Originally Posted by RFSchaller
My 12 is coming up on 10 years and a little over 1100 hours. I noticed a slight “click” every time I got onto the engine after a couple of years, and I shimmed the spar tongues with a .025” shim traced from the spar tab with the bottom of the shim bent under the spar tab. Problem fixed. I am wondering how critical this issue is. Isn’t the stub spar tab basically an anti- rotation device to keep the wing from twisting? If so, is a slight “slop” in fit really a big issue?
The fwd. and aft stubs are there to take twisting/torsional loads but that is not the primary.
The highest loads are an inward compression load at the fwd. stub. That is why there is some pretty heavy structure on that bulkhead member.
amount of fore/aft play in a wing is not a major concern but there is a limit. We have a document that has instructions on how to measure the amount of play and what is the expectable limit. It is not publicly available because we prefer to have direct communication with someone looking at a loose wing issue.
As for adding a .025 shim? I would consider that extreme, and would be way outside of what our acceptable limits are.
Of a bigger concern is that adding a shim in that manor has probably compromised the fit of the stub in the socket.
With the way the parts fit, adding a constant thickness shim will have a negative impact on the proper nesting of the two parts.
For the lack of a better illustration, I have attached a photo of a solar eclipse to mimic the two round parts. Note that as the two radiused edges become displaced, the amount of displacement varies along the edge, with the most difference being at the mid point and no difference at the top and bottom.
This should also be considered if someone decides to remove material at initial wing fitting because they can't quite get the wing inserted fully. This depiction shows that most material should be removed at the mid point and then varying the amount along the edge to where no material is removed at the top and bottom.
Another point related to a .025 thick shim is that if we evaluate the ratio of wing panel length/span to the distance of the shimmed stub from the hinge point of the movement (the main wing spar). If the .025 shim were installed at the rear stub, the dimension ratio is approx. 5:1, so that .025" shim thickness would be equivalent to a fore/aft wing movement of 1/8" out at the tip. We would consider that excessive.