B2-13h Instruments Gyroscopic SR

Student Resource

Subject B2-13h Instruments Gyroscopic

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AA Form TO-19 B2-13h Instruments Gyroscopic

Part 66 Subject

CONTENTS Topic Definitions

ii

Study resources

iii

Introduction

v

Gyroscopes

13.8.2.1

Artificial Horizons

13.8.2.2.1

Slip Indicators

13.8.2.2.2

Directional Gyros

13.8.2.2.3

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DEFINITIONS Define 

To describe the nature or basic qualities of.



To state the precise meaning of (a word or sense of a word).

State 

Specify in words or writing.



To set forth in words; declare.

Identify 

To establish the identity of.

List 

Itemise.

Describe 

Represent in words enabling hearer or reader to form an idea of an object or process.



To tell the facts, details, or particulars of something verbally or in writing.

Explain 

Make known in detail.



Offer reason for cause and effect.


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STUDY RESOURCES E.H.J. Pallett, Aircraft Instruments & Integrated Systems, Chapter 4 Jeppesen Aircraft Instruments and Avionics pp 29 – 41 Avionics Fundamentals, IAP Inc. Chapter 5 B2-13h Student Handout


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INTRODUCTION The purpose of this subject is to allow you to gain knowledge of Aircraft Systems, Instruments – Gyroscopic. On completion of the following topics you will be able to: Topic 13.8.2.1 Gyroscopic Terminology and Characteristics Define Gyroscopic related terms. Explain the following: 

Earth rate and calculate it for various positions on the earth in respect to apparent precession



Rigidity and list the factors which affect it



2 and 3 gimballed gyroscope layouts



Gimbal lock condition; 

methods of avoiding and



rectifying a gyro in this condition



Real drift and apparent drift and list the factors which affect them



Free, tied, earth and rate gyros.

Describe gyroscopic precession and determine the direction of precession resulting from an applied force. Identify the following gyroscopic instrument systems, state their purpose and explain their operation: 

Artificial Horizons



Slip Indicators



Directional Gyros

Describe precautions involved with gyroscopic instruments/components. 13.8.2.2.1

Gyroscopic Instrument Systems: Artificial Horizons

Identify the following gyroscopic instrument system, state their purpose and explain their operation: 

Artificial Horizons;

13.8.2.2.2

Gyroscopic Instrument Systems: Slip Indicators


Slip Indicators (Turn & Bank Indicators) and

13.8.2.2.3

Gyroscopic Instrument Systems: Directional Gyros


Directional Gyros.


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TOPIC 13.8.2.1: GYROSCOPES Newton’s 1st Law of Motion Inertia An object in motion will remain in motion and an object at rest will remain at rest unless acted on by an unbalanced force. This means that if there were no friction, eg space, you could throw/push something and it will continue at that same speed forevermore. In reality in the atmosphere of the earth we have plenty of friction from air and gravity which provides the additional force to oppose the initial motion imparted by you. But the concept is that a moving mass will continue to move in the same direction unless some other force acts upon it.

When a rotor is made to spin at high speed the device becomes a gyroscope possessing two important fundamental properties: • Gyroscopic Rigidity or Gyroscopic Inertia: caused by the inertia of the mass, keeping the axis rigid or pointing in the same direction. • Gyroscopic Precession: describes the application of a force to the gyro and the effect of the angular displacement. Both these properties depend on the principle of conservation of angular momentum, which means that the angular momentum of a body about a given point remains constant unless some force is applied to change it. Angular momentum is the product of the moment of inertia (I) and angular velocity (w) of a body referred to a given point—the centre of gravity in the case of a gyroscope. These rather intriguing properties can be exhibited by any system in which a rotating mass is involved. Although it was left for man to develop gyroscopes and associated devices, it is true to say that gyroscopic properties are as old as the earth itself: it too rotates at high speed and

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So possesses rigidity, and although it has no gimbal system or frame on which external forces can act, it can, and does, precess. 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. The flywheel of a motor-car engine is another example. Its spin axis is in the direction of motion of the car, but when turning a corner its rigidity resists the turning forces set up, and as this resistance always results in precession, there is a tendency for the front of the car to move up or down depending on the direction of the turn. Other familiar examples are aircraft propellers, compressor and turbine assemblies of jet engines; gyroscopic properties are exhibited by all of them.

Elements of the Gyroscope The rotor is a perfectly balanced mass, mounted on a central shaft.

Gyro Rotor Construction The gyro wheel or rotor unit must be perfectly symmetrical and circular about the spin axis. Any other shape would cause an imbalance during rotation. To gain higher momentum and therefore stability, the weight is normally concentrated on the rim. Too much weight causes excessive bearing friction and consequently drift, so a compromise must be made between momentum and friction. Because inertia depends upon the square of the radius, the rotors are made as large as possible with the greatest mass concentrated at the rim.

Gyroscopic Balance The gyroscope must be perfectly balanced to reduce the vibration felt during the high speeds at which they are rotated. Therefore they are both statically and dynamically balanced.


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Static Balance: to be statically balanced, the centre of gravity must be acting upon the spin axis. Dynamic Balance: to be dynamically balanced, the plane of spin must be acting at right angles to the axis of spin. Construction of the rotor will directly affect the rigidity of the gyro. The heavier the rotor is and the closer to the outside rim that the weight can be distributed will contribute to the gyro’s rigidity. We add a frame with bearings and we have created the first axis of spin. This frame will soon become our inner gimbal but unto it is pivoted itself we only have a single axis of spin.

Gyroscopic Rigidity or Gyroscopic Inertia: caused by the inertia of the mass, keeping the axis rigid or pointing in the same direction. Gyroscopic Precession: describes the application of a force to the gyro and the effect of the angular displacement. Both these properties depend on the principle of conservation of angular momentum, which means that the angular momentum of a body about a given point remains constant unless some force is applied to change it. Angular momentum is the product of the moment of inertia (I) and angular velocity (w) of a body referred to a given point—the centre of gravity in the case of a gyroscope. These rather intriguing properties can be exhibited by any system in which a rotating mass is involved. Although it was left for man to develop gyroscopes and associated devices, it is true to say that gyroscopic properties are as old as the earth itself: It too rotates at high speed and so possesses rigidity, and although it has no gimbal system or frame on which external forces can act, it can, and does, precess. 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. The flywheel of a motor-car engine is another example. Its spin axis is in the direction of motion of the car, but when turning a corner its rigidity resists the turning forces set up, and as this resistance always results in precession, there is a tendency for the front of the car to move up or down depending on the direction of the turn. Other familiar examples are aircraft propellers, compressor and turbine assemblies of jet engines; gyroscopic properties are exhibited by all of them.


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

Spinning freedom about an axis perpendicular through its centre (axis of spin XX1) {this means the line from X to X1}

•

Tilting freedom about a horizontal axis at right angles to the spin axis (axis of tilt YY1)

•

Veering freedom about a vertical axis perpendicular to both the spin and tilt axes (axis of veer ZZ1).

Axes of Freedom Engineers have used many and various ways of describing the mounting and axis references of the gyroscope. A three frame gyro was said to have three degrees of freedom which were namely: •

spinning freedom, which enabled a gyroscopes rotor to spin.

•

tilting freedom, where the gyro case or inner gimbal was free to rotate about the horizontal plane, at right angles to the spin axis.

•

veering freedom, where the outer gimbal was free to rotate about the vertical plane, which is perpendicular to both the spin and tilt axes.

The outer gimbal is supported in the frame or case of the gyro system. The modern technical terminology used to express the degrees of freedom of gyroscopes tends towards accepting as fact, that a gyro must spin to show the gyroscopic properties. Therefore, a two frame gyroscope has only one degree of freedom, while the three frame gyroscope has two degrees of freedom. The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer gimbal rings. The whole assembly is known as the Issue B: January 2008

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gimbal system of a free or space gyroscope. The gimbal system is mounted in a frame, so that in its normal operating position, all the axes are mutually at right angles to one another and intersect at the centre of gravity of the rotor.

The gyro must be universally mounted or in gimbals so as to maintain the two degrees of freedom required, that is vertical and horizontal (in this explanation the spin axis of freedom is ignored – although the text refers to two degrees of freedom, it means full freedom of spin, tilt & veer). The construction of the gyro determines the shape and form of the gimbals which in turn depends on how the gyro will be used and in which plane it will be required to sense movement. Gimbals permit the gyro frame (or an aircraft) to move around the gyro while it maintains its original attitude and direction of spin axis. Plane of spin does not require a gimbal as this plane is simply the freedom of the rotor to spin on it’s axis. A gyro cannot detect movement about its plane of spin, eg a DG cannot detect pitch and an AH cannot detect yaw. Each other gyro axis requires a gimbal to provide it with freedom. Only 1 gimbal only permits freedom in only 1 axis (in addition to plane of rotation explained above). A second gimbal is required to provide freedom in both axis’s of tilt and veer. We can limit the gimbals to our advantage in measuring things, eg a rate gyro only has 1 gimbal, but that will be covered in depth later.


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Gyroscopic Inertia or Rigidity The property of rigidity of the gyroscope is its ability to resist any force which tends to change the plane of rotation of its rotor. This means that if a force is applied to try and move the gyroscope to another position the rotor’s axis of spin will try and remain in the constant direction in space. This property is the result of its high angular velocity, and the kinetic energy possessed in the rotor. The gyroscopic inertia or rigidity can be increased by: • increasing the mass of the rotor • increasing the rotor speed • concentrating more mass near the rim of the rotor. This is called increasing the radius of gyration.


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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 applied force, but always at a point 90° away in the direction of rotation. The rate of precession also depends on three factors: •

Strength and direction of the applied force

•

Moment of inertia of the rotor (rigidity of rotor - weight)

•

Angular velocity of the rotor (Rigidity of rotor – speed)

The greater the force, the greater is the rate of precession, while the greater the moment of inertia and the greater the angular velocity, the smaller is the rate of precession. (greater rigidity – smaller rate of precession for equal amount of applied force) 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 and until the directions of rotation and applied force are coincident. At this point, since the applied force will no longer tend to disturb the plane of rotation, there will be no further resistance to the force and precession will cease. gyro will eventually gimbal lock or topple if unrestrained – a rate gyro functions on the basis of precession, but the gyro rotor is restrained by springs so does not gimbal lock – rotor continues to precess against spring pressure whilst turning force is detected by gyro rotor – more on rate gyro’s later. The axis about which a torque is applied is termed the input axis, and the one about which precession takes place in termed the output axis.


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The property of precession only becomes apparent when an external force is applied to a spinning mass. This will cause the plane of rotation to change direction. The change takes place, not at the direction of the applied force, but at a point 90° away in the direction of gyro rotation. The rate of precession also depends upon three factors: Strength and direction of the applied force Rigidity of the rotor; (mass of the rotor, where it is concentrated and speed) The greater the force, the greater is the rate of precession, while the greater the rigidity of the rotor, the smaller is the rate of precession. Since precession is the angular change in position of the plane of rotation (spin) that occurs when the applied force exceeds the rotational force, the direction of the precessional movement is dependent upon the direction of the applied force and the direction of the gyro rotation.


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Unavoidable precession is caused by aircraft maneuvering and by the internal friction of attitude and directional gyros. This causes slow "drifting" and thus erroneous readings. When deflective forces are too strong or are applied very rapidly, most older gyro rotors topple over, rather than merely precess. This is called "tumbling" or "spilling" the gyro and should be avoided because it damages bearings and renders the instrument useless until the gyro is erected again. Some of the older gyros have caging devices to hold the gimbals in place. Even though caging causes greater than normal wear, older gyros should be caged during aerobatic maneuvers to avoid damage to the instrument. The gyro may be erected or reset by a caging knob. Many gyro instruments manufactured today have higher attitude limitations than the older types. These instruments do not "tumble" when the gyro limits are exceeded, but, however, do not reflect pitch attitude beyond 85 degrees nose up or nose down from level flight. Beyond these limits the newer gyros give incorrect readings. These gyros have a self-erecting mechanism that eliminates the need for caging.

Gimbal lock is normally prevented by limiting the movement of the inner gimbal with mechanical stops as shown on the slide. A mechanical stop applied to prevent gimbal locking. This physically prevents the inner gimbal and the outer gimbal from becoming aligned. If the gimbals do reach these stops, the forces acting on the gimbal system cause the system to precess randomly and topple.


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Free or Space Gyros An unrestricted, un-referenced displacement gyro is called a space gyro. These are gyros that have complete freedom about three axis which are all 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 indefinite time assuming that there are no bearing imperfections or external forces such as magnetic fields or gravity. Typical gyro training aids and gyro toys are space gyros. They are not referenced to anything, not even 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 fixed 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. Obviously an un-referenced space gyro is of no use in an aircraft. For a start if the aircraft were sitting still on the ground the gyro would be drifting off at a rate of 15° per hour due to the earths rotation. A gyro in an aircraft must be referenced to the horizon, or the earth. So a space gyro must be controlled to remain rigid, but with respect to the centre of the earth, this is usually achieved by using gravity as a reference to maintain the gyro erect & referenced to the centre of the earth.

Free or Space Gyros These are gyros that have complete freedom about three axis which are all 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 indefinite time assuming that there are no bearing imperfections or external forces such as magnetic fields or gravity.


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Tied Gyros A space gyro would be of no use whatever in establishing the needed vertical and horizontal references required for safe instrument flight. We need to have gyros that have freedom about three axis at right angles to each other, but which are controlled by some external force, so they assume an attitude in respect to some given point. By tying the gyro spin axis to some fixed point, we create what is called a tied gyro. The directional gyro indicator uses a tied gyroscope. The gyro spin axis being tied horizontal in respect to the indicator case and as such is parallel to the aircraft lateral and longitudinal axes. This type of application is called a case tied gyro.

Earth Gyros These are tied gyros whose spin axis is maintained in a vertical position with respect to the earth’s surface by a gravitational device. These types of gyroscopes are called vertical gyros and is the basic element of the gyro horizon or artificial horizon indicator.

Rate Gyros These are tied gyros, which are tied to a particular reference point by springs, creating a gyro which has only one degree of freedom. It is constructed so as to indicate rate of movement about a plane at right angles to both the plane of rotation and the plane of freedom which in this case is the tilting plane. The gyroscope is spring-restricted about the tilt axis so that when the unit is turned about the vertical axis, the amount of displacement due to precession is a measure of the rate of turn.

References Established by Gyroscopes The application of the gyroscope into aircraft systems is to provide two essential reference datums: 

vertical flight reference

Against which aircraft attitude changes are noted and used to indicate both pitch and roll functions of the aircraft. 

directional flight reference

Against which aircraft heading changes are noted and used to indicate movement of the aircraft about the vertical axis. These references are established by gyroscopes having their spin axis arranged vertically and horizontally. The vertical gyro is the sense element of all attitude gyros, whilst the horizontal gyro is the sense element of the directional gyro providing aircraft heading information. From the above description, it can be seen that the three gyro controlled flight instruments used by the pilot are all tied gyros. In each case however, a different method is used to tie the gyro instrument to its appropriate reference point.

Earth Gyro Before a free gyroscope can be of practical use as an attitude reference in aircraft flight 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 to earth is then termed an Earth gyro. Any Space gyro referenced to a parameter is referred to as a tied gyro, so an Earth Gyro (tied to centre of the earth) is a form of Tied gyro.


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Rate Gyros The difference between a displacement gyro, and that provided by a rate gyro: where a displacement gyro utilises a gyros property of rigidity in space and 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. The higher the rate of turn, the greater the precessive force, the greater the movement against spring pressure.


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Displacement Gyros Aircraft in flight are still very much a part of the earth, i.e. all references must be with respect to the earth’s surface. The free or space gyroscope we have been referring to in presenting gyro theory would serve no useful purpose in an aircraft and would have to be corrected for drift with respect to the earth’s rotation, called apparent drift, and for wander as a result of transporting the gyroscope from one point on the earth to another, called transport wander. It will also be noted that the pitch, roll, and directional attitudes of the aircraft are determined by its displacement with respect to each appropriate gyroscope. For this reason, therefore, the gyroscopes are referred to as displacement type gyroscopes. Each one has the three degrees of freedom, and consequently three mutual axes, but for the purpose of attitude sensing, the spin axis of the gyro is discounted since no useful attitude reference is provided when displacements take place about the spin axis alone (displacement around axis of spin is not detected). Thus, in the practical case, vertical-axis and horizontal-axis gyroscopes are further classified as two-axis displacement gyroscopes.


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Gyroscope Applications in Aircraft For use in aircraft, gyroscopes must establish two essential reference datums: •

Reference against which pitch and roll attitude changes may be detected

•

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. Both types of gyroscope utilise the fundamental properties in the following manner: •

Rigidity establishes a stabilised reference unaffected by movement of the supporting body

•

Precession controls the effects of apparent and real drift thus maintaining stabilised reference datums (erection systems to reference to earth).

For use in aircraft, gyroscopes must establish two essential reference datums: a reference against which pitch and roll attitude changes may be detected, and 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. Both types of gyroscope utilize the fundamental properties in the following manner: Rigidity establishes a stabilized reference unaffected by movement of the supporting body, and precession controls the effects of apparent and real drift thus maintaining stabilised reference datums.


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To compensate a space gyro to eliminate earth rate, at the equator we could precess it at 15° per hour, so that it will completely rotate every 24 hours (same as the earths rotation) thus appearing to remain erect with respect to earth. If the gyro is not at the equator, the precession value can still be easily calculated because apparent drift equals 15° sin λ (where λ equals angle of latitude). Can be achieved by electrical torquing signals, or by unbalancing gimbals to cause gyro to drift at desired rate. Control of drift which, relates only to horizontal-axis gyroscopes and can be achieved either by: •

calculating corrections using the earth-rate formula given in the preceding table and applying them as appropriate; e.g. to the readings of a direction indicator:

•

applying fixed torques which unbalance the gyroscope and cause it to precess at a rate equal and opposite to the earth rate we,

•

applying torques having a similar effect to that stated in above, but which can be varied according to the latitude.

A gyro corrected for earth rate or apparent drift will maintains its attitude with reference to the earth, it will continue to point to the centre of the earth even as the earth rotates. This is the name given to the apparent drift which becomes evident in the directional gyroscope due to the earth’s rotation. It is a combination of both apparent tilt and apparent veer. Apparent precession occurs at a rate of 15 degrees per hour x sine of the latitude in which the gyro is operating. Apparent drift compensation is carried out by causing the gyroscope to be precessed in the opposite direction to the earth’s rotation. This is achieved by placing weights in the spin axis of the gyro rotor to unbalance the unit so that the weight force causes the gyro to precess. The rate of precession is determined by the latitude in which the gyro is being operated. Real drift results from imperfections in the manufacture of the gyroscope such as bearing friction, gimbal imbalances. Imperfections can cause unwanted precession which can only be minimised by applying precision engineering techniques.


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Transport 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 The control of transport wander is normally achieved by using gravity-sensing devices to automatically detect tilting of the gyro scope’s spin axis, and to apply the appropriate corrective torques. Examples of these devices are later described.

Transport Rate If a gyro were 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. In the diagrams, the one on the left shows an uncorrected gyro which would display transport rate, the one on the right shows a corrected gyro. Transport rate is corrected by referencing the gyro to the centre of the earth, or gravity.


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Gimbal errors: occur in all directional gyros which, as you will recall, have a horizontal spin axis. The gimbal errors are caused during aircraft maneuvers. They are caused by loss of gimbal relationship under certain conditions, in that the gyro spin axis and gimbals are no longer at 90 degrees to each other. Gimbal errors are not caused by external forces, but by the output from sensing synchros, as a result of the outer gimbal moving as the aircraft rotates about the spin axis. This output causes heading errors on inter-connected instruments during maneuvers however, these errors are eliminated when the aircraft returns to straight and level flight. If the aircraft gyro frame is rotated about the rotor spin axis, the outer gimbal must move to maintain the direction of the rotor spin axis. This movement of the outer gimbal will be detected by the outer gimbal heading synchro.


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Handling


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Ring Laser Gyro Laser gyros are now widely used in aircraft navigation applications. They provide accurate, independent navigational data with high accuracy and reliability. They are still a dead reckoning system, and require no external inputs to function. This type of Inertial Reference Unit is referred to as a strapdown system because it does not require a gyro stabilised platform as described in a conventional INU. Pitch and roll movements which would normally introduce errors in an accelerometer, are provided to the computer and the accelerometer outputs are modified electronically to compensate for attitude changes. This form of Inertial Reference Unit normally provides primary Attitude information, and can also measure altitude (inertially), rate of ascent and descent and groundspeed. Outputs from an IRU are typically distributed over a digital data bus to flight control computers, navigation computers, multi-function displays, etc.

Fundamentals of Laser Operation LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Since its discovery in 1960, LASER technology has expanded rapidly with application in many fields other than aviation including medical, agriculture and engineering. The first step in producing Laser is the ionisation of a gas which may be helium, argon, krypton, neon or xenon. Each gas produces a different colour (and wavelength) of light. A mixture of helium and neon is used in ring laser gyros. This gas mixture is held at low pressure inside a sealed tube exposed to an anode and cathode plate. When a high voltage is applied across these plates the gases ionise, producing a glow discharge similar to fluorescent tubes. In laser gyroscopes the applied voltage is around 3000 volts. What is the difference between laser light and say ordinary white light? Firstly, white light is a mixture of many wavelengths; laser light is a single wavelength which is dependant on the type of gas used. Secondly, ordinary light is scattered in all directions but laser light is a parallel beam. For example, recent experiments using a pencil sized laser light aimed at the moon found it spread to a distance of only two miles over the distance of 250,000 miles. Laser is termed coherent light which means it is of a specific wavelength.


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Construction of the Ring Laser Gyroscope Ring laser gyros are not gyroscopes in the sense that we know them. They are simply two beams of laser light rotating in opposite directions engineered to detect motion and behave like a gyro. Laser ring gyros are constructed so that two laser beams are reflected around a triangle causing the light to travel in an enclosed loop. The light travels in both directions at the same time, so we have a clockwise beam and a counter-clockwise beam. So how can this be used to detect motion? Picture a merry-go-round platform which is stationary, with two people walking around it from the same starting point. One walks clockwise and the other anticlockwise, and both can walk entirely around the merry-go-round back to their starting point by walking 100 steps. However, if the platform is rotated slowly clockwise, the person walking with the platform would need to take shorter steps to complete the journey in the same number of steps. Conversely, the person walking against the direction of rotation would need to take longer steps to complete the journey in 100 steps (like walking on an escalator – more steps to achieve the same distance). This can also be explained using the Doppler principle. A similar phenomenon takes place in our laser gyro. If the gyro is turned clockwise (CW), the CW beam completes the journey in a shorter time. In order to complete the journey in the same number of cycles the beam wavelength must be compressed, that is, the frequency must be increased. Conversely, the counter-clockwise (CCW) beam wavelength must increase (frequency decreased).


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Fibre Optic Gyro Fibre Optic Gyros (FOGs) are a later expansion of the RLG principle. Similar to RLGs, they operate on exactly the same principle of sending two light beams, in different directions around a fibre optic path. Any movement of the Fibre Optic coil in either direction will result in the path of light travelling further in one beam while the other light path travels less distance. Bringing the two light beams out to some form of detector, which looks at the phase of the light, will then give an output signal which will relate directly to the amount of rotation encountered by the coil. This is again using the Sagnac effect as do the RLGs.


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The reason FOGs are finding greater use than RLGs is that they can be made in much smaller and compact installations. Also the path that the beams are required to travel around, can be made much longer than a similar RLG light path, and this increase in path length means that more accurate and smaller changes of rotation can be measured. Typically, FOGs can have fibre optic coils which can measure anywhere from 100 metres to over 5 kilometres in length. The longer the path, means basically the more accurate the FOG can be, which in turn means that the Inertial Measurement Unit will provide far greater navigational accuracy. Most modern day FOGs, because of their much smaller physical size than RLGs, are incorporated with GPS receivers and Air Data units. What this does is to give the aircraft designer a compact, fully self contained, navigation unit capable of very high accuracies. Because of their operation and the virtual absence or nearly all moving parts, the mean time between failure (MTBF) for these types of units is now measured in the order of years between failures. Typical MTBFs are measured in excess of 50,000 hours of continuous operation, which in laymens terms means nearly over 6 years of continuous operation between reported faults. The accuracy of these units is also higher than typical RLG units. Accuracies in the order of 15 metres or less, anywhere in the world, is something that Airlines are very happy to accept.

A typical unit from Northrop Grumman as shown above is currently fitted to the Airbus A380. It weighs less than 8 kilograms, and draws less than 36 watts of power from the aircraft electrical system. This unit combines the functions of GPS, Inertial Reference and Air Data modules into one package. What this does is give a small lightweight package which draws very little power, yet provides extremely accurate navigational accuracy.


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Gyroscope Erection Systems There are two main type of air driven gyro erection systems: • wedge plate • pendulous vane. Both types rely on the gyroscopic precession to move the spinning rotor back to a balanced or erect position.

Wedge Plate The wedge plate system simply deflects the air from the rotor system across a wedge shaped plate. This deflected air when exhausted evenly or balanced across the plate has no effect on the outer gimbal. When an unbalanced deflection occurs due to the movement of the rotor from the vertical, the air is distributed more on one side of the plate than the other. This differential air flow causes a precession effect on the outer gimbal and therefore tries to return the spinning rotor to the vertical position.


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A negative pressure is created so that the cabin air enters the filtered inlet and passes through the channels to the jets. The air from the jets hits the rotor buckets, evenly driving the rotor at approximately 15,000 rev./min. After spinning the rotor, the air passes through a pendulous vane unit attached to the underside of the rotor casing and is finally drawn to the vacuum source.


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Pendulous Vane The pendulous vane unit operates on an air driven gyro spinning around a vertical axis, such as an artificial horizon. This type of erection system ports the exhausted air through four ports located at right angles to each other. The pendulous vanes in the normal position port air equally across all outlets and therefore no precession effect is generated. In an unbalanced situation, the gyro rotor is tilted and the gravitational effect automatically adjusts the position of the pendulous vanes and therefore directs the air differentially. This differential force causes the rotor to precess back to the normal position.

Erection Systems for Electrically Driven Gyros Mechanical For electrically driven gyros, a mechanical form of erection is to manually cage the gyroscope. This action mechanically forces the gyro gimbals to a position at right angles to each other, that is, pitch, roll and yaw. Care must be taken when caging an instrument as damage can result to gimbals and bearings if caging is undertaken incorrectly. Ball Type Erection System There are two main types of ball style levelling or erection systems used on artificial horizons or attitude gyros: • ball cage type • rolling ball type.


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Ball Cage Type The ball cage type uses a number of steel balls usually between 5 and 8 suspended below the gyro and free to roll across a radiused disc with hooks located around the perimeter. These hooks capture the balls as their mass moves and apply a precession force to erect the gyro. In the vertical plane, the mass of balls is centred and therefore no precession force is applied. The precessional force applied will erect the gyro. Rolling Ball Type The rolling ball type system when used as a levelling device incorporates a slotted disc with a ball free to travel inside the slot. The slot is driven in at a uniform speed approximately 30 rpm. When the assembly is tilted the ball exerts an effect on the disc. When tilted, the ball rolls to the lower end and waits until the bottom of the slot catches up. Going uphill the ball is pushed by the disc. This means that the average weight on the downhill side is less than the uphill and therefore produces a torque effect which is related to tilt angle. This torque precesses the gyro assembly in the desired direction to move the gyro back to the vertical position.


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Erection Systems for Electrically Driven Gyros Torque Motors and Sensors The application of torque motors is the most commonly used form of gyro correction and possibly the easiest to use. The torque motor is similar to a small electric motor except that the output shaft applies a twisting motion or force which is called torque, to the erection gimbal system when energised. The power is supplied through levelling switches and the torque motors are situated between the gimbal and the next supporting frame. Because of the force of precession, a gyro spinning about the vertical axis requires the pitch torque motor to be positioned between the inner and outer gimbal and the roll torque motor positioned between the outer gimbal and frame. Mercury Switches A levelling switch mounted parallel to the aircrafts longitudinal and lateral axis (pitch and roll) detect any deviation from level and therefore control current flow to the torque motors by applying a torque opposite to the force creating it. The levelling switches can be either straight or curved, the curved variety requiring a greater force to operate.


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Torque Motors The erection torque motors are squirrel cage motors with a laminated iron rotor, mounted concentrically about the stator. The iron core stator has two windings, a reference winding and a control winding. The reference winding is provided with a constant field and in normal operation, that is, no displacement, it has current flowing via a capacitor and is therefore phase shifted by 90 degrees. The control winding is in two parts and is able to provide a reversible field. This circuit is supplied from the same AC source, as it is directly connected to the supply there is no phase shift, leading the reference winding. The combined magnetic field interacts with the field in the stator and the result controls the direction of the torque motor and provides correctional torque to the gyro assembly. In the normal position there is no current flowing in the torque motor circuits due to the levelling switch being in the level position.

DC Coils and Permanent Magnets Both systems use the resultant flux lines to impart a magnetic field which will tend to magnetically displace the gyro and therefore impart a precessional force. A permanent magnet is used where a constant correction force is required to act, as is the case for the correction of apparent drift. The success of this type of permanent field will depend on the operating environment of the aircraft, and the drift rate of the location. DC coils offer the ability of being able to be switched on or off through the use of cutout switches, limit switches, mercury switches or simply as a means of fast erection of the gyro.


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Erection systems are necessary for erecting and maintaining the gyro spin axis in a vertical or horizontal position relative to the earth’s surface. For an attitude indicator to be accurate the gyro’s spin axis must be kept vertical, this is achieved by the erection devices detecting and erecting the gyro to the local vertical. Due to the construction of erection devices, they will be displaced whenever the aircraft changes airspeed or alters direction. Unless provision is made to counter act the acceleration and turning forces, the erection devices will precess the gyro axis to a false vertical and in doing so will present an incorrect


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indication of the aircraft’s attitude. The importance of erection systems and their correction methods must be clearly understood. Erection Errors The erection errors caused by acceleration, deceleration and turning forces acting on the conducting medium in the electrical leveling switches, or pendulous vanes used in the mechanical erection systems can be compensated for by using the characteristics of rigidity and precession to correct the errors. Erection Error Correction To correct for the erection errors encountered during acceleration or deceleration and turning forces, a correcting torque is applied to the outer gimbal. This results in precession of the inner gimbal to counteract the false vertical error.


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Caging

The gyro can be caged manually by a lever and cam mechanism to provide rapid erection. When the instrument is not getting sufficient power for normal operation, an "OFF" flag appears in the upper right face of the instrument. The instrument permits 360° of rotation about the pitch and bank axes without tumbling the gyro. The expanded motion of the horizon bar provides sensitive pitch indications near the level flight position. When the aircraft exceeds the maximum of 27° in pitch up or down, the horizon bar is held in extreme position and the sphere becomes the new reference. A continued increase of climb or dive angle approaching the vertical attitude is indicated by graduations on the sphere. When the aircraft nears vertical, the sphere begins to rotate 180°. As soon as the aircraft departs from the vertical, the instrument again indicates the attitude of the aircraft. This momentary rotation of the sphere is known as controlled precession and should not be confused with gyro tumbling. The attitude of the aircraft about the roll axis is shown by the Issue B: January 2008

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angle between the horizon bar and the miniature aircraft, and also by the bank index relative to the degree marking on the bezel mask (face plate).

Errors Following recovery from unusual attitudes, displacement of the horizon bar in excess of 5° in pitch and/or bank may result. Once the instrument senses gravitational forces, the erection mechanism will immediately begin to correct the precession errors at a rate of 3° to 6° per second. In a normal turn, centrifugal force acting on the erection mechanism will produce normal precession errors in pitch and/or bank up to 5° on return to straight-and-level flight. Acceleration or deceleration will also result in precession errors in proportion to the duration and magnitude of the speed change. Following acceleration, the aircraft pitch attitude will be lower than the instrument indication; following deceleration, the aircraft attitude will be higher than the pitch indication until the erection mechanism realigns the gyro.


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Aircraft Gyro Vacuum Systems Aircraft gyro instruments can be powered by vacuum (air) or electricity. Electric gyros can run off AC or DC power, depending upon what they were designed for. Air driven gyros can run on positive pressure, or vacuum pressure. Air pressure is provided by an engine driven vacuum pump, and the vacuum (more predominantly) is then plumbed through the gyro instruments to run up the gyro rotors. In vacuum gyro systems the filters are very important items as any contamination entering the gyro will dramatically shorten its serviceable life. Filters must be regularly serviced

Aircraft use a venturi system when they do not have the facility for an engine driven vacuum pump to power their air driven gyros. The venturi tube is an open ended metal tube tapering towards the centre or throat. It is fitted to the fuselage or mainplane with the inlet end in line with the direction of flight, and usually located in the propwash area. In flight, the air is forced into the inlet end of the tube and accelerates through the narrowing section or throat of the tube to a higher velocity. This increase in velocity produces a lower pressure at the throat which is connected through tubing to a suction relief valve and then to the case of the gyro instruments. The case of the gyro is also connected through a filter back to atmosphere. As the pressure in the throat of the venturi is lower than atmosphere, the atmosphere causes a flow of air or suction through the instruments to the venturi throat and back to atmosphere as the air leaves the venturi. Venturi tubes as a vacuum source are normally confined to early light aircraft and some of the later types of simple home built aircraft. The venturi is extremely inefficient and limited in its capacity to drive instruments. Venturi tubes are rated by the amount of vacuum they will produce at 120 Mph or 104 Kts. The two-inch/50 mm venturi is used to produce two inches of mercury suction to drive one turn and bank indicator, while the larger four-inch tubes are used for the directional and attitude gyros. One design of the larger tubes is called the super-venturi or eight-inch venturi. This venturi has an auxiliary venturi in its throat and is capable of more suction for the same speed.


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Problems The most common problem with a venturi is from: • being struck • the tube assembly being damaged • being blocked by a foreign object. Always make sure that the tube and fittings are clean, free from obstructions and have a good physical appearance. With the introduction of new aircraft, the aircraft systems and instrumentation became more complex. Higher speeds and increased altitude required a more sophisticated vacuum supply source. The major problem with the venturi system is the formation of ice in the throat and other damage being sustained by the tube assembly sticking out in the airflow. The vacuum powered gyroscopic flight instruments fitted to the many types of aircraft vary in the demand placed on the vacuum system. The two main types of positive displacement, vane type vacuum pumps which are driven from the engine accessory drives are: • wet type • dry type and are classified according to their construction.

Wet Pumps The earlier vacuum pumps were nearly all of the steel vane type which were lubricated from the engine low pressure oil system. This oil has a one-way passage through the pump and is lost with the discharge air over board, via the vent tube. In some designs this oil is returned to the engine crankcase by separating the oil from the discharge air in an oil separator before the air is allowed to enter the atmosphere which prevents the oil causing streaks along the side of the fuselage.


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The modern pumps are dry; that is, they have their wearing parts made of Teflon and or carbon. The pump rotors are made of a fibre material and the rotor blades are of carbon. The pump housing is high grade cast iron finely machined and in some cases the surface is Teflon coated. These pumps can normally be driven in only one direction which is indicated by an arrow on the housing To prevent mechanical damage to the engine accessory drive system, the pumps have a weak-link shear drive designed to fail should the pump suffer an internal fault.

Problems with Vacuum Pumps Look for a show of oil or evidence of vibration. The vacuum pump should be smooth in operation and an erratic output is characterised by a difficult to adjust suction relief valve. The vacuum pump shear link must be checked for signs of stress and the pump must appear to be in good order. An aircraft vacuum source can be either from a vacuum pump which is engine driven or from a venturi which is located in the propwash, external to the aircraft. The vacuum supply in both cases is a source of low pressure.

An aircraft vacuum source can be either from a vacuum pump which is engine driven or from a venturi which is located in the propwash, external to the aircraft. The vacuum supply in both cases is a source of low pressure. Pressurised air ported over cups in gyro rotor, or vacuum air sucked across cups. Spins gyro rotor up to speed and is also used for gyro erection system – reference gyro to earth to eliminate transport rate. Only ever low pressure air used. Only likely to be incorporated in light aircraft.


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At high altitudes vacuum-driven gyroscopic instruments suffer from the effects of a decrease in vacuum due to the lower atmospheric pressure; the resulting reduction in rotor speeds affecting gyroscopic stability. Other disadvantages of vacuum operation are weight due to pipelines, special arrangements to control the vacuum in pressurized cabin aircraft, and, since air must pass through bearings, the possibility of contamination by corrosion and dirt particles

Vacuum-Driven Gyro Horizon 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. 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, 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 RPM. After spinning the rotor, the air passes through a pendulous vane unit attached to the underside of the rotor casing, and is finally drawn off by the vacuum source.


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Disadvantages of air driven gyro systems •

Dirt and dust are a major problem with air driven instruments and therefore instrument filters and system filters must be checked, cleaned or changed at regular intervals.

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When cigarette smoking was allowed on aircraft, the residue from the smoke was a major problem for gyroscopic air driven instruments.

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Engine driven vacuum pumps must be regularly checked for correct operation.

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Incorporation of mechanical pumps adds an additional piece of equipment requiring servicing, in addition to the aircrafts alternator/generator.

To overcome the disadvantages of the air driven gyroscopic instruments in high performance aircraft, gyroscopic instruments were designed for operation on electrical power derived from the aircraft power supplies. This power is generally 115V 400Hz three phase alternating current as supplied from the aircraft alternators or inverters or 28V direct current, the latter being required for the operation of some turn and bank indicators. The alternating current application has been used for the later types of turn and bank, gyro horizon indicators and the remotely located attitude and directional gyros associated with flight control systems and remote-indicating compass systems. Electrical gyros only need a small amount of power from the existing aircraft power supply hence an additional engine driven component (the vacuum pump) is no longer necessary. AC electrically powered gyros can run much faster than air driven gyros so 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. A particular limitation of air driven gyros over most electrically driven gyros is that the gyro should never be removed from the aircraft until at least 30 minutes have passed from the time the vacuum source was disconnected, or rotor has ceased spinning, as the inertia contained within the rotor, and the relative absence of friction within the bearings, may allow the rotor to spin for up to this length of time. Electrically driven gyros often incorporate a form of electrical or dynamic braking which will slow the gyro rotor very quickly once power is removed.


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The direct current electrical gyro uses a method of construction where the rotor is actually the armature winding and the stator is the permanent magnet. This principle of construction allows the greater mass containing the armature winding to spin as the rotor giving greater rigidity. This direct current application utilises the gyro rotor which contains the armature winding of a small permanent magnet motor. This is supplied with 28 volts DC via two spring loaded brushes contacting a commutator, which is mounted on the rotor armature shaft. The stator is a two-pole permanent magnet and forms part of the gimbal frame. In this application (Turn and Slip indicator) the rotor speed is kept at approximately 4,200 RPM.


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AC Drive Methods and Motor Types AC gyros are usually powered by a three phase 115 volt 400 Hz squirrel cage induction motor, consisting of a rotor and a stator. A normal induction motor has the rotor revolving inside the stator. In this instance the motor has been redesigned so that the rotor rotates on the outside of the stator, this increases the mass of the rotor in order to provide the required inertia. The 115 volts AC is supplied to the stator and a rotating magnetic field is established in the stator. This rotating field cuts the bars in the squirrel cage rotor and induces a current, the effect of which produces a magnetic field around the bars which combine with the stator’s field causing the rotor to spin at approximately 22,500 RPM.

AC Drive Methods and Motor Types AC gyros are usually powered by a three phase 115 volt 400 Hz squirrel cage induction motor, consisting of a rotor and a stator. A normal induction motor has the rotor revolving inside the stator. In this instance the motor has been redesigned so that the rotor rotates on the outside of the stator, this increases the mass of the rotor in order to provide the required inertia. The 115 volts AC is supplied to the stator and a rotating magnetic field is established in the stator. This rotating field cuts the bars in the squirrel cage rotor and induces a current, the effect of which produces a magnetic field around the bars which combine with the stator’s field causing the rotor to spin at approximately 22,500 RPM.


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Electric Gyro Horizon 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 motor the mass of the rotor is concentrated further from the centre, 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 bearing pivot in the front cover glass and in the rear casting. 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 is described in Pallett Aircraft Instruments on page 136, but will not be covered here. When power is switched on a rotating magnetic field 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 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.


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TOPIC 13.8.2.2.1: ARTIFICAL HORIZONS

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 deflections made by the aircraft around this stable point are measured and displayed on the cockpit instruments. Gyro Spin axis is vertical, so plane of spin is horizontal. This permits rigidity in lateral and longitudinal axis’ and the displacement of the gimbals from the stable reference is what provides the roll and pitch readout. The gyro is a tied gyro referenced to the earths 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. Most gyro horizons have a pull to cage knob to re-align the gyro in straight and level flight if it is noted to be drifting off, or if it tumbles or suffers gimbal lock.


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This form of gyro horizon has a fixed back plate or sky plate and the aircraft symbol is attached to the gimbals and moves with respect to the back plate to indicate pitch and yaw attitudes. This was an old method of displaying this information and probably not very often seen in modern times. The pitch restriction at 85° is to avoid gimbal lock. This is not a concern in the roll axis.


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This picture is a more exact illustration of the way the gyro horizon actually provides a display of pitch and roll. The dial is fixed and once upon a time would only have had a horizon line drawn across the middle. In more recent time gyro horizons have been coloured with a light colour above, typically blue, to represent the sky and a darker colour below to represent the ground. When the horizon pointer is up and in the blue it means the aircraft is climbing, and when down in the green it is diving. The horizon bar is restricted in pitch movement up to 85° otherwise gimbal lock will occur, whereas the rolling action is unrestricted. The display can therefore indicate unrestricted full barrell rolls but if a loop were performed the indicator would show a climb up to 85° (when the aircraft nose is almost vertical, not when it’s at the top of the loop) the assembly would then roll 180°. The horizon pointer would indicate straight and level inverted flight 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 flight 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 flight.


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With the vacuum system in operation, a negative pressure is created so that the cabin air enters the filtered inlet and passes through the channels to the jets which are directed onto the buckets carved into the gyro’s rotor. The air from the jets hits the rotor buckets, evenly driving the rotor at approximately 15,000 rev./min. After spinning the rotor, the air passes through a pendulous vane unit attached to the underside of the rotor casing and is finally drawn to the vacuum source. The air is exhausted through the pendulous vane unit, which applies a correctional or precession force t keep the gyro perpendicular to the earths surface. This is achieved through the magic of precession and as the gyro tilts off the vertical an air port is uncovered permitting a greater flow of air from that port as it exits from the gyros rotor. The additional airflow exerts a force on the gyro which is felt 90° in the direction of rotation and will cause the gyro to precess back to the vertical.

Operation The operation of the instrument is basically controlled by the principle of gyroscopic inertia or rigidity. The gyro spin axis is maintained in a vertical position relative to the earth. As the aircraft rolls and pitches in flight, the indication is given on a two colour dial, the top half representing the sky and the bottom half which is darker, represents the ground. The horizontal gyro spins about the vertical axis and therefore it can sense rotation about the roll and pitch attitude of the aircraft. Disadvantages of air driven gyro systems •

Dirt and dust are a major problem with air driven instruments and therefore instrument filters and system filters must be checked, cleaned or changed at regular intervals.

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When cigarette smoking was allowed on aircraft, the residue from the smoke was a major problem for gyroscopic air driven instruments.

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Engine driven vacuum pumps must be regularly checked for correct operation.

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Incorporation of mechanical pumps adds an additional piece of equipment requiring servicing, in addition to the aircrafts alternator/generator.


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To overcome the disadvantages of the air driven gyroscopic instruments in high performance aircraft, gyroscopic instruments were designed for operation on electrical power derived from the aircraft power supplies. This power is generally 115V 400Hz three phase alternating current as supplied from the aircraft alternators or inverters or 28V direct current, the latter being required for the operation of some turn and bank indicators. The alternating current application has been used for the later types of turn and bank, gyro horizon indicators and the remotely located attitude and directional gyros associated with flight control systems and remote-indicating compass systems. Electrical gyros only need a small amount of power from the existing aircraft power supply hence an additional engine driven component (the vacuum pump) is no longer necessary. AC electrically powered gyros can run much faster than air driven gyros so 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. A particular limitation of air driven gyros over most electrically driven gyros is that the gyro should never be removed from the aircraft until at least 30 minutes have passed from the time the vacuum source was disconnected, or rotor has ceased spinning, as the inertia contained within the rotor, and the relative absence of friction within the bearings, may allow the rotor to spin for up to this length of time. Electrically driven gyros often incorporate a form of electrical or dynamic braking which will slow the gyro rotor very quickly once power is removed.


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The need for integrating the functions and indications of certain flight and navigation instruments resulted in the main from the increasing number of specialised radio aids linking aircraft with ground stations. These were developed to meet the demands of safe en-route navigation and to cope with increasing traffic congestion in the air space around the world’s major airports.


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The required information is processed by ‘black boxes’ which can be stowed in electrical compartments and radio racks, but in order that the necessary precision flying may be executed, information must still be presented to the pilot. This requires more instruments and more instruments could mean 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. During that phase of a flight involving the approach to an airport runway, it is essential for a pilot to know, among other things, that he is maintaining the correct approach attitude. Such information can be obtained from the gyro horizon and from a special ILS indicator which responds to vertical and horizontal beam signals radiated by the transmitters of an Instrument Landing System located at the airport. It was therefore a logical step in the development of integration techniques in what are termed Flight Director Systems, to include the information from both the gyro horizon and ILS indicator. The methods adopted for the integration of such information, and the manner in which it is presented vary between systems. A complete system normally comprises two indicators: - flight director, attitude flight director or an approach horizon - course deviation indicator (CDI) or a horizontal situation indicator (HSI). The flight director indicator has the appearance of a conventional gyro horizon, but unlike this instrument the pitch and roll indicating elements are electrically controlled from a remotely located vertical gyro unit. The approach attitude of an aircraft with respect to its ILS signals is indicated by independent pointers monitored by the relevant ILS receiver channels. Displacement of the aircraft to the left or right of the localiser beam is indicated by deflections of the localiser pointer. Glideslope pointer functions in similar fashion.

Attitude Directors are basically Artificial Horizons with command steering bars incorporated. The instrument provides the pilot with an indication of pitch and roll, but also has command bars which can be used to guide the pilot onto a selected course, or to a selected altitude. The command bars appear and the pilot flies the aircraft to align the aircraft symbol with the command bars.


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Flight Director Indicator (FDI) This instrument may be 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 basic function of the FDI 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 fixed aircraft symbol.

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

Bank Pointer This displays the bank angle of the aircraft, and is read against a 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 fly the aircraft symbol towards the command bars. The commands are supplied from the flight director computer, which can receive reference signals from a range of navigation aid receivers or INS

Glideslope Pointer This is located on the left side of the FDI and is used when the aircraft has captured the runway glideslope beams, when landing. The aircraft’s vertical position within the beams 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 on the dot closest to the centre line, the pitch command bar comes into view, and the pilot flies towards it. Figure 3.13 shows the glideslope pointer.


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Localiser Deviation Indicator Localiser pointer shows the aircraft’s position in relation to the localiser beams. When the pointer is in the centre of the scale the aircraft is positioned in the centre of the beams.


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The flight director indicator has the appearance of a conventional gyro horizon, but unlike this instrument the pitch and roll indicating elements are electrically controlled from a remotely located vertical gyro unit. Electrical interconnection of the flight director indicator components primarily concerned with pitch and roll attitude information is shown on the slide. Whenever a change of aircraft attitude occurs, signals flow from pitch and roll synchros to the corresponding synchros within the indicator. Error signals are therefore induced in the rotors and after amplification are fed to the servomotors, which rotate to position the pitch bar and horizon disc (or Sphere, or cylinder) to indicate the changing attitude of the aircraft. At the same time, the servomotors drive the synchro rotors to the ‘null’ position. The second circuit shows the interconnection of the glide slope and localiser pointer with the ILS. 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 deflected downwards against the scale, thus directing the pilot to bring the aircraft down on to the glide path. An upward deflection of the pointer indicates flight below the glide path and therefore directs that the aircraft be brought up to the glide path. The pointer is also referenced against the pitch bar to indicate any pitch correction required to capture and hold the glide path. When this has been accomplished, the glide slope pointer and pitch bar are matched at the horizontal centre position.


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If, during the approach, the aircraft is to the left of the localiser beam and runway centre-line, the localiser pointer is deflected to the right directing that the aircraft be banked to the right. Flight to the right of the localizer beam causes pointer deflection to the left, directing that the aircraft be banked to the left. When either of these directions has been satisfied, the pointer is positioned vertically through the centre position of the horizon disc. Flight director indicator houses a number of servo/synchro devices. Aircraft pitch & roll information from twin gyro platform positions horizon disc & pitch bar. Additional servo/synchro devices to drive command bars driven by signals from flight director computer. Typical remote indicator housing servo/synchro systems to repeat information sensed/processed by a remote equipment rack mounted ‘black box’.


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TOPIC 13.8.2.2.2: TURN AND SLIP INDICTORS

Turn Indicators The turn and bank indicator or turn and slip indicator as it is most often called, is one of the first instruments developed for instrument flying. The instrument actually combines a turn indicator and a slip indicator in the one instrument. In early days of flying the turn-and-bank, when used in conjunction with the aircraft compass made a valuable contribution to the art of IFR flying. It was thus considered the primary blind flying instrument. With developments in aircraft instrument technology the turn-and-bank has been replaced as the primary IFR instrument by the AH, although in some light aircraft the turn-and-bank is still considered a primary flight instrument. In larger aircraft the turn-and-bank has become a secondary instrument or is not used.


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Principle of Operation The rate gyro’s spin axis is horizontal and corresponds with the aircrafts lateral axis, that means the plane of spin is through the longitudinal axis of the aircraft. The rate gyro only has one gimbal mounted within the frame or case of the instrument so it is only permitted one degree of freedom which is in tilt. The pivot point for the gimbal is fore and aft of the gyro rotor so it is pivoted in the longitudinal axis of the aircraft. The gyro senses movement about the yawing axis of the aircraft. It is effectively mounted like a DG, but does not have the freedom of a DG. When the aircraft yaws the gyro wants to remain in its current attitude and alignment, but cannot because there is no gimbal to permit veer. Because the gyro cannot remain pointing in the same direction the turning motion of the aircraft has the same effect as if someone applied a precessive force to the front and rear of the gyro rotor, trying to change it’s heading. This force is felt 90° in direction of rotation, so will precess the gyro so it will tilt over. If the gyro was not restrained by springs it would continue to precess in the tilt axis while ever the yawing motion was felt. Because the gyro is held in place by springs, while ever the yawing motion (or rate of turn) remains constant the gyro precession force will remain constant against spring pressure providing a constant indication of the rate of turn. If the rate of turn is increased the precession force increases tilting the gyro further against spring pressure. When the turning motion ends the precession force is removed so the gyro will return to the original attitude, ie spinning in the vertical plane corresponding with the aircrafts longitudinal axis. Rotor axis parallel to aircrafts lateral axis Yawing motion sensed & due to precession rotor tries to lie over against spring pressure Lie over angle proportional to rate of turn & is opposed/restricted by calibrated spring tension 2 Minute and 4 Minute Turns Gyro doesn’t begin to lay over until after the turn has begun, ie when the heading begins to change. This statement will be referred back to when covering Turn Coordinators.

Spin axis parallel to aircraft lateral axis Issue B: January 2008

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Yawing motion sensed – rotor tries to lie over against spring pressure due to precession Lie over angle proportional to rate of turn Gyro lays over when heading changes Rate of precession (lie over angle) depends on: •

Rate at which heading is changing

•

Rigidity of rotor (rotor speed)

Rate of precession dependant upon speed of gyro rotor (gyro rigidity) Rate of Turn indicator rotor speed critical: •

too fast & instrument under reads

•

too slow & instrument over reads

Rate of turn indicators incorporate rotor speed governor to ensure accuracy of indications


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The Mechanism of a Typical Direct-Current Operated Turn-and-Bank Indicator Direct current is fed to the brushes and commutator via a radio interference suppressor and flexible springs which permit movement of the inner ring. The rotor speed is controlled by two identical symmetrically opposed centrifugal cut-outs. Each cut-out consists of a pair of platinum-tipped governor contacts, one fixed and one movable, which are normally held closed by a governor adjusting spring. Each cut-out has a resistor across its contacts, which are in series with half of the rotor winding. When the maximum rotor speed is attained, centrifugal force acting on the contacts overcomes the spring restraint causing the contacts to open. The armature current therefore passes through the resistors, thus being reduced and reducing the rotor speed. Both cut-outs operate at the same critical speed. 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 field magnet and a field ring. A power-failure warning flag is actuated by a stirrup arm pivoted on the gimbal ring. When the rotor is stationary, the stirrup arm is drawn forward by the attraction between a magnet mounted on it and an extension (flux diverter) of the permanent-magnet stator. In this condition the flag, which is spring-loaded in the retracted position, is depressed by the stirrup arm so that the OFF reading appears through an aperture in the dial. As rotor speed increases, eddy currents are induced in the rotor rim by the stirrup magnet, and at a predetermined speed, reaction between the magnet and induced current causes the stirrup arm to lift and the OFF reading to disappear from view.


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Major Parts Rotor speed and rigidity is crucial for this instrument to indicate accurately. If speed drops off, eg s clogged filter in an air driven system, the gyro will slow down and will then precess further with less force applied (remember factors that contributed to amount of precession, gyro rigidity and level of force applied to the gyro). So a slower gyro will over read. Speed control is reliable in electrical instruments but in air driven systems the pilot must ensure the vacuum gauge is reading correctly. A gauge reading an increased vacuum (meaning nearly 0 psi) would indicate that the system filters were dirty because the pump is evacuating the system due to the fact that air cannot come in through the filter to replace the air the pump is sucking out. This indication of a greater vacuum (almost 0psia) means the gyro rotors of all the air driven instruments would be running slower, and hence the turn and slip indicator readout would be in accurate. Loss of rigidity in the DG and AH would become apparent due to them drifting off more often, but their readouts would still be reasonable reliable, whereas the turn and bank readout would be compromised. Rotor speed is crucial in a turn and bank. No erection method is incorporated in a turn and bank because it is physically prevented from drifting off. It does have an earth rate correction in that the gimbals would be weighted appropriate to counteract the earths rotation, but this form of correction is engineered into the turn and bank at the time of manufacture.

Limitations of the Turn and Slip Indicator Turn and Slip indicator will not respond to an aircraft bank, it will only indicate a turn if a yawing motion is sensed.


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Interpretation of Rate of Turn Indications When the needle is deflected by only one needle width on a two minute turn indicator the aircraft is executing a 2 minute turn, or 180° per minute or 3° per second which is a rate 1 turn. On a four minute turn indicator the pointer must be aligned with a doghouse to execute a two minute turn. On a 4 minute turn indicator if the needle is only displaced by one needle width the aircraft is executing a 4 minute turn, or 90° per minute or 1.5° per second or a turning rate of a rate½.

Turn Indicator Rate 1: 180° per minute Rate 2: 360° per minute Rate 3: 540° per minute Rate 4: 720° per minute. A 2 minute turn is 180° per minute which is a rate 1 turn. A 4 minute turn is 90° per minute which is a rate 0.5 turn.


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Turn Indicators (AI&A pg 34 AF 11-20) The turn and bank indicator or turn and slip indicator as it is most often called, is one of the first instruments developed for instrument flying. The instrument actually combines a turn indicator and a slip indicator in the one instrument. In early days of flying the turn-and-bank, when used in conjunction with the aircraft compass made a valuable contribution to the art of IFR flying. It was thus considered the primary blind flying instrument. With developments in aircraft instrument technology the turn-and-bank has been replaced as the primary IFR instrument by the AH, although in some light aircraft the turn-and-bank is still considered a primary flight instrument. In larger aircraft the turn-and-bank has become a secondary instrument or is done away with in it’s entirety.

Turn Indicator The turn indicator is a gyroscopically based indication using a two axis, or rate gyroscope with a spring balance mechanism. Rigidity keeps the indicator in the neutral or zero position, and as the aircraft turns, the force of gyroscopic precession gives the indication of turn. The spring tension applied to the turn movement can be calibrated and this gives an indication of rate of turn. Its secondary function is to ensure a return to zero once the momentum of the turn has ceased. Turn indicators are classified as Rate 1, 2, 3, or 4. This rating indicates the number of degrees per minute the turn pointer is indicating, for example: Rate 1: 180° per minute Rate 2: 360° per minute Rate 3: 540° per minute Rate 4: 720° per minute. To perform a 2 minute turn in larger aircraft would require too much angle of bank, hence 4 minute turn indicators came into being.


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Turn Coordinators A turn coordinator is an interesting development of the turn and bank indicators just described, and is adopted in lieu of such instruments in a number of small types of general aviation aircraft. 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, thus making the gyroscope sensitive to banking of the aircraft as well as to turning. To achieve this the gyro gimbal is canted nose down about 30°. This has the effect of creating a precessive force when the aircraft displaces in roll, causing the indicator pointer to offset as soon as the aircraft is banked. As heading then changes with the yawing motion the turn coordinator then operates on the same principle as the turn indicator. Turn coordinator gimbal canted nose down about 30° and senses roll and yaw Quicker response to a turning motion More closely respond to the artificial horizons indication of a bank/roll 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 gyroscope is a DC motor operating at approximately 6,000 rev./min. In some types of turn coordinator the gyroscope may be an AC brushless motor operating at constant frequency, and supplied from a solid-state inverter housed within the instrument case. 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.


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Turn and Slip indicator will not respond to an aircraft bank, it will only indicate a turn if a yawing motion is sensed. To make the turn and slip more responsive, that is to get it to indicate when the bank is entered rather than waiting for the yawing motion to register on the gyro. The effect of indicating sooner also coordinates the turn indicator more closely with the AH so the pilots instruments complement each other during turns. Indicates rate of turn, but responds to turn more quickly than rate of turn indicator Rear view of a small aircraft as an indicator When wings aligned with horizontal index marks aircraft wings are level When wings are aligned to the turn indicator marks it indicates a 2 minute turn (2 minutes to turn 360° ) Indicates a turn as soon as the aircraft is banked More closely responds to the artificial horizons indication of a bank/roll 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, ie aircraft starts yawing.

The two instruments look to have opposite displays but in fact are indicating the same thing. To prevent any confusion with the artificial horizon turn coordinators display the annotation “NO PITCH INFORMATION” The inclinometer or slip and skid indicator is a simple mechanical instrument that consists of a ball in a liquid filled glass tube. This tube is curved and the ball reacts to gravity and centrifugal force. It is used by the pilot to coordinate turns by use of aileron and rudder control. If the pilot keeps the ball centred the aircraft is being flown in a coordinated manner, that is he is not turning too quickly for the amount of bank, which will result in the aircraft skidding out on the turn, and he does not have too much bank for his turning speed which would caused the aircraft to slip inwards and loose altitude. The ball indicates these conditions as follows. When the aircraft is turning to fast for the bank angle it will be skidding outwards on the turn Issue B: January 2008

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just like a speeding car, and 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 in. 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. The pilot can use just the turn and bank, compass and ASI to maintain controlled IR flight if he loses his AH, ie turn coordinator will indicate any roll, compass will indicate any changes in heading and the ASI will indicate increase in airspeed for a dive and decrease for a dive (or a VSI will perform the same function. This provides the pilots of light aircraft with a level of redundancy if their primary flight instrument, the AH fails. This is also why it is common in light aircraft to have the turn and bank or turn coordinator powered by one system, eg vacuum powered, and the AH and DG powered by another system, eg electrically Although electrical instruments would have an emergency supply as well, eg the battery. This is where the AC and DC gyro motors are considered, although the AC motor runs at higher speed and is generally a more efficient and accurate instrument, if the aircraft alternator fails the AC gyro motor will only continue to run if the aircraft has an inverter on board to convert the DC battery power to AC for the gyro. Whereas the DC powered gyro can be run from battery power with no additional power supply or power modifying components.


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TOPIC 13.8.2.2.3: DIRECTIONAL GYROS


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B2-13h Instruments Gyroscopic SR

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