Re:Xkcd: Airfoil - So what's the right explanation?
by wazoox on Friday October 08, 2010 @06:14AM
(#1771143)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by Isamu on Friday October 08, 2010 @08:09AM
(#1771358)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by Retric on Friday October 08, 2010 @10:12AM
(#1771690)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by 9ec4c12949a4f3 on Friday October 08, 2010 @10:06AM
(#1771673)
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"
Re:Xkcd: Airfoil - So what's the right explanation?
by 9ec4c12949a4f3 on Friday October 08, 2010 @02:05PM
(#1772631)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by araneae on Friday October 08, 2010 @10:01AM
(#1771657)
"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
Re:Xkcd: Airfoil - So what's the right explanation?
by hristov on Friday October 08, 2010 @06:31AM
(#1771165)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by hristov on Friday October 08, 2010 @02:49PM
(#1772843)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by hristov on Friday October 08, 2010 @06:37AM
(#1771173)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by aufreak3 on Friday October 08, 2010 @08:06AM
(#1771349)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by araneae on Friday October 08, 2010 @10:07AM
(#1771674)
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
Re:Xkcd: Airfoil - So what's the right explanation?
by brc on Friday October 08, 2010 @10:13AM
(#1771694)
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.
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by wbeaty on Wednesday October 20, 2010 @06:00AM
(#1810606)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by simonjoe on Friday October 08, 2010 @10:17AM
(#1771713)
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.
Re:Xkcd: Airfoil - So what's the right explanation?
by alan-crowe on Friday October 08, 2010 @10:47AM
(#1771811)
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?
The Bernouilli and Coanda effects, in most flight configurations are of relatively minor importance, except on some particular aircrafts.
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.
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.
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.
That said, I like this handling of it: http://www.amasci.com/miscon/miscon4.html#wing
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.
Here's why definition about deflection of the air is generally not accepted: http://en.wikipedia.org/wiki/Lift_(force)#Criticisms_of_defl...
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.
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.
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.
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.
http://groups.google.com/group/sci.aeronautics/browse_thread...
It's lift demons.
http://news.ycombinator.com/item?id=1771101
It's basically the same mechanism as helicopters