High-Fidelity Gradient-Based Wing Structural Optimization Including a Geometrically Nonlinear Flutter Constraint
E. Jonsson, C. Riso, B. B. Monteiro, A. C. Gray, J. R. R. A. Martins, and C. E. S. Cesnik
AIAA SciTech Forum, 2022
Higher-aspect-ratio lightweight wings can make aircraft more energy efficient thanks to induced drag reduction. Because such wings exhibit large deflections, affecting the flutter onset, design optimization using linear flutter analysis is inadequate. To address this issue, we develop a framework for including a geometrically nonlinear flutter constraint into high-fidelity, gradient-based wing structural optimization. The framework evaluates the objective function (mass) and the stress and adjacency constraints in a built-up finite element model to capture the structural details of realistic wings. This built-up finite element model is reduced to a low-order beam representation to make geometrically nonlinear flutter analysis tractable for optimization. The geometrically nonlinear flutter analysis considers the wing deformed shape at each flight condition and aggregates the associated damping values into a scalar flutter constraint. The flutter constraint is differentiated with respect to the built-up structural sizing variables using the adjoint method to enable large-scale optimizations. The framework is demonstrated by minimizing the mass of a high-aspect-ratio wingbox subject to the geometrically nonlinear flutter constraint along with stress and adjacency constraints. The geometrically nonlinear flutter constraint introduces a mass penalty up to about 60% of the baseline mass compared with the 10% mass penalty by a linear flutter constraint based on the wing undeformed shape. This methodology can help design new-generation energy-efficient aircraft with high-aspect-ratio wings that require considering geometrically nonlinear effects early in the design cycle to prevent flutter.