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Airplane Ground Vibration Testing - Nominal Modal Model Correlation

A brief overview is given of transport airplane ground vibration testing (GVT) at Boeing Commercial Airplane Group. A GVT is a modal test conducted for the purpose of validating and improving a structural dynamic model of the airplane. A “small signal” model is quickly and conveniently identified from multi-input burst random excitation. Sine excitation is used selectively to adjust the model to account for moderate departure of the airplane from linearity. Linear structural dynamic models of transport airplanes are used to represent and predict vibrating behavior. When coupled with aerodynamic models, they are used to predict flutter and dynamic loads. The structural dynamic model is validated by comparing its predicted modes with the experimental modes identified from a GVT (see Figure 1). The airplane structure is modeled in the tested configuration, including support system, fuel and payload mass properties, and with landing gear down. This validated model is then modified to represent various in-flight configurations and a variety of failure conditions. The structure of transport airplanes is found to depart weakly to moderately from linear behavior, so one issue is to identify a ‘nominal’ linear model (or models) which are representative (and predictive) of the operating structure. We consciously impose the assumption of linearity upon the test structure.