Editing Swept wing (section)

==Structural design==

For a wing of given span, sweeping it increases the length of the spars running along it from root to tip. This tends to increase weight and reduce stiffness. If the fore-aft chord of the wing also remains the same, the distance between leading and trailing edges reduces, reducing its ability to resist twisting (torsion) forces. A swept wing of given span and chord must therefore be strengthened and will be heavier than the equivalent unswept wing.

A swept wing typically angles backward from its root rather than forwards. Because wings are made as light as possible, they tend to flex under load. This [[aeroelasticity]] under aerodynamic load causes the tips to bend upwards in normal flight. Backwards sweep causes the tips to reduce their angle of attack as they bend, reducing their lift and limiting the effect. Forward sweep causes the tips to increase their angle of attack as they bend. This increases their lift causing further bending and hence yet more lift in a cycle which can cause a runaway structural failure. For this reason forward sweep is rare and the wing must be unusually rigid.

There are two sweep angles of importance, one at the leading edge for supersonic aircraft and the other 25% of the way back from the leading edge for subsonic and transonic aircraft. Leading edge sweep is important because the leading edge has to be behind the mach cone to reduce wave drag.<ref>Aircraft Performance and Design, John D.AndersonJr.  1999,{{ISBN|0 07 001971 1}}, p.422</ref> The quarter chord (25%) line is used because subsonic lift due to angle of attack acts there and, up until the introduction of supercritical sections, the crest was usually close to the quarter chord.<ref>Fundamentals Of Flight Second Edition, Richard S. Shevell 1989,{{ISBN|0 13 339060 8}}, p.200</ref>

Typical sweep angles vary from 0 for a straight-wing aircraft, to 45 degrees or more for fighters and other high-speed designs.