For Reynolds numbers that are typical for general aviation aircraft separation is typically turbulent. In this paper the question is addressed if flow control strategies that are successful at low-Reynolds number conditions remain effective at higher Reynolds numbers. Towards this end hybrid simulations based on a one-equation renormalization group model were carried out for a modified NACA643-618 airfoil at a chord Reynolds number of one million. For ten degrees angle of attack a short laminar separation bubble develops near the leading edge and the flow separates turbulent from the suction side downstream of the leading edge bubble. Active flow control by harmonic blowing through a spanwise slot was investigated. The control was found to be only mildly effective or even counterproductive. In particular, the disturbances that were introduced by the control were only weakly amplified or even dampened. The unresolved eddy viscosity in the separated boundary layer lowers the effective Reynolds number and makes the flow less unstable or even stable with respect to two-dimensional disturbances. As a consequence of this, higher blowing ratios are required compared to flow control at low-Reynolds number conditions where the shear-layer instability is stronger.