Wing pitch timing and wing elevation modulate forces and body pitch in forward flapping flight

Aerodynamic performance in flying animals can be controlled not only by changes in wing shape and size, but also by changes in wing motion. In flapping wings, the two wings interact aerodynamically, where the interaction depends on wingbeat phase, wing proximity, and wing attitude. Although, observations of free-flying animals show wings are relatively close during forward flight, how these interactions influence force production remain unclear. Here we used a robotic flapping wing and quantitative flow measurements to test the effect of wing interactions while varying mean wing elevation and pitch timing during upstroke-downstroke transitions. We show that both force magnitude and direction depend strongly on these parameters and their interaction. The fitted response models showed that high wing position combined with early pitching maximized vertical force, whereas low-to-mid wing position with synchronized-to-late pitching increased thrust. Efficiency metrics were highest near mid-to-high wing position combined with late pitching. In addition, we found that transition phases strongly affected thrust generation and produced substantial body pitch torques. These findings demonstrate that small kinematic adjustments can markedly alter aerodynamic performance and be used for tailless flight control. This offers mechanistic explanations for observed animal wing motions and novel strategies for controlling flapping drones.