10 −4 10 −2 10 0 10 2 10 4 30.4 30.5 30.6 30.7 30.8 30.9 31 31.1 31.2 100 30.58 30.6 100 31.0358 31.0360 Energy (log scale) Frequency (Hz) a b (a) (b) tongue3:1 tongue5:1 tongue9:1 Figure 10: FEP of the first (symmetric) wing torsional NNM of the Paris aircraft. NNM shapes at energy levels marked in the FEP are inset; they are given in terms of the initial displacements (m) that realize the periodic motion (with zero initial velocities assumed). 238
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