From RC Helicopter Wiki
A rate mode gyro makes the helicopter more stable in yaw by detecting the helicopter turning, and passing an opposing command to the helicopter's anti-torque control (e.g. the tail rotor) to try to dampen the action. The turning action, coming from a side wind, changes in the torque reaction due to collective pitch or throttle changes, or an explicit command to turn from the pilot, are all treated by the gyro in the same way, and opposed to some degree. The amount by which the commands are opposed is set by the gyro gain, which is controlled either by a control on the gyro, or remotely by an additional servo channel.
Early mechanical gyros (consisting of a spinning mass that tilted by gyroscopic precession when the helicopter turned) worked in rate mode. To improve the yaw rate when turning, while keeping tolerable hold at other times, pilots would often set up a programmable mix to reduce the gyro gain channel value when the the rudder was used. It was common to use a revolution mix to help compensate for changes in the torque reaction that occurred as the throttle was changed.
If the helicopter's rotates slowly while in an out of ground effect hover, then the tail blade pitch at rudder neutral should be adjusted by moving the servo up or down the boom, or changing the length of the tail pitch pushrod.
Modern heading hold gyros can usually be made to work in rate mode by setting the value of the gain suitably. After initially setting up the gyro and helicopter, most pilots seldom use rate mode however.
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