Editing Stall (fluid dynamics) (section)

==In accelerated and turning flight==
[[File:Accelerated stall.gif|thumb|350px|Illustration of a turning flight stall, occurring during a co-ordinated turn with progressively increasing angle of bank.]]
The normal stall speed, specified by the V<sub>S</sub> values above, always refers to straight and level flight, where the [[load factor (aeronautics)|load factor]] is equal to 1g. However, if the aircraft is turning or pulling up from a dive, additional lift is required to provide the vertical or lateral acceleration, and so the stall speed is higher. An accelerated stall is a stall that occurs under such conditions.<ref>{{cite web
  | last = Brandon
  | first = John
  | title = Airspeed and the properties of air
  | publisher = Recreational Aviation Australia Inc
  | url = http://www.auf.asn.au/groundschool/umodule2.html#accel_stall
  | access-date = 2008-08-09
  | archive-url = https://web.archive.org/web/20080731103646/http://www.auf.asn.au/groundschool/umodule2.html#accel_stall <!-- Bot retrieved archive -->
  | archive-date = 2008-07-31
}}</ref>

In a [[banked turn#Aviation|banked turn]], the [[lift (force)|lift]] required is equal to the [[weight]] of the aircraft plus extra lift to provide the [[centripetal force]] necessary to perform the turn:<ref name=Clancy5.22/><ref>McCormick, Barnes W. (1979), ''Aerodynamics, Aeronautics and Flight Mechanics'', p. 464, John Wiley & Sons, New York {{ISBN|0-471-03032-5}}</ref>
:<math>L = nW</math>

where:
:<math>L</math> = lift
:<math>n</math> = load factor (greater than 1 in a turn)
:<math>W</math> = weight of the aircraft

To achieve the extra lift, the [[lift coefficient]], and so the angle of attack, will have to be higher than it would be in straight and level flight at the same speed. Therefore, given that the stall always occurs at the same critical angle of attack,<ref>Clancy, L.J., ''Aerodynamics'', Sections 5.8 and 5.22</ref> by increasing the load factor (e.g. by tightening the turn) the critical angle will be reached at a higher airspeed:<ref name="Clancy5.22">Clancy, L.J., ''Aerodynamics'', Section 5.22</ref><ref>Clancy, L.J., ''Aerodynamics'', Equation 14.11</ref><ref>McCormick, Barnes W. (1979), ''Aerodynamics, Aeronautics and Flight Mechanics'', Equation 7.57</ref><ref>{{cite web
 |url          = http://home.anadolu.edu.tr/~mcavcar/common/Stall.pdf
 |title        = Stall speed
 |archive-url  = https://web.archive.org/web/20110818001351/http://home.anadolu.edu.tr/~mcavcar/common/Stall.pdf
 |archive-date = 2011-08-18
 |url-status     = dead
}}</ref>
:<math>V_\text{st} = V_\text{s} \sqrt n</math>

where:
:<math>V_\text{st}</math> = stall speed
:<math>V_\text{s}</math> = stall speed of the aircraft in straight, level flight
:<math>n</math> = load factor

The table that follows gives some examples of the relation between the [[angle of bank]] and the square root of the load factor. It derives from the trigonometric relation ([[Cosecant|secant]]) between <math>L</math> and <math>W</math>.

:{| class="wikitable" style="text-align:center;"
|-
! Bank angle
! <math>\sqrt n</math>
|-
| 30°
| 1.07
|-
| 45°
| 1.19
|-
| 60°
| 1.41
|}

For example, in a turn with bank angle of 45°, V<sub>st</sub> is 19% higher than V<sub>s</sub>.

According to [[Federal Aviation Administration]] (FAA) terminology, the above example illustrates a so-called '''turning flight stall''', while the term ''accelerated'' is used to indicate an ''accelerated turning stall'' only, that is, a turning flight stall where the airspeed decreases at a given rate.<ref>{{cite web
 |url          = http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=3a0a07257d2f5a7f42a2c1920e63f263&rgn=div8&view=text&node=14:1.0.1.3.10.2.65.40&idno=14
 |title        = Part 23 – Airworthiness Standards: §23.203 Turning flight and accelerated turning stalls
 |access-date   = 2009-02-18
 |publisher    = [[Federal Aviation Administration]]
 |date         = February 1996
 |archive-url  = https://web.archive.org/web/20090505182730/http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=3a0a07257d2f5a7f42a2c1920e63f263&rgn=div8&view=text&node=14:1.0.1.3.10.2.65.40&idno=14
 |archive-date = 2009-05-05
 |url-status     = dead
}}</ref>

The tendency of powerful propeller aircraft to roll in reaction to engine [[torque]] creates a risk of accelerated stalls. When an aircraft such as a [[Mitsubishi MU-2]] is flying close to its stall speed, the sudden application of full power may cause it to roll, creating the same aerodynamic conditions that induce an accelerated stall in turning flight even if the pilot did not deliberately initiate a turn. Pilots of such aircraft are trained to avoid sudden and drastic increases in power at low altitude and low airspeed as it may be difficult to recover from an accelerated stall under these conditions.<ref>{{cite journal | title = Keeping the props turning: Biennial event maintains mu-2 pilot skills, camaraderie | journal = [[AOPA Pilot]] | date = 1 September 2018 | first = Mike | last = Collins | url = https://www.aopa.org/news-and-media/all-news/2018/september/pilot/turbine-keeping-the-props-turning | access-date = 12 November 2019}}</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]].