Learning how to make the seals for the wing control surfaces has been a bit of a process for me. We always use a silicon hinge (see two posts earlier), usually at the underside of the wing. This means that a nicely set seal really helps the plane look good, but of course also helps flight characteristics. In the past I’ve had to redo the seals quite a few times when they didn’t come out well, or I had to spend a lot of time sanding them. The Urupema, my third fully scratch-built plane, has the nicest seals I’ve made so far, and I’ve learned again for next time.

What I’ve learned so far: of course the tape is really important. We use a relatively hard PVC tape that is easily removed once the epoxy resin has cured. Also important is cleaning, especially when you have carbon in the wings. It just takes a bit of carbon dust to give you ugly black spots in the nice white seal. The first few seals I set I made the expoxy too thick, which meant that it didn’t distribute nicely the seals looked ugly. I still thicken the epoxy with micro-balloons and a bit of aerosil (and of course apply colorant), but it can stay quite liquid. Use a good syringe to apply it on the tape. Wait for it to cure to a point where it’s still elastic enough to bend, but no longer liquid. Then insert the tape under the wing (I use a ruler) putting the control surface at around a 20-30 degree downward angle. Fix into position and let the epoxy cure. If you apply colorant, you may want to give it some extra time to really harden out (depending on the temperature in your workshop). I usually leave it for two days, then remove the tape and sand the seals down to size (allowing enough upward movement).

For the Urupema I had a lot of work shortening the seals to get enough upward movement in the control surfaces, something to take into account for next time. They’re also quite thick, but then weight isn’t as much of an issue with this plane. I’m pleased with the result.

For the past few projects I always had one of my friends help with cutting out the control surfaces in the wings and elevator. It’s something that easily goes wrong and then ruins lots of hours of work on the wings. With the Corona lock-down rules, having seen it done a few times and actually having all the required tools in my own workshop I though it was time to do it myself. And it went surprisingly well, without any major hiccups, and was easier than I feared – although it does need a steady hand! I used an ancient Dremel with a 0.8mm bit for the underside of the wings (hinge) and a 1.5mm bit for the upper side of the wings. One of my colleagues in our club made a very clever add-on for the Dremel that allows you to guide it along an aluminium ruler. The rules is positioned and glued to the wing with bits of double sided tape. My mate Georg also very helpfully provided a template for cutting out the servo openings, which made it easy to do those.

After cleaning out the foam, I sanded down the upper leading edge of the wing, so it’s razor sharp, and applied a strip of 60gr glass to protect the exposed abachi wood and give the edge a bit more rigidity. Once that was hardened out I set the hinges using silicon glue. This will now need to rest for approximately 5 days, after which I can start doing the seal on the upper side of the wings.