Gust mitigation through closed-loop control. II. Feedforward and feedback control

Wing-gust encounters arising from flight over complex terrain or adverse weather are unavoidable and cause large lift transients that may result in structural damage due to extreme loads and or fatigue. Gust mitigation through control has the potential to extend the lifetime of air vehicles and wind turbines, although it is challenged by sensor noise, time delays, and low latency requirements. In this study, we report on the successful design and deployment of a closed-loop controller to regulate the lift coefficient of an airfoil equipped with an actuated trailing-edge flap against vortical gust disturbances. The approach is served by a feedforward-feedback control strategy and dedicated hardware. The control strategy relies on the characterization and modeling of the lift response to flap deflections and gust disturbances. In Part I of this series [Phys. Rev. Fluids 7, 024705 (2022)], we quantify and model the lift response to flap deflections. In this second part, we examine and model the gust disturbances on the airfoil lift and combine the models within the control architecture. Gust characterization reveals previously unknown synergy between concurrent gusts with opposite rotation direction, yielding large bursts of lift. The lift response model to gust disturbances is compared to Kussner's and validated against experimental results. The model-based feedforward loop detects the onset and magnitude of gust disturbances, which are generated upstream by a pitching airfoil, through a two-component X-wire located upstream of the DLR-F15 research airfoil. The combined controller proves to be effective for lift regulation during gust encounters, with feedforward rejecting large disturbances, and feedback attenuating high-frequency sensor noise and compensating for model uncertainty.