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with the same effect on the North
seeking pole, so the needle turns by
itself in the same direction as the
turn. At Rate 1, it may look like you
are not turning at all - steeper than
that, you could be going backwards!
On Southerly headings, the machine
will be turning against this
inclination, so will read the opposite
way, so don't straighten up until the
compass has gone past the heading
you want. The Z field effect is also
active here, and is complementary.
To put it another way, during turns
through the nearest Pole, the
compass will be sluggish, so you
need to roll out early. During turns
through the furthest Pole, the
compass will be lively, so roll out
late. A displacement of the magnet
in a clockwise direction viewed from
above causes the compass to underread,
and vice versa.
You therefore get the most turning
errors through North or South, and
the most acceleration errors through
East or West.
In view of the above, it follows that,
before you start relying on the
compass (either to navigate or align
your HI), make sure you are in
steady, level flight. Also, make turns
gently, because the swirling of the
fluid will keep the compass moving
after you've stopped turning.
The Compass Swing
The influence of the aircraft on a
compass is made up broadly of three
components:
·  Hard iron, or metal which can
act as a permanent magnet.
·  Soft iron, or metal which only
produces a magnetic influence
when affected by the Earth's
field. This is because the lines of
force flow more readily through
metals then they do in air.
·  Electrical. Current flowing
through a conductor always
produces a magnetic field.
Even though modern designs reduce
aircraft effects as much as possible,
there are always residuals to resolve.
This is done by measuring the
effects on the aircraft's compass
against a Master compass, and
introducing fields of equal
magnitude but opposite polarity
deliberately to counteract them.
Airfields and maintenance areas have
clear areas in which this can be done.
The aircraft is taxied there and
everything that would be used in
flight turned on. Then the aircraft
compass is compared against a
landing compass on several headings, in
a correcting swing. Then the deviations
are reduced by adjusting the magnets
inside the compass and a calibration
swing is done to see what deviations
are left. These figures are written
down on the deviation card.
Gyroscopes
Typically, three cockpit instruments
are under gyroscopic influence.
These are the attitude indicator
(artificial horizon), heading indicator
(DGI) and turn indicator/coordinator.
74 Canadian Private Pilot Studies
A gyroscope is a rotating mass on an
axis, which may be vertical or
horizontal. The spinning allows the
gyro to maintain its own position in
space (rigidity), regardless of whatever
it is attached to is doing. In other
words, it resists attempts to displace
it from its position. If you attached
one to a camera, for example, and
used the camera in a helicopter, the
helicopter could be bumping around
all over the place due to wind or
pilot input, and the camera would
not move from where the operator
put it. The same principle is used
with the instruments mentioned
above, as we shall shortly see.
Another property gyroscopes have is
precession, meaning that a force
applied to the spinning mass is felt
90° away from where it is applied, in
the direction of rotation:
The control inputs on a helicopter
have to allow for this, because the
rotor disc is nothing but a large gyro
– even though you make an input
for forward flight, the actual
movement applied to the rotor head
is done several degrees beforehand.
A more mundane example comes
from riding a bicycle – when you
apply a force to turn one way or
another, it is done at the top of the
wheels, but the turning movement
appears 90° later, hence the turn.
If a primary precession is impeded
for any reason, the impeding force
will produce a secondary precession in
the direction of the original force (as
in the turn and slip indicator, below).
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
　
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