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The rate of precession varies with
rigidity – the greater it is, the slower
the precession will be. When a gyro
is made to move from its preset
position through precession, it is
said to wander. When the movement
comes from an applied force, it is
called real wander. Unwanted real
wander, or random wander, may arise
from bearing friction or imbalance.
Turbulence alone will not cause
precession, but it will enhance the
effects of random wander.
Apparent wander comes from the
Earth's own movement – in other
words, assuming it stayed in one
place, the gyro may still be pointing
in the same direction, but at a
different part of the Earth after a
short time, because it is spinning as
well. It is a very similar thing to
convergency – if the gyro is aligned
with a meridian, it will be at a
different angle with respect to any
new one:
The rate can be calculated (see
Heading Indicator, below). Things get
more complicated if the gyro is in an
aircraft which itself is moving. The
angle between the axis and the new
meridian will be larger – the
additional bit is transport wander.
If the gyro is spinning with its spin
axis horizontal, any horizontal
wander is called gyro drift. Vertical
wander is called gyro topple, whether
the axis is horizontal or vertical
(these are both wanders in specific
directions). The spin axis of the
instruments described below varies
with the function.
The wander rate, in case you're
interested, can be used in the Arctic
to help keep the aircraft straight
when the astro-compass cannot be
248 Canadian Professional Pilot Studies
used. Calculations are made and the
heading reset, rather than the DGI,
based on entries in the gyro log.
Gyroscopic instruments are made to
spin through suction (heading and
attitude indicator) or electricity (turn
instruments). With the former, air is
sucked out of the casing, and vanes
(small bucket-shapes) on the gyro
mass catch the movement and force
it to go round (vacuum system). There
might be a pump, or a venturi
system (on older aircraft) to reduce
the pressure. Since the venturi
system depends on a tube aligned
with the airflow outside the aircraft,
it is only effective above about 90
kts, and therefore not good enough
for IFR. The suction gauge on the
instrument panel is always part of
the checklist before IFR flight to
ensure you have enough for the
instruments to work properly. The
rest of the vacuum system has a
pump driven by the engine, a relief
valve, an air filter, and enough
tubing for the connections.
Higher rotational speeds can be
obtained with electrical gyros,
however, especially at altitude, and
the casings can be sealed. It is also
possible to design gyros with
complete movement through 360°.
During startup checks, pull and hold
any erection or caging knobs before
turning the power on, as the parts
inside can clash against each other as
they spin up (just one of those little
things a pilot can do to save longterm
maintenance costs).
Artificial Horizon
Otherwise known as the attitude
indicator, this instrument represents
the natural horizon and indicates the
pitch and bank attitudes, that is,
whether the nose is up or down, or
the wings are level or not. It is gyrodriven,
and the spin axis is vertically
mounted so the housing can rotate
around the vertical axis, at right
angles to the one in the DGI.
The horizon bar is connected to the
rear of the frame and to the housing
with a guide pin, so when the housing
moves, the bar remains rigid. In a
climb, the pin drives the bar down –
in a descent it goes up. Rolling
rotates the instrument case.
Electrical instruments use mercury
switches with their own torque motors
(the pitch motor is on the roll axis,
and the roll motor is on the pitch axis)
- fast erection involves giving the
motors a higher error signal, which
can be done any time in
unaccelerated flight. The torque
motors are slow acting to provide
damping, and the gyros spin faster
for more rigidity. Acceleration errors
are minimised because there is no
heavy mass underneath the gyro. If
there are any, they will be due to the
mercury sloshing around in the
switches, and show a climbing turn
to the left during takeoff, because
they normally spin the opposite way
to suction-driven ones.
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