Leveraging Aerodynamic Drag for Tails in Legged Robot Locomotion

The interaction between legged robots and their environment has largely been prescribed by the manipulation of contact forces. Yet nature has shown that other physical phenomenon can aid locomotion, such as swinging a tail to maintain balance or accelerate. Many biological tails are lightweight and furry unlike their robotic counterparts, suggesting a different orientation regulation mechanism than inertial effects. Motivated by this insight we explore the physics and utility of aerodynamic drag in tailed reorientation tasks. We present a model of the dynamics induced by aerodynamic drag and from this derive a metric that allows for direct comparison between aerodynamic and inertial tails. We draw several design and control insights and demonstrate their utility in two dynamic behaviors executed on a legged robot. We show that employing aerodynamic drag alongside inertial effects allows for a 63% reduction in normalized tail inertia when executing an aerial reorientation task. We also show a 12% increase in forward acceleration of the robot despite a 10% increase in system mass from equipping the tail. These results show that aerodynamic drag has the ability to transform the way robots interact with the world by expanding control authority independent of contact.