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{{Use dmy dates|date=March 2023}}
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[[File:1915ca abger fluegel (cropped and mirrored).jpg|thumb|upright=1.05|[[Flow separation|Airflow separating]] from an [[airfoil]] at a high [[angle of attack]], as occurs at a stall.]]
In [[fluid dynamics]], a '''stall''' is a reduction in the [[lift coefficient]] generated by a [[foil (fluid mechanics)|foil]] as [[angle of attack]] increases.<ref name="Crane">Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. {{ISBN|1-56027-287-2}}</ref> This occurs when the [[critical angle of attack]] of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the [[fluid]], foil, and [[Reynolds number]].▼
▲In [[fluid dynamics]], a '''stall''' is a reduction in the [[lift coefficient]] generated by a [[foil (fluid mechanics)|foil]] as [[angle of attack]]
Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases the wing's angle of attack and exceeds its critical angle of attack (which may be due to slowing down below [[Stall (fluid dynamics)#Stall speeds|stall speed]] in level flight). A stall does not mean that the engine(s) have stopped working, or that the aircraft has stopped moving—the effect is the same even in an [[Unpowered flight|unpowered]] [[glider aircraft]]. [[Thrust vectoring|Vectored thrust]] in aircraft is used to maintain [[altitude]] or controlled flight with wings stalled by replacing lost wing lift with engine or [[Propeller (aeronautics)|propeller]] [[thrust]], thereby giving rise to post-stall technology.<ref>Benjamin Gal-Or, ''Vectored Propulsion, Supermaneuverability, and Robot Aircraft'', Springer Verlag, 1990, {{ISBN|0-387-97161-0}}, {{ISBN|3-540-97161-0}}</ref><ref>USAF & NATO Report RTO-TR-015 AC/323/(HFM-015)/TP-1 (2001)</ref>▼
▲Stalls in [[fixed-wing
Because stalls are most commonly discussed in connection with [[aviation]], this article discusses stalls as they relate mainly to aircraft, in particular [[fixed-wing aircraft]]. The principles of stall discussed here translate to foils in other fluids as well.▼
▲Because stalls are most commonly discussed in connection with [[aviation]], this article discusses stalls as they relate mainly to aircraft, in particular
==Formal definition==
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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
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>
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A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a [[Spin (aerodynamics)|spin]]. A spin can occur if an aircraft is stalled and there is an asymmetric yawing moment applied to it.<ref>FAA Airplane flying handbook {{ISBN|978-1-60239-003-4}} Chapter 4, pp. 12–16</ref> This yawing moment can be aerodynamic (sideslip angle, rudder, adverse yaw from the ailerons), thrust related (p-factor, one engine inoperative on a multi-engine non-centreline thrust aircraft), or from less likely sources such as severe turbulence. The net effect is that one wing is stalled before the other and the aircraft descends rapidly while rotating, and some aircraft cannot recover from this condition without correct pilot control inputs (which must stop yaw) and loading.<ref>14 CFR part 23</ref> A new solution to the problem of difficult (or impossible) stall-spin recovery is provided by the [[ballistic parachute]] recovery system.
The most common stall-spin scenarios occur on takeoff ([[departure resistance|departure]] stall) and during landing (base to final turn) because of insufficient airspeed during these maneuvers. Stalls also occur during a go-around manoeuvre if the pilot does not properly respond to the out-of-trim situation resulting from the transition from low power setting to high power setting at low speed.<ref>FAA Airplane flying handbook {{ISBN|978-1-60239-003-4}} Chapter 4, pp. 11–12</ref> Stall speed is increased when the wing surfaces are [[
Stalls occur not only at slow airspeed, but at any speed when the wings exceed their critical angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back normally causes the aircraft to climb. However, aircraft often experience higher g-forces, such as when turning steeply or pulling out of a dive. In these cases, the wings are already operating at a higher angle of attack to create the necessary force (derived from lift) to accelerate in the desired direction. Increasing the g-loading still further, by pulling back on the controls, can cause the stalling angle to be exceeded, even though the aircraft is flying at a high speed.<ref>FAA Airplane flying handbook {{ISBN|978-1-60239-003-4}} Chapter 4, p. 9</ref> These "high-speed stalls" produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.
===Characteristics===
Different aircraft types have different stalling characteristics but they only have to be good enough to satisfy their particular Airworthiness authority. For example, the [[Short Belfast]] heavy freighter had a marginal nose drop which was acceptable to the [[Royal Air Force]]. When the aircraft were sold to a civil operator they had to be fitted with a stick pusher to meet the civil requirements.<ref>Tester Zero One – The making Of A Test Pilot, Wg. Cdr. J.A. "Robby" Robinson AFC, FRAeS, RAF (Retd) 2007, Old Forge Publishing, {{ISBN|978-1-906183-00-4}}, p.93</ref> Some aircraft may naturally have very good behaviour well beyond what is required. For example, first generation jet transports have been described as having an immaculate nose drop at the stall.<ref>Handling The Big Jets – Third Edition 1971, D.P.Davies, Civil Aviation Authority, p.113</ref> Loss of lift on one wing is acceptable as long as the roll, including during stall recovery, doesn't exceed about 20 degrees, or in turning flight the roll shall not exceed 90 degrees bank.<ref>Test Pilot, Brian Trubshaw With Sally Edmondson 1998, Sutton Publishing, {{ISBN|0 7509 1838 1}}, p.165</ref> If pre-stall warning followed by nose drop and limited wing drop are naturally not present or are deemed to be unacceptably marginal by an Airworthiness authority the stalling behaviour has to be made good enough with airframe modifications or devices such as a stick shaker and pusher. These are described in "Warning and safety devices".
==Stall speeds{{anchor|Speed}}==
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Stalls depend only on angle of attack, not [[airspeed]].<ref>{{cite book|title=Stick and Rudder|url=https://archive.org/details/stickrudderexp00lang|url-access=registration|author=Langewiesche, Wolfgang|publisher=McGraw Hill|date=1972|pages=[https://archive.org/details/stickrudderexp00lang/page/18 18–21]|isbn=9780070362406}}</ref> However, the slower an aircraft flies, the greater the angle of attack it needs to produce lift equal to the aircraft's weight.<ref name="phakcp4">{{cite web|url=http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf|title=Pilot's Handbook of Aeronautical Knowledge – Chapter 4|access-date=2014-03-13|publisher=[[Federal Aviation Administration]]|archive-url=https://web.archive.org/web/20130904015736/http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf|archive-date=2013-09-04|url-status=dead}}</ref> As the speed decreases further, at some point this angle will be equal to the [[Critical angle of attack|critical (stall) angle of attack]]. This speed is called the "stall speed". An aircraft flying at its stall speed cannot climb, and an aircraft flying below its stall speed cannot stop descending. Any attempt to do so by increasing angle of attack, without first increasing airspeed, will result in a stall.
The actual stall speed will vary depending on the airplane's weight, altitude, configuration, and vertical and lateral acceleration. [[Slipstream#Spiral slipstream|Propeller slipstream]] reduces the stall speed by energizing the flow over the wings.
Speed definitions vary and include:
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}}</ref>
A notable example of an air accident involving a low-altitude turning flight stall is the [[1994 Fairchild Air Force Base B-52 crash]].
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===Deep stall===
[[File:Deep
[[File:Schweizer 1-36 NASA.jpg|thumb|right|A [[Schweizer SGS 1-36]] being used for deep-stall research by [[NASA]] over the [[Mojave Desert]] in 1983.]]
A ''deep stall'' (or ''super-stall'') is a dangerous type of stall that affects certain [[aircraft]] designs, notably jet aircraft with a [[T-tail]] configuration and rear-mounted engines.<ref>{{cite web | url =http://www.aviationshop.com.au/avfacts/editorial/tipstall/ | work =Aviationshop | title =What is the super-stall? | access-date =2009-09-02 | url-status =dead | archive-url =https://web.archive.org/web/20091013203208/http://www.aviationshop.com.au/avfacts/editorial/tipstall/ | archive-date =2009-10-13 }}</ref> In these designs, the turbulent wake of a stalled main wing, nacelle-pylon wakes and the wake from the fuselage<ref>"Aerodynamic Design Features of the DC-9" Shevell and Schaufele, J. Aircraft Vol. 3, No. 6, Nov–Dec 1966, p. 518.</ref> "blanket" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.
[[Brian Trubshaw|Trubshaw]]<ref>"Low Speed Handling with Special Reference to the Super Stall". Trubshaw, Appendix III in "Trubshaw Test Pilot" Trubshaw and Edmondson, Sutton Publishing 1998, {{ISBN|0 7509 1838 1}}, p. 166.</ref> gives a broad definition of deep stall as penetrating to such angles of attack <math display="inline">\alpha</math> that pitch control effectiveness is reduced by the wing and nacelle wakes. He also gives a definition that relates deep stall to a locked-in condition where recovery is impossible. This is a single value of <math display="inline">\alpha</math>, for a given aircraft configuration, where there is no pitching moment, i.e. a trim point.
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Effects similar to deep stall had been known to occur on some aircraft designs before the term was coined. A prototype [[Gloster Javelin]] ([[United Kingdom military aircraft serials|serial]] ''WD808'') was lost in a crash on 11 June 1953 to a "locked-in" stall.<ref>[http://aviation-safety.net/wikibase/wiki.php?id=20519 ASN Wikibase Occurrence # 20519]. Retrieved 4 September 2011.</ref> However, Waterton<ref name=waterton>"The Quick and the Dead". W. A. Waterton, Frederick Mueller, London 1956, p. 216.</ref> states that the trimming tailplane was found to be the wrong way for recovery. Low-speed handling tests were being done to assess a new wing.<ref name=waterton/> [[Handley Page Victor]] ''XL159'' was lost to a "stable stall" on 23 March 1962.<ref>[http://www.thevictorassociation.org.uk/?p=491 A Tale of Two Victors]. {{Webarchive|url=https://web.archive.org/web/20120322161536/http://www.thevictorassociation.org.uk/?p=491 |date=2012-03-22 }}. Retrieved 4 September 2011.</ref> It had been clearing the fixed droop leading edge with the test being stall approach, landing configuration, C of G aft. The brake parachute had not been streamed, as it may have hindered rear crew escape.<ref>"The Handley Page Victor Volume 2". Roger R. Brooks, Pen & Sword Aviation 2007, {{ISBN|978 1 84415 570 5}}, p. 250.</ref>
The name "deep stall" first came into widespread use after [[1963 BAC One-Eleven test crash|the crash]] of the prototype [[BAC 1-11]] G-ASHG on 22 October 1963, which killed its crew.<ref>"Report on the Accident to B.A.C. One-Eleven G-ASHG at Cratt Hill, near Chicklade, Wiltshire on 22nd October 1963", Ministry of Aviation C.A.P. 219, 1965.</ref> This led to changes to the aircraft, including the installation of a [[stick shaker]] (see below) to clearly warn the pilot of an impending stall. Stick shakers are now a standard part of commercial airliners. Nevertheless, the problem continues to cause accidents; on 3 June 1966, a [[Hawker Siddeley Trident]] (G-ARPY), was [[1966 Felthorpe Trident crash|lost to deep stall]];<ref>{{cite web |url=http://aviation-safety.net/database/record.php?id=19660603-1 |title=ASN Aircraft accident Hawker Siddeley HS-121 Trident 1C G-ARPY Felthorpe |publisher=Aviation-safety.net |date=1966-06-03 |access-date=2013-04-02}}</ref> deep stall is suspected to be cause of another Trident (the [[British European Airways Flight 548]] ''G-ARPI'') crash – known as the "Staines Disaster" – on 18 June 1972, when the crew failed to notice the conditions and had disabled the stall-recovery system.<ref>AIB Report 4/73, p. 54.</ref> On 3 April 1980, a prototype of the [[Canadair Challenger]] business jet crashed after initially entering a deep stall from 17,000 ft and having both engines flame-out. It recovered from the deep stall after deploying the anti-spin parachute but crashed after being unable to jettison the chute or relight the engines. One of the test pilots was unable to escape from the aircraft in time and was killed.<ref>"Winging It The Making Of The Canadair Challenger". Stuart Logie, Macmillan Canada 1992, {{ISBN|0-7715-9145-4}}, p. 169.</ref> On 26 July 1993, a [[Canadair CRJ-100]] was lost in flight testing due to a deep stall.<ref>{{cite web |url=http://aviation-safety.net/database/record.php?id=19930726-2 |title=ASN Aircraft accident Canadair CL-600-2B19 Regional Jet CRJ-100 C-FCRJ Byers, KS |publisher=Aviation-safety.net |date=1993-07-26 |access-date=2013-04-02}}</ref> It has been reported that a [[Boeing 727]] entered a deep stall in a flight test, but the pilot was able to rock the airplane to increasingly higher bank angles until the nose finally fell through and normal control response was recovered.<ref>{{cite web |author=Robert Bogash |title=Deep Stalls |url=http://www.rbogash.com/Safety/deep_stall.html |access-date=4 September 2011}}
Deep stalls can occur at apparently normal pitch attitudes, if the aircraft is descending quickly enough.<ref>Airplane Flying Handbook (FAA-H-8083-3B), [https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/17_afh_ch15.pdf chapter 15], p. 15–13.</ref> The airflow is coming from below, so the angle of attack is increased. Early speculation on reasons for the crash of [[Air France Flight 447]] blamed an unrecoverable deep stall, since it descended in an almost flat attitude (15°) at an angle of attack of 35° or more. However, it was held in a stalled glide by the pilots, who held the nose up amid all the confusion of what was actually happening to the aircraft.<ref>{{cite magazine |author=Peter Garrison |title=Air France 447: Was it a Deep Stall? |url=http://www.flyingmag.com/news/air-france-447-was-it-deep-stall |magazine=[[Flying (magazine)|Flying]] |date=Jun 1, 2011}}</ref>
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===Tip stall===
Wing sweep and taper cause stalling at the [[wing tip|tip of a wing]] before the root. The position of a swept wing along the fuselage has to be such that the lift from the wing root, well forward of the aircraft center of gravity (c.g.), must be balanced by the wing tip, well aft of the c.g.<ref>{{Cite web |url=https://www.flightglobal.com/pdfarchive/view/1964/1964%20-%200018.html |title=Archived copy |access-date=2019-03-06 |archive-date=2019-03-07 |archive-url=https://web.archive.org/web/20190307112308/https://www.flightglobal.com/pdfarchive/view/1964/1964%20-%200018.html |url-status=dead }}</ref> If the tip stalls first the balance of the aircraft is upset causing dangerous nose [[pitch up]]. Swept wings have to incorporate features which prevent pitch-up caused by premature tip stall.
A swept wing has a higher lift coefficient on its outer panels than on the inner wing, causing them to reach their maximum lift capability first and to stall first. This is caused by the downwash pattern associated with swept/tapered wings.<ref>Fundamentals Of Flight – Second Edition, Richard S.Shevell, Prentice Hall 1983, {{ISBN|0-13-339060-8}}, p.244</ref> To delay tip stall the outboard wing is given [[Washout (aviation)|washout]] to reduce its angle of attack. The root can also be modified with a suitable leading-edge and airfoil section to make sure it stalls before the tip. However, when taken beyond stalling incidence the tips may still become fully stalled before the inner wing despite initial separation occurring inboard. This causes pitch-up after the stall and entry to a super-stall on those aircraft with super-stall characteristics.<ref>Handling The Big Jets – Third Edition, D.P.Davies, Civil Aviation Authority, p.121</ref> Span-wise flow of the boundary layer is also present on swept wings and causes tip stall. The amount of boundary layer air flowing outboard can be reduced by generating vortices with a leading-edge device such as a fence, notch, saw tooth or a set of vortex generators behind the leading edge.<ref>Flightwise – Principles Of Aircraft Flight, Chris Carpenter 1996, Airlife Publishing Ltd., {{ISBN|1 85310 719 0}}, p.369</ref>
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==Flight beyond the stall==
As a wing stalls, [[aileron]] effectiveness is reduced, rendering the plane difficult to control and increasing the risk of a spin. Post stall, steady flight beyond the stalling angle (where the coefficient of lift is largest) requires engine thrust to replace lift, as well as alternative controls to replace the loss of effectiveness of the ailerons.
==Spoilers==
{{Main|Spoiler (aeronautics)}}
Except for flight training, airplane testing, and [[aerobatics]], a stall is usually an undesirable event. [[spoiler (aeronautics)|Spoilers]] (sometimes called lift dumpers), however, are devices that are intentionally deployed to create a carefully controlled [[flow separation]] over part of an aircraft's wing to reduce the lift it generates, increase the drag, and allow the aircraft to descend more rapidly without gaining speed.<ref>{{cite web|title=Spoilers|url=http://www.grc.nasa.gov/WWW/K-12/airplane/spoil.html|publisher=
Unlike powered airplanes, which can control descent by increasing or decreasing thrust, gliders have to increase drag to increase the rate of descent. In high-performance gliders, spoiler deployment is extensively used to control the approach to landing.
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==History==
German aviator [[Otto Lilienthal]] died while flying in 1896 as the result of a stall. [[Wilbur Wright]] encountered stalls for the first time in 1901, while flying his second glider. Awareness of Lilienthal's accident and Wilbur's experience motivated the [[Wright Brothers]] to design their plane in "[[Canard (aeronautics)|canard]]" configuration. This purportedly made recoveries from stalls easier and more gentle. The design allegedly saved the brothers' lives more than once.<ref>
The aircraft engineer [[Juan de la Cierva]] worked on his "[[Autogiro]]" project to develop a [[Rotary-wing aircraft|rotary wing aircraft]] which, he hoped, would be unable to stall and which therefore would be safer than aeroplanes. In developing the resulting "[[autogyro]]" aircraft, he solved many engineering problems which made the [[helicopter]] possible.
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* [[China Airlines Flight 140]]
* [[China Airlines Flight 676]]
* [[Yeti Airlines Flight 691]]
* [[Air France Flight 447]]
* [[Colgan Air Flight 3407]]
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* [[West Caribbean Airways Flight 708]]
* [[Northwest Orient Airlines Flight 6231]]
* [[Voepass Linhas Aéreas Flight 2283]]
== Notes ==
{{Reflist|30em}}
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* [[L. J. Clancy]] (1975), ''Aerodynamics'', Pitman Publishing Limited, London. {{ISBN|0-273-01120-0}}
* Stengel, R. (2004), ''Flight Dynamics'', Princeton University Press, {{ISBN|0-691-11407-2}}
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[[Category:Aircraft aerodynamics]]
[[Category:Aviation risks]]
[[Category:Aircraft wing design]]
[[Category:Aerial maneuvers]]
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