Gyroscopic Instruments ATC Chapter 14

Aim To review principals of operation of the gyroscopic instruments

Objectives Describe the gyroscopic principles Describe the power sources of the gyroscopic system State what each instrument indicates and name the power source for each instrument State the effect of system failures on instrument indications

1. Gyroscopic Principles The Gyroscope Any spinning object exhibits gyroscopic properties. A gyroscope is a spinning wheel (or rotor) mounted in a special frame (gimbal) so that its axis is unrestrained in one or more planes. Its properties are useful in indicating changes to direction and attitude. There are two fundamental properties of gyroscopic action Rigidity in space Precession

1. Gyroscopic Principles Rigidity in space Rigidity in space refers to the principle that a gyroscope remains in a fixed position in the plane in which it is spinning. Rigidity is dependant on three factors: The mass of the rotor The speed of the rotation The distance of the mass from the centre of the rotor. The mass and radius of the rotor determine its moment of inertia. Rigidity   where: Moment of inertia  Angular velocity Rigidity is the property of the gyro that allows the gyro to be used as an independent stable reference.

1. Gyroscopic Principles Precession Precession is the tilting or turning of a gyro in response to a deflective force. The reaction to this force does not occur at the point at which it was applied; rather, it occurs at a point that is 90° later in the direction of rotation. This principle allows the gyro to determine a rate of turn by sensing the amount of pressure created by a change in direction. The amount and rate of precession is dependant on three factors: The magnitude and direction of the applied force The moment of inertia of the rotor The angular velocity of the rotor Rate of precession    where:  Torque applied Moment of inertia  Angular velocity

1. Gyroscopic Principles Terminology Drift - is used to describe any movement of the gyro spin axis away from a chosen datum. Real drift - describes a failure of the gyro to hold its space datum due to friction in the gimbal bearings. Apparent drift - describes drift when comparing the direction of the gyro axis with a datum that is itself not fixed. Topple - any movement of the gyro axis in the vertical plane. When a gimbal limit stop is reached, severe and usually rapid misalignment occurs which is also termed topple. Caging - indicates the gyro gimbals are clamped so it can be re-erected. When uncaged, the gyro should work again.

2. Power Sources to the Gyroscope Gyroscope Rotor Drive Sources of power for the gyros include the vane-type engine-driven vacuum pump, vacuum pressure by a venturi tube and electrically powered gyroscopes Most aircraft have at least two sources of power to ensure at least one source of bank information is available if one power source fails (redundancy) Commonly vacuum or pressure systems provide the power for the heading and attitude indicators, while the electrical system provides the power for the turn coordinator

2. Power Sources to the Gyroscope The Engine-Driven Vacuum Pump A typical vacuum system consists of an engine-driven vacuum pump, relief valve, air filter, gauge, and tubing necessary to complete the connections. The gauge is mounted in the aircraft’s instrument panel and indicates the amount of pressure in the system.

2. Power Sources to the Gyroscope The Engine-Driven Vacuum Pump The vacuum or pressure system spins the gyro by drawing a stream of air against the rotor vanes to spin the rotor at high speed. The amount of vacuum or pressure required for instrument operation varies according to aircraft type, but is usually between 4.5 "Hg and 5.5 "Hg, see the aircraft PoH

2. Power Sources to the Gyroscope Vacuum Pressure (Suction) by a Venturi Tube Some aircraft (typically older aircraft such as the tiger moth) have an externally mounted venturi tube to provide vacuum power in place of the engine-driven vacuum pump When air flows through the venturi tube and speeds up due to the shape of the venturi, the static pressure decreases (Bernoulli’s principle). The low pressure area is connected to the pressure instruments to spin the gyroscopes as in the engine-driven system Disadvantages of this system include: The requirement of sufficient aircraft speed to activate the venturi effect The time required to spin up the gyroscopes Parasite drag Susceptibility to icing

2. Power Sources to the Gyroscope Electrically Powered Gyroscopes The rotor of a gyroscope can be built as the armature of an electric motor Regulated DC power will spin the rotor at the design rpm On light aircraft, it is usually only the turn indicator or turn coordinator that is electrically powered Advantages: Self contained and therefore free of dust and moisture entering the system reducing lifespan Does not require operation of engine Improved rigidity (higher inertial moment and RPM) Does not rely of atmospheric pressure (reduced at high altitude) Constant RPM – predictable rate of precession

3. Instruments indications and source Artificial Horizon (AH or AI) Sometimes referred to as the master instrument. Indicates both pitch and bank attitude directly (in miniature) against the artificial horizon. Miniaturization of the outside world means that small movements of the AH represent quite large changes in pitch and bank attitudes. Indirectly, the AH is a guide to airspeed Nose low, high or increasing airspeed Nose high, low or decreasing airspeed Most commonly driven by an engine driven vacuum pump but can be electrically driven or venturi driven.

3. Instruments indications and source Artificial Horizon (AH or AI) Tied gyro - the AI uses a (axis is kept vertical) to which the rotor axis is aligned to the centre of the earth. This is termed an earth gyro. Erection System – To tie the axis to the vertical, the system senses gravity most commonly by use of a pendulous unit on the bottom of the rotor casing. It functions in two ways: The mass causes the gyro to rest with its vertical axis The pendulous vanes act to keep the rotor axis vertical after vacuum is applied and to precess the axis when it is displaced/toppled.

3. Instruments indications and source Artificial Horizon (AH or AI) Gimbal Limits – the AI is free in three axes but is limited by physical stops on at least one axis. The pitch and bank limits depend upon the make and model of the instrument. Limits in the banking plane are usually from 100° to 110°, and the pitch limits are usually from 60° to 70°. If either limit is exceeded, the instrument will topple. A number of modern attitude indicators do not topple. Caging – To hold the gyro gimbals locked at 90° so to re-erect the rotor axis. Not all gyros have caging functions. The erection system will re-establish reference but is usually slow with a rate of 8° per minute for vacuum driven AIs. Standby Attitude Indicator – Some aircraft are fitted with a second AI should a primary system fail. It is independent and usually powered by 26 VAC 400Hz 3 phase from an inverter connected to the battery.

3. Instruments indications and source Artificial Horizon (AH or AI) Errors Acceleration Error – the error induced by aircraft accelerations. It is a small but noticeable error, which is more obvious on vacuum powered indicators. Roll Effect – the mass at the bottom of the gyro tends to lag. This applies a force to the inner gimbal that is transferred to the rotor, resulting in precession at 90° to the direction of lag. The result of this precession is a false indication of right bank. Pitch Effect – the pendulous vanes lag producing a force on the side of the inner gimbal that results in a precession that causes the horizon indicator to move down. The false indication is a climb. Turn Error – During turning, the mass at the bottom of the gyro tends to continue in its original direction. The pendulous vanes sense gravity and turning forces and an error is induced. Error magnitude is dependant on aircraft speed, rate of turn and turn time length. The pendulous vanes are modified and the erection rate is kept low to minimise the turn error.

3. Instruments indications and source Heading Indicator (HI) The HI is a mechanical instrument designed to facilitate the use of the magnetic compass. It is not affected by the forces that make the magnetic compass difficult to interpret. The operation of the HI depends upon the principle of rigidity in space. The direction information is not valid until the pilot selects a datum. Because of precession and drift errors, the heading indicator creeps or drifts from the heading to which it is set. The direction must be reset about every 15 minutes and completed during S&L constant speed flight. Indirectly it indicates Bank angle: HI decreasing, left turn HI increasing, right turn

Earth rate ( per hour) = 15 sin (latitude) 3. Instruments indications and source Heading Indicator (HI) Errors Real Drift – is caused by gimbal bearing friction and imbalance. Gimballing Error – is caused by combined pitch and roll movements/accelerations and contributes to real drift. Apparent Drift due to Earth Rotation – is due to the Earth’s rotation and its orbit around the sun when compared to the gyro being aligned to a point in space. Earth rate ( per hour) = 15 sin (latitude)

3. Instruments indications and source Heading Indicator (HI) Errors Apparent drift - can be compensated for by use of the latitude nut. It is fitted to the inner gimbal and applies a torque to appose the earth rate error at that latitude. Flights north or south of the reference latitude will cause drift. Flights east or west will induce transport error. Transport Error – when the aircraft is flying east or west (with or against the earth’s rotation), the gyro is not compensated for this new rate.

3. Instruments indications and source Flux Gate Compass System The flux gate compass system drives slaved gyros via the characteristic of current induction for heading information. The flux valve is a small, segmented ring made of soft iron that readily accepts lines of magnetic flux. Via a synchro, heading information is relayed to improvements to the HI – Radio Magnetic Indicator (RMI) or Horizontal Situation Indicator (HSI).

3. Instruments indications and source Remote Indicating Compass Three components – pictorial navigation indicator (HSI/RMI), slaving control and compensator unit. The slaving meter indicates the difference between the displayed heading and the magnetic heading. The magnetic slaving transmitter containing the flux valve is mounted remotely, usually in a wingtip to eliminate the possibility of magnetic interference. The slaving control and compensator unit has a push button that provides a means of selecting either the “slaved gyro” or “free gyro” mode.

3. Instruments indications and source Turn Indicator Directly indicates the rate of change of direction. Indirectly it can indicate limited angles of bank usually to 35 degrees. Usually powered by an electrically driven Gyro. Standard rates of turn: Rate 1 = 180°/min or 3°/s Rate 2 = 360°/min or 6°/s Rate 3 = 540°/min or 9°/s

Underspeed causes underread. Overspeed causes overread. 3. Instruments indications and source Turn Indicator (TC) Errors Variation in Rotor Speed – Rigidity is proportional to rotor speed (angular velocity) and therefore changes to rotor speed effect the rate of precession. Underspeed causes underread. Overspeed causes overread. Variation in TAS – If aircraft TAS is different to the calibrated TAS, an error will result. The alignment of the gimbal relative to the horizontal is affected by the angle of bank thus moving the sensitive axis of the rate gyro away from the vertical. Looping Error – If the aircraft pitches during a turn, an error will result in the form of an increased rate of turn indication.

3. Instruments indications and source Turn Coordinator (TC) Development of the turn indicator in which the gimbal is tilted upwards by about 37° allowing sensitivity to roll as well as yaw. The advantage of the TC is its immediate response to roll and yaw and its presentation. Is utilised by many autopilots systems for sensing and control for single-axis (wing leveller) autopilot function. Incorporate ‘off’ flag or warning light to indicate power failure (electrical).

3. Instrument Indications 3. Instruments indications and source 3. Instrument Indications Balance Indicator Directly indicates balance. Usually incorporated with the turn co-ordinator. Indirectly indicates aircraft yaw. Powered by gravity. Slipping Turn Skidding Turn Co-ordinated Turn

3. Instruments indications and source G1000 Directly indicates all of the above parameters. Information is generated by the air data computer, AHRS and magnetometer. All indications are displayed on the PFD and MFD. Attitude and Heading Reference System (AHRS) – Attitude information is derived from solid-state laser systems that are capable of flight at any attitude without tumbling. The heading information is derived from a magnetometer which senses the earth’s lines of magnetic flux.

4. System failures Vacuum Failure Instruments affected Artificial horizon Direction Gyro Indicated by: low VAC pressure reading Annunciator Troubleshooted by: Can get false indications at low RPM

4. System failures Vacuum Failure Artificial Horizon Failure indicted by: The gyro toppling, may happen over an extended period of time as the gyro slows down Red warning flag Direction Gyro Failure indicted by: Red warning flag Inaccurate readings, check against compass

4. System failures Electrical Failure G1000 Failure indicted by: Red X through affected instruments If complete failure occurs the screens may cease to function Turn coordinator Failure indicted by: Red warning flag Note: The balance ball will still be functional

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