The Art of the Helicopter John Watkinson - Karatunov.net

278 The Art of the Helicopter
information and assessing the attitude of the machine from the view of the world outside.
In poor conditions or at night the horizon may be indistinct and without attitude
clues even the finest pilot could lose control owing to disorientation. The gyroscopic
instruments provide sufficient clues that in conjunction with the other flight instruments
the machine can be flown without looking outside at all. Whilst the instruments
are optional in a VFR light helicopter, most have them fitted and it is worth knowing
how they operate as they could make a significant difference if an unanticipated
deterioration in the weather takes place or if cloud is inadvertently entered.
The basic principle of the gyroscope was considered in Chapter 2. In practical instruments
the gyro rotor is spun by an air jet or by an electric motor. Many light planes
have a venturi in the slipstream that generates suction. This is applied to the case of
the gyro instrument. Filtered cabin air is drawn into the instrument through a nozzle
by the suction and blows the rotor round. Clearly this arrangement is not suitable for
a helicopter as there is not necessarily a slipstream. A further alternative is to have a
vacuum pump driven by the engine. The solution adopted in most helicopters is to drive
the gyroscopes electrically. Power is fed through the gimbals by miniature sliprings.
As the operation of the gyro is dependent on the rotational speed, many gyros, but
not all, have a flag coloured red or orange which swings into view if the rotor is not
running fast enough. No flight action should be based on the reading of an instrument
showing a flag.
If a gyroscope is mounted in gimbals and set running with the rotor axis parallel to the
earth’s surface, it will not remain in this condition due to drift. Drift has two unrelated
components. First, the gyroscope is built to finite accuracy and minute imbalances
in the gimbals and bearing friction will result in the spin axis changing slowly in an
unpredictable manner; a genuine drift. Second, as the earth is turning at 15 ◦ per hour
the gyro axis fixed in space will display apparent drift.
Figure 7.14 shows that the effect of the earth’s rotation on apparent drift is a function
of latitude. At the equator the gyro axis and the earth’s axis are parallel and no drift
is observed. However, at the geographic pole, the full 15 ◦ /hr will be observed. The
apparent drift due to earth rotation is a sinusoidal function of latitude. The drift may
Fig. 7.14 Apparent drift in a gyroscope is not a random effect but is due to the earth’s rotation. At the poles
the full rotational rate of the earth (15 ◦ /hr) will be apparent. This falls to zero at the equator as a sinusoidal
function of latitude.