Wing Design

Wing Design Day 16: Support Washers, Trailing Edge Revisions, Support Blocks; Oh My!

This post isn’t about one particular day but rather several weeks of hard work. School, homework, and, for some, college applications, means we can no longer put in 40 to 60 hours per week like we were able to during the summer. With that said, we’re no less enthusiastic and we’re still working hard to get this plane up in the air by this upcoming summer! Let’s go over our most recent changes and the thought process behind them:

SUPPORT BLOCKS AND UPDATED FALSE RIBS

New design using foam support blocks
OLD design using aluminum stringer

We substituted the thin 9/16”x0.035” aluminum stringer that ran the length of the wing with a series of XPS (eXtruded PolyStyrene) foam blocks similar to that used in the Affordaplane. When we initially designed the stringer, we only planned for it to support the minimal force applied to the false ribs due to fabric tensioning. What we didn’t take into account is the buckling force of the rib caused by the moment generated at the spar attachment points when fully loaded to 5.7g (approx 240lb per rib). While our compression struts are designed to counteract this buckling, were they to fail, the rib would immediately cave into itself and that would NOT be good.

Though the rib held 280lb, it needed to be held to prevent it from bending and buckling
The idea behind the foam support blocks is that they are placed midway between the two spars and will effectively quadruple the buckling strength of the rib by reducing the column length by half according to Euler’s column buckling equation. Unlike the stringer, they run from the full width of the rib, providing better contact and rigidity. Though the ribs aren’t intended to carry any compressive load, were the compression struts to fail, the wing wouldn’t explode mid flight. Because the false rib attached to the stringer, the false rib design had to be slightly updated to epoxy into the support blocks.
New squared off false rib design

SUPPORT WASHERS

We discussed the reasons behind using support washers or support slings in our wing design day 15 blog post so we won’t get into those here. Long story short, we stayed up until 4am one night cutting out support slings only to find out our bandsaw skills weren’t good enough and the support slings were either too loose or didn’t fit. We were hesitant to use support washers because they rely on the adhesion between the washer and the rib but our testing proved they were much stronger than needed. While the support washers should technically be 6” in diameter to prevent tearout, we were only able to make them 4 ¾” due to the thickness of the rib. Nevertheless, they are still strong enough during testing so we’re good to go. The support washers are made of 0.016” 2024 T3 aluminum sheet metal which is so thin it can be cut out with scissors. There are technically three different diameters of support washers: rib, aileron, and aileron rib, but that will be covered in a later article.

TRAILING EDGE REVISIONS

We discussed the reasons behind using support washers or support slings in our wing design day 15 blog post so we won’t get into those here. Long story short, we stayed up until 4am one night cutting out support slings only to find out our bandsaw skills weren’t good enough and the support slings were either too loose or didn’t fit. We were hesitant to use support washers because they rely on the adhesion between the washer and the rib but our testing proved they were much stronger than needed. While the support washers should technically be 6” in diameter to prevent tearout, we were only able to make them 4 ¾” due to the thickness of the rib. Nevertheless, they are still strong enough during testing so we’re good to go. The support washers are made of 0.016” 2024 T3 aluminum sheet metal which is so thin it can be cut out with scissors. There are technically three different diameters of support washers: rib, aileron, and aileron rib, but that will be covered in a later article.

WING GENERATOR REVISIONS

 This section is full of a lot of a lot of technical CAD jargon so feel free to skip ahead if that’s not your thing. In short, we updated the wing part studio to now include an option to generate all geometry that would typically be included in the assembly. Though our old wing generator would create all unique parts (a standard design practice), it required the user to copy and arrange them as necessary in an assembly window to find useful data such as CG, weight, etc. Now there is an option to generate all parts, not just unique ones, making the wing generator a lot more user friendly for those without Onshape experience.

That’s all for today! We hope you found this article informative and useful for your own ambitions and projects. Now here’s a photo of Ollie’s cat Sweet Pea saying goodbye.

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