In this paper, a particular class of 2-D aeroelastic systems accounting for structural nonlinearities in pitching displacement and operating in an unsteady aerodynamic incompressible flowfield description is considered. By using the flap hinge torque of a trailing-edge flap surface as the control input, a continuous controller is proposed to suppress the aeroelastic vibrations of the wing section model. The control design based on the choice of the pitching angle as the output variable yields a semi-global asymptotic stability result. Furthermore, the system is theoretically shown to be robust to external disturbances. Numerical simulation results verify the efficacy of the proposed control strategy toward suppressing aeroelastic vibration in both pre- and post-flutter flight speed regimes under a multitude of external disturbances.