Editing Stall (fluid dynamics) (section)

==Formal definition==
[[File:StallFormation.svg|thumb|Stall formation]]

A stall is a condition in [[aerodynamics]] and aviation such that if the angle of attack on an aircraft increases beyond a certain point, then lift begins to decrease. The angle at which this occurs is called the ''critical angle of attack''. If the angle of attack increases beyond the critical value, the lift decreases and the aircraft descends, further increasing the angle of attack and causing further loss of lift. The critical angle of attack is dependent upon the airfoil section or profile of the wing, its [[planform (aeronautics)|planform]], its [[aspect ratio (aeronautics)|aspect ratio]], and other factors, but is typically in the range of 8 to 20 degrees relative to the incoming wind ([[relative wind]]) for most subsonic airfoils. The critical angle of attack is the angle of attack on the [[lift coefficient]] versus angle-of-attack (Cl~alpha) curve at which the maximum lift coefficient occurs.<ref>Clancy, L.J., ''Aerodynamics'', Section 5.7</ref>

Stalling is caused by [[flow separation]] which, in turn, is caused by the air flowing against a rising pressure. Whitford<ref>''Design For Air Combat'', Ray Whitford 1987, Jane's Publishing Company limited, {{ISBN|0 7106 04262}}, p. 15</ref> describes three types of stall: trailing-edge, leading-edge and thin-aerofoil, each with distinctive Cl~alpha features. For the trailing-edge stall, separation begins at small angles of attack near the trailing edge of the wing while the rest of the flow over the wing remains attached. As angle of attack increases, the separated regions on the top of the wing increase in size as the flow separation moves forward, and this hinders the ability of the wing to create lift. This is shown by the reduction in lift-slope on a Cl~alpha curve as the lift nears its maximum value. The separated flow usually causes buffeting.<ref>''Understanding Aerodynamics – Arguing From The Real Physics'', Doug McLean 2013, John Wiley & Sons Ltd., {{ISBN|978-1-119-96751-4}}, p. 322</ref> Beyond the critical angle of attack, separated flow is so dominant that additional increases in angle of attack cause the lift to fall from its peak value.

Piston-engined and early jet transports had very good stall behaviour with pre-stall buffet warning and, if ignored, a straight nose-drop for a natural recovery. Wing developments that came with the introduction of turbo-prop engines introduced unacceptable stall behaviour. Leading-edge developments on high-lift wings, and the introduction of rear-mounted engines and high-set tailplanes on the next generation of jet transports, also introduced unacceptable stall behaviour. The probability of achieving the stall speed inadvertently, a potentially hazardous event, had been calculated, in 1965, at about once in every 100,000 flights,<ref>{{Cite web |url=https://www.flightglobal.com/pdfarchive/view/1965/1965%20-%200721.html?search=stalling |title=Archived copy |access-date=2019-03-03 |archive-date=2019-03-06 |archive-url=https://web.archive.org/web/20190306044446/https://www.flightglobal.com/pdfarchive/view/1965/1965%20-%200721.html?search=stalling |url-status=dead }}</ref> often enough to justify the cost of development of warning devices, such as stick shakers, and devices to automatically provide an adequate nose-down pitch, such as stick pushers.<ref>''Handling The Big Jets'' – Third Edition, D.P. Davies, Civil Aviation Authority, pp. 113–115</ref>

When the mean angle of attack of the wings is beyond the stall a [[spin (aerodynamics)|spin]], which is an [[autorotation (fixed-wing aircraft)|autorotation]] of a stalled wing, may develop. A spin follows departures in roll, yaw and pitch from balanced flight. For example, a roll is naturally damped with an unstalled wing, but with wings stalled the damping moment is replaced with a propelling moment.<ref>''The Design Of The Aeroplane'', Darrol Stinton 1983, BSP Professional Books, {{ISBN|0-632-01877-1}}, p. 464</ref><ref>{{Cite web |url=https://www.flightglobal.com/pdfarchive/view/1978/1978%20-%200550.html?search=april%20going%20for%20a%20spin |title=Archived copy |access-date=2019-03-03 |archive-date=2019-03-06 |archive-url=https://web.archive.org/web/20190306043914/https://www.flightglobal.com/pdfarchive/view/1978/1978%20-%200550.html?search=april%20going%20for%20a%20spin |url-status=dead }}</ref>