Wing Design Days 10-13
Updated: Mar 27, 2020
Drag strut holes and cleanup work
After a full three day weekend's worth of work, Design Team can now comfortably say that they have their first complete draft of the wing design. Some of the key new changes include the removal of the rib lightening holes, the addition of drag strut holes, slight tweaks to the drag strut U brackets, and the addition of thrust washers to the aileron torque tube. While finite element analysis and real-life testing of individual parts are yet to prove the functionality of this design, it includes all necessary parts to the best of our knowledge.
Lightening hole removal and drag strut holes
We had originally hoped that the randomly placed circular lightening holes in the ribs would conveniently align to allow the drag struts to pass through them uninterrupted. When we were testing out our variabilization, however, we soon realized that it only allowed a couple specific rib spacings and wasn't very adaptive. So the bulk of the work during these three full days was spent writing the equations so that when a new rib was added or the number of drag struts was adjusted, the drag strut holes would automatically rearrange themselves to adapt. Unfortunately, this came at the cost of a slight increase in design complexity as each rib now has a uniquely placed drag strut hole and the design no longer uses one universal rib design. This will increase manufacturing time and require that we are more careful during construction, but we couldn't find any way around it. Also, we were unable to get the lightening holes to adjust with the placement of the drag strut holes and they were therefore removed. While this will add some weight to the wing, it will also improve its strength potentially allowing us to use a fewer number of actual ribs and instead substitute in a couple additional false ribs. Our next step in rib design will be to construct a full-scale prototype of a main rib, half rib, and false rib in order to get a rough weight estimate and test their strength. For more information on the different types of ribs, check out our Rib Types article.
U bracket modifications
In order to counteract shear forces on the wing, it is necessary to install some form of drag strut which runs between the leading and aft spars. This is typically done with steel drag and anti-drag wires, but we opted to use solid aluminum struts instead for additional strength and to eliminate the need to check the drag wire tension. To attach the drag struts (grey) and compression struts (light blue) to the leading and aft spars, we opted to use steel U brackets similar to the Affordaplane. Previously, the bolt holes to attach the U bracket to the spars were located directly beneath the drag and compression struts. While this worked for the compression strut (which was concentric with the bolt hole), it interfered with the drag strut. To fix this, we simply extended the U bracket a bit more so the bolt head cleared the drag strut. To save weight, we added a 1" chamfer to this extension.
In previous designs, we also didn't factor in how to keep our aileron centered in its slot and prevent it from sliding from side to side and rotating. We asked around on the Home Built Airplanes Forum and got advice to use a thrust washer (yellow) which we will turn out of Delrin and epoxy to either end of our aileron. Users on the forum also pointed out that there needs to be a little bit of horizontal play in the aileron to accommodate for wing bending. To mitigate this, we made our thrust washers 1/8" thinner as a preliminary measure until we are able to calculate the exact required thickness based on our modeled wing bending.
We realized our old bushing design didn't need to be symmetrical because it was going to be glued in place. We removed the bushing cap part and trimmed the other side of the bushing flush with the rib. This saves weight and money, knocks a useless part off our BOM, and reduces design complexity.
*Editors' note: the thought processes and design choices presented in this article don't necessarily represent those implemented into the final design and are subject to change. Flight Club Aerospace is a group of amateur students with no formal education in any fields of engineering. We present this information for educational purposes only, with the understanding that it is not to be re-created without adequate professional oversight and mentorship. For our latest designs and updates, please see our most recent blog posts.