Robust utter margins can be computed for an aeroelastic model with respect to an associated uncertainty description that describes modeling errors. An on-line implementation to compute these robust margins is considered. The on-line approach generates uncertainty descriptions at test points and can account for time-varying errors in the model. A utterometer is introduced as an on-line tool to indicate a measure of distance to utter during a ight utter test. Such a tool is formulated based on

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... ust utter margin analysis. A ight test of an F/A-18 is simulated to demonstrate the performance of the utterometer. This tool is clearly more informative than traditional tracking of damping trends and provides accurate information about the true utter margin throughout the ight test. The simulation demonstrates characteristics of the utterometer that could improve ight test ef ciency by increasing safety and reducing ight time for envelope expansion. Nomenclature P = plant model P nom = nominal plant model P rob = robust plant model P true = true plant model q = dynamic pressure W in = weighting matrix on input uncertainty D = uncertainty operator D A = parametric uncertainty affecting state matrix D in = dynamic uncertainty affecting plant input dq = perturbation to dynamic pressure l = structured singular value