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The Art of the Helicopter John Watkinson - Karatunov.net

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

in temperature so that mass flow is not reduced as seriously. At high altitude <strong>the</strong><br />

governor will attempt to maintain power by increasing fuelling. <strong>The</strong> reduced mass flow<br />

reduces <strong>the</strong> dilution <strong>of</strong> <strong>the</strong> combustion products by excess air with <strong>the</strong> result that <strong>the</strong><br />

gas temperature entering <strong>the</strong> power turbine tends to rise. Consequently although <strong>the</strong><br />

engine could produce power at very high altitudes, in practice power will have to be<br />

limited to protect <strong>the</strong> power turbine. It should be appreciated that <strong>the</strong> power needed<br />

by <strong>the</strong> airframe may also fall with altitude so this effect is hardly a problem, especially<br />

in helicopters.<br />

Power is controlled through limiting fuel admission to <strong>the</strong> burners. As kerosene in not<br />

very volatile it is ei<strong>the</strong>r atomized at <strong>the</strong> burner nozzle or heated by passing it through<br />

pipes exposed to <strong>the</strong> burning gases on <strong>the</strong> way to <strong>the</strong> burner nozzle. It <strong>the</strong>n vaporizes<br />

on leaving <strong>the</strong> nozzle and burns readily.<br />

Starting a turbine requires an electric motor which will spin <strong>the</strong> gas generator spool<br />

fast enough to make <strong>the</strong> compressor operate. Once a suitable compressor speed is<br />

established (typically 15% <strong>of</strong> flight idle) <strong>the</strong> fuel is sprayed through <strong>the</strong> nozzle and <strong>the</strong><br />

igniter is operated. This results in hot gas generation that will increase <strong>the</strong> turbine<br />

speed until engine power can take over from <strong>the</strong> starter motor. <strong>The</strong> starter motor<br />

and igniter are typically turned <strong>of</strong>f at 50% <strong>of</strong> flight idle. If insufficient compressor<br />

speed is achieved before <strong>the</strong> ignition attempt, combustion pressure will overcome <strong>the</strong><br />

compressor pressure and <strong>the</strong> turbine equivalent <strong>of</strong> a backfire takes place. In some cases<br />

<strong>the</strong> starter motor is permanently connected and becomes a generator when <strong>the</strong> engine<br />

is running.<br />

6.19 Compressors<br />

<strong>The</strong> job <strong>of</strong> <strong>the</strong> compressor is to provide a steady flow <strong>of</strong> air under pressure to <strong>the</strong><br />

burners. Compressor design is fraught with compromise primarily because <strong>of</strong> <strong>the</strong> need<br />

to deliver different flow rates depending on <strong>the</strong> power required. It is relatively easy to<br />

design a compressor that is very efficient under one specific set <strong>of</strong> conditions, but it may<br />

become very inefficient under o<strong>the</strong>r conditions. In practice it may be better to design<br />

a compressor which is a little less efficient, but which maintains that efficiency over a<br />

wide range <strong>of</strong> flow rates. <strong>The</strong> compressor can be centrifugal, axial or a combination <strong>of</strong><br />

<strong>the</strong> two.<br />

<strong>The</strong> centrifugal compressor was used in early helicopter turbine engines as it allows<br />

a shorter assembly and could be designed using experience from turbochargers.<br />

Figure 6.19(a) shows a single-entry centrifugal compressor. <strong>The</strong> air enters axially near<br />

<strong>the</strong> eye or centre <strong>of</strong> <strong>the</strong> impeller at a speed approaching <strong>the</strong> speed <strong>of</strong> sound. <strong>The</strong> blades<br />

may be twisted at <strong>the</strong> eye to allow a smoo<strong>the</strong>r entry thus avoiding compressibility<br />

effects. As <strong>the</strong> air moves away from <strong>the</strong> axis <strong>the</strong> impeller blades impart higher tangential<br />

velocity. In practice <strong>the</strong> compressor impeller delivers air in a direction having both<br />

radial and tangential components. This high velocity is <strong>the</strong>n converted into pressure<br />

using an assembly known as a diffuser: basically a divergent duct operating according<br />

to Bernouilli’s <strong>the</strong>orem. <strong>The</strong> diffuser may at least double <strong>the</strong> pressure at <strong>the</strong> impeller<br />

outlet and more in some designs.<br />

It is possible to construct a double-entry centrifugal compressor as shown in<br />

Figure 6.19(b). This allows twice <strong>the</strong> mass flow with <strong>the</strong> same diameter, but is less<br />

efficient because <strong>the</strong> blades run hotter than in <strong>the</strong> single-entry design which obtains<br />

some cooling at <strong>the</strong> rear <strong>of</strong> <strong>the</strong> impeller. A single centrifugal stage may produce a pressure<br />

ratio <strong>of</strong> up to 4.5 : 1. Stages may be cascaded to produce higher pressure ratios, but<br />

<strong>the</strong>n <strong>the</strong> advantage <strong>of</strong> shortness is lost. <strong>The</strong> centrifugal compressor is relatively easy to

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