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 by Albert Betz and Adolph Busemann, 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. 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. Reasons for sweep There are three main reasons for sweeping a wing: 1. to arrange the center of gravity of the aircraft and the aerodynamic center of the wing to coincide more closely for longitudinal balance, e.g. Messerschmitt Me 163 Komet and Messerschmitt Me 262. Although not a swept wing the wing panels on the Douglas DC-1 outboard of the nacelles also had slight sweepback for similar reasons. 2. to provide longitudinal stability for tailless aircraft, e.g. Messerschmitt Me 163 Komet. 3. most commonly to increase Mach-number capability by delaying to a higher speed the effects of compressibility (abrupt changes in the density of the airflow), e.g. combat aircraft, airliners and business jets. Other reasons include: 1. enabling a wing carry-through box position to achieve a desired cabin size, e.g. HFB 320 Hansa Jet. 2. providing static aeroelastic relief which reduces bending moments under high g-loadings and may allow a lighter wing structure. 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