How do you secure surface mount fins?

[John Taylor]

Not too long ago I learned from others a way to secure surface mount fins securely.
I prefer modifying kits to TTW. Sometimes it's not feasible. So:
What I do now is two parts, first using a thin dremel cutting wheel or utility knife, etch or cut little X's along the root edge ( the surface edge that touches the airframe). This greatly increases the surface area to be bonded.
Then along the fin attachment line on the airframe, (where the fin will attach) punch a line of little holes using a needle. These holes in the airframe will create little rivets further securing the fin.
What do y'all do?

 

[Lee]

I also use a needle to punch holes - in both the fin root edge and B T.
On the BT I punch 3 rows, 1 for the fin and one each for the fillets in order to broaden the foot print of the glueing area

 

[Hobie1dog]

2 posts and I'm already learning something new. I just use a commercial grade epoxy, not the cheap stuff everybody in this hobby uses.

 

[neil_w]

For LPR? I use glue. Back in the day all fins were surface-mount and we were fine.

I’ll remove the glassine from the fin root (sandpaper), then do a double-glue joint with TBII, then a light TBII fillet, then larger fillets with TB Q&T.

Never would consider glue rivets for the typical LPR, unless I have some plans to stress it with very high thrust motors or something like that.

 

[waltr]

On LPR I have found that any white or wood glue is stronger than the cardboard BT. When a fin breaks off it always tears away a layer of the cardboard. Using stronger glue will not help.

The pin holes in the BT sounds like a good idea as this allows the glue to hold onto more than just the BT surface.

On HPR minimum diameter rockets with surface mounted fins, the common method is to apply fiberglass/epoxy from fin tip to fin tip across the BT between the fins.

 

[bjphoenix]

I put on big fillets of yellow blue, 2 layers of thick glue because a lot of the thickness goes away when it dries. It would have to tear off a pretty wide piece of tube to come off. However I understand that this can happen if you have a hard surface landing area. If you need strength then you need through the wall.

 

[Sooner Boomer]

For LPR? I use glue. Back in the day all fins were surface-mount and we were fine.

"Back in the day...". I remember when Centauri came out with composite engines. A friend made a scale model of the D-region Tomahawk. He made it with basswood fins, epoxied on. On liftoff, I think the fins stayed on about 25 feet into the flight and all peeled off. He then used balsa and SigBond (the yellow glue of the day). The fins stayed on after that.

 

[Back_at_it]

I will normally remove the outer layer of the body tube where the fins attach. I don't typically punch the holes in the tube any longer. After popping off a few fins I didn't see the benefit. Had the same rate of failure as just gluing them to the tube after removing the outer layer. If there is room to do TTW then I will generally recut the fins with fin tabs.

As for adhesive. If it's wood to paper, pretty much any wood glue out there will be stronger than the materials you are bonding so take you pick.

 

[MidOH]

Sand off the shine. A line of brown paper under each fin.

Use 30min epoxy that sinks into the paper. Fillet with a small amount of 15 min epoxy. 30min soaks into paper when it gets hot. And that paper minus well be fiberglass now.

 

[Grant_Edwards]

[...] What I do now is two parts, first using a thin dremel cutting wheel or utility knife, etch or cut little X's along the root edge ( the surface edge that touches the airframe). This greatly increases the surface area to be bonded. Then along the fin attachment line on the airframe, (where the fin will attach) punch a line of little holes using a needle. [...]

I use fillets (usually Titebond Quick&Thick). On my rockets, the glue between the fin's root edge and the body tube is just there to hold the fin still while the fillet dries. I think roughing up the gloss finish on the body tube helps...

 

[Joekeyo]

I use fillets (usually Titebond Quick&Thick). On my rockets, the glue between the fin's root edge and the body tube is just there to hold the fin still while the fillet dries. I think roughing up the gloss finish on the body tube helps...

I saw a builder do something similar on a NARCON workshop video. Tacked the fins on with CA; then added fillets for strength and structure. Haven't tried it but sounds like a valid, quick build.

 

[Grant_Edwards]

I saw a builder do something similar on a NARCON workshop video. Tacked the fins on with CA; then added fillets for strength and structure. Haven't tried it but sounds like a valid, quick build.

CA is definitely the quickest way to do that. I'm too lazy to get out the CA, the applicator tips, and the "un-cure". I just use Titebond Q&T with the "double glue" method. That sets in about 10 minutes to the point where you can add the fillets.

 

[Woody's Workshop]

One piece papered fins with mounting tabs is a good way to mount low power surface mount fins.
Cut and fold over leading and trailing edges from both sides. Gives the fins structure for hard landings.
Fillets are an option but I always add them.

 

[John Kemker]

I also use a needle to punch holes - in both the fin root edge and B T.
On the BT I punch 3 rows, 1 for the fin and one each for the fillets in order to broaden the foot print of the glueing area

I, too, have used that method. I was taught it as "epoxy rivets" by Mike Platt of Microbrick

2 posts and I'm already learning something new. I just use a commercial grade epoxy, not the cheap stuff everybody in this hobby uses.

Careful how you generalize! I use Aeropoxy and West Systems, which are both commercial grade. I know many other people in this hobby who do the same, including John Coker.

 

[Hobie1dog]

Careful how you generalize! I use Aeropoxy and West Systems, which are both commercial grade. I know many other people in this hobby who do the same, including John Coker.

I was not going to name companies.

 

[John Kemker]

I was not going to name companies.

Company names don't matter. Not everyone in this hobby uses cheap epoxies.

 

[Buzzard]

From the 1977 Flight Systems Industries catalog. I believe that it was in earlier catalogs. It was when I first heard of the technique. I remove the smooth glasine coating on low impulse competition models. Use that and the rivets for those that would encounter higher dynamics or landing loads.
High power requires these and other techniques like long cure epoxy and generous fillets.

Chas

 

[teepot]

I just built a rocket with surface mounted fins. All I do is apply JB Weld Kwik Weld to the root and stick the fin on. Then epoxy fillets. I don't ruff anything up or anything else. A balsa fin will break before the bond fails. With plywood fins the outside of the tube pulls away a large piece of paper along with the fin. Can't poke holes in an MD or the motor won't go in. For me fins coming off are usually broken rather than the bond failing.

 

[brockrwood]

Sand off the shine. A line of brown paper under each fin.

Use 30min epoxy that sinks into the paper. Fillet with a small amount of 15 min epoxy. 30min soaks into paper when it gets hot. And that paper minus well be fiberglass now.

Tubes with white paper wraps are my bane. I try to sand down to brown paper where the fins are glued on.

 

[Ronz Rocketz]

neil has convinced me that epoxy is a waste of time on LPR. The cured epoxy is more likely to crack or peel off the top layer from the cardboard, even if it has soaked in. Glue dries not cures, so more flexible and less likely to break off. I did use epoxy on my Honest John but only because I thought it looked cool with the airfoil.

 

[neil_w]

neil has convinced me that epoxy is a waste of time on LPR. The cured epoxy is more likely to crack or peel off the top layer from the cardboard, even if it has soaked in. Glue dries not cures, so more flexible and less likely to break off. I did use epoxy on my Honest John but only because I thought it looked cool with the airfoil.

I did?

Admittedly, it sounds like something I would say. I do use epoxy sometimes when I want extreme non-grabbiness, but never for fins (again talking LPR here).