Gyroscopic precession

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Gyroscopic precession is one of the phenomena used to explain one of the quirks of cyclic pitch control.

When you apply forward cyclic, you may have noticed that the point of maximum blade pitch, and therefore maximum lift, is directly right of the main mast, rather than directly aft as might be expected.

As the main rotor is a rotating mass, it acts as a gyroscope. Gyroscopic precession means that the main rotor reacts to the force 90 degrees after it experiences it, causing the rear of the disk to tilt up.

Another explanation is known as 'aerodynamic precession'; in this model, the paddle or blade 'flies' to its new position. So for example, when forward cyclic is applied, the paddle has positive pitch to the right of the helicopter because it needs to be traveling up as it goes along that side, towards the back; and the opposite on the other side.

Depending on the exact design of the head, this phase lag may not be exactly 90 degrees. This can be corrected by altering the swashplate timing.

Gyroscopic precession is also how mechanical gyros measure the yaw rate of the helicopter.

See also

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