointing in the direction that the pilot intended.
A HHG will ‘learn’ the orientation of the helicopter and maintain this heading until the pilot inputs a definite yaw control that will override the gyro. Once a signal has been received from the transmitter, the gyro learns the new heading and keeps the heli pointing in that direction until a new command is received.
Heading Hold Gyros more or less eliminate unwanted changes of direction forced on the helicopter by wind gusts. The gyro will prevent the helicopter from swinging naturally round in to the wind, in the same way as a weather vane does (the wind pushing on the tail will force the helicopter round in to wind), because it will know that the change in yaw command did not come through the receiver, and so was not made by the pilot.
HHGs, once an expensive luxury, are now commonplace and almost all rc helicopter gyros have this feature.