10 An Experimental Modal Channel Reduction Procedure Using a Pareto Chart 107 Fig. 10.7 Mode shapes and frequencies obtained from the experimental modal testing on the UAS The 90 % test also captured all of the natural frequencies that were seen in the base model. However, any motion in the fuselage or vertical stabilizer was undetectable using the 90 % data. The inability to observe this motion could be acceptable since the fuselage and vertical stabilizer don’t experience much motion and all of the modes were detected. This test uses 10 sensors versus 16, resulting in a 38 % reduction in sensors. The 75 % test captured almost all of the natural frequencies that were seen in the base model, missing one mode at 33.8 Hz. However, any motion in the fuselage or tail was undetectable in this test. The inability to detect a mode at 33.8 Hz can be traced to this fact since that mode consists of horizontal and vertical stabilizer motion. This lack of important information leads to the conclusion that this is too much channel reduction and that all of the important surfaces of the aircraft should be instrumented. This test uses 6 sensors versus 16, resulting in a 63 % reduction in sensors. It can be seen that, according to this test, 90 % of the motion should be captured by sensors if all of the modes are to be recognized. The values chosen in this case were chosen because they coincide with the significant structural components of the aircraft while resulting in nearly a 40 % reduction in required channels. Since the correct level is case dependent, this value should be left to user discretion. 10.6 Finite Element Modeling A finite element model was made of the UAV to simulate the dynamic behavior of the aircraft and to find the natural frequencies and mode shapes. ANSYS was used for the modeling where the wings and tail sections were meshed with plate elements and the fuselage with beam elements. The meshed model is shown in Fig. 10.8. In this simple model the wings and tail are constructed of plate elements of constant thickness and composition while the beam elements are a hollow tube with constant wall thickness and composition. The actual composition of the aircraft is much more complex. For example the wings are made of multiple ribs of wood with spars running through them and are covered with a plastic film while the fuselage is made of truss segments connected to plywood sheets. While this structure is very complex, the simplified FE model illustrates much the same behavior with far less modeling effort. A modal analysis was performed on the structure in a free configuration to identify the natural frequencies and mode shapes of the structure, summaries of the results are shown in Table 10.3 and Fig. 10.9. The number of elements used was varied to assess the influence on mode shapes and frequencies and it was observed that adding more degrees of freedom to this model did not change the results significantly. It was found that reducing the number of beam elements used did eliminate any fuselage bending, resulting only in wing and tail motion.
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