Stabilizing air dampers for hovering aerial robotics: design, insect-scale flight tests, and scaling

Most hovering aircraft such as helicopters and animal-inspired flapping-wing flyers are dynamically unstable in flight, quickly tumbling in the absence of feedback control. The addition of feedback loops can stabilize, but at the cost of additional sensing and actuation components. This can add expense, weight, and complexity. An alternative to feedback is the use of passive mechanisms such as aerodynamic drag to stabilize attitude. Previous work has suggested that small aircraft can be stabilized by adding air dampers above and below the center of mass. We present flight tests of an insect-scale robot operating under this principle. When controlled to a constant altitude, it remains stably upright while undergoing cyclic attitude oscillations. To characterize these oscillations, we present a nonlinear analytic model derived from first principles that reproduces the observed behavior. Using numerical simulation, we analyze how changing damper size, position, mass, and midpoint offset affect these oscillations, building on previous work that considered only a single configuration. Our results indicate that only by increasing damper size can lateral oscillation amplitude be significantly reduced, at the cost of increased damper mass. Additionally, we show that as scale diminishes, the damper size must get relatively larger. This suggests that smaller damper-equipped robots must operate in low-wind areas or in boundary-layer flow near surfaces.