This article presents details of the development of a special class of laminate, possessing Extension-Shearing Bending-Twisting coupling, necessary for optimised passive-adaptive flexible wing-box structures. The possibility of achieving a measurable drag reduction in cruise flight, without the cost or reliability issues associated with active control mechanisms, is of significant interest for achieving increased fuel burn efficiency, and meeting associated emissions targets. The introduction of passive Bending-Twisting coupling at the wing-box level has been previously demonstrated through laminate level tailoring with Extension-Shearing coupling only, but the limited design space and the possibility for ply terminations (to produce tapered thickness) effectively rule out this special class of laminate for practical construction. The study is now broadened to consider laminates with Extension-Shearing and Bending-Twisting coupling, beyond the less well-known un-balanced and symmetric design rule or indeed balanced and symmetric designs with off-axis alignment. Results reveal a vast laminate design space with Extension-Shearing coupling that can be maximised without the unfavourable strength characteristics associated with off-axis alignment. Results also reveal that shear buckling strength can be maximised through Bending-Twisting coupling when load reversal is not a design constraint.
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