Nonlinear Response Structural Optimization of a Joined Wing Using Equivalent Loads

Abstract

The joined wing is a new concept of the airplane wing. The forewing and the aft wing are joined together in the joined wing. The joined wing can lead to increased aerodynamic performances and reduction of the structural weight. The structural behavior of the joined wing has a high geometric nonlinearity according to the external loads. Therefore, the nonlinear behavior should be considered in the optimization of the joined wing. It is well known that conventional nonlinear response optimization is extremely expensive; thus, the conventional method is almost impossible to use for large-scale structures such as the joined wing. In this research, geometric nonlinear response optimization of a joined wing is carried out by using equivalent loads. The used structure is a joined wing that is currently being developed in the U.S. Air Force Research Laboratory. Equivalent loads are the load sets that generate the same response field in linear analysis as that from nonlinear analysis. In the equivalent loads method, the external loads are transformed to the equivalent loads for linear static analysis, and linear response optimization is carried out based on the equivalent loads. The design is updated by the results of linear response optimization. Nonlinear analysis is carried out again and the process proceeds in a cyclic manner until the convergence criteria are satisfied. It was verified that the equivalent loads method is equivalent to a gradient-based method; therefore, the solution is the same as that of exact nonlinear response optimization. The fully stressed design method is also used for nonlinear response optimization of a joined wing. The results from the fully stressed design and the equivalent loads method are compared.