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  • Print publication year: 2013
  • Online publication date: May 2013

12 - Stall

Summary

To maintain low drag and high lift, the flow over an airfoil section must remain smooth and attached to the surface. This flow has a rapid acceleration around the nose of the airfoil to the point of maximum suction pressure, and then a slow deceleration along the remainder of the upper surface to the trailing edge. The deceleration must be gradual for the flow to remain attached to the surface. At a high enough angle-of-attack, stall occurs: the deceleration is too large for the boundary layer to support, and the flow separates from the airfoil surface. The maximum lift coefficient at stall is highly dependent on the Reynolds number, Mach number, and the airfoil shape. Figure 8.12 shows clmax values from 1.0 to 1.6 for various airfoils, corresponding to stall angles-of-attack of 10° to 16°. The unstalled airfoil has a low drag and a lift coefficient linear with angle-of-attack. The airfoil in stall at high angles-of-attack has high drag, a loss of lift, and an increased nose-down pitch moment caused by a rearward shift of the center of pressure. The aerodynamic flow field of an airfoil or wing in stall is complex, and for the rotary wing there are important three-dimensional and unsteady phenomena as well.

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