can help explain why precession occurs, it is more
continue to spin with its axle pointed in that direction,
important to know how precession affects gyro
no matter how the case of the gyroscope is positioned
(fig. 4-2, view B). As long as the bearings are frictionless
and the rotor is spinning, the rotor axle will maintain its
The rotor of a gyro has one plane of rotation as long
plane of spin with respect to a point in space. This
as its axle is aligned with, or pointed at, one point in
property of a free gyroscope is termed rigidity of plane.
space. When the axle tilts, turns, or wobbles, the plane
of rotation of the rotor changes. Plane of rotation means
Newton's first law of motion states that a body in
the direction that the axle is aligned or pointed.
motion continues to move in a straight line at a constant
speed unless acted on by an outside force. Any point in
Torque is a force that tends to produce rotation.
a spinning wheel tries to move in a straight line but,
Force acts in a straight line, at or on a point. Torque
being a part of the wheel, must travel in an orbit around
occurs within a plane and about an axle or axis of
its axle. Although each part of the wheel is forced to
rotation. If the force acts directly on the point of an axis,
travel in a circle, it still resists change. Any attempt to
no torque is produced.
change the alignment or angle of the wheel is resisted
by both the mass of the wheel and the velocity of that
Because of precession, a gyro will react to the
mass. This combination of mass and velocity is the
application of torque by moving at right angles to the
kinetic energy of the wheel, and kinetic energy gives the
direction of the torque. If the torque is applied
rotor rigidity of plane. Gyroscopic inertia is another
downward against the end of the axle of a gyro that is
term that is frequently used interchangeably with
horizontal, the gyro will swing to the right or left in
rigidity of plane.
response. The direction in which it will swing depends
on the direction the rotor is turning.
A gyroscope can be made more rigid by making its
rotor heavier, by causing the rotor to spin faster, and by
A simple way to predict the direction of precession
is shown in figure 4-4. The force that tends to change
concentrating most of the rotor weight near its
circumference. If two rotors with cross sections like
the plane of rotation of the rotor is applied to point A at
those shown in figure 4-3 are of equal weight and rotate
the top of the wheel. This point does not move in the
at the same speed, the rotor in figure 4-3, view B, will
direction of the applied force, but a point displaced 90°
have more rigidity than the rotor in figure 4-3, view A.
in the direction of rotation moves in the direction of the
This condition exists because the weight of the rotor in
applied force. This results in the rotor turning left about
figure 4-3, view B, is concentrated near the
the Z axis and is the direction of precession.
circumference. Both gyroscope and gyrocompass rotors
Any force that tends to change the plane of rotation
are shaped like the rotors shown in figure 4-3, view B.
causes a gyroscop to precess. Precession continues as
long as there is a force acting to change the plane of
rotation, and precession ceases immediately when the
force is removed. When a force (torque) is applied, the
Precession describes how a gyro reacts to any force
that attempts to tilt or turn it. Though vector diagrams
Figure 4-3.--Weight distribution in rotors.
Figure 4-4.--Direction of precession.