A gyro is an electronic device used to dampen directional change or hold a specific direction. There are two primary types of gyro operation, rate and heading hold (HH). A rate gyro attempts to stop motion in a given axis, while an HH Gyro will attempt to maintain the current direction in a given axis. Some gyros are capable only of only one mode, while others can be switched between rate and HH mode. Gyros are most commonly found on helicopters where they are used in the yaw (horizontal) axis, though they are also sometimes used on cars, boats, and airplanes to stabilize motion in a given axis and make the model easier to control. Multiple gyros can be used together on aircraft to correct for motion in pitch and roll, as well as yaw.
Rate-mode gyros are generally cheaper than HH gyros and are found on most entry-level helicopters, sometimes integrated into a 4-in-1. Rate mode gyros differ significantly from HH gyros because they will not return to the original heading after movement occurs. For instance, if the wind blows the tail of a helicopter to the left, the rate mode gyro will stop the movement, but the tail will continue to point in the new direction after the motion is stopped.
HH gyros are usually more expensive and sometimes more difficult to set up than rate-mode gyros. Most HH gyros can operate in rate mode and HH mode, and almost all can be switched on the fly using a channel on the receiver. They are usually used on CP helicopters for 3D flying, but are also sometimes used on FP helicopters to provide more precise control.
Most rate and HH gyros have adjustable gain, which tells the gyro how quickly to respond to changes in direction. If the gain is set too low, the gyro will respond relatively slowly to outside forces such as wind. This results in loose or sloppy control in that axis, or slow oscillation. If the gain is set too high, the gyro will respond quickly to an outside force, but will also try to correct for the motion of the response. This results in a feedback loop, and can cause a rapid oscillation as the gyro repeatedly corrects for its own corrections.
A gyro can be made to correct for motion in yaw and roll simultaneously by placing the gyro at a 45 degree angle to both axis. This is only useful when the same control surface is used to control motion in both axis, such as the rudder on a non-aileron airplane.