sheet metal

[msb6]

Anyone know where I can source some extra wide 7075-T6 .063? My doubler measures 97 x 137

 

[rb1957]

I think I've seen 8' wide, but 4' and maybe 6' are more common.


"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

is that a "doubler" or a complete new a/c skin??

 

[msb6]

These are the doublers that wrap around a Cargo Door installation 80 x 60 door

 

[rb1957]

yeah, having done this... split and splice the dblr. Not ideal, but what choice ?

using 7075T6 is a "bold" choice ... presumably the fuselage is under pressure ?

Where are you ? NA ? Europe ?? other ???

Having done this, I imagine you're using several plies of 0.063" thk sheet ? ... I'd recommend not to. I'd recommend biting the bullet (the very large and unpleasant bullet) and use rolled plate, 0.25" thk, machined at the edges. Yep, epic pain in the ass, but multiple plies will cause you no end of pain.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

yeah, isn't 7075-T6 quite susceptible to SCC?

 

[rb1957]

not really ... only very thick plate (6" +) ... been there too !

Sheet is fine, but with a fatigue cost (comapred to good ol' 2024T3).


"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

msb... hope this makes sense...

RE the 7075-T6 0.063-thick sheet for a one-piece doubler around your cargo door: You have not stated whether this will be clad sheet or bare sheet. Also: Is this for a repair or new design [significant production volume, etc]; AND is this to be an internal or external doubler??? Also How much raw sheet material distortion/bow from flat is acceptable, IE: a lot; a little; none???

Size... 0.063x97X137-in is not attainable without premium/special order... IF any company would even attempt to make-it in less than 0.125 thickness. Procurement will take 'at best' months... and will be incredibly expensive per sheet in high volume procurement. OH, and-BTW, the one-or-more sheets are so huge and flimsy, they will require special shipping/handling to avoid wrinkling in transit; then special vacuum grippers for in-shop handling; and highly skilled mechanics to cut-it/deburr and fit-it-in-position... without wrinkling. good thing is, that 0.063 will easily drape over constand-section curvatures... 0.125 would be a lot stiffer.

OH, and by the way that large size will be very difficult to apply finishes onto... such as chromate conversion coating [CCC]... and epoxy primer in midst all of this effort. Few, if any CCC bath tanks will accommodate that size... multiple dips from various directions and special handling tools... to prevent wrinkling... will be essential. OR spray apply CCC over a drip-bath. Gives me chills to think about this...

Now some practical issues... 3/16 shear-head lockbolts or rivets can be installed in shallow countersinks in 0.063 sheet... but anything larger in Dia will require 'CSKs in knife edge'... unless-of-course it is hot-bonded in position without 'chicken fasteners'.

I could go on-an-on about how difficult this is for practical reasons.

Go multi-piece splice-segments.. and ensure over-lap splices are oriented forward overlapping* aft; and upper over-lapping* lower... and always assemble it wet with sealant. AND always be aware of external skin CSK depths... VS skin thickness... too thin... then why bother with the doubler? *lapping to minimize lap-step INTO the relative wind and/or into rain-flow.

I could go on, but I am tired. Good night.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[msb6]

This is what was called out by the mfg who did the Engineering on the mod. I know this is going to be impossible to procure, so I will have to get them involved to split and splice.

 

[rb1957]

let the shite show begin ! always a problem with 3rd party engineering ...

I can hear them ... well, you "can" get the sheet; yes, it'll be very expensive; but you didn't ask us to design something to a budget, so if you want us to change the design, well, that'll cost you more ...

where are you ? (country, region (not city)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

msb...

AND YOUR answer to the following, in my previous post...???
RE the 7075-T6 0.063-thick sheet for a one-piece doubler around your cargo door: You have not stated whether this will be clad sheet or bare sheet. Also: Is this for a repair or new design [significant production volume, etc]; AND is this to be an internal or external doubler??? Also How much raw sheet material distortion/bow from flat is acceptable, IE: a lot; a little; none???

"Nothing is impossible for the man who doesn't have to do it himself." --A.H. Weiler
"Nothing is impossible for one who does NOT have to do the job.” --variation

"Nothing is impossible. Some things are just less likely than others." --Jonathan Winters, comedian, author and artist

AND for fun... and while I'm on a roll...
"People say nothing is impossible, but I do nothing every day!” –Winnie-the-Pooh

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Sparweb]

I'm going to guess that the OP, msb6, is uncomfortable sharing too much, even on an anonymous forum, and it may be hard to give the answers to the questions you ask, Wil. I've been in the same situation as MSB myself, supporting a cargo door mod as "liaison" and being completely baffled at the unrealistic specifications, but having no input in the design.
Speaking to my own observations, in my experience, cargo doors and the associated doublers are usually installed by after-market modifiers or MRO facilities, so I assume that's what MSB is talking about.

If your cargo door designer is a group internal to the organization where you work, try to make it their problem, not yours to source the sheet. This may require escalating the issue when you tell your manufacturing supervisors what the sheet will cost and take so long to arrive. Then they have to defend the issue with the engineering group. None of this is efficient but it will be better than trying to do all the shouting yourself.

If your cargo door designer is a 3rd party and you are just the installers, then this will be even harder. Talk to contracts/legal to determine how re-work for unobtainable materials should be handled. Again, above my pay grade and maye yours too.

Lastly, since both of those courses will probably fail, your last resort is to just get 2 sheets of standard size 12' by 4' and splice them on either side of the door. Hire some other company that actually knows what they are doing to design the splice for you or perhaps consider making it competitive against your 3rd party cargo door designer. The splice could be the easiest part of the design in a cargo door.

 

[WKTaylor]

SW...

My request for a few tech details regarding the sheet metal... not related to 'type acft', who, where, etc... seems pretty benign.

All...

The selection of sheet metal has lots of options... but there are surprises... 'old stand-by alloys and/or tempers' have fallen from favor for various valid reasons.

NOTE1. Prohibited/restricted metallic materials... poor fatigue durability and/or EXCO and SCC resistance... are approximately defined in...
MIL-STD-1587 DESIGN CRITERIA STANDARD - MATERIAL AND PROCESS REQUIREMENTS FOR AEROSPACE WEAPONS SYSTEMS

NOTE2. My company has an expanded prohibited/restricted metallic materials list that also includes[excludes] older/less-available/odd alloys/tempers.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[LiftDivergence]

I realize from reading the preceding posts we might be intentionally light on specific details.

These are the doublers that wrap around a Cargo Door installation 80 x 60 door

But I find it a bit curious... there are really only a few major 3rd parties that do cargo conversion and cargo door STCs... AEI, Precision, IAI come to mind.

While these parties are far from perfect, I find it hard to believe that a big company with a lot of cargo mod experience would have something like this in their production design.

So is it that a "new" cargo door is being installed and these are part of the skin reinforcement. OR is this some other mod that touches/affects and existing cargo door? And if so, is that cargo door OEM or STC?

I've seen and personally designed some pretty big reinforcing repairs in my time. But 137 inches is pretty huge. Assuming typical frame spacing for an airliner, thats what? Almost 7 frame bays?

Also, to elaborate on what I think some folks were alluding to above...I quote from a very typical airliner structural repair manual, Section 51-30-01 concerning material substitution (the substitution factor for CLAD 2024-T3 to CLAD 7075-T6 is 1.0), but:

"THESE MATERIALS CANNOT BE USED AS REPLACEMENTS FOR THE INITIAL MATERIAL IN AREAS THAT ARE PRESSURIZED. THEY ALSO CANNOT BE USED IN THE WING INTERSPAR STRUCTURE OR THE WING CENTER SECTION STRUCTURE."

So either this is a cargo door in a non-pressurized section, or the original skin is 7075-T6 (seems unlikely), or the designers aren't unaware of/don't care about/have designed around this standard limitation?

I'm stretching my memory a bit on this one, but I seem to recall a 737 service bulletin that dealt with scribe/tooling marks around the WTB fairing. And depending on the length and severity, this could lead to some pretty big doublers. And I specifically remember them being broken into smaller sections that were spliced together. But I could be wrong, it might not have been in an SB. I know I've seen it though.

Either way, I think pushback on the huge reinforcement could be warranted but you'll need to stack the deck with a lot of the problems we're bringing up to make a case against it.

Edit: I didn't find the repair I was thinking of, but I also remembered the 737 lap splice mod covered in service bulletin 737-53A1177. This installs a repair doubler and tripler in place of some original lap joints over the whole length of the skin panel. So, truly huge. I looked at the drawing and it does give an optional splice of the doubler and tripler using splice plates, but it is not required. So there is some OEM precedence for installing doublers this large.

The difference is, that was a kit drawing intended to be used for a series affected by a SB. And it's Boeing, so I'm assuming kitting and procurement is not as challenging. Your situation is probably different.

Keep em' Flying
//Fight Corrosion!