Fuselage Cutouts and Skin Doublers

[Allanon81]

I'm a stress engineer working a particularly challenging/interesting problem. We're working a modification to a CL-605 fuselage where a customer wants us to install a service panel, but wants to avoid the use of external doublers. The cutout in the fuselage would span one frame bay and cut one stringer. Our structures design team wishes to install machined c-channel type structure around the edge of the cutout to reenforce it. The idea is the shear that normally would have been carried through that section of skin would be beamed up to a back panel using the machined structure.

I'm looking through Niu and find plenty of examples of cutouts with external/internal doublers installed, but nothing about being able to reinforce the hole using and internal structure.

Is this even possible/practical? I'm thinking that the internal structure will have to be pretty heavy to beam those loads up and not buckle.

Let me know if I can provide further details.

Thanks!

 

[rb1957]

anything's possible given enough Aluminium and enough electrons (to analyze it).

you Can frame the cut-out ... longerons would extend a couple bays either side of the cut-out ... i'd also reinforce the next stringer. i don't think you can reinforce the strgrs effectively (if you back up the existing strgr, how thick is the skin ?? how big a rivet can you install ??). so i think you're into replacing the strgrs with heavier sections.

surprised (as no doubt you are) that the customer is adamant against external dblr. what about a sneeky internal dblr ?? which you're already removing the strgrs, why not ??

 

[Allanon81]

haha... anything's possible, just mathematically improbable... right?

Unfortunately the internal doubler won't work without taking out a large section of frames/stringers. Plus the skin is chem-milled by the OEM to certain thicknesses based on the FEA.

When you say to reinforce the next stringer... are you talking about the stringer above and below the cutout?

I think what they are trying to do is the following:

When you cut a hole in a fuselage and cut a stringer with it, you get two means of shear flow redistrubution to the skin around the cutout. The first is the shear flow of the missing panels gets redistributed to the outer panels. The second is the axial load from the cut stringer gets redistributed to the upper and lower sills, and this redistribution causes additional shear flow in the surrounding panels. I believe they are trying to eliminate the first mean by beaming that shear flow up 3.5 inches to another panel that would act as a replacement to the skin. Framing it might be a lot more expensive than to just add an external doubler.

Thanks for the input!

 

[rb1957]

i meant the strgrs at teh edge of the cut-out (what you were talking about), and the next ones away ... the shear in the skin isn't going to stop at the longeron, it'll bleed into the next bay as well.

also you're not going to reinforce the stringers only at the hole edge ... you have to give them length to pick up the load you're asking them to carry. hence the longerons would be three frame bays long. the 2nd strgr out you could reinforce the bay of the cut-out and blend out over the outer bays. be careful about tapering the cut stringer (ie, don't just saw the damn thing at the edge of the hole).

you've also got to think of pressure loads (reinforce the frames).

part of the key to making a frame work is the corners ... they need to be well gusseted, to be stiff in bending.

this is going to be Way more difficult and expensive than a dblr.

remember DT (structural inspections).

what sort of stuff are you servicing ?

where on the fuse are you ?? hopefully not just behind the rear spar ? hopefully somewhere on the fwd fuse. ?? maybe in an unpressurised area ???

 

[Allanon81]

It's located in the forward fuselage, but in a pressurized area... so yes, pressure loads will have to be considered as well. I've basically told them the same thing... that it will be cheaper to put an external dblr on the stupid thing and be done with it... it's definitely a lot easier to size than to try to figure out how to get all of this structure to work. The corners of the box structure have been my greatest concern since they told me their idea... I just don't see how we can get this to work... fortunatly, I do have all of the loads data from the OEM (both stringer end loading and skin shear flow), so I can at least prove on paper how beafy this thing is going to get.

 

[40818]

Reluctance to have skin doublers normally means a VIP A/C. I would just roll with it, come up with the final estimate based on the simple doubler and also the expensive internally reinforced structure and let them choose. RB's advise is giving you clues as to whats required, just work out what you need to carry the loads and do a couple of different options to allow for the best solution to appear.

 

[Allanon81]

40818, you are correct... this is a VIP business jet. Thanks for the advise... I'm looking into both solutions and providing them both.

Thanks for the input!

 

[der8110]

With the paint application techniques on a Challenger, no one would ever know there was an external doubler. Bondo, high build primer, and a few coats of glossy paint will surely hide it. So much so, they'll be bitchin' about the appearance of the hinge and latches.

Skins are milled to 0.047" so the doubler ought to be 0.050" or 0.063", easily within the range of bondo.

Internal doublers would cause quite a bit of disassembly, shoring the aircraft and peeling back pretty much the entire skin panel. Stringers would get taper shims and frames get new clips to match hole centers. Also with all that acreage, get ready for supporting misdrilled holes. Good luck with it.

 

[40818]

The thing that always struck me with VIP mods was simply that they didn't care about details, if they want a small swimming pool then they would have one....laws of physics dont come into it.If they spend a $100Mil they dont expect to see anything except the glossy pics they have been shown. Industry is somewhat different (the norm) and want efficiency and weight saving. The person who buys aircraft for own use doesn't really care.

 

[Allanon81]

der8110... that was my point... however, where the mod is located, it might interfere a little bit with the forward part of the fairing. That said, it would still be a lot cheaper to add the doubler, trim the fairing a little bit a reinstall.

40818... I've ran into such problems before. We put a shower on a Gulfstream and the customer complained that there was only enough water for one shower... try explaining to him that there's only so much water you can carry on an aircraft and can't exactly pump any additional on while in flight... I also love how the customer thinks that what a sales guy (who has no engineering experience) says is the gospel... even though sometimes they ask for things that just can't practically happen...

 

[rb1957]

i think we all agree this is a lousy way to go. whilst you can't argue with city hall, you can make the solution pretty unpleasant. i dont't think you can reinforce the existing stringers, i think you need new ones ... at least you have large rivets (BB6s) to work with. you need to reinforce (at least, if not replace) the frame flanges. and this just on the edge of the cut-out but also a fair ways away.

i think pressure loads are going to be your biggest concern, shear in the fwd fuse is pretty small.

how are you going to estimate the loads (assuming you don't have access to the OEM reports) ?

 

[Allanon81]

That's the nice thing about this... I do have the OEM reports with the full FEA. It gives me the stringer end loads and panel shear flow for each piece of the fuse for multiple load conditions... you have no idea what I had to go through to get it...

I'm working on sizing the structure now... but management I believe has already decided that this is going to be way to much work and cost to even mess with.

 

[SAITAETGrad]

Allanon81...some of us have been down that road before many times with Bombardier/DeHavilland/Canadair.

For further reading on how poor geometry can get you into serious DTA trouble, I would suggest "Damage Tolerance in Pressurized Fuselages: New Materails and Ftigue Resistant Aircraft Design", the 11th Plantema Memorial Lecture presented by Tom Swift from the 14th ICAF Symposium.

What you have been asked to do is a job for a seasoned hand. I would suggest contracting someone like RB1957 who has amazing experience on such installations on regional/bizjet aircraft and is delegated for 571.

 

[rb1957]

actually i do ... i've had dealing with the "friends in montreal". you must some really good contacts.

so you've convinved someone of the silliness of this ... or better yet convinced someone that they decided it's silly ... always had trouble with that ... tended to get thrown out of the room when i laughed at them

 

[rb1957]

why thank you SA !