Specifications

needs to push against something. In this case, the motor is pushing against the body of the
helicopter. So, when the motor spins the main rotor blades clockwise, the body of the heli-
copter tends to spin counterclockwise. This is consistent with Newton's Third Law of Motion
which states:
"For every action there is an equal and opposite reaction."
In this case, the action is a clockwise rotation of the main rotor blades, and the reaction is the
body of the helicopter turning counterclockwise. So, the tail rotor needs to provide the correct
amount of clockwise thrust thrust to balance the counterclockwise reaction to the clockwise
main rotor. For lack of a better term, we will call this "main rotor counter thrust".
30.1.7.1.2. Turn (Yaw) the helicopter
The second function provided by the tail rotor is to turn (yaw) the helicopter.
If we need to turn left, then we set the tail rotor thrust to slightly less than the main rotor
counter thrust. This means the counterclockwise force (reaction of main rotor) will be greater
than the clockwise force (tail rotor thrust) so the body of the helicopter will turn counterlcock-
wise.
If we need to turn right, then we set the tail rotor thrust to slightly more than the main rotor
counter thrust. This means the counterclockwise force (reaction of main rotor) will be less than
the clockwise force (tail rotor thrust) so the body of the helicopter will turn clockwise.
If we don't need to turn, then the tail rotor thrust is exactly the main rotor counter thrust. For
lack of a better term, we will call this the "turning thrust".
30.1.7.1.3. Yaw stabilization
The third function provided by the tail rotor is yaw stabilization.
When airflow hits the side of the helicopter, the helicopter will tend to "weathervane" into the
airflow because there is more leverage against the tail of the helicopter than the nose.
This airflow can be either a random gust of wind, or the helicopter may be moving sideways re-
lative in still air.
We can use the tail to correct the orientation of the helicopter by increasing or decreasing the
thrust of the tail rotor. For lack of a better term, we will call this the "yaw stabilization thrust".
So the total thrust of the tail rotor should be all three of these variables added together, or:
tail rotor thrust = main rotor counter thrust + turning thrust + yaw stabilization thrust
30.1.7.2. Yaw rate gyros and how they work
For a yaw rate gyro, the functions are controlled by the following devices:
• Counter main rotor torque - transmitter revo mixing
• 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
Technical Appendix
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