We present experimental data on the use of a “pop-up feather” type high lift device (passive flap) on the upper side of the wing. While a number of previous studies have shown the effectiveness of this flap in two-dimensional geometries, we focus here on the use of the flap on finite wings and the three-dimensional flows involved in these geometries. We present parameter studies to determine the optimal size and location of such a flap on a rectangular SD8020 finite wing. We show that the flap has virtually no effect on the wing tip and that the optimal placement of the flap near the wing root is close to the optimum found in two-dimensional airfoil studies.