Thorp Air Command - T18.net

How much of an interference fit do I want, I do intend to fold the wings. If I add up the spacer, spar web, and spar cap I come up with .380. The 315 fitting should be .375 before plating. My problem is my 315 fittings are .390 thick. I have two sets of fittings one from Ken Knowles
the other from Mike Archer both sets are .390. This adds up to a .010 interference. Do I make another 216 spacer .010 thick or surface grind the 315 fitting (there goes the plating) Use a ten pound hammer. Need some guidance from those who have been there, I can't be the first person with this problem.

Jon Sanders

Use whatever thickness spacers you need to use to get a good interference fit . You want ZERO play when the two wings are mated . I have probably folded/unfolded my wings at least 30 times and I still have ZERO play when the wings are mated . I do not recommend grinding the main fittings . Adjusting the fit with spaces should be all you need . I took a quick look at mine and it appears I used a 0.016 spacer in addition to the 216 spacer . Don't hold me to that as it has been a few years ago . Fit and adjust , fit and adjust .

Thank you Rich.
Jon Sanders

I think Rich does have the best tech on this fitting, I did buff mine to fit, they are always coated with grease to help on removal and reinstall, they are tight. Tight is the key word here just as Rich has noted, I have no slop in any direction in the fittings, the pins have to be installed with a slide hammer, and removed the same way, I keep em greasy to aid in this process. I have removed my wings many times over a 2 year period, still no slop. Use the shims, much easier and you can achieve the tolerance you need to be tight, and still yet be able to dis assemble and reassemble, a sheen of grease helps also....Dan

Jon,

My wing joints have a very small gap in the 'sandwich' built up for these fittings. I read in some instructions I had that a small (.010 - .015" ?) shim is used in front of the 315 at bottom, and behind it at top. The idea was to guarantee a very small clearance for pitch rotation around the rear support tube as you separate. Can't say it really works that way or not; my wings are still kind of tight.

What seems to be most significant in my case is matching the dimension between front and rear spars. If that's not right, it won't matter much about the fittings anyway. To be honest, it wouldn't bother me terribly to have an 'easy' slip fit for the 315 and 215 fittings. Here's what really matters to me the most for that joint: Match drill the holes for the two pins and be certain the pins fit very snug. I reamed mine after the heat treatment of the fittings to account for dimensional changes. I again built up the sandwich to 'match ream' with 215 & 315 fittings, spacers, and shims as described above. This was before spar assembly. I tried to attach an image, but you can always look at my photo album. Getting the dihedral angle right between spars was also easier to do afterward. When going to attach these fittings to spars I laid out the spars which because of size are easier to measure. Ask around how much wobble (induced by the pins) that you can feel up and down on various projects at the wing tips. I think most guys would agree about 1/8" maximum. Hope this helps.

Les Krumel,
Albuquerque

If you have a set of T-18C plans you should have the Sunderland Aircraft wing inspection and maintenance manual . It outlines specific procedures and free play at the main spar pins . Attached is a copy of mine for your files .