The Coriolis effect is when the rotor blades speed up or slow down as the center of gravity moves closer or further away from the axis of rotation.
The Coriolis effect is otherwise known as the law of conservation of angular momentum, which states that an object will have the same rotational momentum unless acted upon by an outside force. There are two primary factors involved with the Coriolis effect as it relates to the rotor system. These factors are the distance of the blades center of gravity (CG) from the axis of rotation and the rotational speed of rotor or centrifugal force being applied.
If a blade’s center of gravity moves closer to the axis of rotational axis, that blade’s speed will increase. The Coriolis effect is significant when the blades cone or lead-lag due to flapping. When the blades cone, such as due to high G-loading, the rotor disc diameter decreases, and the CG moves inward on all blades. As the rotor disc diameter becomes smaller, all the blades increase speed. This can be felt as an increase in rotor RPM, without corrective action by the pilot or governor.
In a fully-articulated rotor system, the CG moves closer to the axis of rotation when blades flap up. As a blade flaps up it increases speed and leans forward, which reduces stress on the rotor system. The opposite is true when a blade flaps down. As it flaps down, the CG moves away from the center of axis and the rotation will decrease resulting in the blade lagging.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15
Principles of Helicopter Flight, 2nd Edition, pg. 78
FM 3-04.203-2007 Fundamentals of Flight pg. 1-15, 1-53