page_title: Swept wing
section_title: Forward sweep
hierarchical_section_title: Swept wing / Variant designs / Forward sweep
caption_attribution_description: The No. 2 X-29 technology demonstrator aircraft is seen here during a 1990 test flight. At this angle, the aircraft’s unique forward-swept wing design is clearly visible. The X-29 was flown by NASA's Ames-Dryden Flight Research Facility (later redesignated the Dryden Flight Research Center), Edwards, California, in a joint NASA-Defense Advanced Research Projects Agency-Air Force program to investigate the unique design's high-angle-of-attack characteristics and its military utility. Tufts -- small strips of cloth attached to the surface of the aircraft to visually study the flow of air over the aircraft -- can be seen on the aft fuselage, wing, and tail surfaces of the X-29 in this photo. Angle of attack, or high alpha, refers to the angle of an aircraft's body and wings relative to its actual flight path. This aircraft was flown at Dryden from May 1989 until August 1992.
context_page_description: A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction.
  Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigated in Germany as early as 1935, finding application just before the end of the Second World War. It has the effect of delaying the shock waves and accompanying aerodynamic drag rise caused by fluid compressibility near the speed of sound, improving performance. Swept wings are therefore almost always used on jet aircraft designed to fly at these speeds. Swept wings are also sometimes used for other reasons, such as low drag, low observability, structural convenience or pilot visibility.
  The term "swept wing" is normally used to mean "swept back", but variants include forward sweep, variable sweep wings and oblique wings in which one side sweeps forward and the other back. The delta wing is also aerodynamically a form of swept wing.
context_section_description: Sweeping a wing forward has approximately the same effect as rearward in terms of drag reduction, but has other advantages in terms of low-speed handling where tip stall problems simply go away. In this case the low-speed air flows towards the fuselage, which acts as a very large wing fence. Additionally, wings are generally larger at the root anyway, which allows them to have better low-speed lift.
  However, this arrangement also has serious stability problems. The rearmost section of the wing will stall first causing a pitch-up moment pushing the aircraft further into stall similar to a swept back wing design. Thus swept-forward wings are unstable in a fashion similar to the low-speed problems of a conventional swept wing. However unlike swept back wings, the tips on a forward swept design will stall last, maintaining roll control.
  Forward-swept wings can also experience dangerous flexing effects compared to aft-swept wings that can negate the tip stall advantage if the wing is not sufficiently stiff. In aft-swept designs, when the airplane maneuvers at high load factor the wing loading and geometry twists the wing in such a way as to create washout (tip twists leading edge down). This reduces the angle of attack at the tip, thus reducing the bending moment on the wing, as well as somewhat reducing the chance of tip stall.  However, the same effect on forward-swept wings produces a wash-in effect that increases the angle of attack promoting tip stall.
  Small amounts of sweep do not cause serious problems, and had been used on a variety of aircraft to move the spar into a convenient location, as on the Junkers Ju 287 or HFB 320 Hansa Jet. But larger sweep suitable for high-speed aircraft, like fighters, was generally impossible until the introduction of fly by wire systems that could react quickly enough to damp out these instabilities. The Grumman X-29 was an experimental technology demonstration project designed to test the forward swept wing for enhanced maneuverability in 1984. The Su-47 Berkut is another notable example using this technology. However no highly swept-forward design has entered production.
caption: Grumman X-29 experimental aircraft, an extreme example of a forward swept wing
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