Gyroscopes 3
will produce an initial roll error, which changes to pitch error as the aircraft turns through 90º. In addition, the pendulous lower part of the gyro will try to line up with the resultant acceleration, but this effect will be precessed through 90°, and will give an initial pitch error that changes to roll error as the turn continues. The combined effect of these is that the gyro axis describes a wobbly circle during a 360° turn, offset a few degrees from its correct position.
Gyro Erection Air driven artificial horizons are made pendulous, with their centre of gravity below the suspension point, so that they settle in their gimbals in a nearly erect position when not working, to reduce erection time on start-up. Once the gyro is rotating, simple pendulosity does not help with erection, and the gyro has its spin axis tied to the Earth vertical by a system of pendulous vanes and air jets which make the gyro precess back to the vertical if it is displaced.
These errors and the correction system only apply to air driven AHs
Pendulosity is only there to help before start. After start it induces unwanted errors
After 90º the error will be nose up and bank under reading, after 180º of turn the bank angle will be correct and the maximum pitch up error exists. After 270º the pitch error has reduced but is still present but now the bank over reads. If the turn were continued onto the original heading the errors would be zero. These AH erection errors are called turning errors, figure 3.12.
At the bottom of the rotor case are four air exhaust vents, each normally half covered by a flap, a pendulous vane, which is hinged at the top. When the gyro is vertical, air escapes from all four vents equally. When it is displaced from the vertical, one vent will be closed as the pendulous vane covers the vent and another will be opened. The now unbalanced reaction from the air vents is precessed by 90° in the direction of rotation, to restore the spin axis to the vertical.
Figure 3.12 Some instruments have the vanes adjusted to keep the gyro offset half the expected maximum turning error (typically a 2° to 2½° offset) from the true vertical when erect, to minimise any turning error. This correction system only works for one specified rate of turn, usually rate 1, and one set TAS, typically 250 KT. This system is called compensation tilt.
Figure 3.11
Erection Errors The gyro will be affected by any false indication of the vertical, caused by lateral acceleration in turns or by aircraft acceleration and deceleration. In a turn the erection system will try to erect the gyro to the resultant acceleration, which in a balanced turn will be the aircraft vertical. This Instrumentation
The pendulous vanes of the erection system are displaced by sustained fore-and aft acceleration, for instance on take-off, when a false nose up indication is given. Acceleration also affects the v6.1.5
3.9