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Rotorcraft Flying Menu >Aerodynamics
of Flight >Hovering
Flight > Coriolis Effect (Law of Conservation of Angular
Momentum)
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.
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