Figure 4-3.--Weight distribution in rotors.

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

performance.

(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

Precession

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.

4-3