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

roll rate. <strong>The</strong> mixing levers subtract <strong>the</strong> actual roll rate from <strong>the</strong> desired roll rate to<br />

produce <strong>the</strong> roll rate error. This applies cyclic fea<strong>the</strong>ring in <strong>the</strong> sense that reduces <strong>the</strong><br />

error. Thus <strong>the</strong> error is kept small such that <strong>the</strong> actual roll rate is equal to <strong>the</strong> desired<br />

roll rate.<br />

7.25 <strong>The</strong> Hiller system<br />

<strong>The</strong> second major flybar development was due to Stanley Hiller and this was used in<br />

many <strong>of</strong> <strong>the</strong> early Hiller machines. <strong>The</strong> main difference between <strong>the</strong> Young and <strong>the</strong><br />

Hiller systems is that in <strong>the</strong> Hiller <strong>the</strong> flybar has aerodynamic properties as well as<br />

gyroscopic properties and so is properly called a control rotor. Figure 7.45 shows that<br />

<strong>the</strong> control rotor is mounted transversely on a bearing as before, but <strong>the</strong>re are two major<br />

contrasts with <strong>the</strong> Young system. First, <strong>the</strong> Hiller control rotor has full authority over<br />

<strong>the</strong> cyclic pitch <strong>of</strong> <strong>the</strong> main rotor, such that <strong>the</strong> control rotor axis completely determines<br />

<strong>the</strong> control axis <strong>of</strong> <strong>the</strong> main rotor. Second, <strong>the</strong> Hiller control rotor has small blades or<br />

paddles and it has fea<strong>the</strong>ring bearings. Pitch control arms on <strong>the</strong> control rotor lead to<br />

<strong>the</strong> swashplate so that cyclic pitch control <strong>of</strong> <strong>the</strong> control rotor is obtained. <strong>The</strong> pilot’s<br />

cyclic control actually flies <strong>the</strong> control rotor, whose attitude will <strong>the</strong>n be followed by<br />

<strong>the</strong> main rotor.<br />

<strong>The</strong> following rate <strong>of</strong> <strong>the</strong> control rotor can be made as slow as required by changing<br />

its Lock number, i.e. <strong>the</strong> relationship between <strong>the</strong> mass <strong>of</strong> <strong>the</strong> paddles and <strong>the</strong>ir lifting<br />

area. Thus as before, relatively light blades can be made to respond relatively slowly.<br />

<strong>The</strong> control rotor will not respond rapidly to gusts and so <strong>the</strong> attitude <strong>of</strong> <strong>the</strong> machine<br />

is stabilized as it is in <strong>the</strong> Young system.<br />

<strong>The</strong> Hiller system has <strong>the</strong> advantage that <strong>the</strong> pilot only has to produce enough force<br />

to fea<strong>the</strong>r <strong>the</strong> control rotor. <strong>The</strong> control forces to fea<strong>the</strong>r <strong>the</strong> main rotor are created by<br />

lift on <strong>the</strong> paddles. Thus not only does <strong>the</strong> Hiller system provide following rate control<br />

Fig. 7.45 In <strong>the</strong> Hiller system, <strong>the</strong> flybar is gyroscopic as in <strong>the</strong> Bell system, but damping is aerodynamic. <strong>The</strong><br />

flybar also provides power assistance through <strong>the</strong> application <strong>of</strong> cyclic pitch to <strong>the</strong> paddles.

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