Thorp Air Command - T18.net

maybe a completely nutty idea. But I'll ask it anyway.

I may be wrong, but I recall reading that some folks have experienced "smoking" rivets at the spar. In which I think the solution was to add a 2nd row of rivets. Or maybe I'm remembering that wrong.

I was wondering if anyone has tried to bond the skin to the spar with something like pro-seal or some other boding agent, if that would reduce the working of the skin and rivet?

It seems to me that with the number of wet wing Thorps out there, it would not too much additional work to coat the top of the spar along with the whole wet wing process. Stick the skin to the top of the spar, and then rivet after it sets up. Maybe the negatives of this would out weigh any benefit.

But this all may be my total naivety/ignorance with all of this and is just a silly thought.

I visited a body shop the other day and they were gluing patch panels instead of welding. I ask about how well it would hold, his comment was its stronger than the body metal itself. Can't remember what it was, but and good body supply shop probably can help.

When I was building my wings (probably 30+ years ago) I considered doing just that, but decided against it for a number of reasons. Mostly just because of the mess & difficulty of doing it cleanly when using solid AN rivets. Pulled rivets would make it a little easier in a couple of ways. Pro Seal?... definitely not - especially not with riveting after the stuff sets up. That would be a poor choice for that purpose, IMHO. I was going to use 3M 2016 Epoxy structural adhesive. It is used in the aerospace industry, has excellent properties including strength & a coefficient of expansion that works well with aluminum.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I just looked up 3M 2016 Epoxy Structural Adhesive and drew a blank. I did find 3M Scotch Weld 2216 Epoxy Structural Adhesive, which possibly is what you were describing or is the current offering?
A link:
http://solutions.3m.com/wps/portal/3M/e ... 721&rt=rud
I would consider this adhesive in my wing panel application if using 'Pop' rivets on ribs or along the spar where there is a history of skin creep or rivet smoking.
It's offered in 5 gallon buckets, one gallon cans, quart cans, pint cans and 2 part mixing syringe (47 ml?) .
Here is a product application guide:
http://multimedia.3m.com/mws/media/1539 ... 16-b-a.pdf
Here is a link on surface preparation:
http://multimedia.3m.com/mws/media/9333 ... lletin.pdf
Here is an Amazon link, where the price will take your breath away:
http://www.amazon.com/3M-Scotch-Weld-Ad ... B00HTWCWC6
Ebay prices are much better right now...

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Scott Emery
EAA Chapter 326
T-18 N18TE

Yes... sorry about that. It's been awhile since I've thought about that stuff ... thanks Scott. In my original post I also should have stated "...coefficient of THERMAL expansion...".

One other thing - the EC-2216 offered on the Amazon link you provided is identical chemically to 2216 B/A, but is packaged specifically for aerospace use. I think it might be a little like buying an aircraft part with a TSO as opposed to an identical product without that. So I'm thinking the price may be greater for the EC-2216 than the 2216 B/A.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Check out 3M 8115. It is probably the most common panel bonding epoxy used in body shops. Extremely strong and will stick to dang near anything. Would guess it is what Bill is referring to. It is some pretty amazing stuff. I keep some on hand for those "how the heck am I gonna fix this now" projects.

I should add that those are non aircraft related projects. Just putting in my plug as a fan of the panel bonding epoxy. I have found it to be like JB weld on steroids in a much larger quantity.

I'd like to add an additional thought/disclaimer to the above statement - I am not necessarily advocating the use of adhesive to bond the wing skins to the spars in conjunction with the normal complement of rivets. People who are a lot smarter than I have done the research, and unless a structure is specifically designed to utilize that kind of bonded joint, the end result can actually be weaker than with properly installed rivets alone. I'd take that possibility very seriously. Also, the 8116 is specifically described by 3M as a non-structural adhesive.I just want to caution anyone who might be considering this to really do the research & proceed at your own risk.

There... now I feel better.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Glad that John is feeling better ! There are aerospace adhesives for bonding aluminum , but the cost , equipment and the right temp/humidity required to make such a thing happen are out of reach of us earth born mortals . Besides if one were to go this route probably some sort of testing would have to be done . Probably a flex to failure type test ? Anyone willing to sacrifice a complete wing for the experiment ? Remember I am right all of the time at least 50% .

Stay with rivets , nut and bolts ! Time tested formula ! Use the goo for cosmetic type construction . Now I feel better !

RB

I was reminded a while back that airplane wings flex. They need to flex. If you work at making them not flex, you will transfer the load to another part of the beast. That part may not be designed for that load. Unless you can do all that calculating, please build it as JT and LS designed.
Cubes

I have been reading the old newsletters. And in a previous newsletter (can't seem to find the # of newsletter right now) there was question posed on possibly bonding the skin to the spar to stop smoking rivets. Of great interest to me was the response form Tom Hunter in newsletter 137, (pg 19) where he stated stated that he in fact "bonded" the skin to the spar with Pro seal, (amazingly as my OP speculated). He went on to say that after 400 hours ( in 2007 more then 9 years ago) he has NOT had any evidence of this page smoking rivets.

Does anyone know where that plane may be and if there is still no evidence of smoking rivets?

Could this really be the solution to the smoking rivet issue? Sure seems like a simple ( albeit messy) solution. If In fact it prevents what appears to be smoking rivets.

TommyCat was sold to a man in Australia. Not sure if he is on this list.
Cubes

I am rebuilding the left wing panel on my standard T-18. The skin was originally secured with monel pop rivets. These rivets are actually stronger than driven aluminum rivets of the same diameter, however, they do not 'fill the hole' the way a driven rivet swells up while being driven. This could allow some skin creep.
When I drilled out the rivets, the ones over the spar mostly spun as soon as I put the drill bit to them. The rivets over the ribs were much tighter and did not spin until the drill was nearly thru the heads. I am planning to use driven flush head rivets over dimpled skin & countersunk spar to try to prevent creep on the spar. It's a bit more work, but cheaper. If I end up using monel pop rivets, I would consider using the Scotch Weld 2216. I would not consider Pro Seal simply because 216 is a newer, more specific application product. Although at the Sedona Fly In Dan Doubleday said he had a positive test-to-destruction experience with it while working in a major aerospace application,
The center section wing is the one with a history of rivet smoking, and mine does have the additional rivets called out after the discovery of the problem. Still, many of these monel rivets have lost paint, which 'might' be an indication of rivet/skin creep.
Gary J is rebuilding his T-18 center section (on a cool steel tube jig) and is talking about .032 skin and adding web stiffeners between the spars? This might be a solution to the skin creep problem too.
On the other hand, the monel rivets might create a galvanic cell with the 2024 aluminum, and the slight corrosion is lifting the paint? I just checked the galvanic series chart, and 2024 T3 aluminum is close to the top, whereas Monel is just below 1/2 way down, so a corrosion potential exists.

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Scott Emery
EAA Chapter 326
T-18 N18TE

I have the S (Suderland convertible) wing. It was a wet wing. I am drilling out the outer panels and reskinning. It appears that the dirt few rivets on the main spar of the outboard panel at least lost paint but don't appear to smoke.

The rivets drilled out a few of them have spun on the ribs. But not a Single rivet spun in the pro seal wet wing area. Those rivets were the easiest to drill and remove.

I have not yet drilled out a single rivet on either the front or rear spars. My logic here is those will be the last rivets it drill out. Simply because I want to get good at and perfect my technique so I have less chance to screw up. And this have to replace the spars. Ribs are relatively cheap to buy and replace. Having to drill out and remove and replace a spar cap, not so much!!!!

The thing I like about pro-seal is that it seems to remain flexible while still sticking tenaciously. It has been proven to not be a problem for corrosion.

I think the cause of the smoke is the high frequency vibration of the skin in the prop wash area. Maybe due to the pulse of the prop wash. Pro-seal I think remains sticky enough to dapen out or. Hangs the harmonic vibration while still allowing some movement/flexibility to take up stress which occurs with the wing "working" and bending under load/turbulence. Something a pure 100% structural adhesive may not allow.

There is a specific application chart for Pro Seal products;
http://www.bergdahl.com/sealant/aerospa ... ant-chart/
Faying surfaces would be the wing skin, spar & rib application column, and I see quite a few products to select from.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE