Heading hold mode
From RC Helicopter Wiki
Unlike a rate mode gyro which just tries to dampen yaw of the helicopter, a gyro operating in heading hold mode keeps a record of the heading that the gyro should be pointing to, and will attempt to maintain that heading (by commanding the tail servo to alter pitch of the tail rotor) regardless of any forces that try to turn the helicopter, such as side winds or torque reaction changes. Using the rudder control on the transmitter changes the gyro's desired heading, and so the helicopter will turn to follow.
Heading hold gyros make many maneuvers much easier, such as backwards flight, or traveling pirouetting maneuvers, where the more consistent pirouette rate makes timing the cyclic inputs much easier. Normal forward flight however can be considered to be more complicated, as the tail must be controlled explicitly to keep turns coordinated.
Heading hold gyros can also make learning to hover easier, as the tail doesn't have to be constantly managed to keep it pointing towards the pilot. Some coaches think that this can make the pilot lazy in learning to control the tail, though.
As a heading hold gyro interprets any rudder input as a command to yaw, any revolution mix must be disabled.
Most heading hold gyros have remote gain control, allowing the gyro sensitivity (and so the strength of the correction) to be controlled accurately from the transmitter. Usually if the gain channel value is reduce below "center", then the gyro will switch into rate mode.
It is easy to tell if the gyro is in heading hold mode or not; in heading hold mode rotating the gyro will cause the tail servo to move off to one side and stay there until the gyro is rotated in the opposite direction. With a rate mode gyro, the deflection of the tail servo is proportional to the rate of rotation of the gyro, so when the gyro is stationary, the servo will always return to its center position. This "stickiness" in rate mode makes it very easy to check for correct gyro sense in the pre-flight checks.
Although most heading hold gyros do not work correctly in the presence of trim or subtrim, some cheaper heading hold gyros may require a small amount of rudder subtrim to stop the tail servo creeping to one end of its travel when the helicopter is left motionless on the ground.
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