Thorp Air Command - T18.net

You'll notice that all of these products are specifically referred to as fuel tank sealants. They are not designed to be structural adhesives. What is the elasticity? What is the shear strength? How thin will the sealant layer be when you get it squeezed down and riveted? There are people who are much smarter than I am who have published papers/reports warning about the use of sealants on structural joints specifically designed to be riveted. Wil Taylor is one such individual. One problem comes about with having even a relatively thin and flexible layer of "sealant" between two riveted surfaces, preventing intimate metal-to-metal contact, compromising strength for a number of reasons. I've been playing the devil's advocate here, because I truly believe that there seems to be some enthusiasm for using Pro Seal for a purpose for which it was not designed, on a joint that was not designed to be bonded with such a material. My sole purpose in this ongoing conversation is strictly to share knowledge... I have no dog in this hunt other than that.

Concerning "Tommy Cat", if true, this is the first I've heard of Tom's use of sealant on those joints. It would be very interesting to know if it is true, and also how everything is doing after how many specific hours. Consider that, perhaps, sealant might mask one of the "symptoms" of smoking rivets. Strength might be compromised, but you wouldn't have those smokin' rivets to look at. Tom is gone now, and I don't wish to be disrespectful, but this is a serious subject IMHO, and I think that even a craftsman like Tom could do some things that might not be acceptable (like all of us?) to someone else, like for instance to the designer of this airplane. I have corresponded with a friend who is one of the guys who unloaded Tom's airplane when it arrived in Australia for it's new owner. In his words - "When I pulled the Tommy cat out of the container I checked it over .. Tom Hunter forgot to fit the rudder stop. Also all the control rod ball joints < rose joints were cheap car application type with nylon bearings . I couldn't believe my eyes. The rest of the aircraft was very well built". Forgot to fit the rudder stop?! I don't really know what to make of that. If the new owner participates in this forum, it would be interesting at least to get his input. It may not be a good idea to use that specific airplane as a successful example of using Pro Seal on the skin to spar joints. Or then again, it might. Who knows?

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

BTW - The owner of Tom Hunter's Thorp is now Kevin Peatt of Melbourne, Australia. Jim Peran (J.P.) in OZ told me that "The aircraft has been very well looked after."

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

Well the newsletter note was written by all accounts to be from Tom Hunter himself and reproduced into the newsletter by Roy Ferris. And Tom stated that he in fact put the pro-seal on the spar surface. And in specific reference to the issue of the smoking rivets. I have to take that as being pretty definitive as to the fact that it was in fact the way his airplane was constructed unless proven otherwise.

Is there any way that we can find that current owner and see how many hours are on it and how those rivets stood up to the test of time since the last report 9 years ago?

I agree that one example is not statistically significant. But we also have multiple consistent reports of the rivets appearing to have some sort of movement. And at least one case that they are not moving in which the construction technique was the use of pro-seal and apparently cured that issue. What does that mean? Well I don't know. But a further history from I think 9 more years of history of this would be at least interesting.

I do not think that the pro-seal is really an adhesive. I and this is my own OPINION is that it would be an isolating or dampening effect. In essence a cushion to reduce the vibration of the skin.

If the skin is vibrating or working, then that would indicate that there is no skin to spar strength by the contact of metal to metal. So I'm not sure that is an issue of loss of strength as a result of sticking the skin to the spar versus just friction grip of metal to metal contact. The risk may be the exact opposite. That the seal would ADD strength and prevent movement that is necessary to be allowed to occur.

All I know is that when I tried to pull the inspection covers off the bottom of the wet wing that were coated and stuck together in addition to the nutplates and screws that held the plate in place. It took considerable effort to remove the plate from the ring. Were they structurally adhered together? I would not call it that any more than calk would "adhere" one piece to another as any sealant would or could do. Granted the tenacity of the "stick" was much higher with the pro-seal than calk. But were they essentially welded together? No They were meant to be able to come apart. Something that is structurally adhered together is done with a boding agent that has no intent to be taken apart.

The question in my mind is whether or not it is important for the skin to be allowed to flex or move independent of the spar or not is important or even necessary. And the follow on question, is whether something like Pro-Seal used in that area would prohibit the necessary movement or not. In other words, if some movement is necessary. How much is needed?

I do not know the answers or really even how to find out the answers to these questions. I'm just trying to continue the dialog.

It seems like the rivet "issue" (I won't call it a problem because so far it has not proven itself to be a problem over many decades) has been a long standing point of discussion and many different attempts to eliminate the issue. And in reading the old newsletters one attempt at a different attempt to eliminate it MAY have resulted in a solution. A solution that many have tried do solve. Even to the extent of drilling more holes into the spar and putting in more rivets which over time has also proven to not solve the issue. But we have what appears to be a POTENTIAL relatively easily application that may eliminate the issue. And I just wonder how to investigate the merit of whether or not the addition of something like Pro-Seal is viable or potentially dangerous. Drilling extra holes in the spar to nail the skin down to the spar would seem to me to be a far more extreme structural manipulation than adding pro-seal between the spar and skin. But I may be completely wrong. yes I know filling a hole with a properly installed rivet (IN COMPRESSION) is "as if" there was never a hole drilled at all. In tension (negative G) that is NOT the case. And just logically, physically drilling a hole just doesn't make sense that it is "the same as" if there was never a hole there at all.

I think the goal of this site and the community of Thorpers is to help sustain and even improve the aircraft and refine the breed.

All aircraft, even the very best designed aircraft in the world (which I think the Thorp is one of the best designed aircraft and certainly home built aircraft in the world!) have weaknesses. While we all want to promote the breed. I do not think we should shy away from acknowledging any shortcoming of the aircraft. And we owe it to all, to do our best to try to identify and if possible fix, mitigate or minimize the potential for any shortcomings to the extent possible.

IF the issue of the rivets smoking, or flaking off the paint can be solved. Why shouldn't we look for that solution? But, we have to weigh if the cure is worse than the disease!

IF it is flexing and all that is needed is to change the frequency or something to the prop wash pulse or other cause. Then something short of sticking the skin to the spar might work as well. Maybe simply "sealing" the top skin surface with Pro-Seal as if those bays were to be wetted may change the harmonic frequency and solve the issue without the need to adhere the skin to the spar.

Does anyone know if the rivet issue is also common regardless of the thicknesses of skins (e.g. 0.032 and 0.025)? If it is a harmonic issue, it would seem to me that if the phenomena occurred on both, as well as with and without a 2nd row of rivets, it would seem to rule harmonic vibration as the cause. Because I would highly doubt that the harmonic frequency of the two thicknesses would fall into the same frequency range to be the cause. But surely there IS a cause. To this point, it appears that no one has definitively determined the cause.

It just seems like the issue seems to occur in the approximate area about the width or diameter of the propeller. Which would seem to point to one possibility to be the pulse of the propeller.

Are there other aircraft designs that movement between the skin and the spar is not only known but required? Maybe there are, but I am not aware of any. Yes we have seen smoking rivets in other aircraft. And in most all cases when seen it is detected, an attempt is made to rectify the situation if it can be fixed.

Again I'm not trying to start an argument here. I'm just trying to determine if the situation can be alleviated. For people who already have flying aircraft or their wings are already built, it is a non issue. But for people still in the building process or re-building process, if the issue can be resolved, why not incorporate the solution?

The dilemma seems to be that a significant analysis of vibration and flex and structure needs to be done in order to have a possibly reach a definitive answer. Which unfortunately is unlikely to occur. Unless we can find an aeronautical engineer/guru to study the issue pro-Bono, and conduct the analysis. Which again I suspect the odds of that are somewhere between slim and none.

The second best option is if a brief analysis of whether adding something like pro-seal between the spar and skin will "do no harm". If that could be determined at least making the experiment would only result in a little added weight with the benefit of MAYBE eliminating the rivet issue.

The risk is that by using pro-seal (or some other sticky material) between the spar and skin shifts load/stress to some other point and is worse than doing nothing and dealing with the cosmetic flaking of paint and trail of aluminum dust. Surely NO ONE wants to risk a worse or even dangerous situation!

I hope I'm not stepping on any toes. I'm just asking questions and trying to learn. Which is I think what EAA and more specifically this forum with regard to Thorps is all about.

The default position is to not install "extra" rivets, nor pro-seal or any other adhesive as results of aircraft with multiple thousands of hours with the "smoking" rivet as being nothing but a cosmetic nuisance seems to be proven. But if the cosmetic nuisance can be eliminated safely. It would seem like the thing to do. I'm just trying to determine if there is a reasonable way to find out if adding something like pro-seal can be done safely. If that cannot be reasonably proven. Then sticking with the original design is the only real answer and living with the flaking paint. God knows I've dealt with a lot of worse nuisances in my life than having to touch up paint a few rivets every now and then!

Tom,
I'm not an aero engineer. But I do know that stiffening a structure more than the designer intended is worse than leaving it alone. It brings all sorts of unintended consequences into play, including flutter, flight control effectiveness etc. There would have to be a complete airframe analysis done to convince me that pro seal is ok to use. I'm not going to pay for that analysis. The more I deal with this plane, the more I have come to respect JT for what he designed. I do not support these type of mods. However, it is an experimental. You can do what you want with it because it is your butt on the line.
Cubes

Would adding a 2nd row of rivets constituit a stiffening of the structure? Of course JT himself as I understand it was OK with that option. Which to my understanding was not effective.

Like I said. The default position is to leave things alone as the cosmetic issue has proven to be anything more than that. Cosmetic.

There is a group at KVIS that would use old Pro- seal on each rivet, not as an adhesive and also not as a sealant. The old out of date pro-seal would fill the very small gap around the top of the Rivet where it met the skin, effectively making it smoother when scotch briting after the Pro-goop got stiff. This also kept the paint from cracking around the rivet as the wing flexed the way it is supposed to. It looked real nice and after the Prime coat the rivets were all but hidden, but it was not used as an adhesive,just a hairline gap filler to make paint prep easier. Some of them had mighty fine Paint jobs that were show stoppers and that is Fantastic!! But I got into the air 5-8 yrs before they did doing the standard Rivet process, etching ,sealing and then painting (if what I done counts as a paint job) at least it will not corrode. Use the Rivets, leave the glue in the sack with the Revel Model plane it works good for that. Dan

Well making the rivets "disappear" with the paint finish may look awesome. But when trying to do a repair. It makes it difficult to find the rivet and drill it out. I can say that I was very happy to see all the rivets and especially if I could still see the little rivet head center mark. As I could then center punch that mark and know that I'm drilling the rivet shaft center. If the entire rivet is invisible, then a whole lot of paint in the general area of each rivet has to be removed. At least by seeing the rivet head itself, I only had to take a punch and chip the paint off the rivet head to in most cases reveal the center head manufactured center mark. Maybe that is not a big deal with an invisible head because if the damage was such that drilling out rivets was necessary. You'll probably be repainting the area anyhow.

I often wonder if the cracking of the paint around every rivet is simply from the impossibility of removing all the sanding dust in the crack between the dimpled skin and the rivet head. This area is so small it is nearly impossible to completely ensure the crack is absolutely clean and prepared for paint. it also is hard for the paint to not bridge at the very vertex of the crack and thus the strength is only as good as the paint surface tension when dried. This ultimately causes poor paint adherence around the circumference of the rivet head. Ultimately leading to the paint flaking off.

I'm not sure if dipping or coating each rivet with epoxy primer before setting the rivet would be enough. The use of pro-seal I can see how that would work. As long as paint can remain adhered to the hardened pro-seal. But it would be an extremely messy job. I'm not sure it is worth the mess to coat each rivet head. But I can sure see how that could work to minimize or prevent the paint cracking around the rivet head.

For those inclined to glue the wing panels, the auto industry used panel bond to glue body panels . Its stronger than a weld.

I did a little work drilling out rivets. On the damaged wings I have. I noticed that there are places where the bent metal (caused from the wind damage and flipping over of the source aircraft wings I know have) caused the skin to rip away from the spar. And all of the rivets are still in place and still seem tight, yet the rivet head pulled through the skin.

Which indicates that the rivets are STRONG and that they are not the weak link. At least in my mind.

So if the rivets are so tight that the skin will rip away first, what is causing the rivets to appear to smoke or whatever? Something is causing it. Yet even adding additional row of rivets which would seem to result in a much greater grip and pressure to hold the skin down to the spar has not prevented the phenomena. It just is a mystery I guess.