On some helicopters, for various reasons, sometimes the applied cyclic control leads to a reaction from the main rotor that is out from the input. For example, pushing the cyclic forward may, as well as tilting the disk forward, also cause a small amount of roll to the left or the right. The reaction is said to lead or lag the input, and the amount is called the phase angle.
This can sometimes be fixed by changing the swashplate timing angle by moving the anti-rotation bracket, or by changing the angle of the washout guide by rotating the phase ring, to change the phase at which the inputs are applied to the rotor system. Alternatively two programmable mixes can make a swashplate timing mix, mixing elevator with aileron output, and aileron input with elevator output, so as to apply the opposite phase error to the controls and apparently remove the interaction.
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