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· 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 than they do in air. It has
vertical and horizontal elements,
the latter being residual and not
easily corrected. Modern
compasses are sensitive, down
to .01 gauss, but even that gives
246 Canadian Professional Pilot Studies
excessive hunting (in fact, you
need a DGI when the magnetic
field is below about .06 gauss).
· 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
(which is what the compass
correction card is for – it lists
uncorrectable residual deviation).
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. The
corrections are applied to horizontal
hard magnetism (which changes with
heading) and vertical soft (latitude).
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.
A compass swing should be done on
installation of the compass in the
first place, as per maintenance
schedules, whenever there is any
doubt about accuracy, after a shock
to the airframe or a lightning strike,
or if the aircraft has been left
standing for some time or has
moved to a significantly different
latitude, or (finally), if any major
component or electrical installation
has changed.
Coefficient A is constant on all
headings, and comes from horizontal
soft iron in the aircraft. Apparent A
arises from misalignment of the
lubber line (as opposed to Real A). It
comes from the sum of all deviations
divided by 4. Coefficient B is a fore
and aft force (E-W divided by 2),
and Coefficient C is athwartships (N-S
divided by 2). Their maximum limits
are ±3° for a gyromagnetic compass
and ±15° for a direct reader.
After correction, maximum errors
are 1° for a gyromagnetic compass
and 3° for a direct reading one.
Gyroscopes
Typically, three cockpit instruments
are under gyroscopic influence.
These are the Attitude Indicator
(artificial horizon), Directional
Gyroscopic Indicator (DGI) and Turn
Indicator/Coordinator.
A gyroscope is a rotating mass on an
axis, which may be vertical or
horizontal, suspended in gimbals. 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 applies with the flight
instruments mentioned above, as we
shall shortly see. Rigidity can be
improved with faster spin speeds,
Instruments 247
increasing the gyro's mass or its
radius.
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.
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).
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