This study addresses critical mass property variations in T500 tiltrotor aircraft (500 kg-class) during structural reconfiguration by developing a multi-stage coordinate transformation methodology. The approach resolves inertial coupling effects, validated via multi-axis measurements showing < 0.5% CG error and 92.3% inertia tensor agreement. A bivariate polynomial model achieves high-fidelity CG prediction (<3% MAC residuals, R2 = 0.983), enabling 96.3% stability margin compliance in flight tests. The framework reduces certification iterations by 37%, establishing a robust foundation for variable-configuration aircraft design and control optimization.