Why passenger jet wings are designed to bend

HostIf you have ever had a window seat on one of those big new long-distance jets, and you happened to look out at the wing during takeoff, it can be a little spooky. The wingtips don't just stay flat. They arch way up, almost like a bird’s wings when it's soaring through the air.

HostIt looks like the metal is straining under the weight of the whole plane, which makes you wonder why we build them to be so floppy in the first place.

GuestIt's a striking sight, and it's funny because our brains usually tell us that if something is strong, it should be stiff. We think of a plane as this solid, heavy tube of metal. But in the world of high-stakes building, being stiff is actually a weakness. If those wings were built to stay perfectly straight and rigid, they would need a massive amount of extra material to keep them from snapping under the pressure. All that extra weight would make the plane far too heavy to even get off the ground.

GuestA wing that can bend is actually much tougher because it lets the stress move around. Instead of all that force hitting the one spot where the wing meets the body of the plane, the flex lets the stress spread out across the whole length. It's a lot like how a tall tree stays up in a bad storm. If the trunk stays totally stiff, the wind just pushes and pushes until it snaps at the base. But if the tree sways and bows, it lets that energy go. The wing is doing the exact same thing to stay in one piece.

HostI get the idea of a tree swaying, but a tree is a living thing. A plane is a machine. It feels like if I bend a piece of metal back and forth enough times, it's eventually going to stay bent or just break.

GuestWell, you're not wrong about metal. Older wings were mostly made of aluminum, and metal does get tired. If you bend it too far or too many times, it can wear out. But that's why the wings on those newer planes look so much more dramatic. They're made of carbon-fiber mixes. These modern materials can handle a lot more stretching and pulling without ever losing their shape.

GuestBecause they're so much better at handling that strain, engineers can make the wings longer and thinner, which helps the plane slice through the air with less work. Here is the really wild part, though. Those wings are built with a special twist in them. When the plane is just sitting on the ground, the wings aren't actually in their best shape. They only settle into the perfect, most efficient shape once the air pushes them up into that high arch. So, when the wing looks the most bent out of shape to a nervous passenger, that's actually when it's working at its absolute peak.

HostSo it's actually performing better when it looks like it's about to snap? That's a lot to ask a passenger to trust. Especially when you hit a pocket of rough air and the whole thing starts swinging up and down.

GuestThat swinging is actually your best friend during a bumpy flight. Think of the wing as a giant shock absorber, kind of like the suspension in your car. When you're driving and you hit a pothole, the springs in your car soak up that jolt so you don't feel the full hit in your seat. When a plane hits a vertical gust of wind, that energy has to go somewhere.

GuestIf the wings were rigid, every little bump in the air would feel like a sharp, metal-on-metal strike. You would be tossed around the cabin. By bending, the wings soak up that vertical jump before it can even reach you. They turn a sharp, jarring hit into a smoother, swaying motion. It makes the ride way more comfortable for the people inside, and it also saves the body of the plane from a lot of wear and tear over twenty or thirty years of service.

HostI hear you on the comfort, but there has to be a breaking point. I have seen videos of those wings flapping pretty hard in a storm. Is there a limit to how many feet they can bend before the material just gives up?

GuestThere's a limit, but it's way higher than anything you would ever see in the sky. Before any new plane design is allowed to carry passengers, they have to do what they call an ultimate load test. They take a full-sized plane, put it in a giant rig, and use big machines to pull the wings upward.

GuestInternational safety rules say the wings must be able to hold up one hundred and fifty percent of the highest load they could ever possibly encounter in the worst flight conditions imaginable. In these tests, the wings often bend more than twenty-five feet up from where they started before they finally break. The few feet of movement you see out your window during a storm is just a tiny fraction of what that wing can handle. It's operating well within what we call its elastic zone, which just means it'll always snap back to its original shape without any damage at all.

HostSo even if the wing is arching up high enough to cover the view from the window, it's still nowhere near the point of actually failing.

GuestNot even close. You would need a storm much worse than anything that actually exists on this planet to get anywhere near that breaking point. The wing is designed to find its true, perfect form only when it's under that massive pressure of holding the plane up in the sky.

HostThe next time I'm in that window seat, I'll try to look at those arching wingtips as a sign that the plane is finally in its groove and doing exactly what it was built for.