BUILD DAY: DAY 2
On Saturday, we finished polishing the spar support washers and routing the wing’s primary ribs. As mentioned in previous blog posts, we initially attempted to cut out the rib profiles using a hot wire cutter and the bandsaw. We encountered issues with both methods: the hot wire cutter required constant speed to avoid deforming the foam and the bandsaw, while an upgrade from the hot wire cutter, required a steady hand to produce an accurate profile. Eventually, we found a way that works perfectly for us:
CNC mill a stencil for the foam profile we want out of ¾” MDF or plywood. Use the stencil to create an outline on foam but leaving about an inch on all sides.
Jigsaw out the foam (which was a lot faster and safer than using a bandsaw, even if it dispersed 10x more foam particles everywhere). Use about 5 pieces of double-sided tape that are ¼” in length so the wood stencil and foam can be stuck together but not impossible to separate later on. Also, leave foam sticking out on all sides.
Then use a router table with a 1 ¼” bit to cut out the exact foam profile. The bit has a bearing at the bottom which follows the CNC profile while matching it exactly to the foam piece adhered above. Please note this is a 3 person job. The rib requires two people to handle, and we used another person on vacuum duty since foam dust goes everywhere. Use an Exacto knife to separate the foam from the wood profile and voila! You have a perfectly cut foam rib!
This was definitely the most efficient and accurate out of any other method we tried. During this day, we were able to route out all 10 ribs and an extra!
BUILD DAY: DAY 3
On Sunday, we began to apply fiberglass on the outline of the primary ribs we routed on Saturday. In the blog posts about the wing design process and rib testing, we covered how we first used 3/32” aerospace-grade plywood cap strips on the top and bottom of the ribs. Unfortunately, it didn’t work that well – the plywood had difficulty adhering to the ribs and the ribs failed at 100lbs, half the weight we were anticipating it to be able to carry. The plywood’s overall lack of strength and difficulty to adhere to the foam led us to switch to using fiberglass cap strips instead. We also decided to incorporate glass micro-bubbles into the epoxy mixture for better adhesion to the foam.
To create the epoxy mixture used to adhere the fiberglass cloth to the ribs, we combined one pump West Systems 105 epoxy, one pump West Systems 105 hardener, then twice that volume of glass bubbles. This was only possible with the use of the West Systems pump kit which cost an additional $20 but it’s worth every penny. The process of measuring out the glass bubbles was tedious and occasionally inaccurate, so we had to sacrifice a few cups of epoxy because they were either too runny or thick. We would recommend creating a system with an actual measuring cup and process of extracting the power to limit the amount of glass bubble exposure. Please also note that the glass bubbles are unsafe to inhale and you should follow the SDS (ours is linked) recommended guidelines for your specific product.
Carefully measuring out the glass bubbles
After the epoxy mixture was made, we began to apply the epoxy and fiberglass cloth onto the rib. In our first attempt to apply the epoxy, we applied a coat of epoxy on the entire perimeter of the rib and then layered on the fiberglass cloth. This method wasn’t the best because the epoxy would partially cure before we were able to layer the fiberglass cloth.
We tried a few methods but this is the one that worked the best (check out our time-lapse here!):
- Starting at the trailing edge, apply a thin layer of epoxy to the top of the rib
- Add the 1” fiberglass cloth starting 2 inches forward from the trailing edge. As the cloth is laid down, press it down with a gloved finger being careful not to shift the fabric weave.
- Flip over the rib and continue to the other end
- Flip it once more and continue applying the epoxy and fiberglass until it overlaps by 2 inches. You can use a rotary fiber cutter to cut the fiberglass
- Take your popsicle stick, drag it along the top of the rib making sure to press down firmish to drag out any extra epoxy. If any spots look “dry,” rub in a little more epoxy.
- Flip the rib and do this to the bottom
- Turn the rib on either side and scrape off all the epoxy off the sides that leaked
- Leave it outside to dry for 24 hours (we left a 2” x 4” piece of wood below the rib so none of the epoxy sticks to the ground)
Laying down the fiberglass fabric
An innovative way to cut the fabric once installed
They ended up looking great and super sturdy!
Our final ribs!