A forward-swept wing is an aircraft wing configuration in which the quarter-chord line of the wing has a forward sweep. Typically, the leading edge also sweeps forward. Characteristics The forward-swept configuration has a number of characteristics which increase as the angle of sweep increases. Main spar location The rearward location of the main wing spar would lead to a more efficient interior arrangement with more usable space. Inward spanwise flow Air flowing over any swept wing tends to move spanwise towards the rearmost end of the wing. On a rearward-swept wing this is outwards towards the tip, while on a forward-swept wing it is inwards towards the root. As a result, the dangerous tip stall condition of a rearward-swept design becomes a safer and more controllable root stall on a forward-swept design. This allows full aileron control despite loss of lift, and also means that drag-inducing leading edge slots or other devices are not required. With the air flowing inwards, wingtip vortices and the accompanying drag are reduced. Instead, the fuselage acts as a very large wing fence and, since wings are generally larger at the root, this raises the maximum lift coefficient allowing a smaller wing. As a result, maneuverability is improved, especially at high angles of attack. At transonic speeds, shockwaves build up first at the root rather than the tip, again helping ensure effective aileron control. Yaw instability One problem with the forward-swept design is that when a swept wing yaws sideways (moves about its horizontal axis), one wing retreats while the other advances. On a forward-swept design, this reduces the sweep of the rearward wing, increasing its drag and pushing it further back, increasing the amount of yaw and leading to directional instability. This can lead to a Dutch roll in reverse. Aeroelasticity One of the drawbacks of forward swept wings is the increased chance of divergence, an aeroelastic consequence of lift force on forward swept wings twisting the tip upwards under increased lift. On a forward-swept design, this causes a positive feedback loop that increases the angle of incidence at the tip, increasing lift and inducing further deflection, resulting in yet more lift and additional changes in wing shape. The effect of divergence increases with speed. The maximum safe speed below which this does not happen is the divergence speed of the aircraft. Such an increase in tip lift under load causes the wing to