Welding question

[Will Weidner]

I have what is probably a really dumb question, but it's got me stumped so I'm going to ask it anyway.  Leonard says to tack the fuselage completely before welding.  I might not understand this correctly.  Does this mean not to weld any joints complete before fitting diagonals, etc.?  I'm particularly concerned about the cluster where the rear cabane strut comes down to join the top rear longeron (to the 69" dimension on page 6 of the plans).  There has been a lot of talk on this forum about whether to bend the longeron tubing at this joint.  A lot of people say only the lower longerons should be bent.  Assuming the upper longeron is welded to the rear strut (not bent from 1 piece), how would you go about welding this cluster?  I assume a finger strap would be needed?  If so, you can't fit all the tubes then weld it complete at the end.  Sorry if this is obvious to most people, but I just can't seem to figure it out.  Any help would be greatly appreciated.  Thanks.

[rockiedog2]

>>>Does this mean not to weld any joints complete before fitting diagonals, etc.?
yes. often a tube fits over another tube and i don't weld the joint area under the tube on top.

>>>There has been a lot of talk on this forum about whether to bend the longeron tubing at this joint.  A lot of people say only the lower longerons should be bent. 
 At the 69" station I cut the backbone off there and welded in the 2 top longeron sections that continue forward. And welded in the cabane brace that comes in at that point too; just a normal cluster like all the rest. I didn't use a finger strap there since it's not a simple butt joint but is a cluster with long angles and a lot of weld area. I wouldn't bend one of the top longs there cause of a loss of precision in turning that corner.
 it's generally considered that the "traditional/proper/correct" way is for the wing attach cabanes(sta 2 and 3) to be a cut/weld butt joint where they meet the top longerons. but there are planes out there that have a bend there instead.  as at the 69" station if that turn is made with a bend there is a less precise change in direction there. Where finger straps are needed(the wing attach cabanes at the top longs) I  weld the finger straps on after finish welding those joints. I also added a corner gusset where the front wing attach cabane meets the top longeron.

 
An easy way to cap the end of an open tube like at 69"is to weld a freeze plug there.
when finish welding the fuse i start at sta one and weld that sta in a clockwise direction then back to sta 2 and all the way to the tail. that minimizes the fuse warping. if you go all the way down one side that side will wanta grow.

[Will Weidner]

Joe, thanks so much for that information.  It all makes a lot of sense.  I was really having trouble getting my mind around it.  I was hoping that cutting the upper longeron at the 69" dimension was ok to do because I pre-fabbed the longeron with the fin and stab mount points already drilled and welded in.  It's good to know to treat this as a normal cluster joint.

You mentioned capping the longeron at the 69" point.  Is it necessary to cap it?  If I wanted to put some linseed oil in there, wouldn't it be better if it was open so it could get to all the tubes connected to it?

[rockiedog2]

i usually finish weld all the clusters and leave the forward end of the longerons open wherever i can. pour the tube full of linseed oil then drain it and weld the freeze plug over the end. your weld will probably get pin hole blowouts as the linseed oil expends...just weld the pinholes up as you go and you'll eventually get it welded all round. i always want all my tubes sealed up which means no open ends...more likely to eventually rust even tho coated with oil.
if you really wanta get anal about it you can drill a 1/8" hole(vent and drain) in the tube side near the forward end, pour the oil in, weld the freeze plug on, then drain all the oil out that little hole and put a drive rivet in it.  that way you got every bit of the tube inside coated. i never have done that i just leave a little oil in the tube and then orient it so it coats the freeze plug area.

[Rich Snyder]

Will- Anyone who is new to gas welding something as intricate as a fuselage, (me), can understand your trepidation at this task. My only comment would be to be sure and think a couple of steps ahead all the time. There is usually no problem in fitting the cross members but the diagonals won't fit  once the cross piece is tacked in. There is a certain order that will make life easy. The other way gives you a good amount of practice carefully grinding little tack welds off.
Rich

[Will Weidner]

Joe, thanks for being patient and explaining things so well.  I think I get it now - the cluster at the 69" dimension has the upper longeron running clear through it, which is why the end of it is open.  I was thinking it was butted (with mitered ends) to the rear cabane strut coming down, which explains my confusion about a finger strap.  Running clear through is what you mean, right?  I like to poke around here and on the yahoo website looking for pictures, but couldn't find any clear ones of this area.

Rich, yes, you definitely have to think a lot of steps ahead.  It's amazing how much information is on the plans and the instructions, but there's a lot you have to figure out for yourself.  There are times (like now) when I think I've got it figured out and I have to put on the brakes and think about it.

[rockiedog2]

>>>Running clear through is what you mean, right? 

correct