Time for another Sailplane update! This one will be more geared toward activity in the lab. For new readers, within this class, we are split up into smaller “groups” that are responsible for different aspects of the HPA’s build and design. I am in the Wing Group (which this semester has turned into more of the Wing/ Tail Group). For this semester, our goal is essentially to have the entire wing and tail sections built. What this means for us is that we need to make a lot of carbon tubes so we can start building. So for the past few weeks, lab has consisted of a never ending cycle of cutting carbon, buffing mandrels, laying up carbon, baking carbon, removing the carbon tube (without breaking it, hopefully), and occasionally breaking the new tube in the name of science. In other words, we’ve been making a lot of tubes and then testing (so breaking) them to determine the load that they can withstand.
One of the tubes that we made came out fairly well. Once you bake the tube, you have to remove it from the mandrel it is attached to. To do this, we attached two pieces of wood- one to the bare mandrel, and one to the tube itself- and tied them with rope. We tied the knots in such a way that when the two pieces of wood, when twisted in opposite directions, will tighten the knot holding them to the tube. After twisting the pieces of wood
slowly (in an attempt to not break the tubes and waste carbon- a precious resource in the sailplane lab), you can hear a small “pop” when the tube is finally free from the mandrel. Then it’s just a matter of playing tug of war with the tube (literally, that’s exactly what it feels and looks like). With this tube in particular that we made, after removing it, it was then used for testing. However, the test was very informative, as we did some tweaking to the tube before this test. We found that the carbon tube was breaking under tension, which is something that it shouldn’t do, due to the nature of carbon itself. Nonetheless, this first test found that the tube withstood a bending moment of 29 ft-lbs. This gave us a (conservative) factor of safety of 1.5. This is fairly good, however I’ve been told that with the HPA we usually try and aim for a FOS of about 2. To reach this goal, we tried covering the tube with fiberglass, as an added form of strength for the tube. After testing this, we found that the modified tube held 77 ft-lbs, which is a FOS of 4. While this may seem like it gives us an obvious direction to move in with making our tubes, adding that one fiberglass layup to the tube is extremely time consuming. In the end, we are thinking of possibly just adding another layup of carbon. This might not prove to be as effective as the layer of fiberglass, but it will do the job without wasting too much time. If you would like more information about carbon tubes (and especially if this post looked like it was in another language) you can check out the interesting links below.
*Interesting Links*:
http://www.nanocyl.com/jp/CNT-Expertise-Centre/Carbon-Nanotubes
http://www.westsystem.com/ss/assets/Uploads/Buildingcompositetubes.pd
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