The increase in the maximum lift of an airfoil caused by small, movable tabs mounted on its upper surface has been explored in low-speed, wind-tunnel experiments at a chord Reynolds number of 1.0 x 10(6). These devices, herein called lift-enhancing effectors, have a chord that is 9% that of the airfoil and deploy passively at angles of attack approaching stall. Compared to the clean airfoil, the maximum lift coefficient is increased by approximately 20% with these simple devices. The lift increase is mainly caused by the effectors acting as "pressure dams," allowing lower pressures upstream of their location than would occur otherwise. At an effector the pressure recovers in a stepwise manner and continues downstream toward a trailing-edge value that is higher than that of the clean airfoil. This higher trailing-edge pressure also contributes to the increase in lift by allowing higher pressures over much of the lower surface. It has been shown that, in the absence of separation, properly installed effectors will lay flush on the surface and allow the airfoil to have the same performance in the low-drag range as the clean airfoil.
