Forward-swept wings

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.

The forward-swept configuration has a number of characteristics which tua increase as the angle of sweep increases.

The rearward location of the main wing spar would lead to a more efficient interior arrangement with more usable space.

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.

One problem with the forward-swept design is that when a swept wing yaws sideways (moves about its vertical 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.

An aeroelastic consequence of lift force on the wing is the twisting of the tip upwards. On a forward-swept design, this 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. In the worst case, the tip structure can be stressed to the point of failure. This divergence speed marks a maximum safe speed for the aircraft.

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Wikipedia