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 weight.
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.
This is the exact material replacing foam board on the 20-footer — 1/4″ thick, 48″ wide, 50 feet in a single fanfold roll. No paper skin to peel and wick water while the plane sits outside at an event.
The spec that matters: 1/4-inch extruded polystyrene (XPS) fanfold — close enough to the 3/16″ foam board everyone knows that your building intuition carries over, and one roll covers a giant wing without piecing together dozens of small sheets.
Lower-cost route: any hardware-store 1/4″ XPS fanfold underlayment works — it's sold as siding underlayment in pink, blue, or green, and the brand on the label doesn't matter.
If you substitute a coated or paper-faced board instead, you're back to the delamination that killed last year's plane — glue two scraps of your actual material, cure fully, and try to tear them apart before anything important depends on it.
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. The actual 20-foot build is underway — Part 3 covers Build Week 2: finished elevators, a take-apart tail, 27 laser-cut ribs, and a weight estimate creeping toward the combat limit. Subscribe to catch each stage on video.
Nothing to correct yet — this write-up matches the video. The full-size build is underway: Part 3, Build Week 2, is written up here. Spot something off? Tell me and I'll fix it.
Auto-transcribed from the video, so expect the odd mis-heard word. Here so you can search or skim what was said without watching.
Last year Preston Goes and I built a 10 foot RC airplane and crashed it in front of the entire field at FlightFest. We designed most of the plane on the fly inside the back of an RV in the middle of some crazy storms and luck was definitely not on our side. The glue failed, the wing didn't create enough lift, the power system was mostly a guess and grossly underpowered. Our first real launch was also our final flight. So this year Preston and I are trying again, except the new plane is going to be double the size, 20 feet wide, which means every bad decision from last year is about to become twice as expensive and twice as destructive.
So before I build the full plane, I need to prove that this new design can actually work. I'm testing some completely new materials that I've never used before. I need to find adhesive that won't peel apart overnight, a wing profile that can lift the slow heavy cargo plane. I need a spar strong enough to hold together a 20 foot wing without buckling without adding so much weight that the wings fold and the entire plane comes barreling into the ground. And the build hasn't even started yet.
I already have a mountain of Amazon boxes with motors, electronics, hardware, parts and a credit card statement that is making me wonder if Preston is ever going to ask me to build anything with him again. By the end of this video, I need to finish the design, test the materials, build a working wing section and fly a small prototype using the same construction method. Because next week, I start building the real plane. And once I cut enough materials for a 20 foot wing, there's no cheap way to start over. So me and Preston Gozer teaming up again this year and we're building this massive airplane.
This is a mockup I made out of foam board, but it's not going to be strong enough for a 20 foot wingspan. The design always looks simpler on paper. The hard part is making every piece work together. And after struggling and getting the design just right, I realized that this entire plane now depends on one beam. And this is probably one of the most important pieces of the entire plane, the spar.
The wing spar is what stops your wing from doing this. And as you can see, there's normally some kind of wood structure inside of the wing. So you might think a two by four would be a fantastic wood spar. But when your plane is this big and you need this much wood, that gets really heavy really quick. So what we have to do is split the board down the middle, cut strips and put foam in the inside.
So it becomes a super strong structure that's fairly lightweight. Last year, the power system was mostly a gas and the plane was badly underpowered. So this time I'm sizing electronics around the plane instead of hoping they're enough. The motors have to produce enough thrust to get a heavy cargo plane off the ground. The ESC's have to handle that power without overheating.
And the batteries have to provide enough current without adding so much weight, the airplane can't fly. And because this plane is so large, the servos and wiring also have to be much stronger and more reliable than what I normally use. The goal isn't to make the most powerful airplane possible, it's to build a power system that gives this plane enough margin to actually fly. And even if we have the electronics all figured out, we have to build an airframe that's able to handle that power. Foam seems simple until you build a plane that's 20 feet wide.
This foam board that you can get from Dollar General is fantastic for these small builds. But when you're talking about a 20 feet wide plane, it's going to sit outside with moisture and heat. And this stuff has paper that likes to peel off and isn't great when it gets wet. To solve that, I'm not even going to use this stuff. I got this pink insulation foam here from Menards and it's about the same dimension, but this stuff doesn't have that paper layer.
So it's going to be way more resilient in the long term. And it's also thin enough that we can bend it and form it around our wing. Glue. Last year in the rush of everything, I made one critical design failure that really killed everything. The glue.
So this year, before we build anything, I'm testing which glues actually are going to perform. Last year we used Gorilla Glue, which just peeled off the plastic layer that was on the foam board we were using. So this year, I'm going to try this foam glue and see if it's actually worth it. Is this stuff going to be super annoying to work with? Or is it going to be like the best thing ever?
We've still got Gorilla Glue for our foam to wood applications. And we've got 30 minute epoxy where it really matters. And now that we have a plan for our materials, I want to build something because I've never used these materials before and a 20 foot airplane is not the place to try it for the first time. So I cut off this simple flying wing design, glued it together and added the power up 4.0 module. It didn't fly great at first, but after I adjusted center gravity a little bit, yeah, yeah, yeah, it's doing it.
See if it'll actually turn. No, not really. So after that, I wanted to build a glider that's a little bit more like the actual 20 foot plane. So I scaled it down, cut out some ribs on my bamboo lab H2D laser cutter. Honestly, this thing's so nice just to have for easy prototyping.
Okay, so we've got our wing skin right here. And what we've also got is a couple of strips. And these are actually our wingspars, which they're not super strong on their own. But what I'm going to do is I'm going to take a thin piece of wood and we're going to put it on the top and we're going to put it at the bottom. And that's going to make a really solid structure.
And that is exactly what we're going to do on the full scale version. I'm doing two things. I'm trying to figure out how to use this foam because I've never used it before. And I'm also trying to make sure my construction method actually works because I don't want to have another repeat of last year. And then we're going to take and put some airfoil ribs in here.
And that's going to help shape the wing and we can take our skin and put it over top of it. Yeah, my initial review, that stuff's a pain in the butt. It takes forever. It's sticky and nasty and hot glue is way easier. So now we should have a really strong spar.
Yeah, that's actually pretty solid. I'm impressed. Basically what you're doing here is creating a tension beam. So if you're trying to flex it this way, this is going to be pulled taut. If you're trying to flex it this way, then this is going to be pulled taut.
We've got our wing structure here, which is pretty light and I think really strong. I could probably cover this with some kind of covering foam. I guess it's kind of like we're not going to be able to really tell what it's actually going to be like until we actually build a section of the wing. So this is just kind of to get used to the foam more than anything. We're going to throw together a quick piece of the log just in the tail.
We'll see if it glides. Not too bad. Oh yeah. That's good. Let's take it outside.
A week ago, this airplane only existed in my head. Now we've got design files, material I can actually trust, and a construction method that I think can survive a 20 foot wingspan. There's only one thing left to do. Next week, we start building because this time we can't fail again.
