JE Gordon's masterly popular exposition, Structures - Or Why Things Don't Fall Down, contains a fascinating account of the early wing failures experienced by the Fokker D8 monoplane. The first world war pilots who initially flew the D8 found to their cost that placing the aircraft in a steep dive would literally twist the wings from the fuselage.
Professor Gordon's explanation of the cause for this revolves around two concepts: the centre of flexure of the wings, and the centre of aerodynamic pressure. The wings on the Fokker D8 were constructed from two spars running across the span of the wing, and a series of ribs running across the chord at regular intervals. Both spars initially had the same stiffness.
The upward force on an aircraft wing will make it bend upwards. If an upward force were applied halfway between the two spars, then both spars would bend upwards by an equal amount. Unfortunately, the centre of pressure on an aircraft wing lies approximately one quarter-chord from the leading edge. As a consequence, the forward spar on the Fokker D8 would bend upwards by a greater degree than the rear spar, twisting the wing. This also increased the angle of attack at the ends of the wing, leading to a positive feedback process in which the wings were twisted to ever greater degrees.
The point at which a load must be applied in a beam-like structure to cause no twisting effect is called the centre of flexure. The centre of flexure and the torsional stiffness of the Fokker D8 wings were dependent upon the relative stiffness of the two spars. When the two spars were equally stiff, the centre of flexure lay behind the centre of pressure, causing the wing to twist at the ends as it was bent upwards. Thus, to prevent this, is was ultimately necessary to make the forward spar stiffer than the rear spar. By stiffing the forward spar, the centre of flexure was moved forward to coincide with the centre of pressure, and the wings were prevented from twisting.
Now, one might suggest that the inverse of the twisting effect suffered by the Fokker D8s could actually be very beneficial for the front-wing aerodynamics of a Formula 1 car. If the centre of flexure of a front-wing is designed so that it lies behind the centre of pressure, then when the ends of the wing are bent downwards by the aerodynamic loads, the ends of the wing will also be twisted forwards, increasing the angle of attack of the outermost parts of the wing, and enhancing the ground effect, in a positive feedback process.
Thank goodness the FIA's front-wing deflection tests prevent this type of thing from happening.