Passive dynamics regulates aperiodic transitions in flapping wing systems.

Natural and artificial flapping wing flyers generally do not exhibit chaos or aperiodic dynamic modes, though several experimental and numerical studies with canonical models of flapping foils have reported inevitable chaotic transition at high ranges of dynamic plunge velocity ( ). Here we considered the idealized case of a pitching-plunging flapping foil and numerically investigated the effects of passive pitching dynamics on the fluid forces and dynamical states, and compared it with a fully actuated wing. We found that in comparison to fully actuated foils, aperiodic transition can be avoided even for high when passive oscillations are allowed. Passive pitching modulated the relative foil orientation with respect to the incoming free stream to maintain a lower effective angle-of-attack throughout the stroke and reduced the leading-edge-vortex (LEV) strength. Absence of aperiodic triggers such as flow separation and strong LEVs keep the wake periodic, and chaotic transition is averted. In the presence of fluctuating inflow conditions, passive pitching attenuated the fluid loads experienced by the airfoil thus improving the wing's gust mitigating potential. These findings highlight the favorable properties of passive dynamics in regularizing aerodynamic loads on flapping wing systems and presents viable solutions for artificial flying platforms.