Direction Indicators

The Direction Gyro Indicator (DI) uses a horizontal axis tied gyro, which possesses freedom in three planes, and uses the gyroscope's property of rigidity, to stabilise an azimuth scale. It is manually aligned with the direct reading magnetic compass and, in light aeroplane, provides a stabilised directional reference for maintaining and turning accurately on to a heading.

The DI is non-magnetic and is thus not subject to turning and acceleration errors, dip or magnetic disturbances. The DI provides an accurate dead-beat indication of heading, and shows any deviation from the set heading instantaneously. The DI is also not north seeking, so it must be provided with a directional datum from an outside source, which is normally taken from the direct reading magnetic compass.

It is thus essential that the DI indications be checked, at regular intervals, against the direct reading magnetic compass, because after the initial synchronisation the gyro may wander, particularly after aerobatics. The DI is thus designed to complement the DRC, and not replace it.

Basic Description of the Direction Indicator

The instrument can be either air driven or electrically driven. In the air driven version the instrument consists of an air-driven horizontal axis gyro, rotating at approximately 10,000 rpm.

The azimuth scale is graduated from 0° to 360° in 5° divisions, with main graduations every 10°, and figures every 30°. The scale is then read of against a vertical lubber line.

Below the window is sited a knob, which is used to cage the gyro, and also to rotate the gyro assembly when, setting a given heading.

Operation of the Direction Indicator

The diagram below shows the internal mechanism of a typical air driven version of the instrument.

The rotor spins about its horizontal axis, and is supported in bearings in the inner gimbal ring, which is free to rotate about the horizontal axis through 110°, ie. 55° either side of its central position, when it is uncaged.

The inner gimbal ring is supported in bearings in the vertical outer gimbal ring and is free to rotate in azimuth through 360° about the vertical axis. A nozzle is cited in the outer gimbal, and directs a jet of air onto buckets cut in the rotor periphery.

The action of the air ensures that the rotor reaches its operating rpm after about five minutes, when full suction is developed by the vacuum pump. The air jet also maintains the rotor spin axis in the horizontal plane, as shown below. If the gyro topples, a component of the jet force will act at right angles to the rotor, and will produce a precessing force, which will erect the gyro.

In this version the indicator scale is attached to the outer gimbal, but on newer models, the synchronising gear ring also drives a sequence of gears, which connect the movements of the gyro around its vertical axis, onto a vertical scale, as shown below.

The gyro is initially erected using a caging mechanism, which manually places the gyro spin axis in its horizontal plane. The mechanism consists of a bevel pinion and a caging arm, which are both directly controlled by a caging or setting knob sited on the front of the instrument. When the knob is pushed in the bevel pinion engages with the synchroniser gear ring, and allows the scale to be adjusted in azimuth by rotating the caging knob.

At the same time the caging arm is raised, which locks the inner gimbal ring in its horizontal plane, and prevents the gimbal ring and rotor from toppling during resetting. The caging knob is pulled out to uncage the gyroscope, by disengaging the gears and allowing the caging arm to drop, thus releasing the inner gimbal ring. The inner gimbal should also be locked in its horizontal position during aerobatics, to prevent severe loads being transmitted to the rotor bearings.

In the electrical version, the rotor is part of an AC ‘squirrel cage' induction motor, and rotates at approximately 24,000 rpm. Initial erection is again by use of the caging device, but thereafter the gyro is tied so that it maintains its spin axis horizontal to the earth's surface.

Errors Associated with the Air Driven Direction Indicator

The air driven Direction Indicator is subject to the following errors:

Real Drift and Topple

The gyro is subject to both of these errors, which are caused by mechanical imperfections such as a slight imbalance of the rotor/gimbal system, bearing friction and mechanical or electrical latitude correction.

Apparent Drift

Earth rate (due to earth rotation) and transport wander (due to the gyro being transported in an east/west direction) cause this error. A fixed rate of compensation is calculated for the latitude of operation of the aeroplane for apparent wander, and may be applied by means of balancing nuts, which are attached to the gimbals. The rate of compensation, but no compensation for transport wander is made apart from periodic resetting of the instrument to the magnetic compass heading.

The drift rate can be as much as 15.04°/hour; and for this reason the instrument should be realigned with the magnetic heading shown on the direct reading compass approximately every 15 minutes, whilst the aeroplane is in level and unaccelerated flight. Any topple experienced by the gyro will be corrected for by the air jet erection system.

Gimballing Error

Any misalignment between the aeroplane axes and the navigation system axes will cause this error. This error only exists during manoeuvres, ie. during climbing, descending or banking, but once level flight is resumed, will disappear.

Use of the Direction Indicator (DI)

On small basic aeroplanes and some older aeroplanes, the direction indicator is the primary heading reference used to maintain the required heading, although great care must be exercised when using this instrument, due to the various drift errors that exist.

Prior to departure the DI should be aligned with the magnetic compass using the heading set knob. Also during the pre-flight process, and during taxi for take-of, the two readings should be periodically compared to make sure that the gyro is not showing large drift rates. Once in flight the DI should be again checked and reset against the magnetic compass approximately every 15 or 20 minutes, with the aeroplane in a steady level flight condition.

Advanced Use of the Direction Indicator

In areas where the Earth's magnetic field does not provide a stable heading reference, it is necessary to base any headings on an unmonitored DI. This is a highly sophisticated instrument, where any friction effects are reduced to a minimum, although the instrument still suffers from drift errors. These errors therefore need to be calculated before they can be compensated for.