The effect of changing the pitch to diameter ratio (P/D) of a row of round, wall-normal, zero-net-mass-flux (ZNMF) jets located at the leading edge of a NACA 0015 airfoil was investigated. A parametric study and particle image ve-locimetry (PIV) measurements were conducted on a two-dimensional airfoil in a water tunnel at a Reynolds number of 6.56 × 10 4. Different optimal forcing frequencies and percentage lift increases between the two P/D cases were observed. It is possible that differences in jet interaction mechanisms may have caused the differences in control effectiveness between P/D cases. Time-averaged streamlines indicate a reduction in the size of a recirculation region over the upper surface of the airfoil may be causing the improved lift.