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The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

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

In free<br />

space<br />

(c)<br />

(d)<br />

With<br />

ferrous<br />

object<br />

nearby<br />

Fig. 7.4 (Continued) (c) Deviation is an error in a mag<strong>net</strong>ic compass due to a nearby ferrous object. (d) A<br />

deviation card shows what compass errors exist.<br />

called <strong>the</strong> lubber line. Since <strong>the</strong> compass is invariably mounted ahead <strong>of</strong> <strong>the</strong> pilot, <strong>the</strong><br />

lubber line will be at <strong>the</strong> back <strong>of</strong> <strong>the</strong> unit, and so <strong>the</strong> engraving is rotated 180 ◦ with<br />

respect to <strong>the</strong> mag<strong>net</strong>. Thus if <strong>the</strong> helicopter and pilot and mag<strong>net</strong> are all facing due<br />

north, <strong>the</strong> part <strong>of</strong> <strong>the</strong> disc with ‘N’ on it will be facing <strong>the</strong> pilot.<br />

Compass dip is minimized by pivoting <strong>the</strong> disc well above <strong>the</strong> CM. If <strong>the</strong> dip exerts<br />

a tilting couple, <strong>the</strong> CM moves from below <strong>the</strong> pivot to balance it. <strong>The</strong> disc remains<br />

within 2 or 3 ◦ <strong>of</strong> horizontal as <strong>the</strong> vertical component <strong>of</strong> <strong>the</strong> earth’s field is thus opposed.<br />

<strong>The</strong> disc rotates until <strong>the</strong> mag<strong>net</strong> is parallel to <strong>the</strong> horizontal component <strong>of</strong> <strong>the</strong> earth’s<br />

mag<strong>net</strong>ic field. Oscillation is prevented by <strong>the</strong> viscosity <strong>of</strong> <strong>the</strong> liquid acting on <strong>the</strong> large<br />

periphery <strong>of</strong> <strong>the</strong> disc.<br />

<strong>The</strong> contact pressure between <strong>the</strong> pivot and <strong>the</strong> disc is reduced by buoyancy <strong>of</strong> <strong>the</strong><br />

liquid and this reduces friction. When visualizing how a compass works, it is important<br />

to remember that <strong>the</strong> compass disc tries to keep pointing <strong>the</strong> same way as <strong>the</strong> helicopter<br />

turns around it.

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