Last year Preston and I crashed a 10-foot plane in front of the whole field at Flite Fest. This year we're going double — 20 feet — which means every bad decision costs twice as much. So before cutting a single piece of the real wing, I had to prove the design: the spar, the foam, the glue, and a flying prototype.
| What killed Big Red | The fix, tested before building |
|---|---|
| Glue peeled off the foam's plastic film overnight | Three adhesives tested on the actual materials first |
| Wing didn't make enough lift | Purpose-built airfoil with ribs, prototyped and glide-tested |
| Power system was "mostly a guess" | Electronics sized around the plane, with margin |
| No structure plan for a huge span | Foam-cored box spar, built and flex-tested by hand |
At 20 feet, the wing spar stops being a detail and becomes the design. Your instinct might be "just use a 2×4" — but scale that across a span this big and the weight becomes absurd. A spar has to be strong and light, or the wing folds under its own solution.
What I landed on: split the board down the middle, cut strips, and sandwich foam in between. It sounds too simple to work. It isn't, and here's why:
A spar is a tension/compression beam. When a wing lifts, it bends upward — which stretches one side of the spar and squeezes the other. As I put it while flexing the test piece: bend it one way and the top strip pulls taut; bend it the other way and the bottom does. Those thin wood strips (the spar caps) do all the heavy lifting, while the lightweight foam core's only job is to hold them a fixed distance apart so they can't buckle toward each other. That's the same principle as a steel I-beam or a real aircraft spar — nearly all the strength lives at the top and bottom surfaces, so the middle can be something almost weightless.
Verdict after building one: "Yeah, that's actually pretty solid. I'm impressed." Strong, light, and cheap — and now proven at small scale before it gets committed to a 20-foot version.
Paper-faced foam board is the hero material of this channel — it's perfect for 30-inch builds that cost a dollar and live in a garage. But this plane is going to sit outside in heat and moisture at an event, and that paper skin peels and wicks water. On a small plane that's a bad afternoon; on a 20-foot wing it's the whole project.
So the airframe moves to pink insulation foam (XPS) from the hardware store: similar thickness, no paper layer to delaminate, far more resilient to weather, and thin enough to bend and form around the wing shape.
The exact stuff I'm using: Owens Corning FOAMULAR fanfold underlayment from Menards — 1/4 inch thick, 48 inches wide, and 50 feet long in a single fanfold roll. That thickness lands right next to the 3/16-inch foam board everyone knows, so the building intuition carries over, and one roll is enough material for a giant wing without piecing together dozens of small sheets.
Not near a Menards? Ask for 1/4-inch XPS fanfold underlayment — most home centers carry the same thing under a different brand (it's sold as siding underlayment, usually pink, blue, or green). Any 1/4-inch extruded-polystyrene fanfold works; the brand on the label doesn't matter.
Two things to know if you try this: XPS has no paper skin, so it gets its stiffness from your structure rather than the surface — which is exactly why the spar and rib design matter so much here. And it's chemically fussier than foam board: solvent-based adhesives and some spray paints will eat it, so everything that touches it has to be foam-safe.
Last year, one adhesive decision quietly killed the build — Gorilla Glue grabbed the foam's plastic film instead of the foam itself, and the wings came apart overnight. This year the glue got tested first, on the real materials, before anything important depended on it:
The transferable lesson for any builder: glue two scraps of your actual material, let them cure fully, then try to tear them apart. Five minutes and two scraps will tell you more than any product label — and it's the exact test that would have saved last year's plane.
A 20-foot airplane is the worst possible place to try a technique for the first time — a lesson I learned the hard way when I scrapped a build mid-project for exactly that reason. So before the real wing, I built two small aircraft using the new methods:
That glide was the entire point of the video. It doesn't prove the 20-footer will fly — but it proves the construction method produces a wing that holds its shape and generates lift, which is exactly the thing we couldn't say last year.
The H2D's little laser module cuts ribs fine at this scale, but once you're cutting a full rib set for a real wing it runs out of bed and patience. That's when a dedicated CO2 laser earns its keep — I keep a few OMTech picks in the gear guide, and code ERICROBBRC takes 5% off if you go that route.
Last year's electronics were bought at the event and hoped over. This time the order flips: figure out what the plane will weigh, then choose components that carry it with margin.
This is the same math the Airplane Designer runs — pick Cargo/Scale, enter a span, and it returns the weight estimate, watts, thrust target, and battery size. On a giant it'll also tell you when one motor can't do the job and the plan needs two, three, or four. The calculators cover the individual numbers if you'd rather check them one at a time.
Design done, materials proven, method flight-tested. Next week the actual 20-foot build starts — and once that much material is cut, there's no cheap way to start over. Subscribe to catch the build, and I'll write up each stage here as it happens.
