Chapter 1 - Gyroscope Principles Flashcards
What are the two principal properties of a gyro?
- Rigidity in inertial space
- Precession
What are the three factors which govern gyroscopic rigidity or inertia?
- Rotor mass which is usually greatest at the rotor outer edge
- Rotor RPM
- Radius of rotor mass from gyro axle
What determines the direction of precession of a gyroscope?
90° in the direction of rotation
What does the rate of precession depend on?
- Direction of applied force
- Strength of applied force
- Gyroscopic rigidity
What are the two main types of gyroscope?
- Free gyro
- Tied gyro
Describe a rate gyro?
Inner gimbals are tied or restrained by springs so that the rate of precessing force can be controlled by the spring tension and measured
What are the normal uses of a rate gyro?
- Turn co-ordinators
- Turn and slip indicators
- Inertial platforms
Describe an Earth tied gyro?
Tied by gravity to give a reference to the surface of the Earth
Where are Earth tied gyroscopes usually found?
- Artificial horizons
- Gyro compasses
What are the methods of tying a gyroscope to Earth?
- Vacuum operated gyros
- Electrically driven gyros
- Pendulous
What defines the gyroscopes orientation?
Position of its spin axis
What is the angle between the inner and outer gimbal in straight and level flight?
90°
What information will a horizontal gyro tell you about the aircraft’s movement?
Rotation in yaw
What angular change in a vertical gyro represents the aircraft’s change in pitch?
Change in the angle between the inner and outer gimbal
What angular change in a vertical gyro represents the aircraft’s change in roll?
Change in the angle between the outer gimbal and the frame
What angular change in a horizontal gyro represents the aircraft’s change in direction?
Change in the angle between the outer gimbal and the aircraft
What is the First Law of Gyrodynamics?
If the rotor of a perfect displacement gyroscope is spinning at constant angular velocity and therefore, constant angular momentum, no matter how the frame is turned, no torque is transmitted to the rotor axis
What is the Second Law of Gyrodynamics?
If a constant torque is applied about an axis perpendicular to the spin axis of an unconstrained, symmetrical spinning body, then the spin axis will precess steadily about an axis mutually perpendicular to the spin axis and the torque axis
What might be the causes of real wander?
Bearing friction and an unbalanced rotor
What is gyroscopic resistance?
Equal and opposite force which opposes the initial torque applied to a precessing gyroscope
What is the formula which shows the relationship of rigidity with inertia, speed of rotation and precession force?
Rigidity = (SI)/F
What is the relationship between the precession formula and the rigidity formula?
One is a reciprocal of the other
Describe secondary precession?
When a torque is applied to a gyro, it initially dips at this point before it precesses
Once precession starts, friction in the gimbal bearings causes an opposing torque, which due to the ‘rule of thumb’, causes another precession and a dip through a larger angle
Describe nutation?
- Can only occur with a two degree of freedom gyro
- Can be damped to a large extent by manufacturers
- Most apparent when a gyro starts up
When can secondary precession occur?
- Can only take place when the gyro is already precessing
- Acts in the same direction as the originally applied torque
What is indicated if nutation occurs other than at start-up?
It is a fault
What is secondary precession also known as?
Nutation or nodding
How many degrees of freedom does a displacement gyro have?
Two
What is wander defined as?
Any movement of the spin axis away from the reference frame in which it is set
How is wander further resolved?
Drift - movement of the spin axis in the horizontal plane around the vertical axis
Topple - movement of the spin axis in the vertical plane around the horizontal axis
What are the two types of apparent wander?
Earth wander and transport wander
How can apparent wander be compensated for?
- Precession motors
- Small weights fitted to unbalance the gimbals
What sensing devices are used to correct topple?
Gravity or tilt switches
What can be the accuracy of topple correction methods?
As low as 1°
How does drift differ in the Northern Hemisphere to the Southern Hemisphere?
Precessional force is in the opposite direction
What is the weight called in the pendulum method of tying a vertical gyro to gravity?
Low down
What errors are associated with horizontal acceleration acting on a pendulous unit and on the vanes controlling air jets?
Acceleration forces will cause an indication of climb and vice versa
Describe electrical levelling of correction of errors in vertical gyro?
Two tilt sensitive switches, mounted at 90° to each other so that they are in the roll axis and the pitch axis
What can be the accuracy of electrical levelling of a vertical gyro?
As low as 1°
What are the torque motors usually for levelling a gyroscope?
Split phase induction motor
What problems are associated with tilt switches (using an electrolyte)?
- Longitudinal and lateral acceleration forces
- Exceeding normal limits of roll and pitch
How do gyro designers tend to ensure pendulum errors are eliminated without using a ‘bottom heavy’ solution, which also ensures fast erection of a gyro at run up?
Mechanical caging system to ensure that the gyro is held near to its normal erect position when at rest
What does roll erection cut-out involve?
Two additional curved electrolyte switches connected in series and mounted on the roll axis of the vertical gyro
When will roll (bank) erection cut-out circuits open circuit the levelling circuit?
- Turn greater than 10° of roll
- and/or lateral centrifugal acceleration greater than approximately 0.18g
How does the roll (bank) erection cut-out work?
When the turn is greater than 10°, or lateral acceleration of 0.18g, the electrolyte in the roll switch for that particular movement moves away from the contacts and goes open circuit, thereby removing the AC supply which would feed the servo motor
When will gimbal lock occur?
When the two gimbals orientation is such that the spin axis is in line with an axis of freedom
Where a gyro displays gimbal lock, what will any attempted movement about the lost axis result in?
Real wander, often referred to as toppling
How is gimbal lock prevented?
Stops are fitted between the inner and outer gimbals at approximately 85° climb or dive
In a horizontal gyro, where is the drift maximum?
At the poles
How is drift in a horizontal gyro compensated?
The gyro is precessed at a rate of 15° sin latitude / hour
How many degrees per hour is a horizontal gyro precessed at the North Pole?
15° / hour
How is transport wander actually compensated for in a vertical gyro?
These are Earth tied
In a pneumatic powered vertical gyro, how are acceleration and deceleration force compensated for?
By inclining the spin axis from the vertical
The instrument indications are adjusted on assembly to indicate a level flight datum
What happens to a vertical gyro if it precessed during a banked turn?
Once in straight and level flight, the error in roll transfers to become an error in pitch
How many degrees of freedom does a rate gyro have?
One axis
What is the deflection of the spin axis in a rate gyro a measure of?
Measure of the rate of turning
When does movement of a rate gyro cease?
When the precessional force is equal and opposite to the restraining force
Describe the intergrating gyro?
Single degree of freedom gyro using viscous (liquid) restraint to damp the precessional rotation about the output axis
What are the advantages of the integrating gyro over the displacement gyros?
- Gyro does not suffer nutation
- Small input rate causes a large gimbal deflection
What additional purpose does the damping liquid have in an integrating gyro?
Supports the weight of the inner gimbal so reducing bearing torques
What can the gimbal gain of an integrating gyro be made to be by design?
1:10
What are RLGs made of?
Triangular block of Zerodur
What is the main property of Zerodur?
Temperature stable glass
How does a RLG detect movement?
When stationary, the two beams of the RLG have the same frequency and no signal is produced
When rotation occurs about the axis of the RLG, the two beams travel different lengths, resulting in the frequencies of both beams changing in opposite senses
At the detector, a fringe pattern is produced whereby light and dark bands are seen
Counter circuit calculates interference fringe, which produces a digital readout of angular movement
What is the relationship of accuracy and length of beam?
Greater the length of the laser path, the greater will be the accuracy of the RLG
What is used to counter any thermal expansion of a RLG?
Cavity path length control circuit (CPLC) controls in/out movement of the mirrors via a piezoelectric transducer to maintain the optical path length
What is the major disadvantage of RLGs?
Inability to detect low rotation rates about its sensitive axis
How is the dead band region overcome?
Mechanical dither system which oscillates the unit about its input axis
How many RLGs will a Laser Reference System (LRS) contain?
Three RLGs mounted on a fixed, rigid platform
What is a ‘triad’?
Three orthogonal RLGs of square format manufactured in a single monolithic construction of the low expansion glass ceramic
How is the rotor speed in a pneumatic system controlled?
Reduction or increase in the vacuum system
In AC powered gyroscopes, how is the maximum gyroscopic effect obtained?
The rotor is on the outside of the motor
What kind of motor might an AC powered gyroscope be?
Squirrel cage, hysteresis or a combination of them both
What is the rotor speed of an AC powered gyroscope determined by?
The frequency of the supply
How can a slower rate of rotation be achieved with an AC powered gyroscope?
Using multiple poles
What does high speed allow in gyroscopes?
Enables rigidity to be achieved with relatively small rotors
In a DC powered electrical gyro, what happens when the power to the gyro is switched on?
The governor contacts in the armature are closed, short circuiting the resistor, allowing the maximum voltage to the motor
In a DC powered electrical gyro, what happens when the gyro reaches its operational speed?
The governor contacts in the armature are opened, bringing the resistor in series with the armature, causing a voltage drop at the motor, slowing it down
In rate gyros, what would be the indicating error if the rotational speed is less than the normal operational speed?
Under read
Why do some manufacturers recommend that their gyros are not caged during shipment?
Whilst caged, shocks could transfer force to the assembly and cause damage
How is the caging device of some types of vacuum-operated gyro horizons actuated?
By a screw inserted through the bottom of the case
Prior to installation on the aircraft of a caged gyro, where the cage is actuated by a screw, what must be done?
The screw must be removed, retained for future use, and a shorter length blanking screw inserted in its place
