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known as rigidity in space or gyroscopic inertia. Thus, if the spin axis of a gyro were pointed toward a
star, it would keep pointing at the star. Actually, the gyro does not move, but the earth moving beneath it
gives it an apparent motion. This apparent motion, shown in Figure 3.10, is called apparent precession.
The magnitude of apparent precession is dependent upon latitude. The horizontal component, drift, is
equal to 15o per hour times the sine of the latitude, and the vertical component, topple, is equal to 15o per
hour times the cosine of the latitude.
Figure 3.10. Apparent Precession.
3.6.1.3. These computations assume the gyro is stationary with respect to the earth. If the gyro is to be
used in a high-speed aircraft, however, it is readily apparent that its speed with respect to a point in space
may be more or less than the speed of rotation of the earth. If the aircraft in which the gyro is mounted is
moving in the same direction as the earth, the speed of the gyro with respect to space will be greater than
the earth's speed. The opposite is true if the aircraft is flying in a direction opposite to that of the earth's
rotation. This difference in the magnitude of apparent precession caused by transporting the gyro over
the earth is called transport precession.
3.6.1.4. A gyro may precess because of factors other than the earth's rotation. When this occurs, the
precession is labeled real precession. When a force is applied to the plane of rotation of a gyro, the plane
tends to rotate, not in the direction of the applied force, but 90o around the spin axis from it. This
torquing action, shown in Figure 3.11, may be used to control the gyro by bringing about a desired
reorientation of the spin axis, and most DGs are equipped with some sort of device to introduce this
force. However, friction within the bearings of a gyro may have the same effect and cause a certain
94 AFPAM11-216 1 MARCH 2001
amount of unwanted precession. Great care is taken in the manufacture and maintenance of gyroscopes
to eliminate this factor as much as possible, but, as yet, it has not been possible to eliminate it entirely.
Precession caused by the mechanical limitations of the gyro is called real or induced precession. The
combined effects of apparent precession, transport precession, and real precession produce the total
precession of the gyro.
Figure 3.11. Precession of Gyroscope Resulting From Applied Deflective Force.
3.6.1.5. The properties of the gyro that most concern the navigator are rigidity and precession. By
understanding these two properties, the navigator is well-equipped to use the gyro as a reliable steering
guide.
3.6.2. Directional Gyro (DG). The discussion thus far has been of a universally mounted gyro, free to
turn in the horizontal or vertical or any component of these two. This type of gyro is seldom, if ever,
used as a DG. When the gyro is used as a steering instrument, it is restricted so that the spin axis remains
parallel to the surface of the earth. Thus, the spin axis is free to turn only in the horizontal plane
(assuming the aircraft normally flies in a near-level attitude), and only the horizontal component (drift)
will affect a steering gyro. In the terminology of gyro steering, precession always means the horizontal
component of precession.
3.6.2.1. The operation of the instrument depends upon the principle of rigidity in space of the gyroscope.
Fixed to the plane of the spin axis is a circular compass card, similar to that of the magnetic compass.
Since the spin axis remains rigid in space, the points on the card hold the same position in space relative
to the horizontal plane. The case, to which the lubber line is attached, simply revolves about the card.
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3.6.2.2. It is important at this point to understand that the numbers on the compass card have no meaning
within themselves, as on the magnetic compass. The fact that the gyro may indicate 100o under the
lubber line is not an indication that the instrument is actually oriented to magnetic north (MN), or any
other known point. To steer by the gyro, the navigator must first set it to a known direction or point.
Usually, this is MN or geographic north, though it can be at any known point. If, for example, MN is set
as the reference, all headings on the gyro read relative to the position of the magnetic poles.
3.6.2.3. The actual setting of the initial reference heading is done by using the principle discussed earlier
of torque application to the spinning gyro. By artificially introducing precession, the navigator can set
the gyro to whatever heading is desired and can reset it at any time, by using the same technique.
3.6.3. Gyrocompass Errors. The major error affecting the gyro and its use as a steering instrument is
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