The Art of the Helicopter John Watkinson - Karatunov.net

264 The Art of the Helicopter
classification by the quantity displayed, instruments can also be categorized by the
internal operating principles, and this is more appropriate to a technical description.
The internal operating principle can be magnetic, pressure, gyroscopic, optical or radio
and these principles will be examined in turn.
7.3 The magnetic compass
The simple compass is a small freely suspended magnet that attempts to align itself
with the earth’s magnetic field. Figure 7.4(a) shows that the earth acts as if a large bar
magnet were buried inside it. The north-seeking pole of the compass magnet (generally
just called the north pole) is so called because it points to the north. As opposite poles
attract, clearly the magnetic pole at the north end of the earth is actually a south pole.
It is called the north magnetic pole because of where it is. Figure 7.4 also shows that
as the lines of magnetic force return to the poles, they angle sharply into the ground.
This is called magnetic dip and in the UK lines of force enter the ground at about 65 ◦
to the horizontal. A simple bar magnet suspended at its CM would adopt that angle of
dip. Only the horizontal component of the earth’s field is useful for navigation so dip
is clearly a nuisance. There is no dip at the equator since lines of force are parallel to
the ground there. Near the magnetic poles compasses are useless.
The axis of the earth’s magnetic field is not aligned with the rotational axis, and so
the magnetic poles are some distance from the true poles. The north magnetic pole
has been wandering about in northern Canada in recent history. Maps are made with
latitude/longitude grids that relate to the true poles, and so a line of longitude by
definition points true north. As the magnetic pole is not at the true pole, a magnetic
compass does not point to true north. Variation is measured in degrees and direction.
For example, if the compass points 8 ◦ to the west of true north, the variation
is 8 ◦ W.
In order to use the magnetic heading for navigation, maps are drawn with additional
lines which show the variation. A line joining all points having the same variation is
called an isogonal line. On a map covering a large area, isogonals are drawn every 2 ◦ .
There is one line joining places of zero variation called the agonal line and which
currently passes through Sweden. On charts covering a small area in great detail
the variation is shown once and can be considered to be the same over the whole
chart.
Direction is expressed in degrees clockwise from North or by the cardinal points
North, South, East and West and can be true (T) or magnetic (M). The degree symbol
is often omitted and heading will simply be a three-digit number followed by the
magnetic/true descriptor, e.g. 295M, 310T.
Runways are numbered according to their magnetic direction but rounded to twodigit
accuracy. The runway name is painted at the landing end. Thus a runway running
east–west would have 09 painted at the western end since this would be visible to an
aircraft flying on a course of 90 ◦ . The number 27 would be painted at the opposite end
where it would be seen by an aircraft flying on a course of 270 ◦ .
A simple bar magnet will also oscillate about true north if disturbed because the
earth’s field acts like a weak spring and the magnet has rotational inertia. The balance
wheel of a watch oscillates in the same way. In order to damp oscillations, aircraft compasses
incorporate a liquid. Figure 7.4(b) shows the construction of a typical aircraft
compass. The magnet is actually a number of magnets, typically four, embedded in a
disc made of some non-magnetic material such as plastic. The outside of the disc is
engraved with the magnetic heading which will be read off against a mark on the case