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I’ve read that the Rutan Quickie uses wings which are hot-wired out of foam and covered with fiberglass, but I’ve struggled to find further details. What type of foam are the wings cut from, and are they reinforced/stiffened in any way?

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The foam is closed cell polystyrene, like the blue or pink rigid sheets sold at Home Depot. In later years builders switched to much-more-expensive polyurethane foam because of its fuel/solvent resistance.

The Rutan design just used solid cores split where the spar web goes, with applicable layups of glass/epoxy resin to make the spar web and flanges, and skins.

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The structure is extremely simple and quick to fabricate; the time consuming part is the finishing. To get a finish like a mold-produced part required many many hours of filling and finishing the outer surface.

Builders were attracted to Rutan's process by the apparent simplicity of the construction, but discovered the projects took just as long to complete as a metal or fabric covered plane because of all the finishing hours of sanding and filling.

Epoxy resin can be a problem as well. In the 70s when builders were turning out Vari-Ezes, not much consideration was given to protection from epoxy exposure, until it was discovered the builder could become sensitized to the resin and many had to abandon projects because they would break out if they went anywhere near uncured epoxy once sensitized, protection or no protection. Nowadays people know to use the proper masks and barriers like gloves and special creams to minimize exposure to epoxy fumes.

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    $\begingroup$ And once you are through all that filling and sanding, you could as well use the part as the core of a mold. From that mold you can produce multiple parts which weigh less (no filler!) and are built from the outside in, so you can have useful structure and parts on the inside (pushrods!) instead of a solid foam core. $\endgroup$ Commented Jan 8, 2022 at 20:28
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    $\begingroup$ To be fair, it was intended as a one-off method for the scratch builder back in the pre-kit days.No individual builder was about to build a set of molds. It helped a lot when builders learned to use peel ply and vacuum bagging for layups.Peel ply gets you way closer to the finished product, more or less the surface roughness of the dacron peel ply fabric molded into the resin, and cuts the sanding and filling back drastically as long as your cores were cut accurately. A chum bought a Dragonfly, whose cores were not quite so precise, and micro-filling and sanding wore him out and he sold it. $\endgroup$
    – John K
    Commented Jan 8, 2022 at 22:25
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the quickie wing is indeed a foam core with a fiberglass resin skin. it is reinforced with a single spanwise fiberglass web protruding down from the top surface of the skin into the interior of the foam core, which is securely glued to the underside of the skin.

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