## 3 Flow Control Devices for Gust Load Alleviation
### Traditional Control Surfaces
#### 3.1.2 Spoilers  
Spoilers are multifunctional control devices on the upper wing surface, serving a variety of tasks in the fight control system of modern airliners [49]. Spoilers are typically positioned along the upper surface of the wing. Unlike the high-lift devices mentioned above, spoilers are mainly used as an inflight speed brake by rotating to induce a controlled flow separation and thus reducing the lift and increasing the drag. For the inflight speed brake system, the deflection rate of spoilers is rather low, and the dynamic characteristic is not the key factor. However, for an effective GLA system, the deflection rate should be much higher to counteract the high-frequency gusts. Therefore, the dynamic performance of spoilers under unsteady activations requires better understanding. On a 2-D airfoil, [PERSON] et al. [50] found that the incremental lift loss of the spoiler with the increasing deflection angle is quite nonlinear. [PERSON] et al. [51] conducted parametric influences of the spoiler geometry on its aerodynamic characteristics. It was found that the vertical distance from the spoiler tip to the trailing edge of the airfoil acted a more important role than that of the chord or deflection angle of the spoiler. For the spoiler location, [PERSON] [52] found that placing the spoiler at \(x/c=0.4\) failed to reattach flow along the surface, unlike the spoiler at \(x/c=0.2\) for a given deflection angle of \(\delta=40^{\circ}\) at \(\alpha=4^{\circ}\). [PERSON] et al. [53] suggested spoilers to be located downstream on swept wings in order to achieve analogous effects like that on unswept wings. [PERSON] and [PERSON] [54] conducted a comprehensive study of 2D and 3D spoilers on the unsteady flow patterns. Transient lag effects were observed during spoiler deployment and the lagging effects increased with the spoiler hinge line locating forward to the airfoil leading edge. Additionally, an initial lift overshoot rather than lift reduction was noticed during their experiments with fast spoiler deployment. This was later called as adverse lift effect by [PERSON] [55]. During their experiments, it was also found that spoilers on 3D wings had higher effectiveness than that of the 2D aerofoils. It was pointed out that it was due to reduced reattachment length behind the spoiler because of the interaction of vortices from spoiler tip and edges. The time lag during dynamic spoiler deployment was also observed by [PERSON] [56], who also revealed a hysteresis effect. [PERSON] [55] introduced a non-dimensional time to quantify the spoiler deployment rate as \(s=\frac{U_{\infty}t}{c}\), with \(c\) being the chord length of the airfoil and \(U_{\infty}\) the incoming flow velocity. It was found that the adverse lift effect is dependent to the non-dimensional time. When the non-dimensional time was less than 5, the adverse lift effect would be observed. However, during the experiments on harmonically oscillating spoilers by [PERSON] et al. [57], the adverse lift was observed for deployment times up to 9. During the experimental work to investigate the adverse lift by [PERSON] [58], no adverse lift was observed for deployment times above 8. [PERSON] et al. [59] also found that larger deflection angles did not mean greater changes in lift. It was found that lift remained relatively constant until the spoiler was sufficiently inclined to prevent separated flow downstream of the spoiler from reattaching. However, the low spoiler angle (\(5=1^{\circ}\)) was shown to be ineffective at separating the flow. This is due to the height of the spoiler being too small, which is mentioned previously that the extent of the separated region being proportional to the spoiler height. The finding was opposite by [PERSON] et al. [47], as lower deflection angles produce greater force variation than larger deflection angles. Meanwhile, this statement was given under the result of the unsteady force variation with \(\delta=5^{\circ}\) which was greater than twice the amplitude observed at \(\delta=10^{\circ}\).