Fuselage Cutouts and Skin Doublers
Fuselage Cutouts and Skin Doublers
(OP)
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!
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!





RE: Fuselage Cutouts and Skin Doublers
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 ??
RE: Fuselage Cutouts and Skin Doublers
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!
RE: Fuselage Cutouts and Skin Doublers
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 ???
RE: Fuselage Cutouts and Skin Doublers
RE: Fuselage Cutouts and Skin Doublers
RE: Fuselage Cutouts and Skin Doublers
Thanks for the input!
RE: Fuselage Cutouts and Skin Doublers
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.
RE: Fuselage Cutouts and Skin Doublers
RE: Fuselage Cutouts and Skin Doublers
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...
RE: Fuselage Cutouts and Skin Doublers
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) ?
RE: Fuselage Cutouts and Skin Doublers
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.
RE: Fuselage Cutouts and Skin Doublers
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.
RE: Fuselage Cutouts and Skin Doublers
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
RE: Fuselage Cutouts and Skin Doublers