washout strut

A hang glider sail (wing) is flexible by design, and control and stability depend on having some twist, i.e. the root of the wing is at a higher angle of attack than the tips. Generally upward twist at the wing tip(washout), where the trailing edge is higher than the leading edge, is desirable in a hang glider, but downward twist (wash-in) can lead to excessive spiral instability or even an unrecoverable spiral dive. For this reason many gliders have a length of aluminium tubing (~2 ft.) below or inside the wing near the tip which fits into a socket in the leading edge tube so that it remains at a fixed angle and prevents wash-in; this tube is known as a washout strut.

A hang glider wing needs washout because it can turn very tightly. When turning, the wing tip on the inside of the turn has a lower airspeed than the rest of the glider - it covers less distance in the same amount of time. The production of lift depends on angle of attack and airspeed. If the wing had a fixed angle of attack and was being flown in a turn at an airspeed such that the middle or root of the wing, where the bulk of the load is, were flying just above stall speed, the inside tip would be seriously stalled, and would tend to drop. Since there are no control surfaces on a hang glider to counter the roll (on an airplane one might use the aileron on the outside wing to adjust) a spiral dive would ensue, leading to other problems.

So the wing tip is twisted (and allowed to twist in response to load) so that it will have a lower angle of attack and thus produce lift when on the inside of a turn. It is possible through maneuvers or errant gusts to apply a load to a wingtip such that it could wash-in deeply and quicky lead to serious control problems, so the washout strut is there to prevent twist in the wrong direction. In normal flight the wingtip does not place any load on the washout strut, and one could fly happily without them, as long as conditions and maneuvers were mild.