Glue "channel" instead of "rivets" for balsa fin roots?

[DaveCombs]

An idea just popped into my head (and that alone should make you nervous)...

People sometimes use glue "rivets" - holes poked perpendicular to the root edge of a fin - for extra strength on balsa fins. But has anyone tried this: Instead of poking holes, take your X-Acto blade and cut a shallow slot along the root edge (again, perpendicular to the mounting surface) and shoot some glue up in there to help with the joint?

Good / bad / indifferent / unnecessary?

 

[powderburner]

Poking a few small holes along the length of the fin root is not going to significantly affect the strength of the BT material. However, if we are talking about Estes-style paper/cardboard BT, I think making a lengthwise slice would be enough to weaken the aft BT, especially if cut deep enough to be of any use at getting glue penetration. The fins might be attached better (or, they might not) but the aft BT might fall apart inflight.

If you lightly scuff-sand the BT zone under the fin, and make your fins with a square root cross section (and a good *dry* fit against the local BT), and properly use a decent white or yellow glue, you will not need glue rivets or channels for most any low power designs.

If you are nervous about having adequate fin attachment strength, like for very high performance low power rockets (on up into mid and high power designs) then it's time to change over to TTW construction, externally reinforced roots (fiberglass, etc), or other high power construction methods.

I would vote for "unnecessary"... but keep those ideas poppin'

 

[DaveCombs]

The cut is made in the fin material - not the BT. I do routinely sand BTs to get the glassine coating off.

The question was more of a curiosity than anything, really.

 

[powderburner]

It just didn't occur to me that you were talking about the balsa side of the joint. Wow, that would be some intricate knife-work! (I don't know if my old eyeballs could even see to cut a groove in 1/16 balsa)

Even if cut out of the fin material, I am not sure that you would get any better penetration of glue into the balsa fin root. In fact, it might be worse; if you prep fins in the "standard" fashion (sanding the roots square) the balsa grain would tend to be a bit more loose or open, and probably better able to absorb some water-based glue. If you use an Xacto-style blade to cut a groove in the fin root, I think the blade would tend to crush the wood fibers a bit more, and would effectively close off some of the balsa's internal fibrous structure and would reduce glue penetration.

I still think that a well-fitted (dry) joint is going to be your best bet. If the fin is really really thick and the BT is fairly small, you might want to wrap the BT with sandpaper (facing out) and shape the fin root with a small "dished" surface to get a better dry fit.

Have you tried using a small paintbrush to pick up a *few* drops of water, and moistening the balsa fin root before you apply glue? That's another way to get better glue penetration.

 

[Buzzard]

I have used a similar technique on the fins when I know that there is going to be a decent fillet of glue on the fin joint. I take off the glassine (usually I cut and peel rather than sand) and make crosscuts with a razorsaw on the fin root. I have used this with balsa, ply, and G10. Since the 1/16th or so slots will be covered by the fillet, it adds some strength, especially for thin fin materials. I have cut slits in balsa fins both parallel to the fin root and across the fin root.

Chas

 

[RimfireJim]

Something to think about is the failure mode that you're trying to prevent. Bonded joint failures can be of any of the following: substrate A, substrate B, adhesive surface A, adhesive surface B, cohesive, or a combination of any of these. A substrate failure is where the material itself fails before the glue fails. An adhesive failure is where the bond breaks between the adhesive and the substrate, and a cohesive failure is where the bond breaks within the glue material. The weak link in this chain will determine the strength of the joint, and beefing up an aspect that is not the weakest link won't increase the overall strength.

My experience with balsa to paper tube glue joint failures is that most of them have been substrate failures of the paper, not of the balsa, though sometimes a fraction of the joint is a balsa substrate failure. A few times I've gotten an adhesive failure at the paper/glue interface, probably because I didn't remove the glassine layer properly. I can't recall seeing a balsa/glue adhesive failure. I attribute that to balsa being so porous that the glue has no trouble penetrating and getting a good bond.

So, IMHO, "unnecessary".

 

[bigone5500]

I've flown lots of rockets with and without sanding the glassine and I use (if it really counts) Titebond II. If the fins have broken off, for whatever reason, the body tube has failed to hold together (torn paper). I have never had an adhesive failure. In my opinion, the real strength comes from correctly applied fin/tube fillets.

 

[jj94]

I can't see why the channel method wouldn't work. Try it out on your next build.

As for sanding glassine off, I find that it really does help. Bigone mentioned that the fillets are what really matters, but IMO, fillets aren't as strong when they are bonded to glassine instead of scuffed up paper. I sand glassine off, not for better fin root adhesion, but for a better surface for the fillets. It really helps for me.

 

[Micromeister]

I agree with JJ:
I rarely bother doing much sanding on the body tubes other then maybe run 240 glit stick along the fin lines before attachment.

as for the balsa fin channel. If I were going to go to that much trouble i'd insert a sliver of 1/64th 3ply plywood in the slit and to a thru the wall fin joint. Point is: for LPR even most BP powered MPR models it's just not necessary.
I think DaveComb is operation under a misconception about the epoxy or Glue rivet method:
Dave the Holes have to be in both the balsa and BT for the method to work properly. Actually you should have 3 rows staggeted along the BT/fin line one down the center, the other two along the OUTSIDE of the fin line on both sides of the fins. These two lines are riveted, and fillet smoothed after you fins are attached
This is my preferred method for attaching Lexan fins to cardboard or other material body Odd-Rocs. LOT of flights. not a single fin failure.....even on Hard landings.

 

[Bigander]

I could see it making it worse. Your making more surface area but moving it away from the surface that your gluing to, gaps make an easier failure. I've never glue riveted, if your joint is stronger than your substrate as it is, what's the point?

 

[bigone5500]

I can't see why the channel method wouldn't work. Try it out on your next build.

As for sanding glassine off, I find that it really does help. Bigone mentioned that the fillets are what really matters, but IMO, fillets aren't as strong when they are bonded to glassine instead of scuffed up paper. I sand glassine off,...

You are correct, I should have mentioned that. I do sand the glassine layer with 320grit.