Construction Details - Nav Lights Part 3




Here's the fairing after shaping with the sanding batten. All the sharp corners have been rounded over. Care is needed when sanding foam. You can easily remove too much material and wind up with a coke bottle shape in the middle of your airfoil. Next, the voids under the mounting plates will be filled and any tears or gouges that happened during shaping will be repaired.







Aerosol expanding foam (sold for weathersealing and insulation) can be used to fill the voids. It's fun stuff (keep it away from your kids....) but it continues to expand for hours and takes 24 hours to cure to point you can work it. By that time, it'll have turned your nice smooth fairing into something really ugly, so be ready.... But you just saw off the excess and sand again. This foam cures with a smooth skin, but its interior often has larger bubbles and voids. These I filled with a fairing compound of epoxy and WEST system fairing powder (microballoons). Trowel it on and sand it off. Sanding an epoxy/foam combination can be tricky because the epoxy is significantly harder than the foam, even when mixed with balloons. Again, go slowly, and take care not to remove more foam than you want. Keep looking at your work from many angles. This view is the bottom and also shows the rivited joint between the two plates







When the fairing is filled and smoothed to your satisfaction (remember the two layers of cloth that will cover it ) Cover the whole deal with a single layer of lightweight fiberglass cloth and epoxy resin. Be sure to use epoxy; polyester resin, the usual resin used in glass work, will dissolve all your carefully worked styrofoam. You'll be really annoyed. The glass/resin skin serves several purposes. It gives the foam fairing a tough resilient skin, it ties the whole structure together to provide twisting resistance to the pull of the covering fabric when its shrunk, and it provides a barrier coat to prevent the Poly-tak fabric adhesive from dissolving the foam. After glassing, a final light sanding (careful not to go through the cloth...) finishes the job except for running the wiring, and installing the brace shown here and in the next photo.








Here's the back of the finished assembly. That brace is a really desireable addition to the structure whether or not you're doing a navlight installation. Thanks to Chick Torbett for the idea. It REALLY stiffens the tip bow. You can pick up the whole wing and shake it by the tip bow with no flexing of the bow. Impressive. In this installation I expect (hope) it will also provide substantial resistance to the shrinking cover fabric trying to twist the navlight fairing.

Well, I'm publishing these pages right at this point in the work on my wings. After my EAA tech advisor takes a look and I've got em covered, I'll get you an update. Later....








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