Bend Station #3 to Station #2

[914pete]

Hi guys.  Bending day today.  I bent the tail post up, no problem there.  While I was heating up station #3 I bent in the lower longerons parallel to station #2.  However, I got kind of a weird "S" bend there see pic.  (pic is facing the tail) The cross tube at Station #3 is 24" center to center however, just after it maybe 1" it bows out to 24 1/4" center to center then back to 24" center to center at Station #2.  Is that going to cause problems down the road?  Should I re-heat that area on both sides and try to work both longerons in?

[Vince Carucci]

Does the tube looked compromised between the tac-weld and the 2 inch mark in the photograph? [deformed, cracked, butted, wrinkled, etc.]

[914pete]

Does the tube looked compromised between the tac-weld and the 2 inch mark in the photograph? [deformed, cracked, butted, wrinkled, etc.]

No.  Its a smooth bend just not as sharp an angle as I would have liked.

[Bob S.]

You must have inadvertantly let too much of the tube get the heat. not much you can do with it now...move on down the road and try better on the next bends.

[Will Weidner]

I've said this before on this forum, but new people have joined since then.  When I'm uncertain or apprehensive about something new, I mock it up and try it to see what happens.  In the case of these bends next to clusters, I tacked up a small cluster out of short pieces of scrap.  Then I heated it and bent the tube just aft of the cluster to see what happens.  What I was concerned about is how much heat the cluster sucked out of the heated region.  Also, I wanted to see if I should heat more on the inside or outside of the bend to prevent tearing or buckling.  This quick test answered those questions.  I found I had to concentrate the heat more into the cluster to get the area just aft of the cluster hot.  I also put a little more heat outside the bend than inside.  I suspect what happened to Pete was the cluster sucked more heat out than he expected, moving the cherry red zone away from the cluster.

[Vince Carucci]

Pete,

The [tensile] forces through that intersection are primarily down the strut; into the landing gear/strut bracket; across the 5/8 tube to the other side; and up to the other wing the same way. The forced along the longeron between station 2 & 3 will be much less. If it concerns you further, think about welding in a small gusset. 

It may be that the curve in your tube is simply a hallmark of "hand built".

Vince

PS: Think of it as a learning curve!

[914pete]

Hi Guys, yes, I think it just comes down to experience.  I did practice a bit before bending the longerons but it seems the tubes have minds of their own sometimes.  I bent up the longerons at station #2 to the fire wall and the first bend has a slight outward kink on the inside of the tube, the second bend was perfect. 

Vince, I was thinking the same thing.  I'm not terribly concerned about it.  Its more aesthetics.  I may still weld in a small gusset when all is said and done.

[joecnc2006]

Pete,

I'm kinda thinking like you, that would probable bug me knowing its there although would not affect much, I would more than likely head it up straighten it out let it cool then re-head and re-bend and try get it straight.

[914pete]

Pete,

I'm kinda thinking like you, that would probable bug me knowing its there although would not affect much, I would more than likely head it up straighten it out let it cool then re-head and re-bend and try get it straight.

I was going to do just that but then slept on it.  Next day I was thinking I could easily do more damage re-heating, straightening, cooling, bending etc. etc.

[Will Weidner]

Pete, if it were mine I wouldn't try to fix it.  As long as it's not buckled I would leave it.  Not based on any engineering analysis, but from seeing tube bends on some of these airplanes much further from the cluster on much higher loaded tubes with greater bend angles.  Not text book, but they worked none the less.

[Dan_]

Pete, if it were mine I wouldn't try to fix it.  As long as it's not buckled I would leave it.  Not based on any engineering analysis, but from seeing tube bends on some of these airplanes much further from the cluster on much higher loaded tubes with greater bend angles.  Not text book, but they worked none the less.

I'm not an engineer, but I am rather certain that the lower longerons see tension principally.  Especially in the landing loads.  

If it is kinked, ^{(mild fold over type)} weld over the kink.  

Otherwise, if it were mine I'd just try to make the other side the same...  

Also don't try to make the bend all at once.  Put some force on it in the direction you want it to go,  put the heat where you want it to bend,  feel the tube relax into the bend... Stop putting heat...  Repeat.  

Don't try to bend it in 2 directions at once.