Improved Modal Damping Characterization for Small-Scale Heliogyro Blades

The heliogyro solar sail maneuvers by actuating high-aspect-ratio blades at their root with sinusoidal pitch profiles, producing spin-averaged forces and moments. The lightweight blade material has little inherent damping, requiring active control to attenuate blade vibration caused by maneuvering. Paradoxically, some amount of inherent damping at the blade's higher-frequency modes is needed to add damping to the larger-magnitude low-frequency modes, and so the control design's robustness depends on the accuracy of the blade damping model. Therefore, characterization of blade damping is critical for increasing the Technology Readiness Level for the heliogyro. This paper describes damping characterization tests performed on small-scale heliogyro blades hanging in a high-vacuum chamber, the closest analog possible for centrifugally loaded blades in space. Improvements to the measurements and a new modal damping algorithm, which the authors call the incremental and reduced frequency response function method, allowed separate flap and twist damping ratios to be calculated for the first three modes of these lightly damped blades to within +/- 2% error. This unprecedented precision enables definitive conclusions that the residual air damping is insignificant and that an assumption of a linear viscous torsional blade damping model is insufficient for the design of a robust proximal blade twist feedback controller.