Fillet question - is epoxy necessary on outside Fin-BT for G,H?

The Rocketry Forum

Help Support The Rocketry Forum:

Alan R

Well-Known Member
Joined
Oct 11, 2018
Messages
367
Reaction score
230
Location
Oregon
Was watching the Zephyr build video, will be starting that build in a week or so. Tim puts epoxy fillets on the outside fin -> BT.

For my 29mm builds I epoxy the inside parts (CR-MT, Fin-MT, CR-BT) but use wood glue fillets on the outside Fin-BT joint. Basically just faster, less messy, easier.

Is there that much stress on TTW fins for G,H motors that epoxy fillets are necessary on the outside BT-Fin?
 

Donnager

Well-Known Member
Joined
May 29, 2019
Messages
136
Reaction score
76
Wood glue is stronger than the paper tubes, so I wouldn't worry about it. If you are really going to stress the airframe, you may want to do something, but the biggest 29MM is an I205 or thereabouts.

I've got a couple high power rockets with wood glue construction that perform quite well (38mm). Hard landings are more damaging than the acceleration. I usually do a fillet with wood glue, followed by a second fillet of quick 'n thick. Sometimes a duplicate of either is needed, and then the rest is smoothed with filler of some type.

Lately, I've had more failures associated with recovery than aerodynamic structural issues.
 

BMcD

Well-Known Member
Joined
Nov 12, 2019
Messages
56
Reaction score
22
In my opinion (admittedly uneducated) the external fillets are primarily cosmetic. It does not seem to me that they provide a whole lot of strength, as a pull out force would simply tear the outer paper on the BT allowing the fin and the epoxy to break free. The inside BT-Fin fillet does provide a lot of strength, because any force that would pull the fin out would have to pull the fillet through the thickness of the tube.
 

Greg Furtman

Well-Known Member
TRF Supporter
Joined
Nov 10, 2018
Messages
1,210
Reaction score
565
Location
Webster, Wisconsin
In my opinion (admittedly uneducated) the external fillets are primarily cosmetic. It does not seem to me that they provide a whole lot of strength, as a pull out force would simply tear the outer paper on the BT allowing the fin and the epoxy to break free. The inside BT-Fin fillet does provide a lot of strength, because any force that would pull the fin out would have to pull the fillet through the thickness of the tube.
It seems to me that a BT-fin fillet would makes things more aerodynamic than a nearly 90 degree interface. But then I might be wrong about this.
 

Alan R

Well-Known Member
Joined
Oct 11, 2018
Messages
367
Reaction score
230
Location
Oregon
It does not seem to me that they provide a whole lot of strength, as a pull out force would simply tear the outer paper on the BT allowing the fin and the epoxy to break free.
This is my thought. The Zephyr is a 38mm (G,H). All of the structural bonding is internal, on the motor mount.
It seems to me that a BT-fin fillet would makes things more aerodynamic than a nearly 90 degree interface.
Agree with this too. I always do wood glue fillets on the outside, mostly for looks, but also aerodynamics of filling holes at the BT-fin joint.
 

richP

Well-Known Member
Joined
May 23, 2019
Messages
138
Reaction score
123
Wood glue is plenty for the fillets. I would even go a step further and say that it's plenty for the MMT/fin attachment too, although I would probably use epoxy there just for piece of mind. As long as you prep the surfaces properly, I would not worry in the least with wood glue.
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
I think part of the reason epoxy is often suggested is because it doesn't loose any of its mass as it cures.

I usually use wood glue for fin fillets and internal fillets but I notice that the wood glue fillets look great when wet but by the time they dry they are almost imperceptible. A few times I did a second application just to build up a bit more bulk on the fillet.
 

Greg Furtman

Well-Known Member
TRF Supporter
Joined
Nov 10, 2018
Messages
1,210
Reaction score
565
Location
Webster, Wisconsin
I think part of the reason epoxy is often suggested is because it doesn't loose any of its mass as it cures.

I usually use wood glue for fin fillets and internal fillets but I notice that the wood glue fillets look great when wet but by the time they dry they are almost imperceptible. A few times I did a second application just to build up a bit more bulk on the fillet.
I use WEST System epoxy and use their phenolic microballoons to thicken it up to make my fin fillets. Very lightweight and it doesn't run.

On another post someone mentioned these & it really works well.

https://www.amazon.com/gp/product/B0771DY1LK/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1
 

Nytrunner

Pop lugs, not drugs
Joined
Oct 15, 2016
Messages
6,694
Reaction score
2,106
Location
Huntsville AL
Use a thicker wood glue for the final fillets. Titebond Quick & THick (at Lowes) doesn't sag or shrink near as much as titebond II.

And yes it's basically cosmetic for any through the wall fin constructions
 

Nytrunner

Pop lugs, not drugs
Joined
Oct 15, 2016
Messages
6,694
Reaction score
2,106
Location
Huntsville AL
Good enough for rocket fillets when i don't t want to put on gloves, weigh out 2 different chemicals, and worry about working time limits
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
Is that all you use it for? I guess there's no reason to use it over regular wood glue for general assembly?
 

Nytrunner

Pop lugs, not drugs
Joined
Oct 15, 2016
Messages
6,694
Reaction score
2,106
Location
Huntsville AL
Yep, that is the only reason i have it.

Titebond II is very strong and convenient for most any other paper/plywood construction. But Q&T is the way to go for fillets.

Put down a layer, smooth it with a wet finger, come back in 10 mins and do another right over it. I usually do 1 layer for every 1.5-2 inches of body tube diameter (i used 3 layers on my 5.5" L2).
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
5,947
Reaction score
1,328
I use WEST System epoxy and use their phenolic microballoons to thicken it up to make my fin fillets. Very lightweight and it doesn't run.
1
My understanding is that for paper to paper, wood to paper, and wood to wood that yellow glue provides at least as strong a bond as epoxy. Short curing epoxies have an advantage in helping you build faster.

But your bring up a good point of adding micro balloons to epoxy to make a thicker more aerodynamic and cosmetic fillet,

Anybody ever try adding micro balloons to yellow glue?
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
I'll try some Q&T for fillets for sure! I like one part stuff! Lately I've been using Titebond Polyurethane to coat my balsa fins in a fin press. It works pretty well. Makes a nice flat, hard and smooth surface with no epoxy to mix.
 

BMcD

Well-Known Member
Joined
Nov 12, 2019
Messages
56
Reaction score
22
I'll try some Q&T for fillets for sure! I like one part stuff! Lately I've been using Titebond Polyurethane to coat my balsa fins in a fin press. It works pretty well. Makes a nice flat, hard and smooth surface with no epoxy to mix.
Soooo... what is a fin press?
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
I got the idea from Apogee newsletter #509.

Basically you slather a bunch of epoxy onto your fins and clamp them between two pieces of wood until it cures. You must have some kind barrier on the wood of course. The fins come out after 24 hours with almost a plastic coating. It also ensures there's no warpage since they cure while clamped flat.

The article recommends thin sheets of HDPE plastic, like a flexible cutting mat, but I've found a silicone sheet to work a bit better. I got a silicone place mat from Target for a few bucks. It's a bit easier to peel the cured glue off a silicone sheet than a plastic mat.

Here's the newsletter...
 

Attachments

ebruce1361

The man with the plan. And some duct tape.
Joined
May 14, 2018
Messages
372
Reaction score
156
Location
Pinellas County, Florida
Anybody ever try adding micro balloons to yellow glue?
Not microballoons, but I've achieved pretty much the same effect by mixing fine sawdust with wood glue. Depending on how much you mix in, you can go from a slightly bulkier wood glue that is a bit stronger than the glue by itself, to a ball of stuff the consistency of Play-doh that works well as a crack or gap filler. I have repaired everything from broken antique chairs to a rifle stock by mixing sawdust and plain Elmer's wood glue. Although, in the context of rocketry, I am sorry to say that I have never tried this for fin fillets. I will have to correct this oversight. :facepalm:
 

Alan R

Well-Known Member
Joined
Oct 11, 2018
Messages
367
Reaction score
230
Location
Oregon
I saw this on a thread & bought some. I haven't used it yet but the guy swears by it.
https://www.rocketryforum.com/threads/estes-conquest-build-and-musings.151560/#post-1871339
I might have to try that. I have tons of caulk-type things around here. Need to go read the labels.
Not microballoons, but I've achieved pretty much the same effect by mixing fine sawdust with wood glue.
I've done that for years for random wood repairs. Never thought about using it on fillets.

It's like @Weldo said about the wood glue fillets.. it shrinks or gets absorbed and the nice fillet you painstakingly pulled with yoru little finger just disappears.

Thanks for all the input guys.
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
5,947
Reaction score
1,328
I got the idea from Apogee newsletter #509.

Basically you slather a bunch of epoxy onto your fins and clamp them between two pieces of wood until it cures. You must have some kind barrier on the wood of course. The fins come out after 24 hours with almost a plastic coating. It also ensures there's no warpage since they cure while clamped flat.

The article recommends thin sheets of HDPE plastic, like a flexible cutting mat, but I've found a silicone sheet to work a bit better. I got a silicone place mat from Target for a few bucks. It's a bit easier to peel the cured glue off a silicone sheet than a plastic mat.

Here's the newsletter...
hmmm, wonder if parchment paper would work. Is the silicone placemat re-usable?
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
Yea it is. One of the reasons I went from using a HDPE sheet to silicone was that the residual glue peals off the silicon much easier than the HDPE.

I’m not sure about parchment paper, it’s worth a try. I know wax paper doesn’t work...
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
Oh! Yea that’s cool! I’ll try it too.

I thought wax paper might be good but CA glue sticks to it pretty well. Since I found that out I kind of wrote off using wax paper, but parchment is a possibility.
 

LOC

Well-Known Member
TRF Sponsor
TRF Supporter
Joined
Nov 23, 2016
Messages
184
Reaction score
83
So I'll chime in for $0.02 worth. I don't feel the glue you use is nearly as important as remembering to remove the outer glassine/silicone layer on the tubes. If you can at LEAST sand it off, or even better peel it off. 80% of the "what glue to use" disappears! For mid-power I don't do external fillets. 80% of high-power I don't either. I do prefer the West Systems as mentioned above or the US composites 635 thin epoxy however. Wood glue is great for the "up" part, but seen so many cracked, snapped, or broken joints happen on landing that I'm not a fan. But again, if your weight is low, this isn't a problem! Balance your glue and durability and slowly increase your impulse. You'll start noticing what it is you prefer for your style. I hope that helps?
 

jqavins

Joseph Avins
TRF Supporter
Joined
Sep 29, 2011
Messages
3,402
Reaction score
1,055
Location
Howard, NY
ln addition to cosmetics and a little aerodynamic benefit, external fillets, it seems to me, are useful in that they keep the fin's position stable from side to side, so to speak. If the slot has any clearance to it, the fin can vibrate that tiny little bit; better to keep it still.

On both surface mount and TTW fins, I also use wood glue for the structural fillets. When that shrinks down, I use light weight spackle over it for the cosmetic and aerodynamic fillets.
 

Weldo

Washing Hands
Joined
Feb 28, 2020
Messages
141
Reaction score
47
Location
Pittsburgh, PA
I've heard pretty good things about the lightweight spackle. In the model airplane world lots of guys like it for filling balsa.
 

jqavins

Joseph Avins
TRF Supporter
Joined
Sep 29, 2011
Messages
3,402
Reaction score
1,055
Location
Howard, NY
I use it for that as well as fillets. In fact, I find it's better for that. On fillets sometimes its minimal shrinkage is enough to cause some visible cracks and need a second application. Worth it.
 
Top