Meeting Abstract
The numerous degrees of freedom afforded by the avian wing and its deformable feathers provide birds with impressive flight capacity. Skeletal movements influence the gross aerodynamic shape of the wing, whilst feather bending and interaction allow both continuous and discontinuous surface possibilities. The avian flight control system intricately balances its functional morphology to produce aerodynamic surfaces that can cope with dynamic environments. This work focuses on understanding the selection of wing morphology for two fundamental and repeatable flight test cases. We investigated quasi-steady gliding in and out of ground effect for a barn owl (Tyto alba), tawny owl (Strix aluco), goshawk (Accipiter gentilis) and tawny eagle (Aquila rapax) over multiple flight trials. Each flight was captured with twelve high-speed cameras and a marker-based motion tracking system. This enabled 3D reconstruction of the upper and lower wing surfaces, supplemented by trajectories of key anatomical features (body and feathers). Application of the reconstruction method to a bird-sized calibration object showed that 95% of points were within 3mm of the nominal geometry, with a modal error of 0.2mm. Most flights take place at near-constant horizontal velocity through the measurement region. There is little difference in the measured glide angles (and hence the lift-to-drag ratios for steady flight) between the two flight cases, which is unusual given the aerodynamic enhancements normally conferred by proximity to the ground. We present the associated wing morphologies, including spanwise distributions of wing thickness, camber and twist.
