Three-dimensional wing structure attenuates aerodynamic efficiency in flapping fly wings

The aerial performance of flying insects ultimately depends on how
flapping wings interact with the surrounding air. It has previously been
suggested that the wing’s three-dimensional camber and corrugation help to
stiffen the wing against aerodynamic and inertial loading during flapping
motion. Their contribution to aerodynamic force production, however, is
under debate. Here we investigated the potential benefit of
three-dimensional wing shape in three different-sized species of flies
using models of mCT-scanned natural wings and models in which we removed
either the wing's camber, corrugation, or both properties. Forces and
aerodynamic power requirements during root flapping were derived from
three-dimensional computational fluid dynamics modelling. Our data show
that three-dimensional camber has no benefit for lift production and
attenuates Rankine-Froude flight efficiency by up to ~12% compared to a
flat wing. Moreover, we did not find evidence for lift-enhancing trapped
vortices in corrugation valleys at Reynolds numbers between 137 and 1623.
We found, however, that in all tested insect species, aerodynamic pressure
distribution during flapping is closely aligned to the wing’s venation
pattern. Altogether, our study strongly supports the assumption that the
wing’s three-dimensional structure provides mechanical support against
external forces rather than improving lift or saving energetic costs
associated to active wing flapping.