How engineers shape wind turbine blades to catch wind

HostI was driving out through the hills last week and passed a huge wind farm. From a distance, those blades look like they're barely moving, almost like they're just drifting along. But when you get closer, you see how massive and sleek they really are. Why that specific curvy shape? Why not just make them flat like the paddles on an old-fashioned windmill?

GuestIt's a bit of a trick of the eye. Those tips are actually moving at nearly two hundred miles an hour. If you used a flat board, the wind would just slam into it. You would get a lot of push, but the whole thing would be clunky and slow. To really get power, you have to stop thinking about a sail and start thinking about a wing. The shape you see is called an airfoil.

HostSo is it basically an airplane wing stood up on its end?

GuestYeah. One side is curved more than the other. When the wind hits the front edge, it has to split. The air going over the curved side has to move faster to meet up with the air on the flatter side. Fast-moving air doesn't push as hard as slow air. That difference in push actually pulls the blade forward. We call that lift. It's the same force that keeps a heavy jet in the sky, but here, we're using it to pull a wheel around in a circle.

HostWait, so the blade is being pulled forward? I always pictured the wind just shoving it out of the way.

GuestA flat paddle would work like that, but it's not a great way to do it. If the wind just shoves it, you lose a lot of energy to heat and noise, and it puts a lot of stress on the tower. By using lift, the blade slices through the air. It's much better at catching the energy. But the really strange part is when you look at how the blade changes from the bottom to the top. Have you noticed how they look sort of twisted?

HostI noticed that. I thought it was just to make them look nice, or maybe it was just the way they were built. It looks like a piece of wood that got wet and warped.

GuestIt's very much on purpose. Think about a merry-go-round. If you stand right in the middle, you're barely moving. But if you stand on the very edge, you're flying. A wind turbine blade works the same way. The part near the center hub is moving slowly, but the tip is screaming along.

HostSo the wind feels different depending on where it hits the blade?

GuestThat's the big challenge. To the tip of the blade, the wind feels like it's coming from a totally different side because the blade is moving so fast. So, engineers twist the blade. Near the base, it's thick and angled steeply to catch the slow air and get the whole thing started. But as you move out toward the tip, the blade gets thinner and flatter. It's angled to slice into that fast-moving air. If it didn't have that twist, the tip would actually act like a brake and slow the whole machine down.

HostThat sounds like a lot of work to design. You're making a huge, heavy wing that has to be perfectly balanced but also twisted like a noodle.

GuestAnd it has to be huge. That's the other secret to catching more wind. It's not just about the shape; it's about the reach. If you double the length of a blade, you don't just get double the power. You get four times the power.

HostWhy is that? Is it just because there's more stuff to hit the wind?

GuestIt's about the giant circle the blades draw in the sky. When you make the blade longer, the circle it sweeps through gets much, much bigger. You're catching a way larger wall of wind. The problem is that as those blades get longer, they get heavier and they start to bend. If they bend too much, they could hit the tower they're sitting on.

HostI have seen videos of that happening. It looks like the whole thing just explodes. How do they stop them from being too floppy?

GuestIt comes down to what they're made of. They use things like balsa wood and fiberglass, or even carbon fiber for the really big ones. They build them to be stiff enough to hold their shape but just flexible enough to handle a big gust of wind without snapping. Lately, they have even been looking at the very tips. You might see some newer ones that have a little flick at the end, like the tip of a bird wing.

HostI have seen those on airplanes. They're those little vertical fins at the end of the wings.

GuestThey do the same thing here. When air flows over a wing, it wants to curl around the end and create a little whirlpool. That creates drag, which acts like an invisible hand pulling back on the blade. Those little flicks, or winglets, break up those whirlpools. It sounds like a small change, but when a machine runs for twenty years, those tiny gains add up to a massive amount of extra power.

HostIt's funny because they look so simple and white and clean when you see them from the road.

GuestThat simple look hides the fact that every inch of that surface is shaped to grab every bit of energy it can from a passing breeze.

HostThose giant white arms are basically doing a high-speed dance to turn a gust of air into the light in my living room.