The effect of chordwise flexibility on the aerodynamic performance of microrobot-inspired flapping wings

Unsteady aerodynamic mechanisms involved in the lift production of flapping-and-pitching wings is a developing topic of contemporary research and are yet not fully understood. For biologically-inspired flying machines such as sub-gram flapping wing micro-air-vehicles (FW-MAVs) which operate in flow with Re < 1000, these mechanisms are critical for the achievement of net positive lift, control authority and payload capacity. The aerodynamic performance is sensitive to the wing stiffness, with complex three-dimensional and time-varying flow structures evolving along and behind the wing. Wing flexibility is defined by the dimensionless parameter effective stiffness, the bending rigidity normalized by the dynamic fluid pressure, which is used to design four wings with distinct stiffness. This work uses scaled wings undergoing prescribed sinusoidal flapping and pitching motion with zero mean flow to study the induced flow and forces during hovering. Tomographic PIV is used to visualize the 3D vortex formation and force-generating flow structures while a 6-axis force-torque sensor at the wing root captures instantaneous data. The coefficients of lift and drag are computed for several cycles of flapping for all wing types and compared to the results of the flow visualization.