Flight dynamics and optimal control of unpowered avian landings

Abstract Most birds can transition from aerial to terrestrial locomotion in complex environments. However, to touchdown at safe speeds is challenging as low speeds result in poor aerodynamic control authority, especially in unpowered flight, and how birds manage this is not well understood. Through exploration of the dynamics, we provide an analytical optimal control strategy that minimizes touchdown speed and show that zero touchdown speed is physically possible in unpowered flight. However, this comes at the cost of large approach velocities and displacements, which are not commonly observed in birds, indicating that other factors probably influence their flight behaviour. When flights are constrained to land on the ground, unpowered landings can no longer touchdown with zero speed, thus requiring increases in forelimb or hindlimb skeletomuscular mass to dissipate the remaining kinetic energy. This requirement is a physical constraint that may have shaped avian evolution. Comparing our model with experimental data on perching birds provides new evidence to support a proposal that birds minimize touchdown speed and time spent post-stall. Overall, our results provide a mechanical basis for understanding landing behaviour and highlight important physical constraints that may have affected avian evolution.