Unbalanced layup

[no2metal]

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??

 

[SWComposites]

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.

 

[Jordonlaw]

"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.

 

[RPstress]

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.