This work is part of a research program to improve the efficiency, stability, and productivity of tiltrotor aircraft in order to meet the performance and sustainment goals of the Future Vertical Lift program. The XV-15 is used as a testbed to investigate the impact of rotor aerodynamics on whirl-flutter stability. Improvements to the state-of-the-art in prediction capability of tiltrotor aeroelastic stability through the use of time-domain coupled computational fluid dynamics (CFD) and computational structural dynamics simulations are presented. The Rotorcraft Comprehensive Analysis System (RCAS) is coupled to the rotorcraft CFD suite, Helios. Comparisons are made between linearized stability analysis, transient stability analysis using RCAS and transient stability analysis using CFD/CSD. For transient RCAS analysis, an algorithm based on the log decrement method is then used to determine the system stability. For transient CFD/CSD analysis, Prony’s method is used to determine modal frequency and damping. Good agreement is shown between the three methods of predicting tiltrotor whirl-flutter stability at speeds less than 250 knots. In a high speed case at 377 knots, large differences between the predicted values are displayed between CFD/CSD analysis and comprehensive analysis.