Download PDF A4 Edition

How Helicopters Work
•
Turn (yaw) the helicopter
•
Stabilize yaw
23.6.1.1. Counter main rotor torque
The first function provided by the tail rotor is to counter main rotor torque.
The motor in a helicopter spins the blades clockwise. But in order to spin the blades, the
motor 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 helicopter 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".
23.6.1.2. 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 counterlcockwise.
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".
23.6.1.3. 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 relative 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
23.6.2. 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
125