Gyroscopic Instruments - Gyroscopes Theory
Gyroscopic Flight Instruments
Aircraft Instruments
Gyroscopic instruments, a convenience for VFR flight, are an absolute necessity for IFR flight and provide a direct indication of an aircraft's attitude. Every aircraft is equipped with three gyroscopic instruments and comprise the attitude indicator, heading indicator, and turn coordinator. On most small aircraft, the vacuum system powers the attitude and heading indicators, while the electrical system powers the turn coordinator. This is done for safety reasons since the turn coordinator can function as a backup in case of a vacuum system failure. Gyroscopic instrument operation is based on two fundamental concepts that apply to gyroscopes, namely; rigidity in space and precession.
Figure 1 - Example of a gyroscope

As a mechanical device a gyroscope may be defined as a system containing a heavy metal wheel or rotor, universally mounted so that it has three degrees of freedom: spinning freedom, about an axis perpendicular through its center; tilting freedom, about a horizontal axis at right angles to the spin axis; and veering freedom, about a vertical axis perpendicular to both the other axes. The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer rings. The whole assembly is known as the gimbal system of a free or space gyroscope. The gimbal system is mounted in a frame, so that in its normal operating position, all the axes are mutually at right angles to one another and intersect at the centre of gravity of the rotor.

Operation of the Airspeed Indicator (ASI)
Figure 2 - Turn Coordinator

The system will not exhibit gyroscopic properties unless the rotor is spinning; for example, if a weight is suspended on the inner ring, it will merely displace the ring about its axis YY' because there is no resistance to the weight. When the rotor is made to spin at high speed, however, the device becomes a true gyroscope possessing two important fundamental functions as explained before.

Rigidity in space refers to the principle that a wheel with a heavily weighted rim spun rapidly will reamain in a fixed position in the plane in which it is spinning. 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 3)


Airspeed Indicator (ASI)
Figure 3 - Gyroscope demonstrating precession

Precession is the tilting or turning of a gyro in response to pressure. Unfortunately, it is not possible to mount a gyro in a frictionless environment. A small force is applied to the gyro whenever the airplane changes direction. The reaction to this force occurs in the direction of rotation, approximately 90° ahead of the point where the force was applied. This causes slow drifting and minor erroneous indications in the gyroscopic instruments. The rate of precession basically depends on three factors; the strength and direction of the applied force, the moment of inertia of the rotor, and the angular velocity of the rotor. The greater the force, the greater is the rate of precession, while the greater the moment of inertia and the greater the angular velocity the smaller is the rate of precession.

Precession of a rotor will continue, while the force is applied, until the plane of rotation becomes coincident with that of the force. At this point there will be no further resistance to the force and so precession will cease.

The axis about which a force is applied is termed the input axis, and the one about which precession takes place is termd the output axis.

References established by gyroscopes
For use in aircraft, gyroscopes must establish two essential reference datums: one for the detection of pitch and roll attitude changes, and the other for the detection of changes about the vertical axis, i.e. a directional reference. These datums are established by using vertical and horizontal spin-axis gyroscopes respectively. Both types utilize their fundamental properties in the following manner: rigidity provides a stablized reference unaffected by movement of the supporting body, and precession controls the effects of apparent and real drift thus maintaining stabilized datums.

Operation of the Airspeed Indicator (ASI)
Figure 4 - Gyroscopic Reference

It will also be noted that the pitch, roll, and directional attitudes of an aircraft are determined by its displacement with respect to each appropriate gyroscope. For this reason, therefore, the gyroscopes are referred to as displacement-type gyroscopes. Each one has three degrees of freedom and, consequently, three mutual axes, but for the purpose of attitude sensing, the spin axis is discounted since no useful attitude reference is provided when displacements take place about the spin axis alone. Thus, in the practical case, the two types of gyroscope are further classified as two-axis displacement gyroscopes.