The forewing and the hindwing of a dragonfly have different geometry that could be an evolutionary specialization for better aerodynamic performance via sophisticated wing pitch control. Under different extent of wing pitching by the wing root musculature, the fore- and hindwings could exhibit different shape deformation and aerodynamic characteristics as a result of passive shape deformation. We measured the flow around the flapping wings using time-resolved particle image velocimetry (TR-PIV) to investigate the consequences of shape and the pitching mechanisms of the wings on the aerodynamics of dragonflies. The flow fields and pitching angle variations of the naturally actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. We found that the trailing edge vortex dynamics and the wake were affected by the wing shape only for the in-vivo experiment with muscle induced pitching. Under the in-vivo with muscle induced pitching, the hindwing took more part in generating horizontal momentum with larger pitching magnitude, due to the larger chord length compared with forewing. Meanwhile, when there was only pitching due to the wing membrane deformation of artificially actuated flapping, a slight difference in the surrounding flow structures was found between the hindwing and the forewing, and the net flow in one period was reduced nearly to zero. These results provided quantified evidence to the extent and importance of the pitching motion of the wings in dragonfly flight. The results of this work can be useful for the design of wings, their actuation mechanism, and the in-flight kinematics control of flapping wing micro air vehicles (MAVs).

蜻蜓的前翅和后翅具有不同的几何形状，可以通过先进的机翼俯仰控制来实现更好的空气动力学性能，这是一种进化的专业化技术。在机翼根部肌肉对机翼的俯仰程度不同的情况下，由于被动形状变形，前翼和后翼可能呈现出不同的形状变形和空气动力特性。我们使用时间分辨粒子图像测速仪（TR-PIV）测量了拍打机翼周围的气流，以研究形状和机翼俯仰机制对蜻蜓空气动力学的影响。将蜻蜓的自然致动翼的流场和俯仰角变化与仅通过拍打运动人工致动的同一翼的流场和俯仰角变化进行了比较。我们发现仅在肌肉诱发俯仰的体内实验中，后缘涡旋动力学和尾流才受到机翼形状的影响。在体内肌肉诱发俯仰的情况下，由于与前肢相比，弦长更大，因此后翼在产生更大俯仰幅度的水平动量中发挥了更大的作用。同时，当仅因人为操纵的襟翼的机翼膜变形而产生俯仰时，在后翼和前翼之间发现周围的流动结构略有差异，并且在一个周期内的净流量几乎减小至零。这些结果为蜻蜓飞行中机翼的俯仰运动的程度和重要性提供了量化的证据。这项工作的结果对于机翼的设计，其致动机制，