Transitional boundary layer over an airfoil in the combined pitch-plunge oscillating motions at low Reynolds number is experimentally investigated. The study is based on the data obtained from both surface hot-film and surface static pressure. Different oscillation zones prior to, within, and beyond the static stall angle of attack of the airfoil are considered. The hysteresis loops for the laminar separation bubble, transition, relaminarization points, as well as the airfoil lift coefficient are investigated and are compared for different types of dynamic motions. Two different types of combined pitch-plunge motions, constructive and destructive, are considered. In the constructive motion, the amplitude of the equivalent angle of attack increases, while in the destructive one the reverse happens. From the present it is concluded that clockwise or counterclockwise direction of the hysteresis loops is determined by the apparent mass and by the wake effects when oscillating below and within the static stall angles of attack for this airfoil. It is further presumed that the apparent mass effect is more pronounced for the combined motions for the lower pitch amplitude cases. Moreover, massive flow separation and dynamic stall vortices when oscillating within and beyond the static stall angles of attack are believed to play a significant role in the hysteresis loops. It is found that the destructive combined motion results in a large hysteresis of the flow phenomena over the surface of the model. The trends of the hysteresis loops for laminar separation points may differ compared to the transition points in combined pitch-plunge motion.

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