An approach, based on the indicial method, is described for computing the unsteady lift, pitching moment, hinge moment and the drag on a thin airfoil in subsonic compressible flow due to arbitrary motion of a trailing-edge flap. first, exact indicial responses at small values of time due to trailing-edge flap deflection are obtained from linear unsteady subsonic theory in conjunction with the aerodynamic reverse flow theorems. Using an assumed analytic form for the indicial functions, these results are then used to help obtain complete asymptotic approximations for the indicial responses due to impulsive flap deflection and flap rate motion. The airloads due to arbitrary flap motion are subsequently obtained in state-space form, which is a form suitable for many forms of aeroelasticity analyses. Validation of the method is conducted with experimental data for oscillating flap motion at various subsonic Mach numbers.
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