The application
The study of the normal modes for vibration of a helicopter rotor blade provides important information for the dynamical behaviour of the blade during operation. The modal analysis is typically done with a setup as shown in Figure 1. On one end, the blade is fixed to a rigid structure. On the other end, the blade can move freely.
A shaker is used to load the blade dynamically and strain gages are used to monitor the blade via the bending at different locations along the blade. By sweeping the frequency of the shaker, the different resonance frequencies, corresponding to the different modes, can be mapped. Figure 2 shows the typical strain signal during a chirped frequency sweep near the fundamental mode, which occurs at 12 Hz for this particular blade.
Scanning over a more extended frequency range evidently allows one to identify also the higher order modes of the blade.
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