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those areas.
As the magnetic pole and lines of
force do not coincide with either the
true poles or lines of longitude, there
is a system of accounting for magnetic
variation, discussed in Navigation.
A direct reading compass has a
pivoted magnet that is free to align
itself with the horizontal component
of the Earth's magnetic field. It must
have certain properties to be able to
do this, namely:
· Horizontality. The needle must
dip as little as possible. This is
done by making its centre of
gravity lie below the pivot point,
with pendulous magnets, which
opposes the vertical component
of the Earth's magnetic force
(Z). Although there is still a
residual dip, if it is less than 3°
at mid-latitudes, it is OK. There
is also a collar and sleeve
assembly that stops it falling
apart when inverted.
· Sensitivity. This can be improved
by increasing the length and/or
the pole strength of the magnet.
However, two short magnets
will do just as well, and they can
also be employed as the weights
under the pivot point
mentioned above. Pole strength
can be increased by using
special alloys. In addition, you
could use a jewelled pivot to
reduce friction, a suspension
fluid which both lubricates it
and reduces the effective weight
of the whole assembly.
· Aperiodicity. The ability to settle
quickly after a disturbance,
which is helped by the
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
72 Canadian Private Pilot Studies
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:
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
Instruments 73
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,
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