Aerodynamics of Flight with Feathered Wings - Insights via Experiments and Computational Models

Feathered wings in birds serve multiple roles across a wide range of flight conditions, including varying weight, speed, altitude, and control. Key among these roles is lift and thrust generation for weight support, acceleration and maneuvering. In low-speed maneuvers like takeoff, hovering, or perching, midsize and large birds face the challenge of negative lift generation during the upstroke. However, primary flight feathers often spread apart, creating gaps that allow airflow and reduce resistance contributing to the negative lift. Wing pitching also helps by adjusting the effective angle of attack to a more favorable range. While these adaptations are known qualitatively, there is limited quantitative understanding of their contributions to lift during low-speed flight. In the current study, we approach this problem via experimental measurements and computational models. High-speed videos of a hawk in slow-speed forward flight are used to establish the kinematics of the wing and dynamically changing topology of the primary feathers. Computational models employ a high-fidelity sharp interface immersed boundary method with a relatively simple model of a flapping wing which is made up of several feathers that can move passively due to flow-induced forces. Our results demonstrate improved lift generation behavior during upstroke quantitatively and provide valuable insights and data for understanding the role of feathers in bird wings.