From insects to machines

Insects and hummingbirds remain unmatched in their aerodynamic ability to hover in place in addition to other acrobatic feats such as flying backward and sideways by exploiting flapping-wing motion. Although this remarkable ability is key to making small-scale aircraft, flapping-hovering behavior has been difficult to reproduce artificially because of the challenging stability, power, and aeroelastic phenomena involved. Recent interest in small-scale unmanned air vehicles, especially those capable of hovering like insects and hummingbirds, is driven by many potential applications. A number of flapping machines have been developed, but only two are capable of untethered hovering flight. A key challenge is to demonstrate a stable untethered flapping-hovering ability at a weight and power approximating that of insects and birds where flapping-hovering flight is observed in nature. Here we demonstrate, for the first time, a passively stable 24-g machine capable of flapping-hovering flight at a Reynolds number similar to insects (Re = 8 times 10³). This architecture, particularly the passive stability, may help in the design of insect-sized hovering vehicles as well as shed light on the aeroelastic dynamic principles underlying insect flight.