7 ms·

What You Learned About How Planes Fly Was Probably Wrong

93 points by danielrm26 17y ago

danbmil99 17y ago
I say they fly because the shape of the wing makes them fly. If you built an airplane with a wing shape that didn't fly, it wouldn't sell. Therefore, it's really a case of natural selection.
jacquesm 17y ago
I built a fair sized (5m, 16') windmill and this was one of the most surprising things I learned while designing the blades. Windmill blades and airplane wings have a lot in common. I wrote some python software for it to model the curvature and get an idea of how to get the maximum effect out of a blade cut from a given blank. The neat thing was that without the software being interactive we'd have never clued in to some of the possibilities. In case anybody is interested here is a snapshot of the python program: http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.png.html http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.p...
- vaksel 17y ago
  do you have pictures of the windmill?
  - jacquesm 17y ago
    sure: http://pics.ww.com/v/jacques/renewables/windmill/ http://pics.ww.com/v/jacques/renewables/windmill/ That documents the whole building from start to finish, including the 3d computer controlled router/plasmacutter we built in order to fabricate all the parts. The whole thing including making the tools took about a year and half. A single picture of the completed machine is here: http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jpg.html http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jp... It's a variable pitch 3 blader with a 'drum' type rotor that holds 18 2x1x.5" neos. Total power about 2000 Watts, design power was 2.5 KW so not perfect but still pretty good.
- lutorm 17y ago
  Sure, a propeller, or a windmill, or a sail, are all airfoils that experience lift. They just don't "fly", ie keep something suspended.
- JabavuAdams 17y ago
  Cool. Have you ever used the Manipulate[] function in Mathematica? It blew my mind the other day. You can just wrap any expression (including plots), with something like Manipulate[expression, {k, 0, 10}], and you get the regular output with a slider for k in the range [0,10]. If expression is a plot, it'll redraw as you drag the slider.
cesare 17y ago
Wow. I had completely forgotten how cool it was playing with a hand outside the car window as a kid. It really was an aha moment. A bit dangerous, I know.
vinutheraj 17y ago
A better more scientific and mathematically detailed explanation can be found if you google "how wings work" - http://www.iop.org/EJ/article/0031-9120/38/6/001/pe3_6_001.pdf http://www.iop.org/EJ/article/0031-9120/38/6/001/pe3_6_001.p... Thank you to the OP for pointing out the flaw in the popular explanation !
- danielrm26 17y ago
  Yes, I linked to a similar post at the bottom of mine--as well as a few other resources. Many get put off by a four-page explanation, however, and move on to other things. I like to try and give the full punch in a single page and then provide resources to go into more detail if so desired.
yardie 17y ago
I thought everyone knew this j/k. Really when I studied the Bernoulli principle in highschool it didn't make a lot of sense to me at the time. If the curvature of a wing was so important then how did objects with no curvature at all gain any lift? For example gliders. Bernoulli lift only really comes into effect once you have level flight. Then the curvature alone can provide enough lift to support the weight of the plane without inducing drag. The Koanda effect is where ailerons and flaps come into play. The produce additional lift but also contribute more drag. As the angle of attack increases laminar flow drops. Using the authors example, placing a glass in a stream of water redirects the flow, but you'll notice there is a bit that sort of fans out. That is the equivalent of spoiled air. Too much of it and you've lost all laminar airflow, bye bye lift. Anyways, I'm getting out of my area of expertise. It's been ages since I studied aerospace and my AIAA bible is in my parents' garage.
- screwperman 17y ago
  Bernoulli's principle also doesn't explain how planes can fly upside-down.
arakyd 17y ago
It's true that the usual explanation with the Bernoulli effect is wrong, but the idea that the Coanda effect is important for normal flight is also wrong. The relationship between wing camber, angle of attack, and lift is a little more complicated than the post, or the presentation it references, implies. See http://av8n.com/how/htm/airfoils.html#sec-other-fallacies http://av8n.com/how/htm/airfoils.html#sec-other-fallacies for more details. A few planes have been built with the Coanda effect in mind (http://en.wikipedia.org/wiki/Boeing_YC-14 http://en.wikipedia.org/wiki/Boeing_YC-14), but in general it isn't very important.
- raquo 17y ago
  Hm. a hundred years after first plane took off, we still argue how exactly do they fly...
  - pg 17y ago
    Not because there's something especially mysterious about it; it's just an instance of the more general rule that (a) people misunderstand anything complicated and (b) this doesn't stop them from expressing opinions about it. There is one thing that's slightly different in this case: the misleading coolness of diagrams of the Bernoulli effect. One can easily see how in the hands of a popular writer, that could get transformed from an interesting optimization for wing cross-sections to "how planes stay up." I was fooled by this till a later age than I'd like to admit. It never occurred to me to ask how planes fly upside down, if the shape of the airfoil is how they generate lift.
    - luckyland 17y ago
      There's that, and the fact that both Bernoulli and Newton's laws offer a thorough explanation of flight.
    - lutorm 17y ago
      Well, but it is complicated. Hydrodynamics are not intuitive, and the fact is that none of these rules that are being thrown around are sufficient to solve the problem. They all just express different aspects of it. You can't say, is it the Bernoulli effect or is it the fact that the wing makes the air behind it go down. Those are both different ways of expressing the same thing. You can't have one without the other. This is why I like the "See How it Flies" discussion.
dattaway 17y ago
Another perspective how wings are vacuum lifted: vacuum air flow has a low mass, but very high velocity. I work with vacuum packaging equipment and we often see a few 90 turns in the hoses hits the flow like a brick wall at each turn. The speed of the vacuum near 1 torr carries tremendous energy and is enough to rip silicone caulking from inside the joints. Air at double the atmospheric pressure won't see the high velocities to do any damage. I can imagine the high speed downward air flow giving tremendous push.
randallsquared 17y ago
The reason I didn't believe the curved-upper-surface explanation as a kid was that meant that the volume of low-pressure air would have to be similar to the volume of a blimp for it to be useful for lifting the plane, even if it were as efficient as hydrogen; the wings would have to be many times the size of the fuselage.
jussij 17y ago
From the article: > Ask yourself why planes can hang tons of massive crap (engines, bombs, etc.) off of the bottom of their wings if the bottom of the wing is so important for flight I guess that since the top of the wing generates the fast flowing air which then generates the low pressure that generates the lift, by slowing the air on the bottom of the wing with bombs and engines is only going to increase this pressure differential and hence increase the lift (at the expense of drag). The definitely don't seem to hang anything off the top of the wing.
- yardie 17y ago
  Structurally, it's easier for something to hang (well in places that have gravity as a constant) than it is to prop up. You could mount something above the wings but than you would need additional hardpoints to keep it from falling over and smashing into the wing. If it's hanging you only need one hardpoint.
alexvasi 17y ago
Geek Cruise lecture "Why Airplanes Fly - A Modern Myth" (audio only, 3 parts): http://podcast.geekcruises.com/index.php?search_string=airplane&Submit=Search&search=1 http://podcast.geekcruises.com/index.php?search_string=airpl...

93 points by danielrm26 17y ago

I say they fly because the shape of the wing makes them fly. If you built an airplane with a wing shape that didn't fly, it wouldn't sell. Therefore, it's really a case of natural selection.

jacquesm 17y ago

I built a fair sized (5m, 16') windmill and this was one of the most surprising things I learned while designing the blades. Windmill blades and airplane wings have a lot in common. I wrote some python software for it to model the curvature and get an idea of how to get the maximum effect out of a blade cut from a given blank. The neat thing was that without the software being interactive we'd have never clued in to some of the possibilities. In case anybody is interested here is a snapshot of the python program: http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.png.html http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.p...

vaksel 17y ago
do you have pictures of the windmill?
- jacquesm 17y ago
  sure: http://pics.ww.com/v/jacques/renewables/windmill/ http://pics.ww.com/v/jacques/renewables/windmill/ That documents the whole building from start to finish, including the 3d computer controlled router/plasmacutter we built in order to fabricate all the parts. The whole thing including making the tools took about a year and half. A single picture of the completed machine is here: http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jpg.html http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jp... It's a variable pitch 3 blader with a 'drum' type rotor that holds 18 2x1x.5" neos. Total power about 2000 Watts, design power was 2.5 KW so not perfect but still pretty good.
lutorm 17y ago
Sure, a propeller, or a windmill, or a sail, are all airfoils that experience lift. They just don't "fly", ie keep something suspended.
JabavuAdams 17y ago
Cool. Have you ever used the Manipulate[] function in Mathematica? It blew my mind the other day. You can just wrap any expression (including plots), with something like Manipulate[expression, {k, 0, 10}], and you get the regular output with a slider for k in the range [0,10]. If expression is a plot, it'll redraw as you drag the slider.

cesare 17y ago

Wow. I had completely forgotten how cool it was playing with a hand outside the car window as a kid. It really was an aha moment. A bit dangerous, I know.

vinutheraj 17y ago

A better more scientific and mathematically detailed explanation can be found if you google "how wings work" - http://www.iop.org/EJ/article/0031-9120/38/6/001/pe3_6_001.pdf http://www.iop.org/EJ/article/0031-9120/38/6/001/pe3_6_001.p... Thank you to the OP for pointing out the flaw in the popular explanation !

danielrm26 17y ago
Yes, I linked to a similar post at the bottom of mine--as well as a few other resources. Many get put off by a four-page explanation, however, and move on to other things. I like to try and give the full punch in a single page and then provide resources to go into more detail if so desired.

yardie 17y ago

I thought everyone knew this j/k. Really when I studied the Bernoulli principle in highschool it didn't make a lot of sense to me at the time. If the curvature of a wing was so important then how did objects with no curvature at all gain any lift? For example gliders. Bernoulli lift only really comes into effect once you have level flight. Then the curvature alone can provide enough lift to support the weight of the plane without inducing drag. The Koanda effect is where ailerons and flaps come into play. The produce additional lift but also contribute more drag. As the angle of attack increases laminar flow drops. Using the authors example, placing a glass in a stream of water redirects the flow, but you'll notice there is a bit that sort of fans out. That is the equivalent of spoiled air. Too much of it and you've lost all laminar airflow, bye bye lift. Anyways, I'm getting out of my area of expertise. It's been ages since I studied aerospace and my AIAA bible is in my parents' garage.

screwperman 17y ago
Bernoulli's principle also doesn't explain how planes can fly upside-down.

arakyd 17y ago

It's true that the usual explanation with the Bernoulli effect is wrong, but the idea that the Coanda effect is important for normal flight is also wrong. The relationship between wing camber, angle of attack, and lift is a little more complicated than the post, or the presentation it references, implies. See http://av8n.com/how/htm/airfoils.html#sec-other-fallacies http://av8n.com/how/htm/airfoils.html#sec-other-fallacies for more details. A few planes have been built with the Coanda effect in mind (http://en.wikipedia.org/wiki/Boeing_YC-14 http://en.wikipedia.org/wiki/Boeing_YC-14), but in general it isn't very important.

raquo 17y ago
Hm. a hundred years after first plane took off, we still argue how exactly do they fly...
- pg 17y ago
  Not because there's something especially mysterious about it; it's just an instance of the more general rule that (a) people misunderstand anything complicated and (b) this doesn't stop them from expressing opinions about it. There is one thing that's slightly different in this case: the misleading coolness of diagrams of the Bernoulli effect. One can easily see how in the hands of a popular writer, that could get transformed from an interesting optimization for wing cross-sections to "how planes stay up." I was fooled by this till a later age than I'd like to admit. It never occurred to me to ask how planes fly upside down, if the shape of the airfoil is how they generate lift.
  - luckyland 17y ago
    There's that, and the fact that both Bernoulli and Newton's laws offer a thorough explanation of flight.
  - lutorm 17y ago
    Well, but it is complicated. Hydrodynamics are not intuitive, and the fact is that none of these rules that are being thrown around are sufficient to solve the problem. They all just express different aspects of it. You can't say, is it the Bernoulli effect or is it the fact that the wing makes the air behind it go down. Those are both different ways of expressing the same thing. You can't have one without the other. This is why I like the "See How it Flies" discussion.

dattaway 17y ago

Another perspective how wings are vacuum lifted: vacuum air flow has a low mass, but very high velocity. I work with vacuum packaging equipment and we often see a few 90 turns in the hoses hits the flow like a brick wall at each turn. The speed of the vacuum near 1 torr carries tremendous energy and is enough to rip silicone caulking from inside the joints. Air at double the atmospheric pressure won't see the high velocities to do any damage. I can imagine the high speed downward air flow giving tremendous push.

randallsquared 17y ago

The reason I didn't believe the curved-upper-surface explanation as a kid was that meant that the volume of low-pressure air would have to be similar to the volume of a blimp for it to be useful for lifting the plane, even if it were as efficient as hydrogen; the wings would have to be many times the size of the fuselage.

jussij 17y ago

From the article: > Ask yourself why planes can hang tons of massive crap (engines, bombs, etc.) off of the bottom of their wings if the bottom of the wing is so important for flight I guess that since the top of the wing generates the fast flowing air which then generates the low pressure that generates the lift, by slowing the air on the bottom of the wing with bombs and engines is only going to increase this pressure differential and hence increase the lift (at the expense of drag). The definitely don't seem to hang anything off the top of the wing.

yardie 17y ago
Structurally, it's easier for something to hang (well in places that have gravity as a constant) than it is to prop up. You could mount something above the wings but than you would need additional hardpoints to keep it from falling over and smashing into the wing. If it's hanging you only need one hardpoint.

alexvasi 17y ago

Geek Cruise lecture "Why Airplanes Fly - A Modern Myth" (audio only, 3 parts): http://podcast.geekcruises.com/index.php?search_string=airplane&Submit=Search&search=1 http://podcast.geekcruises.com/index.php?search_string=airpl...