It is necessary to know the unsteady properties determined from the vortex dynamics because the unsteadiness is known to make the force greater. The purpose of this study is to investigate the influence of vortices on transient aerodynamic characteristics of a three-dimensional airfoil whose angle of attack is varied impulsively. In the present experiment, the angle of attack of the airfoil is impulsively changed from 0 deg to the angle set under or beyond the static stall angle. The vortical flow fields were measured by using a stereoscopic PIV technique, and the image of three-dimensional vortex structure was depicted by plotting iso-vorticity surface calculated from the velocity data. The flow separates from the upper surface of the airfoil where the angle of attack increases beyond static stall angle under stationary conditions. On the other hand, when the airfoil is raised to a large angle of attack, high C_Lmax is attained by changing the rate of an angle of attack with the small non-dimensional rise time T*. The small value of T* indicates fast change in an angle of attack during incidence variation. The tip vortex (TIV) becomes stronger with increasing the angle of attack, and the whole separation on the upper surface of the airfoil is suppressed by the TIV after the airfoil stops. Therefore, C_L gradually decreases and the value of C_L comes to that of the stationary condition asymptotically.

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