Upstroke wing clapping in bats and bat-inspired robots offers efficient lift generation

Wing articulation is critical for the efficient flight of bird- and bat-sized animals. Inspired by the flight of Cynopterus brachyotis, the lesser short-nosed fruit bat, we built a two-degree-of-freedom flapping wing platform with variable wing folding capability. In the late upstroke, the wings 'clap' and produce an air jet that significantly increases lift production, with a positive peak matched to that produced in the downstroke. Though ventral clapping has been observed in avian flight, the potential aerodynamic benefit of this behaviour is yet to be rigorously assessed. We used multiple approaches-quasi-steady modelling, direct force/power measurement and particle image velocimetry (PIV) experiments in a wind tunnel-to understand critical aspects of lift and power variation in relation to wing folding magnitude over Strouhal numbers at St = 0.2-0.4. While lift increases monotonically with folding amplitude in that range, power economy (ratio of lift/power) is more nuanced. At St = 0.2-0.3, it increases with wing folding amplitude monotonically. At St = 0.3-0.4, it features two maxima-one at medium folding amplitude (approx. 30°) and the other at maximum folding. These findings illuminate two strategies available to flapping wing animals and robots-symmetry-breaking lift augmentation and appendage-based jet propulsion.