Bat-inspired wing clapping during upstroke improves lift and power economy

Inspired by the flight of the frugivorous bat Cynopterus Brachyotis, we designed and built a two degree-of-freedom flapping wing platform to study the aerodynamic benefits of wing folding. We tested this physical model in a wind tunnel over a Strouhal number range of 0.2 – 0.4 - typical for animal flight. Motion tracking is employed to ensure that the wings follow their prescribed motion, while aerodynamic forces and moments are measured using a six-axis load cell. The power required from the motors to generate the motion is also monitored. The measurements are compared with predictions of a quasi-steady model based on blade element momentum theory (BEMT). Finally, PIV is measure the air jet produced when the wings come together below the body at the start of the upstroke. The PIV measurements are analyzed using a control volume analysis. It is found that wing folding improves overall lift generation and power economy - the ratio between lift and power expenditure.