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PA18 Fuselage Centerline

Bucrepus

Registered User
I'm building a PA18 fuselage from scratch. After tacking the fuselage together, my tail post is 1/8 inch off centerline. What is a permissible tolerance?
 
it will move more as you weld it....(& weld cool and shrink)

it moves to the side that cools....

not sure how to explain it, but if it's not in a jig, you can probably correct that by choosing which side to weld first(gas welding, it moves ALLOT!)....

but I have no guess right now, on which side to recommend to weld first, to get there... out of practice...
 
and truthfully the tail post IS ON THE CENTER LINE, so it just some bays in front of that that are off the 1/8".... which won't matter

the jig we used made from a virgin surplus PIPER un-flown fuselage ends up with tail post about 1" off center(measured from out at wing strut attach on wings)
 
Mike,

Are you talking about measuring from the rear wing attach fitting, to the center of the tail post on each side. If the measurement is equal the centerline is good. One inch sounds like excessive for a factory fuselage. But this is my first SB build.
 
Mike,

Are you talking about measuring from the rear wing attach fitting, to the center of the tail post on each side. If the measurement is equal the centerline is good. One inch sounds like excessive for a factory fuselage. But this is my first SB build.

No, from the strut attach fitting out on wing(rear)


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You won't find a fuselage that is exact. My repair jig and all the others I have seen have a flat pad for the tail spring fitting. I have repaired a lot of fuselages and they end up just left or right of center. The important part is to be in the correct horizontal plane and to be straight vertically.
 
Thanks for the replies. Just a little concerned being the my first supercub fuselage. I didn't want a big barn door tied to the rudder to correct off centerline.
 
If you are gas welding, you can expect about 1/16” shrink at each cluster. TIG, perhaps half that. Weld up the fuselage, tramel everything out and see where it is out of line, then play the torch on clusters that you want to shrink to bring it back in line where you want it. It’s surprising how much you can move these things with just a little heat.


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The key to welding out light material is to keep moving around and not put to much heat in any one location(side to side,front to back, top to bottom). As you weld you will see the assembly move during the process. The long side if needed can be drawn back with heat and the proper cooling(wet rag to rub the area). I’m sure someone will have an opinion on this but it has been working in the industrial piping world for many many years.
 
I have taking all the talking points into consideration, and put into practice. I TID each cluster moving from die to side working from top to bottom starting at the front of the fuselage. Last night I measured from the center line of the left rear top wing attach fitting and then from the right side to centerline of tail post. Measuring turned out with 1/32 difference, I think things are looking up.
Steve Pierce will this be ok?
 
I have taking all the talking points into consideration, and put into practice. I TID each cluster moving from die to side working from top to bottom starting at the front of the fuselage. Last night I measured from the center line of the left rear top wing attach fitting and then from the right side to centerline of tail post. Measuring turned out with 1/32 difference, I think things are looking up.
Steve Pierce will this be ok?

Left/right variation can be fixed after the fact by bending the vertical stabilizer leading edge. Your 1/32” should be fine. Up and down variation could throw you stabilizer travel off, so it’s more important. One of the super cubs I used to fly had so many splices in the Longhorns and ended up being a little short on nose up trim. Flew it like that towing gliders for some 18 years until the owner pulled it apart again. That rebuild converted the flat back 18A to a standard 18 configuration. Just last year I finally pulled the Restricted certificate and issued a Standard certificate for it.


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..Up and down variation could throw you stabilizer travel off, so it’s more important. ...

yes this is more important.... bottom longerons stretch, and you fix by splicing and shortening them to get stab travel back to book number.... but don't shorten them too much or it won't be able to slow down.... keep it to the correct number.... (I learned this, by giving one an extra 1/4" down at rear stab attach/tailpost/lower longeron ... and later date putting a new tail section on same plane using the correct measurements.... customer Immediately said that fixed his couldn't slow it down issue that appeared with first repair....)
 
I have taking all the talking points into consideration, and put into practice. I TID each cluster moving from die to side working from top to bottom starting at the front of the fuselage. Last night I measured from the center line of the left rear top wing attach fitting and then from the right side to centerline of tail post. Measuring turned out with 1/32 difference, I think things are looking up.
Steve Pierce will this be ok?


Now your bragging. :lol:
 
Last night I measured from the center line of the left rear top wing attach fitting and then from the right side to centerline of tail post. Measuring turned out with 1/32 difference, I think things are looking up.
Steve Pierce will this be ok?
Did your tape measure sag more on one side when you measured? ;-)
 
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