1.1 Gyroscopic InstrumentsThe gyroscope (Gyro) forms an essential element in artificial horizons,automatic pilot systems and gyro compasses.In navigation it provides a means by which an aircraft’s direction and attitude can be determined.

DefinitionIt is well known that aspinning top will remain upright which it is set spinning at high speed,but as its speed of rotation slows down it eventually topples.

Any rotating mass has gyroscopic properties which produce special effects that it does not have if it is sationary.masses which produce gyroscopic effects include jet engines,spinning coin,a bicycle wheel,the earth,in fact any mass that rotates.

Although all rotating masses have gyroscopic properties,in engineering terms the name gyroscope is used extensively to describe a rotating mass-or –rotor-mounted in such a way that the spin axis of rotor is free to rotate about one or more axes at right angles to the spin axis..1.1.1 Gyroscopic Properties

As mechanical device a gyroscope may be defined as a system containing a heavy metal wheel (rotor), universally mounted so that it has three degrees of freedom:

Spinning freedom: About an axis perpendicular through its centre (axis of spin XX).

Tilting Freedom: About a horizontal axis at right angles to the spin axis (axis of tilt YY).

Veering Freedom: About a vertical axis perpendicular to both the other two axes (axis of veer ZZ).

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 center of gravity of the rotor.

The system will not exhibit gyroscopic properties unless the rotor is spinning. When the rotor is spinning at high speed the device becomes a true gyroscope possessing two important fundamental properties:

1. Gyroscopic Inertia (Rigidity).

2. Precession.

1.1.2 Rigidity

The property, which resists any, force tending to change the plane of rotor rotation. It is dependent on:

1. The mass of the rotor.

2. The speed of rotation.

1.1.3 Precession

The angular change in direction of the plane of rotation under the influence of an applied force. The change in direction takes place, not in line with the force, but always at a point 90º away in the direction of rotation. The rate of precession also depends on:

1. The strength and direction of the applied force.

2. The angular velocity of the rotor.

Figure 32 shows a gyroscope.

Gyroscope.Figure 33 shows the characteristics of gyro rigidity.

Gyro RigidityFigure 33

Gyro A has its spin axes parallel with the Earth's spin axes, located at the North Pole. It could hold this position indefinitely.

Gyro B has its spin axes parallel to the Earth's spin axes, but located at the Equator. As the Earth rotates, it would appear to continually point North.

Gyro C is also situated at the Equator. As the Earth rotates, it appears to rotate about its axes, however it is the Earth that is rotating and not the gyro.

This rigidity can be used in a number of gyro instruments including the directional gyro.

1.1.4 Precession

If an external force is applied to a spinning gyro, its effect will be felt at 900 from the point of application, in the direction of gyro rotation. This is known as precession. It can be seen in Figure 34, that if a force is applied to the bottom of the rotating wheel, it will rotate about its horizontal axis.

This property is not wanted in some instruments, such as directional gyros. The use of precession is used in turn indicators, which will be covered later.

Gyro PrecessionFigure 34

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 center of gravity of the rotor.a)Free Gyro:A gyro having complete freedom in three planes at right angles to each other.this is also sometimes known as ‘space’ gyro.

b)Tied Gyro:A gyro having freedom in three planes at right angles to each other but controlled by some external source.all gyros on aircraft are tied.c)A tied gyro controlled by gravity to maintain its position relative to the earth.the artificial horizon gyro is an earth gyro.d)Rate Gyro:A gyro having one plane of freedom at right angles to the plane of rotation ,so constructed as to measure rate of movement about the plane at right angles to both the plane of rotation and plane of freedom .these gyros will have a spring to bring them back to datum once precessional force is removed.

Instruments that use either the rigidity or the precession of gyros are:

1. Gyro Horizon Unit.

2. Attitude Director Indicator.

3. Standby Horizon Unit.

4. Direction Indicator.

5. Turn and Slip Indicator.

6. Turn Co-ordinator.

Gyro Horizon Unit

The Gyro Horizon Unit gives a representation of the aircraft’s pitch and roll attitudes relative to its vertical axis. For this it uses a displacement gyroscope whose spin axis is vertical. Figure 37 shows a displacement gyro and the two axis of displacement.

Displacement GyroFigure 37

Indications of attitude are presented by the relative positions of two elements, one symbolising the aircraft itself, the other in the form of a bar stabilized by the gyroscope and symbolising the natural horizon. Figure 38 shows a typical Gyro Horizon Unit.

Gyro Horizon UnitFigure 38

The gimbal system is so arranged so that the inner ring forms the rotor casing and is pivoted parallel to an aircraft’s lateral axis (YY1); the outer ring is pivoted at the front and rear ends of the instrument case, parallel to the longitudinal axis (ZZ1). The element symbolizing the aircraft may either be rigidly fixed to the case, or it may be externally adjustable for setting a particular pitch trim reference.

1.2 Attitude Director Indicator (ADI)

The ADI presents a symbolic three-dimensional display of the aircraft’s attitude, combined with lateral and vertical steering commands. The aircraft’s attitude is displayed by the relationship of a stationary airplane symbol with respect to a moveable horizon line. The horizon line is carried on a sphere, which is servo driven in pitch and roll. The sphere is marked off in increments of 5 degrees, and is coloured blue to represent sky above the horizon line, and black or brown/orange to represent ground below the horizon line. The sphere is unbalanced in the roll axis so that on loss of power it rotates to approximately 90 degree left bank indication. Cross pointer bars are used to indicate flight director commands and are brought into view by operation of the flight director switches (FD BARS). The horizontal (pitch) bar indicates below the miniature airplane symbol to command pitch up attitude. The vertical (roll) bar indicates to the right center display to command right roll, and to the left of center of display to command left roll. Both bars are biased out of view when the FD BARS are off, but the FD flag will not appear unless a power loss is experienced.

Aircraft position relating to a glideslope is given by a pointer moving over a vertical display. Aircraft position above the glideslope beam is indicated by the pointer being positioned below the glideslope scale index, and aircraft position below the glideslope beam is indicated by the pointer being positioned above the glideslope beam. The loss of the glideslope valid signal will cause the glideslope warning flag (GS) to come into view. The glideslope indicator and warning flag are mounted on the right hand side of the ADI presentation.

Localiser deviation is indicated by lateral movement of the localiser pointer, and is a read on a fixed horizontal scale. The pointer indicates to the right of the fixed scale index if the aircraft is to the left of the localiser beam and to the left of the index if aircraft is to the right of the localiser beam. The loss of either the localiser valid input or tuned to localiser input will bias the localiser pointer from view. Loss of the localiser valid signal causes the localiser (LOC) flag to move into view. The localiser indicator is positioned at the bottom of the ADI display, above the inclinometer.

Slip information is conventionally displayed on the ball type inclinometer mounted on the indicator at the bottom of the ADI display. Instantaneous testing of the sphere and flight director is accomplished by pressing the TEST switch.

The sphere should indicate:

(a) 10° ± 5° Pitch Nose up.

(b) 20° ± 5° Roll to the Right.

(c) ATT and FD flag in view.

(d) FD Bars Indicate Nose Up and Roll to the Right.

Figure 43 shows an Attitude Director Indicator (ADI)

Attitude Director Indicator (ADI)Figure 43

1.3 Standby Attitude Indicators

The standby attitude indicator provides a continuous visual indication of the aircraft attitude in the pitch and roll axes.

1.3.1 Description and Operation

The standby attitude indicator display comprises a two-coloured drum supported in an outer gimbal, a roll marker mounted on the outer gimbal shroud and a roll scale and aeroplane index mounted on the front cover behind the dial glass. A white line dividing the two colours on the drum, blue representing the sky and dark orange representing the earth, represents the horizon. Attitude is indicated by the position of the drum relative to the aircraft symbol. A graduated scale on the drum, which can indicate 60 degrees of dive or 80 degrees of climb, indicates pitch angle. Roll angle is indicated by a white marker relative to the roll scale which is graduated at zero degrees and 10, 20, 30, 40, 50 and 60 degrees left and right of zero. A fast erection knob is provided on the bottom right-hand side of the instrument face and is a purely mechanical caging device. Figure 46 shows a Standby Attitude Indicator and its location.

Standby Attitude Indicator H 301Figure 46

1.4 Direction Indicators

This indicator was the first gyroscopic instrument to be introduced as a “Heading Indicator” .The instrument uses a horizontal axis gyroscope and, being non-magnetic, is used in conjunction with a magnetic compass.

In its basic form, the outer ring of the gyro carries a circular card, graduated in degrees, and referenced against a lubber line fixed to the gyro frame. When the rotor is spinning, the gimbal system and card are stabilized so that, by turning the frame, the number of degrees through which it is turning may be read on the card. Figure 50 shows a Directional Indicator.

Directional Indicator Figure 50

In the directional gyro, the rotor is enclosed in a case, or shroud, and supported in an inner gimbal which is mounted in an outer gimbal, the bearings of which are located top and bottom on the indicator case. The front of the case contains a cut-out through which the card is visible, and also a lubber line reference.

The caging/setting knob is provided at the front of the case to set the indicator onto the correct heading (magnetic). When the setting the heading, the inner gimbal has to be caged to prevent it from precessing as the outer gimbal is rotated. Figure 51 shows the construction of a directional gyro.

Directional GyroFigure 51

1.5 Turn & Slip Indicator

This indicator contains two independent mechanisms:

1. A gyroscopically controlled pointer mechanism for the detection and indication of the rate at which an aircraft turns.

2. A mechanism for the detection and indication of slip/slide.

A gimbal ring and magnifying system, which moves the pointer in the correct sense over a scale calibrated in what is termed “Standard Rates”, actuate the rate of turn pointer. Although they are not always marked on a scale, they are classified as follows:

1. Rate 1 - Turn Rate 180º per minute.

2. Rate 2 - Turn Rate 360º per minute.

3. Rate 3 - Turn Rate 540º per minute.

4. Rate 4 - Turn Rate 720º per minute.

Figure 52 shows a typical Turn & Slip indicator.

Turn & Slip Indicator Figure 52

For the detection of rates of turn, a rate gyroscope is used and is arranged in the manner shown in figure 53.

Rate Gyro Turn IndicatorFigure 53

It differs in two respects from the displacement gyro as it only has one gimbal ring and a calibrated spring restraining in the longitudinal axis YY1. When the indicator is in its normal operating position the rotor spin axis, due to the spring restraint, will always be horizontal and the turn pointer at the zero datum. With the rotor spinning, its rigidity will further ensure that the zero position is maintained.

When the aircraft turns to the left about the vertical input axis the rigidity of the rotor will resist the turning movement, which it detects as an equivalent force being applied to its rim at point F. The gimbal ring and rotor will therefore be tilted about the longitudinal axis as a result of precession at point P.

As the gimbal ring tilts, it stretches the calibrated spring until the force it exerts prevents further deflection of the gimbal ring. Since precession of a rate gyro is equal to its angular momentum and the rate of turn, then the spring force is a measure of the rate of turn.

Actual movement of the gimbal ring from its zero position can, therefore, be taken as the required measure of turn rate.

1.5.1 Bank Indication

In addition to the primary indication of turn rate, it is also necessary to have an indication that an aircraft is correctly banked for the particular turn. A secondary indicating mechanism is therefore provided, which, depends for its operation on the effect of gravitational and centrifugal forces. A method commonly used for bank indication is one utilising a ball in a curved liquid-filled glass tube as shown in Figure 26.

In the normal level flight the ball is held at the center of the tube by the force of gravity. Let us assume the aircraft turns left at a certain airspeed and bank angle. The indicator case and the tube move with the aircraft and centrifugal force (CF) in addition to that of gravity acts upon the ball and tends to displace it outwards from the center of the tube. However, when the turn is executed at the correct bank angle and matched with airspeed, then there is a balanced condition between the two forces and so the resultant force (R) hold the ball in the center of the tube.

If the airspeed were to be increased during the turn, then the bank angle and centrifugal force would also be increased. As long as the bank angle is correct for the appropriate conditions, the new resultant force will still hold the ball central.

If the bank angle for a particular rate of turn is not correct (under-banked/over-banked), then the aircraft will tend to either skid or slip. In the skid condition the centrifugal force will be the greatest, whereas in the slip condition the force of gravity is greatest.

Figure 54 shows bank indication for various aircraft bank conditions.

Bank IndicationsFigure 54

1.6 Turn Co-ordinator

The final instrument in this group is the turn co-ordinator. Basically, its mechanism is changed slightly from the turn and slip indicator, so that it senses rotation about the longitudinal axis, (bank) as well as the vertical axis, (turn). This gives a more accurate indication to the pilot, of the turning of the aircraft.

Figure 55 shows a Turn co-ordinator indicator.

Turn co-ordinator IndicatorFigure 55

1.6.1 Warnings

1. ATT Flag

Indicates an internal failure of the ADI or a Gyro Attitude (VRU) failure.

2. FD Flag

Indicates an internal failure of the command bars for any axis or flight director failure.

3. LOC Flag

Indicates a loss of the localiser valid signal, or insufficient signal with index off scale.

4. Glideslope Flag

Indicates loss of the localiser valid (G/S) signal with index off scale.

1.6.2 Attitude Distribution

Figure 44 shows a block schematic of the attitude transfer switching circuit and shows the distribution of the attitude information. The transfer switching is drawn in the ‘NORMAL’ position fed from 28V ESS DC.

Switching allows either gyro to supply both ADI attitude displays and the autopilot. The flight data recorder and weather radar are hard wired to No 1 gyro.

Primary outputs are used exclusively for the ADI attitude displays. Buffered secondary 3 wire outputs are used for the autopilot, FDR and ADI cross-switching. The latter arrangement prevents a faulty ADI being paralleled with the other ADI thus causing the loss of both.

The instrument comparator monitor (ICM) provides comparison of the ADI attitude displays. A two wire roll signal is also fed to the ICM to increase the heading warning threshold in turns.