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Tuesday, August 31, 2010

Busy Weekend

On Friday afternoon I installed the centering rings on my engine tube. Since I plan to install an Aero Pack Engine Retainer I had to alter the assembly instructions slightly. The instructions call for the centering rings to be mounted 1/8" from each end of the engine tube, but the motor retainer requires 3/8" for mounting. I checked the fit with the rear centering ring moved 1/4" forward and found that that puts the forward edge of the ring right at the back of the fin slots in the tube, so proceeded to assemble the engine tube in this configuration. I then prepped the bad area of fiberglass on my payload bay tube for patching by sanding out the bad area and feathering out the surrounding area.

On Saturday I fiberglassed, patching my payload bay tube and adding a layer of the fiberglass sleeving to the engine tube between the centering rings. The primary motivation for adding the fiberglass to the engine tube is to account for the extra thickness of the airframe tube when mounting the fins. There is an added benefit in that the fiberglass should also strengthen the connection between the engine tube and the centering rings.

One of these days I will learn that fiberglass doesn't like making 90° bends. I cut the sleeve for the engine tube quite a bit longer than the tube in order to allow for fraying as I worked the sleeve over the centering ring and stretched it out on the engine tube. I thought I'd be able to bend the sleeve up along the centering rings. This proved harder than I anticipated. Once I had my epoxy applied I found that I couldn't keep the fiberglass in contact with the tube as it approached the centering rings. I had a length of the shrink tubing available that was the right length, but I couldn't see how I was going to keep it from contracting lengthwise while I shrunk it as the centering rings would keep me from holding it with my pliers. I decide that I had to try using the tubing anyway, as I wasn't getting what I wanted without it. When I tried to pull the shrink tubing on over the engine tube the fiberglass pulled off the engine tube and ended up a wadded mess inside the shrink tubing. I pulled the shrink tubing back off and straightened out the fiberglass. I ended up doing this twice as a second attempt yielded the same results. I finally resorted to cutting two lengths, each about 1" in length, from the shrink tubing and working them onto the engine tube. After sorting  out the fiberglass yet again I put one at each end of the engine tube and shrank them in place, using my stirring stick from my epoxy to push the tubing sections towards the centering rings as I heated them. I did a better job of this apparently at the forward centering ring than at the aft one. The finished product has a fairly tightly radiused curve in the glass as it transitions up onto the centering ring with the gap under the radius filled with epoxy. At the aft end the radius isn't as tight and there are areas that are not epoxy filled.

On Sunday I installed the engine tube in the airframe and proceeded to install the fins. I had to use my Dremel with an engraving cutter to cut out the radiused fiberglass at the aft centering ring in order to get the fins to insert into the slots. The poor bonding at the aft centering ring worked in my favor in this case, making it easy to remove the obstruction.

On Monday I used Loctite Repair Putty to form fillets between the fins and the airframe. I worked on sanding out the fillets, but ran out of time. More sanding tonight.


Photo Album: Fiberglassing 103

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