Gyroscopic Instruments

Question 1

What are the three gyroscopic instruments?

Answer 1

• Attitude Indicator AI (Artificial Horizon)
• Direction Indicator
• Turn Indicator or Turn Coordinator

Question 2

What is a gyroscope?

Answer 2

A spinning mass that is mounted in a case (gimbals) which gives it freedom to move on all three planes.

Spinning part = rotor

Rotor remains in the same position which gimbals freely move around the rotor.

Question 3

Rigidity

Answer 3

• Gyroscopes = heavy for their size, and spin at a high rate.
• Able to maintain their alignment with the plane of rotation.
• Allows the gyroscope to maintain its fixed alignment in space so that it is not easily moved by an external force.

Question 4

What is rigidity dependent on?

Answer 4

• Mass - the greater the mass, the greater the rigidity of the rotor
• RPM or rotaional velocity - the faster the spinning or rotaional velocity of the rotor, the greater the rigidity
• Radius - the greater the radius of the rotor, the greater the rigidity.

Question 5

Precession

Answer 5

When an external force applied to a gyroscope is felt 90 degrees further on in the plane of rotation fro where the force was applied.

Question 6

What is precession dependent on?

Answer 6

• Rotational velocity of the gyro (RPM) - the greater the RPM, the rate of precession will be less. RPM and rate of precession are inversely proportional.
• Angle of the external force - If the external force is applied at a right angle to the plane of rotation, the precession will be maximum. If the force is applied at a lower anle the rate of precession will be less.
• Strength of external force - the greater the strength, the greater the rate of precession.

Question 7

Space gyro

Answer 7

• Moves about all three planes
• Can align itself with a point in space e.g. a star
• Not used in aircraft instruments

Question 8

Tied gyro

Answer 8

• Moves about all three planes
• Tied or held to one plane by and external force (airflow)
• Used for the Direction Indicator (DI)

Question 9

Rate gyro

Answer 9

• Moves about two planes
• Uses any movement about the third plane to measure the rate of change.
• Helpful for a Turn Coordinator and Turn Indicator (measure rate of turn)

Question 10

Earth gyro

Answer 10

• Moves about all three planes
• Held to one plane by the force of gravity
• Spin axis always points to center of earth
• Used on the Artificial Horizon (AH)/Attitude Indicator (AI)

Question 11

TERDAT

Answer 11

• Align vertically
  T (tied) D (DI)
  E (earth) A (AH)
  R (rate) T (turn)

Question 12

How is an aircraft’s gyroscope driven?

Answer 12

• Electrically
  OR
• Suction pump

Question 13

Electrically driven gyroscope system

Answer 13

Use small motor to drive gyro. Commonly used in light aircraft for the turn coordinator and sometimes artificial horizon.

Question 14

Pneumatic driven gyroscope system

Answer 14

Air is sucked through the system by a vacuum pump

Question 15

Disadvantages of a pneumatic (suction) system compared to electric system.

Answer 15

• More maintenance compared to electric
• Electric system spins gyro at a higher velocity, greater accuracy and larger limits
• Electrical system doesn’t rely on the engine

Question 16

Venturi tube

Answer 16

As the aircraft flies, the air passing through the venturi accelerates, increasing the dynamic pressure and reducing the static pressure at the throat of the venturi.

• Prone to icing
• Suction redcues with an increase in altitude.

Question 17

Rate 1 turn

Answer 17

• Turn coordinator and indicator
• 3 degrees per seocnd, or 360 degrees in 2 minutes

Question 18

Formula to calculate AoB required to complete a rate 1 turn.

Answer 18

10% of TAS = 7 = AoB for rate 1 turn

Question 19

Turn indicator

Answer 19

• Rate gyro
• Movement in one axis other than the plane of rotation
• Horizontal spin axis - vertical plane of rotation
• Measures the rate of movement about the normal axis

As the a/c turns, force felt on gyro, springs balance precession force. Movement is fed through mechanisms to the indicator bar on the instrument.

Question 20

Turn coordinator

Answer 20

Same information as turn indicator, presented differently.

• Electrically driven

Gyro gimbal is set at 30 degrees to the longitudinal axis, whereas TI is parallel to the longitudinal axis.

• Does not directly indicate AoB

Question 21

Balance ball

Answer 21

• Slip/skid indicator
• Shows direction of g force
• Not powered by gyro

Question 22

TI/TC errors

Answer 22

• Incorrect rotor speed - A decrease in rotor RPM makes the TI under-read, and vice versa.
• Pitch and yaw - Will over-read if subjected to g-force. If turn is not balanced, instrument will not read accurately

Question 23

TI/TC serviceability checks

Answer 23

• Taxi checks
• Not visible algs on instruments
• Check gyro rotation
• Balance ball centered when on level ground
• Timing of balamced rate one turn - 2 mins for 360 degrees or 1 min for 180 degrees

Question 24

Direction Indicator

Answer 24

• Overcomes errors associated with the compass.
• In most light aircraft
• Uses tied gyro
• Movement about all three planes, motion is tied in one plane

Outer gimbals allow movement around the normal axis and the inner gimbals allow movement about the longitudinal axis.

Question 25

Real drift (real wander) - DI

Answer 25

- Caused by firctional forces, wear and tear - Slowly indicates the incorrect heading - Maximum deviation allowed = 4 degrees per 15 minutes or 16 degrees per hour

Question 26

Apparent drift (apparent wander) - DI

Answer 26

- Due to rotation of the Earth - Over time, the DI will no longer be aligned with magnetic north - At poles, you'll experience apparent drift of 15 degrees - At equator, you'll experiecne apparent drift of 0 degrees

Question 27

Incorrect suction - DI

Answer 27

- Incorrect amount of suction, possibly leading to low rotor speed or excessive rotor speed. - Deviates more easily

Question 28

Gimbal error - DI

Answer 28

When the axis of the outer gimbal is not aligned with the true vertival axis passing through the centre of the rotor. Whenever the aircraft is banked and/or pitching up or down. - WIll under-read or over-read Gimbal error increases with an increase of bank angle or pitvh angle, and that it is negligible when flying straight and level.

Question 29

Electric vs Suction Driven DI

Answer 29

- Some DI are electrically driven - Electrically driven has a higher gyro RPM - greater rigidity - More freedom in pitch and roll before toppling - Less wear and tear - Better and faster systems

Question 30

DI serviceability checks

Answer 30

- Taxi checks - Must not precess more than 4 degrees in 15 minutes and 16 degrees per hour.

Question 31

Attitude Indicator AI

Answer 31

Allows pilot to see attitude and bank angle of the aircraft in relation to the horizon. Gyro lies flat or horizontal, meaning its plane of rotation is horizontal and its spin axis is vertical. - Uses Earth gyro - moves about all three planes

Question 32

Pendulous system

Answer 32

Attached to the inner gimbal case and is hung penduously. Unit maintains the true vertival by utilising exhause ports. Air used from spinning the gyro is exhausted from ports, precessed at 90 degrees in the diretion of rotation and moves the pendulous unit back to the true vertical.

Question 33

Electrical gyroscopes

Answer 33

- Most modern attitude indicators are electrically driven. - Uses torque motor and levelling switch systems to replaces the usual sucking in of air.

Question 34

Limitations of AH and AI

Answer 34

- Prone to toppling (AH) - Suction driven AI has has pitch limitation of 55 degrees and roll limit of 90 degrees and a roll limit of 60 and 110 degrees. - Electrically driven AI has a pitch limit of 85 degrees abd no roll limit.

Question 35

Pendulosity

Answer 35

- Occurs due to the low centre of gravity in the pendulous unit - Lags upon acceleration - acceleration error Errors that may be caused: Turn to the right - nose up Turn to the left - nose down Straight acceleration - climbing right turn Straight deceleration - descending left turn

Question 36

Erection error

Answer 36

Any movement in vanes will change what the gyroscope thinks is vertical - therefore creating a false vertical.