The whirl flutter stability of a scaled tiltrotor wind tunnel model (TRAST) has been studied. A detailed wing-pylon structure model has been implemented, and integrated with a wing extension. The effect of a wing extension on tiltrotor whirl flutter stability has been investigated. Sizing studies of wing extensions are conducted based on stiffness required. The integrated rotor, wing-pylon and wing extension model represents all inertia, stiffness, and load path details which may provide more accurate prediction of wing mode damping of the system. The results indicate that a wing extension can generate significant aerodynamic damping for both wing beam and torsion modes. The downstop spring stiffness has significant influence on wing beam mode damping ratio. The geometric orientations such as sweep and cant angles have mild effect. A thin plate is used to tune the wing extension beam mode frequency to be closer to wing beam and torsion frequencies. The results show that tuning may help enhance both wing beam mode and torsion mode damping.