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Rotorcraft Flying Menu >Aerodynamics
of Flight >Hovering
Flight > Gyroscopic Precession
The spinning main rotor of a helicopter acts
like a gyro-scope. As such, it has the properties of gyroscopic
action, one of which is precession. Gyroscopic preces-sion is
the resultant action or deflection of a spinning object when
a force is applied to this object. This action occurs approximately
90° in the direction of rotation from the point where the
force is applied. [Figure 3-8]
Figure 3-8. Gyroscopic precession principle—when
a force is applied to a spinning gyro, the maximum reaction
occurs approx-imately 90° later in the direction of rotation.
Let us look at a two-bladed rotor system to
see how gyroscopic precession affects the movement of the tip-path
plane. Moving the cyclic pitch control increases the angle of
attack of one rotor blade with the result that a greater lifting
force is applied at that point in the plane of rotation. This
same control movement simul-taneously decreases the angle of
attack of the other blade the same amount, thus decreasing the
lifting force applied at that point in the plane of rotation.
The blade with the increased angle of attack tends to flap up;
the blade with the decreased angle of attack tends to flap down.
Because the rotor disk acts like a gyro, the blades reach maximum
deflection at a point approxi-mately 90° later in the plane
of rotation. As shown in figure 3-9, the retreating blade angle
of attack is increased and the advancing blade angle of attack
is decreased resulting in a tipping forward of the tip-path
plane, since maximum deflection takes place 90° later when
the blades are at the rear and front, respectively.
Figure 3-9. With a counterclockwise
main rotor blade rota-tion, as each blade passes the 90°
position on the left, the maximum increase in angle of attack
occurs. As each blade passes the 90° position to the right,
the maximum decrease in angle of attack occurs. Maximum deflection
takes place 90° later—maximum upward deflection at
the rear and maxi-mum downward deflection at the front—and
the tip-path plane tips forward.
In a rotor system using three or more blades,
the move-ment of the cyclic pitch control changes the angle
of attack of each blade an appropriate amount so that the end
result is the same.
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