Coriolis Effect (Law of Conservation of Angular Momentum)

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Coriolis Effect, which is sometimes referred to as con-servation of angular momentum, might be compared to spinning skaters. When they extend their arms, their rotation slows down because the center of mass moves farther from the axis of rotation. When their arms are retracted, the rotation speeds up because the center of mass moves closer to the axis of rotation.

When a rotor blade flaps upward, the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place in order to conserve angular momentum. Conversely, when that blade flaps down-ward, its center of mass moves further from the axis of rotation and blade deceleration takes place. [Figure 3-5] Keep in mind that due to coning, a rotor blade will not flap below a plane passing through the rotor hub and perpendicular to the axis of rotation. The acceleration and deceleration actions of the rotor blades are absorbed by either dampers or the blade structure itself, depend-ing upon the design of the rotor system.

Figure 3-5. The tendency of a rotor blade to increase or decrease its velocity in its plane of rotation due to mass movement is known as Coriolis Effect, named for the mathe-matician who made studies of forces generated by radial movements of mass on a rotating disc.

Two-bladed rotor systems are normally subject to Coriolis Effect to a much lesser degree than are articu-lated rotor systems since the blades are generally “underslung” with respect to the rotor hub, and the change in the distance of the center of mass from the axis of rotation is small. [Figure 3-6] The hunting action is absorbed by the blades through bending. If a two-bladed rotor system is not “underslung,” it will be subject to Coriolis Effect comparable to that of a fully articulated system.

Figure 3-6. Because of the underslung rotor, the center of mass remains approximately the same distance from the mast after the rotor is tilted.