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The Art of the Helicopter John Watkinson - Karatunov.net

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(a)<br />

(b)<br />

(c)<br />

Engines and transmissions 197<br />

Static<br />

‘Droop’<br />

Fig. 6.4 (a)Following a fixed increase in power demand due to, for example, an increase in collective pitch,<br />

<strong>the</strong> RRPM may fall, because <strong>the</strong> feedback loop needs an error to increase power. This static droop can be<br />

eliminated using feedforward. (b)An input from <strong>the</strong> collective lever is used to anticipate approximately <strong>the</strong><br />

amount <strong>of</strong> power needed. <strong>The</strong> feedback system <strong>the</strong>n has to make smaller power corrections that it can do with<br />

a smaller RRPM error. (c)Transient droop due to <strong>the</strong> time needed for <strong>the</strong> gas generator to increase speed.<br />

difference <strong>of</strong> <strong>the</strong> system. Increasing <strong>the</strong> gain will allow tighter control <strong>of</strong> speed but it<br />

also increases <strong>the</strong> chance that <strong>the</strong> system will become unstable so that it swings between<br />

too fast and too slow without stabilizing. A better solution is to combine governing and<br />

correlating systems so that <strong>the</strong>re is both feedback and feedforward. Figure 6.4(b) shows<br />

how this is done. <strong>The</strong> power setting is determined by <strong>the</strong> sum <strong>of</strong> two signals. One <strong>of</strong><br />

<strong>the</strong>se is <strong>the</strong> feedforward signal from <strong>the</strong> collective lever. This provides approximately <strong>the</strong><br />

right power setting. <strong>The</strong> second signal is <strong>the</strong> RRPM error feedback from <strong>the</strong> governor<br />

that slightly adapts <strong>the</strong> power setting to minimize <strong>the</strong> speed error. As most <strong>of</strong> <strong>the</strong> power<br />

setting is provided by <strong>the</strong> feedforward signal, <strong>the</strong> static droop will be minimal.<br />

Piston engines are geared to <strong>the</strong> rotor and so run at near-constant RPM. <strong>The</strong> power<br />

delivered is changed with a change <strong>of</strong> torque and with a naturally aspirated engine this<br />

can be done instantly by adjusting <strong>the</strong> throttle or, in <strong>the</strong> case <strong>of</strong> a Diesel, changing

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