"Axial Fan" impellers in the relative motion of the airfoil blade, between the gas and the airfoil is to flow around airfoils. In Airfoil Flow Analysis, the angle defined relative movement direction of the chord line (ie, the connection of the center of curvature of the airfoil trailing edge) of the angle of attack (or angle of attack), red angular size is about the impact of airfoil the most important factor in flow characteristics. When the angle of attack is zero, the blades have a greater lift and smaller frictional resistance. When the angle of attack increases, the dorsal surface of the tail blade flow generating separation force and increased resistance (mainly physical resistance) greater increase in blade lift-drag ratio decreases. When the angle of attack increases to a critical value, the flow separation point forward, the separation zone to expand, resulting in decreased lift dramatically increase drag. This phenomenon is known as flow around a stall (or stall). For this work rely on external fan off the stream is to produce a non-stable condition of the important reasons.