To accurately predict the aero-servo-elastic response of helicopter blades fitted with active-control flaps (ACFs) under complex aerodynamic excitation, a high-fidelity coupled modelling framework is developed that consistently integrates three-dimensional large-deformation beam dynamics, flap-actuator dynamics, and unsteady rotor aerodynamics. The structural subsystem employs geometrically exact beam elements formulated in a floating frame of reference; the associated nodal degrees of freedom fully capture bending, torsion, and axial deformation. Root flexibility, viscous damping, and multiple hinge kinematics are incorporated through homogeneous transformation matrices embedded in the finite-element derivation. The ACF is introduced as an additional rigid panel. Aerodynamic loads are evaluated with a hybrid source–doublet panel / free-vortex approach. The rotor surface is discretized into source–doublet panels, the near wake is represented by a fixed vortex lattice, and the far wake is modelled as a free-wake filament system, thereby retaining the essential unsteady features of the flow. The numerical campaign begins with hover and forward-flight cases without flap actuation, where computational results are validated against wind-tunnel measurements to establish grid- and time-step independence and to define a baseline vibration level. Parametric sweeps over flap-input frequency and amplitude subsequently reveal pronounced amplifications in both torsional and flapping responses as the excitation parameters vary. The influence of flap chordwise and spanwise dimensions on the frequency-domain response is then examined, and an upper bound on flap length is proposed using a vibration-divergence peak criterion. The resulting framework thus provides a rigorous basis for the safe assessment of low-noise, low-vibration rotor concepts and for the future development of real-time, model-predictive vibration-suppression algorithms.