20.01.2013 Views

The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

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

Fig. 6.41 Electrical system <strong>of</strong> a light helicopter showing <strong>the</strong> position <strong>of</strong> <strong>the</strong> ammeter and <strong>the</strong> master switch.<br />

<strong>The</strong> state <strong>of</strong> <strong>the</strong> battery and alternator can be assessed using an Ampere meter, always<br />

abbreviated to ammeter, which is connected as shown in Figure 6.41. When <strong>the</strong> engine<br />

is stopped, electrical loads are supplied by battery, and current flows out through <strong>the</strong><br />

ammeter which will register discharge (−). When <strong>the</strong> engine is started, <strong>the</strong> battery will<br />

have been partially discharged by <strong>the</strong> starter motor, and current will flow back from<br />

<strong>the</strong> alternator to charge <strong>the</strong> battery, registering charge (+). As <strong>the</strong> battery becomes<br />

recharged <strong>the</strong> magnitude <strong>of</strong> <strong>the</strong> charging current will fall and <strong>the</strong> ammeter will return<br />

to <strong>the</strong> null position where it will remain. If <strong>the</strong> ammeter displays a discharge with flight<br />

RPM, <strong>the</strong> alternator is not supplying enough current, it is switched <strong>of</strong>f or some load<br />

has been applied beyond its capacity. A common explanation is that <strong>the</strong> belt driving<br />

<strong>the</strong> alternator has broken or is slipping.<br />

Figure 6.41 also shows that <strong>the</strong> master switch isolates all electrical devices from <strong>the</strong><br />

battery (except <strong>the</strong> clock). In some cases <strong>the</strong> total electrical load is too large for a switch<br />

and a relay or contactor is used. This is a small version <strong>of</strong> <strong>the</strong> starter motor solenoid<br />

and is controlled by a small current from <strong>the</strong> switch itself. Generally only one wire feeds<br />

current to each device and current returns through <strong>the</strong> metal frame <strong>of</strong> <strong>the</strong> machine.<br />

If a failure <strong>of</strong> <strong>the</strong> insulation occurs somewhere in <strong>the</strong> electrical wiring, current could<br />

flow direct to <strong>the</strong> frame with little resistance to stop it. <strong>The</strong> high fault current could cause<br />

a fire in <strong>the</strong> wiring. This is prevented by splitting <strong>the</strong> wiring into sections and protecting<br />

each with a fuse or circuit breaker. In <strong>the</strong> case <strong>of</strong> a fault, <strong>the</strong> section containing <strong>the</strong> fault<br />

will carry an excessive current that melts <strong>the</strong> fuse or mag<strong>net</strong>ically trips <strong>the</strong> breaker. <strong>The</strong><br />

current is cut <strong>of</strong>f and <strong>the</strong> fire is prevented. Equally importantly, <strong>the</strong> supply is maintained<br />

to all o<strong>the</strong>r sections; only devices using current from <strong>the</strong> affected section will lose power.<br />

A fuse is cheap and effective, but it has to be replaced if it fails. A fuse should only be<br />

replaced with one <strong>of</strong> <strong>the</strong> same rating. If a fuse is replaced with one <strong>of</strong> a higher rating,<br />

<strong>the</strong> protection it gives is lost and <strong>the</strong> wiring could burn in <strong>the</strong> case <strong>of</strong> a fault. A circuit<br />

breaker pops out when tripped and can be reset by finger pressure. In addition breakers<br />

can be used as switches by pulling <strong>the</strong>m out. A tripped breaker does not always indicate

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!