# How It Works: Science and Technology - Vol. 8

By Wendy Horobin | Go to book overview

Gyrocompass

A high-precision Sperry
CL II directional
gyroscope used in the
compass systems of many
aircraft. It is also used as
part of the directional
equipment of some rockets
and submarines.

The gyrocompass is a true-north directional indicator used extensively in merchant and naval vessels. It is one of the most used navigational aids, as it provides a true-north indication regardless of any rolling, pitching, or yawing of the vessel and is entirely unaffected by any of the disturbances that commonly affect magnetic compasses. The gyrocompass is usually installed below deck, and its indication is relayed around the ship to operate ancillary equipment such as steering and bearing repeaters, course recorders, and gyropilots.

The basis of the gyrocompass is a gyroscope controlled in such a way that its spin axis is made to seek and maintain alignment with the geographic meridian (north-south line). This alignment is achieved by combining Earth's rotation and the force of gravity with inertia and precession (the regular movement of the axis of a rotating body as a result of an applied angular force, such that the axis describes a cone in space).

Theory of operation

Earth rotates about its polar axis from west to east with an angular velocity of one revolution in 24 hours, that is, 15 degrees per hour. At any point on Earth's surface, this angular velocity can be resolved into two components: a component aligned to the local vertical, known as the vertical Earth rate, and a horizontal component aligned to the meridian and known as the horizontal Earth rate.

The magnitude of these components varies with latitude. Vertical Earth rate varies as the sine of the angle of latitude and is 15 degrees per hour at the poles and zero at the equator, while the horizontal Earth rate varies as the cosine of the angle of latitude and is zero at the poles and 15 degrees per hour at the equator.

The gyroscope used in the gyrocompass is electrically driven and mounted in gimbals in such a way that it has freedom to move about both a vertical and a horizontal axis. The gyroscope can be considered as a space-stable element because its axes will remain pointed in the same direction widi respect to inertial space unless acted upon by a force. Earth is not a part of inertial space but rotates within it. For this reason, the directions in which the axes of a gyroscope point, with respect to an observer stationary on Earth, will appear to change as Earth rotates, although they are in fact remaining constant with respect to inertial space.

Being a space-stable element, the gyroscope senses the rotation of Earth: the vertical axis senses the vertical Earth rate and the horizontal axis the horizontal Earth rate. Therefore, the effect of Earth's rotation on the axes of the gyroscope varies with the latitude. This idea can best be appreciated by considering the behavior of the gyroscope at various geographical locations.

If the gyro is located at the North Pole with its spin axis horizontal, the rotation of Earth will be measured entirely by the vertical axis, and to an observer using the spinning Earth as a refer-

The apparent tilting of
a gyrocompass in relation
to Earth. Because of
Earth's rotation, a
gyrocompass that is freely
mounted on Earth's surface
appears to change axis. In
the diagram, the axis of
the gyrocompass at
midnight is horizontal in
relation to Earth but by
8 A.M. it has moved
through 120 degrees.

-1028-

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How It Works: Science and Technology - Vol. 8

• Title Page *
• Contents *
• Gold 1013
• Governor 1017
• Grass-Cutting Equipment 1018
• Gravity 1020
• Gun 1023
• Gyrocompass 1028
• Gyroscope 1030
• Hair Treatment 1032
• Halogen 1034
• Hang Glider 1037
• Hearing 1041
• Heart 1045
• Heart Pacemaker 1048
• Heart Surgery 1049
• Heat Engine 1053
• Heat Exchanger 1054
• Heating and Ventilation Systems 1056
• Heat Pump 1063
• Helicopter 1065
• Hi-Fi Systems 1071
• High-Speed Photography 1077
• Holography 1080
• Hormone 1084
• Horticulture 1088
• Hosiery and Knitwear Manufacture 1090
• Hydraulics 1100
• Hydrocarbon 1105
• Hydrodynamics 1109
• Hydroelectric Power 1112
• Hydrofoil 1116
• Hydrogen 1118
• Hydroponics 1120
• Hygrometer 1123
• Ignition System, Automobile 1124
• Image Intensifier 1128
• Immunology 1132
• Induction 1138
• Inertia 1142
• Information Technology 1147
• Ink 1151
• Index i
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