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Trenman

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On my 98mm mongoose kit, my fins are very very rough. This came as a surprise to me because my 38mm mongoose had very smooth, finished fins can anyone help me undertand why that is and what the best process to smooth them out would be? Thanks :)
 

cherokeej

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I would imagine that the fins are left rough to allow for doing a composite lay-up. You want the fins to have some tooth for the epoxy to grab. You should seriously score and rough up the surface of the a/f tube for the fillets and the lay-up, too. I'd suggest chopped Kevlar in the fillets to give them structure, followed by tip-to-tip carbon and a thin glass veil, with peel-ply and batting used, vacuum applied if possible. That will leave a smooth enough surface for priming, and keep the fins from fluttering in the breeze. Shear forces on fins are hardly worth considering... It's the side forces that start them to fluttering that will kill an airframe dead.
 

Trenman

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I would imagine that the fins are left rough to allow for doing a composite lay-up. You want the fins to have some tooth for the epoxy to grab. You should seriously score and rough up the surface of the a/f tube for the fillets and the lay-up, too. I'd suggest chopped Kevlar in the fillets to give them structure, followed by tip-to-tip carbon and a thin glass veil, with peel-ply and batting used, vacuum applied if possible. That will leave a smooth enough surface for priming, and keep the fins from fluttering in the breeze. Shear forces on fins are hardly worth considering... It's the side forces that start them to fluttering that will kill an airframe dead.
Ive been looking at laying a cf/kevlar combo twill tip-to-tip. Do you know of a good link with a detailed how to guide?
 

cherokeej

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Yup. On John Coker's site. You might want to bookmark his main page, but here's the lay-up page...

http://www.jcrocket.com/tttjig.shtml

Note the use of peel-ply and batting, to remove all excess epoxy. You'll find that Kevlar and Kevlar hybrids are a lot tougher to wet out than just carbon or glass. Work at it, and be sure to completely wet out the fabric. You do not want any dry spots.

Success in this technique is in the preparation, and in the materials. Use a good, slow curing epoxy, not the adulterated stuff from the hobby store.

Pre-cut everything. Make paper patterns to fit and use them to cut the carbon/Kevlar and glass panels. Pre-cut the peel-ply and a panel of batting, and have your weight bags ready to put into position, or the vacuum set-up ready to go, before you even start to mix epoxy for the fillets. I like to use small lengths of dowel placed in the valley over the fillets, between the weight bags and the batting, to keep the lay-up pressed against the fillets.

Have your tools ready. Brushes, mixing cups, popsicle sticks, triple-beam for properly measuring the epoxy components... Everything at the ready, before you start.

Good luck with the Mongoose.
 

kramer714

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You might want to rethink the kevlar-carbon hybrid for the outer layer of a fin. The outer layers give you the most stiffness in bending, kevlar is significantly less stiff that carbon and will actually make the fine more likely to flutter. Kevlar needs to be dry before laminating and can be more difficult to wet out.

Couple other reasons why Kevlar isn't a great idea for outer layers,

Sanding - if you need to sand the surface smooth, Kevlar int the right answer it fuzzes up and can be difficult to smooth out, primers don't stick to it as well as a good carbon/epoxy laminate either.

Strength- Kevlar will decrease the strength of your fins not increase it. Here is how to think of it, think rubber bands attached to window glass (ok im reaching here but work with me), the carbon composite (window glass) is about three times stiffer than the Kevlar (rubber band), when it starts to bend, the carbon takes up much more of the load than the Kevlar, the Kevlar won't actually take up much load until the carbon has failed. Some sporting goods applications use a combination so after the carbon fails the Kevlar holds the pieces together, but this is after a failure.

oh one side note that isn't talked about much, double cup!
Weigh out and mix resin in a single cup, I use uncoated paper buckets (popcorn buckets) for mixing, mix the resin fully, then pour it all into a SECOND cup, scrape as much as you can into the second cup and then with a NEW mixer (I use tongue depressors mcmaster 54845T85 with one end cut straight) re mix it. Good shop practice..
 
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cherokeej

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You might want to rethink the kevlar-carbon hybrid for the outer layer of a fin. The outer layers give you the most stiffness in bending, kevlar is significantly less stiff that carbon and will actually make the fine more likely to flutter. Kevlar needs to be dry before laminating and can be more difficult to wet out.

Couple other reasons why Kevlar isn't a great idea for outer layers,

Sanding - if you need to sand the surface smooth, Kevlar int the right answer it fuzzes up and can be difficult to smooth out, primers don't stick to it as well as a good carbon/epoxy laminate either.

Strength- Kevlar will decrease the strength of your fins not increase it. Here is how to think of it, think rubber bands attached to window glass (ok im reaching here but work with me), the carbon composite (window glass) is about three times stiffer than the Kevlar (rubber band), when it starts to bend, the carbon takes up much more of the load than the Kevlar, the Kevlar won't actually take up much load until the carbon has failed. Some sporting goods applications use a combination so after the carbon fails the Kevlar holds the pieces together, but this is after a failure.

oh one side note that isn't talked about much, double cup!
Weigh out and mix resin in a single cup, I use uncoated paper buckets (popcorn buckets) for mixing, mix the resin fully, then pour it all into a SECOND cup, scrape as much as you can into the second cup and then with a NEW mixer (I use tongue depressors mcmaster 54845T85 with one end cut straight) re mix it. Good shop practice..
While I'll agree that Kevlar and Kevlar hybrids are a lot tougher to wet out properly, and you dang sure can't sand 'em, I must disagree with your analysis of their use on fins.

Kevlar doesn't stretch. At all. That's its advantage in a lay-up.

Look at a G10 fin from the front. Measure one side, from the root to the tip. It's a fixed measurement. Now, bend that fin into an arc. Unless that fin is infinitely thin, the radius of the outside arc is greater than the radius of the inside arc. You've stretched the outside of the fin, and compressed the inside. Now, do a composite lay-up on that fin, and try to stretch that side. You're putting the lay-up under tension. But the carbon, Kevlar, or hybrid material doesn't stretch. Do the lay-up on both sides, and the fin won't bend.

IMHO, the only real drawbacks to using Kevlar or a hybrid is getting it wet out, and trying to sand it.

James
 

kramer714

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Everything stretches..

how stiffness the fin is is a function of the stiffness of the materials (modulus E), the stiffness of the composite in shear (Shear Modulus - G), and the geometry (goes into Moment Of Inertia - I).

Stiffer materials stiffer fin,
higher Moment Of inertia - stiffer fin

When you bend the fin, the outer fibers do stretch a little and the inner fibers do stretch a little, plus the layers shear a little between them.

How likely a fin is to flutter is dependent on the same factors, E,I, and G.

For the same thickness fin, a graphite fin will be stiffer and have a higher natural frequency than a Kevlar fin or fiberglass fin.
 

cjl

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While I'll agree that Kevlar and Kevlar hybrids are a lot tougher to wet out properly, and you dang sure can't sand 'em, I must disagree with your analysis of their use on fins.

Kevlar doesn't stretch. At all. That's its advantage in a lay-up.

Look at a G10 fin from the front. Measure one side, from the root to the tip. It's a fixed measurement. Now, bend that fin into an arc. Unless that fin is infinitely thin, the radius of the outside arc is greater than the radius of the inside arc. You've stretched the outside of the fin, and compressed the inside. Now, do a composite lay-up on that fin, and try to stretch that side. You're putting the lay-up under tension. But the carbon, Kevlar, or hybrid material doesn't stretch. Do the lay-up on both sides, and the fin won't bend.

IMHO, the only real drawbacks to using Kevlar or a hybrid is getting it wet out, and trying to sand it.

James
Kevlar is nowhere NEAR as stiff as pure carbon though. It's tougher for impact resistance, but as far as stiffness, the Kevlar is definitely inferior to pure CF. It's also much more difficult to get a good finish on Kevlar, so I would agree for the recommendation to ditch it.
 

WDG

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While I'll agree that Kevlar and Kevlar hybrids are a lot tougher to wet out properly, and you dang sure can't sand 'em, I must disagree with your analysis of their use on fins.

Kevlar doesn't stretch. At all. That's its advantage in a lay-up.

Look at a G10 fin from the front. Measure one side, from the root to the tip. It's a fixed measurement. Now, bend that fin into an arc. Unless that fin is infinitely thin, the radius of the outside arc is greater than the radius of the inside arc. You've stretched the outside of the fin, and compressed the inside. Now, do a composite lay-up on that fin, and try to stretch that side. You're putting the lay-up under tension. But the carbon, Kevlar, or hybrid material doesn't stretch. Do the lay-up on both sides, and the fin won't bend.

IMHO, the only real drawbacks to using Kevlar or a hybrid is getting it wet out, and trying to sand it.

James
Everything stretches..

how stiffness the fin is is a function of the stiffness of the materials (modulus E), the stiffness of the composite in shear (Shear Modulus - G), and the geometry (goes into Moment Of Inertia - I).

Stiffer materials stiffer fin,
higher Moment Of inertia - stiffer fin

When you bend the fin, the outer fibers do stretch a little and the inner fibers do stretch a little, plus the layers shear a little between them.

How likely a fin is to flutter is dependent on the same factors, E,I, and G.

For the same thickness fin, a graphite fin will be stiffer and have a higher natural frequency than a Kevlar fin or fiberglass fin.
Both of these guys made some valid points, but I think because one thing is true does not mean another is not true. Kevlar has extremely little stretch to it. It has a very high tensile strength but very little yield strength. It holds or breaks - not much in between. (When building machines that require exact timing with a timing belt, we use kevlar reinforced urethane belts. If it is something that could see sudden starts and stops or has other reasons for sudden changes in torque, we use steel or poly reinforced urethane. The kevlar belts snap. The others give a little but stay together.) As far as laminates go, almost any composite material (if made well) regardless of the materials, will have different (not necessarily better or worse - but definitely different) characteristics than either (any) of the composite materials alone. I will go a step further on making a sandwiched laminate. In order for the laminate to bend (flutter) one side has to give in tension and the other in compression. The greater the distance between the two sides of the sandwich, the more rigid the laminate. As long as the core doesn't crush, it can be just about anything. With a fin, you have a very rigid piece of material with not necessarily a very rigid line of connection. So flutter can come from hinging on that line - fin doesn't have to bend at all for that.) So why don't we just put guy wires between the fins? Because it is difficult to find something rigid enough and it adds drag. A thicker laminated fin will be more rigid than a thinner one but will provide more drag. Going tip to tip we are basically doing two things at once - making funny v-shaped guy wires (encased in epoxy) and creating a sandwiched laminate. Besides a nice rough finish (I like bead blasting) don't be afraid to drill a bunch of holes through your fins to really give something for your binder to bite into. If possible, do both sides of a fin in the same layup - that way your epoxy plugs (and use milled fiber (glass or carbon) or even kevlar pulp if you have the patience) are essentially a single piece. Now when it all sets up it isn't a bunch of layers so much as one composite piece. LOL - there is that word again - composite - you gotta love it.

I used kevlar/cf for tip to tip. (I was thinking about going with a natural finish and was going for the cool looking factor-lol) It was a pita to cut and get set in place, however, my project just had big fins not designed to go mach, so I didn't mind adding a few layers of laminating epoxy over it before sanding. Yes, I used filler, too. Bondo actually makes a pretty decent kevlar filled filler. (These fins were consisted of 1/8" G-10 core (peppered with holes), sandwiched between 2 layers of 5/32" birch plywood. The plywood started at the air frame and the G-10 went through the wall to the motor mount, and then some thick kevlar/cf cloth tip to tip (and a ton of epoxy over it all). I had 1-3/4" deep bevels all the way around on the fins.)

Now if you are going to all of this trouble to make your fins flutter free, think real hard before you omit a very nice bevel - at least on the leading edge. If you are going much beyond mach, the trailing edge will virtually be in a vacuum anyway. A nice straight edge on a good bevel makes a big difference in fin flutter. The nose cone and leading edges of the fins are where the shock waves start when going supersonic. (Well, your launch lugs or rails buttons will also be doing that if you use them.)
 
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