Editing Stall (fluid dynamics) (section)

==Variation of lift with angle of attack==

[[File:Lift curve.svg|right|thumb|upright=1.1|An example of the relationship between angle of attack and lift on a cambered airfoil. The exact relationship is usually measured in a [[wind tunnel]] and depends on the airfoil section. The relationship for an aircraft wing depends on the planform and its aspect ratio.]]

The graph shows that the greatest amount of lift is produced as the critical angle of attack is reached (which in early-20th century aviation was called the "burble point"). This angle is 17.5 degrees in this case, but it varies from airfoil to airfoil. In particular, for aerodynamically thick airfoils (thickness to [[Chord (aircraft)|chord]] ratios of around 10%), the critical angle is higher than with a thin airfoil of the same [[camber (aerodynamics)|camber]]. Symmetric airfoils have lower critical angles (but also work efficiently in inverted flight). The graph shows that, as the angle of attack exceeds the critical angle, the lift produced by the airfoil decreases.

The information in a graph of this kind is gathered using a model of the airfoil in a [[wind tunnel]]. Because aircraft models are normally used, rather than full-size machines, special care is needed to make sure that data is taken in the same [[Reynolds number]] regime (or scale speed) as in free flight. The separation of flow from the upper wing surface at high angles of attack is quite different at low Reynolds number from that at the high Reynolds numbers of real aircraft. In particular at high Reynolds numbers the flow tends to stay attached to the airfoil for longer because the inertial forces are dominant with respect to the viscous forces which are responsible for the flow separation ultimately leading to the aerodynamic stall. For this reason wind tunnel results carried out at lower speeds and on smaller scale models of the real life counterparts often tend to overestimate the aerodynamic stall angle of attack.<ref>{{Cite book|last1=Katz|first1=J|title=Low-Speed Aerodynamics: From Wing Theory to Panel Methods|last2=Plotkin|first2=A|publisher=Cambridge University Press|year=2001|pages=525}}</ref> High-pressure wind tunnels are one solution to this problem.

In general, steady operation of an aircraft at an angle of attack above the critical angle is not possible because, after exceeding the critical angle, the loss of lift from the wing causes the nose of the aircraft to fall, reducing the angle of attack again. This nose drop, independent of control inputs, indicates the pilot has actually stalled the aircraft.<ref>Clancy, L.J., ''Aerodynamics'', Sections 5.28 and 16.48</ref><ref>Anderson, J.D., ''A History of Aerodynamics'', pp. 296–311</ref>

This graph shows the stall angle, yet in practice most pilot operating handbooks (POH) or generic flight manuals describe stalling in terms of [[airspeed]]. This is because all aircraft are equipped with an [[airspeed indicator]], but fewer aircraft have an angle of attack indicator. An aircraft's stalling speed is published by the manufacturer (and is required for certification by flight testing) for a range of weights and flap positions, but the stalling angle of attack is not published.

As speed reduces, angle of attack has to increase to keep lift constant until the critical angle is reached. The airspeed at which this angle is reached is the (1g, unaccelerated) stalling speed of the aircraft in that particular configuration. Deploying [[flap (aircraft)|flaps]]/slats decreases the stall speed to allow the aircraft to take off and land at a lower speed.