Many people seem to be confused about how gyros work. In order to understand how a gyro works, it is necessary to first understand the relationship between the main rotor and the tail rotor.
Most helicopters have a clockwise main rotor, so for this section, we will assume the main rotor blades are spinning clockwise.
Also, some helicopter use a variable pitch tail rotor and some helicopters use a variable-speed motor for the tail rotor, so this section will use the terminology "increase/decrease tail rotor thrust" to accomodate both cases.
The three basic function of the tail rotor are:
Counter main rotor torque
Turn (yaw) the helicopter
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".
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".
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
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 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.
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 no longer directly controls the tail thrust. The rudder signal tells the heading hold gyro how many degrees per second to turn, and the HH gyro will do whatever is necessary to move the tail to this position.
Note that the revo mixing MUST be disabled for the heading hold gyro to work properly. If the revo mixing is enabled, then the heading hold gyro will interpret it as a signal to turn the helicopter.
Imagine we have a helicopter with a properly configured yaw rate gyro and the motor is disconnected and it is on the ground where it cannot turn. If we hold left rudder on the transmitter for one second and then center the stick, the servo will move to one extreme servo position for one second and then center.
Now, imagine we have the same helicopter with a properly configured heading hold gyro, and the motor is disconnected and it is on the ground where it cannot turn. Imagine that the setting for this heading hold gyro is full left stick is 180 degreees per second.
If we hold left rudder on the transmitter for one second and then center the stick, the heading hold gyro will know the helicopter should turn counterclockwise 180 degrees.However, since the helicopter is on the ground and cannot turn, the tail servo will stick at one extreme and stay there - the heading hold gyro will keep trying to turn the helicopter. If we manually pick up the helicopter and turn it counterclockwise 180 degrees, the servo will finally center.
Note that both types of gyros only stabilize YAW. Neither type of gyro will stabilize roll or pitch. Technically speaking, a helicopter gyro contains an angular acceleration sensor for only one axis.