Aerodynamics and stability of hawkmoth forward flight with flexible wing hinge

Flying insects can achieve remarkable and robust flapping aerodynamic performance and flight stability in various environments partially owing to the passive mechanisms of their wings and bodies, but a careful analysis associated with flexible wing hinge for forward flight is missing. Here we develop a fluid-structure interaction model that couples the one-torsional-spring-based elastic wing-hinge dynamics with the flapping aerodynamics to study the aerodynamics and flight stability of the hawkmoth, Manduca sexta, with flexible wing hinge at various flight velocities. The results show that the leading-edge vortex, the body vortex, and their interactions are responsible for augmenting the vertical force production interactively at all flight velocities, enabling a 6.5% increase at fast flight speeds. The elastic storage enabled by the flexible wing hinge exhibits a J curve, achieving high power efficiency at intermediate forward flight velocities. We verify that the realistic wing-hinge stiffness leads to optimal aerodynamic performance in terms of vertical force production and power cost. External disturbance rejection based on the flexible wing hinge is highly robust in multiple directions, independent of the forward flight velocity. This study highlights the importance and significance of flexible wing hinges in biomimetic designs for flapping micro aerial vehicles.