Can anyone recommend texts/papers/websites/etc that provide guidelines for finite element modeling of aerostructures, preferably with regard to NASTRAN? I've been doing FEA for a long time, but very little of my experience is with aerostructures/boxbeam modeling, analysis, and (especially)the results interpretation for downstream use in detail stress work. From the research I've done so far, it seems the big aero companies (I don't work for one) have their own published guidlines on proper use of NASTRAN; I was wondering if there are more publicly available sources with similar content. The FEA of aerostructures seems like a world all its own (e.g., model your spars with rods and shear panels, include only this much skin, and interpret your output just so, in order to do subsequent detail stress work on web buckling and rib crushing). Hopefully you folks that do this kind of thing for a living (work for FEA group in a big aero company?)will know just what I mean, and point me in the right direction. Thanks all.
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NASTRAN/FEA GUIDELINES FOR AEROSTRUCTURES? 17
- Thread starter LIstress
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Aerospace
- Sep 6, 2005
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From my interpretation of your post, your current work is wing spar and ribs??
If this is the case then your main FE model will be a global tip-to-tip wing with a single quad element per panel, with bars replicating stiffeners?? Am i right??
If so then your typical approach is to develop a decent global FEM to which to apply all your SMT loads to, which will in turn give you results on you spars, ribs, covers etc. Detail stressing will still be covered by hand analysis.
There isnt any big great wonderfull book to which to use as a basis to design aircraft FEM to unfortunately. The old fashioned route of experience is gernerally used.
If this is the case then your main FE model will be a global tip-to-tip wing with a single quad element per panel, with bars replicating stiffeners?? Am i right??
If so then your typical approach is to develop a decent global FEM to which to apply all your SMT loads to, which will in turn give you results on you spars, ribs, covers etc. Detail stressing will still be covered by hand analysis.
There isnt any big great wonderfull book to which to use as a basis to design aircraft FEM to unfortunately. The old fashioned route of experience is gernerally used.
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- #3
Aero:
Thanks for responding.
Yep--spars and ribs for now; skins, bulkheads, fittings etc not far down the road either. You are exactly right about the modeling approach too.
We're given the loads in the form of Nastran output, for subsequent use in more classical type analyses. Once I started working the job and researching the topic a bit, I started to experience the nuances of this branch of the FEM/NASTRAN world, thus prompting my questions.
What does SMT stand for?
Thanks for responding.
Yep--spars and ribs for now; skins, bulkheads, fittings etc not far down the road either. You are exactly right about the modeling approach too.
We're given the loads in the form of Nastran output, for subsequent use in more classical type analyses. Once I started working the job and researching the topic a bit, I started to experience the nuances of this branch of the FEM/NASTRAN world, thus prompting my questions.
What does SMT stand for?
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Not aware of any public guidelines.
However, here are a few tips on what I'm aware of standard industry practice.
Not many people use shear elements. 99% of wing-box-type models are shells (use CQUAD4s in Nastran; CQUAD8s look pretty but NASTRAN's are not as good as the CQUAD4s, which are a highly refined formulation developed over many years).
I've seen a global (whole-wing-box type) model with as few as 6 shell elements down the depth of the spar web. This seems a bit mean to me, but it worked for them.
For composite materials, smeared properties on MAT8s with PSHELLs are often used, rather than have a whole load of PCOMPs.
Often in a wing model things like the spar flanges and wing cover stiffeners will be modelled with bars (use CBEAMs not CBARs if you ever want to go non-linear). However, it's probably more sensible to use more QUADs these days; bars used to be used to save CPU, which basically isn't relevant any more. Stiffeners should be at least two QUAD4 elements deep (some say three). Generally element corner stress recovery should be turned on. These days this seems passably accurate in NASTRAN, but keep an eye on unusual geometry. "Strain gauge" bars (very low area and E to recover stresses along element edges) can used instead.
Offsets are often used, but make for trouble, as they can't be used with buckling runs or non-linear (though wing box type analysis is rarely taken non-linear). They also screw up structural-thermal runs. This is one reason not to use bars for stiffeners, as they should really be offset inwards. Un-offset bars for spar flanges is ok.
Usually simple outer mold line (OML) geometry is used. Things like rib feet are usually not modelled at the wing model scale, nor are fasteners. However, if you do have the time to put in some fasteners it can save a lot of effort later on.
For a wing, fin or tailplane box the leading edge is not usually modelled, and nor are the trailing edge control surfaces. Care is taken so that the TE surfaces do not pick up load due to wing bending. One exception to this was the DC10 and MD11 outboard flap; this had four flap tracks, and so it bent with the wing. Loads from things like flap tracks and engine pylons are usually applied as discrete forces at flap track and pylon attachment locations.
Things like access panels (e.g., manholes in the lower wing skin or upper tailplane) are not usually modelled. If they are, it's usual to connect them with fastener elements (CBUSHes in NASTRAN these days).
Often the basic aero SMT loads are applied using rigid elements (RBE3s) at rib positions. However, sometimes the actual air pressures are used, along with extra pressures or line loads for load on non-modelled surfaces (LE and TE).
Other tips: for linear runs start K6ROT (drilling freedom suppression) off at 1.0 or 10.0. Do not use AUTOSPC.
-RP.
However, here are a few tips on what I'm aware of standard industry practice.
Not many people use shear elements. 99% of wing-box-type models are shells (use CQUAD4s in Nastran; CQUAD8s look pretty but NASTRAN's are not as good as the CQUAD4s, which are a highly refined formulation developed over many years).
I've seen a global (whole-wing-box type) model with as few as 6 shell elements down the depth of the spar web. This seems a bit mean to me, but it worked for them.
For composite materials, smeared properties on MAT8s with PSHELLs are often used, rather than have a whole load of PCOMPs.
Often in a wing model things like the spar flanges and wing cover stiffeners will be modelled with bars (use CBEAMs not CBARs if you ever want to go non-linear). However, it's probably more sensible to use more QUADs these days; bars used to be used to save CPU, which basically isn't relevant any more. Stiffeners should be at least two QUAD4 elements deep (some say three). Generally element corner stress recovery should be turned on. These days this seems passably accurate in NASTRAN, but keep an eye on unusual geometry. "Strain gauge" bars (very low area and E to recover stresses along element edges) can used instead.
Offsets are often used, but make for trouble, as they can't be used with buckling runs or non-linear (though wing box type analysis is rarely taken non-linear). They also screw up structural-thermal runs. This is one reason not to use bars for stiffeners, as they should really be offset inwards. Un-offset bars for spar flanges is ok.
Usually simple outer mold line (OML) geometry is used. Things like rib feet are usually not modelled at the wing model scale, nor are fasteners. However, if you do have the time to put in some fasteners it can save a lot of effort later on.
For a wing, fin or tailplane box the leading edge is not usually modelled, and nor are the trailing edge control surfaces. Care is taken so that the TE surfaces do not pick up load due to wing bending. One exception to this was the DC10 and MD11 outboard flap; this had four flap tracks, and so it bent with the wing. Loads from things like flap tracks and engine pylons are usually applied as discrete forces at flap track and pylon attachment locations.
Things like access panels (e.g., manholes in the lower wing skin or upper tailplane) are not usually modelled. If they are, it's usual to connect them with fastener elements (CBUSHes in NASTRAN these days).
Often the basic aero SMT loads are applied using rigid elements (RBE3s) at rib positions. However, sometimes the actual air pressures are used, along with extra pressures or line loads for load on non-modelled surfaces (LE and TE).
Other tips: for linear runs start K6ROT (drilling freedom suppression) off at 1.0 or 10.0. Do not use AUTOSPC.
-RP.
- Thread starter
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LIstress
There are very few public documents on FEA in aerospace, but here are the ones I have come across. By the way, the AGARD report has 4 papers in it. The best ones are by Ian Taig of BAE.
Guidelines to FE Practice - national Agency for Finite Element Methods and Standards, National Engineering Laboratory, W.M. Mair, Scotland, July 1984
Practical Application of FEA to Aircraft Structural Design, AGARD-LS-147, France, 1986
Of course, THE best and most complete document I have ever seen on exactly how to model aircraft frames, skin and stringers, etc. is Boeing's FEA handbook but of course its proprietary. The basic practices of modeling for "coarse" grid loads models is fairly well established in industry so if you have an opportunity to study an existing FEM that would help you significantly. However, each airframe is design differently of course. There are always exceptions to rules. For example, some aircraft do use structural leading edges typically in the vertical tail so this too gets a little complicated.
good luck
There are very few public documents on FEA in aerospace, but here are the ones I have come across. By the way, the AGARD report has 4 papers in it. The best ones are by Ian Taig of BAE.
Guidelines to FE Practice - national Agency for Finite Element Methods and Standards, National Engineering Laboratory, W.M. Mair, Scotland, July 1984
Practical Application of FEA to Aircraft Structural Design, AGARD-LS-147, France, 1986
Of course, THE best and most complete document I have ever seen on exactly how to model aircraft frames, skin and stringers, etc. is Boeing's FEA handbook but of course its proprietary. The basic practices of modeling for "coarse" grid loads models is fairly well established in industry so if you have an opportunity to study an existing FEM that would help you significantly. However, each airframe is design differently of course. There are always exceptions to rules. For example, some aircraft do use structural leading edges typically in the vertical tail so this too gets a little complicated.
good luck
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- #9
crackman
Thanks for the references, and a star for you (hope it got there by now); I'm going to try and track them all down.
As for the Boeing document, are you referring to D6-25400-006, "Finite Element Modeling Guidelines for Aircraft Structural Analysis?" I've seen references to it in other literature, but never the document itself. I do have an interesting observation (and question) about the document, however.
If you search the NASA reports server for D6 25400, you'll obtain hits for some manuals to a Boeing developed '70s-era finite element code called ATLAS. Therein are some modeling guidelines, but not the entire set of document's- worth. Do you know if the current Boeing document evolved from this? If so, is it still substantially the same in its modeling advice (which is less affected by the passage of time, as opposed to hardware specific issues), or has it evolved significantly?
Thanks for the references, and a star for you (hope it got there by now); I'm going to try and track them all down.
As for the Boeing document, are you referring to D6-25400-006, "Finite Element Modeling Guidelines for Aircraft Structural Analysis?" I've seen references to it in other literature, but never the document itself. I do have an interesting observation (and question) about the document, however.
If you search the NASA reports server for D6 25400, you'll obtain hits for some manuals to a Boeing developed '70s-era finite element code called ATLAS. Therein are some modeling guidelines, but not the entire set of document's- worth. Do you know if the current Boeing document evolved from this? If so, is it still substantially the same in its modeling advice (which is less affected by the passage of time, as opposed to hardware specific issues), or has it evolved significantly?
- Thread starter
- #10
Crackman:
Do you know how I can get a readable copy of "Practical Application of FEA to Aircraft Structural Design, AGARD-LS-147, France, 1986?" PDF is best of course, but a readable hardcopy works too. I was able to obtain a hardcopy only through DTIC, and much of it is unreadable due to poor copy quality. Thanks.
Do you know how I can get a readable copy of "Practical Application of FEA to Aircraft Structural Design, AGARD-LS-147, France, 1986?" PDF is best of course, but a readable hardcopy works too. I was able to obtain a hardcopy only through DTIC, and much of it is unreadable due to poor copy quality. Thanks.
You might be able to order this in the Online bookstore
search for "practical application of finite element analysis"
search for "practical application of finite element analysis"
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Andries:
I hope you don't feel that I'm unresponsive, but I've posted a few responses in the last few days that haven't made it to the list; I don't know why. Hopefully this post will get up there, so that I can say thank you since I did get the package.
I hope you don't feel that I'm unresponsive, but I've posted a few responses in the last few days that haven't made it to the list; I don't know why. Hopefully this post will get up there, so that I can say thank you since I did get the package.
LIstress
Sorry about not getting back to you. Yes, the D6-25400 document is the same one I was speaking of except that its been revised since the time it was released by NASA. It has a very in-depth appendix which is probably the most detailed guideline I've seen which presents several appropriate ways of modeling airframe structure (fuselage frames, stringers, wing spars, wing skins, etc.) along with constraints, loading schemes, etc. However, I do not believe that version is available thru NASA. Glad to hear you were able to obtain the Taig papers.
Sorry about not getting back to you. Yes, the D6-25400 document is the same one I was speaking of except that its been revised since the time it was released by NASA. It has a very in-depth appendix which is probably the most detailed guideline I've seen which presents several appropriate ways of modeling airframe structure (fuselage frames, stringers, wing spars, wing skins, etc.) along with constraints, loading schemes, etc. However, I do not believe that version is available thru NASA. Glad to hear you were able to obtain the Taig papers.
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Download the PDF copy of AGRAD LS-147, "Practical Application of Finite Element Analysis to Aircraft Structural Design" with TAITS's two papers at:
Image quality not the best...
Image quality not the best...
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