The effects of oscillatory blowing as a means of delaying separation are discussed. Experiments were carried out on a hollow, flapped NACA 0015 airfoil equipped with a two-dimensional slot over the hinge of the flap. The flap extended over 25% of the chord and was deflected at angles as high as 40 deg. The steady blowing momentum coefficients could be varied independently of the amplitudes and frequencies of the superimposed oscillations. The modulated blowing was a major factor in improving the performance of the airfoil at much lower energy inputs than was hitherto known. Optimum benefits in performance were obtained at reduced frequencies, based on the flap chord, of an order of unity. Significant increase in lift as well as cancellation of form drag were observed. The increase in Reynolds number did not have an adverse effect on the data.