The Art of the Helicopter John Watkinson - Karatunov.net

280 The Art of the Helicopter
instrument is turned bodily about a vertical axis, the rigidity of the gyro will keep the
gimbals fixed, and so the action of moving the instrument case around them will rotate
the scale and indicate the angle through which the unit has turned.
The gyro axis is maintained horizontal by tying the gyro to the vertical gimbal ring.
In an air-driven gyro there is already a stream of exhaust air leaving the rotor shroud.
In an electrically operated gyro the rotor can be designed to act as a blower to produce
an air jet. The air stream from the shroud impinges upon a wedge-shaped plate on the
vertical gimbal ring. If the gyro axis (or the horizontal gimbal ring) is not at right angles
to the vertical ring, the airflow will produce an imbalanced force on the vertical ring,
tending to turn it. The direction of this force is such that it will cause the gyroscope
slowly to precess in a direction that will bring the inner ring to rest at 90 ◦ to the outer
or vertical ring.
As a result the gyro axis is aligned with the long-term average attitude of the helicopter
about the roll axis. Compared to the slow rate of precession in the tying process,
manoeuvres of the helicopter are very rapid and average out. In flight a turn would
be accompanied by banking, but the gyro would maintain its axis because the inner
gimbal can pivot in the outer.
Although more accurate in a manoeuvre than a magnetic compass, the DI does suffer
from minor geometric errors. The outer gimbal has two sets of bearings at 90 ◦ , and
essentially is acting as a Hooke joint between the gyro and the display card. As has
been seen elsewhere in this volume, the Hooke joint is not a constant velocity joint,
but suffers from cyclic angular errors when the input and output axes are not parallel.
The result is that the DI reading is slightly out due to what is called gimballing error.It
should be pointed out that gimballing error is very small and disappears when straight
and level flight resumes.
The occurrence of gimballing error depends on the orientation of the gyro axis with
respect to the helicopter’s yaw axis. In the general case where the gyro axis is not
transverse to the flight path, a roll will cause gimballing error. In the special case where
the gyro axis is transverse, a combined roll and pitch change will be needed to cause a
gimballing error.
Figure 7.14 showed that with the DI gyro axis horizontal, the rotation of the earth
causes apparent drift at a rate given by multiplying the earth rate, 15 ◦ /hr, by the sine
of the latitude. For example, on the south coast of England at Lat. 51 ◦ N there will be
a DI earth drift rate of just under 10 ◦ /hr.
In some units it is possible to compensate the DI for earth rate mechanically. The inner
gimbal ring is deliberately imbalanced by a nut on an arm so that gravity produces a
slight rolling torque on the gyro. The gyro will precess this into a yaw and when correctly
adjusted the rate of yaw will be equal to the earth rate for the latitude at which the
machine is normally operated so that the drift is cancelled. In more sophisticated DIs,
there may be an adjustable earth rate compensator having a control which is set to the
current latitude and a switch to select the sense of the compensation for the appropriate
hemisphere. A latitude-dependent current is generated and sent to a torque motor in
the DI fitted between the inner and outer rings. This applies torque to roll the inner
ring, but the gyro precesses this into yaw. If the correct torque is applied for the latitude,
the resultant precession rate will equal the earth rate and the DI drift will be minimal.
In principle if the groundspeed, latitude and heading are known transport error can
also be cancelled. This may be useful for flying near the magnetic poles.
However, most DIs rely on periodically being reset to the same reading as the magnetic
compass and are fitted with a control knob which is pushed in and turned to force
the gyro round to the correct heading. Pushing in the knob also forces the gyro axis to
be at right angles to the vertical ring, a process known as caging. Clearly the machine