803: Airfoil

Explain xkcd: It's 'cause you're dumb.
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!"


In the first panel a cross sectional drawing of a plane wing with the air moving around the wing showing a 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. The theory goes that, because the air on top of the wing is traveling faster, it must, as a result of Bernoulli's Principle, create an area of lower pressure above the wing; this causes lift (that is, the wing rises) because the higher pressure below the wing (symbolized by thick "up" arrow) pushes it up more than the low pressure above the wing. This is what the teacher Miss Lenhart is teaching as is revealed in the next panel.

As it turns out, this is, to put it mildly, a vast oversimplification of how lift is truly created. Because then a student asks a particularly insightful question: Why, if the theory is true, can planes fly upside down? (If the simple airfoil theory is all that permits planes to stay up in the air, then flying upside down should reverse the pressures — pushing the plane down and causing it to crash.) Miss Lenhart thinks about it and clearly has no answer.

The final set of panels posit three potential responses from Miss Lenhart, upon realizing her theory has been disproved:

In the right one, Miss Lenhart realizes that perhaps the model she's been using to explain how an airfoil works is wrong (or, at a minimum, too simple). She is curious about it and suggests that this is an area for further exploration, and encourages additional study — in effect, rewarding the student for their insight. It seems that Miss Lenhart has taken the right course as it is shown later in 843: Misconceptions that she wished her students to generally avoid any common misconceptions. The title text also mentions that this is a common misconception and it is (currently) the first mentioned on list of common physics misconceptions on Wikipedia.

In the wrong panel, Miss Lenhart, out of apparent embarrassment, avoids the question entirely, saying simply that it's complicated (and implying that such questions are outside the student's understanding). This way to continue a discussion where you wish to be right was much later used in 1731: Wrong.

In the very wrong panel, not only does Miss Lenhart avoid answering the question, she attempts to distract them (or even punish them for asking such an insightful question - note that in this panel, Miss Lenhart has clenched her fists, suggesting anger) by telling the kids that Santa Claus isn't real but in fact that he is really their parents — something that would obviously distress children if they still believe in Santa Claus (in addition to distracting them from the question they've asked) and constitute harsh punishment for pointing out the teacher's ignorance. Of course most children old enough to be taught about the airflow around plane wings should be too old to believe in Santa. However, if she just wished to tell them a bit about planes she may have drawn this drawing even in very early grades making the Santa trick effective.

The title text suggests additional reasons for re-thinking the common theory as to how airfoils create lift. It points out that (1) it is absurd to believe the air has to get across the airfoil's two sides in the same amount of time, and (2) the Wright brothers plane's wings were curved the same amount on both sides of the airfoil (the Wright Flyer's wings were concave, like an arch; and thus the curves were in the same direction, not reflected vertically), meaning that the distance that the air needs to travel to hug along each face of the wing is not the dispositive factor in creating lift.

The strip is correct in noting that lift is a far more complicated process than the simple theory posited by Miss Lenhart. While the role of Bernoulli's Principle (that is, the difference in pressures) cannot be entirely discounted, the theory here is vastly too simple. As an initial matter, as suggested by the title text, there is no reason that the air on top of the wing should be compelled to "keep up" with the air on the bottom of the wing. Indeed, as demonstrated by the illustration below, in the time that the air below the wing travels across, the air on top of the wing has not only traveled the length of the entire top of the wing (a distance that may be farther than the distance under the wing, due to its shape), but often additional distance.

Karman trefftz.gif

Lift may be more usefully described as resulting from the deflection of air, although this explanation still does not explain how symmetrical wings will work (at least, absent effects caused by a change in the "angle of attack") nor how a plane may fly upside down. The Wikipedia article on lift provides a more detailed explanation. It in fact gives an explanation as to these two issues. It explains that with zero angle of attack, a symmetrical wing will not generate lift (though it is possible that other factors may generate other slight upward force, such as updrafts, the shape of the plane, and the angle of the engine relative to the wings). It also explains that an asymmetrical (or "cambered") wing may adjust angle of attack to compensate and still generate lift.

Finally, to answer the question in the second panel in a general sense: most planes can't fly upside down for an extended period of time. While many aerobatic aircraft can sustain inverted flight with negative g forces, some others can achieve an inverted attitude only momentarily, and are experiencing positive g forces. Usually the reason for this is not the wings, which function perfectly fine upside down (albeit sometimes at lower efficiency), but the engines, which may not get fuel or oil under such conditions. It has to also be noted that if angle of attack were ignored, movable control surfaces would be useless. Almost any airplane can do a barrel roll or Aileron roll, given sufficient altitude (a Boeing 707 prototype once did this, and so did the Concorde in a demonstration).


[In a frame-less picture to the left of the first panel there's a picture of a cross section of an airfoil (a plane wing), with a small black arrow pointing down on the wing from above and similar but larger arrow pointing up on the wing from below. Two lines beginning close to each other at the right respectively moves over and under the wing ending in arrow heads to the left. Just before and after the wing four small lines crossing the long arrows indicate approximately where the path of the lines stop being parallel. Above the drawing there is a caption. Below, in a speech bubble with an arrow pointing towards the next panel to the right, is the text that the teacher Miss Lenhart has just used to describe the drawing.]
Handling a student who challenges your expertise with an insightful question:
Miss Lenhart: 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.
[Miss Lenhart is shown standing while a student asks a question from off-panel.]
Student (off-panel): But then why can planes fly upside down?
[Miss Lenhart is pondering the question. Beat panel. Three long and curved arrows point out from the right frame of this panel, leading to each of the next three panels which are arranged vertically above each other, making the comic much deeper in this column than in the first two.]
[In the top panel Miss Lenhart turns away from the students taking a hand to her chin. Overlaid on the top of the panel there is a small frame with a caption:]
Miss Lenhart: Wow, good question! Maybe this picture is simplified—or wrong! We should learn more.
[In the middle panel Miss Lenhart stands as before. Overlaid on the top of the panel there is a small frame with a caption:]
Miss Lenhart: It's... complicated.
Miss Lenhart: And we need to move on.
[In the bottom panel Miss Lenhart visibly ball her hands in to fists and leans a little forward looking more down. Overlaid on the top of the panel there is a small frame with a caption:]
Very wrong:
Miss Lenhart: 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.

I suspect most school children have seen a paper airplane, maybe even built one. Those wings are, for practical purposes, 2D. Nitpicking (talk) 15:56, 6 March 2023 (UTC)dead