Most gyroscopes possess two properties – rigidity in space and precession.
Unlike the compass, which seeks out the north magnetic pole, the gyro simply is aligned by the pilot to a point in space. If we align the gyro against the compass, at that precise moment the gyro will indicate the direction of the north magnetic pole. But in fact we have aligned the horizontal axis of the gyro with a point in space, a ‘star’ perhaps.
Imagine the aircraft is located at the North Pole. As the earth revolves about its axis the gyro will drift (apparent precession) at the same rate as the rotation of the earth (360 degrees in 24 hours). Our ‘star’ which is located perpendicular to the axis of the earth is stationary but the earth is revolving. To us it appears that the gyro is drifting.
If on the other hand the aircraft is located at the equator, then our alignment to the ‘star’ is parallel to the rotational axis of the earth and drift or apparent precession will not be detected.
Apparent precession at the poles of 360 degrees per 24 hours equates to 15 degrees per hour at the poles and zero at the equator. At intermediate latitudes, apparent precession will vary, the rule being:
Apparent precession = 15 deg Sine Latitude per hour, and
Decreases in the Northern Hemisphere
Increases in the Southern Hemisphere.
Unfortunately most gyros also experience ‘real precession’ caused by imperfections in the mechanical components of the instrument. Real precession will of course modify its alignment capability.
Fifteen-minute checks and appropriate adjustment by the pilot will keep simple directional gyros aligned.
------------------
Blue Hauler