Wing Attach Fitting Welded In Wrong Spot

[TonyG]

Hi all,

I'm working on my Stits SA6B project, and have turned my attention to the wings. This was obtained as a partial project, and I've just noticed a possibly serious error on the part of the original builder. The right rear wing attach fitting is located too far forward on the fuselage, and so isn't aligned with the rear spar as it should be. See pictures below.



Correct (forward) fitting


Incorrect (rear) fitting

I would say the discrepancy is 1/4-3/8". Mostly what concerns me is how far off the fitting the aft-most spar strap is. The forward one is adjacent to the fitting, albeit "inside" the bracket rather than in front of it, but at least the bolt is only loaded in shear through this part. The rear strap is cantilevered off the fitting by the bolt.

My thoughts are:
1. Cut and relocate the fitting, which is all but impossible without serious reconstructive surgery to a major cluster of structural members.
2. Weld a shim to the aft face of the fuselage fitting to relieve the bending on the bolt. This still leaves the forward strap of the rear spar on the wrong side of the bracket, but very close to it. Here is an illustration of the idea.


Proposed fix.

Any thoughts? It's a deviation to a critical structural component, but I don't have a lot of options. I don't like that the fuselage-side bracket won't be "captured" by the spar straps as per design, but I don't know how big a deal that is. I could put a bushing on the bolt, captured in the middle of the fitting, which would effectively transfer any longitudinal loads into the rear half of the fitting without putting the loads on the nut.


-Tony

[Aaron Novak]

Hi all,

I'm working on my Stits SA6B project, and have turned my attention to the wings. This was obtained as a partial project, and I've just noticed a possibly serious error on the part of the original builder. The right rear wing attach fitting is located too far forward on the fuselage, and so isn't aligned with the rear spar as it should be. See pictures below.



Correct (forward) fitting


Incorrect (rear) fitting

I would say the discrepancy is 1/4-3/8". Mostly what concerns me is how far off the fitting the aft-most spar strap is. The forward one is adjacent to the fitting, albeit "inside" the bracket rather than in front of it, but at least the bolt is only loaded in shear through this part. The rear strap is cantilevered off the fitting by the bolt.

My thoughts are:
1. Cut and relocate the fitting, which is all but impossible without serious reconstructive surgery to a major cluster of structural members.
2. Weld a shim to the aft face of the fuselage fitting to relieve the bending on the bolt. This still leaves the forward strap of the rear spar on the wrong side of the bracket, but very close to it. Here is an illustration of the idea.


Proposed fix.

Any thoughts? It's a deviation to a critical structural component, but I don't have a lot of options. I don't like that the fuselage-side bracket won't be "captured" by the spar straps as per design, but I don't know how big a deal that is. I could put a bushing on the bolt, captured in the middle of the fitting, which would effectively transfer any longitudinal loads into the rear half of the fitting without putting the loads on the nut.


-Tony

Tony,

I would advise against the shim idea as that would be causing an oblique loading of the attach fitting. Perhaps gusseting and reinforcement could be added to counteract this loading, however the best approach to me would be to correct the error in the fuselage (to match the plans). In the end its probably not as bad of a re-work as you are thinking.

[Frank Giger]

Duct tape. Lots of duct tape sandwiched a quarter inch thick.

Okay, more seriously:

I agree that while shimming the mount as you suggest will work, it's not optimal due to the weird loading on the joint - over time it could crack, and having a wing depart from the aircraft would have adverse effects to the flight characteristics (or so I've read).

If the fuselage is square as it is I understand your hesitation on entering the rabbit hole of unintended snowballing of corrections. Nothing makes for piles of junk like a fix to the fix needed to fix something caused by a repair.

One thing to consider is to make a gusseted mount and weld it to the fuselage. If one looks at the problem mount the start of the gusseting solution was actually started with the top mount coming out to the wing; it just wasn't pursued in the rear. To get a little beefier, one could oversleeve the fuselage tube to match the gusset.

I think you'd be fine with that since this is a low speed non-aerobatic aircraft.

I will, of course, defer to experts who will now post that I'm trying to kill you with bad advice.

[Jeff Point]

You are looking only at fixing the fuselage half of the joint. How about the wing? Could you build a new wing attach fitting in which the front plate is shimmed forward that 1/4" in order to fit on the outside of the fuselage fitting? You'd have to shim the new wing fitting to fit the wing spar (ala aluminum spar Cub type wings) and you'd still have the shim behind the rear part of the fitting (as per your idea) but now the wing fittings would both fit outside of the fuse fittings as designed, and that should eliminate the oblique load issue that Aaron mentioned. (I'm not an engineer, so Aaron please check me on this if I'm wrong.)

This way you don't have to mess with the fuselage structure at all. Building a new wing fitting will be some work, but far less work than your idea and you don't mess with a critical piece of structure (ie- if you screw up the new wing fitting, toss it and build another one.)

[martymayes]

Tony, I don't see a problem with adding a spacer like you suggest. Might even consider fitting it to the longeron and welding there as well. Will certainly add weight. The wing is externally strut braced so in the overall scheme, it won't make much difference. Perhaps you can find someone who can verify the loads at that attachment?

Neat project BTW.

[TonyG]

Aaron, that's the answer I was afraid of! Frank, you understand my mentality on the issue as well. My concern is just how integrated that fitting is. The fitting is welded into place and then a strap is formed and welded over it to capture it against the tubing (see plans excerpt below). That thing ain't coming off without cutting out the whole cluster, I fear. I do agree (and appreciate the confirmation) that it seems a little sketchy to load that fitting obliquely when it was designed to take vertical loads symmetrically across the two flanges. Jeff, I thought about modifying the wing as well, at least adding a spacer between the spar and the forward strap so that it rests on the correct side of the fuselage fitting, but the load will still act through the same plane and result in torque on the fitting. It's just transferred slightly differently through the bolt. I still like the idea of getting the strap to the correct side of the fitting though.





Marty, after posting the original post I thought about extending/welding the shim all the way to the longeron and should have suggested that to begin with. Anybody else have any input on that? It results in a very beefy flange, but would help move the load further back and closer to where it should be transferred into the fuselage and (hopefully) eliminate/reduce the torque on the fitting. It greatly changes the rigidity of the flanges relative to each other (which could be remedied by doing the same to the forward flange), but I don't expect that to be a huge issue in this case. Also, a rough back-of-the-envelope calculation suggests that the rear spar fitting might only take about 1/4 the normal load of the forward spar fitting, which is built exactly the same and suggests quite a bit of margin in the rear fitting that is incorrect.

I'm ready and willing to take advice, and if that means I have to do some serious surgery then so be it - I just want to exhaust all my options before committing to that!


-Tony

[martymayes]

Marty, after posting the original post I thought about extending/welding the shim all the way to the longeron and should have suggested that to begin with. Anybody else have any input on that? It results in a very beefy flange

I wouldn't keep it the same thickness all the way to the longeron but instead bevel it so it would be ~1/16" thick where it butts the tube. This would make for easier welding as you wouldn't have that thick steel wicking heat away. In fact, I'd bevel it everywhere 1/4" thickness wasn't required.

Tony I know of another type plane where if you use later style wings on the earlier style fuselage, the wing fittings don't line up and it's common to install spacers to make it work. That type of fitting is very common and it's doubtful Ray Stits designed it as a "zero tolerance" fitting. A fractional spacer will have minimal impact on overall strength.

[TonyG]

I wouldn't keep it the same thickness all the way to the longeron but instead bevel it so it would be ~1/16" thick where it butts the tube. This would make for easier welding as you wouldn't have that thick steel wicking heat away. In fact, I'd bevel it everywhere 1/4" thickness wasn't required.

Tony I know of another type plane where if you use later style wings on the earlier style fuselage, the wing fittings don't line up and it's common to install spacers to make it work. That type of fitting is very common and it's doubtful Ray Stits designed it as a "zero tolerance" fitting. A fractional spacer will have minimal impact on overall strength.

Marty, I was thinking about the challenge of welding 1/4" steel to 0.035" tube as well. Not super easy, but doable with practice and careful heat management. The benefit to maintaining the thickness all the way to the longeron would be that you would also be transferring the load into the longeron further back, closer to the design. Agreed, though - this isn't a precision instrument, it's an aircraft designed with margins and an understanding that an unskilled fabricator would be forging and assembling components by hand.


-Tony

[martymayes]

Tony, with what you have demonstrated in this thread, I have complete confidence in your ability to solve the issue. Press on!

[TonyG]

Tony, with what you have demonstrated in this thread, I have complete confidence in your ability to solve the issue. Press on!

Ah jeez, people always have more confidence in me than I do, myself!