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Click HERE to go to the Andy Douglas Streamliner page.
Click HERE to go back to the shell rendering page

Andy Douglas' streamliner - the plug


Plug construction method
One classic method for building a plug is to make a stack of horizontal airfoil shapes out of common 2-inch-thick Styrofoam insulation board, then use a rasp and sandpaper to smooth them into the final shape. This is how Warren Beauchamp did the Barracuda fairing.

While this works, it's also somewhat messy (lots of foam bits from the sanding) and labor intensive. I intend to borrow a page from the homebuilt aircraft industry and employ the same method used to carve the foam cores of airplane wings: hotwiring (i.e. using a heated wire to cut the styrofoam board into a precise shape, guided by rigid templates affixed to either face of the foam).

The CAD file I've got consists of 24 vertical cross-sections four inches apart. I printed each out full-size and glued it to a piece of Masonite, then cut out the Masonite to create a series of rigid, full-sized templates (using vertical templates instead of horizontal airfoil shapes keeps the template size small and manageable). Originally I was going to cut the paper templates to shape before gluing them to the Masonite, but in an all-too-rare flash of common sense, it occurred to me that I need not do that.

The picture below shows a typical template. The grid lines are used as an alignment reference. The two thicknesses of blue foam are held together with hot glue... the line has been drawn as a guide for applying the glue.



To create each section of the plug, I take two thicknesses of common 2" Styrofoam insulation board, glue them together, and attach adjacent templates to the two faces of the resulting 4" thick foam block. Precise alignment is necessary to avoid having a warped shape. This will be accomplished by placing reference marks on the templates and aligning them, then inserting three or four rods through both templates and the foam.

Then, using a hotwire saw (a thin, heated wire held taut in a frame... I bought mine at a hobby shop), I'll trace around the edge, holding the wire down to both templates as it moves. This will result in a tapered block of foam that is close to being the final shape.

The photo below shows the saw and the first test piece under construction. This is very rough... it gets much easier with practice, though the ridges that are visible are almost unavoidable. These can be sanded, but actually need not be, because the foam shell material can't follow irregularities that small anyway.



To provide reference lines on the finished plug, the templates will have shallow notches at regular intervals that are consistently placed from template to template (some of these are visible in the template photo above... the first [very rough] groove is visible in the test piece photo). These notches will create thin grooves all along the outside of the template that will make alignment of the blocks simple. They'll also make handy reference points for trimming the shell foam accurately.

The blocks will be stacked to form the final shape, using the matching reference lines for alignment. Some sanding will be necessary, but it should be minimal compared to the more traditional method. The plug will then be ready for molding the Zotefoam per John Tetz's method.

One of the great advantages of using John's foamshell technique is that the plug need not be finely finished. As anyone who's done a molded fiberglass shell can tell you, finishing the plug is most of the work and can take hundreds of hours. With the foamshell, once you get rid of the gross high and low spots, you're done.

To come...
Description of how the fairing splits apart for transport.... how to do colorful graphics without paint... general notes on fastening panels and mounting the fairing.