803: Airfoil

Explain xkcd: It's 'cause you're dumb.
Revision as of 23:22, 24 November 2012 by Davidy22 (talk | contribs)
Jump to: navigation, search
This is a fun explanation to prepare your kids for; it's common and totally wrong. Good lines include "why does the air have to travel on both sides at the same time?" and "I saw the Wright brothers plane and those wings were curved the same on the top and bottom!"
Title text: This is a fun explanation to prepare your kids for; it's common and totally wrong. Good lines include "why does the air have to travel on both sides at the same time?" and "I saw the Wright brothers plane and those wings were curved the same on the top and bottom!"


This comic is about the common teaching that an airfoil works because the air on top of the wing must travel faster to keep up with the air flowing across the bottom of the wing. They then will say that because the air on top of the wing is traveling faster it creates an area of lower pressure above the wing and it therefore goes up because the higher pressure below the wing pushes it up more than the low pressure above the wing is pushing it down. This is how many people think an airfoil works but it is not right. The next panel is a student asking why planes can fly upside down if that is true and the teacher thinks about it. Then there are three different routes to take, the right one, the wrong one and the very wrong one. In the right one, the teacher realizes that perhaps the model is wrong and that the reason for why and airfoil works that she is teaching is wrong. In the wrong panel, the teacher avoids the question and says that it's complicated. In the very wrong panel, not only does the teacher avoid answering the question she goes completely off topic and tells the kids that Santa Claus isn't real and that Santa Claus is really their parents. The title text includes ways to debunk the reason for airfoils working that is taught. It points out that it is absurd to believe the air has to get across the airfoil's two sides in the same amount of time and also points out that the Wright brothers plane's wings were curved the same amount on both sides of the airfoil and therefore it wouldn't even have a difference in the distance the air needs to travel to get across the wing.


>Handling a student who challenges your expertise with an insightful question:
[There's a picture of the cross section of an airfoil, with an arrow above and below, pointing from right to left. Layered on top of these arrows, pointing up and down at the cross section, are a larger arrow below and a smaller arrow above.]
(This panel just contains text, and has a speech curlique hanging towards the person in the next panel.)
Teacher: So, kids, the air above the wing travels a longer distance, so it has to go faster to keep up. Faster air exerts less pressure, so :the wing is lifted upward.
Student: But then why can planes fly upside down?
(The teacher is standing, pondering the question. Three arrows point out of this panel, leading to each of the next three panels which are :arranged vertically.)
(This is a label at the top of the panel, not a character speaking.)
(This is the character speaking.)
Teacher: Wow, good question! Maybe this picture is simplified -- or wrong! We should learn more.
Teacher: It's... complicated.
Teacher: And we need to move on.
Very wrong:
Teacher: Santa Claus is your parents.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


This has been dubbed "the equal transit-time fallacy." ((((((_(_(__(___billb(a)amasci.com___)__)_)_)))))) (talk) 07:59, 10 June 2022 (UTC)

For more information have a look at my paper here-https://arxiv.org/abs/2110.00690 AerospaceDoctor (talk) 03:09, 30 September 2022 (UTC)

Since gases are unique in the way they convert pressure change to heat or cold, none of the above can be true. The schema shows more columns of air under the wing than pass above. How come? We know that an actual wing pulls air down from the layers above rather than bludgeon the air in front of it.

What happens is that vortices are engendered. Wing design is all about getting them past the control sections (the major problem to aircraft design in the 1940's.)

I used Google News BEFORE it was clickbait (talk) 01:15, 26 January 2015 (UTC)

It is a convention in fluid dynamics that the flow always comes from the left. Nobody working in this field would ever draw a picture with the flow coming from the right side. So this is a hint by its own that the image may be wrong.

Regarding the last comment: The image is from wikipedia. The original comment tells us that the wing influences the flow even at a great distance. The flow at the top is still a lot faster than below the wing. 21:01, 3 June 2015 (UTC)

My high school math teacher said there were three ways for a teacher to say "I don't know" -- "I don't know", "It's beyond the scope of the course", and "You wouldn't understand it if I told you." So, whenever we asked a question like the one in the comic, he'd say "It's beyond the scope of the course, and you wouldn't understand it if I told you anyway! Dumb kids!" We loved that guy. 17:01, 5 July 2018 (UTC)

This airfoil problem is a 2D problem and the classical Bernoulli's equation is a 1D equation. If you use velocity vector(speed and direction) instead speed the equation works.