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Unbalanced layup

Unbalanced layup

Unbalanced layup

(OP)
Is composite wing mainly of quasi layup construction? and is it only for the only skins or the stiffeners like spars, ribs and stringers. I came across a presentation mentioning using unbalanced layup on the wing design to overcome the twisting, is this just simply add on extra plies on a quasi layup skins??

RE: Unbalanced layup

No, wing skin layups are typically biased towards having more fibers in the primary load carrying direction; i.e., spanwise.  Unbalanced means there are an unequal number of +45 and -45 plies, which induces twisting of the wing when it bends in the spanwise direction.  This bending-twist coupling can be used to counteract aerodynamic induced twisting of the wing.

RE: Unbalanced layup

"Unbalanced means there are an unequal number of +45 and -45 plies"

Well in fact any unequal number of off-axis plies (except 90s of course). You get unbalanced layups with unequal numbers of +30, -30 plies for example.

RE: Unbalanced layup

A/C box skins (wing, tailplane, fin) might be as much as 60% spanwise fibre in some places. This needs unidirectional material (unbiased all cloth layups can be at most 50% 0 degree). Such a skin might vary between 40% and 60% spanwise. It depends on the other forces in the box. +-45 degree plies need to be present to take box torsion. Typical percentages might be 30 to 50%. Chordwise fibres are needed as well, particularly near significant load inputs such as trailing edge control surface hinges and actuators. Skin stiffeners can be more 0 degree, as they don't need as many angle plies or 90 degree ones.

Spar webs need lots of 45 degree plies. Ribs need a good mix, with (usually) quite a few fibres in the vertical direction to resist Brazier crushing loads and tension due to over-fuel pressures and crash pressures.

Before the aerodynamic effects of gross structural coupling between incidence and torsion were understood some terribly dangerous monoplanes were designed. In WWI quite a few German pilots were killed by such effects.

Deliberate material-based bend-twist coupling was probably most famously used on the X29 with its forward swept wing.

While you can achieve such coupling with uneven numbers of + and - angle plies, it can also be done by using the distribution through the thickness and to keep the same number in each direction. (This is probably more practical for a more monolithic structure than a box.) Just put all the +angle plies nearer the centerline of the beam than the -ones (or vice versa, depending on what you're trying to do). Under bending, the +plies above the neutral axis try to shear one way and those below it shear the other (giving twist). The same goes for the -plies, giving twist in the opposite direction. However, the plies furthest away from the neutral axis are more effective, giving a nett twist in their direction.

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