Wing morphing allows gulls to modulate static pitch stability during gliding

TLDR

Birds must stabilize their flight path during gliding, yet the effects of wing morphing on stability remain poorly understood. We investigate how morphing the gull elbow joint in gliding flight affects their static pitch stability. We identified gliding wing configurations from field observations and cadaver measurements, then tested wings across the elbow range in a wind tunnel to measure pitching moments. Measurements revealed that as wind speed and gusts increased, gulls flexed their elbows to adopt wing shapes with increased spanwise camber, and that wings with extended elbow angles had low camber and high passive stability, whereas flexed elbows produced increased camber and reduced static pitch stability, demonstrating that gliding gulls can transition across a broad range of static pitch stability characteristics using a single joint angle.

Abstract

A gliding bird's ability to stabilize its flight path is as critical as its ability to produce sufficient lift. In flight, birds often morph the shape of their wings, but the consequences of avian wing morphing on flight stability are not well understood. Here, we investigate how morphing the gull elbow joint in gliding flight affects their static pitch stability. First, we combined observations of freely gliding gulls and measurements from gull wing cadavers to identify the wing configurations used during gliding flight. These measurements revealed that, as wind speed and gusts increased, gulls flexed their elbows to adopt wing shapes characterized by increased spanwise camber. To determine the static pitch stability characteristics of these wing shapes, we prepared gull wings over the anatomical elbow range and measured the developed pitching moments in a wind tunnel. Wings prepared with extended elbow angles had low spanwise camber and high passive stability, meaning that mild perturbations could be negated without active control. Wings with flexed elbow angles had increased spanwise camber and reduced static pitch stability. Collectively, these results demonstrate that gliding gulls can transition across a broad range of static pitch stability characteristics using the motion of a single joint angle.