re: today's xkcd
Oct. 8th, 2010 05:05 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
crankyoldmanSince I've gotten asked.
It's about airfoils and misconceptions.
Pretty funny joke, considering that even at a university level some people still get confused about airfoils. The short answer is that fluid dynamics is COMPLICATED. In 3D space, there are a lot of factors in dealing with them.
Why do they get confused? Because the math is right, and math is sometimes hard to translate into language. Particularly the English language, I've learned. (yes, there are languages that make math easier to understand... they are countries that tend to kick the US's butt in math scores. but that's another topic). If the math were wrong, we'd have no planes flying!
But the longer answer in how airfoils create lift is not just based on flow movement; it's based on circulation. The problem with the standard (wrong) explanation is that they just don't describe Bernoulli's Principle right. Bernoulli's principle is really about momentum and circulation. And the base equation is a simplification of much more complicated fluid dynamics, but I won't go into that.
So, when you're flying, the flow doesn't just stay all neat and tidy everywhere. In the wake of a wing, there are vortices.
Vortices are in a swirly sort of movement, which is kind of like an angular momentum thing. Remember the Newtonian principle of equal and opposite reactions? Well, the same goes for moments, which are just another fancy force thing. In order to make up for creating swirlies, a circulation effect forms around the wing.
So you've got the forces of the flow, and the circulation. You add up the vectors and the net velocity on top is much faster than the bottom, resulting in a net force up (as well as the pressure being lower than the ambient).
Same thing as the explanation, but just brought about by something more complicated than "it takes it longer to go over a curved surface and needs to catch up". No no no. In fact, imagining units of air going over the top, they'll actually BEAT similar units going along the bottom.
So what does this have to do with flying upside down? Well, some airfoils are perfect for this, like this one, used in acrobatic planes.
But you can even fly upside down with non-symmetric airfoils. You have to have the right angle of attack to do that, though. So when a plane is upside down, the pilot has the stick in a position to create a positive angle of attack. It's just less efficient to fly that way, not impossible. Often this will result in some loss of altitude, unless a plane is an acrobatic variety.
Aerodynamics was one of my weaker areas, though, and I'm likely to have missed something.
Here are some links to other sites that talk about the proper way to think about lift:
- Airfoil lifting force misconception
- The airfoil myth
It's about airfoils and misconceptions.
Pretty funny joke, considering that even at a university level some people still get confused about airfoils. The short answer is that fluid dynamics is COMPLICATED. In 3D space, there are a lot of factors in dealing with them.
Why do they get confused? Because the math is right, and math is sometimes hard to translate into language. Particularly the English language, I've learned. (yes, there are languages that make math easier to understand... they are countries that tend to kick the US's butt in math scores. but that's another topic). If the math were wrong, we'd have no planes flying!
But the longer answer in how airfoils create lift is not just based on flow movement; it's based on circulation. The problem with the standard (wrong) explanation is that they just don't describe Bernoulli's Principle right. Bernoulli's principle is really about momentum and circulation. And the base equation is a simplification of much more complicated fluid dynamics, but I won't go into that.
So, when you're flying, the flow doesn't just stay all neat and tidy everywhere. In the wake of a wing, there are vortices.
Vortices are in a swirly sort of movement, which is kind of like an angular momentum thing. Remember the Newtonian principle of equal and opposite reactions? Well, the same goes for moments, which are just another fancy force thing. In order to make up for creating swirlies, a circulation effect forms around the wing.
So you've got the forces of the flow, and the circulation. You add up the vectors and the net velocity on top is much faster than the bottom, resulting in a net force up (as well as the pressure being lower than the ambient).
Same thing as the explanation, but just brought about by something more complicated than "it takes it longer to go over a curved surface and needs to catch up". No no no. In fact, imagining units of air going over the top, they'll actually BEAT similar units going along the bottom.
So what does this have to do with flying upside down? Well, some airfoils are perfect for this, like this one, used in acrobatic planes.
But you can even fly upside down with non-symmetric airfoils. You have to have the right angle of attack to do that, though. So when a plane is upside down, the pilot has the stick in a position to create a positive angle of attack. It's just less efficient to fly that way, not impossible. Often this will result in some loss of altitude, unless a plane is an acrobatic variety.
Aerodynamics was one of my weaker areas, though, and I'm likely to have missed something.
Here are some links to other sites that talk about the proper way to think about lift:
- Airfoil lifting force misconception
- The airfoil myth