Pressure+Internal loads 2

[airmail]

Hi all!
I am looking for some advice about compliances in stiffness and strength of an aerodynamic surface. In particular, wing leading edge.
I have to prove that panel's deflection is below 1 mm. To ensure this, is it necessary that loads of the panel and pressures be taken into account at the same time? I mean, critical in-plane flows + pressure is my load case? Or, on the contrary, in-plane flows for strength justification and pressure for stiffness proving?

Best wishes.

 

[israelkk]

Usually aerodynamic surface sees only pressure distribution, what other loads are you reffering to?

 

[airmail]

For example, loads coming from wing torsion box (through attachment ribs), in a manoeuvre, windmilling, gusts...

 

[Sparweb]

With the flexure of the wing under load, there will be sympathetic load in the leading edge whether you want it or not. I agree that this load must be considered in conjunction with the pressure from the airflow. You cannot divorce the stress from the load that produces it. It would violate the first requirement of structural analysis: the applied loads on a structure in equilibrium with be equal and opposite to the reactions. The loads are specified by the reg's. It's up to you to explore the structural consequences of those loads.

If you're working on a wing, for example, with a 2-spar box structure, and ignoring the LE and TE for the primary analysis, the sympathetic stress in the LE and TE deserve to be checked afterward to see how much bucking will be induced by the deflection of the wing (flexure+torsion simultaneously). I would picture some shear being induced into the LE and TE panel. Wrinkling of these surfaces will mean that the aerofoil changes shape, probably for the worse. The aerodynamic pressure that is associated with the load case (ie., low pressure over the top of the LE) may help, but I doubt it would help much.


Steven Fahey, CET

 

[airmail]

Steven, thank you for your opinion.

A question that comes to me now is next: Is it reasonable that the fact of having or not only in-plane loads (in panels)has influence in panel's deflection (caused only by out-of-plane loads).

I think that if the panel does not buckle and if it's strength enough, normal to plane loads should be the only ones that have influence in panel's deflection. So, panel's strength and stiffness can be studied separately. (Obviously, always without panel's buckling).
Another thing: what's sympathetic loads? (you're right: I am not english speaker)

Regards.

 

[SuperStress]

The simulataneous application of in-plane and out-of-plane loads is the classic definition of a beam-column. There most definitely WILL be interaction between them. Let me know if you need references for beam-column interactions and I'll post some for you.

"Sympathetic" loads might also be called "induced" loads. Basically we're talking about how the leading edge (in this case) gets loaded by the wing box simply because it is attached to it. It is "going along for the ride" and picking up unintentional loading.

Hope that helps.

SuperStress

 

[airmail]

Thanks, Super.

I've decided to enter both loads: in plane and normal. Now, my results vary a lot. I'm obtaining big displacements (vertical deflections). So, probably, linear analysis is not reliable.

I haven't found some criterion in allowable displacements for laminated panels.

Any advice? Shouldn't I set so low bounds to panels' deflection (max: 1 mm) and be focused on ensure, for example, no buckling?

Thanks again. Regards.

 

[Sparweb]

You won't find a universally accepted deflection for your panel because there isn't one, no matter what it is. Some purposes may allow lots of deflection, others require very little.

A lot of information should be considered before answering your question. What's the critical buckling stress in the panel? What's it made of and how thick is it? How sensitive to deflection is the particular area? Where is this panel on the wing? I can't see any of these parameters from here; only you can. A better "feel" for the numbers and the loads/stresses/deflections that they represent is required to give you further advice. One way to obtain a better feel is to turn off the computer, open Bruhn, and work out the problem without FEA. If there's still debate, test a full scale mock up of the panel, and watch it wrinkle under the critical load cases.



Steven Fahey, CET