Editing Delta wing (section)

==General characteristics==
===Structure===
The long root chord of the delta wing and minimal area outboard make it structurally efficient. It can be built stronger, stiffer and at the same time lighter than a [[swept wing]] of equivalent [[aspect ratio]] and lifting capability. Because of this it is easy and relatively inexpensive to build—a substantial factor in the success of the [[Mikoyan-Gurevich MiG-21|MiG-21]] and [[Dassault Mirage|Mirage]] aircraft series.<ref>{{Cite book |last1=Sahoo |first1=Devabrata |url=https://books.google.com/books?id=TYshEQAAQBAJ&dq=%C2%A0simplicity+delta+wing+stronger+stiffer&pg=PA50 |title=Advances in Aerospace Technologies |last2=Sharma |first2=Abhishek |last3=Rajput |first3=Shailendra |date=2024-10-23 |publisher=CRC Press |isbn=978-87-7004-638-1 |pages=50 |language=en}}</ref>

Its long root chord also allows a deeper structure for a given [[aerofoil]] section. This both enhances its weight-saving characteristic and provides greater internal volume for fuel and other items, without a significant increase in drag. However, on supersonic designs the opportunity is often taken to use a thinner aerofoil instead, in order to actually reduce drag.

===Aerodynamics===

====Low-speed flight and vortex lift====
Like any wing, at low speeds a delta wing requires a high [[angle of attack]] to maintain lift. At a sufficiently high angle the wing exhibits [[flow separation]], together with an associated high drag.<ref name=":0">{{Cite book|title=High Angle of Attack Aerodynamics: Subsonic, Transonic, and Supersonic Flows|url=https://archive.org/details/highangleattacka00romj|url-access=limited|last=Rom|first=Josef|date=1992|publisher=Springer New York|isbn=9781461228240|location=New York, NY|pages=[https://archive.org/details/highangleattacka00romj/page/n24 15]–23|oclc=853258697}}</ref>

Ordinarily, this flow separation leads to a loss of lift known as the [[Stall (fluid dynamics)|stall]]. However, for a sharply-swept delta wing, as air spills up round the leading edge it flows inwards to generate a characteristic [[vortex]] pattern over the upper surface. The lower extremity of this vortex remains attached to the surface and also accelerates the airflow, maintaining lift. For intermediate sweep angles, a retractable "moustache" or fixed [[leading-edge root extension]] (LERX) may be added to encourage and stabilise vortex formation. The ogee or "wineglass" double-curve, seen for example on [[Concorde]], incorporates this forward extension into the profile of the wing.

In this condition, the centre of lift approximates to the centre of the area covered by the vortex.

====Subsonic flight====
In the subsonic regime, the behaviour of a delta wing is generally similar to that of a [[swept wing]]. A characteristic sideways element to the airflow develops. In this condition, lift is maximised along the leading edge of the wing, where the air is turned most sharply to follow its contours. Especially for a slender delta, the centre of lift approximates to halfway back along the leading edge.

The sideways effect also leads to an overall reduction in lift and in some circumstances can also lead to an increase in drag. It may be countered through the use of leading-edge slots, wing fences and related devices.

====Transonic and low-supersonic flight====
[[File:194th Fighter-Interceptor Squadron - Convair F-106A-135-CO Delta Dart 59-0136.jpg|thumb|Convair made several supersonic deltas. This is an [[Convair F-106 Delta Dart|F-106 Delta Dart]], a development of their earlier F-102 Delta Dagger]]
With a large enough angle of rearward sweep, in the [[transonic]] to low [[supersonic]] speed range the wing's leading edge remains behind the [[shock wave]] boundary or [[shock cone]] created by the leading edge root.

This allows air below the leading edge to flow out, up and around it, then back inwards creating a sideways flow pattern similar to subsonic flow. The lift distribution and other aerodynamic characteristics are strongly influenced by this sideways flow.<ref name="mason10-1">Mason, Chap. 10, pp. 9–12.</ref>

The rearward sweep angle lowers the airspeed normal to the leading edge of the wing, thereby allowing the aircraft to fly at high [[subsonic flight|subsonic]], transonic, or supersonic speed, while the subsonic lifting characteristics of the airflow over the wing are maintained.

Within this flight regime, drooping the leading edge within the shock cone increases lift, but not drag to any significant extent.<ref>Boyd, Migotzky and Wetzel; "A Study of Conical Camber for Triangular and Sweptback Wings", Research Memorandum A55G19, NACA, 1955.[http://nix.nasa.gov/search.jsp?R=19930090334&qs=N%3D17%2B4293246867]{{dead link|date=June 2021|bot=medic}}{{cbignore|bot=medic}}</ref> Such conical leading edge droop was introduced on the production [[Convair F-102A Delta Dagger]] at the same time that the prototype design was reworked to include [[Area rule|area-ruling]]. It also appeared on Convair's next two deltas, the [[F-106 Delta Dart]] and [[B-58 Hustler]].<ref>Mason, Chap. 10, p. 16.</ref>

====High-speed supersonic waveriding====
At high supersonic speeds, the shock cone from the leading edge root angles further back to lie along the wing surface behind the leading edge. It is no longer possible for the sideways flow to occur and the aerodynamic characteristics change considerably.<ref name="mason10-1" /> It is in this flight regime that the [[waverider]] design, as used on the [[North American XB-70 Valkyrie]], becomes practicable. Here, a shock body beneath the wing creates an attached shockwave and the high pressure associated with the wave provides significant lift without increasing drag.