Sheet Metal Forming for Aircraft skins

[crystallographic]

Originally Posted by AllyBill

Not welded 7075 so can't comment there.
2024 can crack if you weld it in one of its higher tempers depending on what you are trying to do, don't for instance try to insert a circular patch, but when properly annealed it welds like any other grade and doesn't crack at all. When I say properly annealed I mean you take it to the heat treatment plant and have them do a full anneal and furnace cool, make your weld with strips of native material then have the heat treatment plant put the desired temper back. Never had a problem with it.

As a slight aside, the skins of the Handley-Page Victor wing were extensively spot-welded with no issues and they were 2024.

Will

Spot welding of 2024 T3 is very common on US airplanes. Going back to pre-WW2, many cowlings and "semi-structural" elements have been made using spot welding. One of the largest spot-welded airplane structural assemblies I have seen is the rear half fuselage of the Convair B36 "Peacemaker" - the behemoth that replaced the excellent Northrop YB49.
https://en.wikipedia.org/wiki/Convair_B-36_Peacemaker

I have seen a ski-plane ski marked "24S" (pre-1953 designation for 2024 T3) and it was gas welded. I have not seen other examples of tig or gas welding of 2024 alloy in aircraft production. I do know that the Aluminum Association rates 2024 weldability as a "D" - compared to alloy 2219, a ballistic material used in the early A10 Warthogs as a welded tub to protect the pilot - later replaced by welded titanium.

This article I grabbed on a quick search describes fairly recent aluminum welding recommendations:
https://books.google.com/books?id=DZ...uminum&f=false
It does not mention 2024 as being weldable.

Rockmount Research, on the other hand, sells a filler metal designed for 2024 and has for many years. It's specific applications in aerospace are above my pay-grade. I believe the filler is designated as "Neptune TIG."

I heard some years back that Trek Bicycles (and maybe Cannondale?) had their weld engineers work out welding of 2024 for their bike frames. So far, I have not spoken to anyone there about that.

I guess the bottom line is that some can weld it, but at the levels of the Aluminum Association and their tech reps, welding 2024 is rated as a "D" - or as Paul Dickerson told me once, "Screw it!" ... or rivet or bolt or bond it.

[BTromblay]

Hi Tim,
I work on airplanes as well, welcome to the forum.

Bill

[tfindlow]

Wow - lot's of feedback for a simple introduction...

As to what I'm up to - I'm working on a aluminum semi-monocoque tailcone that bolts to a chromoly-steel tube framework for the forward fuselage. Similar to the way that the Mooney M20 or North American T-6 are built. The tailcone is 2024-T3 alclad sheet, over aluminum frames, riveted together. The steel tube is gas welded 4130.

Like most tailcone designs - it's almost all flat-wrap; meaning no compound curves are necessary. However at the front of the cone, the straight line of the tailcone bottom needs to curve up to meet the straight line of the fuselage floor aft of the wing. And at the same time the lower corner radius on the tailcone needs to come to a "close to square" corner where the frame attaches to the lower longerons. This means there's one part of the tailcone which isn't a developable surface.

BellyPanel1.jpg

I'm currently trying to figure out how to go about constructing this - considering that my metal shaping skills are rudimentary at best.

I'm designing in Rhino-4; and have used the advanced flattening tool to "Squish" the proposed surface down onto into 2-D. The tool indicates that most of the work is in stretching and some small areas requiring shrinking; about 3% elongation each way. You can adjust the analysis to have all stretch and no shrink - but the amount of elongation goes up. I don't know whether this is within the range of forming for 2024-T3 for .032 or .040 sheet thickness, or whether it will have to be 2024-O and heat treated afterward.

BellyPanelFlat.jpg

In any case I'm trying to figure out an appropriate technique to apply to the problem. A lower skill threshold would be preferable for a novice like me - a 7 year apprenticeship is probably not on the cards at this time of my life. So I'd love any pointers as to where best to to aim my research...

[AllyBill]

Daresay there's a good reason why not, but at a glance I wonder why you don't design that bump out of there and make the bottom flat like the top.

Will

[tfindlow]

Aerodynamics - square corners produce vortices and drag. Not that there aren't many aircraft out there that have rectangular, square cornered fuselages (Chris Heinz's CH-701 for example). The corner curve helps stabilize the fuselage shell against buckling as well.

[AllyBill]

No, l was meaning the small bulge on the underside. Will that not refuce down to a flat skin?

Will

[BTromblay]

Hi,
I feel the part is doable with 2024-T3 material. I recommend mocking it up with paper first to confirm your CAD drawings. I recommend .04" instead of .032" as you will need to blend out tool marks and you could thin the material to much in 032.

I would recommend the use of a English wheel or planish hammer, do you have this kind of tooling?

What is the make one model of your project.?

CHEERS,

BILL

[tfindlow]

It's not suppose to bulge -- this is my first attempt at getting a panel that lines up with the tangents at the adjacent edges; it would be nice to get a lovely convex shape that meets all the constraints - I just haven't figured out how to sweet-talk Rhino into giving me that. I've been trying to get the MatchSrf command to do the job - but as you can see the result is a little off true.