Sizing Fuselage Frames using FEA Results

[DRDearth]

I've run acoss a number of engineers questioning how to use results from FEA models to Size Fuselage frames for bending moment. I've always thought one typically takes Shear loading in the frame between longerons (q lbs/inch) X distance between longerons (inch)==> this gives total shear force (V lbs). Then multiply this total shear X frame depth (dx)to give total Moment (M in-lbs) i.e. M = V x dx. But when shear loads come out of a FEA model that idealizes the frame with outer & inner caps as Rods and the frame web as a membrane (or shear panel), Then the question arises how one relates these Rod loads (lbs) & panel shear (lbs/in) to estimate total moment in the Frame.

What would be nice is a sample problem that illustrates how one can use the FEA results of Rod axial loads and panel shear loads to compute bending moment in the Frame. Any comments out there?

David R. Dearth, P.E.
Applied Analysis & Technology

 

[corus]

Generally the results (forces etc) from a FEA analysis depend on the relative stiffness of the stucture. I would be wary of factoring up or down the size fo a frame in relation to the results as the change in size of the frame would affect the stiffness and hence the forces etc. If anything use the results as a guide and then reassess the structure with the new sizes to check if they meet design standards.

corus

 

[JStephen]

I'm not sure I follow your reasoning at all on the calculation of the moment. In the M=Vdx, the dx ought to be differential of the length, not the depth?

But regardless, if you have all the internal forces at a particular cross section, it's not hard to find the resulting moment- perhaps tedious if there's a lot of elements.

 

[GregLocock]

Or, of course, change the model so that it uses the actual section properties, rather than building up a composite element.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DRDearth]

To All.. Thanks for the commments. Aircraft Fuselage frames are typically circular maybe a "Z" section. Usually the frame section is idealized using the Frame web modeled as a Membrane and the frame inner & outer caps are idealized as Rod elements. The bending moment in the Frame can be estimated taking the differnce in axial loading between the Inner & Outer Caps (Rods) X the depth of the frame. The depth being the inboard/outboard distance between inner & outer caps (the rods). However, bending moment can also be approximated taking shear loading (lbs/in) around the circular section X length between longerons X inboard/outboard depth. This is outlined in Bruhn & Perry for example. The problem becomes more complex in actual FEA models so it sometimes becomes difficult to sort out. AKA the source of this Thread.

David R. Dearth, P.E.
Applied Analysis & Technology

 

[GregLocock]

Well, I'm not quite sure you'd use a hand calculation methodology in FEA other than as a check, but if that is really what you want to do, write a postprocessor to pull the moment out at the end of each element in the results deck and add them up. The best language to use for that is Fortran, second best is QBASIC, in my (relevant) experience.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DRDearth]

Thanks for the note.. the questions I posted are really for the purposes of making ROM santiy checks of FEMs. In the actual case FEM of aircraft structures will react loading more complex than the simple case I outlined.

David R. Dearth, P.E.
Applied Analysis & Technology

 

[Stressdude]

I am faced with the same problem in my current assignment.

What I did is to use like what David mentioned to obtain the moment. Now this is the FEM's moment. Besides moment, you'll also have axial loads, which is the summation of the outer and inner chord (idealized as rods) loading.

Depending on the actual sectional properties, you'll have additional moments when you shift the axial loads from the FEM centriod to the actual section's centriod.

As for the shear stress in the web element, I'll use it to check the web for panel buckling.

Using the sectional properties of the frame, Mc/I + P/A will give you the chord's tension and compressive stress. Tension stress is checked against material tensile properties while compression is checked for lateral instability using Johnson-Euler's column buckling check, ref. Bruhn's.

Besides the frame's primary loading, I'll check if the skin has gone into intermediate diagonal tension, as this will dump secondary loads to the frame chords. See Bruhn's Section C11 on curve skins for more insight to the problem.

If I missed out anything, feel free to add.

Gordon