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.

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.

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.

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.

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.  Steve

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. Steve

or that:

part2/

[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

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.