Unsteady rotational loads of hovering flexible flapping wings with varying pitch axis location

Insects propel using flexible wings, flapping at the wing root and undergoing twist about torsion axis. The inherent wing flexibility combined with varying pitching axis location leads to non-linear interactions between wing inertia and aerodynamic loads. Understanding the impact of pitch axis location on passive torsional response and aerodynamic efficiency becomes pivotal. Therefore, current study employs chordwise flexible rectangular wings with an aspect ratio of 2 subjected to flapping and pitching motions. Flexibility was quantified by the non-dimensional structural parameter, effective flexural stiffness which depends on wing material, thickness and geometry. Pitch axis location from leading edge to mid-chord and non-dimensional rotation time for flapping (0.1, 0.5) and pitching (0.08,0.16,0.24) were systematically adjusted. Results indicate the rotational forces scale with chordwise offset. However, the exhibited scaling did not follow linear trend with pitch axis position and pitching velocity as observed in rigid wings. Further, force generation was found to be optimal for a pitch axis location between quarter and mid-chord. The influence of phase lag between leading and trailing edge due to wing flexibility was found dependant on pitch rotation time hinting on the structural dominance during wing rotation. Current study emphasizes rotational force generation scaled with pitch axis location, and material flexibility to mimic the adaptive performance of insect flight.