Development of flutter constraints for high-fidelity aerostructural optimization

High fidelity computational modeling and optimization of aircraft has the potential to allow engineers to produce more efficient designs requiring fewer unforeseen design modifications late in the design process. In order for the optimization algorithm to generate a useful design, all the relevant physics must be considered, including flutter. This is especially important for the high-fidelity aerostructural optimization of commercial aircraft, which is likely to result in wing designs that are prone to flutter. To address this issue, we developed a flutter constraint formulation suitable for gradient-based optimization. This paper investigates the feasibility of using a Doublet-Lattice Method (DLM) based flutter constraint for high-fidelity aerostructural optimization. The p-k flutter equation is solved using a determinant iterative method to obtain the eigenvalues. The Kreisselmeier–Steinhauser (KS) function is used to aggregate the damping values for individual modes into a single value that is used as the flutter constraint. To study the behavior of the flutter constraints using this method, we optimize a simple flat plate problem and perform a flutter analysis for a full transport aircraft using the uCRM configuration. We compute accurate and efficient derivatives for the DLM and the coupled derivatives with respect to structural sizing variables, as well as wing shape variables. These derivatives are computed using an automatic differentiation method and validated using the complex-step method. However, it is found that the current formulation using determinant iteration root finding method, is not adequate or robust, even for the simplest problems and reformulation is required.