On the interpretation of bending-torsion coupling for swept, non-homogenous wings

The wide range of conflicting definitions of the axis on a general swept, non-uniform, non-homogenous wing about which there is no bending/torsion coupling are reviewed. A generalization of these definitions is made, with an emphasis on whether deflections and loads on a local streamwise section or the entire wing are considered. It is demonstrated, using a flexibility matrix approach, that the loading case must be considered in order to be able to accurately determine the flexural axis of a typical wing structure. The methodology is demonstrated using three numerical models: a simple swept wing, an aluminum wing box, and a tow-steered, variable stiffness, composite plate wing. Finally, the sensitivity of the flexural axis is considered and it is shown that the global flexural axis is much more sensitive to modeling or measurement errors than the local flexural axis.