Spar attach fittings, flox or microballons?

[jiott]

I've heard they are filled with Helium for major weight reduction-use liberally

[southwind32]

having never used Micro balloons, and failing to ask Debra, how much of the micro balloons do you use? Are there instructions on the bottle?

[aviator79]

I'm getting an Interior!

I'm getting a whole plane! (Some assembly required.)

[t j]

having never used Micro balloons, and failing to ask Debra, how much of the micro balloons do you use? Are there instructions on the bottle?

I don't have my manual any more but it seems it was about a tea spoon or so was enough in the hysol mixture for one spar attach bracket.

On another note, they are tinny, light, and like micro ball bearings. Be very careful not to spill any. Also don't sneeze towards them. They will fly every where. It's difficult to gather them back up.

Don't get them near your nose or mouth. They are easily inhaled and quite harmful if inhaled.

[efwd]

The qty of the microballoons to hysol is not specified in the manual. You use only enough to provide for a layer of balloons to remain between the two dissimilar metals. It is very little. The previous stated qty would be sufficient. There is no information on the jar you will receive.
Eddie

[southwind32]

Ok, thanks for the info. I think when I get them, I'll take 1 micro balloon and mic it, then figure the area of the spar attach, and multiply it out to see how many micro balloons I need to count out to form a layer, and then maybe add 1 or 2 extra to be on the safe side............NOT !!
About a teaspoon sound better

Thanks again.
Brian

[efwd]

LOL, had me scared for a second.

[jrevens]

Things change over the years of course, but what I got in my kit was a small zip lock bag with a very small quantity of micro balloons - maybe a teaspoon. I happened to have a big half-gallon can of the stuff from my model airplane days, and so I used a little of that too. I've thought a bit about the whole theory of using them for separation between dissimilar metals as Kitfox suggests, and my personal analysis and opinion is that it is probably not really the best idea. Again, this is my opinion only. First of all, when they are added to the epoxy mix they decrease the strength of the bond... probably an insignificant amount considering the small quantity used, but there all the same. Next, in order to have corrosion you need moisture. If the fittings and spar, for instance, are clean and dry when assembled properly with the epoxy, you will have a very nice air & water tight coupling at the contact surfaces, and there will be a thin layer of epoxy between the majority of the surface areas with or without micro balloons anyway. Those surfaces are not a perfect fit and a thin layer of epoxy is necessary to form a strong bond anyway - you don't and can't squeeze it all out even if you wanted to (and you don't). So, what about the little places of point contact that will be there between the two surfaces? They will be there with or without micro balloons in the mix - the balloons slip & slide with the epoxy and move out of the way to a certain extent. Lastly, what about all the S.S. rivets going into the aluminum spar... do you think there will be a layer of micro balloons between the surfaces of the expanded rivets and the aluminum? I don't, and those holes and rivets are all over the fittings. Those rivets are really making very good metal to metal contact when they are expanded. So, even if you had this "perfect' layer of micro & epoxy between the faying surfaces, you still have these rivets which would really cause concentrated points where electro-chemical corrosion could occur. Check your spar attach fittings to spars with an ohmmeter and I think you'll see that there is all kinds of metal to metal contact. I could be very wrong, but that's how I see it now. 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.

[efwd]

Good point John. You brought up a point that I have been pondering myself. What about those Stainless rivets in aluminum structures?
Eddie

[jiott]

Interesting points to ponder John. Based on what I have read in the marine industry, epoxy and microballoon mixtures are commonly recommended for a bedding in attaching fittings to somewhat uneven surfaces. They claim it thickens the epoxy so it is less likely to be squeezed out, and also some sources claim it actually strengthens the bond. Based on this, I believe the benefit it provides on our spar fittings is more of a good solid gap filling bed that also provides dissimilar metal separation due to squeeze-out resistance, rather than a separation of only a microballoon diameter. Will cotton flox do the same thing we wonder? I don't know, except I do know that the marine industry definitely specifies microballoons rather than flox. Flox seems to be specified mostly for sag resistance rather than separation in my research.

Regarding SS rivets in aluminum and steel, none of these materials are too far separated on the galvanic corrosion chart, meaning the corrosion activity is not too active, and as you say, without an electrolyte (moisture) the activity potential is much reduced.