Figure 4-5.--Continuous precession.

right and continue turning until the weight is re-

gyroscope precesses until it is in the plane of the

moved.

force. When this position is reached, the force is

about the spinning axis and can cause no further

FORCE OF TRANSLATION

precession.

If the plane in which the force acts moves at the

Any force operating through the center of gravity of

same rate and in the same direction as the precession it

the gyroscope does not change the angle of the plane of

rotation but moves the gyroscope as a unit without

causes, the precession will be continuous. This is

changing its position in space. Such a force operating

illustrated by figure 4-5, in which the force attempting

through the center of gravity is known as a force of

to change the plane of rotation is provided by a weight,

translation. Thus, the spinning gyroscope may be moved

W, suspended from the end of the spin axle, X. Although

freely in space by means of its supporting frame, or case,

the weight is exerting a downward force, the torque is

without disturbing the plane of rotation of the rotor. This

felt 90 away in the direction of rotation. If the wheel

condition exists because the force that is applied through

rotates clockwise, as seen from the weighted end,

the supporting frame acts through the center of gravity

precession will occur in the direction of arrow P. As the

of the rotor and is a force of translation. It produces no

torque on the gyro rotor.

gyroscope precesses, it carries the weight around with

it so that forces F and F1 continuously act at right angles

EFFECT OF EARTH'S

to the plane of rotation, and precession continues

ROTATION

indefinitely. In other words, the rotor will turn to the

As just explained, a free-spinning gyroscope

can be moved in any direction without altering the

angle of its plane of rotation. If this free-spinning

gyroscope is placed on the earth'is surface at the equator,

with its spinning axis horizontal and aligned east and

west, an observer in space below the South Pole would

note that the earth rotates clockwise from west to east

and carries the gyroscope along. As the earth rotates,

rigidity of plane keeps the gyroscope wheel fixed in

space and rotating in the same plane at all times. Figure

4-6 shows how this gyroscope would appear. Assume

that the gyroscope is set spinning at 0000 hours

with its spinning axis aligned east and west and

parallel to the earth's surface. At 0600, 6 hours after the

gyroscope was started, the earth has rotated 90 and

the axle of the gyroscope is aligned with the original

starting position. At 1200 the earth has rotated 180,

while the gyroscope returns to its original position. The

figure shows how the gyro completes a full cycle in a

24-hour period.

APPARENT ROTATION OF THE

GYROSCOPE

An observer on the earth's surface does not see

the operation of the gyro in the same way as an

observer in space does. On the earth, the gyro

appears to rotate, while the earth appears to stand

still. As the earth rotates, the observer moves with

it, so the gyroscope seems to rotate around its

horizontal axis. The effect the observer sees on the

Figure 4-5.--Continuous precession.

earth is called apparent rotation and also is referred

4-4