Wing twist and folding work in synergy to propel flapping wing animals and robots

We designed and built a three degrees-of-freedom (DOF) flapping wing robot, Flapperoo, to study the aerodynamic benefits of wing folding and twisting. Forces and moments of this physical model are measured in wind tunnel experiments over a Strouhal number range of St = 0.2–0.4 - typical for animal flight. We perform particle image velocimetry (PIV) measurements to visualize the air jet produced by wing clapping under the ventral side of the body when wing folding is at the extreme. The results show that this jet can be directed by controlling the wing twist at the moment of clapping, which leads to greatly enhanced cycle-averaged thrust, especially at high St or low flight speeds. Additional benefits of more thrust and less negative lift are gained during upstroke using wing twist. Remarkably, less total actuating force, or less total power, is required during upstroke with wing twist. These findings emphasize the benefits of critical wing articulation for the future flapping wing/fin robots and for an accurate test platform to study natural flapping wing flight or underwater vehicles.