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A Post-Flutter Response Constraint for Gradient-Based Aircraft Design Optimization

AIAA 2021-3082
Session: Aeroelastic and Aerostructural Optimization I
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Considering flutter as a constraint in aircraft design optimization has the potential to enable energy-efficient vehicle designs with high-aspect-ratio wings while meeting flutter requirements early in the design cycle. However, nonlinear aeroelastic effects in high-aspect-ratio wings may cause self-sustained oscillations to arise even before flutter occurs (subcritical bifurcation). To address this problem, this paper presents a novel post-flutter response constraint for gradient-based aircraft design optimization. The constraint prevents self-sustained oscillations inside the flight envelope independent of the flutter boundary. Moreover, it avoids transient or iterative calculations and can handle discontinuities due to mode switches and hump modes. The formulation is demonstrated by optimizing a typical aeroelastic section with a nonlinear torsion spring and an initially subcritical bifurcation. The results show that the post-flutter response constraint suppresses the subcritical bifurcation while keeping the flutter constraint active, so avoiding a conservative solution.