Towplane strut problem

Earlier, John Sp wrote:

...Play seems to be about 10 thou (+-.25mm)
which translates to about .75 inch of
"Waggle" At the wing tip...

I'm not an engineer, but somehow that doesn't add up. Simple geometry
suggests that in order for 0.010" of play at the root to yield 0.75" at
the tip, the ratio of the depth to the semispan would have to be around
75:1.

Operating from basic specs and a grainy 3-view, it looks to me like the
semispan is around 17 feet and the depth (height from the wing mount to
the strut mount) is around 4 feet. That yields an actual semispan/depth
of only around 4:1. With that arrangement, +/- 0.010" of play at the
bolt yields a vertical play at the wingtip of around +/- 0.040".

Some semi-random comments on this situation, in no particular order:

* There's a thread on a similar topic on the discussion boards at
http://www.supercub.org but it seems to concern J-3 rather than PA-18.
However, I'll bet that you would more quickly get a good response on
that board than here.

* Was there perhaps a typo in this post? If the .75 is mm instead of
inches, then it all makes sense. On the other hand, I can't imagine
worrying about .75mm of wingtip vertical play...

* I know from measuring wing mounting play on various gliders that it
is very hard to measure wingtip vertical play separate from wing
flexibility. It's hard to tell where mechanical slop ends and
limberness begins.

* It's possible that you are actually seeing +/- 0.010" of play at the
strut bolt and 0.75" of vertical play at the tip. But only if there are
also other sources of play such as the fuselage-to-wing mounting, or
the strut mount provisions inside the wing.

* A certain amount of play is inevitable. There is simply no way of
making the wing mounting provisions completely tight without using
interference fit fasteners.

Good luck, and best regards

Bob K.
http://www.hpaircraft.com