Aerodynamic interference depends on stroke plane spacing and wing aspect ratio in damselfly model wings

The fluid dynamics of aerodynamic force control in insects depends on how oscillating wings interact with the surrounding air. The resulting flow structures are shaped by the flow induced by the wing’s instantaneous motion but also on flow components resulting from force production in previous wing strokes and the motion of other wings flapping in close proximity. In four-winged insects such as damsel- and dragonflies, the flow over the hindwings is affected by the forewing downwash. In these animals, a phase-shift between the stroke cycles of forewing and hindwing modulates aerodynamic performance of the hindwing via leading edge vortex destruction, changes in local flow condition and the wake capture effect. This review is engaged in the significance of wing-wake interference for force control, showing that in damselfly model wings the strength of phase-dependent force modulation critically depends on the vertical spacing between forewing and hindwing stroke planes and the aspect ratio of both wings. We conclude that damsel- and dragonflies reach maximum steering capacity for body posture control when forewings and hindwings flap in close proximity and have similar length. The latter findings are of significance for the evolution and diversification of insect wings because they might explain why forewings and hindwings are little different in the order Odonatoptera.