MARS projects - Stiffeners for foam shells
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Making high-density foam stiffeners for foamshell fairings
by John Tetz

This page details the construction of molded high-density Zote foam stiffeners to improve control of the shape of foamshell fairings. The particular project described here refers to the orange foamshell fairing, but is also being used in the new Momentum-based streamliner, shown on the vacuum molding page. For source information on the foam material used, see the foamshell page.

My orange foamshell (OFS) is a successful design, but I've always had some problems with surface waviness. Zote foam is remarkably stiff when molded into tight curves, but is rather floppy in areas where there surface is flatter. To solve this problem I decided to add some lightweight stiffeners, made using high-density Zote foam (HD 80). This type of stiffener is called a longeron (longerons are the thin longitudinal stringers typically used in aircraft construction: they tie the aircraft's fuselage bulkheads together and make a stiff structure).

The longerons start as long strips of HD 80 Zote foam. They are heated and formed into a U shape, 1inch overall width x  0.75" high  x 0.15 to 0.2" thick. This U-shaped cross section makes for a much stiffer final product than would be possible with a simple flat piece of foam. To mold them into this U shape, the foam is pressed between a pair of female and male wooden forms, which were made out of 2x4 lumber on a radial arm saw. The mold blocks are not the full length of the longeron... they're only about 8-10 inches long. The image below shows a plan for making the mold blocks. The smaller (male) U shapes fit into the larger (female) U shapes.

Click the image for a larger version

Since it's difficult to properly heat long pieces of foam, the longerons are formed a bit at a time. First, cut a piece of foam long enough for the whole longeron. Then,
heat an 8-inch long section of the foam strip using a hot air gun, lay it on top of the female half of the mold and push the male mold into the female mold.

While each section is in the mold, heat the material for the next section (the material that extends beyond the end of the mold). By the time the next section is heated, the foam in the mold will be cool. Then lift the male half out, move the newly heated foam over the female half, and re-press the male half into it. This step-and-repeat process can form long lengths.

After the entire length of the foam has been molded, I suggest doing the cycle over again in order to get a more uniform result.  The foam takes a better set on the second cycle.

The final task is to trim the edges of the foam precisely by using the edge of the female mold as a guide.

For gluing, mark where the longeron will go then lay some masking tape along the outside of the marked lines. When the contact cement is applied, it will not show beyond the sides of the longeron.

Fitting the longerons
With the shell hanging vertically, it formed a rather natural foil shape. I then glued a longeron along the top edge of the bomb bay doors back to the bottom of the seat frame.

The outside of the seat frame touches the inside of the shell. Therefore the longeron had to be cut then continued towards the tail. (They are the black strips in the vertical photo).

Click the photo for a larger version

I added another pair about half way up the sides (along the bottom of the side door on the left side) and a similar one on the right side. These start close to where the shoe is in the up stroke and continues past the waist area of the seat towards the tail. I ended them just after the seat because they would interfere with panniers. See interior photo below.

Click the photo for a larger version

I was planning to run some vertical ribs but so far it looks like they may not be necessary. I also made V-shape stiffeners about 5/8" tall, and used them inside the canopy to help control that moving and flexible shape. I also added stiffeners down at the bottom of the nose area just in front of the BB doors, as that area was kind of floppy. It's now quite stiff.

The overall foil shapes on the shell look a lot smoother and are much stiffer and more stable. There were lots of concave and convex areas which are more or less gone. I bet if I had started out with a smooth shell shape, none of those concave areas would exist.

I almost think if the vacuum molding works we might be able to get away with side thickness down towards 0.4", and maybe as low as 0.35" with the use of longerons and vertical ribs. The nice thing about a female molding is the internal braces can be easily and accurately added to the foam while it's still in the mold. The braces would add a minor amount of weight but probably give us good stiffness and control over foil shape. I still see a 5-6 lb. shell weight with mounting brackets bringing it to the 7 lb. area.