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suspension liquid. The two
magnets employed above are
also useful here, as they keep
the mass of the assembly near
the pivot, reducing inertia. Light
alloys reduce inertia even more.
Being magnetic, the compass will be
affected by all the fields generated by
the aircraft itself, causing a
phenomenon called Deviation, which
is discussed in the Navigation chapter
(see also The Compass Swing, below).
To try and eliminate errors,
particularly magnetic dip, a remote
indicating gyrocompass may be used,
which is slaved to a DGI (see below).
The master unit is mounted near the
rear of the aircraft, so it is removed
from as much influence as possible
(hence the term remote). It contains a
gyroscope under the influence of a
magnetic element.
E2B
A typical E2B compass, as used in
most aircraft today, consists of a
floating inverted bowl suspended on
a pedestal in kerosene (for damping):
The bearings are marked on the
outside of the bowl, and there are
two parallel magnetized needles
inside, suspended under the pivot
point, as mentioned above.
The centre of gravity's position
below the suspension point gives rise
to errors when accelerating or
turning, caused by magnetic dip (the
reason for a suspended bowl in the
first place) and inertia.
Acceleration Errors
These are caused by inertia on East-
West headings. Because the C of G
of the compass is under the pivot
point, accelerating makes the bulk of
the compass lag behind the machine
and displace the C of G:
Because you are going East or West,
the North bit of the compass is
pointing to the side of the aircraft,
and the pivot and C of G are
therefore side by side instead of
being in line:
Instruments 245
The pivot point (which is nearest to
the Pole) is forced forward and the
C of G goes backwards, making the
North point spin further off line.
The needle is forced clockwise to
read less than 90° during the turn.
A deceleration would have the
opposite effect. There is also a
complementary effect from the
vertical component of the Earth's
magnetic force (Z) which imposes a
turning force on the dipped end of
the magnet – since the magnet can
only turn by rotating about the pivot,
the effect is created in the same way.
The watchword here is ANDS –
Accelerate North, Decelerate South, or
SAND in the Southern Hemisphere.
Accelerations produce apparent
turns towards the nearest Pole, and
decelerations towards the Equator.
Turning Errors
These happen during turns through
North or South – the compass lags
on Northerly headings and leads on
Southerly ones making it look as if
you're turning slower through North
and faster through South. Since a
turn could be regarded as an
acceleration, for the same reasons as
mentioned above, as you bank facing
North, the pivot point and C of G
are still displaced in the same way,
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. Has
vertical and horizontal effects.
Corrected with magnets.
　
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