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The Art of the Helicopter John Watkinson - Karatunov.net

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Fig. 7.13 A chaser system which allows <strong>the</strong> position <strong>of</strong> an altimeter mechanism to be measured without<br />

adding friction. See text.<br />

movement and <strong>the</strong> power available is small and it is difficult to drive a conventional<br />

encoder directly. Instead a form <strong>of</strong> electromechanical amplification is used. Figure 7.13<br />

shows one example. <strong>The</strong> sensing capsule is mounted on a platform that can be moved<br />

by an electric motor. <strong>The</strong> moving diaphragm <strong>of</strong> <strong>the</strong> capsule carries only a low mass<br />

vane or plate placed in a light beam. <strong>The</strong> light falls on differential photocells so that<br />

very small <strong>of</strong>f-centre movements <strong>of</strong> <strong>the</strong> diaphragm produce an electrical signal because<br />

more light falls on one cell and less on <strong>the</strong> o<strong>the</strong>r. This signal is amplified and supplied to<br />

<strong>the</strong> electric motor such that <strong>the</strong> motor is driven in a direction that recentres <strong>the</strong> optical<br />

sensor. Alternate types <strong>of</strong> sensing may be used, for example a differential transformer<br />

as will presently be described.<br />

A motor used in this way is known as a chaser. Effectively <strong>the</strong> chasing action means<br />

that <strong>the</strong> diaphragm <strong>of</strong> <strong>the</strong> capsule remains in <strong>the</strong> same place. Pressure changes must<br />

<strong>the</strong>n be reflected in <strong>the</strong> position <strong>of</strong> <strong>the</strong> motor drive, which has enough power to drive <strong>the</strong><br />

pressure encoder. In addition to <strong>the</strong> encoder output, <strong>the</strong> speed <strong>of</strong> <strong>the</strong> motor may be<br />

measured to produce a rate output. When used for <strong>the</strong> altitude hold function in an<br />

autopilot, <strong>the</strong> chaser motor may be disconnected and <strong>the</strong> signal from <strong>the</strong> optical sensor<br />

will be used to make <strong>the</strong> aircraft climb or descend as needed to maintain altitude.<br />

Altitude is sensed using an evacuated capsule, and means are required to allow <strong>the</strong><br />

appropriate reference pressure to be set. Airspeed is sensed using a capsule exposed on<br />

one side to dynamic pressure at <strong>the</strong> pitot head and to static pressure on <strong>the</strong> o<strong>the</strong>r.<br />

For automatic approach or landing systems a RADAR altimeter is a necessity. In fact<br />

it is not an altimeter as it does not consider air pressure; it is a height meter because<br />

it works by timing <strong>the</strong> reflection <strong>of</strong> a radio signal to give height AGL which is <strong>the</strong><br />

requirement for landing.<br />

7.11 Gyroscopic instruments<br />

Gyroscopic instruments are those depending on a rapidly rotating flywheel for <strong>the</strong>ir<br />

operation and include <strong>the</strong> direction indicator (DI), <strong>the</strong> turn and slip indicator (T + S),<br />

also known as <strong>the</strong> turn and bank indicator, and <strong>the</strong> artificial horizon. <strong>The</strong>se display, in<br />

various ways, <strong>the</strong> attitude and heading <strong>of</strong> <strong>the</strong> helicopter. It is possible to fly a helicopter<br />

in good conditions without <strong>the</strong>m, relying only on <strong>the</strong> mag<strong>net</strong>ic compass for heading<br />

Control 277

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