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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Gyroscopic Flight Instruments

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Altimeter trend vector.
Figure 7-17. Altimeter trend vector.

Gyroscopic Flight Instruments

Several flight instruments utilize the properties of a gyroscope
for their operation. The most common instruments containing
gyroscopes are the turn coordinator, heading indicator, and
the attitude indicator. To understand how these instruments
operate requires knowledge of the instrument power systems,
gyroscopic principles, and the operating principles of each

Gyroscopic Principles
Any spinning object exhibits gyroscopic properties. A wheel
or rotor designed and mounted to utilize these properties is
called a gyroscope. Two important design characteristics of an
instrument gyro are great weight for its size, or high density,
and rotation at high speed with low friction bearings.

There are two general types of mountings; the type used
depends upon which property of the gyro is utilized. A freely
or universally mounted gyroscope is free to rotate in any
direction about its center of gravity. Such a wheel is said to
have three planes of freedom. The wheel or rotor is free to
rotate in any plane in relation to the base and is balanced so
that, with the gyro wheel at rest, it remains in the position
in which it is placed. Restricted or semi-rigidly mounted
gyroscopes are those mounted so that one of the planes of
freedom is held .xed in relation to the base.

There are two fundamental properties of gyroscopic action:
rigidity in space and precession.

Rigidity in Space
Rigidity in space refers to the principle that a gyroscope
remains in a .xed position in the plane in which it is spinning.
An example of rigidity in space is that of a bicycle wheel.
As the bicycle wheels increase speed, they become more and
more stable in their plane of rotation. This is why a bicycle is
very unstable and very maneuverable at low speeds and very
stable and less maneuverable at higher speeds.

By mounting this wheel, or gyroscope, on a set of gimbal
rings, the gyro is able to rotate freely in any direction. Thus,
if the gimbal rings are tilted, twisted, or otherwise moved,
the gyro remains in the plane in which it was originally
spinning. [Figure 7-18]

Regardless of the position of its base, a gyro tends to remain rigid in space, with its axis of rotation pointed in a constant direction.
Figure 7-18. Regardless of the position of its base, a gyro tends to
remain rigid in space, with its axis of rotation pointed in a constant

Precession is the tilting or turning of a gyro in response to a
deflective force. The reaction to this force does not occur at
the point at which it was applied; rather, it occurs at a point
that is 90° later in the direction of rotation. This principle
allows the gyro to determine a rate of turn by sensing the
amount of pressure created by a change in direction. The rate
at which the gyro precesses is inversely proportional to the
speed of the rotor and proportional to the deflective force.