text: By wind tunnel experiments, it has been found that the behavior of an airfoil under unsteady motion is quite different from that under quasi-steady motion. Flow separation is less likely to happen on the upper airfoil surface with a larger value of AoA than the latter, which can increase the maximum lift coefficient to a certain extent. Three primary unsteady phenomena have been identified to contribute to the delay in the onset of flow separation under unsteady condition:
  * During the condition where the AoA is increasing with respect to time, the unsteadiness of the flow resulting from circulation that is shed into the wake at the trailing edge of the airfoil causes a reduction in the lift and adverse pressure gradients compared to the steady case at the same AoA;
  * By virtue of a kinematic induced camber effect, a positive pitch rate further decreases the leading edge pressure and pressure gradients for a given value of lift;
  * In response to the external pressure gradients, there are also additional unsteady effects that occur within the boundary layer, including the existence of flow reversals in the absence of any significant flow separation.
  The development process of dynamic stall on 2D airfoil can be summarized in several stages:
  * Stage 1: the AoA exceeds the static stall angle but the flow separation is delayed due to the reduction of adverse pressure gradients produced by the kinematics of pitch rate.
  * Stage 2: flow separation and the formation of a vortex disturbance is cast-off from the leading edge region of the airfoil. This vortex, called leading edge vortex (LEV) or dynamic stall vortex (DSV), provides an additional lift for the airfoil so long as it stays over the upper surface, and also noteworthy increases in nose-down pitching moment (moment break, moment stall) while it moves downstream across the chord.
  * Stage 3: a steep decrease of the lift coefficient (lift break, lift stall) occurs as the DSV passes into the wake. 
  * Stage 4: full separation of the flow on the upper surface of the airfoil can be observed, accompanied by the peak of nose-down pitch moment.
  * Stage 5: the full flow reattachment is achieved as the AoA gradually decreases until it is fairly smaller than the static stall angle. The reasons for the lag are, firstly, the reorganization of the flow from fully separated to reattached, and secondly, the reverse kinematic "induced camber" effect on the leading edge pressure gradient by the negative pitch rate.