Why do Wings Work?

A discussion on airfoils and lift generation.

Jan 21, 2021

To be honest, I’m pretty tired right now, so I’ll be brief – you’d actually be surprised, it’s kind of simple.

We all know generally what wings look like. Triangular, swept back, aerodynamic looking – here, look at this picture if you forget. [1]

How large the wings are is important, but that by itself doesn’t cause the plane to lift off. The most important thing is the wing’s airfoil. That’s the shape of the cross section of the wing. Imagine looking at the wing from the very tip, at eye level. Now imagine you take a vertical slice of the wing. The resulting shape is the airfoil. Here’s an example. [2]

This, this is the good stuff. These airfoils can have many different shapes for different applications, but all of them have what’s called a “camber”. This is most simply how the shape looks bent upwards. These airfoils generate lift by directing air above and below them. The air that goes over the airfoil has a longer distance to travel compared to the air that takes the bottom path. Because it’s going farther, it speeds up. As a result, this shape causes the air above to go faster than the air below. We discussed air pressure laws in a previous newsletter but in short, as the speed of the air increases, its pressure decreases. So, the air above is at a lower pressure than the air below. Think of driving in a car on the highway. If you open the window, and don’t have all of your paper and plastic bags secured, they’ll be sucked out of the window. The air moving past the car has a lower pressure than the air inside the car. Same thing with the wing – the airfoil is essentially sucked upwards. Now this only yields enough upwards force to overcome the weight of the plane when you’re going fast enough, so that’s why we need long runways to get up to speed before we can lift off. The different shapes that you can have yield different qualities – some can give you more lift for your speed, these ones usually come with an increase in drag. This is why the little flaps that fold down are so genius. When down, the flaps change the shape of the airfoil and allow it to generate more lift, but also increase drag. This is why pilots use them both during takeoff (for extra upwards force) and landing (to slow down faster). Here’s what the airfoil looks like with the flaps down. [3]

The characteristics of this airfoil are what determine the “coefficient of lift” for the wing. This is usually found through wind tunnel tests and cannot really be directly calculated. But, once you have the experimental coefficient of lift, you can use it in an equation we’ve seen before:

‘CL’ is the coefficient of lift. The higher you can make it, the more lift your wings can generate, and the easier it is to stay off the ground. ‘A’ is the surface area of the wing. So, like we touched on earlier, the larger your wing the more lift it can generate. These days airfoils are about as good as they can be, so now science nerds spend their time varying them throughout the wing to get optimal “lift distributions.” But that’s for a very separate newsletter.

So, there it is, the very core of how a wing works, and how planes fly. That is what the Wright brothers discovered when they took their maiden flight. Little fun fact about their plane: it sucked. Not sure if you’ve seen the pictures but it went like 400 feet. Pretty whack. The reason why it sucked so bad was all of those little supports they had were super draggy. They slowed the plane down so much that it could only fly for a little bit before losing altitude. You’d think the guys that figured out how to fly could also figure out how to design an aerodynamic truss. Anyways, I digress. Thank you so much for reading, please let me know if you have any ideas for future newsletters, as always, I would love to hear from you. Happy Wednesday!

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