Figure 4. Principal Panel Mode (22 Hz), Isometric View (left) and Top View (right) The measurement of panel resonances was not the primary objective of the free boundary condition modal tests as the principal goal was to provide validation data for the global bending and torsional modes. Although some aspects of the instrumentation design and test execution could have been improved for measurement of panel resonances, these preliminary test results provide some key observations that have motivated the subsequent work. These include: (1) Panel resonances are relatively low frequency modes, (2) A series of panel resonances of increasing complexity are present, and (3) Panel resonances are present in a section of the structure that is important for buckling design considerations and which is also susceptible to damage. Some natural questions which we have investigated [2] and seek to further investigate include the ability to predict panel resonances in high-fidelity finite element models and their potential uses for blade evaluation, structural health monitoring, and design. B. BSDS Seismic-Mass-on-Airbags Boundary Condition Modal Test A second series of modal tests was performed on the BSDS blade. The new test was devised to validate BSDS structural models, particularly in the root region. A root boundary condition consisting of a seismic-mass-on-airbags was utilized in this test to provide a boundary condition that could be accurately characterized for inclusion in the structural analysis. This approach is very important to eliminate error in the boundary condition, which has a large effect on the modal parameters. In addition, this boundary condition provides a service-type boundary condition because the root region of the blade is strained and the seismic mass can move as dictated by the dynamics of the boundary condition. The boundary condition is modeled with a simple mass-damper-spring model. A full description of the test and validation results is reported in References 7 and 8. A photo of the test concept is shown in Figure 5. 96
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