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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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Introduction to helicopter dynamics 105<br />

Fig. 3.36 (a) In a two-bladed rotor <strong>the</strong> blades flap with a 180 ◦ relationship. <strong>The</strong> fundamental flapping cancels,<br />

whereas <strong>the</strong> second harmonic flapping adds. Two bladed rotors are prone to 2P hop. (b) <strong>The</strong> third harmonic<br />

cancels in a two-bladed rotor.<br />

rotor, or indeed for a rotor with a higher number <strong>of</strong> blades. In Figure 3.37(b) is shown<br />

<strong>the</strong> effect for <strong>the</strong> third flapping harmonic. Here <strong>the</strong> third harmonic waveform is in <strong>the</strong><br />

same phase for each blade and so <strong>the</strong>re will be summation.<br />

Now that <strong>the</strong> general principle is clear, <strong>the</strong> result for any number <strong>of</strong> blades and any<br />

harmonic can readily be established. This is summarized in Figure 3.38(a) where it will<br />

be seen that <strong>the</strong> lowest hopping frequency is numerically equal to <strong>the</strong> number <strong>of</strong> blades<br />

and that <strong>the</strong>se frequencies may be multiplied by integers to find <strong>the</strong> higher hopping<br />

modes.<br />

When in-plane forces are considered, <strong>the</strong> results are quite different. Figure 3.38(b)<br />

shows <strong>the</strong> result <strong>of</strong> various harmonics with different numbers <strong>of</strong> blades. <strong>The</strong>se forces<br />

will be heterodyned by rotor speed so that <strong>the</strong> result on <strong>the</strong> hull <strong>of</strong> a rotor frequency nP<br />

will be vibration at (n − 1)P and (n + 1)P whereas <strong>the</strong> frequency P is absent. Following<br />

<strong>the</strong> principles <strong>of</strong> vector summation explained above, certain harmonics cancel at <strong>the</strong><br />

hub. Note that as with hop forces, <strong>the</strong> lowest frequencies at which rocking forces can<br />

exist are numerically <strong>the</strong> same as <strong>the</strong> number <strong>of</strong> blades, with <strong>the</strong> harmonics obtained by<br />

integer multiplication. <strong>The</strong> difference between Figure 3.38(a) and Figure 3.38(b) is that

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