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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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Engines and transmissions 243<br />

<strong>the</strong> AH-64 Apache. In <strong>the</strong> Chinook, shown in Figure 6.34(b), <strong>the</strong> front rotor thrust is<br />

carried by <strong>the</strong> transmission, whereas <strong>the</strong> rear rotor has a thrust bearing at <strong>the</strong> top <strong>of</strong><br />

<strong>the</strong> fin. <strong>The</strong> drive shaft is fitted with a splined coupling so that hull flexing does not<br />

stress <strong>the</strong> transmission.<br />

6.28 Multi-engine transmissions<br />

<strong>The</strong>re are a number <strong>of</strong> reasons for installing more than one engine in a helicopter. One<br />

obvious result is increased power, but clearly this is only obtained if all <strong>the</strong> engines are<br />

working. Ano<strong>the</strong>r reason may be to increase safety. If one engine fails, <strong>the</strong> o<strong>the</strong>r(s) will<br />

continue to provide some power. Ano<strong>the</strong>r possibility is to improve range or economy<br />

by shutting down one engine in cruise.<br />

In a multi-engine helicopter, each engine will have its own one-way clutch so that <strong>the</strong><br />

loss or seizure <strong>of</strong> one engine does not prevent <strong>the</strong> transmission turning. In most cases<br />

twin engines are fitted for safety reasons. Single engine helicopters are not permitted<br />

to fly over built-up areas. Ideally in <strong>the</strong> case <strong>of</strong> an engine failure <strong>the</strong> machine would<br />

be able to continue normal flight. However, this would mean that each engine would<br />

have to deliver <strong>the</strong> same power as a single engine. In <strong>the</strong> case <strong>of</strong> turbine engines this<br />

is very inefficient because in normal flight both engines would be delivering only half<br />

<strong>the</strong>ir rated power. However, <strong>the</strong> power needed to drive <strong>the</strong> compressors would be twice<br />

<strong>the</strong> case for a single engine and this would impair <strong>the</strong> fuel consumption.<br />

In practice a helicopter only needs full power for a short time, typically at take-<strong>of</strong>f with<br />

a full fuel load and landing at a hot/high destination. For <strong>the</strong> rest <strong>of</strong> <strong>the</strong> flight less power<br />

would be acceptable after a failure by using lower speeds and reduced rates <strong>of</strong> climb.<br />

Turbine engines are very reliable and failures are relatively uncommon. Consequently<br />

instead <strong>of</strong> overengining a helicopter, it is more sensible to design engines that can be<br />

overrated for short periods <strong>of</strong> time. Thus in addition to <strong>the</strong> continuous power rating<br />

an engine would have a higher ‘contingency’ rating which it could only tolerate for a<br />

few minutes. <strong>The</strong>re may also be an even higher emergency rating that might only be<br />

sustainable for half a minute. If an engine enters one <strong>of</strong> <strong>the</strong>se conditions, an indicator<br />

operates which can only be reset on <strong>the</strong> ground and a timer runs to measure <strong>the</strong> degree<br />

<strong>of</strong> overload.<br />

<strong>The</strong> use <strong>of</strong> <strong>the</strong>se contingency power ratings in some cases may put <strong>the</strong> engine under<br />

such stress that it will need immediate overhaul, but <strong>the</strong> frequency with which this<br />

happens is so low that <strong>the</strong> saving in fuel when <strong>the</strong> engines are working normally is <strong>of</strong><br />

more consequence.<br />

Figure 6.35 shows how a twin-engine helicopter might take-<strong>of</strong>f from a ro<strong>of</strong>top helipad<br />

in a built-up area. <strong>The</strong> take-<strong>of</strong>f is conducted upwards and backwards at first so that <strong>the</strong><br />

helipad remains in <strong>the</strong> pilot’s view. If, during this initial climb, an engine fails <strong>the</strong> pilot<br />

has <strong>the</strong> option <strong>of</strong> returning to <strong>the</strong> pad and <strong>the</strong> time for which contingency power is<br />

needed would be quite short. <strong>The</strong> height reached in <strong>the</strong> initial climb has to be such that<br />

if an engine failed just as <strong>the</strong> machine moved into forward flight it could use <strong>the</strong> power<br />

<strong>of</strong> <strong>the</strong> remaining engine and <strong>the</strong> power obtained by losing height to reach minimum<br />

power speed without falling below <strong>the</strong> height <strong>of</strong> <strong>the</strong> pad or nearby buildings.<br />

If ano<strong>the</strong>r engine <strong>of</strong> <strong>the</strong> same type is fitted to a single engine helicopter, <strong>the</strong> safety<br />

aspect will be improved, but <strong>the</strong> fuel economy will suffer because <strong>the</strong> losses <strong>of</strong> two<br />

compressors are being borne. At a sufficient height, one engine could be shut down so<br />

that <strong>the</strong> o<strong>the</strong>r runs more efficiently. In <strong>the</strong> event that <strong>the</strong> running engine fails, provided<br />

<strong>the</strong> second engine can be started promptly, flight could continue to a convenient point.

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