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Rev Lovejoy

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OK, I feel really stupid to have to ask this. I am 37, and while I have plenty of hobbies and talents, modeling has not been one. I am just getting back into this whole rocketry thing. It is quickly becoming apparent that gluing fins to rockets is going to be the challenging part in my projects. This may seem obvious to you all, but I'm that-new.

I have a fin marking tool that came with an edmonds product, that is curved to the same degree as the tube, making it lay flat on the tube. Brilliant.

So I ask: when gluing a flat surface to a curved one, is there something I can do to make the joint flush? Or at this small scale, is the mass of the glue bond enough?

Sorry if this is so elementary as to go without saying, but I tend to deconstruct and analyze everything I do.
 

sj_h1

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On small scale it really doesn't make much of a difference. A glue fillet will hide anything you do anyway. If you are really concerned, wrap a piece of sandpaper around the and sand the fin. You really won't have to do much to get the curve shape on the fin root.
 

powderburner

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On small scale it really doesn't make much of a difference. A glue fillet will hide anything you do anyway. If you are really concerned, wrap a piece of sandpaper around the and sand the fin. You really won't have to do much to get the curve shape on the fin root.
OK, I think what he meant was to wrap the sandpaper around the body tube backwards (with the grit-side facing out) and use the whole thing like a curved sanding block to shape the fin root. Putting a used motor case inside the body tube would help.

For most low-power rockets, the fins are pretty thin and even though it is not a perfect geometric fit you can get away with gluing a square fin root to a round body tube. You would have to get up to something like 1/8, or 3/16, or thicker fin material attached to a BT5 or BT20 before you really have to worry about shaping a curved fin root surface.

And sj_h1 is also right about those fillets covering up the root.

Welcome to rocketry, and TRF! Feel free to ask lots of questions
 

gpoehlein

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Oh, and don't forget to use a double glue joint with either white or yellow glue. To do this, put a very thin layer of glue on both the root of the fin and on the line where you will glue the fin. Let dry for a half minute or so, then apply another thin layer to the fin root and glue in place. The glue will grab very quickly and you will be able to glue on all the fins in short order. Be sure to leave the rocket upright (I stick the nose end on one of the Estes Fin Alignment Guides to dry - if you lay it on its side, the fins will sag before they dry). Double glue joints are VERY strong - usually the fin will break or the paper of the tube tear off before the glue joint will fail - especially if you put on glue fillets after the fins dry.
 

mach7

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Greg speaks the truth, double glue joints are VERY strong. I use a slightly different method that achieves the same results.

Apply glue along the root end of the fin.

Press the fin onto the body tube, over the alignment marks.

Remove the fin.

Let both the fin and body tube "rest" for 5 min. The glue will become tacky.
make sure nothing else touches the glue. I stand the body tube upright and rest the fins with the glue side overhanging a flat surface, like my cutting mat.

After 5 min apply another bead of glue to the root end of the fin, press back onto the body tube. Hold for a few seconds and let rest at least 30 min.

Repeat for the other fins.

After everything has dried, apply a thin coat of glue along the seams and smooth.

You will then have a nice fin/body join that is strong.

We have people here who are much better than me at filleting fins, and there are a few different techniques. Someone should chime in with them.

Hope this helps.

Welcome to the forum, have fun, and Balsa rules!
 

powderburner

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All the info about double-glued root joints is correct. Believe it or not, although it sounds like a bunch of silly extra work, it is probably the fastest way to get the fins on.

One other thing I would add: scuff sand the plasticine layer on the outside of the BT material. Estes puts this on their tubes so they will be easier to finish, or some other goofy reason, but the thin plastic layer also makes it harder for the fin root glue to penetrate and grip. If you give the fin root region (on the body tube) a light sanding to scuff through the plastic film, your glue will give you a much stronger joint.

Do the same thing with the launch lug (at least, the side of the LL that is facing the BT) and with the area of the body tube under the LL.
 

MarkII

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Yes, but keep in mind that you only need to scuff the tube up LIGHTLY. Use extra fine grade sandpaper, apply it with a light touch, and just do enough to take off the shine. No more than that.

MarkII
 

Pantherjon

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I use the double glue method as well on my fins..The technique is great for getting good solid joints..What I usually do is lay out all the fins then go down the line placing a thin layer of glue along the root edges. Once I get done with the last fin the first one is usually tackey enough for the second layer and onto the tube. THen the 2nd one gets it's 2nd layer, etc..I usually put a pretty healthy(not REAL thick) layer of glue as the 2nd one and when I apply onto the tube some excess oozes out and makes for good initial fillets that I smooth down the fin/body joint with my finger tip.
 

luke strawwalker

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Oh, and don't forget to use a double glue joint with either white or yellow glue. To do this, put a very thin layer of glue on both the root of the fin and on the line where you will glue the fin. Let dry for a half minute or so, then apply another thin layer to the fin root and glue in place. The glue will grab very quickly and you will be able to glue on all the fins in short order. Be sure to leave the rocket upright (I stick the nose end on one of the Estes Fin Alignment Guides to dry - if you lay it on its side, the fins will sag before they dry). Double glue joints are VERY strong - usually the fin will break or the paper of the tube tear off before the glue joint will fail - especially if you put on glue fillets after the fins dry.

Amen to that! I never tried double-glue joints until about a year ago... impatience of youth had me convinced that was the LONGEST way to make a glue joint, and we all want to get done ASAP ya know... it's counterintuitive but double-glue joints are actually the fastest way to glue fins on. It doesn't take long at all for the thin layers of glue on the tube and fin edge to dry, and then the thin film of glue on the parts grabs almost instantly like superglue but without the brittleness and risks of gluing yourself to your work that CA's have...

Here's another tip that helps IMMENSELY with the double-glue joint process-- save an old egg carton from the fridge next time it's empty and going in the trash-- invert it and using your hobby knife, slit 4-6 of the egg cups across them from the center to the long side. Now you have a handy holder for your fins as you apply the glue to the root edge and it'll hold them level while the glue sets up. The long "valley" between the egg cups is quite handy for holding the body tube while the glue on the fin lines on it tacks up in preparation for double glue joints as well.

Also, don't forget that the quickest and easiest way to get really nice looking fins (and strong too!) is to cover them with printer paper glued on after they're cut and sanded. That's a GREAT tip I learned on these forums!

Later and good luck! OL JR :)
 

luke strawwalker

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Greg speaks the truth, double glue joints are VERY strong. I use a slightly different method that achieves the same results.

Apply glue along the root end of the fin.

Press the fin onto the body tube, over the alignment marks.

Remove the fin.

Let both the fin and body tube "rest" for 5 min. The glue will become tacky.
make sure nothing else touches the glue. I stand the body tube upright and rest the fins with the glue side overhanging a flat surface, like my cutting mat.

After 5 min apply another bead of glue to the root end of the fin, press back onto the body tube. Hold for a few seconds and let rest at least 30 min.

Repeat for the other fins.

After everything has dried, apply a thin coat of glue along the seams and smooth.

You will then have a nice fin/body join that is strong.

We have people here who are much better than me at filleting fins, and there are a few different techniques. Someone should chime in with them.

Hope this helps.

Welcome to the forum, have fun, and Balsa rules!
Glad you mentioned that!

The best thing I've found for filleting fins in typical LPR kits (and probably on bigger ones too unless you're glassing or using epoxy anyway) is TITEBOND MOULDING AND TRIM GLUE. Look in your handy big-box harware store (Lowe's or Home Depot) for this stuff with the wood glue and stuff like that. It's in a gray bottle almost identical to the Titebond II and Titebond III wood glues.

The stuff comes out about the consistency of caulk-- pretty thick, like toothpaste. It goes on like a dream from the bottle and is quick and easy to smooth into perfect fillets with only your finger and a paper towel, and any extra is easily wiped off with a wet paper towel. It dries clear and doesn't hardly shrink at all during drying, so unlike white or yellow glue fillets that go on thick and dry down to a skinny little layer barely visible and sunken, which requires a second or third application to get the fillet built up enough, this stuff is 'one pass and you're done' fillets. None of the stink, mess, or allergen exposure problems of epoxy, and none of the running, dripping, and shrinkage of wood or white glue fillets. GREAT stuff!

Don't know how we ever made it without some of these things... :) OL JR :)
 
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