Convertible unmanned aerial vehicle (UAV) promises a good balance between convenient autonomous launch/recovery and efficient long range cruise performance. Successful design of this new type of aircraft relies heavily on good understanding of powered lift generated through propeller-wing interactions, where the velocity distribution within propeller slipstream is critical to estimate aerodynamic forces during hover condition. Current study analysed a propeller-wing combination with a plain flap. A 5-hole probe measurement system was built to construct 3 dimensional velocity field at a survey plane after trailing edge. The study has found that significant deformation of propeller slipstream was present in the form of opposite transverse displacement on extrados and intrados. The deformation could be enhanced by flap deflections. Velocity differences caused by the slipstream deformation could imply local variation of lift distribution compared to predictions from conventional assumptions of cylindrical slipstream. The research underlined that the mutual aspect of propeller-wing interaction could be critical for low-speed aerodynamic design.