Lost a fin: repair and prevention: glue rivets?

My newly finished Astron Elliptic II had a rough first day of flying. On the third flight one of the fins detached, like so:


I have to say this was a surprise to me; the fins were attached pretty well I thought, and this ticket only flies on A engines. It looks like the glue held but the surface layer of the BT was pulled off with the fin.

So, how to repair? And how to prevent in the future?

First, repair: should I just glue the whole thing back on? There's nice porous exposed paper on the fin and the BT, so it should glue well, but I don't know if I should be trimming the old fillets off the fin and basically starting again.

Also, for repair *and* prevention, I'm thinking this is what glue rivets are for. But although I've seen them mentioned a lot, I've not seen any detail on how to do them. How far apart should the holes be? How large? What is the gluing procedure?

Would it be reasonable to use some glue rivets in making this repair?

I usually just glue it back on. Not saying that is the best, but that's what I do.

Oh that's to bad, she was a nice looking rocket.
I don't think you'll ever get it back on as nice as it was with out redoing the whole (most of) rocket again, I'd just glue it on as is.

I've had similar happen, I'd say scuff it up a bit on either piece and wood glue it back together. Not the strongest option, bit it'll preserve some of the aesthetics at least. The astron elliptic is one of my favorites, tough little rocket- I think mine has 25+ flights on it, good luck with the repair!

Nate

Put it back together with yellow wood glue. Usually I like to really scuff the area under the fin, all the way through the top glassine layer, that way the glue soaks into the tube a bit more. If you notice the glue only really bonded on the section with the paper stuck to it. The trailing edge landing first allow the fin to be popped up and away. Most times I like to mark out where the fin root will be and use an Xacto knife to cut the glassine totally away from the area beneath the fin and part of the fillet.

neil_w said:

My newly finished Astronaut Elliptic II had a rough first day of flying. On the third flight on of the fins detached, like so:
View attachment 277889View attachment 277890

I have to say this was a surprise to me; the fins were attached pretty well I thought, and this ticket only flies on A engines. It looks like the glue held but the surface layer of the BT was pulled off with the fin.

So, how to repair? And how to prevent in the future?

First, repair: should I just glue the whole thing back on? There's nice porous exposed paper on the fin and the BT, so it should glue well, but I don't know if I should be trimming the old fillets off the fin and basically starting again.

Also, for repair *and* prevention, I'm thinking this is what glue rivets are for. But although I've seen them mentioned a lot, I've not seen any detail on how to do them. How far apart should the holes be? How large? What is the gluing procedure?

Would it be reasonable to use some glue rivets in making this repair?

Click to expand...

I agree with most of the other posters here for repairing this model. If it is a minimum diameter model (Epoxy Rivet Method) is not a good idea as the internal rivet heads would block proper motor installation.
Looking at your model it does not appear the fins have fillets? If this is the case, adding fillets to both sides of each fin will greatly increase the ability of this models fins to absorb hard landings. Fillets will not stop all fin breaking on landing but will greatly decrease the frequency.

Here's how I'd go about repairing this models broken fin. First with either a spare piece of the same OD body tube with a solid internal mandrel or empty motor casing I wrap (single wrap) 240grit sandpaper around the body then carefully sand off the rough pieces of body-tube and excess paint maintaining the curvature of the body tube on the root edge of the fin. If using the model itself for this process carefully tape the sandpaper between the two remaining fins. Once you have the fin sanded to your liking. Gently sand the body tube area where the fin broke off. Try to get a smooth raw cardboard area without removing too much paint outside the immediate damaged area. Using a small angle or channel re-mark the fin line on the body tube. Run a good line of White or Yellow Carpenters glue on the fin root edge. Touch it along the alignment line, separate the two pieces and allow the glue to dry. Once this first glue is dry to the touch apply another decent glue line to the fin root, press the fin to the body tube along the alignment line previously glued. Allow the fin to dry completely. now apply a good bead of glue along both sides of the fin, Using a finger tip whip this wet glue into a radius and allow to dry, Repeat if necessary. I've found Yellow Carpenters glue works well for fillets. If you want a bit more flexability try Devcon 5 or 30 minute epoxy for these fillets. To form the Radii (wearing a rubber glove) dip your finger in Rubbing alcohol before running the fin/body line. Hope this helps a little.

Astronaut Elliptic II

Click to expand...

Curse you autocorrect!

Micromeister said:

I agree with most of the other posters here for repairing this model. If it is a minimum diameter model (Epoxy Rivet Method) is not a good idea as the internal rivet heads would block proper motor installation.

Click to expand...

It's not minimum diameter, it's a 13 mm mount inside a BT20. The fins are 1/16".

I'm still trying to understand how one uses glue rivets on a LPR model with wood glue. Is it written up in detail anywhere? I haven't been able to find it yet.

Looking at your model it does not appear the fins have fillets? If this is the case, adding fillets to both sides of each fin will greatly increase the ability of this models fins to absorb hard landings. Fillets will not stop all fin breaking on landing but will greatly decrease the frequency.

Click to expand...

There are fillets, and they're still attached to the detached fin. For the remaining fins, the fillets are kind of in the shadows, hard to see in the photo I posted. Maybe I should be making them bigger, but as far as I can tell the issue here was that the combined fin/fillet assembly pulled the surface layer of the BT clean off. That is why I thought glue rivets might be a solution to this, since they'd extend the attachment all the way through the tube rather than just on the surface.

Glue rivets are nothing more than poking holes in the airframe under the fin root and/or the fillet area which allows the glue to push through the holes. Good gluing technique is stronger than the two materials without resorting to the "rivets".

Gluing it back with yellow glue would be strong (I have done this effectively before). Especially if you put a thin layer on both surfaces and let them air dry 1-2 minutes before pressing them together.

Alternatively, glue rivets using Gorilla Glue (GG) could work (Full disclosure, I have never done this before). Be sure to squirt a small amount into the hole and lightly dampen your surfaces (water cured). The GG will expand inside the BT and create a "rivet head". Make sure to keep pressure on the fin until it is cured as GG likes to expand, so you don't want it pushing the fin away from the BT when it expands.

Hi,
This was discussed towards the end of this thread, with the "rivets" described as "pinholes."

https://www.rocketryforum.com/showt...h-pinhole-method&highlight=big+dog+glue+rivet

The TL;DR summary is, stagger the pinholes (or alternate) across the center line, with about 1/4" spacing between holes.

I was doing this for a Big Dog, medium power rocket, for a smaller rocket it would be overbuilding (no such thing?) but could be a good learning experience. I would definitely try using fillets on your fins, but if stuff keeps busting off, you may want to "up your game" and try pinholes/rivets.

Ah, that is the thread I was looking for, thanks. I'll probably try some pinholes on this repair.

That whole thread actually has some great info and discussion of techniques.

It also starts off with the classic line "I've been using yellow glues and the double glue method on my LPR birds since the Dead Sea was only sick."

Well you have some good advice on the repair but I would add that sometimes a design element (fins with a relatively long span) can defeat the best laid plans. The Estes Goblin comes to mind as a model with the tendency to lose a fin on landing (at least for me :blush. For this particular repair, I would clean up that fin root and go with small epoxy fillets, as Micromeister suggested. :2:

did you paint or prime the body tube then glue on the fins?

Primed first, but the root areas were masked off, so the glue joint (double-glued) was to the raw tube. It was a white craft tube; I don't think in this case that I particularly roughed up the tube before gluing.

Again, it seems as if the glue held to the tube, it's just that the outer layer of the tube came off with the fin. I'll have to look at it more closely and see if that was true down at the bottom of the fin where the detachment may likely started.

neil_w said:

Curse you autocorrect!


It's not minimum diameter, it's a 13 mm mount inside a BT20. The fins are 1/16".

I'm still trying to understand how one uses glue rivets on a LPR model with wood glue. Is it written up in detail anywhere? I haven't been able to find it yet..

Click to expand...

OK! if the model is not minimum diameter then "Epoxy Rivet" attachment could possibly help a bit.
The Epoxy Rivet Method works because dried epoxy while strong also has a bit of flexibility that dried Glues (white, yellow or what-have-you) do not.
The process is fairly straight forward. First draw an alignment line where you will center your fin, on each side of this line within about 1/16" drill or punch 1/32" holes alternating down each side of the center line. If the fin has enough thickness one can also drill or punch tiny holes in the root edge of the fins. Along the bottom edge of the fin about 1/16" up from the root edge tiny holes are drilled through the thickness of the fin. After all this is done using a good 2 part epoxy (Devcon 5 or 30 miunte) epoxy is applied to the root edge and pressed into place centered on the line with the epoxy squeezed into the alternating body tube holes. Once cured the epoxy fillet is applied filling the side holes completely which will not show at all when finished. Below are a couple pictures of this method used to attach Lexan fins to cardboard body tubes. These may make it a bit easiler to visualize.

For small and mini rockets, I've used superglue/CA on breaks like this with good result since you're only bonding the lost bt layer back on. It also dries clear, so the repair isn't so noticeable. YMMV.

Using the glue rivet method with wood glue is certainly better than no rivets at all.

Even with decent fillets, the typical surface mounted fin failure involves some delamination of the body tube when the fin is knocked off. Glue rivets of any kind add another set of mechanical fastening through the paper layers of the tube.

tab28682 said:

Using the glue rivet method with wood glue is certainly better than no rivets at all.

Even with decent fillets, the typical surface mounted fin failure involves some delamination of the body tube when the fin is knocked off. Glue rivets of any kind add another set of mechanical fastening through the paper layers of the tube.

Click to expand...

I hate to disagree Tom:
But using any type White or yellow Carpenters Glue is almost like not using any rivet at all. Yes, they may help a little in preventing some delamination of the Craft-tube but because these glues have not structural sheer strength they simply snap at the fin root along with the fillet which will still tear up some cardboard outside the rivet line(s).
Using pin holes rather then drilled 1/32" or larger holes makes it even worse as these very thin "glue rivets" have even less sheer/ shock strength.

Again the reason the "Epoxy Rivet" method works so well is that most 2-part epoxies retain some flexibility for a very long time after curing, allowing the joints and fillets to move a little with the fin material. The same can not be said for any glue, especially all Wood glues and CA's. They just become brittle to any shock load placed on them.

Micromeister said:

I hate to disagree Tom:
But using any type White or yellow Carpenters Glue is almost like not using any rivet at all. Yes, they may help a little in preventing some delamination of the Craft-tube but because these glues have not structural sheer strength they simply snap at the fin root along with the fillet which will still tear up some cardboard outside the rivet line(s).
Using pin holes rather then drilled 1/32" or larger holes makes it even worse as these very thin "glue rivets" have even less sheer/ shock strength.

Again the reason the "Epoxy Rivet" method works so well is that most 2-part epoxies retain some flexibility for a very long time after curing, allowing the joints and fillets to move a little with the fin material. The same can not be said for any glue, especially all Wood glues and CA's. They just become brittle to any shock load placed on them.

Click to expand...

We will have to agree to disagree. The additional strength of the wood glue rivets is still an improvement over the simple delamination of the paper airframe tube.

Note that I did not say that they were as good or better than epoxy.

I've never finished one build because I used glue rivets and a fin ended up being crooked. Even with just standard white glue, I couldn't get the fin to come off no matter what I did. The balsa fin itself broke before the bond with the body tube. I use these "rivets" as standard practice to repair older rockets when a fin breaks off, and they really make a strong bond.

For clean breaks like this, I simply use thin CA. It's served me well for many years, and if there are no paint chips missing, makes for a barely noticeable repair.

In the spirit of experimentation I decided to start with what seemed like the simplest approach: just gluing the whole mess back on with Titebond II.

I am not really confident that it's strong enough, but I'll give it a shot. If it comes off again I'll try something different.

I just decided that this rocket didn't justify any high-effort approaches until I confirm that the lazy approach isn't good enough. We'll see how it goes, though it could be a few months before I get a chance to try it out.

In the meantime I learned plenty from this thread, so thanks for all the discussion.