Thorp Air Command - T18.net

I truly appreciate everyone's input. I can now make an educated decision as to what needs to be done on my plane. Thank you all again.

Feisty, my T-18 also has paint failure on top of pulled (pop) rivets on the wing.
The wing spar has a double row of rivets from the fuselage outboard for about a foot, then a single row outboard of that. I don't know if that qualifies for the double row riveted wing, somebody please fill us in?
However, some pulled rivets on the wing-to-ribs also have paint failure, and these are very low stress joins.
I suspect paint prep, the small hole in each rivet or possible dissimilar metal galvanic issue between the rivet head, mandrel and/or skin? I see no loose rivet.
I'm all ears for a suggested paint repair, surface prep and primer process...

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

All good comments guys - I'm with you. And I can add 2 cents. If it were me I'd leave it alone for a while, keep a close eye on it, and see first if it was just the paint also.

If it needed repair I'd probably use the Cherry rivets, but here's some thought to go with it: Typically, we always debur all the holes we drill to lower the risk of cracking. I did countersinks per John Thorps design on the spar webs, which is technically pretty thin as well. On your wing skins you can't really dimple any new holes unless you tear a lot apart, that's a non-starter since it would probably create more damage in the process. Nice thing about dimples is that the bent material takes up some of the shear and reduces stress at the edge of the hole. If you countersink new holes, they're going to really have sharp edges that you can't debur - and that's prone to cracking. Would you consider a new row with some kind of round-head pop rivets? Certainly Cherry makes those too. Still can't debur underneath but holes are smaller and maybe not as sharp. Is that reasonable? I wouldn't worry about strength in the spar cap, just getting a good grip and the stress in the skin.

Les Krumel,
Albuquerque

Good thought Les, thank you. The inability to debur (short of opening the wing) does bother me. I tend to think its not a paint issue because the paint has only popped off the rivets in this one row of rivets on each wing. I would be more inclined to say its a paint issue if rivets in other places were popping paint too.

So I have been corrected off line. That's the value of this group. We have some no kidding experts here that I really appreciate learning from. I was told (and passed on) that there is no difference in strength of a properly drilled and filled rivet hole. That MAY be true in compression, but not in tension. You do weaken the spar in tension with any hole. If you get down to brass tacks, the spar has hundreds of holes in the caps. But those holes and their location was thoroughly analyzed and planned for.
So the bottom line is drill no hole in the cap unless you take responsibility for the structural strength of that hole.
I learned something which always makes me happy.
Cubes

Ryan,
The subject of suspect rivets on the top of the spar on the inner wing has been talked about for years. Hence Bob Highleys discussion with JT many years ago. My advice still stands in that if you suspect rivets working, do something about it. I have heard guys drilling out rivets and replacing with CM, adding a double row, all the way to building a new wing. The paint opened around the edge of my rivets in that location as well, but not the outer wing. I still believe it is a prep issue.
Cubes

great info Cubes. Thanks for posting. I wouldn't have intuitively thought that about the spar holes in tension. I need to decide what right for me and do it. Thanks again.

I've just drilled out the flush head monel CM style rivets on my left outer wing. Some of these rivets were starting to lose paint over the main spar, but the same type rivets were still holding paint just fine over most of the ribs.
Drilling out the rivets holding the skin to the ribs mostly resulted in the rivet head not spinning at first. They were all solid.
Drilling out the rivets holding the skin to the spar mostly resulted in the rivet heads spinning as soon as the drill started in.
This wing panel was bent up from a ground loop. All these rivets held, maybe one at the edge ripped out where the panel contacted the ground. Some were under considerable tension when I drilled them out, and finally let go with a loud bang!
I plan to rivet the new skin onto the new spar with driven AN426AD-4 flush rivets in the hope that they will remain tight over time, I may use monel pull rivets on the ribs and aileron where driven rivets are difficult or not possible. They are quite strong, very easy to work with, and drill out quickly & perfectly.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Hi Scott - for whatever it's worth to you, I think you would be well advised to very carefully inspect (perhaps with a dye penetrant?) and possibly replace your spar fittings on that wing if you haven't already. They may look perfectly fine to the naked eye, but be damaged or weakened. There has been at least one fatal loss of a Thorp that had been involved in a serious ground loop previously, when a wing departed the airplane in flight at 7000'. There was some speculation about that ground loop incident at the time. Just something important to consider IMHO.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Thanks John;
The first determination was inspecting the center section for deformation.
Then while removing the panel, how the bolts came out, and the quality of fit. These all were fine, with each bolt requiring a firm press and the bolts coming out smoothly. The H beam piece is not deformed and fits the clevis perfectly.
All the rib bolts and the 2 web rivets came out perfectly.
When the rivets were drilled out and the skin removed, the main & auxiliary spar damage was observed to not extend near the fittings.
Most of the ribs look re-usable, although I will probably replace all the nose ribs due to the previous set being too pointy at the leading edge.
I intend to examine the bolt holes and tabs under magnification and die penetrant test the piece.
So your advice is well taken.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE