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

Aerodynamic performance in airplanes and flying animals can be controlled by changes in wing shape and size, but during flapping flight another key component is wing motion. Observations of free-flying animals reveal natural wing motions, but testing causal mechanisms requires controlled manipulation of flapping kinematics. In flapping wings, aerodynamic interactions between the two wings are expected to depend on wingbeat phase, wing proximity, and wing attitude. How these interactions influence force production remains unclear. Here we used a robotic flapping wing and quantitative flow measurements to test the effect of wing interactions between wings flapped above or below the body in combination with pitch timing during upstroke-downstroke transitions. We show that both force magnitude and direction depend strongly on these parameters. High wing position combined with early pitching enhanced vertical force, whereas low position and late pitching increased thrust. Maximum aerodynamic efficiency was achieved flapping around the horizontal plane and pitching late. 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.