This study addresses the aerodynamics of propellers used in small unmanned aerial vehicles (UAVs) during transition between two flight modes: forward-flight and near-hover. Wind tunnel experiments were conducted for two angles of attack. Velocity fields behind retreating and advancing blades were measured using Particle Image Velocimetry (PIV). The vortex center and the core boundary were accurately identified by the method based on the normalized angular momentum. The major difference in an advancing blade as compared to a retreating one is in a stronger downwash behind the advancing propeller pushing the entire wake downward. Slopes of vortex center lines for the advancing blade are about two times steeper than in the retreating one. Peaks of velocity components are located right behind the propeller plane. On the advancing blade side, the vertical velocity component is approximately two times greater than on the retreating blade side. An increase in the vortex core size with the distance traveled is observed.