Robotic Avian Wing Explains Aerodynamic Advantages of Wing Folding and Stroke Tilting in Flapping Flight

Avian flapping strategies have the potential to revolutionize future drones as they may considerably improve agility, increase slow speed flight capability, and extend the aerodynamic performance. The study of live birds is time-consuming, laborious, and, more importantly, limited to the flapping motion adopted by the animal. The latter makes systematic studies of alternative flapping strategies impossible, limiting our ability to test why birds select specific kinematics among infinite alternatives. Herein, a biohybrid robotic wing is described, partly built from real feathers, with more advanced kinematic capabilities than previous robotic wings and similar to those of a real bird. In a first case study, the robotic wing is used to systematically study the aerodynamic consequences of different upstroke kinematic strategies at different flight speeds and stroke plane angles. The results indicate that wing folding during upstroke not only favors thrust production, as expected, but also reduces force-specific aerodynamic power, indicating a strong selection pressure on protobirds to evolve upstroke wing folding. It is also shown that thrust requirements likely dictate the wing's stroke tilting. Overall, the proposed biohybrid robotic flapper can be used to answer many open questions about avian flapping flights that are impossible to address by observing free-flying birds.