Meeting Abstract
Birds adopt a variety of wing and tail configurations during gliding flight, yet it is currently unclear whether these configurations are inherently stable or unstable. Stability influences manoeuvrability and may therefore have a significant impact on behaviours such as foraging, obstacle avoidance and predator evasion. By combining photogrammetric 3D surface reconstructions of a free-gliding barn owl (Tyto alba) and peregrine falcon (Falco peregrinus) with X-ray computed tomography (CT) scans of similar sized cadavers of the same species, it was possible to accurately estimate the in-flight aerodynamic shape of the birds and their inertial properties. Linear flight dynamics models were then generated using Athena Vortex Lattice (AVL), a computational aerodynamics tool used for aircraft design. The results showed that both the barn owl and peregrine were highly longitudinally statically unstable in glide for all three flights recorded for each species. The peregrine altered its wing and tail configuration with speed and featured varying degrees of camber, twist, sweep and dihedral that resulted in distinct changes to its degree of longitudinal and lateral-directional stability. The dynamic modes showed both similarities and differences with conventional aircraft configurations. Both birds had a stable phugoid and roll-subsidence modes, with a mildly unstable spiral mode. They also had a highly unstable longitudinal mode typical of highly manoeuvrable combat aircraft and similarly would require very fast corrective responses to control. The measured inherent instability of these birds, and the enhanced manoeuvrability conferred, may reflect the need for these predatory birds to catch highly manoeuvrable prey either in the air or on the ground.
