As the aerospace industry strives for unprecedented levels of efficiency, sustainability, and performance, traditional design and testing methodologies are proving insufficient. This session will highlight the latest breakthroughs in high-fidelity simulations and Multidisciplinary Design Optimization (MDO), which are now seamlessly integrated to create highly accurate and predictive digital twins of aircraft systems. We will explore how these simulations are enabling engineers to rapidly iterate on complex designs, from novel aerodynamic configurations and lightweight composite structures to advanced propulsion systems. The session welcomes presentations and discussions on a range of topics, including: 1) High-order schemes for fluid dynamics: e.g., finite-difference/element, Discontinuous Galerkin (DG), and Flux Reconstruction (FR) methods for more accurate and efficient CFD; 2) Aeroelastic analysis: e.g., with a focus on high-aspect-ratio and flexible wings, which are critical for enhancing aerodynamic efficiency; 3) High-fidelity structural analysis: e.g., material and geometrical nonlinear analysis to better predict the behavior of complex structures under various situations; 4) Multi-physics simulation: Covering complex interactions such as fluid-structure interaction (FSI) and combustion science, essential for integrated system analysis; 5) Data-driven methods: Utilizing reduced-order models (ROMs) and machine learning to accelerate the design cycle and manage massive datasets. Discussions will also address computational resource challenges and data-management needs in large-scale simulations. Designed for researchers and professionals at the intersection of aerospace engineering and computational science, this STS highlights the shift from physical prototyping toward virtual design paradigms—paving the way for innovative, sustainable aircraft development.