Editing Stall (fluid dynamics) (section)

==Types==

===Dynamic stall===
{{anchor|Stall delay}}
Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong [[vortex]] to be shed from the leading edge of the aerofoil, and travel backwards above the wing.<ref>{{cite journal |last1=Buchner |first1=A. J. |last2=Soria |first2=J. |title=Measurements of the flow due to a rapidly pitching plate using time resolved high resolution PIV |journal=Aerospace Science and Technology |volume=44 |pages=4–17 |year=2015 |doi=10.1016/j.ast.2014.04.007|bibcode=2015AeST...44....4B }}</ref><ref>{{cite journal |last1=Khalifa |first1=Nabil M. |last2=Rezaei |first2=Amir S. |last3=Taha |first3=Haithem E. |date=2021 |title=Comparing the performance of different turbulence models in predicting dynamic stall |journal=AIAA Scitech 2021 Forum |pages=1651 |doi=10.2514/6.2021-1651|isbn=978-1-62410-609-5 |s2cid=234321807 }}</ref> The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.<ref name="Filippone">{{cite web|url=http://aerodyn.org/Dstall/dstall.html |title=Dynamic Stall, Unsteady Aerodynamics |access-date=25 March 2016 |url-status=unfit |archive-url=https://web.archive.org/web/20071229110350/http://aerodyn.org/Dstall/dstall.html |archive-date=29 December 2007 }}</ref>

Dynamic stall is an effect most associated with helicopters and flapping wings, though also occurs in wind turbines,<ref>{{cite journal |doi=10.1017/jfm.2018.112 |title=Dynamic stall in vertical axis wind turbines: Scaling and topological considerations |journal=Journal of Fluid Mechanics |volume=841 |pages=746–66 |year=2018 |last1=Buchner |first1=A-J. |last2=Soria |first2=J. |last3=Honnery |first3=D. |last4=Smits |first4=A.J. |bibcode=2018JFM...841..746B |s2cid=126033643 |doi-access=free }}</ref> and due to gusting airflow. During forward flight, some regions of a helicopter blade may incur flow that reverses (compared to the direction of blade movement), and thus includes rapidly changing angles of attack. Oscillating (flapping) wings, such as those of insects like the [[bumblebee]]—may rely almost entirely on dynamic stall for lift production, provided the oscillations are fast compared to the speed of flight, and the angle of the wing changes rapidly compared to airflow direction.<ref name="Filippone" />

Stall delay can occur on [[airfoils]] subject to a high angle of attack and a three-dimensional flow. When the angle of attack on an airfoil is increasing rapidly, the flow will remain substantially attached to the airfoil to a significantly higher angle of attack than can be achieved in steady-state conditions. As a result, the stall is delayed momentarily and a lift coefficient significantly higher than the steady-state maximum is achieved. The effect was first noticed on [[Propeller (aircraft)|propellers]].<ref>{{cite book
|last=Burton
|first=Tony
|author2=David Sharpe |author3=Nick Jenkins |author4=Ervin Bossanyi
 |title=Wind Energy Handbook
|url=https://books.google.com/books?id=4UYm893y-34C&pg=PA139
|year= 2001
|publisher= John Wiley and Sons
|isbn= 978-0-471-48997-9
|page=139
}}
</ref>

===Deep stall===
[[File:Deep stall.svg|alt=A diagram with the side view of two aircraft in different attitudes demonstrates the airflow around them in normal and stalled flight.|thumb|Diagrammatic representation of a deep stall. Normal flight (above), Deep stall condition - T-tail in "shadow" of wing (below)]]
[[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. Aircraft with rear-mounted nacelles may also exhibit a loss of [[thrust]].<ref name="TaylorPg9">{{cite journal |url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660017791.pdf |title = A Systematic Study of the Factors Contributing to Post-Stall Longitudinal Stability of T-Tail Transport Configurations |access-date = 24 September 2018 |author=Taylor, Robert T. & Edward J. Ray |journal = NASA Langley Research Center |date = 15 November 1965 |page=9}}</ref>  T-tail [[propeller aircraft]] are generally resistant to deep stalls, because the prop wash increases airflow over the wing root,<ref name=Parachutes>{{cite tech report|url=http://www.airborne-sys.com/files/pdf/spin_stall_parachute_recovery_systems_ss_17543100.pdf |title=The system approach to spin/stall parachute recovery systems&ndash;a five year update |access-date=2015-12-15 |archive-url=https://web.archive.org/web/20160304212837/http://www.airborne-sys.com/files/pdf/spin_stall_parachute_recovery_systems_ss_17543100.pdf |archive-date=2016-03-04 |url-status=dead |last=Taylor |first=Anthony&nbsp;"Tony"&nbsp;P. |institution=Irvin Aerospace}}</ref> but may be fitted with a [[precautionary principle|precautionary]] vertical tail booster during [[flight testing]], as happened with the [[A400M]].<ref name=boeing>{{Cite web |url=http://www.sfte2013.com/files/75234188.pdf |title=Archived copy |access-date=2015-12-18 |archive-url=https://web.archive.org/web/20150120151624/http://www.sfte2013.com/files/75234188.pdf |archive-date=2015-01-20 |url-status=dead }}</ref>

[[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.

Typical values both for the range of deep stall, as defined above, and the locked-in trim point are given for the [[Douglas DC-9]] Series&nbsp;10 by Schaufele.<ref>"Applied Aerodynamics at the Douglas Aircraft Company-A Historical Perspective". Roger D. Schaufele, 37th AIAA Aerospace Sciences Meeting and Exhibit, January 11–14, 1999/Reno, NV. Fig.&nbsp;26. Deep Stall Pitching Moments.</ref> These values are from wind-tunnel tests for an early design. The final design had no locked-in trim point, so recovery from the deep stall region was possible, as required to meet certification rules. Normal stall beginning at the "g break" (sudden decrease of the vertical [[load factor (aeronautics)|load factor]]<ref name=boeing/>) was at <math display="inline">\alpha = 18^\circ</math>, deep stall started at about 30°, and the locked-in unrecoverable trim point was at 47°.

The very high <math display="inline">\alpha</math> for a deep stall locked-in condition occurs well beyond the normal stall but can be attained very rapidly, as the aircraft is unstable beyond the normal stall and requires immediate action to arrest it. The loss of lift causes high sink rates, which, together with the low forward speed at the normal stall, give a high <math display="inline">\alpha</math> with little or no rotation of the aircraft.<ref name=trubshaw>"Accident Report No. EW/C/039, Appendix IV in "Trubshaw Test Pilot". Trubshaw and Edmondson, Sutton Publishing 1998, {{ISBN|0 7509 1838 1}}, p. 182.</ref> [[BAC 1-11]] G-ASHG, during stall flight tests before the type was modified to prevent a locked-in deep-stall condition, descended at over {{convert|10000|ft/min|m/s|-1}} and struck the ground in a flat attitude moving only {{convert|70|ft|m|-1}} forward after initial impact.<ref name=trubshaw/> Sketches showing how the wing wake blankets the tail may be misleading if they imply that deep stall requires a high body angle. Taylor and Ray<ref name="TaylorPg20">{{cite journal |url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660017791.pdf |title = A Systematic Study of the Factors Contributing to Post-Stall Longitudinal Stability of T-Tail Transport Configurations |access-date = 24 September 2018 |author=Taylor, Robert T. & Edward J. Ray |journal = NASA Langley Research Center |date = 15 November 1965 |page=20}}</ref> show how the aircraft attitude in the deep stall is relatively flat, even less than during the normal stall, with very high negative flight-path angles.

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&nbsp;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&nbsp;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&nbsp;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&nbsp;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&nbsp;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&nbsp;April 1980, a prototype of the [[Canadair Challenger]] business jet crashed after initially entering a deep stall from 17,000&nbsp;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&nbsp;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}}</ref> The crash of [[West Caribbean Airways Flight 708]] in 2005 was also attributed to a deep stall.

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=1 Jun 2011 |access-date=18 October 2011 |archive-date=28 September 2011 |archive-url=https://web.archive.org/web/20110928092254/http://www.flyingmag.com/news/air-france-447-was-it-deep-stall |url-status=dead }}</ref>

[[Canard (aeronautics)|Canard-configured]] aircraft are also at risk of getting into a deep stall. Two [[Velocity XL|Velocity]] aircraft crashed due to locked-in deep stalls.<ref>Cox, Jack, ''Velocity... Solving a Deep Stall Riddle'', EAA Sport Aviation, July 1991, pp. 53–59.</ref> Testing revealed that the addition of [[leading-edge cuff]]s to the outboard wing prevented the aircraft from getting into a deep stall. The Piper Advanced Technologies PAT-1, N15PT, another canard-configured aircraft, also crashed in an accident attributed to a deep stall.<ref>[http://aviation-safety.net/wikibase/wiki.php?id=10732 ASN Wikibase Occurrence # 10732]. Retrieved 4 September 2011.</ref> Wind-tunnel testing of the design at the [[NASA Langley Research Center]] showed that it was vulnerable to a deep stall.<ref>Williams, L. J.; Johnson, J. L. Jr. and Yip, L. P., ''Some Aerodynamic Considerations For Advanced Aircraft Configurations'', AIAA paper 84-0562, January 1984.</ref>

In the early 1980s, a [[Schweizer SGS 1-36]] sailplane was modified for [[NASA]]'s controlled deep-stall flight program.<ref>[http://www.dfrc.nasa.gov/Gallery/Photo/Schweizer-1-36/HTML/index.html Schweizer-1-36 index: Schweizer SGS 1–36 Photo Gallery Contact Sheet<!-- Bot generated title -->] {{Webarchive|url=https://web.archive.org/web/20080529161135/http://www.dfrc.nasa.gov/Gallery/Photo/Schweizer-1-36/HTML/index.html |date=2008-05-29 }}.</ref>

===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>