A310 Fuselage Cross Section to find Inertia and Centroid Values

[sb1786]

Hello All,

I am trying to find the bending stress at an antenna location. The fromula is stress = Moment*centroid/Inertia.

I am having a lot of trouble finding a model or data to model a cross section of the fuselage at location staion 2163. This would help me find the inertia and centroid values I am after.

Anyone knows how I can go about with this. Any tips, help, previous experience?

Thanks

 

[eng3000]

Do you have an actual aircraft to work on or is this for something like an STC? If you have an actual aicraft, then ask the owner/operator to request the relevant drawing from Airbus. The same goes if you have a potential customer.

The aircraft IPC may be able to give you some useful information too.

 

[rb1957]

stop now ... drop your pen, move away from the screen.

stressing a fuselage (for an antenna installation) has little to do with ...
"The fromula is stress = Moment*centroid/Inertia.

I am having a lot of trouble finding a model or data to model a cross section of the fuselage at location staion 2163. This would help me find the inertia and centroid values I am after."

you need to worry about hoop stress from pressure.

you post demonstrates that you need professional (tho' not psychological) help. do Not read AC43-13. start with Bruhn and Niu.

sigh

 

[sb1786]

eng300
I did try to go to Airbus for this information but no luck... They do not give this information out... The only information is a drawing with the length of skin sheet. No profile drawing...

rd1957
Read "guidelines for developing Damage tolerance Based Inspection Program" by SAC.

This guidline shows that hoop stress (already calculated) longitudinal and bending stress must be considered amongst other loading conditions...

I was not going to layout all the loads needed in the post... All I am missing is the bending stress. stress = Moment*centroid/Inertia is just a small part of the analysis.


In other words, the question still remains how can I calsulate the moment of inertia and centroid of the fuselage at location station 2163.

Thanks

Do not worry professional help is not needed

 

[sb1786]

BTW rd1957 thanks for the reference to Bruhn and Niu it is quite usefull...

 

[rb1957]

oh well ... good luck ... i'm aware of that IMHO needlessly complicated method for determining fuselage stress spectra based on stresses due to bending.

a simplistic way would be to use the "gross" definition of the fuselage shell (radius (measure with a tape measure if you have to), shell thickness) then add lumps for the stringers. i mean, there are so many "guesses" in the method a few more shouldn't hurt !

 

[sb1786]

rb1957

I did think of that but it would take quite a long tape... lol

I guess the best thing is to find a document that has these general geometric values from Airbus... I know it exists but they won't give it. So frustrating.

 

[rb1957]

i still think "accurate" dim'ns are "gilding the lilly" ... use your best guess, diameter to the nearest foot.

 

[der8110]

you are on the right track.

looking at the outer shell of skin and stringers is quite conservative, floor structure does add quite a bit of heft. Usually this is approach not done because of time constraints; but if you have the time...

Other end of the spectrum is a full FEM of the fuselage; comparisons are why I say the outer shell (even if just the skin) is conservative. There's even an FAA Tech Center report that even describes the technique you're using, with examples of a few airliners, dammed if I could find it now.

Go to the SRM, that should be able to identify stringer profiles all the way around. You are probably in a constant-section area, that is, applicable a couple bays forward and aft. CAD can get you the MOI and centroid. Bending moment is going to be harder; the aforementioned FAA report had a technique for that too. As far as the nearest foot - even if you don't have an SRM, you have a tape measure which should get you within a quarter. Stringers can be estimated quite well with common measuring tools too. Just look at what pr/t will do for fatigue life or crack growth if you're off a foot.

The alternatives to finding bending stress are, well, proportional to the effort. Like I implied, if you have a patron supporting you, keep going.

 

[sb1786]

der8110

Thanks for the reply. I was trying to find the information in SRM but nothing had the measures I am looking for...

Also, all the profile of the A310 fuselage I could find show that it is a perfect circle of radius 111 in... which doesn't seem possible. A perfect circle?

As for the stringers location this is also a challenge... I can only estimate this value and cannot find a document that holds that information.

For the stringers cross section I have the information.

Unfortunatly, I do not have access to aircraft to take my measures using a tape... What can I do to get information into CAD?

 

[rb1957]

why not a perfect circular fuselage? it's most structurally efficient, and the europeans are known for their pursuit of efficiency (well, some of the mainlanders, i suspose).

but for the method you have why not ? and use the stringer areas near your installation, there should be some sort of GA of the fuse (for stringer pitch), else use what you near your installation, and your best guesses, and a factor afterwards. at the end of the day, it shouldn't matter much, the method is So hokey.

personally, i prefer to work with what i know and make the best guesses i can ... i don't calculate the effects of the gust/manoeuvre spectrum (there are So many assumptions hidden in the method you're using), i prefer to say, i know the hoop stress, let's assume that a factor of 1.2 accounts for these effects; so i use something like 1.2*hoop stress as my equivalent 1/flt stress. if i think i'm in a more sensitive area (say behind the rear spar on the upper fuselage shell) i'd probably use 1.5*.

 

[sb1786]

rb1957 and der8110,

I am fairly new to all this and you guys seem to have a lot of experience from your posts... Can you recommend any readings for steps to a good stress report and steps to a good DTA. Like what you should consider what you shouldn't?

Thanks

 

[rb1957]

i'd start with the guys who are going to approve your work. what are they expecting to see ? how familiar are you with the plane and with these guys ? mind you, they're probably as over-worked as they guys over here, so they might not be able to give you much time. do not go on a DT course untill you've done some work, and understand some of the issues. i'll assume that you're not working on your own, who's around that can help ? (if you're young, don't be afraid to show ignorance, nor a willingness to learn)

it's Very hard to design an antenna installation over the 'net.

avoid an external doubler if you can (how thick is the skin?).

the basic DT problem is a crack initiated by load transfer into either the doubler or the supporting channel. no external doubler means that the external (visual?) inspection is viable; an external doubler means (almost certainly) a special inspection program (external NDI).

to calculate the load transfer, use Niu (something like pg 238). the difficulty is the cyclic loading. this is dominated by hoop stress, with gust/maneouvre cycles on top. the difference is not particularly large, and can be accounted for by a factor on hoop stress (if your cert. guys agree !).

but there are a host of details ... how close are the rivets ? is secondary bending an issue ??

 

[verymadmac]

We usually do this type of problem by a comparative analysis to a patch repair or antenna installation in the SRM. IF they are quite similar you can getaway without any DTA. The fuselage splices are another good place for comparison.

 

[Dan320]

From the A320 SRM the cross section is drawn like this:

upper circle radius 1975 mm

lower circle radius 1926 mm

lower circle center is located 240 mm below upper circle center.
This generates a cusp like the MD-80, only less pronounced.

I think, but I am not sure, the cross sections of A310 and A320 are identical.

Hope this is of some use.

Dan

 

[sb1786]

Hey Dan,

From my understanding the A320 is a narrow body and A310 a wide body familly aircraft. This makes it impossible for the cross sections to be identical. Also, the radius of an A310 is 111in.

Thanks for the info on a320 anyway!

 

[737eng]

I remember coming across something by Broek that detailed how to accomplish what you are doing. Good read if you can find it.

I would recommend picking up Broek's "Practical Use of Fracture Mechanics" and "Fatigue of Structures and Materials" by scijve (sp?)

 

[taglia]

Last year there was a code named RAPID, and RAPID 2besides other things, to do some works with antena holes in fuselages.
I found it in FAA web page, don´t know if it´s still there.

 

[Dan320]

Try to reverse-engineer an SRM-repair in the area?
The SRM-repairs are already analyzed and come with an inspection schedule. (Talking about A320 here)
On the other hand, most times the crown area is not among the SRM-repairs.
I know this is simplistic, but...

 

[sb1786]

Taglia,

I have been loooking for this RAPID software but cannot get my hands on it... I only found a copy of the HELP or how to use the software pdf... Do you have a copy and can direct me to it?

DAN,

Taking a similar repair from the SRM was my first instinct and step. However it is not enough for authorities and I must come up with a DTA.

Thanks