Foam Construction Theory

[Brent Gair]

Let me be a bit more specific on my theoretical foam construction as mentioned in another thread. I say "theoretical" because I have not used this idea for an airship. However, I'm fairly well versed in the basics of stacked foam construction so this isn't something I'm just pulling out of my...

This thumbnail (which links to a more easily seen larger pic...I hope), shows one variation of the idea:

http://img355.imageshack.us/img355/434/blm10tu.th.jpg (http://img355.imageshack.us/my.php?image=blm10tu.jpg)

It's pretty straight forward. Each frame or bulkhead section can be cut from hard styrene sheet. I actually use styrene sheets intended for light fixtures. It's cheaper and harder than most model plastic. This styrene essentially serves as a template for each section.

Standard home insulating styrofoam is used as the fill between the styrene frames. This foam would be cut very close to the shape of the styrene frames so as to require a minimum of shaping.

Although not shown in my quick drawing (to prevent clutter), I would use something like aluminum tubing right through the center of the ship from nose to tail. This would serve to keep everything straight. Each piece of foam and styrene would have a central hole drilled and they would stack together on the aluminum tube.

As each layer of foam and styrene is added to the stack, they would be glued together (polyurethane glue is excellent for this).

Final shaping would be done by block sanding the foam down to the level of the sytrene templates. The foam is soft and easily sanded. The harder styrene will hold it's shape so as to lessen the chance of oversanding.

The final product can be fiberglassed with lightweight glass cloth and epoxy.

[Charles Adams]

This would work great -- if the hull were smooth and round. I would suggest another approach which would be pretty easy to accomplish, although it might work best for really large models.

You could make a frame from lite ply or balsa, cutting the bulkheads with notches at each "corner," and then install wood stringers to form the shape of the airship. Next, cut thin, long, wedge-shaped pieces of foam and glue them in between the stringers, allowing them to stick up and out beyond the frame.

Finally, take a hot-wire rig and, using the wood stringers as a guide, cut and smooth the foam "strip" to the contour of the hull. Do this one longitudinal section at a time until the entire hull is filled in and shaped. This should yield a nearly perfect foam hull with great strength!

Again, this might be very tricky for a small model due to the difficulty of cutting the thin foam "inserts" needed to fit in between the stringers. You could develop a template for these, cut from wood, for each section of the hull. (The inserts would vary in size depending on the diameter of the bulkhead in each section.) Then use the template to cut a stack of foam inserts to the correct shape using a hot wire cutter.

The more I think about it, the more this could be an ideal method for building a large master to be used for making fiberglass cast hulls. Only 1/2 of the hull needs to be built!

I think the trickiest part of this method would be figuring out the exact size and shape of each insert and that could be best done by creating a computer model of the hull. By mocking up the frame sections, the exact shape of the longitudinal fill strip can be calculated in the computer and that can be used to output a suitable template. Lasercut this in wood and it should be very precise.

[Charles Adams]

This is a great topic and brings up all kinds of possibilities. Another idea: Use your method to create a "core" of shaped foam which is smooth as if lathed (like a torpoedo). Now glue stringers onto this. Finally, fill in the gaps in between the stringers with A+B foam or some sort of filler and then use the stringers as a guide to smooth it all to shape.

[Brent Gair]

This would work great -- if the hull were smooth and round .

On the contrary...if I intended to make a smooth and round ship, I would turn it on a lathe! If something has that "blimp" shape, a lathe is the only answer. That's what I use on my smooth skinned 1950's era scifi ships.

If you look at the larger drawing, each styrene frame is a template. Note that I even describe it as a polygon. The frames aren't round. Imagine the project at it's most basic: a 2" thick block of foam sandwiched between a pair of styrene polygons. The foam would be sanded down so that a plane would be formed between the "flats" of each polygon. That could be a truly flat plane (as in the early ships) or could be sanded to a slight curve depending on requirements.

This require no stingers. The polygon cross section is arrived at by cutting and block sanding the foam to match those styrene polygons.

[Charles Adams]

I got it. I am just imagining how hard it would be to sand it down and get nice and clean "facets" all around the hull without having stringers as a guide.

[Brent Gair]

I did this very quick proof of concept test. I used 1" foam and paper templates with 12 sided polygons. The subject is about 4.5" in diameter so any more sides wouldn't photgraph well. This took about 40 minutes.

http://img207.imageshack.us/img207/1613/bpof17zn.th.jpg (http://img207.imageshack.us/my.php?image=bpof17zn.jpg)

It should get easier as the scale increases. This is about half the size I would normally work with (I'd use 2" foam).

Remember, sanding the facets only requires accuracy between each template/frame. Let's suppose I use this method to build a 48" hull (that's a size I would like) that would be about 24 frames and 24 2" thick blocks of foam. When placed on a cental core, they would all be aligned.

Then, when sanding, I sand each block of foam between aligned flats on each template . In other words, I sand 2" at a time. I don't have to block sand a 48" facet (facet ain't the best word but you know what I mean) from nose to tail. Stringers as guides aren't necessary. Just sand from one template to the next...2" at a time. Everything was already lined up when the sections go onto the central core. It's a matter of making sure the sections are stacked in alignment (I use a common hardware store straight line laser for many models). If the templates are alligned, the facets are aligned and you only have to be able to sand straight for 2" at a time.

[Charles Adams]

Looks good! The next thing I was going to say was give it a try and see if it works. The smaller the model and the greater the number of sides of the cross section, the trickier it might get. But if it works for you, then go for it! :D

Another reason I like the tissue or silkspan idea is because many airships had wire rigging just under the skin that shows in places. If you cut little notches in your wood frame and rig it with thread, you can very effectively simulate this effect. In addition, many older Zeps had a somewhat "crinkly" texture to the covering which looks a lot like tissue paper! So I am thinking it should create an excellent sense of scale.

Again, the point of this forum and this thread is to discuss the different possibilities. . . :D

[Jamie]

Hi, I’ve just joined the site and need to make a model for a class at the Smithsonian Institution I’m going to be teaching. I’m intrigued by your technique and would like to use it, but the link to your first picture is broken. Could you repost it please? Thanks very much.

[Charles Adams]

Here you go. . . :)

[Jamie]

Great! Thanx, J-

[MT Hopper]

Brent the link to the quick proof of concept photo is broken and only displays the infamous red X.

Cheers from the Heart of North America
Will