The dream of soaring in the sky like a bird has captivated the human mind for ages. Although many failed, some eventually succeeded in achieving that goal. These days we take air transportation for granted, but the physics of flight can still be puzzling.
In this article we’ll investigate what makes airplanes fly by looking at the forces generated by the flow of air around the aircraft’s wings. More specifically, we’ll focus on the cross section of those wings to reveal the shape of an airfoil – you can see it presented in yellow below:
We’ll find out how the shape and the orientation of the airfoil helps airplanes remain airborne. We’ll also learn about the behavior and properties of air and other flowing matter. In the demonstration below, you can see a fluid flowing around a gray cube. Using the slider to change just one property of this substance, we can end up with vastly different effects on the liveliness of that flow:
Over the course of this blog post we’ll build some intuitions for why these different effects happen to airfoils and other objects placed in flowing air. We’ll start this journey by looking at some of the methods we can use to visualize the motion of the air.
Visualizing Flow
If you’ve ever been outside in a grassy area on a windy fall day, you may have witnessed something similar to the little scene seen below. The slider lets you control the speed of time to observe in detail how the falling leaves and the bending blades of grass are visibly affected by the wind sweeping through this area:
We intuitively understand that it’s the flowing air that pushes the vegetation around, but note that we only observe the effects that the wind has on other objects – we can’t see the motion of the air itself. I could show you a similarly windy scene without the grass and leaves, and I could try to convince you that there is something going on there, but that completely empty demonstration wouldn’t be very gratifying.
Since the air’s transparency prevents us from tracking its movement directly, we have to come up with some other ways that can help us see its motion. Thankfully, the little outdoor scene already provides us with some ideas.
Notice that as the wind hits a blade of grass, that blade naturally bends in the direction of the blowing gust, and the faster that gust, the stronger the bending. A single blade indicates the direction and speed of the flow of air in that area.
In the next demonstration we’re looking at the same grassy field from above. When seen from this perspective, all the blades form short lines that are locally aligned with the wind. The more leaned over a blade of grass is, the longer the line it forms. We can mimic this behavior with a collection of small arrows placed all over the area, as seen on the right side:
... continue reading