The center-of-gravity (CG) envelope of tiltrotor aircraft defines critical flight safety boundaries, with longitudinal CG deviations beyond±0.2MAC reducing short-period damping ratios by 40% and elevating Dutch-roll frequencies by 25%, degrading flight quality from Level 1 to Level 3. This study proposes a coupled lateral-longitudinal CG envelope prediction framework through nonlinear flight dynamics modeling integrating rotor-body aerodynamics, gyroscopic coupling, and dynamic inflow modeling. Key capabilities include: (1) bifurcation analysis of equilibrium states to track CG envelope evolution during 0–90 \(^\circ \) nacelle tilt conversions, and (2) eigenvalue-based stability criteria defining operational limits across helicopter-transition-airplane regimes. Experimental validation confirms the T500 maintains CG positions within ±5% static margins during conversion, exhibiting: (a) cross-axis phase margins >6 dB/45 \(^\circ \) , and (b) <10% control power variation between extreme CG configurations.