According to current art, high-lift devices such as leading-edge slats are essential during the takeoff and landing stages of large transport planes. Because of complexity of the mechanical positioning mechanism and of the specific geometry, there are regions on the main wing that are significantly affected by the slat edge and associated (though not resolved in this study) tip vortices. Under adverse conditions, the vortices degrade the exposed region of the wing performance, which results in lower lift and higher drag. In this experimental study, an airfoil with leading-edge slat was tested for performance enhancement of a thick airfoil section with and without operating active flow control system positioned at the 20% chord location. It was found that the slat improved the overall performance and that the active flow control alone can deliver similar performance gains as the slat for this type of wing section with achievable low actuator control input. Part-span slat configurations, covering 31.8 and 77.3% of the span, were also tested. It was found that there are positive effects of the active flow control system on the airfoil three-dimensional flow field and that, in principle, active flow control can reduce or even eliminate the slat-edge adverse effects altogether.