Fuselage - Flush Repair 2

[MrHiLok]

I am designing a flush repair for fuselage section. The skin sustained damage and was removed by cutting out the damaged fuselage skin . An external patch repair was accomplished. Now there is a requirement to do a flush repair. To do a flush repair will require running the doubler under stringer. There are shear ties which will be affected by the doubler.
Can I run the doubler under the shear ties? The repaired configuration will be, fuselage skin, two fillers, doubler and then shear ties attaching the frame.
Thank you very much for your comments.

 

[rb1957]

you can do pretty much anything.

if you have to do an internal dblr, then i think you need to do two stringer splices as well.

cut away the internal structure ... the stringers, the shear ties (these are frame cleats, right? "shear ties" could mean shear clips to the floating frame, a la B707).

then install the dblr, splice the stringers (SRM repair?), adjust the shear ties (for the thickness of the dblr).

I don't recommend just "wedging" the dblr between the skin and stringer (ie de-rivet stringers, slide doubler inbetween, re-rivet). I know some people do this, I know it's been done on approved STCs, just not on STCs I work on.

Quando Omni Flunkus Moritati

 

[MrHiLok]

Thank you rb1957 for your comment.
Forgot to mention in my previous post about splicing the stringer. The doubler will run under the spliced stringer.
I am more concern about the shear ties. The original design is the shear ties are connected to the skin and the frame. In the flush repair, the shear tie will be over the doubler. This increase in thickness, how that will affect the transfer of load from skin to the frame? And how about fastener bending? The increase in the stack up at the shear tie due to doubler is 0.226 inch.

 

[rb1957]

"The increase in the stack up at the shear tie due to doubler is 0.226 inch." ... the doubler's a 1/4" thick !?

so you've got a floating frame, with shear ties (or clips) attaching the frame outer chord to the fuselage skin, yes?

your concern is the effect on the fasteners, increased stack-up? you could look at the fasteners as a beam, with increased bending due to the dblr; you could make a free body of a tie, showing how the off-set between applied shear and shear reaction are balanced (bolt bending, couple between bolts ??) ...

Quando Omni Flunkus Moritati

 

[stressguyMike]

Best to consult the SRM (structural repair manual) for the aircraft that you are working on. If you abide by an SRM repair, you are covered for flight and ultimate loads, damage tolerance, and fatigue.

 

[MrHiLok]

Thank you every one for their valuable comments.
The repair which I am designing is based on the repair principles of the SRM. The damage is at a frame and the repair doubler will extend over a lap joint. There are shear ties at the location, chem mill pockets.
There are not many flush repairs in the OEM SRM, which I can use for guidance. I will be sending the repair to OEM for final review and approval.
Before sending the repair design to OEM, I want to get repair design to a stage which will require minimum work from the OEM.

 

[rb1957]

"I want to get repair design to a stage which will require minimum work from the OEM." ... that can be quite difficult to do, unless you design something very similar to how the OEM would do it. To get an OEM to endorse a repair means they take (at least in their own minds) responsibility for it. Your difficulty in designing the repair is that you don't Know the loads int he area, you can make shrewd (conservative) estimates/assumptions. And the repair is now more complex than first thought (at least in my mind) ... now there's frame damage as well.

my 2c, if you intend to pass the repair by the OEM for their "ok", then i think you're better off getting them to design the repair in the first place. The option is to get your own RDA/RDC ... whatever a repair certificate is called where you are.

Quando Omni Flunkus Moritati

 

[Sparweb]

Chem-milled skins... spanning a skin lap joint and multiple frames.... I can see the complexity of the repair (both the installation and its substantiation) becoming very difficult.
Are you in a position to question the "... requirement to do a flush repair..." as you put it in your original question?
The aircraft owner may not realize the cost and extent of the work required to make the repair flush. Which requirements come into play... aerodynamic smoothness, static pressure ports, passengers noticing the big dirty patch...? Sometimes the best engineering approach can be to challenge the "need", rather than to solve the so-called "problem". If the requirement comes instead from a ridiculously frequent cycle of inspections, then maybe it's a repair re-design that is on order, but not necessarily a flush one. These are questions - I'm am just challenging assumptions at this point.
I don't like external skin patches any more than you do (many reasons) but sometimes, if the safety and integrity of the repair is not actually going to be improved, why all that work?


STF