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Composite Fin Layup

Old 05-22-2006, 12:48 AM
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justinh
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Default Composite Fin Layup

I'm in the process of making a mold for the vertical fins on a Hotspot. I'm not quite sure what weights of cloth I should be using for the layup, any advice would be appreciated. I'm hoping to incorporate skin hinges as described here http://www.nextcraft.com/foamwingsandparts.html anybody have experience doing this type of thing? Here's a rendering of what the part will look like and a pic of the airplane.
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Old 05-25-2006, 01:08 AM
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ptxman
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Default RE: Composite Fin Layup

Nice bird. I guess an answer would be related to how much vibration & load you expect. Assuming you are going to make a female mold from which you will vac bag the flying surfaces, a guestimate would be 1/16" balsa core skins (or rohacell or similar core material) bagged with maybe 1-2 oz/yd internal & external glass cloth. Of course you need spars & closeouts in this sort of structure & CF tow caps on the spars is a good way to add high strength at minimal weight. I think +/- this general layup is close to what typical gas pylon racers would do for tail feathers & get lots of strength & rigidity in that application, you could confirm with them.

Re the hinge thing, this sort of construction from female molds lends itself well to this. Its a bit different than the link you provided only in the way than the wing is built, but same basic idea. Check these links, electric F5B model but same idea:

http://www.rcgroups.com/forums/showthread.php?t=141255

http://www.rcgroups.com/forums/showt...l&pagenumber=1
Old 05-25-2006, 01:25 AM
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justinh
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Default RE: Composite Fin Layup

Thanks, it WAS a nice bird... before the unfortunate meeting with the asphalt [:@]

There shouldn't be much of any vibration. I know people have had problems with flutter in this area on this model though so I'd like them to be strong. I don't have a vac bag setup, how important is that vs just laying it up in the mold besides weight savings? I've got female molds milled from aluminum. Trying to come up with what internal structure I should put inside right now as well. I made a little 4x4 test mold today and it seemed to come out ok. I think I can get the process right if I can figure out the right layup schedule and get a good structure inside.
Old 05-25-2006, 01:52 AM
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Default RE: Composite Fin Layup

Well I guess on something small & relatively flattish like a stab mold cavity you might be able to fake it. But with what Im not sure, ... lead filled bean bags? You need something with weight & which is conformable to the cavity shape & wont impart any dents or creases. The stab mold cavity is relatively shallow & not deviating as much as say the LE nose of a wing mold, you could probably get away with it.

The problem with any weight based system is the force is acting downward only & is really quite minimal compared to vacuum pressure. Vac bagging acts perpendicular to the mold surface which is exactly what you want to conform a skin to the shape, especially if the shape is compound like a radiused tip or as the LE profile rolls up or any camber. Also consider the math: say you are operating at 12 psi vac (82% of atmospheric) & say your stab surface area is 10" x 4" = 40in2. That vacuum pressure is equivalent to a 12 * 40 = 480 lb weight acting on this area. So something like a 20 pound dumbell, which seems decently heavy, is only 4% of this equivalent pressure on the layup. You might not get good uniform contact with the glass & wont pull much excess resin out.

Milled aluminum molds? Thats a luxury production item! You MUST get a decent bagging system, thats like hauling gravel in the back seat of Ferrari! <g>
Old 08-09-2006, 10:31 PM
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Default RE: Composite Fin Layup

got the molds done finally. still trying to figure out what materials to use for layup
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Old 08-10-2006, 12:37 AM
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davidfee
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Default RE: Composite Fin Layup

Those aren't done... time to start polishing!
Old 08-10-2006, 11:15 AM
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Default RE: Composite Fin Layup

The issue you will have is that since it is a fairly flat and tall component, you need more wall strength to prevent it from bending, twisting, fluttering....

You need some sort of core like balsa or rohacell foam so that the skins are rigid and take the flight loads. I've seen a few tails done without a skin core and they were wobbly messes and seemed dangerous. This is where a vacuum system comes in. You may be able to laminate a piece of balsa down on the flat surface without vacuum but as soon as you hit a moderate curve (or worse a compound curve) your most likely going to end up with voids and gaps. Now, they do sell some stuff called core-mat which they use in boats and other heavy duty applications. I've used it in fuselages but it soaks up the epoxy and can end up heavy. I've done it without vacuum but it required a lot of micro-balloons around the edges to keep things stuck down and smooth the transition of the top layer of glass over everything.

I think the alternatives to a vacuum bagged core would be pretty heavy and not much stronger than a properly done layup.

Ya know, with aluminum mold capabilities you could make some friends real quick who'd help you with a vacuum job.
Old 08-10-2006, 11:38 AM
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Default RE: Composite Fin Layup

I think I've got a vacuum system to use for this now. Trying to decide what cloth to use and how to layout some spars and internal structure inside. I'm thinking maybe a couple layers of 4oz cloth?
Old 08-10-2006, 12:59 PM
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daven
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Default RE: Composite Fin Layup

In my opinion, a couple layers of 4oz is way too much.

I was thinking an outer layer of .75 oz, then a layer of 1.4 or 2 oz cloth, then your balsa skin (1/16 or even 1/32") with a layer of 1.4 oz or 2 oz on the inside. Once thats laid up, put in a bag and suck everything into the molds (up to 25" of mercury if you can pull it). When thats dry, build up your internal structure and add your torque rods for your skinned hinges and glue it all together.
Old 08-11-2006, 05:22 PM
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Default RE: Composite Fin Layup

That should be perfect. You can use a piece if 1/4"-3/8" balsa laminated with 1.4oz glass +/- 45* bias weave as your sheer web and sub spar. You'll end up with a very rigid and light part. Pre paint the molds with a white primer and you should have a beautiful part when it's done.
Old 09-01-2006, 02:52 AM
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justinh
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Default RE: Composite Fin Layup

here's my first attempt. I did a layer of .75oz followed by a layer of 2oz then ~1/20 balsa then finally 2oz on the inside. The part came out pretty stiff, I think it should work fine. Anybody see any glaring mistakes? =p

Justin
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Old 09-01-2006, 09:26 AM
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dolanosa
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Default RE: Composite Fin Layup

The one glaring mistake I see is that you are drinking Yellow Tail Merlot. You should be drinking something better since you finished that layup.

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