Hummingbird-bat hybrid wing by 3-D printing*

Hovering hummingbirds have inspired small flapping-wing aerial robots. Natural flyers, including hummingbirds and bats, undergo torsional wing deformation during flapping flight owing to complex wing structure, while previous artificial wings were relatively simple and difficult to design the torsional flexibility. In this paper, we proposed a hummingbird-bat hybrid (HBH) wing in which torsional flexibility was implemented by an available fabrication technology. The HBH wing had a torsional arm at the leading edge inspired by a torsional wrist of a hummingbird. A bat-like stretchable wing membrane was also employed not to constrain the wing torsion. The membrane was supported by wing shafts of which bending stiffness was designed based on that of the feather shaft of a hummingbird. The three-dimensional (3-D) shape of the torsional arm and wing shafts was created by 3-D printing. The effect of the torsional arm and stretchable membrane on lift generation and deformation was evaluated using an electric flapping mechanism. It was confirmed that the torsional arm actually enhanced the passive wing torsion. The stretchable wing membrane further promoted the torsion effect of the torsional arm. Consequently, the HBH wing did not increase lift, but efficacy, defined as lift per input power, was greatly improved by 14% at most compared with the wing without a torsional arm.