Towplane strut problem

Our gliding club towplane (Piper Super Cub) has developed play in the
Fuselage / Main strut connection - Portside. The bolt and clevis (Fork)
seem to be OK bolt is a reasonably snug fit in the fork. - I presume the
play is in the hole in the Bracket welded to the fuselage. Play seems to
be about 10 thou (+-.25mm) which translates to about .75 inch of "Waggle
" At the wing tip. Can anyone help with the following -



What is the permitted wear at this point ? (If any)

What approved repair methods can be used here?



Thanks in advance



Best wishes from Cape Town



John Spargo

"John Sp" wrote in message
...
Our gliding club towplane (Piper Super Cub) has developed play in the
Fuselage / Main strut connection - Portside. The bolt and clevis (Fork)
seem to be OK bolt is a reasonably snug fit in the fork. - I presume the
play is in the hole in the Bracket welded to the fuselage. Play seems to
be about 10 thou (+-.25mm) which translates to about .75 inch of "Waggle
" At the wing tip. Can anyone help with the following -



What is the permitted wear at this point ? (If any)

What approved repair methods can be used here?

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 am not sure if this is something you want to play around with
yourself. I would consult a licensed aircraft mechanic. There may be a
repair scheme for this or it may have to be replaced. A mechanic
familiar with the aircraft would know. Having worked on aircraft
engines, nothing gets fixed without an engineer working out an approved
repair. Or as we used to say " no engine flys till the paperwork weighs
as much as the engine" :-)

John Sp wrote:
Our gliding club towplane (Piper Super Cub) has developed play in the
Fuselage / Main strut connection - Portside. The bolt and clevis (Fork)
seem to be OK bolt is a reasonably snug fit in the fork. - I presume the
play is in the hole in the Bracket welded to the fuselage. Play seems to
be about 10 thou (+-.25mm) which translates to about .75 inch of "Waggle
" At the wing tip. Can anyone help with the following -



What is the permitted wear at this point ? (If any)

What approved repair methods can be used here?



Thanks in advance



Best wishes from Cape Town



John Spargo

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

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.

On Tue, 14 Jun 2005 20:50:15 +0200, "John Sp"
wrote:

Our gliding club towplane (Piper Super Cub) has developed play in the
Fuselage / Main strut connection - Portside.

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