The gyro horizon, or artificial horizon as it is sometimes called, indicates the pitch and bank attitude of an aircraft relative to the vertical, and for this purpose employs a displacement gyroscope whose spin axis is maintained vertical by a gravity – sensing device, so that effectively it serves the same purpose as a pendulum but with the advantage that aircraft attitude changes do not cause it to oscillate.

2. Indications of pitch and bank attitude are presented by the relative positions of two elements, one symbolizing the aircraft itself, and the other in the form of a bar stabilized by the gyroscope and symbolizing the natural horizon. Supplementary indications of bank are presented by the position of a pointer, also gyro-stabilized, and a fixed bank angle scale. Two methods of presentation are shown in fig below.

3. The operating principle may be understood by referring to Fig below. The gimbal system is arranged so that the inner ring forms the rotor casing, a and is pivoted parallel to the aircraft’s lateral axis YY1 ; and the outer ring is pivoted parallel to the aircraft‘s lateral axis ZZ1. The outer ring pivots are located at the front and rear ends of the instrument case. The element symbolizing the aircraft may be either rigidly fixed to the case, or externally adjusted up and down for pitch trim setting.

In operation the gimbal system is stabilized so that in level flight in the three axes are mutually at right angles. When there is a change in the aircraft’s attitude, it goes into climb say, the instrument case and outer ring will turn about the axis YY1, of the stabilized inner ring.

4. The horizon bar is pivoted at the side and to rear of the outer ring, and engages an actuating pin fixed to the inner ring, thus forming a magnifying lever system. In a climb attitude the bar pivot carries the rear end of the bar upwards causing it to pivot about the stabilized actuating pin. The front end of the bar and the pointer therefore move downwards is relative to the symbolic aircraft element, a climbing attitude is indicated.

Changes in the lateral attitude of the aircraft i.e. banking, displaces the instrument case about the axis ZZ1 and the whole stabilized gimbal system. Hence, lateral attitude changes are indicated by movement of the symbolic aircraft element relative to the horizon bar, and also by relative movement between the bank angle scale and the pointer.

Freedom of gimbal system movement about the roll and pitch axes is 360 ^o and 85 ^o is to prevent ‘gimbal lock’ .

Vacuum -Driven Gyro Horizon

5. A typical version of a vacuum- driven gyro horizon is shown in Fig below.. The rotor is pivoted in ball bearings within a case forming the inner ring, which in turn is pivoted in a rectangular – shaped outer ring. The lower rotor bearing is fitted into a recess in the bottom of the rotor casing, whereas the upper bearing is carried in a housing which is spring –loaded within the top cap to compensate for the effects of differential expansion between the rotor shaft and case under varying temperature conditions .

6. A background plate which symbolizes the sky is fixed to the front end of the outer ring and carries the bank pointer which registers against the bank- angle scale. The outer ring has complete freedom through 360 o about the roll axis. A resilient stop limiting the +0r – 85 o pitch movement is fitted on the top of the rotor casing.

7. The horizon bar and pointer are an accurately balanced assembly pivoted in plain bearings on the side of the outer ring and slotted to engage the actuating pin projecting from the rotor case. Pitch attitude changes are indicated by the pointer set at right angles to the bar and positioned in front of the ‘sky plate’.

8. In the rear end cover of the instrument case, a connection is provided for the coupling of the vacuum supply. A filtered air inlet is also provided in the cover and is positioned over the outer-ring rear-bearing support and pivot, which are drilled to communicate with a channel i0n the outer ring. This channel terminates in diametrically-opposed spinning jets within the rotor casing, the underside of which has a number of outlet holes drilled in it.

9. With then vacuum system in operation, a depression is created so that the surrounding atmosphere enters the filtered inlet and passes through the channels to the jets. The air issuing from the jets impinges on the rotor buckets, thus imparting even driving forces to spin the rotor at approximately 15,000 rev./min. in an anticlockwise direction as viewed from above. After spinning the rotor , the air passes through a pendulous vane unit attached to the underside of the rotor casing, and finally drawn off by the vacuum source. The purpose and operation of the pendulous vane unit is described under ‘ Erection Systems for Gyro Horizons’

Electric Gyro Horizon

10. The reason why electrical driven gyro was developed are as listed below: -

(a) Higher speeds:- The need to increase the rigidity of the gyro for better accuracy by increasing the rotation speed was felt .The air driven gyro had a rotor speed of 18000 RPM as compared to 30000 for the electrical driven gyro.

(b) Neater design :- The air driven gyros were bulky and had a large number of limitations on installation and utilization. The electrical version are less bulky and better designed and occupy less area.

(c) Greater operational limits:- The air driven gyro was limited in pitch to + - 60 deg and to +- 110 deg in bank. To have more freedom of movement it was essential to have a topple resistant gyro possible with the electrical one.

(d) Moisture corrosion :- Due to moisture content in the air driving the air driven artificial horizon the chances of corrosion could not be ruled out.

(e) Failure warning device:- The lack of a failure warning device was a major reason to develop the driven newer electrical artificial horizon.

11. An example of an electric gyro horizon is shown in Fig below. As will be noted, it is made up of the same basic elements as the vacuum-driven type, with the exception that the vertical gyroscope is a 3- phase squirrel-cage induction motor (consisting of a rotor and a stator). One of the essential requirements of any gyroscope is to have the mass of the rotor concentrated as near to the periphery as possible, thus ensuring maximum inertia. This presents no difficulty where solid metal rotors are concerned, but when adopting electric motors as gyroscopes some rearrangement of their basic design is necessary in order to achieve the desired effect. An induction motor normally has its rotor revolving inside the stator, but to make one small enough to be accommodated within the space available would mean too small a rotor mass and inertia. However, by designing the rotor and its bearings so that it rotates on the outside of the stator, then for the same required size of the rotor the mass of the rotor is concentrated further from the center, so that the radius of gyration and inertia are increased . This is the method adopted not only in gyro horizons but in all instruments and systems employing electric gyroscopes. The motor assembly is carried in a housing which forms the inner gimbal ring supported in bearings in the outer gimbal ring, which is in turn supported on a bearings pivot in the front cover glass and in the rear casting. The horizon bar assembly is in two halves pivoted at the rear of the outer gimbal ring and is actuated in a manner similar to that already described earlier.

12. The 115 v 400 Hz 3-phase supply is fed to the gyro stator via slip rings, brushes and finger contact assemblies . The instrument employs a torque-motor erection system, the operation of which described later. When power is switched on a rotating magnetic field is set up in the gyro stator which cuts the bar forming the squirrel-cage in then rotor, and induces a current in them. The effect of this current is to produce magnetic fields around the bars which interact with the stator’s rotating field causing the rotor to turn at a speed of approximately 20,000 – 23,000 rev./min. Failure of the power supply is indicated by a flag marked OFF and actuated by a solenoid.

Erection Systems For Gyro Horizons.

13. These systems are provided for the purposes of erecting the gyroscope to its vertical position, and to maintain it in that position during operation. The systems adopted depend on the particular design of gyro horizon, but they are all of the gravity-sensing type and in general fall into two main categories: mechanical and electrical. The construction and operation of some systems which are typical of those in current use are described on the following pages.

Pendulous Vane Unit

14. This unit, employed with the air -driven instrument described earlier,. It is fastened to the underside of the rotor housing and consists of four knife-edged pendulously suspended vanes clamped in pairs on two intersecting shafts supported in the unit body. One shaft is parallel to the axis ZZ 1 of the gyroscope. In the sides of the body there are four small elongated ports, one under each vane.

15. The air, after having spun the gyro rotor, is exhausted through the ports, emerging as four streams; one forward, one rearward and two lateral. The reaction of the air as it flows through the ports applies a force to unit body. The vanes, under the influence of gravity, always hang in the vertical position, and it is this feature which is utilized to govern the airflow from the ports and to control the forces applied to the gyroscope by the air reaction. When the gyroscope is in its normal vertical position as shown, the knife-edges of the vanes bisect each of the ports (A,B,C, and D), making all four port openings equal. This means that all four air reactions are equal and the resultant forces about each axis are in balance.

16. If now the gyroscope is displaced from its normal vertical position, for example, its top is tilted towards the front of the instrument as a ©; the pair of vanes on the axis YY ₁ remain vertical, thus opening the port (D) on the right –hand side of the body and closing that (B) on the left . The increased reaction of the air from the open post results in a torque being applied to the body in the direction of the arrow, about axis XX ₁.

17. This torque is equivalent to one applied on the underside of the rotor and to the left, or at the top of the rotor at point F as shown at (d). As a gyroscope rotor always moves at a point 90 o away from the point P back to the vertical when the vanes again bisect the ports to equalize the air reactions.

Torque Motor and Leveling Switch System

18. This system is used in a number of electrically-operated gyro horizons and consists of two torque control motors independently operated by mercury leveling switches, which are mounted, one parallel to the lateral axis, and other parallel to the longitudinal axis. The disposition of the torque motors and switches is illustrated diagrammatically in Fig below. The laterally mounted switch detects displacement of the gyroscope in roll and is connected to its torque motor so that a corrective torque is applied around the pitch is detected by the longitudinally mounted leveling switch, which is connected to its torque motor so that corrective torques are around the roll axis.

19. Each leveling switch is in the form of a sealed glass tube containing three electrodes and a small quantity of mercury. They are mounted in adjustable cradl3es set at right angles to each other on a switch block positioned beneath there gyro housing. The tubes are filled with an inert gas to prevent arcing at the electrodes as the mercury makes contact and also to increase the rupturing capacity. The Torque motors comprise a squirrel-cage type laminated-iron rotor mounted concentrically about a stator, the iron core of which has two windings, one providing a constant field and called the ‘reference winding’, and the other in two parts so as to provide a reversible field. and called the ,control winding’. Both windings are powered from a step-down auto-transformer connected between phases A and B of the 115 V supply to the gyro horizon. The electrical interconnection of all the components comprising the system is indicated in Fig below.

20. When the gyro is running and in its normal operating position, the mercury in the leveling switches lies at the center of the tubes and is in contact with the center electrode. The two outer electrodes, which are connected across the control windings of the torque motor stators remain open. The auto- transformer reduces the voltage to a selected value(typically 20V) which is then fed to the center electrode of the switches and the reference windings of the torque motors. Thus , in the normal operating position of the gyroscope, current flows through the reference windings only.

21. Let us consider what happens when the gyroscope is displaced about one of its axes, to the front of the instrument, say, and about the pitch axis YY ₁. The pitch-leveling switch will also be displaced and the mercury will roll to the forward end of the tube to make of the control winding of the pitch torque motor causing current to flow through it in the direction indicated in Fig above. The stator of the roll torque motor will also be displaced inside its fixed rotor ,but will receive no current at its control winding since the roll-leveling switch is unaffected by displacement about the pitch axis.

22. The necessary corrective torque to the gimbal system must be applied by the pitch torque motor, and in order to do this , then magnetic field of its stator must be made to rotate. The voltage applied to the reference winding is fed via a capacitor, and in any alternating –current circuit containing capacitance, the phase of the current is shifted so as to lead the voltage by 90 ^o. In the circuit to control winding there in the control winding by 90 ^o. In the circuit to the control winding there is no capacitance; therefore, the voltage and current in this winding are in phase, and since the reference and control windings are both fed from the same source , the same source, then the reference winding current must also lead that in the control winding by 90^o. This out-of-phase arrangement, or phase quadrature, applies also to the magnetic field set up by each winding.

23. Thus, with current and flux flowing through the control winding in the direction resulting from the gyro displacement considered, a resultant magnetic field is produced which rotates in the stator in an anticlockwise direction. As the field rotates, it cuts the closed-circuit bar-type conductors of the squirrel-cage rotor causing a current to be induced in them. The effect of the induced current is to produce magnetic fields around the bars which interact with the rotating field in the stator creating a tendency for the rotor to follow the stator field.

24. This tendency is immediately opposed because the rotor is fixed to the instrument case; consequently, a reactive torque is set up in the torque motor which is exerted at the rear bearing of the outer ring. We may consider this torque as being exerted at a point on the gyro rotor itself so that precession will take place at a point 90 ^o ahead in the direction of rotation. This precession will continue until the gyro and mercury switch are once again in the normal operating position. It will be clear from Fig above that displacement of the gyroscope in the opposite direction will cause current to flow in the other part of the leveling-switch control winding, thus reversing the direction of the stator magnetic field and the resulting precession.

Fast-erection Systems

25. In some types of electrically-operated gyro horizon employing the torque-motor method of erection, the arrangement of the leveling switches is such that, if gyro rotor axis is more than 10 o from the vertical, the circuits to the torque motors are interrupted so that the gimbal system will never erect. For example, in one design a commutator switch, known as a bank erection cut-out , is carried on the outer gimbal ring about the roll axis , and serves to reduce erection cut-out , is carried on the outer gimbal ring about the roll axis, and serves to reduce erection errors during turns involving bank angles greater than 10^o, by opening the circuits to both leveling switches . Thus, if on resuming level flight the gimbal system has not remained accurately stabilized or as to be within the 10^o angle, the erection cut-out will maintain the erection system in the inoperative condition.

26. Furthermore, it is possible for the gyroscope of a gyro horizon to have ‘toppled’, or to be out of the vertical by too great an angle prior to starting the instrument; then due to the low erection rate of the system normally adopted, it would take too long before the required accuracy of indication was obtained. In order, therefore, to overcome these effects and to bring the gyroscope to its normal operating position as quickly as possible, a fast-erection system may be provided. Two typical systems in current use are described in the following paragraphs.

Fast-erection Switch

27. This method is quite simple in operation. The switch consists of several contacts connected in the power supply lines to the erection-system torque motors and leveling switches. Under normal operating conditions of the gyro horizon, the switch remains spring-loaded to the ‘off’ position and the low-voltage supply from the auto-transformer passes over one closed contact of the switch to the erection system, the other contacts remaining open.

28. Whenever the gyroscope goes beyond the appropriate angular limits, the erection system circuit must be restored and the gyroscope’s position brought back to normal as quickly as possible. This is achieved by pushing in the switch so that the contact in the low-voltage supply line opens to isolate the erection system from the auto-transformer, and the upper contacts close. The closure of these contacts completes the circuit to the torque motors and leveling switches, but the power supply to them is now changed over from the low-voltage value to the full line voltage of 115 V from one of the phases. This results in an increase of current through the stator windings of the torque motors, and the greater torque so applied increases the erection rate from the normal value of 5 o per minute to between 120^o and 180^o per minute, depending on the particular and design.

29. There are two important precautions which have to be observed when using this switch. Firstly, the switch must not be depressed for longer than 15 seconds to prevent overheating of the stator coils due to the higher current. The second precaution is one to be observed under flight conditions; the switch must only be depressed during straight and level steady flight and / or shallow angles of climb or descent. If acceleration or deceleration forces are present, the gyroscope will precess and produce false indications of pitch and bank attitude.

Electromagnetic Method of Fast Erection

30. In this method, a circular-shaped electromagnet is secured to the inside of the instrument case above an umbrella-shaped armature mounted on the gyro rotor housing. The armature is of approximately the same diameter as the magnet. Control of the electromagnet and the erection time is achieved by an auxiliary power control unit containing a three-phase transformer , bridge rectifier, thermally-operated time-delay relay and a standard d.c. relay. all interconnected as shown in the circuit diagram .

31. When the normal 115 V alternating-current supply is initially switched on, it is fed to contacts 1 and 2 of the standard relay, and from one phase, through the time- delay relay, to the bridge rectifier. The direct current obtained from the rectifier is then supplied to the coil of the electromagnet, which, on being energized, produces a magnetic field radiating symmetrically from a small center pole to a circular outer pole. If, at the moment of switching on the power supply the gyro rotor housing and hence the armature are tilted away from the center of the magnet then the magnetic field is no longer symmetrical with respect to the center of the armature. Under these conditions , therefore, a greater force exerted on one side of the armature than the other, and the applied torque is in such a direction as to cause the gyro housing to erect to the vertical and bring the top of the armature into line with the center of the magnet before the rotor is up to full speed.

32. In addition to passing through the electromagnet, the direct current from the rectifier also passes through the coil of the standard relay which is thus energized at the same time as the electromagnet . The resulting changeover of the relay contacts causes the 115 V supply to be fed to the tapping points 2 and 3 on the transformer primary winding. This has the effect of reducing the number of turns of the winding; in other words, the transformer is of the set-up type, the voltage of the secondary winding in this particular application being increased to 185 V.

33. After approximately 20 seconds, the time-delay relay opens and disconnects the direct current from the electromagnet. The standard relay then de-energizes and switches the gyro rotor circuit from the transformer to the normal 115 V supply, the rotor running up to full speed some seconds later.

Erection Rate

34. This is the term used to define the time taken, in degrees per minutes , for the rotor axis of a vertical gyroscope to take up its vertical position under the action of its gravity-sensing erection system. For the ideal gyro horizon, the erection rate should be as fast as possible under all conditions, but in practice such factors as speed, turning and acceleration of the aircraft, and earth’s rotation all have system is acted upon by centrifugal forces and is displaced to make the gyro follow it by precession. Therefore, the maximum erection rate that can be used is limited by the maximum error that can be tolerated during turns. Them minimum rate is governed by the earth’s rotation, speed of the aircraft, and random changes of precession due to bearing friction, variations in rotor speed, and gimbal system unbalance.

35. Thus, erection systems must be designed so that for small angular displacements of the rotor axis from the vertical, the erecting couple is proportional to the displacement, while for larger displacement it is made constant. It is also arranged that the couple gives equal erection rates for any rotor axis displacement in any direction in order to reduce the possibility of a slow cumulative error during manoeuvres. Normal erection rates provided by some typical erection systems are 8^o per minute for vacuum-driven gyro horizons and form 3^o to 5^o per minute for electrically-driven gyro horizons.

Errors Due to Acceleration and Turning

36. As we have already learned, the erection devices employed in gyro horizons are all of the pendulous gravity- controlled type. This being so, it is possible for them to be displaced by the forces acting during their acceleration and turning of an aircraft, and unless provision is made to counteract them the resulting torques will precess the gyro axis to a false vertical position and so present a false indication of an aircraft attitude. For example, let us consider the effects of a rapid acceleration in the flight direction, firstly on the vane type of erection device and secondly on the leveling-switch and torque-motor type .

37. The force set up by the acceleration will deflect the two athwartships-mounted vanes to the rear, thus opening the right-hand port. The greater reaction of air flowing through the port applies a force to the underside of the rotor and the torque causes it to precess forward about the axis YY 1. The horizon bar is thus displaced downwards, presenting a false indication of an ascent. With the leveling-switch and torque-motor type of erection device, the acceleration force will deflect the mercury in the pitch leveling switch to the rear of the glass tube. A circuit is thus completed to the pitch torque motor which also precesses the gyroscope forward and displaces the horizon bar to indicate an ascent.

38. In both cases the precession is due to a natural response of the gyroscope , and the pendulous vanes and the mercury always return to their neutral positions, but for so long as the disturbing forces remain, such positions apply only to a false vertical. When the forces are removed the false indication of ascent will remain initially and then gradually diminish under the influence of precession restoring the gyro axis to its normally true vertical. It should be apparent from the foregoing that ,during periods of deceleration, a gyro horizon will present a false indication of a descent.

39. When an aircraft turns, false indications about both the pitch and bank axes can occur due to what are termed ‘gimballing effects’ brought about by forces acting on both sets of pendulous vanes and both leveling switches. There are, in fact, two errors due to turning erection errors and pendulosity errors.

Erection Errors

40. As an aircraft enters a correctly banked turn, the gyro axis will initially remain in the vertical position and an accurate indication of bank will be presented. In this position, however, the longitudinally mounted pendulous vanes, or roll leveling switch, are acted upon by centrifugal force. The gyroscope will therefore be subjected to a torque applied in such a direction that it tends to precess the gyro axis towards the aircraft perpendicular along which the resultant of centrifugal and gravity forces is acting. Thus, the gyroscope is erected to a false vertical and introduces an error in bank indication.

41. An analysis of the error can be made with aid of Fig above, which illustrates the case of an aircraft turning to starboard through 360^o from a starting point A. The centrifugal force experienced by the gyro axis in the false vertical position during the turn is constant and at right angles to the instantaneous heading at a constant rate during a 360^o turn, the top of the gyro axis will trace out a circular path which is turn, the top of the gyro axis will trace out a circular path which is 90^o in advance of the aircraft heading. The circle at the left of the Fig represents the path of then gyroscope axis, and any chord of this circle will indicate the tilt of the axis in relation to the true vertical. The chord AB’, for example, represents the direction of tilt after the aircraft has turned through 90 o. In relating this tilt to the gyroscope and the response of its gravity-controlled erection devices to the turn, it can be resolved into two components ,one forward and the other to starboard. Thus, in addition to an error in bank indication an error in pitch is presented when the aircraft is at point B of its turn. In a similar manner, the chord AC’ indicates the direction of tilt after 180 o; at this point the tilt is maximum and the bank error has been reduced to zero, leaving maximum error in pitch indication. The direction of tilt after 270 o is indicated by chord AD’, and resolving this into its two components as at point D, we see that the pitch error is the same as at B but the bank error is in the opposite direction. On returning to point A the tilt of the gyro axis would be zero.

Compensation for Erection Errors

42. Erection errors may be compensated by one of the following three methods: (i) inclination of the gyro spin axis, (ii) erection cut-out, and (iii) pitch-bank compensation.

Inclined Spin Axis

43. The method of inclining the spin axis is based on the idea that ,if the top of the axis can describe a circle about itself during a turn, then only a single constant error will result. In its application, the method is mechanical in form and varies with the type of gyro horizon, but in all cases the result is to impart a constant forward (rearward in some instruments) till to the gyro axis from the true vertical. The angle of tilt varies but is usually either 1.6° or 2.5°. In vacuum-driven types the athwartships-mounted pendulous vanes are balanced so that the gyroscope is precessed to the tilted position; in certain electric gyro horizons the pitch mercury switch is fixed in a titled position so that the gyroscope is precessed away from the true vertical in order to overcome what it detects as a pitch error. The linkages between gyroscope and horizon bar are so arranged that during level flight the horizon bar will indicate this condition.

44. The effect of the tilt is shown in Fig where point A represents the end of the true vertical through the center of the rotor, and AA’ represents the direction of tilt (forward in this case). During a turn to starboard the top of the gyro axis describes a circle about point A at the same rates as the aircraft changes heading. The amount of tilt and its direction in relation to the aircraft during the turn are therefore constant.

Erection Cut-Out

45. The erection cut-out method is one applied to certain types of electric gyro horizon and operates automatically whenever the aircraft banks more than 10 in either direction. It consists basically of a commutator made up of a conducting segment and an insulated segment, and two contacts or brushes connected in series with the bank leveling switch. The commutator is located on the bank axis at the rear of the outer gimbal ring, and in the straight and level flight condition the two brushes bear against the conducting segment thus completing the power supply circuit to the leveling switch.

During a turn there is relatively movement between the commutator and brushes due to banking, and when the bank exceeds 10° the insulated segment comes under one or other of the brushes and so interrupts the supply to the bank leveling switch. Displacement of the mercury by centrifugal force cannot therefore energize the relevant torque motor and cause precession to a false vertical.

46. Owing to the function of the cut-out, no erection about the bank axis is possible if the power supply is switched on to the instrument when its gimbal system is tilted more than 10° about this axis. However, the supply can be connected by means of ‘fast erection’ circuit which by-passes the cut-out in the manner described earlier.

Pitch-Bank Erection

47. The third method, generally referred to as ‘pitch-bank erection’, is a combined one in which the bank leveling switch is disconnected during a turn and its erection system is controlled by the pitch leveling switch. It is intended to correct the varying pitch and bank errors and operates only when the rate of turn causes a centrifugal acceleration exceeding 0.18g, which is equivalent to a 10° tilt of the bank erection switch. The system is shown schematically in Fig 5.20, and from this we note that two additional mercury switches, connected as a double-pole changeover switch, are provided and more interconnected with the normal pitch and bank erection systems.

48. Let us consider first a turn to the left and one creating a centrifugal acceleration less than 0.18g. In such a turn, the mercury in the bank leveling switch will be displaced to the right and will bridge the gap between the supply electrode and the right-hand electrode, thus completing a circuit to the bank torque motor. This is the same as if the gyro axis had been tilted to the right at the commencement of the turn; the bank torque motor will therefore precess the gyro to a false vertical, left of the true one. At the same time, the gyro axis tilts forward due to gimballing effect, and the mercury in the pitch leveling switch, being unaffected by centrifugal acceleration, moves forward and completes a circuit to the pitch torque motor, which precesses the gyro rearwards. The two curved changeover switches, which are also mounted about the bank axis, do not come into operation since the mercury in each switch is not displaced sufficiently far to contact the right-hand electrodes. Thus, with centrifugal acceleration less than 0.18g there is no compensation.

49. Consider now a turn in which the centrifugal acceleration exceeds 0.18g. The mercury in the bank leveling switch is displaced to the end to the tube and so disconnects the normal supply to the bank torque motor, ie. it now acts as on erection cutout. However, the pitch leveling switch still responds to a forward tilt and remains connected to its torque motor, and as will be noted from the diagram, it also connects a supply to the lower of the two changeover switches. Since the mercury in these switches is also displaced by the centrifugal acceleration, a circuit is completed from the lower switch to the bank torque motor, which precesses the gyro axis to the right to reduce the bank error. At the same time, the pitch leveling switch completes a circuit to the pitch torque motor, which then precesses the gyro axis rearward so reducing the pitch error. Thus, during turns a constant control is applied about both the pitch and roll axis by the pitch leveling switch; hence the term ‘pitch-bank erection’.

50. The changeover function of the curved mercury switches depends on the direction of tilt of the gyro axis in pitch. This is indicated by the broken arrows in Fig above; the gyroscope and the pitch leveling switch now being tilted rearward, the latter connects a supply to the upper of the two changeover switches and changes its direction to the bank torque motor causing it to precess the gyroscope to the left.

51. The change in direction of the supply to the bank torque motor is also dependent on the direction of the turn, as a study of Fig above will show. As in the erection cut-out method of compensation, this system requires a ‘fast-erection’ facility to bring the gyro axis to the true vertical when it is tilted more than 10° in either bank or pitch.

Since the forces and torques acting on the gyroscope depend on the aircraft’s speed and rate of turn, the then obviously all erection errors will vary accordingly, and this makes it rather difficult to provide compensation which will eliminate them entirely. It is usual, therefore, particularly for instruments employing the inclined axis and bank-cut-put method of compensation, to base compensation on a standard rate one turn of 180° per minute at an airspeed of 200 m.p.h. At other rates of turn and airspeeds the errors are small.

Pendulosity Errors

52. Pendulosity, or ‘bottom heaviness’ as it is sometimes called, it often deliberately introduced in gyro horizons so that the gyroscope will always be resting near its vertical position. This helps to reduce the erection time when starting and also it prevents the gimbal systems from spinning about the bank and pitch axes during run-down of the rotor. However, it can be acted upon by accelerating and decelerating forces in straight and level flight, and centrifugal forces during turns; consequently, it is an additional source of error; i.e.pendulosity error.

53. When acceleration takes place the base of the rotor assembly tends to lag behind owing to inertia, i.e. it tends to swing directly rearwards. In following the force through with the aid of the 90° precession rule, it will be see, however, that the rotor assembly will precess about the bank axis to port or starboard depending on the direction of rotor rotation. A deceleration has the opposite effect.

54. The pendulosity error resulting from a turn may be analyzed in a manner similar to that of erection errors. In Fig an aircraft is again considered as turning to starboard through 360° from the point A. As the turn is entered the centrifugal acceleration tends to swing the base of the rotor assembly to port, causing precession of the gyro about the pitch axis, which again depends on the rotation of the rotor. In this instance, the gyro axis tilts forward to a false vertical and the instrument indicates an apparent climb. Throughout the turn, the top of the gyro axis traces out a circular path which, unlike that resulting from turning effects on erection systems, is synchronized with the aircraft’s heading change. As before, any chord of the circle from the point at which the turn commenced indicates the tilt of the gyro axis in relation to the true vertical, and varying errors in bank and pitch indication will be presented.

Compensation for Pendulosity Errors

55. Compensation is usually effected by adopting the inclined – axis method, the inclination in this case being about the bank axis, and the direction being dependent on that of rotor rotation. The amount of inclination is governed by the type of instrument, two typical values being 0.5° and 1.75°. The effect of the compensation, shown by the full circle in Fig, is exactly the same as that produced by inclining the gyro axis in pitch, i.e. the top of the axis traces out a circular path about itself to produce a single constant error.

CONCLUSION

56. Having seen the constructions, functioning and the errors of the artificial horizon we can see the great utilization of the instrument. With the advent of modern Inertial Navigation systems the artificial horizon is now being used as a standby system in most modern aircraft .