Towplane strut problem

Being a pilot myself, I think you have more of a concern that merely fixing the play in the strut end. If it has been wiggling the the hole, there's a high likelihood of fretting corrosion and fatigue in the element around the hole.

I hate to say this, but you might need to check if there are any Airworthiness Directives out on that model, about that part.

LLoyd

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

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
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  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.

John: I would suggest taking a good look at the wing attach fittings too. They may also have some damage. Best place to look for repair data is the SRM for the Super Cub

Craig C. snipped-for-privacy@ev1.net

I was going to guess portside then I reread your message. I used to tow with a PA-12. I'm guessing that upon release, your pilot is doing a slipping spiral on the wingtip like I did. Lots of vibration and shuddering. Possibly the cause of the wear. I know that doing that makes controling the airspeed a matter of feel rather than instrumentation. Perhaps a little overspeed is mixed in?

I was taught to do that to prevent cold shocking the engine and to get back on the ground ASAP. As we only had one sailplane, I wasn't in a hurry to get down, but it was fun to do and the people on the ground loved to see me do it.

I learned to fly in a J-3 and I have owned various steel tube airplanes over the years... I have helped to jig, drill, and ream the fuselage tabs and mount biplane wings... I doubt that there is any wear allowed by Piper at that lower spar attach tab... The hole in the fuselage tab should be a close tolerance fit for the bolt... I always considered the wing tip shake test to require no relative movement of the wing in relation to the strut/fuselage - other than the usual springing/bouncing of the entire airframe with a vigorous shake... With any slop in the strut flutter becomes an issue... I urge that you folks get an experienced steel tube airframe mechanic to make the repair soon...

denny