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Is Rutan's method of foam core construction obsolete, or is it used on other fibreglass designs as well?

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  • $\begingroup$ Still used when appropriate at Scaled Composites. It’s more associated with home built aircraft since molds aren’t required. $\endgroup$
    – Eric S
    Mar 17, 2021 at 18:45

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I was a regular at Oshkosh during Rutan's heyday in the 70s, and had a chum that was building a Dragonfly, which used the same construction method. Rutan's mold-less process is only suitable to one-off projects, like prototype parts, or amateur built airplanes. It's a sub-optimal process from an engineering perspective because it is very time consuming to produce an ascetically decent component, and tends to be somewhat heavier than molded sandwich parts.

Rutan advertised extreme speed at creating a structure compared to metal or wood/fabric. It was much faster to create a wing or fuselage... but then all the time saving was eaten up in finishing. The method leaves a rough and somewhat uneven outer surface (due to the hot wire cutting process, where the wire bends under stress as it's cutting, producing a dished result when cutting a curved profile) that requires hours, and hours, and hours of filling and sanding (using looooong sanding blocks) with a lightweight filler made from epoxy and micro-spheres (hollow glass beads) to get anything close to the finish you get with a mold.

The hot wired foam cores, done by hand, always have surface continuity imperfections that need lots of filler, plus the outer surface of the epoxy tends to be rough. You have to sand very carefully to avoid going down into the glass cloth. The original layup method didn't use vacuum bagging and produced a heavy part with excess epoxy resin. Later on builders started to vacuum bag the layups and use peel-ply fabric (polyester fabric similar to aircraft covering fabric, which epoxy won't stick to) with an absorbent material to soak up excess epoxy and provide a better outer surface (which ended up with about the smoothness of the peel-ply fabric, the outer surface being an impression of the peel-ply weave.

My chum sold the Dragonfly project before finishing it. He just didn't have the time to do all the finishing work.

If you are going to build your own airplane from glass and foam, that method is pretty much the only option. Beside Rutan's designs, the EZ's and Quickies, and the Viking Dragonfly, I can't think of any other designs off hand. There was another scratch built homebuilt that used sheets of polyurethane foam bent to the skin curvature on a form, then layups done on the skin, that produced something closer to a modern fibreglass sandwich system without using molds, but I can't recall the name.

Here's is a complete guide to the Rutan moldless method.

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