On the Benefits of Engaging Coupled-Adjoint to Perform High-Fidelity Multipoint Aircraft Shape Optimization

Aircraft shape optimization must be performed with multiple flight conditions in order to produce a robust aerodynamic design. Under these flight conditions, the aircraft is subjected to different aerodynamic loads which translate into different static aeroelastic equilibria. As the optimization proceeds, the external shape is modified and consequently alters each aero-structural coupling. Modeling the latter in the adjoint-based optimization process therefore appears necessary to properly take flexibility effects into consideration, not only to get the right state variables but also to compute consistent coupled sensitivities. Nonetheless, relying on a coupled-adjoint increases the wall-clock time of the optimization process with respect to a more classical uncoupled aerodynamic approach. This paper therefore assesses the benefits of the coupled approach with respect to an uncoupled one, by quantifying them on an industrially-representative long range aircraft test case. A high-fidelity 5-point optimization problem is considered, relying on RANS CFD for the aerodynamics and on a FEM model of the aircraft for the structural analysis. The optimizations consist in minimizing the weighted drag coefficient subject to lift, pitching moment and geometrical constraints with respect to 110 variables controlling the twist law and cross-sectional camber laws. The two approaches are compared on cost efficiency, geometrical proximity and by analyzing the far-field drag breakdown improvements for each point of the problem.