106 W.H. Semke et al. Z X Y Z X Y Z X Y Z X Y Fig. 10.5 Clockwise from top left: base test, 95 % test, 75 % test, 90 % test Z X Y Fig. 10.6 Shaker excitation locations Table 10.2 Summary of natural frequencies for the experimental modal analysis Base 95 % 90 % 75 % Description 12.4 12.4 12.4 12.4 Mode 1 wing bending 17.3 17.5 17.5 17.5 Tail torsion, antisymmetric wingtip bending 20.8 20.8 20.8 21.0 Wingtip, wing, tail, HS, VS torsion, wingtip bending 26.7 26.0 26.0 25.9 Wing torsion, slight HS bending 28.9 28.2 28.1 28.1 Symmetric wing torsion 30.2 30.8 30.8 30.7 Antisymmetric wingtip bending, tail wag 33.8 36.5 36.1 – HS, VS torsion 39.9 39.7 39.8 39.8 HS bend/torsion, mode 2 wing bending, VS torsion 45.0 44.9 44.9 44.7 Mode 2 wing bending, HS bend/torsion, VS torsion was excited with random excitation. ModalVIEW was then prompted to record data from the accelerometers and when ModalVIEW was done sampling the shaker was turned off. This was repeated so that two data sets were gathered at each excitation location. Once all of the vibration data was gathered, analysis was performed using ModalVIEW. The FRFs generated were analyzed and a list of natural frequencies was created. ModalVIEW was then used to build a model of the Super Hauler. The accelerometers were assigned to their respective nodes and degrees of freedom. The modal response of the structure can be animated illustrating the motion of each mode shape at its natural frequency. A summary of the natural frequencies found in the tests can be seen in Table 10.2 and the corresponding mode shapes can be seen in Fig. 10.7. It can be seen that all of the natural frequencies that were observed in the base model were also observed in the 95 % test. However, as would make sense from the channels that were removed, any motion in the fuselage was undetectable using the 95 % data. Since there was relatively little activity in the fuselage when compared to the rest of the structure, the inability to observe that motion is fairly insignificant. This test only uses 11 sensors versus 16 sensors for the base test resulting in a 31 % reduction in sensors.
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