The stability implications of drag minimization by tail action modelled in the gliding barn owl ( Tyto alba )

Tail posture influences lift, drag, trim and stability for birds, yet the interaction between them as the tail spreads and pitches remains unclear, even during steady gliding. In this study, we investigated the aerodynamic consequences of tail morphing, exploring the interactions between weight support, drag, longitudinal trim and stability using data obtained from computational fluid dynamics simulations of high-fidelity, photogrammetry-derived geometry of a free-gliding barn owl. Assuming drag to be minimized over a range of speeds, the tail should be more spread and pitched at low speeds, and less so at high speeds. This influences the proportion of weight supported by the tail; in order to prevent net aerodynamic pitching moment and maintain longitudinal moment equilibrium, the relative position of the centre of gravity must shift. These effects shorten the negative static margin at higher speeds, making the model bird less unstable, limiting the reduction in pitch divergence doubling time that would otherwise have been coupled with the increase in speed. The drag-minimizing model owl is aerodynamically unstable at all speeds, but the feedback and control challenges of maintaining steady glides at high speeds are partially ameliorated and lower than would be predicted without a morphing airframe.