4 J. E. Blackham and M. S. Allen 65 65.5 66 66.5 67 67.5 68 Frequencies (Hz) 10 1 10 2 Comparisons of Compiled FRFs Non-Linear Data Linear Data (a) Mode 1 - 66.55 Hz 114.5 115 115.5 116 116.5 117 117.5 118 Frequencies (Hz) 10 1 10 2 Comparisons of Compiled FRFs Non-Linear Data Linear Data (b) Mode 2 - 116.26 Hz 222 222.5 223 223.5 224 224.5 225 Frequencies (Hz) 10 1 10 2 Comparisons of Compiled FRFs Non-Linear Data Linear Data (c) Mode 3 - 223.14 Hz and Mode 4 - 223.79 Hz Fig. 4 Zoom in on Frequency Response Function for the first 4 modes of the frame and wing structure A few of the modes included in-plane motion. To identify these modes, the average of the FRFs for each of the accelerometers (i.e. the drive points) was plotted and is shown in Figure 6. Most of the modes are dominant in the accelerometers located on the wing and in the Z-direction (i.e. Acc. 2 - 5). The two exceptions are modes 9 and 14 at about 511 and 876 Hz, which are far more visible in the X-direction accelerometer 6 than in the Z-direction. The plots of the mode shapes in Figure 7 can be used to ascertain which might be most sensitive to nonlinearity due to the bolted joints. For example, Mode 1 would exert a load that would want to open and close the bolted joints. This type of motion typically does not change the effective frequency or damping much [11], and the FRFs in Fig. 8 seem to support this. Mode 2 also shows little change in frequency or damping, and this seems to be corroborated by the fact that the mode shape
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