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How Helicopters Work
•
Turn (yaw) the helicopter - rudder stick
•
Stabilize yaw - yaw rate gyro
A yaw rate gyro is a very simple device. It only senses the turn rate (angular acceleration)
and 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
person 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 helicopter 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
helicopter 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 function 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.
23.6.3. 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.
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