Flying insects and many aquatic animals rely on flapping for locomotion. A feature commonly
observed is that wings or fins, respectively, usually appear to be stiffer at the root, and more
compliant at the tip. Recent computational works on idealized propulsors have shown that
such distribution of flexibility may lead to better performances compared to the
homogeneous case [1,2], both in thrust production and efficiency, with peaks related to fluid-
structure resonances. Here, we conduct precision experiments on tapered elastomeric
flappers moving in silicon oil. Different sets of tapered flappers are carefully fabricated to fix
some chosen parameter, such as the total mass or, more importantly, the mean stiffness.
The control over the latter quantity is something that has been often overlooked in recent
experimental studies on flexible propulsor. Our method fills this gap and enables us to
investigate the role of the distribution of stiffness accurately. Our results qualitatively concur
with the above considered computational work, showing band-broadening for power and
thrust production at the resonant peak, as well as an overall higher efficiency for samples
with increasing tapering. This study simultaneously furthers our understanding of flapping
propulsion and might orient the design of small aquatic or air vehicles in the future.

Experimental Investigation on the Effect of Stiffness Distribution on the Hydrodynamic Performance of Flexible Flappers

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