xkcd identifies a very common misconception about how airfoils work! (via Vinnchan and jasonas14) #
Search results for: “lift”
How Wings Create Lift
One of the topics in fluid dynamics almost everyone has come across is the explanation of how airplanes produce lift. Using Bernoulli’s principle–which relates velocity and pressure–and a picture of an airfoil, your average science text will say that a bit of air going over the top of the airfoil has to travel farther than a bit of air going under the airfoil, and that, therefore, the air over the top travels faster than the air under the airfoil.
Unfortunately, this is misleading and, depending on the wording, outright wrong! The hidden assumption in this explanation is that air that goes over the top and air that goes under the bottom have to reach the trailing edge of the airfoil at the same time. But why would that be? (As one of my profs once said, “There is nothing in physics that says there is Conservation-Of-Who-You-Were-Sitting-Next-To-When-You-Started.”)
Take a look at the video above. It shows an airfoil in a wind tunnel using smoke visualization to show how the air moves. Around the 0:25 mark, the video slows to show a pulse of smoke traveling over the airfoil. What happens at the trailing edge? The smoke going over the top of the airfoil is well past the trailing edge by the time the smoke going under the airfoil reaches the trailing edge!
It’s true that air goes faster over the top of the airfoil than the bottom and that this causes a lower pressure on top of the airfoil (as Bernoulli tells us it should) and that this causes an upward force on the airfoil. But which causes which is something of a chicken-and-egg problem.
A more straightforward way, in my opinion, of explaining lift on an airplane is by thinking about Newton’s 3rd law: for every action, there is an equal and opposite reaction. Take a look at the air’s movement around the airfoil as the angle of attack is increased around 1:00 on the video. Just in front of the airfoil, the air is moving upward. Just after the airfoil, the air is pointed downward. A force from the airfoil has pushed the air down and changed its direction. By Newton’s 3rd law, this means that the air has pushed the airfoil up by the same amount. Voila! Lift!
Fire From Below
A slight change in perspective can do wonders. In this video, the Slow Mo Guys look at a burning flame from below. They accomplish this by mounting a gas grill upside-down. This small change means that buoyancy can’t simply lift heat and exhaust gases away from the flame source. Instead, the flow pushes out and around the edges of the grill.
The views are, as always, amazing. The billowing flames are mesmerizing–often closer to laminar than turbulent. And the added spectacle of cinnamon combusting in the later segments really does make for the kind of visuals you’d expect in a sci-fi movie. (Video and image credit: The Slow Mo Guys)
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Particles Separate When Flowing Downhill
When particle-laden fluids like a mudslide flow downhill, even well-mixed particles can wind up separating. To explore how this works, researchers put glass spheres–of two different sizes but equal density–into silicone oil and let it flow down an incline. Their initially well-mixed oil soon turned red as the larger red particles overtook the smaller blue particles near the front. Looking at the flow from the side, the team observed a Brazil-nut-effect-like behavior where the larger particles move toward the top of the flow. That’s where the flow speed is fastest, and the particles are congregating there despite being denser than the oil carrying them! (Video and image credit: Y. Ba et al.)
Milano Cortina 2026: Ski Jumping Suits
Ski jumping is in the news this Olympic cycle after rumors that male competitors may be cheating in order to wear larger suits. In particular, the suggestion is that male athletes are injecting fillers into their genitals before their pre-season 3D body scan in order to appear large enough to allow them to wear a larger suit. This comes after two Norwegian ski jumpers were punished for illegally restitching the crotches of their suits to make them larger.
Ski jumping is a sport that relies heavily on aerodynamics; during the flight phase, jumpers try to maximize their lift-to-drag ratio so that they stay aloft as long as possible. A 2025 study underscores the importance of suit size in this calculus. In the work, the researchers used a baseline suit that was 4 centimeters larger in circumference than their jumper–the loosest configuration that regulations allow. They compared that suit’s flight performance (in wind tunnels and simulation) to a suit 2 cm larger and one 2 cm smaller. The extra 2 centimeters of circumference made a notable difference: the larger suit increased the drag by ~4% and lift by ~5%. That was enough, in their simulation, to let a jumper fly an extra 5.8 meters.
It’s worth noting, though, that the study was looking at the effects of adjusting the suit’s circumference along the entire length between the arm pits and the knees; they never changed anything about the suit’s crotch. I don’t think there’s enough scientific data to say that packing a bit more there would really offer aerodynamic advantages. And the risks of such injections are non-negligible. (Image credit: T. Trapani; research credit: M. Virmavirta et al.; via Ars Technica)
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Instabilities in a Particle Flow
![Composite image of bed layers for 4 different particle density ratios. Text reads, "The wave amplitude and growth rate increase with particle density ratio but only if [the density of large particles is greater than the smaller particle density]."](https://fyfluiddynamics.com/wp-content/uploads/KHbed3-1024x576.png "Composite image of bed layers for 4 different particle density ratios. Text reads, \"The wave amplitude and growth rate increase with particle density ratio but only if [the density of large particles is greater than the smaller particle density].\"")
![Composite image of bed layers for 4 different particle density ratios. Text reads, "The wave amplitude and growth rate increase with particle density ratio but only if [the density of large particles is greater than the smaller particle density]."](https://fyfluiddynamics.com/wp-content/uploads/KHbed3.png)
Even though particles are not (strictly speaking) a fluid, they often behave like one. Here, researchers investigate what happens when two layers of particles–with different size and density–slide down an incline together. The video is tilted so that the flow instead appears from left to right.
When the larger, denser particles sit atop a layer of smaller, lighter particles, shear between the two layers causes a Kelvin-Helmholtz instability that runs in the direction of the flow. This creates a wavy interface that lets some small particles work upward while large particles shift downward.
At the same time, a slice across the flow shows that plumes of small particles are pushing up toward the surface, driven by a Rayleigh-Taylor instability. The researchers also look at what happens when the particles are fluidized by injecting a gas able to lift the particles. (Video and image credit: M. Ibrahim et al.; via GFM)
Bouncing Indefinitely
On the surface of a gently vibrating liquid, a droplet can bounce indefinitely without coalescing, kept aloft by an air film too small to see. As long as the droplet lifts off before the air layer drains out from under it, the droplet won’t contact the water below. Now scientists have shown that this is possible with a solid surface, too.
Using an atomically smooth mica plate, researchers were able to bounce a droplet indefinitely without wetting the surface. At higher vibration rates (below), the droplet essentially hovers in place, bouncing so quickly that we simply see its shape vibrating in response to the surface. (Image and research credit: L. Molefe et al.; via APS)
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Buccaneer Archipelago
Off western Australian, hundreds of low-lying islands and coral reefs jut into the ocean as part of the Buccaneer Archipelago. Tides here have a range of nearly 12 meters, so water rips through the narrow channels as the tide ebbs and flows. These fast flows lift sediment that dyes the water a bright turquoise. (Image credit: M. Garrison; via NASA Earth Observatory)
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Flettner Rotors Spin Anew
In the 1920s, the world saw a new sort of marine propulsion, ships with one or more tall, smokeless cylinders. These Flettner rotors, named for their inventor, would spin in the wind, generating lift to propel the boat, much as a sail would. (The difference is that the rotor uses the Magnus effect.)
The market crash that kicked off the Great Depression spelled an end to the rotorship, but the idea is getting revived as industries search for greener forms of ship propulsion. Although the Flettner rotor still uses fuel (to spin the rotor), it can complete a voyage on only a small fraction of the fuel needed for conventional propulsion. (Image credit: Getty Images; via PopSci)
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Smoke Bomb
With a flurry of motion along its pectoral fin, a sting ray lifts the sand nearby and disappears into the turbid cloud. This tactic helps the animal both hide and escape. In a similar move, sting rays and other bottom-dwelling fish can bury themselves in sand.(Image credit: Y. Coll/OPOTY; via Colossal)
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