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Slightly warped fins

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DAllen

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Mmmkay...working on my L2 project, cut my fins out of some 1/4" birch plywood and they are a little warped. We're talking about 1/8" curve out of straight over the length (or root edge) that is 14" long.

In order to straighten the fins out, the only advice I found online is to buy 1/8" ply and glue them together with the warping facing each other. Welp, I am not real interested in buying more ply and re-cutting my fins. That just seems like a lot of waste. Is there any reason I can just glass the fins (both sides) and mash them between 2 straight boards? I'll use wax paper between the boards and the fins of course so they don't all stick together. Anyone else try this? I was going to glass the fins tip to tip anyways.

-DAllen
 

daveyfire

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I tried that with some 1/4" ply that was more severely warped, with one layer of 6 oz. glass. Sadly, it didn't work out too well; they were still warped ±3/16" or so, and as such I ended up recutting the entire fin set from flatter stock.

One thing I would try if I was doing it again -- perhaps curing the parts in an oven would allow the plywood adhesive to soften slightly and the fins to flatten? Not sure on that one, but it sounds plausible.
 

Handeman

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I can't say as I've ever deal with that particular problem, but DAllen's results is exactly what I would have expected. Unless the warp is because one side has been moistened and drying it while it's held flat can cure it, I think you'll have to remake the fins to get flat ones. Once wood warps, even plywood, it is almost impossible to straighten permanently.
 

Commonwealth.Net

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The plywood we cut on our LASER is often the shape of a sand hill!
Flat, perhaps months ago!

To resolve your warp issue, soak the wood in warm water for several minutes.
Place the wet wood between two hard flat surfaces! I like to use 1/4" thick plate glass. Add weight to make sure the wood is FLAT, I use several pounds of "stuff" on top of the glass.

Let them dry out, may take a full day. The wood will be FLAT!

As soon as you paint or add any moisture they will warp again!
SO YOU MUST paint both sides at the same time with the same
amount of paint.

Good Luck
 

FROB

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<snip>... Is there any reason I can just glass the fins (both sides) and mash them between 2 straight boards?... Anyone else try this?
-DAllen
Yup, and it worked fine - but i was using 1/4" pine plywood (lighter) and 2 layers of 6-oz glass. I also had all outer edges of the fins tapered to a fine point- not sure if that has an effect or not, but those were essentially the conditions under which it worked for me.
 

DAllen

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Yup, and it worked fine - but i was using 1/4" pine plywood (lighter) and 2 layers of 6-oz glass. I also had all outer edges of the fins tapered to a fine point- not sure if that has an effect or not, but those were essentially the conditions under which it worked for me.
That is exactly the kind of ply I am using. Hmmm...I think I will try the water method first and if that doesn't work maybe try the fiberglass unless someone else has ideas. I probably won't be working on this for a few days.

-DAllen
 

madsen

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To resolve your warp issue, soak the wood in warm water for several minutes.
Place the wet wood between two hard flat surfaces!
What he said!--follow his advise. It will work. My only difference is sometimes it takes a bit longer for everything to dry when I do it. Also the advice of applying liquids to both sides at the same time--absolutely correct! It can be a mess/pain to apply epoxy to both sides at the same time--but if you add liquid to just one side and try to let it dry--warp city.
 

Rich Pitzeruse

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You're really worried about a 1/8" curve over a 14" span? I wouldn't fool around trying to fix that, I'd just use them as is.
 

troj

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First off, why the concern? If it's asthetics, then worry about it. Otherwise, don't sweat it.

If you're using through-the-wall fins, the process of gluing it into the fin slots will fix a lot of it. If not, it can still be fixed during the gluing process, it'll just take a little bit more work.

All that said, if it were me, I wouldn't worry about it. The worst it will do to you is generate some spin.

[edit, adding this]To paraphrase the sentiment a lot of people I know would use, "Just fly the bloody thing"[/edit]

-Kevin
 

TWRackers

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Oh great, now I'm starting to have flashbacks....

I went through exactly the same thing a few months ago with the L3 fins I cut out of 5-ply ¼" plywood. I don't know if they were warped from the get-go or if they somehow acquired warps, but all four fins were warped in different directions, although not as much as you described. I tried several tricks involving combinations of heat, moisture, and pressure to flatten them out, and nothing worked.

In the end I bit the bullet (metaphorically speaking ;)) and bought some Midwest Products 5-ply plywood in the 1/8" (actually 3 mm) thickness, and with some help from one of our composites guys at work we made a sandwich of two layers of the plywood with a couple of layers of 3 oz fiberglass and West Systems epoxy in between, and let them cure under vacuum on a large metal plate for 24 hours. From these I cut new fins, and they were (and still are) FLAT. I mean REALLY FLAT. So I effectively have 11-ply 6 mm thick fins, which only needed a thin shim of 0.4 mm thickness to fit the ¼" slots in my Giant Leap Dyna-Wind tubing.

I'd suggest going ahead and making new fins instead of trying to flatten the existing ones. Even if you succeed in getting them flat, they may not stay that way; I don't even know if fiberglassing would prevent a fin that wants to warp from doing so.
 

troj

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I'd suggest going ahead and making new fins instead of trying to flatten the existing ones. Even if you succeed in getting them flat, they may not stay that way; I don't even know if fiberglassing would prevent a fin that wants to warp from doing so.
If warped fins truly are "the end of rocketry as we know it", then do this, to avoid warped fins....

Get quality plywood that's half the thickness of what you want. Laminate two pieces of said plywood together, with a layer of 4oz or so fiberglass in between. Lay on a FLAT surface, put lots of weight on it, and let it cure.

Then, cut the desired fin shape.

Stronger than heck and it won't warp.

That said, it's a lot of work and expense for most rockets. Most of the time, warping doesn't matter. And plywood fins can warp over time based on how you store them, heat effects, etc.

-Kevin (who still sticks with his original "just fly the thing" answer)

PS: My Phantom 4000 had warped fins out of the box. Didn't worry about 'em, never have, never will. Never had a problem flying it on a variety of motors.
 

Handeman

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....snip
-Kevin (who still sticks with his original "just fly the thing" answer)

PS: My Phantom 4000 had warped fins out of the box. Didn't worry about 'em, never have, never will. Never had a problem flying it on a variety of motors.
I understand how it wouldn't matter most of the time, but what about when the big motor get put into it and you start topping 0.8 Mach? Will warped fins increase the drag and make it harder/prevent passing Mach? Besides the spin effects, are warped fins more or less susceptible to fin flutter at high speeds?
 

troj

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I understand how it wouldn't matter most of the time, but what about when the big motor get put into it and you start topping 0.8 Mach? Will warped fins increase the drag and make it harder/prevent passing Mach? Besides the spin effects, are warped fins more or less susceptible to fin flutter at high speeds?
Yes, it will increase drag. But so do any other imperfections in the assembly/finish.

Go to a big launch, and watch the number of rockets that have at least some roll to them. Typically caused by a fin that's just slightly off, or slightly warped, or an imperfection somewhere that generates enough drag to induce spin.

If you're pushing performance, then yes, it matters. Otherwise, it's no big deal.

-Kevin
 

DAllen

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[edit, adding this]To paraphrase the sentiment a lot of people I know would use, "Just fly the bloody thing"[/edit]
Of all the responses so far that one mad me laugh the most. The more I think of it I guess it really isn't that big of a deal. I was just hoping to create a rocket that doesn't spin at all and that is probably going to be more work than it's worth.

Kevin also points out a key consideration for this question which is regarding performance. I plan on flying this bad boy on everything from full I's to full K's and that will depend on how big the field is and how much money I have in my wallet. It's almost 6' long and 5.38" diameter and will be setup for dual deploy so I'm pretty sure even the biggest K won't push it into the transonic region.

-DAllen
 

FROB

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One of the biggest misconceptions around is that the fins have to be absolutely bulletproof to withstand the flight. In truth, the flight is the easy part, 99.8% of the time. Usually, its on landing that they snap off.
Depending on the recovery system you plan on using and the field where you typically fly it's something to keep in mind when building (and factor in that Murphy's law #27b of rocketry states that "on landing the rocket will strike the hardest object on the field-be that a rock, a your car, or someone's noggin- with it's most vulnerable part"- )
 

Rich Pitzeruse

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Here are the fins from the worst kit I ever built in my life......


These fins were warped quite a bit. I debated cutting new fins...but everything else in the kit was such rubbish....I decided to just build it.

It flys just fine, and doesn't spin going up. Yeah, it has more drag than it would with flat fins, and probably flys 20' lower because of it. ;)
 

DaveCombs

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Good advice from someone who knows a thing or two about being warped... :D
 

Handeman

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....snip
It's almost 6' long and 5.38" diameter and will be setup for dual deploy so I'm pretty sure even the biggest K won't push it into the transonic region.

-DAllen
Don't be to sure of that. I plugged the diameter, 8 lbs, and a Cd of .7 into Wrasp with a K1275R and got about 90% Mach.

temp2.jpg
 
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