2 spar differential bending

[GASTRESS82]

Does anyone know a good reference for a simple method for calculating how shear and bending moment transfer between two spars as their moments of inertia change going inboard. I have the independent shear and bending on each spar as well as the balanced moments based on percentage of moment of inertia. Where I'm struggling is how the residual torque is calculated and how the transfer of shear/bending happens. In this case the spars are parallel and the leading edge has some sweep. So the height of the forward spar becomes smaller than the height of the aft spar going outboard.
Thanks.

 

[rb1957]

I guess i'd start by distributing the shears in accordance with statics ... you'll have ribs to join the two spars together, sort of like a beam. then the question of displacement compatibility comes into play ... the two spars can't deflect independently ('cause of the ribs) if the ribs are rigid they'll force the apars to deflect the same, causing shear to move from one spar to the other, and to create a fwd/aft shear couple to balance.

Quando Omni Flunkus Moritati

 

[SuperStress]

You've asked a very complex question to which there are no simple answers. Load follows the stiffest path, plain and simple. Finding the stiffest path is not always obvious. It has to do not only with the geometry of your spars, but also the stiffness of their attachments to the fuselage. What is the backup structure? Is it a continuous wing tip-to-tip, is there a center wing box, or are the wings cantilevered out from fuselage attachments?

Also, as rb1957 states, rib geometry/spacing/stiffness also comes into play. Your two-spar arrangement is a torque box. Review the sections in Bruhn about this construction. I guess my best advice is to do your detailed load path work first. Then your stress analysis will fall out naturally.

SuperStress

 

[Worldtraveller]

I'm going to agree with what the others have said above. It's a complex problem, but one that can be solved analytically. Specifically, Bruhn section A21 is probably where you want to start. You can start outboard and step through the loads bay by bay (this is where spreadsheets excel (no pun intended) and get coupled shear and bending moment diagrams for each spar. The shear diagrams should look essentially like step functions where the ribs transfer shear between spars.

Actually, that sounds like an interesting exercise. I might try to work up a spreadsheet in open office and see what the results look like.