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The Art of the Helicopter John Watkinson - Karatunov.net

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264 <strong>The</strong> <strong>Art</strong> <strong>of</strong> <strong>the</strong> <strong>Helicopter</strong><br />

classification by <strong>the</strong> quantity displayed, instruments can also be categorized by <strong>the</strong><br />

internal operating principles, and this is more appropriate to a technical description.<br />

<strong>The</strong> internal operating principle can be mag<strong>net</strong>ic, pressure, gyroscopic, optical or radio<br />

and <strong>the</strong>se principles will be examined in turn.<br />

7.3 <strong>The</strong> mag<strong>net</strong>ic compass<br />

<strong>The</strong> simple compass is a small freely suspended mag<strong>net</strong> that attempts to align itself<br />

with <strong>the</strong> earth’s mag<strong>net</strong>ic field. Figure 7.4(a) shows that <strong>the</strong> earth acts as if a large bar<br />

mag<strong>net</strong> were buried inside it. <strong>The</strong> north-seeking pole <strong>of</strong> <strong>the</strong> compass mag<strong>net</strong> (generally<br />

just called <strong>the</strong> north pole) is so called because it points to <strong>the</strong> north. As opposite poles<br />

attract, clearly <strong>the</strong> mag<strong>net</strong>ic pole at <strong>the</strong> north end <strong>of</strong> <strong>the</strong> earth is actually a south pole.<br />

It is called <strong>the</strong> north mag<strong>net</strong>ic pole because <strong>of</strong> where it is. Figure 7.4 also shows that<br />

as <strong>the</strong> lines <strong>of</strong> mag<strong>net</strong>ic force return to <strong>the</strong> poles, <strong>the</strong>y angle sharply into <strong>the</strong> ground.<br />

This is called mag<strong>net</strong>ic dip and in <strong>the</strong> UK lines <strong>of</strong> force enter <strong>the</strong> ground at about 65 ◦<br />

to <strong>the</strong> horizontal. A simple bar mag<strong>net</strong> suspended at its CM would adopt that angle <strong>of</strong><br />

dip. Only <strong>the</strong> horizontal component <strong>of</strong> <strong>the</strong> earth’s field is useful for navigation so dip<br />

is clearly a nuisance. <strong>The</strong>re is no dip at <strong>the</strong> equator since lines <strong>of</strong> force are parallel to<br />

<strong>the</strong> ground <strong>the</strong>re. Near <strong>the</strong> mag<strong>net</strong>ic poles compasses are useless.<br />

<strong>The</strong> axis <strong>of</strong> <strong>the</strong> earth’s mag<strong>net</strong>ic field is not aligned with <strong>the</strong> rotational axis, and so<br />

<strong>the</strong> mag<strong>net</strong>ic poles are some distance from <strong>the</strong> true poles. <strong>The</strong> north mag<strong>net</strong>ic pole<br />

has been wandering about in nor<strong>the</strong>rn Canada in recent history. Maps are made with<br />

latitude/longitude grids that relate to <strong>the</strong> true poles, and so a line <strong>of</strong> longitude by<br />

definition points true north. As <strong>the</strong> mag<strong>net</strong>ic pole is not at <strong>the</strong> true pole, a mag<strong>net</strong>ic<br />

compass does not point to true north. Variation is measured in degrees and direction.<br />

For example, if <strong>the</strong> compass points 8 ◦ to <strong>the</strong> west <strong>of</strong> true north, <strong>the</strong> variation<br />

is 8 ◦ W.<br />

In order to use <strong>the</strong> mag<strong>net</strong>ic heading for navigation, maps are drawn with additional<br />

lines which show <strong>the</strong> variation. A line joining all points having <strong>the</strong> same variation is<br />

called an isogonal line. On a map covering a large area, isogonals are drawn every 2 ◦ .<br />

<strong>The</strong>re is one line joining places <strong>of</strong> zero variation called <strong>the</strong> agonal line and which<br />

currently passes through Sweden. On charts covering a small area in great detail<br />

<strong>the</strong> variation is shown once and can be considered to be <strong>the</strong> same over <strong>the</strong> whole<br />

chart.<br />

Direction is expressed in degrees clockwise from North or by <strong>the</strong> cardinal points<br />

North, South, East and West and can be true (T) or mag<strong>net</strong>ic (M). <strong>The</strong> degree symbol<br />

is <strong>of</strong>ten omitted and heading will simply be a three-digit number followed by <strong>the</strong><br />

mag<strong>net</strong>ic/true descriptor, e.g. 295M, 310T.<br />

Runways are numbered according to <strong>the</strong>ir mag<strong>net</strong>ic direction but rounded to twodigit<br />

accuracy. <strong>The</strong> runway name is painted at <strong>the</strong> landing end. Thus a runway running<br />

east–west would have 09 painted at <strong>the</strong> western end since this would be visible to an<br />

aircraft flying on a course <strong>of</strong> 90 ◦ . <strong>The</strong> number 27 would be painted at <strong>the</strong> opposite end<br />

where it would be seen by an aircraft flying on a course <strong>of</strong> 270 ◦ .<br />

A simple bar mag<strong>net</strong> will also oscillate about true north if disturbed because <strong>the</strong><br />

earth’s field acts like a weak spring and <strong>the</strong> mag<strong>net</strong> has rotational inertia. <strong>The</strong> balance<br />

wheel <strong>of</strong> a watch oscillates in <strong>the</strong> same way. In order to damp oscillations, aircraft compasses<br />

incorporate a liquid. Figure 7.4(b) shows <strong>the</strong> construction <strong>of</strong> a typical aircraft<br />

compass. <strong>The</strong> mag<strong>net</strong> is actually a number <strong>of</strong> mag<strong>net</strong>s, typically four, embedded in a<br />

disc made <strong>of</strong> some non-mag<strong>net</strong>ic material such as plastic. <strong>The</strong> outside <strong>of</strong> <strong>the</strong> disc is<br />

engraved with <strong>the</strong> mag<strong>net</strong>ic heading which will be read <strong>of</strong>f against a mark on <strong>the</strong> case

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