  The flexibility of biological propulsors such as wings and fins is believed
to contribute to the higher performance of flying and swimming animals compared
with their engineered peers. Flexibility seems to follow a universal design
rule that induces bending patterns at about one-third from the distal tip of
the propulsor's span. However, the aerodynamic mechanisms that shaped this
convergent design and the potential improvement in performance are not well
understood. Here we analyze the effect of heterogenous flexibility on the
flight performance of tumbling wings. Using experiments, numerical simulations,
and scaling analysis, we demonstrate that spanwise tip flexibility that follows
this empirical rule leads to improved flight performance. Our findings
attribute this improvement to flutter-induced drag reduction. This mechanism is
independent of the wing's auto-rotation and represents a more general trait of
wings with non-uniform tip flexibility. We conclude by analyzing the effects of
both spanwise and chordwise non-uniformity on the flight performance of
tumbling seedpods.
