Some welding questions

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I'm researching and educating myself in preparation for a build. I've a couple questions regarding welding the fuselage (most likely a LE XL).
  • It seems most assemble and tack the entire fuselage then finish weld. This helps keep everything straight and in place? Would it make sense to assemble and weld as you go, so there are fewer parts in the way?
  • When welding a cluster, would one weld the main parts (horizontal and vertical) before installing and welding diaganol pieces? Or, would you tack everything and weld the whole cluster at once?
  • A search led me to a photograph of holes drilled to allow the tubing to be filled with oil, then drained, after welding, for rust-proofing. Do most do this? How many fill / drain points?
 

PTAirco

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Tack welding the whole thing and then finishing the clusters is the way I would go. You could completely weld each joint for the side panels and then add the cross pieces , but it is not necessary and I find it detrimental; it adds too much mass to the cluster which makes it harder to get good penetration for the final pieces.

The idea of drilling tubes at each joint and flushing it with oil is pointless in my opinion; once a joint is welded it is hermetically sealed with zero moisture left inside. Connecting the tubes with holes merely provides a way for moisture and rust to spread once a crack appears somewhere.
 

revkev6

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I'm researching and educating myself in preparation for a build. I've a couple questions regarding welding the fuselage (most likely a LE XL).
  • It seems most assemble and tack the entire fuselage then finish weld. This helps keep everything straight and in place? Would it make sense to assemble and weld as you go, so there are fewer parts in the way?
  • When welding a cluster, would one weld the main parts (horizontal and vertical) before installing and welding diaganol pieces? Or, would you tack everything and weld the whole cluster at once?
  • A search led me to a photograph of holes drilled to allow the tubing to be filled with oil, then drained, after welding, for rust-proofing. Do most do this? How many fill / drain points?
I'm not 100% sure on the tack all first question but I do know that in my experience welding tubing you must have holes drilled to let expanding gasses release. if you do not have a place for the gases to release they will bubble out the last of your weld on each tube
 

PTAirco

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I'm not 100% sure on the tack all first question but I do know that in my experience welding tubing you must have holes drilled to let expanding gasses release. if you do not have a place for the gases to release they will bubble out the last of your weld on each tube
I keep hearing this and have never seen such a thing happen. And I have built, repaired and worked on many steel tube fuselages. I used to be certified to weld on aircraft in the UK.

Sure, the air inside the tube gets heated up, but until the very last fraction of inch of weld, it has somewhere to go. It gets hot and expands from the welding heat, escapes around the joint and the weld simply closes up without any problem. Once the weld cools down the pressure inside the tube drops below atmospheric, so even if you heated up that tube again by starting a new weld, the pressure rise does not cause a problem.

You do not need to drill holes anywhere.
 

Head in the clouds

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I keep hearing this and have never seen such a thing happen. And I have built, repaired and worked on many steel tube fuselages. I used to be certified to weld on aircraft in the UK.

Sure, the air inside the tube gets heated up, but until the very last fraction of inch of weld, it has somewhere to go. It gets hot and expands from the welding heat, escapes around the joint and the weld simply closes up without any problem. Once the weld cools down the pressure inside the tube drops below atmospheric, so even if you heated up that tube again by starting a new weld, the pressure rise does not cause a problem.

You do not need to drill holes anywhere.
I'm with you on that one PTA. In fact I doubt there'd be any air (certainly not oxygen anyway) left in the tube, it'd be all burnt up and just about every other gas inside would be evacuated.

In times gone by they certainly did drill holes and for that exact reason, but I think the reason is flawed. I've welded many 4130 fuselages and never had a 'blowout'.

In my earlier days I did try to completely weld out areas of a fuselage before adding other members and it was a disaster, the joints pulled and the shape was lost. I found that the best way was to make a jig from a flat sheet of MDF and use screws to hold all the parts of one side of the fuselage in place, make the screw heads clamp the tubes to the board. Cut out holes about 6" diameter (or shaped rather than round if necessary where there are shorter parts), so that you can access both sides of the clusters. Then lightly tack all parts together. Turn the jig over and tack the other side more strongly. Turn it back and weld out all you can get at, turn again and do the same. Remove from jig and complete the welds.

Repeat for the second fuselage side.

Stand the two sides vertical in a simple 3D jig, check diagonals to be sure it's square and tack in the cross-members where the two sides are parallel. Tack in the diagonals in that same area. Fully weld out what you have. Pull the tail ends together and jig them firmly, do the same with the nose if it isn't parallel, tack the cross-members, tack the diagonals. Weld out carefully, do not overheat or kinks will form at each weld because the metal is under stress from the curves. If the curve is substantial support the longerons very strongly each side of the weld to reduce kinks. Where curves are present always weld the outside of the curve first and let it cool so that it supports the tube when you weld the inside.

Afterwards I de-stress my welds by gently heating each in turn with a blowtorch until the colour of straw is attained.

Have fun.
 

TFF

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Linseed oil was the old way, now just weld it up. Tack it all then finish weld. I do better welding as far as I can on a cluster then go to the opposite one and do the same thing. I can weld ok, but I am not a welder, so I try to hedge my weaknesses. If I keep after a cluster, I will end up overheating it. The biggest secret is try to get comfortable before starting a bead. I de stress the joints too.
 

PTAirco

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Depending on the design of the fuselage, you could also build it up by making the top and bottom first, jigging it up and then adding the side pieces; the Pitts Model 12 is built that. It depends on the particular fuselage; on my biplane everything is aligned to the bottom two crossmembers so it was helpful to build them on a flat base first.

There is no such thing as a straight welded fuselage, you just have to accept that there will be some distortion, like the longerons bowing in between clusters. You can cure almost all of that by reheating things carefully. If you bring the long side of a bowed longeron to a dull red heat and let it cool slowly, the heated side will shrink. Sometimes quite dramatically, so it's best to do it bit by bit.
 

clanon

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What about the 4 quadrants (opposite quarters of the tube) to avoid deformations...?
 

Marc Bourget

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This order of battle is discussed in good detail in "Construction of Tubular Fuselages" and from a different perspective in Elzea's "Aircraft Welding" I'd recommend both to anyone "new" or not formally trained in OA welding of thin tubular sections

mjb
 
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Thanks gentlemen, you've clarified a lot. For the sake of discussion, I'll be welding a Legal Eagle XL fuselage. I will be TIG welding, as I own a nice TIG welder. I am an amatuer welder with a limited skill set particular to my trade. I will procure direction and inspection of a local expert.

Any input on TIG welding? Filler rod(s), torch (maybe a flexible wp-9), cup size, lense, tips, and techniques...?
 
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This order of battle is discussed in good detail in "Construction of Tubular Fuselages" and from a different perspective in Elzea's "Aircraft Welding" I'd recommend both to anyone "new" or not formally trained in OA welding of thin tubular sections

mjb
Perfect, I love books. Do you recommend both for TIG welding?
 

Head in the clouds

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Thanks gentlemen, you've clarified a lot. For the sake of discussion, I'll be welding a Legal Eagle XL fuselage. I will be TIG welding, as I own a nice TIG welder. I am an amatuer welder with a limited skill set particular to my trade. I will procure direction and inspection of a local expert.

Any input on TIG welding? Filler rod(s), torch (maybe a flexible wp-9), cup size, lense, tips, and techniques...?
Miller Co, welders have some excellent TIG tutorials on Youtube, and you can't beat getting someone to show you the ropes. I'd suggest paying someone for an hour of their time in a specialty welding shop, custom car builder etc
 
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Miller Co, welders have some excellent TIG tutorials on Youtube, and you can't beat getting someone to show you the ropes. I'd suggest paying someone for an hour of their time in a specialty welding shop, custom car builder etc
Great idea. I am acquainted with a real pro. I expect an hour of his time would prove as valuable as many many hours of research and practice. I'll give him a call.
 
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I read Construction of Tublar Steel Fuselages. Lots of good, easy to understand info. The book generated one question. He says when you prep a cluster for welding (say cleaning everything with 220 emery cloth) it will rust very quickly. Yet, he advises against any corrosion protection until after you weld (TIG) everything, to not contaminate your weld. His info kind of conflicts, "Don't let it rust", "Don't protect against rust".

It might take a month or two to construct and weld the fuselage. Do I worry about protecting the clusters from rust between fitting and welding? A couple of solutions come to mind: I wipe the clusters with a light oil, then degrease before welding. Or, I could skip the prep until ready to weld, maybe prep the joints with some light sand blasting just prior to welding. Maybe it really isn't a big concern in Utah where humidity is usually low?
 

PTAirco

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I clean the tubing to shiny, bare metal, starting with a wirebrushing, followed by fine emery paper. Then tack weld the whole thing, which is likely to take a few days at least. In a damp atmosphere you can then almost watch the rust forming, especially with 4130. One reason I like living in California now and not Britain anymore - here you can leave bare 4130 sitting around almost indefinitely.

After it's all tacked and you're ready to do the final welding, I again wirebrush each joint (there will be some slag/oxidation from the tacks which you want to get rid off) and wipe down with acetone before welding. With gas welding, the difference between welding clean and dirty tubing is night and day; the clean stuff just welds so much smoother. When I say wire brushing, I mean powered brushing; I use an angle grinder with a 4 1/2" brush. Doing it by hand is futile. But don't go overboard either, especially on 0.028" tubing.

Once everything is welded up, most people get the whole thing sandblasted. I do it the hard way, more wirebrushing and emery paper. If you do get it blasted, take it to somebody who knows about aircraft. It is too easy to remove substantial amounts of metal if you're not careful.

Light rust that formed on an otherwise clean surface is really not a big deal; at least when you weld with a torch. Since the welding is relatively slow, the oxides have time to float to the surface and don't contaminate the weld. I have experimented with that; welding striaght through badly rusted surfaces and it worked pretty well, but not something you want to do if you can avoid it. But my Henrob (Dillon) torch is supposed to be able to do that; its design forms a kind of gas shield around the tip. Can't be sure the same happens with TIG, though; I'm no expert on that.
 
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Okay. Prep and clean while assembling and tacking, then lightly wire wheel prior to final welding. Thanks!

Where do I buy tubing? Do I need to specify "aircraft grade"? Local metal supply, or somewhere specializing in aircraft materials?
 

PTAirco

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Generally you can't beat Aircraft Spruce & Specialty:

4130 ALLOY STEEL ROUND SEAMLESS from Aircraft Spruce

If you can find it cheaper locally, tell them - they will match the price. Some specialty metal suppliers will charge you an arm and a leg if you mention "aircraft". We DO NOT build aircraft, remember that when buying anything; tell them it's for race cars or jungle gyms or bicycles or anything, but NOT aircraft.
 

steveair2

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I've had trouble doing a aluminum lap weld with gas. At Oshkosh this year I ask for a demonstration of the Henrob, now Cobra torch. This guy did a airworthy lap weld, without even cleaning the aluminum. All he did was put flux on the rod. I think this torch is on my new tool list.
 
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