Xkcd: Airfoil - So what's the right explanation? | Hacker News Reader

Xkcd: Airfoil - So what's the right explanation?

xkcd.com

46 pointsmhw40 comments

Well, actually the Bernouilli effect has its part, and so has the Coanda effect; however most of these explanation concentrate on an artificial setup, without taking in to account the actual angle that the wing makes with the air stream. When you learn to fly, you're first told that the angle of attack is the most important parameter to get lift. See the wikipedia page : http://en.wikipedia.org/wiki/Angle_of_attack

The Bernouilli and Coanda effects, in most flight configurations are of relatively minor importance, except on some particular aircrafts.

Consider a steep angle of attack on takeoff. The lower side of the airfoil is more exposed to the onrush of air, and clearly this pushes the airfoil upwards. But the significance of the Coanda effect is that the air also wraps around the top of the airfoil, adhering to the surface, so that it too exits downward. If it didn't adhere to the surface (which happens in a stall) then you would lose all the downward force of that half of the airflow.

Downward force on a wing reduces lift.

A laminar flow over a wing can maintain a lower pressure above the wing than a chaotic flow. The best simple way to think about this is pooring water from a 2 liter bottle. As you tip it over the pressure of the air inside the bottle decreases and new air is pushed into the bottle filling the partial vacume. However, if you tip it to far you get chaotic behavior and the average pressure increases.

I failed a test in highschool because I said bernouilli didn't make the planes fly. I backed it up with research. Encarta had this video that even stated "bernouilli is just a side-effect of flight, it isn't responsible"

Failed it? Did it only have one question on it?

I brought out my lawyers (parents) on it. Teacher felt he should fail me on the science test because I refused to accept what was in the book.

The agreement was that part wouldn't be marked on my paper only, and telling other students about what I found in encarta and some other stuff it wasn't going to happen.

As a side note, it wasn't actually highschool, this was grade 7.

As a private pilot and Cessna 182 owner I can say, with absolute authority, you're all wrong.

Planes don't fly because of the Bernouilli Effect, or the Coanda Effect, or angle attack, or any of these other technical explanations.

Planes fly because of money. As soon as you stop throwing money at them, they stop flying.

Ask any plane owner and I think they'll agree.

Student pilot here, this man speaks the truth.

"It's complicated" actually kind of describes it. From the NASA website: "The real details of how an object generates lift are very complex and do not lend themselves to simplification...To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations." - http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

That said, I like this handling of it: http://www.amasci.com/miscon/miscon4.html#wing

I have seen many examples of this incorrect explanation in textbooks and posters. It boggles the mind how it has survived for so long. I actually shockingly saw the same incorrect explanation in one episode of NOVA (yes it has gone downhill).

The biggest problem with this diagram is that the air leaving the wing is horizontal, which is (a) wrong and (b) really screws up a kid's understanding of Newton's laws. If the diagram correctly showed the air being forced down by the wing, then a kid could figure out that the wing creates a downward force on the air and the air creates an equal and opposite upward force on the wing.

It seems to me it's you who got things wrong. Explanation about wing forcing air down is very VERY wrong.

Then how is momentum preserved?

How so? The wing has to exert a downward force on something if it wants to go up? If not air then what ?

the lift comes from pressure differential between "top" and "bottom" surface of the wing - that's the definition used by professionals in the field.

Here's why definition about deflection of the air is generally not accepted: http://en.wikipedia.org/wiki/Lift_(force)#Criticisms_of_defl...

The link you provided does not support what you are saying. The link you provided actually agrees with me saying that the theory about air being deflected down is correct. It merely says that while correct, this explanation is not rigorous enough to do engineering. Which is totally right. But I was not trying to do any engineering, I was trying to provide a simple explanation about how a wing works and how that relates to Newton's third law of motion.

I'm glad that you found yourself vindicated and will refrain from interfering with your righteousness.

The Coanda effect. Here's Jef Raskin's essay explaining it: http://web.archive.org/web/20070928072421/http://jef.raskinc...

This cannot be the Coanada effect because the airflow leaving the wing is completely horizontal. If the Coanada effect was present the air travelling over the top surface of the wing would leave the wing at a downwards angle and that would cause lift.

and this is why the picture is wrong.

If you want to satisfactorily explain this to a school kid, it seems like the two most significant factors are a) the angle of attack during take-off - like packing the air under the wing while creating a vacuum above it and b) the curvature taking over the job of "vacuum creation" during steady flight.

A flat-wing plane with its wings slicing the air won't have any lift in steady flight. The only way for such a wing to get lift is to use some angle of attack.

The lift on a curved wing is enough to support its weight when flying fast, but too subtle to literally lift the plane way up starting from lower speeds as on the ground.

Does that about capture the essence of it?

Invoking the Coanda effect seems somewhat unnecessary. For example, if you have a wing fitted with zillions of tiny holes through which a small amount of air is being constantly ejected (to counteract the Coanda effect), it still seems (in my mind) possible for the wing to get lift using the angle of attack and curvature.

.. and that upside down flight needs flaps on the wings.

You're wrong for the right reasons. This would only be a counter-example to the Coanada effect if the xkcd diagram was a correct example of lift. The diagram in this comic would actually produce zero lift, see: http://www.amasci.com/miscon/miscon4.html#wing

Just read the Wikipedia page on Lift: http://en.wikipedia.org/wiki/Lift_(force)

From that page, I now know that the mistake shown in the xkcd comic is called the "Equal Transit-Time Fallacy". There is no requirement for an airflow, once separated, to speed over the top of the wing to match the airflow underneath. In fact the top-layer airflow moves faster than the bottom layer, and arrives at the trailing edge sooner.

..and that's precisely the revelation that made me realise that all of my previous physics teachers had been lying to me.

The Fly With Me podcast had a great episode where they tried to get to the bottom of how a wing really generates lift.

http://joepodcaster.libsyn.com/fly-with-me-episode-25

The host is an airline pilot and he brought in a science podcaster to help him interview a professor of aeronautical engineering. Things went pretty smoothly until the professor started talking about in ring integrals and circular flows of air around the entire wing forcing the interviewers to give up and say "Look, it's really complicated." They put the whole unedited explanation at the end of the podcast for anyone crazy enough to listen to it. :)

The bottom line was that there are 3 things going on at the same time, Bernouilli, Newtonian action/reaction, and the Conada effect (I think, this is where he started talking about ring integrals and blowing everyone's minds). The problem is that even the experts can't say which of those is the principle reason for lift, and which ones are actually side effects from the "real reason." Fortunately the math doesn't care and works anyway, so they can still design and model airfoils very well.

Hilarious! This is the usual problem with rigorous physics/engineering: we don't understand it ourselves, and rather than being honest about it, we quote math at you. It's like doctors in 1700 falling back to Latin in order to hide their embarrassing ignorance from the public. In reality, "If you can't explain it to your grandmother, then you don't understand it yourself." (A. Einstein.)

Similarly, if you asked me some eletronics questions about a piece of complicated circuitry that I didn't understand, I could baffle you with BS. It's simple, I just pull out the equations from the SPICE component models. (As if owning the printout of the software can tell us anything about the behavior of the simulated circuit!!)

So, whenever you see an expert pull this crap, give them the above Einstein quote.

Well, if you're willing to accept that a wing–as described–produces lift in it's 'natural' upward direction (see other replies for some links as this step is a bit complicated), then there's only one way a plane can fly upside down. That reason is that it can also create lift opposite that direction.

Wings are not static. Wings either use flaps or some mechanism that causes them to bend that alters how much lift they produce at a given speed, angle of attack, etc.. The thing is that the reference frame you choose is important.

If you have 0 lift, you're in free fall…a phenomenon that a lot of planes can do. If you can push whatever that distortion in wing shape (flaps or bending) a bit farther, then you can dive faster than freefall, that is producing lift in the downward direction.

Now, just turn the plane upside down and do the same thing.

The freakier thing is that helicopters can theoretically fly upside-down. Same thing: if they can drop faster than free fall, they can fly upside down. The catch is that it's a VERY unstable equilibrium that is a huge stress on basically all of the parts of the motor, steering mechanism, structure, etc..

Or, it's because your parents' are Santa Claus. That's really equivalent.

There are microscopic pixies that hold onto the wing and beat their little wings really fast.

Here is a good (and long) overview from the NASA with wind tunnel simulator Java applets: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

Mary Shafer explained it best many years ago:

http://groups.google.com/group/sci.aeronautics/browse_thread...

It's lift demons.

Isn't is basically the same deal as with a rocket, except that instead of the reaction mass being in a tank, the airplane takes the air in front of it and bats in downwards with the wing?

I think that's angle of attack.

Science Misconceptions in Textbooks and Popular culture

http://news.ycombinator.com/item?id=1771101

http://www.terrycolon.com/1features/fly.html

It's basically the same mechanism as helicopters

http://www.youtube.com/watch?v=6UlsArvbTeo

More than anything doesn't it come down to the forcing of air downward results in lift upwards?

I believe so yes - all the other details are just about stability and performance.

a plane can fly just fine upside-down for the same reason you can steer downwards while right-side up.

.. while correct, the question is "what is that reason?"

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