Meeting Abstract
Hovering hummingbirds generate aerodynamic lift during both downstroke and upstroke by rotating their wings along with spanwise direction. Since their wings are composed of radially-spread multiple feathers, passive flexural deformation of each feather is expected to contribute the rotational twist of the wings. Flexural stiffness of the feathers and its distribution in the hummingbird wing, however, are still largely unknown. In this study, we directly measured the flexural stiffness of feather shafts of a museum specimen of hummingbirds as well as several other species of small birds by cantilever bending tests for multiple locations aiming to quantify the features of distributed flexural stiffness of the hummingbirds. Moreover, in order to investigate its effect on lift generation in hovering, we created wing models from polyimide films and UV-laser-cut CFRP (carbon fiber reinforced plastic) artificial feather shafts mimicking the measured flexural stiffness. The bending tests revealed that the flexural stiffness exponentially increased with the distance from the feather tip. It was also found that the inner feathers were notably less stiff than the other leading feathers in hummingbirds, unlike other small birds. By comparing the wing model for hummingbirds with that for other small birds using an electric flapping mechanism mimicking hummingbird hovering, we found that increased flexibility of the inner feathers induced further rotational torsion at distal region, resulting in increase in lift and decrease in power consumption. This suggests that stiffness of the hummingbird feathers have evolved to adapt for their unique hovering flight.
