Alignment of Gyroscopes
Orientation of a gyro in space can be defined by specifying the alignment of the axis of rotation.
A vertical gyro has its spin axis in the earth vertical.
A horizontal gyro has its axis in the earth horizontal.
However earth horizontal is a plane and not a line so this does not give sufficient information to fully define the axis.
To fully define we have to add the azimuth information.
e.g. A horizontal gyro with its axis aligned with true north.
The rigidity of a gyro system will tend to keep the spin axis fixed in space.
Any movement away from this fixed direction is called wander.
Gyro wander can be either drift or topple.
These are earth references.
Gyro drift occurs when the spin axis turns in the earth horizontal plane.
Gyro topple occurs when the axis tilts in any earth vertical plane.
Whenever the gyro spin axis moves away from its initial defined orientation in space the gyro is said to suffer from real wander.
Real wander can be deliberately induced by applying an external correcting force e.g. alignment of tied gyros.
Real wander can be caused by imperfections in the gyroscope, unbalanced gimbals or bearing friction.
A perfect gyro with no external forces acting on it will not suffer from real wander.
Having said that perfect gyros do not suffer from real wander there are many occasions when they appear to.
This is because our orientation in space has changed while the gyro's orientation has not.
This is apparent wander.
A Horizontal gyro at the equator with its axis aligned to the local meridian shows no apparent drift as it is carried round on the rotating earth.
A Horizontal gyro at the North Pole shows an apparent drift of 15° per hour as the earth rotates under it.
The apparent drift (zero at equator and 15° per hour at poles) is a function of Latitude.
So Apparent Drift = 15 x Sin (Latitude)° per hour.
A vertical gyro at the equator appears to become a horizontal gyro then becomes a vertical gyro again as it is carried round on the rotating earth.
It is showing apparent topple at a rate of 15° per hour.
A Vertical gyro at the North Pole shows no apparent topple as the earth rotates under it.
The apparent topple (zero at poles and 15°/hr at equator) is a function of Latitude.
So Apparent Topple = 15 x Cos (Latitude)° per hour.
A gyro which begins as a horizontal gyro aligned with the local meridian at an intermediate Latitude shows both apparent drift and apparent topple as it is carried around on the rotating earth.
Its apparent drift is 15 x Sin (Lat) and its apparent topple is 15 x Cos (Lat) per hour.
If a gyro is aligned to north on one part of the earth and then moved to another it will be out of alignment.
This is due to convergency between the two points.
This is a form of apparent drift and is called "Transport Wander".
Flights north or south produce no transport drift but will affect the total apparent drift (due to latitude change).
Flights to the east increase the total apparent drift (in the Northern Hemisphere) and those to the west will reduce it.