Analysis/modeling methodology, aileron skins

[JJ22mW]

I've got a question regarding what the industry standard way is to apply air loads to flight control surfaces such as ailerons that have the typical sheetmetal skin/ ribs/spar design. On the project I'm working on, the loads are applied directly to the ribs/spars in the loads FEM, and nothing to the skin. Is this common practice? I can see that if you apply loads to skins, a linear FEM would cause the flat skin bays to react in bending, ie no membrane, which is not realistic for thin skins. But by not applying pressures to the skins, is this potentially unconservative because no out of plane loads are superimposed with the inplane loads in the analysis when checking skin buckling?

I guess this leads to another question:
If a flat plate has a pressure applied to it and causes it to go membrane, when you superimpose the inplane shear and compression loads, does the plate have a higher buckling allow than if no normal pressure loads were considered? I see how the membrane tensions could help out, but you also now have an induced curvature that may affect buckling strength.

Thanks for your responses!

 

[SWComposites]

A loads FEM is generally intended to obtain internal loads in the structure, which are then used in separate stress analyses. If you need to run a buckling analysis, then a different FEM is likely needed (with higher mesh density and different, more detailed loads application).

And pressure loads will likely reduce the buckling loads, due to the pressure induced curvature; but this is now a "beam-column" type case and an eigenvalue buckling analysis may be unconservative; a non-linear analysis may be required if the pressure loads are significant.

 

[rb1957]

if you have a coarse grid model (1 element per rib bay/ strgr bay) then the result is the same ... the pressure load is applied at the nodes, on the ribs and stringers.

if you have a fine gird model (4+ elements per rib/strgr panel) then there'll be a node off the ribs and stringers (somewhere in the middle of the panel). if you apply pressure to the skin, this node'll complain 'cause there isn't much bending (out-of-plane) stiffness behind it; ie the skin is flexible, so the load should be applied to the ribs and stringers.

i think you're right to consider the real world effects of the air pressure load on the skins, and this should be in the post-processing stage ('cause FEM either won't do it (in the coarse grid model) or will complain bitter (in the fine grid model). of course it'll tend to increase buckling stiffness (i think) 'cause the pressure on the thin skin will cause in-plane membrane tension stresses ... but then the deflection of the panel will reduce the buckling stiffness.

something else to ponder is that most test rigs will apply the pressure load to the spars and ribs, and not distributed loading onto the surface ... so this bending effect is not represented.

but also note, that the load cases for ailerons (and control surfaces ingeneral) are quite arbitary ... checked pitch (oppposite surface deflection applied), triangular pressure distributions ... so there's a bunch of conservatism there.

and these surfaces are stiffness designed ... the skins "should" have plenty of strength margin in them.

but rememebr, ailerons have been stressed for nearly 100 years now and the methods have generally worked out ! that said, we keep pushing the corners of our capability/knowledge so one day we'll make a mistake !! hopefully not on my watch !

phew !