Aerodynamic analysis of complex flapping motions based on free-flight biological data.

The wings of birds contain complex morphing mechanisms that enable them to perform remarkable aerial maneuvers. Wing morphing is often described using five wingbeat motion parameters: flapping, bending, folding, sweeping, and twisting. However, the specific impact of these motions on the aerodynamic performance of wings throughout the wingbeat cycle, and their potential to inform engineering applications, remains insufficiently explored. To bridge this gap and better incorporate the properties of coupled motions into the design of biomimetic aircraft, we present a numerical investigation of four flapping-based coupled motions during different flight phases (i.e. take-off, level flight, and landing) using a pigeon-like airfoil model. The wingbeat motion data for these four coupled motions were based on real flying pigeons and divided into: flap-bending, flap-folding, flap-sweeping, and flap-twisting. We used computational fluid dynamic simulations to study the effects of these coupled motions on the flow field, generation of transient aerodynamic forces, and work done by different motions on flapping. It was found that, first, the flap-bending motion causes unstable changes in the effective angle of attack (AoA), which affects the attachment of the leading-edge vortex (LEV), thereby producing more lift at smaller bending angles. Next, the flap-folding motion causes the LEV to attach to the wing earlier and regulates the detachment of vortices. Significant changes in the folding angle are used to influence lift generation and the flap-sweeping motion has minimal effect on the flow field structure across the three flight phases. Finally, flap-twisting motion leads to notable changes in the effective AoA, allowing for dynamic adjustments to control aerodynamics at different stroke stages, resulting in less drag during take-off and more drag during landing. This study enhances the understanding of the aerodynamic performance of bird with coupled motions in different flight phases and provides theoretical guidance for the design of bionic flapping-wing aircraft with multi-degree-of-freedom wings.