Spar attach fittings, flox or microballons?

[jrevens]

Hi Jim & Eddie,

Stainless steel rivets and aluminum have been used successfully for many decades in experimental aircraft designs that I know of. However, stainless steel can actually be quite far from aluminum on the galvanic corrosion chart... it depends on if it is "active" or "passive". But as has been said, it is critical to seal the contact area from moisture and/or prime the metal, traditionally with something like zinc chromate - an excellent protective coating for aluminum in contact with dissimilar metals - now considered a health hazard and difficult to get anymore. I have been in the habit of dipping stainless steel rivets in zinc chromate primer before inserting and pulling them on other airplane projects that I've done. Not only for corrosion protection, but to make them a little easier to drill out if it becomes necessary, without spinning in the hole.

I think that perhaps the marine industry uses a lot of micro balloons because of weight and especially ease of sanding or shaping. You're talking about applications where large quantities, percentage wise, of micro balloons are used. Flox filled epoxy can be quite tough and hard to sand. As far as electrically insulating two metal surfaces, the flox is also non-conductive (as is epoxy of course).

I don't think that the tiny amount of micro balloons specified in this application does much to make the epoxy a "good solid gap filling bed" any more than straight epoxy.

[HighWing]

My thinking is much like Johns. I have used “tons” of micro-balloons over the years. Typically used in surface smoothing over fiberglass layups. I was first introduced to it when a neighbor was building a Verieze. Thus was long before my Kitfox days – late 70s. You could always walk down a row of hangars and recognize the ones occupied by the plastic airplane guys. White dust covering the apron out front. They would roll them out front for the sanding, then back in to apply more “micro” in the low spots. In my minds eye, micro-balloons were like miniature glass Christmas tree ornaments – very fragile, thin walled, microscopic glass spheres. This is where the light weight and low structural strength (very easy to sand) comes in. In my mind’s eye, they would easily crush to very fine glass powder under the compressive forces of riveting. I have often thought that something like the milled glass fibers available as a thickening and strengthening agent might work a bit better for the purpose. All this said, however, 4000 plus kits out there and never a whisper of an issue with the recommended technique.

[jiott]

I am wondering if maybe we should be using quite a bit more "micro" in our epoxy mixtures to actually get the thickening squeeze-out resistance. I don't remember the build manual actually saying much about how much to use. We all assume very little is needed, but maybe more is better. I had quite a bit of microballoons left over when my project was done. This is all academic talk because as Lowell said, 4000 kits and not a whisper of any issues. It hasn't been mentioned, but I think the powder coating on the spar fittings has as much to do with dissimilar metal separation and the lack of corrosion issues as anything. When you rough up a powder coated surface for bonding I think it is important NOT to get down to bare metal in any spots.

[David47]

?...Any structural guys out there with a good working knowledge of this issue? I'd like to hear if I'm off-base about this. BTW, I did use micro on my build, and I'm certainly not saying anyone shouldn't based on these thoughts, but I'm curious about the whole thing.

Yes John, I'll put my hand up as an aircraft structures design guy with 40+ years as one ... and you're right. The use of micro balloons has not a lot to do with corrosion protection. It's a bulking and filling measure. As Lowell suggests, If you want some strength, add chopped fibre. If you want more strength, use epoxy and directional tapes or fabric. S/S fasteners contacting the sides of the hole when the rivet expands will most likely have a very thin layer of adhesive between fastener and hole sides anyway, which is a good thing.

Using S/S rivets in aluminium has been the practice in home built aircraft for many years. As Jim said, the two materials are reasonably close on the galvanic corrosion chart, but having said that, I've wet installed all my rivets using an epoxy primer. Just a personal preference. For corrosion to start, you need as a minimum, moisture and a potential difference between the two metals to start pushing ions around. In this case, the adhesive acts as the barrier between the two, similar to the use of a fibreglass ply between a carbon skin and an aluminium alloy rib, spar, frame or longeron. So I'm not surprised that there haven't been corrosion issues in the steel spar doublers to aluminium tube joints. But as a further precaution for my build, I've also heavily coated this area with primer just to make sure I don't get moisture ingress. Again, a personal preference. .....

[Mikerv9a]

Getting ready to install the spar attach fittings (doublers) and the strut attach fitting. The instructions say to use a little bit of micro balloons. Is flox ok to use in this case, that's what I've been using in other mixtures, I have that, but don't have any micro balloons.
Thanks
Brian

As a builder of a Long-EZ, I can tell you that microballoons is ONLY used for fill and joining pieces of foam together. It has no structural/tear strength and is subject to cracking . Flox is flocked cotton and is only used to create rigid structural joints. It cures as hard as solid epoxy. I don’t buy that flox is not “compatible” with dissimilar metals, it’s just epoxy that is saturated with the flox to keep it from flowing out of the space where it’s used.
If this is a structural bond you’re making, I would NOT use microballons.

[jrevens]

As a builder of a Long-EZ, I can tell you that microballoons is ONLY used for fill and joining pieces of foam together. It has no structural/tear strength and is subject to cracking . Flox is flocked cotton and is only used to create rigid structural joints. It cures as hard as solid epoxy. I don’t buy that flox is not “compatible” with dissimilar metals, it’s just epoxy that is saturated with the flox to keep it from flowing out of the space where it’s used.
If this is a structural bond you’re making, I would NOT use microballons.

Thanks for confirming what I wrote and thought, Mike. The fittings in question are attached with a lot of rivets, and the epoxy should add additional strength to the joint, so why not make the joint as strong as possible and not decrease the strength with micro, while still sealing the contact area between the dissimilar metals from air and moisture? My original comments were just voicing what have been my thoughts from the beginning of my Kitfox project. The original idea to use micro-balloons might have been advanced by someone with incorrect or incomplete knowledge of the structural issues involved. Obviously, history has shown those joints, assembled that way, have been safe and strong enough. I don't mean to insult or disparage anyone's intelligence, but IF there is a better way to do something, I think we all would like to know about it.

[Guy Buchanan]

Adding microballoons to epoxy weakens it. However, sometimes tiny glass beads are added to epoxy for bond line thickness control. If your perfect assembly squeezes out all the epoxy, you have no adhesive bond, and therefore no strength. The glass beads maintain a .005" thickness minimum bond thickness thereby enabling epoxy adhesion. If your epoxy is being used to simply keep out water and thereby inhibit corrosion then adding microballoons will lighten things a little and maybe help keep the epoxy in place by thickening it a little. Strength is irrelevant.

Regarding corrosion. Most anyone who has worked on ocean racing yachts can attest that aluminum hates stainless and will, over time, turn to powder under the affects of galvanic corrosion. However it's a fair amount of time and requires much more salt water than I hope most of our wings are seeing. Anyone buying an aircraft that's spent a major part of its life on floats on salt water would be well advised to boroscope the wings though, to make sure there's no evidence of corrosion.

[jrevens]

Adding microballoons to epoxy weakens it. However, sometimes tiny glass beads are added to epoxy for bond line thickness control. If your perfect assembly squeezes out all the epoxy, you have no adhesive bond, and therefore no strength. The glass beads maintain a .005" thickness minimum bond thickness thereby enabling epoxy adhesion. If your epoxy is being used to simply keep out water and thereby inhibit corrosion then adding microballoons will lighten things a little and maybe help keep the epoxy in place by thickening it a little. Strength is irrelevant...

Good comments, Guy. My parting thoughts are these... based on several things, including the tiny quantity of micro I got in my kit and the lack of any real accurate guidance as far as mixing ratio with the epoxy, I think that the probability is slim that anything really beneficial, including maintaining separation, weight reduction, or thickening the epoxy, is being accomplished in this instance, and therefore it may be a waste of time and possibly a strength reducing one at that. If the joint has been designed for adequate strength with rivets alone, then strength of the epoxy is irrelevant.

[jrevens]

Yes John, I'll put my hand up as an aircraft structures design guy with 40+ years as one ... and you're right. The use of micro balloons has not a lot to do with corrosion protection. It's a bulking and filling measure. As Lowell suggests, If you want some strength, add chopped fibre. If you want more strength, use epoxy and directional tapes or fabric. S/S fasteners contacting the sides of the hole when the rivet expands will most likely have a very thin layer of adhesive between fastener and hole sides anyway, which is a good thing. ...

...

Good comments... thank you, David!

[ken nougaret]

John, you mentioned "IF" the design was strong enough for rivets alone .... Does anyone know if tests have been done based on only rivets only?