Gyroscopic Instruments Flashcards
What are the three gyroscopic instruments?
- Attitude Indicator AI (Artificial Horizon)
- Direction Indicator
- Turn Indicator or Turn Coordinator
What is a gyroscope?
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.
Rigidity
- 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.
What is rigidity dependent on?
- 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.
Precession
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.
What is precession dependent on?
- 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.
Space gyro
- Moves about all three planes
- Can align itself with a point in space e.g. a star
- Not used in aircraft instruments
Tied gyro
- Moves about all three planes
- Tied or held to one plane by and external force (airflow)
- Used for the Direction Indicator (DI)
Rate gyro
- 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)
Earth gyro
- 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)
TERDAT
- Align vertically
T (tied) D (DI)
E (earth) A (AH)
R (rate) T (turn)
How is an aircraft’s gyroscope driven?
- Electrically
OR - Suction pump
Electrically driven gyroscope system
Use small motor to drive gyro. Commonly used in light aircraft for the turn coordinator and sometimes artificial horizon.
Pneumatic driven gyroscope system
Air is sucked through the system by a vacuum pump
Disadvantages of a pneumatic (suction) system compared to electric system.
- 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
Venturi tube
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.
Rate 1 turn
- Turn coordinator and indicator
- 3 degrees per seocnd, or 360 degrees in 2 minutes
Formula to calculate AoB required to complete a rate 1 turn.
10% of TAS = 7 = AoB for rate 1 turn
Turn indicator
- 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.
Turn coordinator
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
Balance ball
- Slip/skid indicator
- Shows direction of g force
- Not powered by gyro
TI/TC errors
- 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
TI/TC serviceability checks
- 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
Direction Indicator
- 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.
Real drift (real wander) - DI
- 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
Apparent drift (apparent wander) - DI
- 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
Incorrect suction - DI
- Incorrect amount of suction, possibly leading to low rotor speed or excessive rotor speed.
- Deviates more easily
Gimbal error - DI
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.
Electric vs Suction Driven DI
- 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
DI serviceability checks
- Taxi checks
- Must not precess more than 4 degrees in 15 minutes and 16 degrees per hour.
Attitude Indicator AI
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
Pendulous system
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.
Electrical gyroscopes
- Most modern attitude indicators are electrically driven.
- Uses torque motor and levelling switch systems to replaces the usual sucking in of air.
Limitations of AH and AI
- 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.
Pendulosity
- 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
Erection error
Any movement in vanes will change what the gyroscope thinks is vertical - therefore creating a false vertical.
