The Art of the Helicopter John Watkinson - Karatunov.net

e modified if the gyro is in a moving aircraft. The track of the aircraft will have an
east/west component and movement in this direction will have to be added to the rate
at which the earth turns. This is known as transport error. The term transport wander
will also be found, but this is misleading as the effect is entirely deterministic.
A completely free gyro will suffer from three forms of error: an unpredictable error
due to non-ideal construction, and predictable errors due to the rotation of the earth
and to being transported with respect to the earth. In a helicopter the low airspeed and
relatively short flights mean that transport error is generally insignificant.
For earth-related activities such as flying, something will have to be done about drift.
The solution depends upon the application, but essentially the rigidity of the gyro is
used to offer short-term stability, whereas the long-term stability comes from elsewhere.
A gyro externally stabilized in this way is described as tied. Where the long-term stability
comes from the earth’s gravitational field, the result is called an earth gyro.
7.12 The direction indicator
The direction indicator is a gyroscopic device that is designed to overcome acceleration
errors in magnetic compasses and to be easy to read. Figure 7.15 shows the general
arrangement of a DI. The rotor axis is maintained horizontal and the shaft is mounted
in a pair of gimbals where the outer one pivots about a vertical axis. A gearing system
couples the rotation of the outer gimbal to the disc or card visible to the pilot. If the
Fig. 7.15 The DI (direction indicator) is a gyroscopic instrument in which the gyro axis is maintained horizontal.
As the aircraft changes heading, the gyro axis will remain the same and can drive a pointer to indicate the new
heading.
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