Building a cowling from scratch

[Drobie]

I've ordered some Newton plans that I plan on scaling down to 1/12" or 1/16". The plans are of a late model turbine.

I don't think I'll have any trouble building the frame and hull, but one aspect of the project that I don't yet know how I'm going to do is to build the whole cowling structure.

I suppose it isn't much different that building an airplane fuselage (right, Andy?), but I've never done that either.

Do you just building bunch of frames and cover them? Doug Forrester mentions carving out of foam to build a mold.

Knowing there is more than one way to skin the cat, any tips or ideas along these lines would be appreciated.

[JimClark]

Doug I carved one out of balsa blocks and then have it vacuum formed by one of the guy's in our club. Very easy to work with and to rough out then I have a Xacto carving set to get the details correct then sand it finally.
Jim clark

[AndyKunz]

Jim's method is very good, especially if you have a friend who will do the molding for a reasonable cost.

When I made the AMSOIL boat I should have taken pictures of the cowling. Oh well.

What I did was make two side views of the shape out of balsa and glued them to a block of blue foam (they took some time to make because I wanted them to fit to the deck tightly). I used a hot wire to cut that shape out.

Then I glued the top view of the thing to the bottom of the foam block and cut again.

The hull has an open cockpit, so I made a cockpit floor out of wood and glued it inside to serve as a sanding/carving limit.

I also made two bulkheads, one right behind the cockpit and one at the extreme rear.

Sand, carve, sand, carve, until the shape is right.

I laid up with EPOXY resin (West Systems) and 3oz glass cloth, I think 2-3 layers. After it cured, it was sand, sand, sand again.

After done, I melted out the foam with acetone, although gasoline would have worked better. I left the bulkheads in place (at least the rear one, ask Barry if the other is there), the cockpit floor, and about 3/8" around the perimeter at the deck line.

Andy

[Drobie]

Originally posted by AndyKunz
glued them to a block of blue foam...I used a hot wire to cut that shape out.
Andy

Is "blue foam" that the sheet insulation you get in any building center, or is there a specific product you use?

[Doug Forrester]

Doug
The "blue foam" that most of us refer to is usually a Dow product called Styrofoam SM. It is the most commonly used sheet insulation product the you can usually get at Home Depot. It is especially fine grain and lends itself well to hotwire cutting and sanding. It will produce an exceptionally fine grain smooth surface if sanded with a succession of finer grades of sandpaper in the same manner as you would do with wood.
I don't use a hotwire to cut blue foam, I cut it with a bandsaw. It will produce a clean cut with a fine tooth blade.

[Doug Forrester]

There are many ways to produce a cowl. The shape of the cowl usually has a bearing on the best method to build it. Some techniques are best suited to simple shapes and others are better with complex cowls. Also whether you are producing a "one-off" cowl or are interested in producing many dictates the method.
If I'm making a one-off cowl, I'll usually carve a male plug from balsa or foam (I know I used foam on the HydroMite, but I actually prefer balsa) and then I hard skin the plug. This hardskin can be glass cloth, some other type of fabric, tissue, paper or nylon stocking (the mind boggles) and can be adhered with many different glues, resins, etc. I never use polyester resin on anything. I can't have the fumes and smell in the house. I also rarely use epoxy resin to apply a surface laminate of glass cloth to balsa. It's very difficult to apply it in a small enough quantity to the fabric and not add too much weight. I generally use thin CA glue on glass cloth. I know that it sounds expensive but a little goes a long way. If you incline your glass/balsa surface and run a bead of CA across the top it will run down and spread quite far. After the cloth is all wet out and cured, you will be surprised at how little CA it takes. The weave of the cloth can then be filled with a lightweight latex-based filler and sanded smooth.

I'll usually lay up a couple of layers of cloth this way and then with a ball style cutter in the Dremel, I'll hog out a lot of the wood from the inside. You'll be able to get good enough to take out at least 90% of the wood because when the cutter hits the glass it makes a different sound and you lay off in that area. Also, hold the cowl up to the light and you should be able to see the areas where it's getting thin.
If I'm interested in making several cowls, I'll make the same balsa plug but I'll use the bare balsa plug to vacuum-form styrene cowls. The techniques involved were discussed recently on the regular RRR Q&A board.

[AndyKunz]

Here's a very good write-up on the glass & resin technique.

http://www.ezonemag.com/articles/1997/jimglass.htm

He has other tips and techniques at http://home.fuse.net/ryan/download.htm

I can't use CA anymore due to acquired allergy. One whiff will keep me sick for days :(

Andy

[Drobie]

Originally posted by Doug Forrester
Doug
The "blue foam" that most of us refer to is usually a Dow product called Styrofoam SM.

I was at Home Depot and Menards (a big regional competitor) and both of them only had Corning polystyrene products.

I don't know if you used the term "Styrofoam" generically.

I bought a sheet anyway for another project. I'm going to build a retriever boat with plans I picked up through RCBM.

I went swimming for boats twice last year. The July swim wasn't bad, but the one in late September was a little cool. I'm too old for that stuff and I certainly don't want to take a dip in April or May.

[Doug Forrester]

Doug
The Owens-Corning equivalent of Dow Styro SM (blue) is pink coloured. It is virtually the same stuff.

[AndyKunz]

FYI - the density of the blue and pink foams varies, just like balsa!

Andy

[Drobie]

One more question guys...

If you want to join a couple of layers of this foam, what kind of adhesive will join but won't melt it.

[AndyKunz]

OLD FORMULA (all black can) 3M #77 spray. The new formula (black & orange can) contains acetone which will melt it slightly. You can use the new stuff if you are VERY careful with it.

Epoxy can also be used if you go with a thin coat (weight). Epoxy doesn't hot-wire well, though.

Basically, non-solvent-based contact cements work. There are a number of them out there even at Sears, Home Depot, etc that will work.

Andy

[Drobie]

I just pulled out a catalog from "The Composites Store", www.cstsales.com.

In browsing it I noticed that carry Bob Smith's CA's and they indicated that the "odorless" variety is "foam safe".

[AndyKunz]

Yes, it is foam safe, but it is just as stinky as the regular stuff when the hot wire hits it! Keep the fumes away from you!

Andy

[Doug Forrester]

Doug
If you're looking to join pieces or layers of foam that are fairly large surface area joints, the CA is almost impossible to work with/spread and expensive! As Andy says, the contact cements work well and even aliphatic resin glues (Titebond) work well although take a long time to set.

[Dunk]

There are foam safe construction adhesives available at the hardware store. The one I use comes in a caulking tube and works slick. They should say "Foam Safe" right on the tube.