In-flight data measurements are available for various flight conditions, including subsosnic, transonic and supersonic flight regimes. Research at MIT focuses on extracting the useful information from the data and using this information to predict the flutter boundary. To get structural response data of the F-18 SRA, NASA uses DEI exciters that are mounted at the tips of the wings. These exciters create a force acting on the aircraft. To measure the response of the aircraft, ten sensors located in different parts of the aircraft are employed. Thus, the problem can be formulated as a System Identification of a 2 input 10 output system.

A Time-frequency analysis method was developed which improves System Identification techniques for frequency sweep excitation signals. Forcing with linear and logarithmic frequency sweep excitation signals allows the use of time-frequency system identification methods for the aeroelastic analysis. A GUI was developed to facilitate the time-frequency decomposition of a signal. The GUI also aids interactive "de-noising" of the signals in time-frequency domain. "De-noised" input and output signals can be used to generate transfer function estimates.

The transfer functions obtained are used to identify natural frequencies and damping ratios of the structural modes. The estimates of the natural frequencies and damping ratios for different flight conditions are used to fit the analytical model of the F-18 SRA. This model is available from Finite Element Analysis and unsteady aerodynamic forces approximation. The "fitted" model is used to predict the flutter boundary.

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