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main shaft, which is connected to a
ratio arm, then a ranging arm, to the
indicator. When altitude decreases,
the ratio arm slides to the end of the
ranging arm, which reduces the
ASI's involvement in the whole
affair. As you go higher, it slides to
the root, giving it more influence.
In the climb, the speed of sound will
decrease because the temperature
does. Therefore, you must reduce
the TAS to keep the Mach Number
constant. A climb at constant RAS
means an increasing TAS and a
decreasing speed of sound (from the
temperature), so TAS and Mach No
will increase.
The machmeter does not suffer
from density error because it cancels
out on both sides of the equation.
However, it is prone to instrument
and pressure errors.
Vertical Speed Indicator
There is a capsule inside this, too,
but it is connected only to the static
system. However, there is a restrictor,
or calibrated leak between the inside
and outside of the capsule that
makes the pressure outside lag
behind that on the inside.
The capsule is compressed or
expanded one way or another and
the "suitable linkage" transfers the
movements to the dial to show
climb or descent:
A complex choke system
compensates for temperature,
density and air viscosity, using two
capillary tubes to give a laminar flow
and two sharp-edged orifices for a
turbulent flow. Errors that result
from the two types are of opposite
sign and cancel each other out.
An instantaneous VSI uses a static
input to an acceleration pump to
reduce lag, which unfortunately
introduces turning errors.
Instruments 243
The VSI is a trend and a rate
instrument, showing direction of
movement (up or down), and how
fast you're going, in hundreds of feet
per minute. In short, it detects the
rate of change of static pressure with
height, based on pressure difference
between the capsule and case.
Aside from the usual position error,
it suffers from lag error, which may
last up to 6-8 seconds before the air
in- and outside the capsule stabilizes,
and reversal error, which occurs when
abrupt changes cause movement
briefly in the opposite direction.
If the static source becomes blocked,
pressure differentials disappear and
the instrument reads zero.
The Compass
The Earth has its own magnetic
field, which resembles a doughnut,
in that the lines of force are more or
less parallel with the curvature of the
Earth but increase their angle
towards the Poles until they move
vertically downwards in a circle
surrounding the true pole:
As the compass needle tries to
follow the lines of force, you will
find it trying to dip near the Poles, to
a point where it is almost vertical
and actually unreadable. This is why
true tracks and headings are flown in
those areas. Magnetic dip is the angle
between the horizontal and vertical
forces acting on a compass needle
toward the nearer pole.
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
244 Canadian Professional Pilot Studies
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