Lessons On Wing Rib Construction
It’s finally time to start building! Okay maybe not quite yet but we are starting the prototyping phase of our design process. The San Francisco shop (located in Rudy’s garage) is now cutting out
wing rib prototypes and the Hillsborough shop (located in Justin’s backyard) is about to start some gusset testing. We decided to make prototypes for several reasons:
Prototyping gives us insight into how difficult and time consuming making the final product will be. By measuring how much time it takes to make only a couple parts, we can get a better picture of how long it will take to make the full quantity.
Prototyping is a great way to practice the skills we’ll need to make the final product.
Some parts such as our XPS foam wing ribs are of our own design and therefore need to be prototyped to make sure they will work, as well as strength tested to ensure their viability and calculate the amount needed.
Being able to actually hold a tangible finished product can reveal tolerance and rigidity issues as well as other unanticipated problems.
Our wing ribs are made of a 1” thick DOW brand XPS (eXtruded PolyStyrene) foam core with a 3/32” mahogany aerospace-grade plywood cap strips on the top and bottom surfaces.
The mahogany plywood looks awesome and definitely improves the sex appeal of the ribs. Unfortunately, the ribs will ultimately be covered by fabric but we’ll all sleep better at night knowing that our plane looks good on the inside as well. Birch ply is also an affordable substitute, but they didn’t have any large enough sheets in stock at AircraftSpruce (not as cool looking though).
There are TONS of different ways to construct wing ribs from bent aluminum tubes to complex composite molds but we ultimately decided on the foam and plywood design because it’s relatively easy to construct and wouldn’t require any expensive tools or complex skills. Foam ribs are also inexpensive and (unofficial) testing suggests they are also quite strong. Other rib geometries and construction techniques will be covered in detail in a later blog post.
Our rib making process is quite simple (a main reason we chose it) and here are the basic steps:
Cut the rib profile out of 1” XPS foam. We used a band saw but you can also use a hot wire, jig saw or hacksaw, or other cutting device depending on how jank you’re willing to go and how good your sanding skills are to clean it up. The ideal would be to use a CNC mill, but we didn’t have access to one with a large enough bed (6’ 6” minimum)
Cut 1” strips out of 3/32” plywood (we recommend going with the quality AA aerospace grade stuff to avoid all imperfections and possible points of failure). These strips should be slightly longer than your chord. We used a table saw for this step, although most other kinds of saws will work for this, although they likely won't produce as straight of a cut.
Mix epoxy in a paper (it will melt plastic) cup using a popsicle stick. Cover the top and bottom of the foam rib with epoxy using a disposable brush, we used around one pump of epoxy, using West System’s kit with pumps, to cover one entire rib. Go light on the epoxy, we found that heavier coats produced a lot of drip which in turn create lots of issues.
Lay the wood cap strips down over the top and bottom of the foam. Clamp and let dry. We recommend cutting airfoil profiles out of mdf and using the excess as a clamp.
Anyways, we learned a lot while constructing our first prototype ribs and we thought we’d share some of our lessons with y’all so you can *hopefully* avoid making the same mistakes.
Lesson #1: The hot wire is not your friend
Right off the bat we struggled cutting our rib profiles out of the XPS foam using our janky home made hot wire cutter.
While many home builders opt to manually draw out their airfoil profile by hand, we decided to CNC mill a stencil out of Medium Density Fiberboard (MDF) to use as a stencil for the hot wire cutter.
This seemed like a great idea at first but when applied, the hot wire was a little too hot and ended up burning the MDF stencil and deforming the foam if not fed at the exact right speed.
Since our stencil was only on one side of the foam, the hot wire also tended to tilt inwards while making a cut so the top and bottom surfaces of the wing rib were tilted. A lot of much more experienced home builders on the HomeBuiltAirplanes forum swear by hot wire cutting but we concluded that it would take more practice than its worth to master.
To be fair, our hot wire cutter was also a really janky one Ollie threw together in his free time, and a square hot wire table would have definitely been a better option. Anyways, after hot boxing the garage with carcinogenic foam smoke we decided to give the band saw a try. Instead of cutting along the MDF stencil, we simply traced it onto the 2’x8’ XPS foam sheets and then cut them out on Rudy’s band saw. To make it a little more manageable in the saw, we actually cut out the rough area of foam surrounding the rib drawing and then made a final detail cut. The band saw ended up being the ultimate solution to all our problems.
Unlike the handheld hot wire cutter, the band saw had a table which the blade was more or less perfectly perpendicular to which eliminated the inward pitching issue.
Unlike the hot wire, the band saw didn’t need to be fed at a constant rate speed or else overheat so we could take breaks mid cut and generally take our time to get the best looking final product.
At first the idea of cutting foam with a single wire seemed appealing because it offers additional precision in tight corners but the length of the band saw blade was actually really helpful in keeping our cuts straight and smooth along the entire surface of the rib.
Lesson #2: Use clamps not tape
We thought the capstrips would be more flexible than they were in reality and we had planned to hold them in place while drying with only painters tape. In practice, the painters tape wouldn’t stick well to the ribs because any epoxy ooze would get on the tape and remove its stickiness. Two rolls of expensive painters tape later we finally got the cap strips attached but a clamping setup would have been helpful. We actually tried to use regular bar clamps but they kept sliding off because of the curved profile of the rib. Going forward, we’re going to use the MDF stencil to cut out a rib negative to clamp to the rib and to apply even pressure to both capstrips while drying.
And with all that said, here is our final product…. Or, at least our final prototype. Anyways, stay tuned for an upcoming testing blog, where we walk you through our test results and how we destroyed this beautiful assembly one bucket of sand at a time.