Specifications

it cannot sense the absolute orientation of the helicopter. In technical terms, it "dampens" the
tail movement.
Imagine you are blindfolded, and are standing on a frozen lake wearing smooth shoes. A per-
son will try to turn you, and you are only allowed to resist the turning force by digging in your
shoes into the smooth slippery ice. Basically, you cannot resist the turning force very
much,and once you have been turned, you do not know the original orientation.
This is very much like a yaw rate gyro.
Therefore, a yaw rate gyro can only provide partial yaw stabilization thrust. Usually the amount
of yaw stabilization thrust is controlled by the gyro gain. Increasing the gain will make the heli-
copter more resistant to random turning, but it also decreases the pirouette rate because the
gyro will fight against both random and intentional yawing movement.
A yaw rate gyro cannot provide "heading hold" capability because it only pushes against the
turning movement but will slip somewhat, and once it's been turned it cannot return the heli-
copter to the original orientation.
A yaw rate gyro "slips" when trying to countering tail movement, so it cannot effectively counter
main rotor torque. Therefore, the main rotor counter thrust is provided by the revo mixing func-
tion on the transmitter.
The revo mixing allows you to set the tail rotor thrust for each throttle position so the tail rotor
thrust exactly counters the main rotor torque. There is no formula for setting these values; they
must be empirically set by trial and error.
The turning thrust is governed by the rudder stick on the transmitter, the same as in a heading
hold gyro.
30.1.7.3. Heading hold gyros and how they work
For a Heading-hold gyro, the functions are controlled by the following devices:
• Counter main rotor torque - heading hold gyro
• Turn (yaw) the helicopter - rudder stick via heading hold gyro
• Stabilize yaw - heading hold gyro
A heading hold is more sophisticated than a yaw rate gyro and functions completely differently.
The first big difference between a heading hold gyro and a yaw rate gyro is that the heading
hold gyro has a microprocessor on-board and can remember how much the helicopter has
turned. Therefore if a random wind gust turns the helicopter, it can always return the helicopter
to the original orientation.
Therefore, the heading hold gyro can supply the correct main rotor counter thrust automatically
because it doesn't "slip". When you apply throttle and the tail starts to move because there's
more main rotor torque, the heading hold gyro can increase the tail rotor thrust to turn the tail
back to its original position.
Also, the heading hold gyro can provide the correct yaw stabilization thrust because it doesn't
"slip", and therefore it can retain the correct orientation at all times.
The second big difference is the rudder signal from the transmitter does not directly control the
tail rotor thrust. Instead, the rudder signal is considered a high-level command to "turn the tail
Technical Appendix
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