The Art of the Helicopter John Watkinson - Karatunov.net

arranged on opposite sides of the pendulum so that if the pendulum swings one way
the area of one orifice is reduced whilst that of the other is increased; the situation is
reversed if it swings the other way.
If the pendulum is central, the two orifices have equal areas revealed, and the thrust
from the air jets will cancel. Should the gyro tilt with respect to gravity the pendulum
will swing away from the centre position and the jet reaction will no longer balance. The
net reaction is at right angles to the angular error, but this is exactly what is necessary
as it causes the gyro to precess and right itself. As there are two orthogonal systems
working simultaneously, the gyroscope maintains itself along the earth’s gravitational
field. The erection system needs only to be able to work somewhat faster than the
earth rate, and so the air jets do not need to be very powerful. In fact it is better if the
erection process is slow because then momentary disturbances of the pendula caused
by manoeuvres and turbulence are simply too rapid for the gravity-sensing system to
respond to and they are filtered out. In some gyros the earth tying is done by tilt switches
that operate torque motors acting on the gimbals. In more sophisticated instruments
the erection process is switched off if the tilt switches tilt by more than a few degrees
as this must be due to a manoeuvre.
As the gravity sensing is so subtle, a gyro takes about ten minutes to settle if started
up with random orientation. In order to speed the process, the instrument is fitted with
a caging knob that is operated when the aircraft is level. This operates a system of cams
to force the gimbals back to the approximately correct attitude. When the cams retract,
the gravity sensing finishes the job. The caging knob should not be operated when the
machine is not level as a false attitude will be indicated. Gyros with tilt switch erection
systems may have a fast erection mode for use on the ground or in level flight in which
the precession rate is increased.
Figure 7.18 shows the general arrangement of the artificial horizon. The outer gimbal
is pivoted on the roll axis of the aircraft and the inner gimbal is pivoted on the pitch axis.
The gyro axis is vertical. Movement of the inner gimbal relative to the outer operates
Fig. 7.18 In an artificial horizon, a gravity-sensing gyroscope (a) is mounted on a forked gimbal (b), which is
free to turn in the roll axis bearing (c). If the machine rolls, the horizon on card (d) will remain horizontal. If the
machine pitches, the card and gyro will remain vertical, but the card will move in the viewing window with
respect to the symbolic ‘wings’ (e). The crank (f) reverses the sense of the card movement so that when the
machine dives the ‘ground’ on the card moves up.
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