From RC Helicopter Wiki
The gyro (short for gyroscope) is a small electronic device that is connected between the receiver's rudder channel (usually channel 4) and the tail servo, used to stabilize the helicopter's heading by detecting rotation of the helicopter and sending opposing commands to the helicopter's anti-torque system.
Early helicopter gyros acted as simple yaw rate dampers: the gyro consisted of a spinning mass that tilted by gyroscopic precession when the helicopter yawed; the gyro electronics measured this deflection, and sent commands to the tail servo to move the nose in the opposite direction (assuming the gyro sense had been set correctly, else the helicopter would spin uncontrollably). The amount by which the gyro tries to oppose yawing is known as the gyro gain.
Modern gyros gyros use solid state electronics (typically based around tiny vibrating crystals) to measure the yaw and can operate in two modes:
- In rate mode, the gyro will attempt to oppose any rotation that it detects, including yaw induced by the pilots commands. A high gain will tend to hold the tail better, but will severely limit the maximum pirouette rate; conversely a low gain will give a higher pirouette rate, but the tail will easily weathervane into any wind. Some pilots would set up a programmable mix to reduce the gain as more rudder was applied to get a good pirouette rate, and reasonable tail hold. A revolution mix is also highly desirable to help hold the heading during collective pitch and throttle changes. Cheaper gyros may only offer rate mode operation.
- In heading hold mode, the gyro will attempt to keep the helicopter's heading constant unless commanded to rotate by the pilot. Heading hold mode offers much more consistent pirouette rates, and almost eliminates the helicopter's tendency to weathervane into any wind; good heading hold gyros can keep the helicopter's heading constant through the most demanding of maneuvers As a result, the helicopter's tail must now be 'flown' when turning to make coordinated turns.
While early model helicopters did not use gyros (relying on the pilot to perform all anti-torque corrections), the gyro is very useful because so many things can easily upset a helicopter's heading. As the helicopter has little rotational inertia, it will begin to turn quickly, disorienting the pilot. Typical things which can disturb the anti-torque balance include any change in head speed from collective or cyclic inputs (altering the torque reaction as well as the tail rotor thrust), and gusts of wind, including the helicopter's own downwash (causing the helicopter to weathervane, or the tail rotor to become more effective). With a good heading hold gyro managing the tail, the pilot's workload is substantially reduced, and they can concentrate on flying the helicopter.
Depending on the gyro, the gyro gain, can sometimes be controlled by an adjustment on the gyro, but more usually by configuring a channel on the transmitter. The gain controls how much the gyro tries to control the tail; too low and the helicopter will not hold its heading; too high and the tail will "wag" quickly left and right. More advanced gyros have many other settings. When using a gyro with transmitter gain control, the gyro will have an extra lead with a single wire; this should be plugged into your gyro gain control channel on the receiver (usually channel 5). Normally, if a screwdriver adjusted gain control is present on the gyro, it will be ignored if you use the remote gain control.
More expensive gyros offer much better control of the tail, showing less gyro creep, stronger control authority (holding backwards and sideways maneuvers better and stopping the tail blowing out), and more consistent pirouette rates (making pirouetting maneuvers such as chaos easier) compared to cheaper gyros. Initial setup with a more expensive gyro may also need to be less precise to achieve equivalent performance to a cheaper gyro.
- Futaba GY401 - a popular, well performing heading hold gyro, although relatively expensive.
- xmhobbies digital gyro G401B - users have reported performance comparable to the GY401, at a fraction of the price ($36 as compared to more than $110 for the GY401) | Flight Video | User review
 See also
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