Active Gust Load Alleviation (GLA) has been an active area of research since past few decades. The main objective of using active GLA are to stabilize the system, to reduce the structural vibration and fatigue loading, and to increase the ride quality in presence of gusts. Aircraft structures, especially wings, have to be flexible to reduce the overall weight of the aircraft and interaction of such structure with gusts has a detrimental effect on stability, structure life, and maneuverability. 1 Different types of active GLA techniques have been implemented and found effective in accomplishing the required objectives. For example, an active load alleviation system called Active Lift Distribution Control System (ALDC) was implemented on Lockhead’s C-5A aircraft to increase the fatigue life impacted by incremental bending moment during maneuver and gust loading. 2 An active control system (ACS) was used in Lockhead L-1011-500 aircraft to incorporate maneuver load and gust load alleviation without changing the structure when wing span was extended to reduce the drag. 3 An active control technology (ACT) was used for Boeing B-1 Lancer aircraft to reduce the structural vibrations due to gust and to subsequently increase the ride quality. 4 These control systems employ inertial (accelerometers, gyroscope etc.) and/or aerodynamic sensors for gust response and/or gust measurement and use different types of control surfaces such as aileron, spoilers, elevators, rudders, and vanes to achieve the desired control objectives. Most of the conventional GLA systems use measurement or estimation of the structural response to the changes in aerodynamic loading, which has inherent delay due to structural inertia. Direct measurement or estimation of aerodynamic characteristics has advantages over conventional sensing systems because it reduces the delay significantly. 5, 6 Air Force Research Laboratory (AFRL) has recently developed bioinspired artificial hair sensors7–10 (AHS), which are capable of extracting the surface flow features and are ideal for measuring/estimating aerodynamics characteristics without delay. These hair sensors have been found to be effective in predicting the real-time aerodynamic parameters through simulation study11, 12 and have been verified with experimental data. 13 Possible applications of these sensors have also been investigated, for example, detection the boundary layers, 14 developement of the control algorithms for snapshot linear feedback control, 15 and gust load alleviation of highly flexible aircraft. 16 A feedforward control is very effective in rejecting aerodynamically-forced disturbance in situations when the disturbance is measurable. 17 Trends in feedforward control for gust load alleviation is increasing in recent years. Necessary disturbance or gust measurement devices for such feedforward control include LiDAR, 18, 19 and alpha probe20, 21. LiDAR provides upstream data for the atmospheric turbulence that is fed to the feedforward control, therefore, it requires wind evolution model to represent the actual gust that interacts with the aircraft. Alpha probe, on the other hand, consists of a small vane integrated on the aircraft body that aligns in the direction of flow thereby reporting the corresponding angle of attack. Gusts load alleviation on commercial aircraft through flight tests using feedforward controller and above mentioned disturbance measurement devices has also been reported. 22

To demonstrate and evaluate the effectiveness of AHS surface flow measurement for active GLA system, this work characterizes the control limitations inherent to a newly-developed 2-D wind tunnel pitch and plunge system. The wing …