Strengthening Plywood Fins

How do you guys go about it? I'm currently building an ESTES Argent and this is my first time dealing with plywood fins. Normally with balsa I would skin the fins and coat the edges with CA. Do people do that with plywood too?

Plywood is very strong, and won't break like balsa. For my Red Max, I did add a thin layer of glass to toughen the fins up though - helps with "hanger-rash" and getting smacked around in the car. A simple procedure is to lay out parchment paper on a flat smooth surface, put down some fiberglass (3oz is great, but 6oz can be good if a fine weave), wet it out, put the fin on top, then put glass on the other side, wet out, more parchment paper, then another flat smooth board on top. Add weight to the top evenly - boxes of soda cans, gallon jugs full of water, etc.. Will leave a smooth surface on both sides that need little sanding or filling.

If you plan on flying it with motors up to and including I-impulse, nothing other than good construction TTW style and good internal and external fillets are needed. Plywood doesn't need the treatments balsa gets due to its tighter grain structure. Priming with a high solids primer will give good results. Also epoxy is not needed on PSII kits unless you just want too, wood glue like Titebond is plenty strong enough (even stronger than the wood and cardboard).

Thanks... I figured it may not need it based on how little flex the fins have to begin with. I will end up using a couple of coats of sanding sealer on them too so that should help as well.

As as for attaching them to the rocket... I tend to overbuild even on lower engine rockets and attach any potential high stress points with epoxy. The plan for the fins is to attach them initially with some titebond ii and then epoxy fillet them to the motor mount and do epoxy fillets on the outside.

Pull_and_Twist said:

Thanks... I figured it may not need it based on how little flex the fins have to begin with. I will end up using a couple of coats of sanding sealer on them too so that should help as well.

As as for attaching them to the rocket... I tend to overbuild even on lower engine rockets and attach any potential high stress points with epoxy. The plan for the fins is to attach them initially with some titebond ii and then epoxy fillet them to the motor mount and do epoxy fillets on the outside.

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I like using the epoxy for external fillets just because I get a better fillet, all of my upcoming kits PSII kits will be getting the dual deploy treatment, and all will eventually fly with at least H-impulse motors.

If you absolutely feel ply fin needs strengthening then fiberglass lamination is the next thing, I used 3mm Baltic Birch for the fins of my 3" FrenzyXL clone with a layer of 2oz and a layer of 7oz FG on each side and they were much stiffer, that rocket I have simmed on L motors and it still doesn't get enough fin flutter to (according to FinSim) to tear the fins off it at 1100 fps. However the weight penalty for FG is pretty good, even better than FG is Carbon Fiber.

So far my Pro Series (Partisan, Leviathan, Ventris, and Argent) held up to various MPR flights. The fins were a little warped in the kits even. It's only 3-ply plywood, so not the best plywood has to offer, but may be just fine as is w/ sanding sealer.

For my scratch builds I use 6mm 5-ply because it is cheap at Michaels, Their 3mm is 3-ply and similar to what I got in my Pro Series kits. Good 5-ply 1/8" costs a little more.

Since this is my first larger rocket I think I'm going to just use the sealer as I've never worked with fiberglass. Can anyone point me to a thread or resource about glassing fins as I wouldn't mind incorporating that into my next build if not in this one?

1/8" plywood fins break when the rocket hits the ground if there is any weight to the thing - like a LOC 4" V-2. (Ask me how I know. Go ahead.)

A way to reduce that is to use a large, resilient fillet. Slow epoxy with some micro-balloons.

Or cut the fins off short. Heh.:jaw:

Pull_and_Twist said:

Since this is my first larger rocket I think I'm going to just use the sealer as I've never worked with fiberglass. Can anyone point me to a thread or resource about glassing fins as I wouldn't mind incorporating that into my next build if not in this one?

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Glassing fins prior to installation is really easy, cut FG cloth, apply epoxy to cloth, lay on fin (I did both layers at once), place fin FG side down in position on non-stick surface (mylar sheet or drafting acetate from Hobby Lobby), place FG on top of second side, once all fins are done cover with acetate/mylar and add weights to top. When you flip the fin to add the FG to second side the fin will grab via surface tension the acetate and attempting to move it means repositioning the FG, so make sure it lands where you want it to be until cured.

Post #5 has picture of process https://www.rocketryforum.com/showt...nzyXL-clone-that-was-going-to-be-a-Thor-clone

rharshberger said:

Glassing fins prior to installation is really easy, cut FG cloth, apply epoxy to cloth, lay on fin (I did both layers at once), place fin FG side down in position on non-stick surface (mylar sheet or drafting acetate from Hobby Lobby), place FG on top of second side, once all fins are done cover with acetate/mylar and add weights to top. When you flip the fin to add the FG to second side the fin will grab via surface tension the acetate and attempting to move it means repositioning the FG, so make sure it lands where you want it to be until cured.

Post #5 has picture of process https://www.rocketryforum.com/showt...nzyXL-clone-that-was-going-to-be-a-Thor-clone

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That sounds easy enough... and the weight will keep the flat parts flat... but how would I deal with the rounded leading/trailing edges? Every time I've worked with epoxy it flows (albeit slowly) and it seems like it would screw up the rounded edges that are not facing up.

The edges would have to be sanded as the FG skins are oversized to make sure you get full glass coverage. Another method would be to wait for partial cure and trim and clean before epoxy cures hard then cover and re-weight until fully cured.

So after some research and hearing your tips, I am thinking I may attempt glassing the Argent fins after all. Before doing that though, I am curious as to whether I will need to get any special types of fiberglass specific epoxy or resin... or will any ol' 30 min curing epoxy work?

Also, how difficult is it to sand fiberglass?? Can it be hand sanded... or do I have to break out the power tools? I use a orbital palm sander to remove the woodfiller i put on my tube spirals but have never used it on fins before. I guess I can always you my dremel as well, since i would only need it for the edges.

Finally, is this the type of fiberglass cloth everyone is referring to? I figure i'll hit up the Home Depot tomorrow and see what types they have in store.

Edit: Really the Estes PSII kits are just fine the way they come as they are fairly light for their size, however adding FG also adds weight, and will change the motors they can be flown on, rockets built with LOC tubing are literally twice as heavy (go figure, the tubes are twice as thick).

Epoxies and the like:
The 30 minute epoxy from your local hobby shop will work or better yet are epoxies by Aeropoxy, West Systems, or my favorite US Composites 635 Medium hardener. The Bondo cloth works but you may have to do some filling after the epoxy cures because of the deep weave, I use it with a layer of 2oz for the top layer, then it covers the weave (bondo cloth is like 7 or 10oz per yard cloth). If you have a local RC aircraft hobby shop they will have some good supplies for trying FG, MY local HobbyTown USA has both .75oz and 2oz in packages (each package has about 9sq.ft.)they also have the Bob Smith laminating resin. Just don't use a polyester resin like HD and Lowes sells in a quart can. The Bob Smith resins are also frequently branded with a particular hobby shops label, the 20min Finish Cure is the BSI brands laminating resin.
https://www.bsi-inc.com/hobby/hobby.html

Your power sander will work OK but a belt or disc sander is better for cured epoxy, whenever I can I try and trim away the excess before the epoxy fully cures and can be removed easily with a Xacto knife, with the BSI epoxy that may be tough to do since the hard cure time is so short. The US Composites, Aeropoxy, and West Systems epoxies have much longer cure times (many at least 12 hours) so you can check a test strip to see when its no longer tacky but still pliable.

My regular suppliers of Epoxy is US Composites, and for FG cloth I frequently use Raka Inc, Fiberglast, or Fiberglass Supply.

FYI the lighter cloths are more difficult to work with when laminating tubing but on flat parts are not too bad, you can apply the epoxy directly to the fin and lay the cloth on it. If you fiberglass the fins then it will be necessary to use epoxy on the fin to tube fillets and joints.

John Coker has a good video on glassing a tube https://www.jcrocket.com/tube-wrapping.shtml and other really good videos on HPR construction techniques.

Many times I have laminated fiberglass onto ply using my FoodSaver (cheap vacuum bagging method) and had really good results. Light kits with light ply really need light FG cloth to avoid unnecessary weight gain, and a little cloth well laminated goes a long way to adding strength. I buy most of my lamination needs from Soller Composites. They have some sleeves and heat shrink tape that work great for airframes as well (which would be overkill for this kit).

I have launched my Agent with H engines multiple times, with no additional attention as to fin strengthening. To be frank, I never have flown it other than HP.
If you do glass your fins, which will add some weight, just remember to check the location of your CG (or better yet re-sim it once you have the new weight).
It's not a problem with an over stable rocket, but when you get to the short stubby ones, it could become a factor.

Thank you all for your input... after sleeping on it I've decided to just go with what I know and just seal the fins and skip the FG, especially since I don't plan on flying it with anything greater then a G motor.

I am very interested in the process though and will be utilizing it on my next build and all your suggestions and insight have been tremendously helpful.

Viperfixr said:

Many times I have laminated fiberglass onto ply using my FoodSaver (cheap vacuum bagging method) and had really good results. Light kits with light ply really need light FG cloth to avoid unnecessary weight gain, and a little cloth well laminated goes a long way to adding strength. I buy most of my lamination needs from Soller Composites. They have some sleeves and heat shrink tape that work great for airframes as well (which would be overkill for this kit).

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Lol, with a FoodSaver? How do you go about doing that? I don't know if my wife would approve of me using her FoodSaver to fiberglass rocket fins though. :kill:

On a side note, if anyone is curious about my progress... I have a build thread going on... https://www.rocketryforum.com/showthread.php?127239-ESTES-Argent-Build-Thread

John Coker has a video about laminating fins and he uses a Foodsaver to do it.