System Identification and Control of a Circulation Control Airfoil for Gust Load Alleviation

The effectiveness of active gust load alleviation systems strongly correlates with the performance of their actuators and can benefit from high-performance actuation systems that can rapidly produce a significant change in lift to counteract gust-induced lift change on the wings. Circulation control actuators are potentially capable of generating the required lift change significantly faster than conventional ailerons, promising a more effective alternative for a gust load alleviation system. To enable aeroservoelastic simulations and control design for such a system, low-order dynamic models of a circulation control actuator are developed from wind tunnel data using system identification, at a chord-based Reynolds number of Re = 1.5 center dot 10(6) and M-infinity = 0.14. A linear first-order model adequately describes the actuator frequency response, with the length and volume of the internal pressurized air system downstream of the valve having an impact on the actuator dynamics. The load alleviation performance of the designed feedforward controller, compared with two simple ad-hoc controllers, is calculated with an RMS-based metric relative to the open-loop gust loads, including uncertainties. The best performance, up to 35% improvement, is obtained for an upward gust with initial upper steady blowing. The assumption of linear superposition of the gust and actuator response, which was used to design the controller, is proved to be accurate. A low level of initial steady blowing, which corresponds to the nominal condition necessary for recovering the baseline performance of the circulation control airfoil, is also shown to mitigate a strong non-linearity in the vicinity of c(mu) = 0 which otherwise heavily degrades the performance of controllers designed using linear methods.