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.
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IMHO, yes. I would reduce the span, increase the root chord, and glass the fins tip-to-tip.Would this be likely to flutter?
It's aircraft grade plywood (birch).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.
root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.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)
What is the body diameter?root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.
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 theIMHO, 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.
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/.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
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.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/.
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.root chord 41 mm tip chord 13 mm fin span 35 mm sweep length 34 mm.
Yeah its aricraft grade birch plywood. I'm using wood glue for fillets but ill use large ones.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.
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.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
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.they tend to be very well characterized with little variation. That is certainly the case for both birch plywood
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 could also paper the fins.
I dont think im quite ready to start using carbon fiber or fiberglass in laminating yet.Carbon fiber is a better choice. A single layer of 5.7 ounce on each side will do a lot to stiffen the fin.
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.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.I dont think im quite ready to start using carbon fiber or fiberglass in laminating yet.
Im going with white glue and standard paper.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.
Moral: don't use hobby 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.
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.Moral: don't use hobby plywood.
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.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
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.Moral: don't use hobby plywood.
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/).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.
You need to go to a better hobby storeHobby store plywood is not Baltic birch...