10 Aeroelastic Analysis Using Ground Vibration Test Modes 153 Table 10.3 Comparison of free-free and soft-suspension FEM modes FEM Bungee FEM Free-Free FEM Bungee FEM Free-Free Freq Cross Mode Mode Freq Freq Pct Ortho No. No. (Hz) (Hz) Diff 1 3 0.41 0.00 -100.0 80 2 5 0.80 0.00 -100.0 99 3 4 0.85 0.00 -100.0 84 4 2 1.18 0.00 -100.0 88 5 6 1.22 0.00 -100.0 81 6 1 3.44 0.00 -100.0 86 7 7 5.50 5.47 -0.4 100 8 8 7.49 7.47 -0.3 100 9 9 10.71 10.69 -0.3 100 10 10 10.93 10.92 -0.1 100 11 11 13.51 13.51 0.0 100 12 12 14.72 14.72 -0.1 100 13 13 21.74 21.74 0.0 100 14 14 21.93 21.93 0.0 100 15 15 28.79 28.78 0.0 100 16 16 29.59 29.58 0.0 100 17 17 30.84 30.81 -0.1 100 18 18 37.62 37.62 0.0 100 19 19 38.24 38.24 0.0 100 Table 10.4 Comparison of critical flutter frequencies and velocities for analytical models FEM Free-Free - 6 RBM FEM Bungee - 6 RBM FEM Bungee - 4 RBM FRS Bungee - 4 RBM Flutter mode description Frequency (Hz) Velocity (ft/s) Frequency (Hz) Velocity (ft/s) Frequency (Hz) Velocity (ft/s) Frequency (Hz) Velocity (ft/s) Rigidbody pitch – – 3.2 207 3.2 207 3.2 204 Wing symmetric 13.0 441 13.0 442 13.0 442 13.2 444 Wingantisymmetric 13.8 454 13.7 454 13.7 454 14.1 456 Tail symmetric 29.8 404 29.9 404 29.9 404 30.0 417 Tail antisymmetric 30.8 410 30.8 410 30.8 410 31.3 413 Table 10.4 summarizes the initial analytical investigation into flutter predictions performed using the correlated FEM in both free-free and soft-suspension boundary conditions. Additionally, the results using the modes extracted directly from the FRS and a subset of the soft-suspension FEM modes in which the same four RBMs as extracted from the FRS are included. As shown in Table 10.4, the wing and tail critical flutter modes have only minimal differences in frequency and critical velocity for all configurations. For the configurations with soft-support RBMs, an additional rigid body pitch instability is predicted. Figures 10.7 and 10.8 present the velocity-versus-damping-and-frequency (V-G-F) diagrams for the free-free and soft-suspension FEMs, which show that only the RBMs have major differences in frequency and damping trends.
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