In the present study, a membrane oscillating flapper was connected to the wingtip of a conventional rigid wing. Stroke-average aerodynamic forces were measured for the range of flapping frequency, showing significant increase in the lift coefficient and lift-to-drag ratio for the wing with flapper. Wake structures were visualized and analyzed using smoke-wire techniques and 2D phase-locked PIV. Major vortex patterns were deduced from the observations: two undulating vortex tubes and a vortex ring. Experimental data indicate that two undulating vortices are periodically closed by a cross-stream vortex. The vortex ring sheds from the flapper during the second half of the upstroke and on pronation and dissipates at a short distance behind the wing. The circulation of undulating vortices and of the ring decreases significantly with increasing distance from the wing downstream, while the decrease of the vortex core size is minimal.