Editing Stall (fluid dynamics) (section)

==Stall speeds{{anchor|Speed}}==
[[File:AltitudeEnvelopeText.GIF|thumb|right|upright=1.4|[[Flight envelope]] of a fast aeroplane. Left edge is the stall speed curve.]]
[[File:Airspeed indicator.svg|thumb|right|upright=0.7|The [[airspeed indicator]] is often used to indirectly predict stall conditions.]]

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.<ref name=Davies>{{cite book |last1=Davies |first1=David P. |title=Handling the Big Jets: An Explanation of the Significant Differences in Flying Qualities Between Jet Transport Aeroplanes and Piston Engined Transport Aeroplanes, Together with Some Other Aspects of Jet Transport Handling |date=1971 |publisher=Air Registration Board |url=https://books.google.com/books?id=TKZTAAAAMAAJ |language=en |isbn=0903083019 |edition=3rd}}</ref>{{rp|61}}

Speed definitions vary and include:
* V<sub>S</sub>: Stall speed: the speed at which the airplane exhibits those qualities accepted as defining the stall.<ref name="Davies"/>{{rp|8}}
* V<sub>S0</sub>: The stall speed or minimum steady flight speed in landing configuration.<ref name="rgl.faa.gov">{{cite web|url= http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/a2fdf912342e575786256ca20061e343/$FILE/AC61-67C.pdf|title= Advisory Circular|access-date= 14 March 2022|author= FAA|author-link= Federal Aviation Administration|work= rgl.faa.gov|date= 25 September 2000 |archive-url= https://web.archive.org/web/20211103092541/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/a2fdf912342e575786256ca20061e343/$FILE/AC61-67C.pdf|archive-date= 3 November 2021|url-status= live}}</ref> The zero-thrust stall speed at the most extended landing flap setting.<ref name="Davies"/>{{rp|8}}
* V<sub>S1</sub>: The stall speed or minimum steady flight speed obtained in a specified configuration.<ref name="rgl.faa.gov"/> The zero thrust stall speed at a specified flap setting.<ref name="Davies"/>{{rp|8}}

An [[airspeed indicator]], for the purpose of flight-testing, may have the following markings: the bottom of the white arc indicates V<sub>S0</sub> at maximum weight, while the bottom of the green arc indicates V<sub>S1</sub> at maximum weight. While an aircraft's V<sub>S</sub> speed is computed by design, its V<sub>S0</sub> and V<sub>S1</sub> speeds must be demonstrated empirically by flight testing.<ref>''Flight testing of fixed wing aircraft''. Ralph D. Kimberlin {{ISBN|978-1-56347-564-1}}</ref>