Morphing wings provide a potential avenue to improve aerodynamic performance of aircraft operating at multiple design conditions. Nevertheless, morphing wing design is constrained by the mutually exclusive goals of high load-carrying capacity, low weight, and sufficient aerodynamic control authority via conformal shape adaptation. This trade-off can be addressed by exploiting the stiffness selectivity and shape “lock-in” properties enabled by using bistable beam-like elements within compliant structures. In this paper, we present an aero-structural optimization method to realize morphing structures with selective stiffness and shape “lock-in” capability from embedded bistable elements. We leverage an embeddable beam element with an invertible curved arch that provides stiffness selectivity and camber variation to the proposed rib geometry. Optimization objectives and constraints are designed to maximize the structure’s stiffness change and camber morphing “lock-in” effect when operating at two distinct flight conditions. Using the optimization results, we manufacture a wing section demonstrator with selective stiffness and “lock-in” morphing featuring two optimized ribs, a load-carrying skin made of a carbon reinforced laminate, Macro-Fiber Composite (MFC) actuators, and a servo-controlled mechanism for switching the bistable elements’ states. The power and energy requirements of actuating and holding a target deflection are experimentally measured and compared. The results show that the bistable elements can assist in holding a target deflection at a reduced energy cost. Finally, we test the experimental demonstrator in a low-speed wind tunnel demonstrating the load carrying capability and lift variation achieved from switching states.
Bistable composite laminates have been widely studied for applications in morphing structures due to their ability to exhibit different shapes and distinct degrees of stiffness about each stable state. Laminates with low aspect ratios (Length/Width) suffer from decreased bending stiffness and easily lose bistability. The present work addresses this challenge by introducing parallel slits along the width of the laminate, thereby relieving the deformation and enabling natural curving of the laminates. We demonstrated this by developing a numerical model of a two-section bistable composite laminate with clamped boundary condition. The design of the slits is chosen by conducting stress analysis on the laminate and using the Tsai-Wu failure criterion to assess failure at the critical locations. Finally, a parametric study is conducted on the laminate with slits to determine the maximum width that preserves bistability.
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