Nastran - Delumping of Internal Loads 1

[VN1981]

Hi,
It seems like we (employer) might have missed an important step in extracting internal loads from Aircraft FEM models...delumping! I found Ian Tiag's fantastic Aircraft FEM notes and I came across the concept of delumping of internal loads when 2D-1D elements are used for web & chord/stiffeners representation. I've spent the last 2 weeks going over the same material & I was able to dig out more information about the above procedure, but I still have questions. Some of them may sound obvious & stupid...so please bear with me

1. Is delumping procedure in use even today in major OEM practices?

2. Is delumping performed for any combination of 2D-1D (Cshear/Plate-CROD/CBEAM) or is it particularly reserved for CROD element I am assuming the answer is YES, but just looking for clarification from experienced users.

3. If I have lets say 30 elements (30 each of 2D representing SKIN & 30 each of 1D representing STR) along the span of wing between 2 rib stations and if I have to perform delumping manually, it becomes tedious. Perhaps the big OEMs have in-house tools or Excel macros to extract/communicate information from NASTRAN and spit out delumped loads, but any tips on how to perform delumping operation with minimal effort?

4. Lastly, any open source material where I can learn more on the above procedure?

Thanks in advance,
- VN

 

[santoshaero]

Hi RB1957, thanks for responding and answering my questions. I am afraid i have understood only part of your reply and have further questions on some of your explanation.

I will start with what is have understood, based on your reply:

1) Because of the greater depth in fuselage compared to wing, the bending stress is less for fuselage.
2) The stress in stringer in sum of this overall bending stress and local bending stress due to stringer offset from fuselage or wing datum.
3) Since the fuselage thickness is much less than wing thickness the stringer offset is less in fuselage compared to wing. This reduced offset means less bending moment and stress.

Kindly clarify if this is in correct order and this is what you meant.

Now the part which i have doubt on:
4) The offset would be measured from skin centreline (for fuselage or wing) to centroid of stiffener cum skin (I mean including 30t of skin). Since fuselage skin thickness is less compared to wing thickness, the centroid would actually be higher in fuselage stringer compared to wing stringer. This would in turn mean more moment offset in fuselage compared to wing, and thus higher stress across depth of stringer.

kindly clarify if you agree with this. If the above is true, then the modelling should be same for stringer in fuselage and skin.

5) In my work of stress analysis, i have sued GFEM, and noticed in wing structures, like Rib, Spars, stringers, vertical stiffeners, Flanges are modelled as CBR and Webs, Plates as CQUAD4. Is it same in fuselage structure as well?

 

[aerostress82]

Just wanted to add this here:
While delumping, never ever forget the diagonal tension effects and the loss of the shear and bending load on the panels (if the structure calls for it). Looks like some main aerospace companies had serious problems due to ignoring this effect.

Delumping is really related to how you model your FEA. Either you use a CROD, CBAR, CBEAM & CSHEAR, CQUAD depeding on your type of structure (ie. fuselage, wing rib, wing skin), it literally comes down to your whole set of hypothesis regarding your assumptions related to your load paths or transient response of your structure for some loadcases or manuvers.

I feel like this question is at a very basic level and the answers would be related to the knowledge level of the principal stress engineer overseeing the project. Hope this explains your parameters and helps with the approach to this problem better..

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[aerostress82]

Are you still looking for a reply on this? Just realized I had actually replied to another post of yours. I may do some more digging in the next weeks if this hand calculation is still a problem. Just let me know please.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer