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Building Tips and technical articles. => Building techniques => Topic started by: Tim Wescott on December 04, 2014, 11:54:18 AM
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This question is sparked by this thread: http://stunthanger.com/smf/index.php/topic,37501.0.html (http://stunthanger.com/smf/index.php/topic,37501.0.html), but I didn't want to pull Jon's thread off-topic.
As far as I know, the best way to achieve a good stiffness to weight ratio in a thin-sheet structure (like a cowl) is to have a thin layer of something strong and rigid, then a thicker layer of something with good shear strength, then another thin layer of something strong and rigid. It's why honeycomb fillers were invented, and why places that sell carbon fibers sell structural foam.
But as far as I know, no one makes a material that is suitable for this sort of thing that is also flexible. Is there anything that you can put into a highly-curved layup, like a cowl, that would do this job? Clearly it would have to be something that's flexible during layup, but which cures out rigid -- I'm envisioning a cloth made out of lightweight fibers, or tubes, or miniature foam spaghetti, or something else that will wet out with the epoxy and cure into something rigid, but which wouldn't lend much mass for its bulk.
The only other alternatives that I know of for a cowl or other similar structure would be to mold or otherwise form the inner portion (expensive), use lots of layers in the layup (heavy), or leave all that out and have yourself a flexible cowling.
So, is there an answer here? Or do you just make a flexible cowling and live with it?
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I've used polystyrene or Rohacell foam between layers of Kevlar or carbon cloth. This year's stunter used a hump (1 - cosine shape) of Foamular pink polystyrene foam about 1/8" thick and 1/2" from front to back on the inside of the front, fixed part of the cowl. The inner layer of cloth goes over the hump and joins the outer layer at the front and back. It's plenty stiff. I think I cut the foam hump almost to the final shape before putting it into the mold. Dealing with a flat sheet of something would be more difficult.
Hey, I got an idea as I was writing this. How about molding a balsa preform over a male form a little smaller than the cowl using the method Bob Hunt is famous for? Then chamfer the edges and stick it in the mold between layers of carbon cloth. Robby, come on over and let's try it.
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Ooh. Ooh. I got another idea. Figuring that Igor would chime in here having already done this, I wondered how he would do it. He'd probably use his NC router. If one had an NC router, he could make two wood plugs, one 1/16" (1.5mm for Igor) smaller all around than the other, then pour the cowl mold over the larger and make the balsa preform over the smaller. Of course if one had a real NC mill, he could make male and female molds out of aluminum or gold whatever he has around and mold super cowls.
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If you had an NC router you could route the mold and the foam insert. Then you could mold the CF, shove the foam insert into it, and let it cure. Then (assuming you made it so it was well indexed) you could put the assembly back into the NC router, thin out the foam however you wanted, and vacuum bag your second layer of CF.
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Nobody uses NC stuff anymore. It's all CNC. Yes, there's a difference. NC is tape controlled, a PITB.
I was thinking of making a female mold, layup with epoxy/CF, then spraycan foam insulation where you want it. Carve/sand away what foam you don't want, and apply another layer of epoxy/CF or FG. The bad thing is that those spraycan foam things are pretty much one shot deals, because they plug up the nozzle. Probably a marketing plan? >:( Steve
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Nobody uses NC stuff anymore. It's all CNC. Yes, there's a difference. NC is tape controlled, a PITB.
Picky, picky.
I was thinking of making a female mold, layup with epoxy/CF, then spraycan foam insulation where you want it. Carve/sand away what foam you don't want, and apply another layer of epoxy/CF or FG. The bad thing is that those spraycan foam things are pretty much one shot deals, because they plug up the nozzle. Probably a marketing plan? >:( Steve
Or brush on Gorilla Glue, then spritz with water.
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Sorry, wrong thread!
Jonathan
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I've made a few cowlings.
Lost foam method. Glued pink foam to a piece of plywood. Glued a dowel as close to the center as possible.
Stuck the thing in a drill press and used a hand held sanding block to shape it. Had a cardboard template for the radius.
Not sure, but I believe I used 6 oz, fiberglass cloth. Couple of coats of polyester laminating resin on the foam before the cloth. I would use Epoxy now.
When cured, acetone removes the pink foam. Cut the hole, trim the end sand the inside and your good to go.
I'll be making another cowling shortly for a Mystery Ship I'm designing. I'll do it the same way, but as I said, I'll use Epoxy.
Four screws or magnets to hold it in place, I have screws on this model, it's rigid.
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when moulding real airplane parts,, we created stiffeners,, basically just like ribs of foam,, as was suggested,, your glass,, a stiffner ( I am thinking a central spine,, then a couple cross ribs) then another layer of cloth,,
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I've made a few cowlings.
Lost foam method. Glued pink foam to a piece of plywood. Glued a dowel as close to the center as possible.
Stuck the thing in a drill press and used a hand held sanding block to shape it. Had a cardboard template for the radius.
Not sure, but I believe I used 6 oz, fiberglass cloth. Couple of coats of polyester laminating resin on the foam before the cloth. I would use Epoxy now.
When cured, acetone removes the pink foam. Cut the hole, trim the end sand the inside and your good to go.
I'll be making another cowling shortly for a Mystery Ship I'm designing. I'll do it the same way, but as I said, I'll use Epoxy.
Four screws or magnets to hold it in place, I have screws on this model, it's rigid.
Since polyester resin dissolves the usual types of styrene foam...you didn't use polyester resin if you did this and it worked. Just for general interest, don't ruin your foam pattern by trying it. If you happen to have some polyester resin, you can try it on any sort of Styrofoam peanuts, meat trays, or electronics packaging stuff you need to see go away. D>K Steve
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Since polyester resin dissolves the usual types of styrene foam...you didn't use polyester resin if you did this and it worked. Just for general interest, don't ruin your foam pattern by trying it. If you happen to have some polyester resin, you can try it on any sort of Styrofoam peanuts, meat trays, or electronics packaging stuff you need to see go away. D>K Steve
Steve,
Yes, you are correct and thanks for the heads up on that.
I remember now. I gave the foam a couply of coats of Epoxy. Otherwise, as you said, the Polyester resin would have an issue with the foam.
I do know, and I'm positive, I used Polyester resin for the glass cloth lay up. Polyester resin is all we used back then for glass cloth.
This model has been framed many years. It's base is the original Adrian Paige kit, not the improved or current one that is now available.
I may finish it for CL Scale.
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Gee Bee Z? We have a replica in the Museum of Flight down at Boeing Field. It's lemon yellow...hate it. Finding the correct yellow is a problem, since there was no color film back then, but the local Randolph dealer says there were no bright colors pre-War II, as the color paint pigment technology wasn't as advanced. No titanium oxide, no chromium oxide, etc.
One thing about your model...the tail surfaces. A slab of balsa with ribs and covering is a horrible structure, and I cringe every time I see that done. It's often done, always sucks, and I always cringe. What's the power on that bad boy? The GBZ is an awesome looking racer. 8) Steve
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Ooh. Ooh. I got another idea. Figuring that Igor would chime in here having already done this, I wondered how he would do it.
:-)))
like that:
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or that:
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but if you once have that CNCed wood plug, why to bother with female mold, I simply cover it by paper and problem solved :- )))
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part2/
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part3/
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[quote author=Tim Wescott ...as far as I know, no one makes a material that is suitable for this sort of thing that is also flexible. Is there anything that you can put into a highly-curved layup, like a cowl, that would do this job? Clearly it would have to be something that's flexible during layup, but which cures out rigid -- I'm envisioning a cloth made out of lightweight fibers, or tubes, or miniature foam spaghetti, or something else that will wet out with the epoxy and cure into something rigid, but which wouldn't lend much mass for its bulk.
The only other alternatives that I know of for a cowl or other similar structure would be to mold or otherwise form the inner portion (expensive), use lots of layers in the layup (heavy), or leave all that out and have yourself a flexible cowling.
So, is there an answer here? Or do you just make a flexible cowling and live with it?
[/quote]
Old thread but anyway;
There is a material you describe, it's called Coremat. With quick search the thinnest available seems to be 1,4mm.
But I'm not convinced that that is a good product for small parts like cowlings.
At least Rohacell sheet can be heat formed before molding but with tight double-curves like in cowling, you propably have to make it from several smaller pieces.
I'd say the best way is to pre-form wet light balsa (0,8..1mm) over a plug or in mold and once dry, suck it in mold between 2 thin glass cloths.
That's how Yatsenko's do it.
Also, thin honeycomb is very flexible.
L
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Gee Bee Z? We have a replica in the Museum of Flight down at Boeing Field. It's lemon yellow...hate it. Finding the correct yellow is a problem, since there was no color film back then, but the local Randolph dealer says there were no bright colors pre-War II, as the color paint pigment technology wasn't as advanced. No titanium oxide, no chromium oxide, etc.
One thing about your model...the tail surfaces. A slab of balsa with ribs and covering is a horrible structure, and I cringe every time I see that done. It's often done, always sucks, and I always cringe. What's the power on that bad boy? The GBZ is an awesome looking racer. 8) Steve
Steve,
I was searching "magnets" and stumbled onto this Thread but never replied to your Post.
Sorry about that, honestly.
You mentioned the tail surfaces. The stab will actually have working flying wires above and below as did the actual aircraft. Should keep it a bit stiff.
Power is the vintage OS 80 double plug R/C engine.
This thread is old. There's a newer Thread, probably at "Scale" where I actually got around to converting this model to CL.
Yes I know. I moved on to something more interesting.
Forgive me, however, the Gee Bee Z will get completed.
Thanks for the reply.
Charles
P.S.
Isn't Igor's work a killer. The man has talent beyond belief. H^^
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there are a lot of interlayers you can use -
I used this stuff on actual car parts -
http://www.ebay.com/itm/321774960671?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
or you can use something like coremat
https://www.fibreglassshop.co.nz/collections/fibreglass-cloths-tapes-reinforcements/products/coremat
light - flexible and safe with polyester resins.
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Tim- "Tap Plastics" has all the stuff you need, and much more. We have several in the Seattle area. Would not be surprised if Aircraft Spruce has it also. We don't have one of those. H^^ Steve