Adaptive control of a nonlinear two-dimensional wing-flap system operating in an incompressible flowfield is studied. An output feedback control law is implemented, and its performance toward suppressing flutter and limit cycle oscillations, as well as reducing the vibrational level in the subcritical flight speed range, is demonstrated. The control law proposed here is applicable to minimum phase systems, and conditions for stability of the zero dynamics are provided. The control objective is to design a control strategy to drive the pitch angle to a setpoint while adaptively compensating for uncertainties in all of the aeroelastic model parameters. It is shown that all of the states of the closed-loop system are asymptotically stable. Furthermore, an extension is presented to include flap actuator dynamics. Simulations have been presented to validate the efficacy of the proposed strategy. Pertinent conclusions have been outlined. Nomenclature b = semichord of airfoil c h , c α = structural damping coefficients in plunge and pitch c lα , c mα = lift and moment curve slopes per angle of attack c lβ , c mβ = lift and moment curve slopes per control surface deflection h = plunging displacement I α = mass moment of inertia of airfoil about elastic axis k h , k α = structural spring stiffness in plunge and pitch L = aerodynamic lift M = aerodynamic moment m = mass of airfoil t = time U = freestream velocity x, y = horizontal and vertical coordinates x α = dimensionless distance from elastic axis to midchord, positive rearward α = pitch angle β = flap angle ρ = air density

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