Friday, January 21, 2011

Things Come Together

Well, I'm back at school after a month-long (!) winter break. I enjoyed the time off, but the fact that there was so much of it at once meant after a few weeks I was looking forward to being back. In addition to the fact that we're going to be missing out on some three-day weekends to makeup the class days from the long break, Aerodesign Team knew that once we got back, we'd be only six weeks out from the Design, Build, Fly paper deadline. Thus, there was a lot of work going on over break, and I'm proud to show off some of what I was up to with the other members of the Connections Group.
Teaser for Below the Jump
The main goal over break was validating the design of the tail connections we designed earlier in the year. This tail connection holds the tail boom onto the fuselage. Thus, it has to withstand forces along the boom axis, various twisting forces (moments) caused by the control surfaces, bending forces on the tail boom and not twist in place too much. We'd designed and produced a part to handle the transmission of these forces. We originally looked at aluminum, but weight restrictions required a switch to nylon for this latest version. It's very rewarding to be able to hold it and know that we took this from requirements to design to testing.

With this new part in hand, we were able to do a lot of validation test. Our first tests were using the piece of plywood shown in the image above to construct a replica of the receiving portion of the bulkhead this part will attach to. We then tested fit and ease of use with the part. It goes together smoothly, and the fit is very good--not much wiggle or anything, which is good because small errors here are magnified by the lever arm of the boom.

The next step was adding loads. We first tested that it could take rotational moments. We drilled a hole through the section of the part where it will attach to the boom, and applied first a moment of 7.25 Newton-meters, then 14.5 Newton-meters using weights suspended from a rod inserted through the hole. The part took the moments without any issues. The applied moments should be far larger than any forces encountered in flight, so we feel confident with our design in this area. In addition to the teaser photo above, another photo of our test setup can be seen below.

Side View of Moment Resistance Testing
The next step was testing resistance to axial forces. For this, we held the plywood bulkhead simulator parallel to the floor and suspended weights from a string through the hole in the part to apply forces parallel to what would be the boom axis. Again we tested at two levels, first with 4.9 N, then with 9.8 N. Again we had no issues and the part took the load. We believe these should exceed any forces the tail will place on the connection in flight. An image of this testing is below.
Finally, we trimmed the plywood bulkhead down and assembled a simulated fuselage end, and glued the test part into some spare carbon-fiber tube of the appropriate diameter. This allowed us to test adding the ability to support moments applied by the tail during pitch and yaw maneuvers. Again, we were very pleased with the ability of the design to support forces--if fact, I'd say I think our test rig would break before the connection we built. Thus, I feel pretty sure that as long as the aft end of the plane is well-built and the bulkheads attached to the carbon-fiber skin correctly, our connections won't be a failure point. Additionally, the section assembles very quickly--it takes only a few seconds to make the structural connection, and then maybe a few more would be required to hook up the control wiring.

In fact, with the performance we saw from some parts which we used here (one component of the connection was something we hadn't used before) we're looking at redesigning the wing connections to incorporate lessons learned. The wing connections are basically designed, but one element was a little clumsy in the first pass. A redesign to incorporate our tail connection test experience could cut the assembly time there in half. It feels good to be at a point where we can now refine our design with some confidence in the basic performance, and I'm looking forward to the construction of a full prototype. The next few weeks will be busy, but if they're as rewarding as these weeks were, then it'll be worth the work.

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