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. AIAA Aviation 2 NOMENCLATURE L C lift coefficient 2force FI inner vortex tube FO outer vortex tube f flapping frequency, Hz L lift force m tip of membrane flapper r tip of rigid wing RI inner part of vortex ring RO outer part of vortex ring S total wing area, mm 2 t time, sec T stroke period, T = 1/f, sec V tunnel velocity, m/s xyz model-fixed frame of reference  air density, kg/m 3  angle of attack, deg  pitching angle, deg Downloaded by UNIVERSITY OF CAMBRIDGE on June 16, 2016 | http://arc.aiaa.org |

In the present study, an oscillating membrane flapper was pivotally attached to the tip of a conventional rigid wing. Stroke-averaged aerodynamic forces were measured for the range of the flapping frequency, showing significant increases in the lift coefficient and lift-to-drag ratio for the wing with a flapper. Major vortex patterns were deduced from observations of smoke-wire visualization and 2D phase-locked particle image velocimetry (PIV). The centerline of the primary vortex wanders in the counterclockwise direction. On the contrary, its core rotates in the same sense of rotation as a wingtip vortex in a conventional wing. The secondary weaker vortex of opposite rotation lasts for a half stroke. The vortex ring sheds from the flapper during the second half of the upstroke and pronation. The outer parts of the vortex system are much stronger than the inner ones. The circulation and size of vortices decrease significantly at the most distant station from the wing. Strong vertical jets were found in smoke-wire visualization and confirmed with velocity and vorticity fields obtained by PIV. These jets are formed between undulating vortices and inside of the vortex ring. The jet airflow moves away from the flapper and downward or upward depending on the flapping direction.

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