The frequency response of a helicopter rotor aeroelastic system in forward flight is studied by a three-dimensional unsteady wake model coupled with a rigid-bladed flapping rotor. This three-dimensional wake model is extracted from Peters' finite-state inflow theory. Fourier series are applied to the blades flapping angles and rotor inflow state coefficients to extend the coupled equations to frequency domain. The frequency response of these coupled system lifts and control derivatives are compared with two-dimensional results and show that in forward fight, the lifts in each resonant area are lower than the vertical flight condition, and reveal that the wake layering is not diminished in forward flight. The wake effect still plays an important role as it does in hover.