Gyro sense refers to the direction of the correction the gyro applies in response to (usually unintended) rotation of the helicopter. If your gyro sense is set incorrectly for your mechanical setup, the helicopter will spin rapidly on take off.
A gyro with the sense set correctly will, on detecting clockwise rotation, apply an adjustment to the tail rotor to try to turn the helicopter anticlockwise (and vice versa), so stabilizing the helicopter's heading.
A gyro with the sense set incorrectly will, on detecting clockwise rotation, apply an adjustment to try to turn the helicopter clockwise (and vice versa), sending the helicopter into an uncontrollable pirouette.
Most gyros have a switch that allows you to change its sense (often called 'reversing the gyro'); if this switch is missing then mounting the gyro upside down will achieve the same effect.
Correct gyro sense can be tested by first putting the gyro into heading hold mode, then picking up the helicopter and turning it through 90 degrees, and then checking the tail blades to ensure they have responded correctly. On a typical model helicopter with a clockwise turning main rotor, rotating the helicopter clockwise should lead to a decrease in tail rotor blade pitch, and rotating it anticlockwise should lead to an increase.
As a gyro detects only rotational movement, it does not matter critically where it is placed on the helicopter, or the orientation of it, except for the upper (or lower) face, which must be uppermost (or lowermost).
The following will cause the apparent sense of the gyro to change, so check the sense and your construction whenever you change one of them:
- Twisting the tail belt the wrong way;
- Mounting the gyro upside down;
- Moving the tail servo from one side of the boom to the other;
- Moving the tail pushrod from one side of the servo to the other;
- Changing the links to the tail blade grips from leading edge to trailing edge.
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