Cluster welding strategies

[sonicapollo]

Looking at various videos, there are apparently different ways to weld up a fuselage. I'm not talking about the actual process, more about what you weld and when. 

Let's take the typical longeron scenario. You have two cross pieces and a diagonal inside. You can't fit the diagonal after you tack the cross members.  Do you tack the front bar (or maybe even finish weld the top?) and then fit the diagonal and aft crossbar, followed by tacking? Obviously the crossbar under the diagonal will not get welded to the longeron where it is covered by the diagonal. I imagine this is normal and not an issue because the diagonal will get welded to both pieces.  Are there any instances when this occlusion would be an issue?

I've noticed some people finish weld quite a bit as they go, others seem to just tack. Are there any welds you have to finish weld, or can you tack everything? I imagine you at least should finish weld the firewall and mounts before you put it in the jig. After that could I just tack everything?  

Sorry if anything above seems silly. Having no plans at the moment some obvious things may escape me.

[CHARLES DEBOER]

I tac welled everything except station #1 and the engine mount on my XL.  It is much easier to weld complete on the bench.  I think it would have been easier if I had  made a jig on the work bench to weld the upper longeron and wing attachment bar in a flat position first.  Next welled the bottom fuse longerons complete and tac the intercostals joining the top and bottom longerons.

[scottiniowa]

Looking at various videos, there are apparently different ways to weld up a fuselage. I'm not talking about the actual process, more about what you weld and when.

Let's take the typical longeron scenario. You have two cross pieces and a diagonal inside. You can't fit the diagonal after you tack the cross members.
But yes, you can- with constant forward thinking, you place tacks where they will NOT be in the way for the next tube, this will take a bit of practice but soon you will visualize what will come next and have tacks that won’t be in the way.   That being said, if you have a tube tack that is in a place that inter fears with the next tube, you can make this next tube fit up to your tack (would rather not do, but can be done) 
 Do you tack the front bar (or maybe even finish weld the top?) and then fit the diagonal and aft crossbar, followed by tacking?
While much of this can be debated, Generally you can tack up the entire fuselage, this makes “going back” possible, where a 1/2 or 3/4 welded up cluster almost impossible to change once this far along. I have done many entire fuselages that hold together/strait and true through the whole process, then once all together…you can start the process of finish welding.
Obviously the crossbar under the diagonal will not get welded to the longeron where it is covered by the diagonal. I imagine this is normal and not an issue because the diagonal will get welded to both pieces.  Are there any instances when this occlusion would be an issue?
99% of the clusters can be viewed as a complete unit when welded. see above on the general tacking process. I can’t think of an area, where I would say, I want this all welded, before I add more tubes to the cluster.  Generally, clusters are designed to be strong as a fully welded unit.  They are not designed to be welded in certain stages like you elude to. If so there would be “call out-instructions” in your plans.  There are none of those in your plans.

I've noticed some people finish weld quite a bit as they go, others seem to just tack.
the latter tends to work best.
Are there any welds you have to finish weld, or can you tack everything?
yes, the latter
I imagine you at least should finish weld the firewall and mounts before you put it in the jig. After that could I just tack everything?  
this is about the only area that really needs that kind of attention. and I can’t see any harm in doing it that way. Make sure it is square  ( to the dimensions you intended) before you insert into your frame.

Sorry if anything above seems silly. Having no plans at the moment some obvious things may escape me.
nothing silly…NO plans? not sure what is meant by that? if you have these questions when your building, much of it will sort itself out.
cheers and best of success.
Scott

From Scott in RED

[Bugsmasher]

Why would you not want the circumference of every tube welded fully? You have to fit every tube anyway...so no added labor involved. The loads on each node of any cluster of a tube junction has to undergo a lot of changing loads at any given point in their life. I would highly suggest fully welding each tube so it can fulfill the designers objectives. Let alone have a safe airframe and not be uncertain. Funny thing you cut a cluster from a 80's Citabria or 48 J-3, saw into the cross section of it to inspect it and they are fully welded.

[scottiniowa]

Why would you not want the circumference of every tube welded fully? You have to fit every tube anyway...so no added labor involved. The loads on each node of any cluster of a tube junction has to undergo a lot of changing loads at any given point in their life. I would highly suggest fully welding each tube so it can fulfill the designers objectives. Let alone have a safe airframe and not be uncertain. Funny thing you cut a cluster from a 80's Citabria or 48 J-3, saw into the cross section of it to inspect it and they are fully welded.

That works too, but for many this can/could make for a huge warping headache,  unless of course you have fixtures like your 80’s Cit. or 48 J-3. of which almost NO one does.   All that said, the Eagle XL frame is so simple, you could get by with a great deal of different methods.

Though seldom ever talked about, if there is a failure of frame components in a crash, almost never is there a failure in a cluster.  ( it could be close to one though)  

As for designer objectives- I am not going to try guess what they “intended", but not once have I ever seen a welding/method/procedure called out as a “ do it this way”    I think you will find anyone who has welded more than 1 or 2 frames, work into a method of excellent tacks on all the clusters, then finish weld.  As always, there is probably 6 different ways to do this.  Best of success.

[sonicapollo]

I appreciate the well thought out post.  That pretty much covers it. I think I'll build the firewall and finish weld it with the mounts. I'll then tack everything else an finish weld it in a homemade rotisserie. 

If i do it this way, should I wait until I finish weld the fuse before I put on the landing gear brackets?  I think that would be a smart move. That, plus then I can just build the whole fuse flat on the table.