Passive wing rotation in flexible flapping wing aerodynamics

Insect wings are flexible. For rigid wings lift enhancing unsteady aerodynamics mechanisms, such as delayed stall via leading-edge vortices (LEVs), wake-capture, and rotational forces, characterize the lift generation of a hovering insect. We have uncovered a novel mechanism that fruit fly size insects can utilize to further increase the lift by adjusting its wing shape passively: A pair of a LEV and a trailing-edge vortex shed in the previous stroke induces a downward wake upstream of the wing, which acts as a wind gust and reduces the effective angle of attack. By streamlining its shape, the flexible wing is then able to mitigate the negative influence from this wing-wake interaction, resulting in a higher lift compared to its rigid counterpart. Furthermore, we show that for the flexible wings, the lift generation is the highest for the symmetric rotational mode, which is consistent with the kinematics exhibited by insects, such as fruit flies and honeybees. These results complement the existing insect flight aerodynamics and have the potential to be adapted for the development of flapping wing Micro Air Vehicles.