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The Art of the Helicopter John Watkinson - Karatunov.net

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Rotors in practice 147<br />

Fig. 4.27 Effects <strong>of</strong> dragging blade motion. (a) A sudden application <strong>of</strong> drive torque may result in all three<br />

blades dragging in <strong>the</strong> same phase. (b) When <strong>the</strong> dragging <strong>of</strong> each blade is at 120 ◦ phase to <strong>the</strong> next, <strong>the</strong> hub<br />

moves in a circle. (c) If <strong>the</strong> phase relationship <strong>of</strong> (b) is changed, <strong>the</strong> direction <strong>of</strong> whirling is reversed. (d) Where<br />

two blades drag in anti-phase and one blade is stationary, <strong>the</strong> hub is driven in a line. Note that all <strong>of</strong> <strong>the</strong>se<br />

examples are in rotating co-ordinates.<br />

In <strong>the</strong> absence <strong>of</strong> preventive measures, if a resonant frequency in <strong>the</strong> hull is <strong>the</strong> same<br />

as that <strong>of</strong> a whirling frequency, one <strong>of</strong> two things can happen. Ei<strong>the</strong>r <strong>the</strong> hull will act as<br />

a vibration absorber as was described in section 3.27, and <strong>the</strong> whirling will be opposed,<br />

or <strong>the</strong> rocking will become increasingly violent until <strong>the</strong> machine ei<strong>the</strong>r comes apart or<br />

turns on its side. <strong>The</strong> phenomenon was first observed when a taxiing autogyro struck<br />

a rock with a wheel and literally disintegrated.<br />

Needless to say this Jekyll and Hyde behaviour in seemingly similar circumstances<br />

means that <strong>the</strong> mechanism involved must be extremely subtle. Whilst <strong>the</strong> results <strong>of</strong><br />

ground resonance are painfully obvious, <strong>the</strong> mechanism causing it is not obvious at<br />

all. Until a ma<strong>the</strong>matical basis for <strong>the</strong> phenomenon was found, designers proceeded<br />

empirically. For example, Frank Piasecki obtained an improvement on his first<br />

machine by filling <strong>the</strong> tyres with cork. It was concluded early that ground resonance<br />

is purely a mechanical phenomenon and that aerodynamic forces are not significant.<br />

<strong>The</strong>re are some parallels with whirling phenomena observed in o<strong>the</strong>r disciplines such<br />

as steam turbines.

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