Gyro Principles in Aircraft PDF

Precession of a rotor will continue while the force is applied until the plane of rotation is in line with the plane of the applied force. The axis about which a torque is applied is termed the input axis, and the one about which precession takes place is termed the output axis. The...

Precession of a rotor will continue while the force is applied until the plane of rotation is in line with the plane of the applied force. The axis about which a torque is applied is termed the input axis, and the one about which precession takes place is termed the output axis. There are, however, many mechanical examples around us every day, and one of them, the bicycle, affords a very simple means of demonstration. If we lift the front wheel off the ground, spin it at high speed and then turn the handlebars, we feel rigidity resisting us and we feel precession trying to twist the handlebars out of our grasp. Gimbal Lock When the spin axis of the rotor becomes aligned with the axis of the outer gimbal, that is, the inner and outer gimbals are aligned, the gimbals become locked. This condition, due to precession, tries to force the inner gimbal to rotate at the same speed as that of the rotor. Gimbal lock is normally prevented by limiting the movement of the inner gimbal with mechanical stops. This physically prevents the inner gimbal and the outer gimbal from becoming aligned. If the inner gimbal reaches these stops, the forces acting on the gimbal system cause the outer gimbal to rotate 180 degrees, a process called “toppling.” The mechanical stops are usually placed at about 85°. Aviation Australia Mechanical stops 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 69 of 191 CASA Part Part 66 - Training Materials Only  Toppling Unavoidable precession is caused by the friction of gimbal bearings during aircraft manoeuvring. This cause is known as drifting and thus gives erroneous readings. When deective forces are too strong or are applied very rapidly, gyro rotors topple over, rather than merely precessing. This should be avoided because it may jump from the bench and destroy itself. At a minimum, it will damage bearings and render the instrument useless until the gyro is erected (gimbals restored perpendicular to each other) again. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 70 of 191 CASA Part Part 66 - Training Materials Only  Terminology Free or Space Gyro An unrestricted, unreferenced displacement gyro is called a space gyro. These gyros have complete freedom about three axes which are acting at right angles to each other (spin, tilt and veer). This enables the gyro to maintain its position relative to some point in space for an indenite time, assuming that there is no gimbal or rotor bearing imperfections or external forces such as magnetic elds or gravity. If you were to sit and watch a perfectly balanced and frictionless space gyro, it will appear to rotate or drift away from the perpendicular, but in reality, the rotor is remaining rigidly xed in space, and as the Earth rotates, the frame rotates around the rotor, appearing to the viewer on Earth as though the gyro is rotating. Earth Rate The effect above is termed Earth rate. At the equator, the gyro appears to precess at 15° per hour so that it will rotate 360 degrees every 24 hours (the same as the Earth’s rotation). Earth rate 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 71 of 191 CASA Part Part 66 - Training Materials Only  Transport Wander/Rate Assume now that the gyroscope is transported from one point on the planet to another, with its spin axis aligned with the local vertical component of gravity. It will have appeared to an observer on the Earth that the spin axis of the gyro scope has tilted – this is transport wander. If a gyro were to be transported from the North Pole to the equator, it will appear as though it has tilted 90°. In fact, you have moved and not the gyro. Transport wander Drift Earth rate and transport wander are termed apparent drift. The gyroscope appears to drift although the gyro is holding its orientation in space (rigidity) whilst the Earth is moving beneath it. Precession caused by physical factors such as bearing friction and gimbal imbalance is termed real drift. Earth Gyro Before a free gyroscope can be of practical use as an attitude reference in aircraft ight instruments and other associated navigational equipment, drift and transport wander must be controlled so that the gyroscope’s plane of spin is maintained relative to the Earth; in other words, it requires conversion to what is termed an Earth gyroscope. A space gyro referenced Earth is then termed an Earth gyro. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 72 of 191 CASA Part Part 66 - Training Materials Only  Tied Gyro A space gyro referenced to any other parameter is referred to as a tied gyro, so an Earth gyro (tied to centre of the Earth using gravity) is a form of tied gyro. Displacement Gyros The pitch, roll and directional attitudes of the aircraft are determined by its displacement with respect to the appropriate gyroscope. For this reason, therefore, the gyroscopes are referred to as two-axis displacement-type gyroscopes. The spin axis of the gyro is discounted since no useful attitude reference is provided when displacements take place about the spin axis. Rate Gyro The difference between a displacement gyro and that provided by a rate gyro is that a displacement gyro uses a gyro’s property of rigidity in space to measure displacement around it; a rate gyro relies on a gyro being subjected to precessive forces against spring pressure to determine rate of movement. The higher the rate of movement, the greater the inertial force applied to the gyro resulting in precession. © Aviation Australia Rate gyroscope 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 73 of 191 CASA Part Part 66 - Training Materials Only  Gyroscope Applications in Aircraft Introduction to Gyroscope Applications Aircraft gyroscopes establish two essential reference datums: A reference against which pitch and roll attitude changes may be detected A directional reference against which changes about the vertical axis may be detected. These references are established by gyroscopes having their spin axes arranged vertically and horizontally, respectively. Vertical and horizontal gyro 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 74 of 191 CASA Part Part 66 - Training Materials Only  Gyro indicators Both gyroscopes use the fundamental properties of rigidity to establish a stabilised reference unaffected by movement of the supporting body and precession to control the effects of apparent and real drift, thus maintaining stabilised reference datum. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 75 of 191 CASA Part Part 66 - Training Materials Only  Correction of Drift The following can control drift for horizontal-axis gyroscopes: Applying xed torques which unbalance the gyroscope and cause it to precess at a rate equal and opposite to the Earth rate Applying a torque that has a similar effect to that stated above but which can be varied according to the latitude Referencing the gyro to the centre of the Earth. A gyro corrected for Earth rate or apparent drift will maintain its attitude with reference to the Earth. Apparent drift Transport rate is corrected by referencing the gyro to the centre of the Earth, i.e., the use of a gravity- sensing device. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 76 of 191 CASA Part Part 66 - Training Materials Only  Transport wander Gyro Erection Systems Bearing friction, gimbal imbalance, the Earth’s rotation and movement of the aircraft over the Earth’s surface create forces that precess a gyro from its normal position. This precession is referred to as drift. A gyro erection system maintains the gyroscope rotor and gimbals aligned at right angles to each other. The systems adopted depend on the particular design of gyro: A directional gyro requires its rotor spin axis to be aligned horizontally so it can detect movement of the aircraft in azimuth. The function of the erection system is to maintain the inner gimbal at right angles to the outer gimbal. An attitude gyro requires its rotor spin axis to be aligned vertically so it can detect aircraft movement in pitch and roll relative to the Earth’s horizon. This requires both the inner and outer gimbals to be aligned with gravity. The erection system uses the property of precession of the gimbal system to maintain the rotor spin axis at the desired attitude. The forces are minimal, with normal erection rates between 1 and 5 degrees a minute. The forces are created by air jets, electrical torque motors or deliberate changes to gimbal balance. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 77 of 191 CASA Part Part 66 - Training Materials Only  Caging Older designs of attitude gyros did not have 360 degrees of freedom in roll and had manually operated caging devices to hold the gimbals in place during rotor start and before take-off. Directional gyros need to be reset to the magnetic heading at frequent intervals during ight, and this is done using a caging knob. Attitude and directional gyros should be caged during aerobatic manoeuvres to avoid damage to the instrument. The caging knob resets or erects a gyro by restoring gimbal alignment. Many attitude gyro instruments manufactured today have complete freedom in the roll axis. Pitch attitude is limited to 85 degrees nose up or nose down from level ight. These instruments do not tumble out of control when the gyro limits are exceeded and have high-speed, self-erecting mechanisms that eliminate the need for manual caging. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 78 of 191 CASA Part Part 66 - Training Materials Only  Aircraft Vacuum Gyro System Introduction to the Vacuum Gyro System Air-driven gyro rotors can run on a very low positive pressure or a very low vacuum. A vacuum can be supplied from a venturi mounted in the airstream. Air pressure or vacuum can be provided by an engine-driven pump. In air-driven gyro systems, the lters are very important items, as any contamination entering the gyro will dramatically shorten its serviceable life. Pressurised air is ported over cups in the gyro rotor, or vacuum air is passed across the cups. This causes the gyro rotor to spin up to speed and is also used for the gyro erection system to reference the gyro to the Earth by eliminating apparent drift and transport rate. Gyro vacuum system At high altitudes, a vacuum-driven gyroscopic instrument suffers from the effects of a decrease in vacuum due to the lower atmospheric pressure, and the resulting reduction in rotor speeds affects gyroscopic stability. Other disadvantages of vacuum operation are weight due to pipelines and special arrangements to control the vacuum in pressurised aircraft. Since air must pass through bearings, the possibility of contamination by corrosion and dirt particles also arises. Early aircraft used a venturi system to provide the level of vacuum required to drive the gyros. A problem with this system is that you do not get suction until the aircraft achieves a reasonable airspeed. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 79 of 191 CASA Part Part 66 - Training Materials Only  The venturis came in two sizes generally: a 2-inch and a 4-inch version. The size referred to the amount of vacuum that was produced: 2 inches or 4 inches of mercury. Vacuum system using a two-inch venturi 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 80 of 191 CASA Part Part 66 - Training Materials Only  Air Flow and Erection in Gyro In the rear end cover of the instrument case, a connection is provided for the coupling of the vacuum supply. With the vacuum system in operation, air is evacuated from the case and is replaced by the surrounding atmosphere, entering via the ltered inlet and passing through channels to the jet on the rotor housing. The air issuing from the jets impinges on the rotor buckets, thus imparting even driving forces to spin the rotor at approximately 15 000 rpm. Air-driven gyroscope After spinning the rotor, the air passes over a knife edge mounted on the outer gimbal. If the inner gimbal is not maintained perpendicular to the outer gimbal, the airstream will impinge more on one side of the knife edge and less on the other side. This generates a torque between the two gimbals. As the inner gimbal is free to move on its bearings, it will precess until the airstream impinges equally on each side of the knife edge. If the gyro is supplied with air pressure, the connections on the case are reversed. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 81 of 191 CASA Part Part 66 - Training Materials Only  Flapper door erection system If the gyro tilts to the left, then door “A” opens and allows air to escape through orice “B.” This provides a precessing force to re-erect the gyro to the local vertical. When the gyro is again vertical, all apper doors are closed and no more air escapes. Disadvantages of Air-Driven Gyro Systems Dirt and dust are a major problem with air-driven instruments, and therefore, instrument lters and system lters must be checked, cleaned or changed at regular intervals. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 82 of 191 CASA Part Part 66 - Training Materials Only  Electrically-Driven Gyro Introduction to Electrically-Driven Gyro To overcome the disadvantages of the air-driven gyroscopic instruments in high-performance aircraft, gyroscopic instruments are designed for operation on electrical power derived from the aircraft power supplies. High-speed rotors (21 000 rpm) of displacement gyros require 115-V, 400- Hz, three-phase AC, supplied from the aircraft alternators or inverters. Lower speed rate gyros can be operated using 14/28-V DC. The alternating current application has been used for the later types of turn and bank. Electrical gyros only need a small amount of power from the existing aircraft power supply. AC electrically powered gyros can run much faster than air-driven gyros, so they provide a more rigid gyroscopic reference. Electrically driven gyros incorporate more solid-state components and therefore require less maintenance effort compared to pneumatically driven gyros. Electrical Gyro Construction This system is made up of the same basic elements as the vacuum-driven type, with the exception that the electrical gyroscope has a xed three phase stator and a squirrel cage induction motor for the rotor. 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. By designing the rotor and its bearings so that it rotates on the outside of the stator, then for the same required size of motor, the mass of the rotor is concentrated further from the centre so that the radius of gyration and inertia are increased. DC electric gyro construction 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 83 of 191 CASA Part Part 66 - Training Materials Only  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 bearing pivot in the front cover glass and in the rear casting. AC electric gyros may use 115-V, 400-Hz, three-phase supply which is fed to the gyro stator via slip rings and brushes or by nger contact assemblies. When power is switched on, a rotating magnetic eld is set up in the gyro stator which cuts the bars forming the squirrel-cage in the rotor and induces a current in them. The effect of this current is to produce magnetic elds around the bars which interact with the stator’s rotating eld, causing the rotor to turn at a speed of approximately 20 000–23 000 rpm. Failure of the power supply is indicated by a ag marked “off” and actuated by a solenoid. Electric power is also supplied to torque motors and switching circuits to maintain the erection of the gyro. DC Gyro Systems Advantages and Disadvantages Direct current has the following advantages: Rugged, light-weight instrument Simple operation Low cost. Simple damping system direct current has the following disadvantages: A maximum rotor speed of approximately 4200 rpm, which is tolerable in a turn-and-bank indicator but does not give sufcient inertia (rigidity) for an articial horizon or directional gyro Commutator and brush wear with the associated arcing and sparking Noise (interference) Higher current. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 84 of 191 CASA Part Part 66 - Training Materials Only  AC Gyro Systems Advantages and Disadvantages Alternating current has the following advantages: Higher speed, giving greater inertia and rigidity Lower current consumption Less noise Greater accuracy. Alternating current has the following disadvantages: More complex construction and operation Reliance on static inverter for AC supply if no aircraft supply available Higher cost. Gyro Limitations A particular limitation of all gyros is that the gyro should never be removed from the aircraft until at least 30 minutes has passed from the time the power source was disconnected. This is to allow the rotor to cease spinning, as the inertia contained within the rotor and the relative absence of friction within the bearings may allow the rotor to spin for this length of time. Movement during rundown can cause uncontrolled toppling, damaging gimbals and bearings. Electrically driven gyros may incorporate a form of electrical or dynamic braking which will slow the gyro rotor very quickly once power is removed. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 85 of 191 CASA Part Part 66 - Training Materials Only  Articial Horizon Introduction to Articial Horizon An articial horizon (AH), also known as a gyro horizon, provides pilots with an indication of an aircraft’s pitch and roll attitude. In order to measure a movement, you need a reference, and in this instance, the gyro becomes the reference or stable point. The amount of movement or deection made by the aircraft around this stable point is measured and displayed on the cockpit instruments (displacement gyroscope). The gyro spin axis is maintained in a vertical position or horizontal plane of rotation relative to the Earth. This permits rigidity in lateral and longitudinal axes, and the displacement of the gimbals from the stable reference is what provides the roll and pitch readout. The gyro is an Earth or tied gyro referenced to the Earth’s gravity to maintain the vertical spin axis should imperfections or errors cause the gyro to drift. The erection system will re-align the gyro with respect to gravity. Fixed sky plate articial horizon The xed sky plate articial horizon, of an older design than the air-driven articial horizon, demonstrates the way the gyro horizon provides a display of pitch and roll. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 86 of 191 CASA Part Part 66 - Training Materials Only  The sky plate is xed to the outer roll gimbal and is painted totally black. A roll angle pointer is attached to this plate and reads against a xed scale. The inner gimbal has a pendulous vane assembly attached below the rotor housing which incorporates two pairs of pendulous vanes. With the roll and pitch gimbal aligned to the vertical, each vane partially covers its respective slot. If the inner or outer gimbal is not at vertical, one vane of a pair covers the slot, while the other vane opens the slot. Air used to rotate the gyro rotor escapes from the pendulous vane assembly through the opened slots. The jets of air coming from the vents generate a torque (force) that precesses the gyro to maintain a vertical with respect to gravity. In some of the older air-driven gyros, the outer gimbal ring may not have complete freedom through 360° about the roll axis. A mechanical stop limits the inner gimbal pitch movement to ±85° (which stops gimbal lock) and is tted on the top of the rotor casing. A pin attached to the inner gimbal protrudes through a slot in the outer gimbal and actuates a counterbalanced arm, to which is xed the horizon bar. The horizon bar is referenced to a miniature aircraft xed centrally inside the glass bezel. In level ight, the horizon bar is aligned centrally and laterally with the miniature aircraft and provides the pilot with a ‘trailing view’ of the aircraft and attitude with respect to the horizon. When the aircraft is climbing, the horizon bar moves towards the bottom of the plate and the miniature airplane appears to be above the horizon, which depicts a nose-up attitude. This older design has a pull-to-cage knob to re-align the gyro in straight and level ight if it is noted to be drifting off or if it tumbles or suffers gimbal lock. Articial Horizon Stabilised Sky Plate Articial horizon stabilised sky plate 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 87 of 191 CASA Part Part 66 - Training Materials Only  The more conventional design uses a stabilised spherical element that displays pitch and roll attitude against the miniature aircraft symbol. The upper half of the element is coloured blue to display climb attitudes, while the lower half is black or brown to display descending attitudes. The white dividing line between the two halves is engraved with a circle at the centre of the line and represents the true horizon. Each half is graduated in ten degree increments up to 80° climb and 60° descent. Bank angle is indicated by a pointer and scale in the normal manner. As in the older design, the miniature aircraft symbol remains xed, and the horizon moves behind it. Articial horizon operation The outer gimbal bearing support is designed to provide unrestricted roll movement. Pitch is still restricted to 85° to avoid gimbal lock. If a loop were performed, the indicator would show a climb up to 85° (when the aircraft nose is almost vertical, not when it is at the top of the loop) and the entire gyro assembly would roll 180°. As the aircraft pulls up past the vertical and is at the top of the loop, the horizon pointer will indicate straight and level inverted ight corresponding with the aircraft being upside down at the top of the loop. As the aircraft comes down to complete the loop, the horizon bar again shows the aircraft heading for the ground until it is pointing almost straight at the Earth (85° nose down) when it will again spin 180°. This means the aircraft symbol will continue pointing at the Earth (indicating a dive). As the aircraft recovers to straight and level ight again at the bottom of the loop, the whole assembly will be back in its original attitude with the horizon bar again showing straight and level ight. This has the same visual effect as the ball rotating fully through 90°, but because this will induce gimbal lock, the entire assembly must be rotated to then wind back down the other side of the sphere. This is not a concern in the roll axis, as there is full 360° movement. The display can therefore indicate unrestricted full barrel rolls. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 88 of 191 CASA Part Part 66 - Training Materials Only  Attitude Director Introduction to Attitude Director Many aircraft currently in service employ integrated ight systems that can display pitch and roll attitude data from remotely mounted vertical gyroscopes. The display, in these cases, uses two servomechanism circuits to drive the spherical element in pitch and roll. The need for integrating the functions and indications of certain ight and navigation instruments resulted from the increasing number of navigation functions developed to meet the demands of safe enroute navigation and to cope with increasing trafc congestion in the air space around the world’s major airports. This required more instruments, which required more panel space. The method of easing the problem was to combine related instruments in the same case and to compound their indications so that a large proportion of intermediate mental processing on the part of the pilot could be bypassed and the indications more easily assimilated. The compound instrument became known as an Attitude Director Indicator (ADI) or an Attitude Reference Indicator (ARI). They all have slightly different displays, but they all operate in the same way. The function of the ADI is to supply the pilot with the aircraft’s attitude and steering information. This represents a view from behind the aircraft looking forward. Steering command and aircraft attitude are displayed around a xed aircraft symbol. Attitude director 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 89 of 191 CASA Part Part 66 - Training Materials Only  Attitude Sphere The sphere is free to move 360° in roll and, depending on type, 90° or 360° in pitch. Gimbal lock limitation minimised or eliminated. Roll Attitude Pointer This displays the bank angle of the aircraft and is read against the roll scale on the case of the instrument. Command Bars There are two command bars: one for pitch and one for roll. They are called command bars because they command the pilot to y the aircraft symbol towards the command bars. The commands are supplied from the ight director computer, which can receive reference signals from a range of navigation aid receivers, including GPS and INS. Glideslope Pointer This is located on the left side of the FDI and is used during the landing phase when the aircraft has captured the runway glideslope beam. The aircraft’s vertical position within the glideslope beam is shown by the pointer. When the pointer is on the centre line, the aircraft is in the centre of the glideslope. When the pointer is above the centre line, the aircraft is below the glideslope. Localiser Deviation Indicator The localiser pointer shows the aircraft’s lateral position in relation to the localiser beam. When the pointer is in the centre of the scale, the aircraft is positioned in the centre of the beam. If the pointer moves to the left of the centre line, the aircraft is to the right of the beam. Speed Command Pointer A speed command pointer on the left of the attitude display will show the difference between the actual and desired aircraft speed. Additional information, depending on manufacturer and features required, such as radio altitude, decision height and an inclinometer can be incorporated. Electronic ight instrument systems can include altitude, airspeed, vertical airspeed and trafc collision and avoidance systems. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 90 of 191 CASA Part Part 66 - Training Materials Only  Flight Director Components The methods adopted for the integration of such information and the manner in which it is presented varies between systems. A complete system normally consists of the following: Flight Director (FDI) or Attitude Director Indicator (ADI) Course Deviation Indicator (CDI) or a Horizontal Situation Indicator (HSI) Mode select panel Flight director computer. A ight director indicator houses a pitch and roll synchro, coupled to servomechanisms, to drive the pitch and roll displays. The difference between a gimbal synchro and an indicator synchro is an error signal which is amplied and routed to the respective servomotor, which rotates to position the pitch bar or horizon disc or sphere to indicate the changing attitude of the aircraft. At the same time, the servomotors drive the indicator synchro rotor to the null position, which directly matches the null position of the synchro on the gimbal. Attitude director operation The right-hand attitude directors diagram shows the interconnection of the glideslope and localiser pointer with the ILS receiver. During an ILS approach, the receiver on board the aircraft detects the signals beamed from ground transmitters in vertical and horizontal planes. If the aircraft is above the glide path, signals are fed to the meter controlling the glide slope pointer, causing it to be deected downwards against the scale, thus directing the pilot to bring the aircraft down on to the glide path. During approach, if the aircraft is to the left of the localiser beam and runway centreline, the localiser pointer is deected to the right, directing that the aircraft be banked to the right. When the direction has been satised, the pointer is positioned vertically through the centre position of the horizon disc. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 91 of 191 CASA Part Part 66 - Training Materials Only  Navigation Mode Select Panel The command bars present information dependent on the pilot’s selection on the mode selector panel. The pitch command knob allows the pitch command bar to be adjusted to the aircraft’s level ight attitude. If the mode selector is off, the command bars are removed from view. Fight director model select panel Conventional meter movements position the glideslope, localiser, ight director steering command pointer and valid ags. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 92 of 191 CASA Part Part 66 - Training Materials Only  Flight Director Computer The ight director computer processes the following: Pitch and roll information from the attitude source Compass heading, desired course, course deviation, or desired heading information from the HIS Navigation information from the selected navigation receiver Information from the central air data computer Information from the pilot’s ight director mode select panel. Flight director schematic The output from the ight director computer drives two command or steering pointers on the ADI. The pilot interprets the position of these command pointers and adjusts the attitude of the aircraft so that the command pointers remain aligned with the miniature aircraft on the ADI display. By doing so, the aircraft will y the predetermined enroute course or runway approach path. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 93 of 191 CASA Part Part 66 - Training Materials Only  Flight director indicator display 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 94 of 191 CASA Part Part 66 - Training Materials Only  Standby Attitude Indicator When an aircraft uses remotely mounted vertical gyroscopes, there is an airworthiness requirement to be met in case the circuits controlling the display of aircraft attitude fail. An independent gyro horizon still nds a place on the instrument panel but in the role of a secondary or standby attitude indicator. The standby articial horizon is electrically operated and supplied with 115-V, three-phase AC from a static inverter, which in turn is powered by 28-V DC from the battery. Indicators have a pitch trim adjustment and a fast-erection facility, both being controlled by a knob in the corner of the indicator bezel. When the knob is rotated, the aircraft symbol may be positioned vertically through ± 0.5 inch, thereby establishing a variable pitch trim reference. Gently pulling the knob out and holding it cages the instrument or energises a fast-erection circuit. As with any facility of this nature, time limitations are imposed on its operation, so to prolong the service life, the circuit breaker for the inverter is pulled when the aircraft is not in service. Standby attitude gyro 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 95 of 191 CASA Part Part 66 - Training Materials Only  Direction Indication Introduction to Direction Indication A basic form of a direction indicator is the Directional Gyroscope (DG). This instrument gives the pilot directional information using a gyro rotor with a horizontal spin axis (vertical plane of rotation) mounted in two gimbals. Directional gyro construction DGs are a much better ight reference when turning onto a magnetic heading. This is because the heading change is displayed dynamically, whereas the magnetic compass will only indicate the new heading after it has realigned with the Earth’s magnetic eld. The gyroscope, when used for heading reference, has several advantages over a magnetic compass: Not subject to the turn or acceleration errors Always dead beat in its indication, which means that the indication moves to the new position without over swing or under swing oscillations In the higher latitudes, its indications are more reliable than the compass because the compass is under the inuence of a greater vertical component of the Earth’s magnetic eld, which makes it try to tilt. The directional gyroscope does not take the place of a standby compass but rather is of assistance to it. During ight, when the aircraft is straight and level, the instrument is regularly caged and reset to the compass heading to ensure that the correct alignment is maintained. An air driven DG spins at approximately 12 000 to 15 000 rpm. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 96 of 191 CASA Part Part 66 - Training Materials Only  When uncaged, the outer gimbal has unrestricted 360° of movement, while the inner gimbal is limited to 85 degrees either side of the perpendicular. The inner gimbal is maintained erect to the outer gimbal by utilising the torque generated by the rotor exhaust air acting across a wedge assembly mounted on the outer gimbal. The compass card provides the pilot with an indication of aircraft heading. Heading Indicator Later designs of the DG use a bevel gear arrangement to produce a vertical display. This type of indicator is called a heading indicator, or more appropriately, a horizontal situation indicator, as the pilot views the heading of the aircraft looking from above, which provides a more intuitive presentation. The lubber line is at the top of the dial, and the aircraft nose points straight up, representing straight ahead. As the aircraft performs a turn, the card rotates. © Jeppesen Vacuum direction gyro A caging button on the front of the instrument allows the two gimbals to be locked together and turned so that the needle can be aligned with the compass heading. The gyro wheel may be air driven or electrically driven. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 97 of 191 CASA Part Part 66 - Training Materials Only  Directional Gyro and Horizontal Indicator Comparisons The readout on a heading indicator is opposite to that of the DG readout. The DG readout was effectively the opposite way a pilot would normally visualise a change in heading. On the DG window readout, the 330° is displayed to the right of north, whereas if you wanted to turn onto 330° (which is more west of a northerly heading), you would turn to the left, not to the right. The heading indicator removed this confusion from the display. Using similar headings which are visible on this indicator, you will notice 330° is to the left of north. Heading indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 98 of 191 CASA Part Part 66 - Training Materials Only  Remote Reading Compass System In the same manner that the vertical axis gyro became remotely mounted from the attitude indicator, so too has the horizontal spin axis directional gyro become remotely mounted from the Horizontal Situation Indicator (HSI). The directional gyro azimuth position is relayed through synchros and an amplier to power a servomechanism that drives the compass card. A magnetic detecting element, called a ux valve, automatically maintains the alignment of the gyro to the Earth’s magnetic eld by applying signals to a torque motor that precesses the outer gimbal. This removes the need for the pilot to update the card heading to agree with magnetic heading and also counters real and apparent drift. Pallett Remote reading compass schematic By incorporating the ux valve, the gyro stabilised heading became known as a remote indicating compass system or a magnetic heading reference system. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 99 of 191 CASA Part Part 66 - Training Materials Only  Horizontal Situation Indicator Introduction to Horizontal Situation Indicator Horizontal situation indicator (HSI) The Horizontal Situation Indicator (HSI), with the gyro stabilised magnetic compass card as its primary indicator, also has deviation indicators incorporated in the same manner as the attitude director indicator. The HSI is called an integrated instrument because it combines several different types of displays which would normally be found in separate instruments. The HSI may receive inputs from various sources, including radio navigation systems, air data systems and heading reference systems. The pilot is able to select the data to be displayed on the instruments. The aircraft is pictorially represented by a small xed symbol at the centre of the instrument. The lubber line at the top of the instrument represents the nose of the aircraft, and the azimuth card displays the magnetic heading of the aircraft. The aircraft heading information may be sourced from different systems, including the inertial navigation system, in which case true heading is displayed. A ag with “mag” or “true” displays behind the lubber line. The course pointer, driven by the course set knob, indicates the selected bearing to the station or waypoint. The course deviation bar indicates the offset distance from the selected track. A heading bug, positioned by the heading knob, displays the desired heading. The interconnection of the various elements comprising an indicator is shown on the horizontal situation indicator schematic. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 100 of 191 CASA Part Part 66 - Training Materials Only  Aviation Australia Horizontal situation indicator schematic Electronic Horizontal Situation Indicator The modern EHSI is modelled after the older electromechanical version and displays much of the same information. In addition to the standard compass rose, associated pointers and indications, an EHSI may also offer a moving map display with superimposed weather radar information. Electronic ight instrument system display 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 101 of 191 CASA Part Part 66 - Training Materials Only  Turn-and-Slip Indicator Introduction to Turn-and-Slip Indicator The turn indicator component detects rate of turn by the use of a rate gyro. The slip indicator consists of a ball in a curved liquid-lled tube and provides the pilot with an indication of aircraft slip or skid. Together, they enabled the pilot to correctly coordinate aircraft turns. With the introduction of advanced ight instruments and systems, the turn component of the indicator has become redundant in all but light aircraft. The slip indicator is usually incorporated into the attitude director indicator. Aviation Australia Turn-and-slip indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 102 of 191 CASA Part Part 66 - Training Materials Only  Rate Gyro A rate gyro has only one gimbal ring, which is restrained in its neutral position by a spring or springs. The gyro rotor is mounted in the gimbal with its axis of spin parallel to the lateral axis of the aircraft (YY in the drawing below). Two bearings, supporting the gimbal ring, allow rotation about the aircraft’s longitudinal axis. A pointer is attached to the gimbal and aligns with a zero rate turn mark on the dial. The pointer is able to deect left or right of this zero mark. © Aviation Australia Rate gyro operation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 103 of 191 CASA Part Part 66 - Training Materials Only  Turn-and-Slip Indicator Principle of Operation With the rotor spinning, its property of rigidity will maintain the gimbal horizontally, and the turn pointer will indicate zero. When the aircraft yaws (turns about the XX axis as shown in the diagram), because there is no gimbal to permit veer, the turning motion of the aircraft has the same effect as if someone applied a precessive force (F) to the front and rear of the gyro rotor. This force is felt at 90° in the direction of rotation, precessing the gyro and tilting it over (P). If the gyro was not restrained by springs, it would continue to precess in the tilt axis while the yawing motion was present. Turn indicator The precession is countered by the spring, and the amount of deection of the gimbal is directly related to the precessive force and the spring tension. The precessive force is a function of gyro rigidity and turn rate, and provided the gyro rigidity is known, the pointer will move to a position where the dial will indicate the rate of turn. Rotor speed directly impacts gyro rigidity and must be controlled to a ne tolerance for accurate readings to be produced. A high rotor speed causes greater rigidity and a greater precessive force, to counter spring tension causing the indicator to over read. A rotor speed below specication results in an under-reading rate of turn. If the rate of turn is increased, the precession force increases, tilting the gimbal further against spring pressure. When the turning motion ends, the precession force is removed, so the gyro will return to the original attitude. The direction of turn determines in which direction the pointer will move, and this gives the pilot not only the rate of turn but also the direction. The position of the restraining spring allows the indicator to display a turn to the left or right. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 104 of 191 CASA Part Part 66 - Training Materials Only  Aviation Australia Rate gyro restraint springs Air-Driven Turn-and-Slip Indicator The air-driven turn-and-slip indicator rotor speed is governed by a regulator in the supply line. The air supply is directed onto the rotor cups by a nozzle mounted on the instrument case. Aviation Australia Air-driven turn-and-slip indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 105 of 191 CASA Part Part 66 - Training Materials Only  Electrically-Powered Turn-and-Slip Indicator Direct current is fed to the commutator brushes via a radio interference suppressor and exible springs which permit movement of the inner gimbal. The rotor speed is controlled by two identical symmetrically opposed centrifugal cut-outs. Angular movement of the gimbal ring is transmitted to the pointer through a gear train, and damping is accomplished by an eddy-current drag system mounted at the rear of the gyro assembly. The system consists of a drag cup, which is rotated by the gimbal ring between a eld magnet and a eld ring. © Aviation Australia Electrically-powered turn-and-slip indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 106 of 191 CASA Part Part 66 - Training Materials Only  Interpretation of Rate-of-Turn Indication On indicators with one central mark, deection of the pointer by one pointer width indicates it will take two minutes to turn through 360°, which is known as a rate 1 turn. Indicators with two marks either side of the centre mark indicate a four-minute turn when the pointer is deected one pointer width, that is, the pointer aligns with the space between the marks or a turning rate of a rate ½. When the pointer aligns with a doghouse, it indicates a rate 1 turn. Rate turn presentation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 107 of 191 CASA Part Part 66 - Training Materials Only  Inclinometer or Slip-and-Skid Indicator The indicator is a simple mechanical instrument that consists of a ball in a liquid lled glass tube. This tube is curved and the ball reacts to gravity and centrifugal force. It is used by the pilot to co-ordinate turns by use of aileron and rudder control. If the pilot keeps the ball centred, the aircraft is being own in a coordinated manner. Coordinated turn When the aircraft is turning too fast for the bank angle, it will be skidding outwards on the turn just like a speeding car. The centrifugal or inertial forces will cause the ball to move to the outside of the index marks, corresponding to the direction the aircraft is skidding. Skidding turn 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 108 of 191 CASA Part Part 66 - Training Materials Only  If the angle of bank is too high, the ball will drop below the index marks due to the force of gravity, again corresponding to the direction in which the aircraft is slipping or dropping. Slipping turn 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 109 of 191 CASA Part Part 66 - Training Materials Only  Turn Coordinator Introduction to Turn Coordinator A turn coordinator is a development of the turn-and-bank indicator and is adopted in lieu of such instruments in a number of small types of general aviation aircraft. A turn coordinator still indicates rate of turn but indicates a turn more quickly, as it responds as soon as the aircraft commences to bank. The turn presentation is different in that it uses the rear view of an aircraft as an indicator symbol, and when the wings are aligned with the horizontal index marks, the aircraft wings are level. When the wings are aligned to the turn indicator marks, it indicates a rate 1 turn (a two-minute turn). The primary difference, other than the display presentation, is in the setting of the precession axis of the rate gyroscope. The gyroscope is spring restrained and is mounted so that the axis is at about 30° with respect to the aircraft’s longitudinal axis (forward or aft tilt), thus making the gyroscope sensitive to banking of the aircraft as well as to turning. © Jeppesen Turn coordinator Since a turn is normally initiated by banking an aircraft, then the gyroscope will precess and thereby move the aircraft symbol to indicate the direction of the bank and enable the pilot to anticipate the resulting turn. The pilot then controls the turn to the required rate as indicated by the alignment of the aircraft symbol with the graduations on the outer scale. Co-ordination of the turn is indicated by the ball-type indicating element remaining centred in the normal way. The annotation ‘no pitch information’ on the indicator scale is given to avoid confusion in pitch control which might result from the similarity of the presentation to a gyro horizon. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 110 of 191 CASA Part Part 66 - Training Materials Only  Turn Coordinator Display Compare the turn coordinator display with the AH display. Using the turn coordinator with the 30° canted gimbal, it will indicate a turn in unison with the AH, whereas the turn indicator would not begin to indicate a turn until aircraft heading begins to change, that is, the aircraft starts yawing. The two instruments look to have opposite displays but, in fact, are indicating the same thing. A notice written on the bottom of the turn coordinator, “no pitch information,” reminds pilots that this instrument is supplying turn information only. © Aviation Australia Turn coordinator display 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 111 of 191 CASA Part Part 66 - Training Materials Only  Gyro Handling Introduction to Gyro Handling Gyroscopes are extremely delicate items, and the smallest amount of mishandling can cause damage which will make them either inaccurate or unserviceable. Gyros must always be handled in a delicate manner. A good motto for the safe handling of gyros is: “Handle like rotten eggs” (break one and there’ll be a big stink). A gyro should never be removed while it is spinning or running down. With no power source to maintain erection and the relative absence of friction within the bearings, the gyro will topple easily, allowing the gimbal to spin as fast as the rotor, causing damage to the bearings or gimbal assemblies. Some older systems could not to be moved for up to 30 minutes after vacuum source or electric power was disconnected. Some newer aircraft gyros have dynamic braking and can run down in a very short space of time, for example, within 1 minute. Dirt and dust are a major problem with air-driven instruments, and therefore, instrument lters and system lters must be checked, cleaned or changed at regular intervals. When caging a gyro, for example, setting a DG to align with a magnetic reference or setting an articial horizon as part of a pre-ight check, the gyro is caged, or the gimbals are made to align at 90° to each other. This manual re-alignment must be carried out carefully and with a single steady action. If functional tests of a gyro are required, never move an operating instrument in a violent or jerky fashion. A high gyroscope failure rate is directly related to rough or improper handling. Gyros are delicate and cannot withstand the shock of being dropped, jarred or struck by pieces of equipment. Do not place gyros on any hard surface. If you have to pack a gyroscope to ship it to a repairer, always make sure there is at least 100 mm of foam rubber between the gyroscope and the shipping container’s outer shell. To prevent damage to a gyro, the instrument should be transported to and from the aircraft in its original shipping container. If this is impractical, the gyro should be hand carried carefully in an upright position. All gyroscopes, whether serviceable or not, are to be handled as though they are serviceable. Using proper handling procedures during removal prevents additional damage. Most malfunctioning instruments can be repaired and returned to service. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 112 of 191 CASA Part Part 66 - Training Materials Only