1/16" plywood fin flutter at mach 1.7

[TRFfan]

Would this be likely to flutter? Im building it right now and i was wondering if it was safe to fly it on a pro24 6G motor.

View attachment 315521

 

[dixontj93060]

I don't use OpenRocket, but if you provide dimensions, I'd be happy to simulate in FinSim.

-Tim (who is very good at ripping plywood fins off rockets)

 

[mikec]

Would this be likely to flutter?

IMHO, yes. I would reduce the span, increase the root chord, and glass the fins tip-to-tip.

I wish I could express confidence in FinSim, but I can't. For one thing even if it's correct analytically, the properties of plywood are too variable from type to type.

 

[TRFfan]

IMHO, yes. I would reduce the span, increase the root chord, and glass the fins tip-to-tip.

I wish I could express confidence in FinSim, but I can't. For one thing even if it's correct analytically, the properties of plywood are too variable from type to type.

It's aircraft grade plywood (birch).

 

[TRFfan]

I don't use OpenRocket, but if you provide dimensions, I'd be happy to simulate in FinSim.

-Tim (who is very good at ripping plywood fins off rockets)

root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.

 

[dixontj93060]

root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.

What is the body diameter?

 

[TRFfan]

its an estes BT-50 (OD 24.8mm ID 24.1mm).

 

[TRFfan]

I could also paper the fins.

 

[ksaves2]

IMHO, yes. I would reduce the span, increase the root chord, and glass the fins tip-to-tip.

I wish I could express confidence in FinSim, but I can't. For one thing even if it's correct analytically, the properties of plywood are too variable from type to type.

I agree with that. I have it and have run some sims but I'm not so certain that the materials I'm using match the approximations in the program. I've been told "it's easy" to input the
data from different materials but I have yet to see someone state how the mathematical constructs are input. Kurt

 

[dixontj93060]

I agree with that. I have it and have run some sims but I'm not so certain that the materials I'm using match the approximations in the program. I've been told "it's easy" to input the
data from different materials but I have yet to see someone state how the mathematical constructs are input. Kurt

To do simulation from other materials you have to use a lamination analysis program to get the physical properties of the composite material. I use: https://www.thelaminator.net/.

 

[ksaves2]

To do simulation from other materials you have to use a lamination analysis program to get the physical properties of the composite material. I use: https://www.thelaminator.net/.

Nice link but how do you know that the data you input approximates what you are holding in your hand? Is all plywood the same? Are their differences in G10/12 that vary between lots that can affect the outcomes? I don't know the answers.
I concede the closer that ones input data approximates the materials used the better off the FinSim prediction will be. Kurt

 

[dixontj93060]

root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.

I'm assuming span of 35mm is really semi-span (length from body to tip)? If so, and you are using decent adhesive, healthy fillets and birch ply (versus light ply), you are just OK with 1.7 mach.

 

[TRFfan]

I'm assuming span of 35mm is really semi-span (length from body to tip)? If so, and you are using decent adhesive, healthy fillets and birch ply (versus light ply), you are just OK with 1.7 mach.

Yeah its aricraft grade birch plywood. I'm using wood glue for fillets but ill use large ones.

And yeah its the length from root to tip.

 

[dixontj93060]

Nice link but how do you know that the data you input approximates what you are holding in your hand? Is all plywood the same? Are their differences in G10/12 that vary between lots that can affect the outcomes? I don't know the answers.
I concede the closer that ones input data approximates the materials used the better off the FinSim prediction will be. Kurt

I'm sorry, you don't use this for plywood. It is used for a layup. You input the dimensions of your laminate and the physical properties of each layer and stack definition and then the composite physical properties are reported and can then be used in FinSim.

If you are just interested about physical properties of base materials, they tend to be very well characterized with little variation. That is certainly the case for both birch plywood and G10/G12. You can find these base parameters in any number reference books or online (e.g., for plywood: https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf).

 

[mikec]

they tend to be very well characterized with little variation. That is certainly the case for both birch plywood

For commercial-grade construction plywood this may be true, but it's likely not true for hobby plywood. See https://sites.google.com/site/chris...nd-development/Fin_Flutter.pdf?attredirects=0 for a nice report by Chris Flanigan about plywood testing, fin flutter, etc, and note especially table 3.1-2.

 

[ksaves2]

Thanks load for those link guys. Kurt

 

[markkoelsch]

I could also paper the fins.

Carbon fiber is a better choice. A single layer of 5.7 ounce on each side will do a lot to stiffen the fin.

 

[TRFfan]

Carbon fiber is a better choice. A single layer of 5.7 ounce on each side will do a lot to stiffen the fin.

I dont think im quite ready to start using carbon fiber or fiberglass in laminating yet.

 

[markkoelsch]

I dont think im quite ready to start using carbon fiber or fiberglass in laminating yet.

Do you have access to laminating epoxy and something like 6 ounce glass? Seriously, you could do that easily (or carbon), and the fins will be vastly stiffer. It would add very little weight, and would very likely give your fins the added stiffness they need.

 

[mikec]

I dont think im quite ready to start using carbon fiber or fiberglass in laminating yet.

Fin flutter aside, if you're not doing something to keep the fins attached to the tube like tip-to-tip lamination, the fins are likely to rip off.

If you don't want to go the standard epoxy/fiberglass route, you might consider white glue and tissue paper reinforcement. Can't hurt and will probably stiffen things up considerably.

 

[TRFfan]

Fin flutter aside, if you're not doing something to keep the fins attached to the tube like tip-to-tip lamination, the fins are likely to rip off.

If you don't want to go the standard epoxy/fiberglass route, you might consider white glue and tissue paper reinforcement. Can't hurt and will probably stiffen things up considerably.

Im going with white glue and standard paper.

 

[ksaves2]

Hmmm, You might be pushing it a bit. I'd be concerned about the fin root to cardboard adhesion even if you used a good epoxy with fillets. I take it they're going to be surface mount fins? A rocket this size (small) I'd think 2 oz cloth
with laminating epoxy should offer a fair chance it would survive. Yes it would add some weight to the back end you'd need to take into account but Mach 1.7? That might be a bit much to expect. The critical thing is the cardboard glue interface
is the weakest link no matter what glue or epoxy you use. Full span laminating can really build resilience. On larger projects laying cloth that goes up in three layers from the fincan, 1/3rd up the fins followed by a layer going 2/3rds up and finally
full span. Yeah, that's 9 pieces of fiberglass cloth for a 3 fin can and 12 pieces for a 4 fin can. That is overkill for you at this size though and I expect a single layer of 2 oz cloth would be sufficient to try. That would be 3 easy pieces of fiberglass
cloth and some laminating epoxy. Make sure your fin root fillets are thoroughly dry then wet (with epoxy) and lay out the cloth two fins at a time



Here are shots of a minimum diameter 38mm rocket with a 3 layer fincan with 2 oz cloth. Twelve pieces of cloth for the three layers. Loafed along on a J350 to 850mph. Used an Eggtimer TRS for recovery and tracking. Used a single break ejection
with a reefed parachute that was released by an Archtype (now PrairieTwister) Rocketry cutter. The main chute is tightly bundled in the protector and cinched with a ziptie. There is a drogue that is attached to the apex of the main that is ejected at
apogee that pulls the main out of the protector once the cutter does its thing. That black glob at the base of the loop on the main is some stiff foam that abuts against the ziptie that keeps the main chute in the protector so the drogue can't
pull it out until it's released. Works like a charm. (Plywood fins too.)

Oh, I just realize those chips in the fincan was after a launch where the Mayhem Rocketry railguide bound on a dirty rail and the rocket blew right through the railguide and destroyed both pieces. Was my fault for using a dirty rail. I was surprised
I was left with a few paint chips and the "hang" didn't destroy the rocket fincan. Lamination really makes things strong. Best of luck. Kurt

 

[dixontj93060]

For commercial-grade construction plywood this may be true, but it's likely not true for hobby plywood. See https://sites.google.com/site/chris...nd-development/Fin_Flutter.pdf?attredirects=0 for a nice report by Chris Flanigan about plywood testing, fin flutter, etc, and note especially table 3.1-2.

Moral: don't use hobby plywood.

 

[retortec]

Your fins and fillets will most likely survive the flight but the tube itself will not without some sort of reinforcement. Most likely scenario is the fins will rip off the top laminate. Taking a paper tube well past mach gets real interesting when you also consider the motor casing cooking everything from the inside. Creating some sort of fin can is the best bet. Most of my attempts use through the wall construction with a coat of warmed 30min epoxy inside and out. A small strip of silk can be painted into the epoxy on the out side for further reinforcement. After all that sets use epoxy putty or JB weld to create the fillets. Final priming and sanding can blend everything together. Treating the rest of the tube inside with warmed CA will help prevent buckling. Internal reinforcements for the recovery and AV bay will help. G loads above 65 even with this kind of construction gets sketchy. Good luck.

 

[cerving]

IMHO, 1/16" ply fins would probably flutter and break off at that velocity. Get some 1/16" fiberglass sheet and cut your fins out of that, and use a good grade of epoxy like RocketPoxy or Proline to glue and fillet them. Take a look at your sim's too, and see if you can reduce the span of your fins... they look pretty big for a rocket with that aspect ratio.

 

[ksaves2]

Moral: don't use hobby plywood.

Yeah, That orange rocket above was the last of my 1/8" 5ply plywood I could find. If I would have known it was going away, I would have bought 2 dozen large sheet. Only thing I and find now is 3 ply. A lot of that is warped.
Holee Molee. Just did a search and found some 5 ply at Balsa USA. Put an order in and we'll see. I told 'em if it's 3 ply, I don't want it and to cancel the order. We'll see. Kurt

 

[dixontj93060]

Yeah, That orange rocket above was the last of my 1/8" 5ply plywood I could find. If I would have known it was going away, I would have bought 2 dozen large sheet. Only thing I and find now is 3 ply. A lot of that is warped.
Holee Molee. Just did a search and found some 5 ply at Balsa USA. Put an order in and we'll see. I told 'em if it's 3 ply, I don't want it and to cancel the order. We'll see. Kurt

Yeah, as long as your dimensions aren't too crazy, online for birch ply is the way to go. There is a dimension close to 12"x12" that fits stacked nicely in a USPS Priority box so shipping can also be fairly reasonable even for some heavy material.

 

[mikec]

Moral: don't use hobby plywood.

What Chris is calling "hobby plywood" in his report is the best-quality stuff you can get for model aircraft applications, not so-called "craft" plywood. At 1/16" you probably just can't get enough plies to not show a major difference in directional strength. Even Aircraft Spruce 1/16 is only 3-ply.

 

[dixontj93060]

What Chris is calling "hobby plywood" in his report is the best-quality stuff you can get for model aircraft applications, not so-called "craft" plywood. At 1/16" you probably just can't get enough plies to not show a major difference in directional strength. Even Aircraft Spruce 1/16 is only 3-ply.

Seems like he would have specified aircraft ply in his report instead of hobby ply. Hobby store plywood is not Baltic birch; often some type of poplar. Aircraft ply typically is Baltic birch veneer but cheaper/softer poplar core internally and uses water resistant adhesive (https://aircraftproducts.wicksaircr...-and-poplar-plywood-aircraft-grade-mil-p-6070). Real Baltic birch is the highest quality and strength using the Baltic birch for all layers and has no internal voids (https://www.woodworkerssource.com/b...-birch-plywood-why-its-better-when-to-use-it/).

 

[mikec]

Hobby store plywood is not Baltic birch...

You need to go to a better hobby store . What I believe Chris was reporting about (and I've talked to him in person several times at launches and seen what he's using in his rockets) is something like "Micro-Cut Aircraft Grade (Birch) Thin Plywood Sheets" from Midwest Products Company (https://www.hobbylinc.com/htm/mid/mid5244.htm), not the crummier stuff one sees at places like Michael's. But my point is that once you get as thin as the 1/16th the OP was talking about, you are looking at a major loss of strength, both absolute and directional, even for the best grades of plywood available.