**3.8. Self-activated deployable flap**

Recently, aerodynamic researchers have focused on a technique to palliate the adverse effect of flow and increase the lift coefficient by inspiring the biological flows from birds' wings. Therefore, a self-activated spanwise flap near the trailing edge of the airfoil as a biomimetic device for control of flow separation has been developed as seen in **Figure 11** [58]. Regarding the principle of the self-activated flap, it starts to pop-up because of backflow to cope with critical conditions of flight when flow separation occurs on the suction surface of the wing. Thus, the wing can be prevented from an abrupt increase in the angle of attack because of perching maneuvers or gusts. Rosti et al. [59] investigated physical mechanism of the flow field over NACA0020 airfoil having an elastically mounted flap at Reynolds number of 2 × 10<sup>4</sup> . It was founded that these flaps could overcome the effect of dynamic stall breakdown causing the abrupt lift loss. In addition, a more positive aerodynamic response such as increasing of lift amount was obtained during the ramp up motion of movable flap. Arivoli and Singh [60] and Schluter [61] also studied self-activated deployable flaps. Their results demonstrated that deployable flaps played crucial roles in terms of aerodynamic performance even though they had a little effect in Reynolds number and heavy stall conditions.

**Figure 10.** Flow progress at trailing-edge airfoil having a Gurney flap [55].
