Recommendations for papering fins?

[Raythain]

I've seen some recommendations to paper fins for strength, and thought the MPR kit I just pulled off the build pile might be a good time to try it out. I always like the option of flying higher and faster . I haven't seen clear answers to these points in the threads I've read.

a) If I'm doing any fin shaping, I'm assuming I'll do this before applying the paper. Can't add much strength if part of the skin is sanded off, right?

b) I'm also assuming that I should paper the TTW fin tabs when a fin has those. If the tab is carrying thrust to the airframe, those would need the reinforcement as well, yes?

c) Should the paper cover the root edge as well as the other fin edges? I usually don't touch those with anything that isn't gluing to the airframe and/or engine mount.

Feel free to hit me with other tip and tricks, I'll take more things to try on scrap balsa to see what works. Thanks!

 

[neil_w]

a) If I'm doing any fin shaping, I'm assuming I'll do this before applying the paper. Can't add much strength if part of the skin is sanded off, right?

I don't think the paper at the very edges of the fin add any particular strength. Sanding afterwards should be fine, and has the benefit of really working the edge of the paper into the wood. Soak the edges with CA when you're done and you'll be fine.

b) I'm also assuming that I should paper the TTW fin tabs when a fin has those. If the tab is carrying thrust to the airframe, those would need the reinforcement as well, yes?

You can, but I don't think the tabs are a likely failure point. The big advantage of papering is to make the exposed area of the fin much stiffer.

c) Should the paper cover the root edge as well as the other fin edges? I usually don't touch those with anything that isn't gluing to the airframe and/or engine mount.

Feel free to hit me with other tip and tricks, I'll take more things to try on scrap balsa to see what works. Thanks!

I would never cover the root edge with paper, there's no reason to.

Experimenting on scrap is a good idea. Papering with regular printer paper (~20 lb) works great, papering with cardstock (65 lb or 110 lb) will make it even stronger and tend to stay smoother (copy paper can conform to the wood grain, and end up a bit bumpy). Make sure to account for the added weight of the paper when checking stability.

 

[Bill S]

I played around with using printer paper and cardstock (110lb) on my Executioner's fins. I would definitely suggest just papering the sides; I did the cardstock such that it wrapped around the front of the pre-shaped fin and it was a real pain to do. Printer paper would have been easier to shape. Definitely pre-shape the fins first.

I kind of went nuts, and did 2 fins up, one papered with printer paper, and one with the cardstock just to see how the weights were.
Bare fin: .14oz
with paper not glued on, just for reference: .19oz.
With paper glued on with white glue: .22oz

Bare fin: .14oz
with cardstock not glued on, just for reference: .27oz
With cardstock glued on, using 30-min epoxy spread on like white glue: .43oz. This resulted in a heavy but very strong fin.

I did find that even with card stock, that some of the wood grain imperfections showed through, requiring me to use a bit of putty and sand. The problem was that the card stock was matte, and even after priming and painting, it wasn't as smooth as I would have liked to see. Maybe a better sealant prior to primer like wood sealant (I'm getting ready to try out the BIN white primer I got from Lowes), then sand smooth?

In any case, the above figures should give you an idea as to what to expect when papering.

 

[SolarYellow]

Has anyone used vellum?

 

[neil_w]

If you search for vellum on the forum you will find many, many hits. Not clear to me if there is any "official" guidance on using it, but I think some folks have said they have used it with success.

 

[K'Tesh]

https://www.rocketryforum.com/threads/papering-balsa-ply-fins-a-foolproof-method.65628/

 

[astronwolf]

Has anyone used vellum?

I have used vellum and tissue with epoxy. Fins and vellum or tissue and epoxy on a sheet glass tool, placed in a vacuum bag, and vacuum applied with a food saver. Mold release was a plastic film. That's for weight critical applications where I am trying to squeeze out excess epoxy.

For less weight critical applications I just set the whole layup between two sheets and squashed it with a pile of books. You can get as complicated or as simple as you want.

Most often I like to use the paper glue that scrap-bookers use (doesn't wrinkle the paper) and just smooth on the paper.

Balsa grain may show through thinner paper like vellum or tissue. I found that cardstock is best for the smoothest balsa fins. Using a less grainy substrate like basswood or filling the balsa grain might help with that.

 

[ep29030]

I did cardstock-reinforced fins on my Der Big Red Max kit. Here is the link to the build:
https://www.rocketryforum.com/threads/yet-another-der-big-red-max-build.176979/#post-2372440

 

[Rex R]

Use a 1" foam paint brush to apply glue thin coat, you want to keep your pinkies (& paper skin) clean and dry.

 

[lakeroadster]

I've seen some recommendations to paper fins for strength, and thought the MPR kit I just pulled off the build pile might be a good time to try it out. I always like the option of flying higher and faster . I haven't seen clear answers to these points in the threads I've read.

Since you are looking for strength... swap those balsa fins for basswood.

 

[bjphoenix]

I think printer paper works well enough for fins up to and including the average BT-60 size rocket. I paper mine for strength, not to help with finishing. I round the fin edges, don't airfoil them. I used to put nice airfoils on my fins but the relatively sharp edges are too fragile for me, they get damaged by handling, transport, landing.
I put a layer of TitebondII on one side of the fin, wipe off almost all of it to leave a uniform layer, put the paper on and squeegee it down quickly, then clamp the fin between 2 smooth pieces of wood. I put a piece of thin plastic between the glued side and the wood so the glue won't stick to the wood. I do one side quickly because if you take too long the paper will soak up the moisture from the glue and start to wrinkle. I'll leave this one clamped about 15 minutes then do the other side, then leave the whole thing clamped for half a day. Then I sand the edges to round them. I seal my edges with wood filler, sand that then eventually get a couple of coats of primer on the fins. If I have imperfections I'll add wood filler in spots, but I don't try to make them perfect.
I would run paper reinforcing all the way to the fin root, or all the way to the end of the fin tab but not between fin root and tube. Ive seen fins broken off at the surface of the tube so extending the paper to the tube and within the glue fillets will help. It is helpful to prevent splitting along the grain too which is the main reason I paper them.
Printer paper has a grain direction, it is easier to tear in one direction than the other. Since I am trying to reinforce fins to prevent splitting along the grain I orient my paper so the strong direction of the paper is perpendicular to the grain direction of the balsa. I did experiments with various bits of paper I had, including thin vellum, and didn't see any difference. To go to the next level in strength would require thicker paper.

 

[Ronz Rocketz]

I'm a proponent of label paper. Less messy and doesn't warp the balsa. The trick is to use a label sheet to remove the dust from the balsa before applying another sheet of label. I bought the label sheets by the large pack so have many sheets to spare. After I remove the edges (I think an Emery stick works great), I apply a light layer of thin CA (with PPE) and wipe off with a towel. This is tricky because I use a towel to wipe the CA but the towel will stick to the CA if you're too slow. I also run a bead of thin CA along the edges. The thin CA creates a hard fin that can be sanded smooth. I also like how they feel. I don't believe I've cracked one of these fins yet since using this method. The Boosted Bertha has taken several hits but no cracks.

An alternative I've used is thin styrene sheets. I've tried TB2 but this warped the balsa on the DBRM so using epoxy. I use an old credit card to spread the epoxy. I used the styrene/epoxy on the plywood fins in my Doorknob clone and has been holding up. I haven't tried styrene/epoxy on balsa yet.

 

[Back_at_it]

While I'm not a fan of papering in general, I do understand the need for it in some applications. I've used general copy paper but that it typically a nightmare of wrinkles. I've used the instructions that come with Estes kits with some better success but lately, if I feel the need to paper I'll use 120# card stock and thin 20 min. epoxy.

I'll spread the epoxy on the fin then apply the card stock and let that tack up before flipping and doing the same on the opposite side. After a few minutes I'll cover the fin in wax paper and place weight on top and let it cure. After it's cured I'll sometimes add a thinned coat of epoxy to the outside of the cardstock for a more finished appearance.

 

[neil_w]

I'm a proponent of label paper. Less messy and doesn't warp the balsa.

I am as well, and use it on (almost) all my builds. However it is definitely not as strong as glued-paper. I *have* broken a label-papered fin (once):

Would it have broken if it were glue-papered? Dunno. I think the extra layers of glue add significant strength.

But for sure, label-papered balsa is stronger than un-papered.

The trick is to use a label sheet to remove the dust from the balsa before applying another sheet of label.

I use blue tape for that. I think it helps a lot, certainly I have never had label paper come loose from a fin.

 

[Daddyisabar]

3 M Lable paper is easy peasy for the lazy builder. NO PAIN, NO PAIN! Strong, durable and good for funky shaped fins.

 

[Ronz Rocketz]

I am as well, and use it on (almost) all my builds. However it is definitely not as strong as glued-paper. I *have* broken a label-papered fin (once):
View attachment 559377
Would it have broken if it were glue-papered? Dunno. I think the extra layers of glue add significant strength.

But for sure, label-papered balsa is stronger than un-papered.

I use blue tape for that. I think it helps a lot, certainly I have never had label paper come loose from a fin.

I've busted papered fins (mostly recently my old Mini Executioner) and I've warped fins with glue (DBRM) so using thin CA to harden the fin.

I had the sticker sheet come loose until you mentioned the blue tape a while ago to remove the dust. I was amazed at how much dust comes off the balsa.

 

[Raythain]

Thanks all, lots of great things to try on my fin sheet leftovers.

Papering with regular printer paper (~20 lb) works great, papering with cardstock (65 lb or 110 lb) will make it even stronger and tend to stay smoother (copy paper can conform to the wood grain, and end up a bit bumpy). Make sure to account for the added weight of the paper when checking stability.

Definitely planning on measuring and throwing it in OR before I try to fly anything really fun. I'll have to see what weight cardstock we've got by the printer.

I did cardstock-reinforced fins on my Der Big Red Max kit. Here is the link to the build:

Great build thread, really nice documentation on the fins.

Since you are looking for strength... swap those balsa fins for basswood.

I thought about it, but I don't have extra fin stock lying around yet. Papering has the benefit of not requiring a trip to the hobby store to make progress on the build.

I'm a proponent of label paper. Less messy and doesn't warp the balsa.

Interesting idea. Built-in adhesive! One more thing to make the wife ask why I need it for rocketry.

Strong, durable and good for funky shaped fins.

I'll keep it in mind. I haven't come anywhere near your heights (depths?) of oddroc scummery. Canted tractor motors with a flexible tail, I bow before your mindsim powers.

 

[ep29030]

We normally use basswood instead of balsa for improved finishing, rather than increased strength. Have not compared them, but I'd put my money on paper-reinforced balsa over basswood when it comes to strength. I use 1/16" or 1/8" basswood on kits where I don't want to fill lots of wood grain to get smooth fins. It's great for that purpose.

 

[lakeroadster]

We normally use basswood instead of balsa for improved finishing, rather than increased strength. Have not compared them, but I'd put my money on paper-reinforced balsa over basswood when it comes to strength. I use 1/16" or 1/8" basswood on kits where I don't want to fill lots of wood grain to get smooth fins. It's great for that purpose.

No need to guess. Below are the material properties of Balsa vs. Basswood. Basswood is 2 to 2.5 x the weight of balsa, but is 3 times stronger in almost all parameters. You can easily substitute Basswood at 1/2 the thickness of balsa with only a slight weight penalty and an improvement in overall strength. No brainer....
View attachment 486553

Basswood is 3 times stronger than balsa. Your statement may be true, depending on what glue you use, and what type of paper, but all that adds weight and at some point the law of diminishing returns kicks in.

 

[SolarYellow]

Basswood is 3 times stronger than balsa. Your statement may be true, depending on what glue you use, and what type of paper, but all that adds weight and at some point the law of diminishing returns kicks in.

It's been discussed elsewhere that the strength and stiffness in bending of a beam are proportional to the cube of the height. So, using round numbers and a factor of "bass = 3x balsa," 3/32 balsa will weigh about half as much as 1/16 bass while being equally strong. That's without reinforcement. Making a composite lamination by papering the balsa concentrates the additional strength and stiffness at the extreme surfaces of the structure, so it's far more efficient than a uniform density of similar overall weight.

The most efficient structure would be the lightest core with the strongest, stiffest skins. So contest-grade balsa or foam laminated with fiberglass or carbon fiber. HL glider peeps have been doing carbon over foam for decades. We don't do that in rocketry because of our focus on minimum drag and the fact that when we get into high power, optimum mass is often heavier than our rockets come out with "non-ideal" construction anyway. Also, a lot of us just want nice, fun launches and aren't trying to squeeze every foot of apogee out of every gram of APCP or BP.

 

[lakeroadster]

It's been discussed elsewhere that the strength and stiffness in bending of a beam are proportional to the cube of the height. So, using round numbers and a factor of "bass = 3x balsa," 3/32 balsa will weigh about half as much as 1/16 bass while being equally strong. That's without reinforcement. Making a composite lamination by papering the balsa concentrates the additional strength and stiffness at the extreme surfaces of the structure, so it's far more efficient than a uniform density of similar overall weight.

The most efficient structure would be the lightest core with the strongest, stiffest skins. So contest-grade balsa or foam laminated with fiberglass or carbon fiber. HL glider peeps have been doing carbon over foam for decades. We don't do that in rocketry because of our focus on minimum drag and the fact that when we get into high power, optimum mass is often heavier than our rockets come out with "non-ideal" construction anyway. Also, a lot of us just want nice, fun launches and aren't trying to squeeze every foot of apogee out of every gram of APCP or BP.

Look no further than wind turbine blades... a balsa core, covered in fiberglass.

But that seems a bit extreme for the OP's MPR kit, and thus we are back to my point about the law of diminishing returns, or in other words: K.I.S.S.

 

[OZRoc]

I'm a proponent of label paper. Less messy and doesn't warp the balsa. The trick is to use a label sheet to remove the dust from the balsa before applying another sheet of label. I bought the label sheets by the large pack so have many sheets to spare. After I remove the edges (I think an Emery stick works great), I apply a light layer of thin CA (with PPE) and wipe off with a towel. This is tricky because I use a towel to wipe the CA but the towel will stick to the CA if you're too slow. I also run a bead of thin CA along the edges. The thin CA creates a hard fin that can be sanded smooth. I also like how they feel. I don't believe I've cracked one of these fins yet since using this method. The Boosted Bertha has taken several hits but no cracks.

An alternative I've used is thin styrene sheets. I've tried TB2 but this warped the balsa on the DBRM so using epoxy. I use an old credit card to spread the epoxy. I used the styrene/epoxy on the plywood fins in my Doorknob clone and has been holding up. I haven't tried styrene/epoxy on balsa yet.

+1 for label paper. Slightly different method. Cut paper slightly wider than fin outline. This allows you to just whack it down on the fin any old how. Press roll the paper with a hard craft roller. Trim the excess label off. Do the same to the other side. Shape / sand the edges how you want using 120# then 320# or 400#. Apply thin CA with a cotton bud. Easier to create a pool of CA on a piece of scrap plastic to dip the bud in while applying. When dry re-sand with 400#.
Leaves all edges hard and smooth. I even do ply fins in this manner as is much easier to cover the grain than mucking about with sealer.
Cheers, Mark.

 

[DigBaddy]

Yet another option .... 1/64" birch ply skins (glued with TBII) over balsa make a heck of a strong fin. Comes out to about the same weight as basswood fins. Sandable primer fills the grain for good finishing too.

 

[ep29030]

As a general rule, composite materials provide greater stiffness for weight over a single material. In rocketry we normally attempt to get the most strength for least weight, sport and scale rockets sometimes being excepted. I have a 4" Red Max upscale I did years ago with 3/16" foamcore board overlayed with 2 layers of 6oz/yd plain weave fiberglass. Very light and stiff fins. My 2.5" Cherokee-D upscale is 1/4" balsa covered with 1 oz/yd carbon mat-also very stiff and light. Purchased kits use solid material for practical (and cost) reasons, and that's fine for sport flying. You go to composite combinations when you want to optimize strength for weight.

I think it's time to buy a larger HPR kit and swap the fins for balsa or foam with a composite skin, just to compare weights of materials, then fly it on a big motor to test the composite. I'm thinking about a LOC kit for the test. BTW, my (Yet Another) Big Der Red Max build provides fin weights along with weights for the plywood upgraded fins. For that one, the balsa/cardstock fins come in about 11 grams lighter than birch plywood (26 vs 37 grams).

 

[ep29030]

Also, more strength (and weight) is not always better. Once you have a design that meets the flight objectives and can withstand the maximum flight stresses, any improvements over that can reduce performance, add cost and build time to the project.

 

[wizardofflight]

Does anyone still use tissue paper? Many years ago I used tissue and model airplane dope to seal and strenghten balsa fins.

 

[bjphoenix]

It's been discussed elsewhere that the strength and stiffness in bending of a beam are proportional to the cube of the height. So, using round numbers and a factor of "bass = 3x balsa," 3/32 balsa will weigh about half as much as 1/16 bass while being equally strong. That's without reinforcement. Making a composite lamination by papering the balsa concentrates the additional strength and stiffness at the extreme surfaces of the structure, so it's far more efficient than a uniform density of similar overall weight.

The most efficient structure would be the lightest core with the strongest, stiffest skins. So contest-grade balsa or foam laminated with fiberglass or carbon fiber. HL glider peeps have been doing carbon over foam for decades. We don't do that in rocketry because of our focus on minimum drag and the fact that when we get into high power, optimum mass is often heavier than our rockets come out with "non-ideal" construction anyway. Also, a lot of us just want nice, fun launches and aren't trying to squeeze every foot of apogee out of every gram of APCP or BP.

Stiffness is a function of thickness cubed but strength is a function of thickness squared.

I do see fins broken at the root edge and I assume the grain was running the correct direction so these fins would benefit from overall strengthening. However I am more concerned about splitting along the grain which is why I do the papering. I like using plywood for midpower, too bad there isn't anything equivalent available for LPR. Papered balsa is the closest I've come to it. For my next fin strengthening I think I'll try cardstock and see how that works out. Different woods have different ratio of strength parallel to grain compared to perpendicular to grain. Balsa seems to be really bad about this, I'm not so sure about basswood. Don't the Executioner and Big Daddy both come with basswood? The basswood seems to be much stronger perpendicular to grain.

I recently did 5 LPR builds at the same time, a total of 23 fins. It took me some time to do all of those fins but there was some efficiency in doing them as a big batch. I only had a couple of fin surfaces with wrinkles that I wanted to patch.

 

[ep29030]

On the paper front, years ago the ultimate paper for strength vs weight was Japanese silkspan. Have not seen/read of anyone using it for years, now we have light composite materials. It was very thin and used primarily by competitors to reinforce glider wings. Here is a 2017 article on its use for planes: http://www.flyrc.com/silkspan-blast-past-new-twists/

 

[Alan15578]

On the paper front, years ago the ultimate paper for strength vs weight was Japanese silkspan. Have not seen/read of anyone using it for years, now we have light composite materials. It was very thin and used primarily by competitors to reinforce glider wings. Here is a 2017 article on its use for planes: http://www.flyrc.com/silkspan-blast-past-new-twists/

Thanks for that. I did not know silkspan was still available. I don't know if silkspan finnishes provides the highest strength to weight ratio. I would have to see the data. Silkspan tends to be thick and porous, taking in a lot of heavy dope. I am not familiar with the different weights or grades of silkspan, and I have not used any of the low odor dope substitutes. I have used silkspan back when I was flying control line aircraft. I have used actual silk, sealed with plasticized dope for flexy B/Gs. Elsewhere, I mentioned that I get the best results using the thinest paper and thinned brush on adhesive, meaning tissue and dope. Mostly I use the tissue on balsa contest gliders. I did cover the stock surface mount fins of a Maxi Alpha with tissue and it has survived flight with E20 and F45 ProJet Motors. I probably would have used silkspan there, if I had any handy.

I think the ongoing discussion of papering fins, is not about what is lightest or best for performance, but what can be done with readily available garbage and common materials.

 

[ep29030]

I did see an article that described covering glider wings with epoxy/brown paper bags (vacuum bagged). I thought that was an interesting use of paper.