Editing Stall (fluid dynamics) (section)

{{Short description|Abrupt reduction in lift due to flow separation}}
{{Use dmy dates|date=March 2023}}
{{Use British English|date=March 2023}}
[[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]] exceeds its [[critical angle of attack|critical value]].<ref name="Crane">Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. {{ISBN|1-56027-287-2}}</ref> The critical angle of attack is typically about 15°, but it may vary significantly depending on the [[fluid]], foil – including its shape, size, and finish – and [[Reynolds number]].

Stalls in [[fixed-wing aircraft]] are often experienced as a sudden reduction in lift. It may be caused either by the pilot increasing the wing's angle of attack or by a decrease in the critical angle of attack. The former may be due to slowing down (below [[Stall (fluid dynamics)#Stall speeds|stall speed]]), the latter by [[icing (aviation)|accretion of ice]] on the wings (especially if the ice is rough). 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>

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.