Editing Swept wing (section)

=== Disadvantages ===
{{More citations needed section|date = November 2021}}

When a swept wing travels at high speed, the airflow has little time to react and simply flows over the wing almost straight from front to back. At lower speeds the air ''does'' have time to react, and is pushed spanwise by the angled leading edge, towards the wing tip. At the wing root, by the fuselage, this has little noticeable effect, but as one moves towards the wingtip the airflow is pushed spanwise not only by the leading edge, but the spanwise moving air beside it. At the tip the airflow is moving along the wing instead of over it, a problem known as ''spanwise flow''.

The lift from a wing is generated by the airflow over it from front to rear. With increasing span-wise flow the boundary layers on the surface of the wing have longer to travel, and so are thicker and more susceptible to transition to turbulence or flow separation, also the effective aspect ratio of the wing is less and so air "leaks" around the wing tips reducing their effectiveness. The spanwise flow on swept wings produces airflow that moves the stagnation point on the leading edge of any individual wing segment further beneath the leading edge, increasing effective [[angle of attack]] of wing segments relative to its neighbouring forward segment. The result is that wing segments farther towards the rear operate at increasingly higher angles of attack promoting early stall of those segments. This promotes tip stall on back-swept wings, as the tips are most rearward, while delaying tip stall for forward-swept wings, where the tips are forward. With both forward and back-swept wings, the rear of the wing will stall first creating a nose-up moment on the aircraft. If not corrected by the pilot the plane will pitch up, leading to more of the wing stalling and more pitch up in a divergent manner. This uncontrollable instability came to be known as the ''[[Sabre dance (pitch-up)|Sabre dance]]'' in reference to the number of North American [[F-100 Super Sabre]]s that crashed on landing as a result.<ref name="historynet.com">{{cite web |url = http://www.historynet.com/deadly-sabre-dance.htm |title = Deadly Sabre Dance |date = 11 July 2011 |publisher = historynet.com |access-date = 11 November 2020}}</ref><ref>Ives, Burl. "Burl Ives Song Book." Ballantine Books, Inc., New York, November 1953, page 240.</ref>

Reducing pitch-up to an acceptable level has been done in different ways such as the addition of a fin known as a ''[[wing fence]]'' on the upper surface of the wing to redirect the flow to a streamwise direction. The [[MiG-15]] was one example of an aircraft fitted with wing fences.<ref name="Gunston Russian p188">Gunston 1995, p. 188.</ref> Another closely related design was the addition of a [[dogtooth extension|dogtooth notch]] to the leading edge, used on the [[Avro Arrow]] interceptor.<ref>Whitcomb 2002, pp.&nbsp;89–91.</ref> Other designs took a more radical approach, including the [[Republic XF-91 Thunderceptor]]'s wing that grew wider towards the tip to provide more lift at the tip. The [[Handley Page Victor]] was equipped with a [[crescent wing]], with three values of sweep, about 48 degrees near the wing root where the wing was thickest, a 38 degree transition length and 27 degrees for the remainder to the tip.<ref>Brookes 2011, pp. 6–7.</ref><ref>Lee, G.H. [http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201386.html "Aerodynamics of the Crescent Wing."] ''[[Flight International|Flight]]'', 14 May 1954, pp. 611–612.</ref>

Modern solutions to the problem no longer require "custom" designs such as these. The addition of [[leading-edge slat]]s and large compound [[flap (aircraft)|flaps]] to the wings has largely resolved the issue.<ref name="Smith1975">[http://www.arvelgentry.com/amo/High-Lift_Aerodynamics.pdf High-Lift Aerodynamics, by A. M. O. Smith, McDonnell Douglas Corporation, Long Beach, June 1975] {{webarchive|url=https://web.archive.org/web/20110707172637/http://www.arvelgentry.com/amo/High-Lift_Aerodynamics.pdf |date=7 July 2011 }}</ref><ref>{{citation |first=F.|last= Handley Page |url=https://www.flightglobal.com/pdfarchive/view/1921/1921%20-%200844.html |title= Developments In Aircraft Design By The Use Of Slotted Wings | archive-url=https://web.archive.org/web/20121103181345/http://www.flightglobal.com/pdfarchive/view/1921/1921%20-%200844.html |archive-date=3 November 2012 |work=Flight |date= 22 December 1921 | page= 844 |via=Flightglobal Archive |volume= XIII |number=678 |url-status=live }}</ref><ref name=perkins-hage>Perkins, Courtland; Hage, Robert (1949). ''Airplane performance, stability and control'', Chapter 2, John Wiley and Sons. {{ISBN|0-471-68046-X}}.</ref> On fighter designs, the addition of [[leading-edge extension]]s, which are typically included to achieve a high level of maneuverability, also serve to add lift during landing and reduce the problem.<ref>{{cite web |last1=Lee |first1=Gwo-Bin |title=Leading-edge Vortices Control on a Delta Wing by Micromachined Sensors and Actuators |url=http://www.las.inpe.br/~jrsenna/AerospaceMEMS/Contr-Ensaios-voo/caltech1.pdf |publisher= American Institute of Aeronautics and Astronautics |access-date=18 October 2018}}</ref><ref>''Effects of Wing-Leading-Edge Modifications on a Full-Scale, Low-Wing General Aviation Airplane.'' Nasa TP, 2011.</ref>

In addition to pitch-up there are other complications inherent in a swept-wing configuration. For any given length of wing, the actual span from tip-to-tip is shorter than the same wing that is not swept. There is a strong correlation between low-speed drag and [[aspect ratio (wing)|aspect ratio]], the span compared to chord, so a swept wing always has more drag at lower speeds. In addition, there is extra torque applied by the wing to the fuselage which has to be allowed for when establishing the transfer of wing-box loads to the fuselage. This results from the significant part of the wing lift which lies behind the attachment length where the wing meets the fuselage.