What You Learned About How Planes Fly Was Probably Wrong

http://danielmiessler.com/blog/why-planes-fly-what-they-taught-you-in-school-was-wrong

by danielrm26 • 17 years ago

93 40 17 years ago

jacquesm3• 17 years 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.p...

vaksel• 17 years ago

do you have pictures of the windmill?

jacquesm3• 17 years ago

sure: 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.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.

lutorm6• 17 years 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.

JabavuAdams2• 17 years 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• 17 years 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• 17 years 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.p...

Thank you to the OP for pointing out the flaw in the popular explanation !

danielrm26• 17 years 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.

yardie2• 17 years 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• 17 years ago

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

arakyd2• 17 years 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 for more details.

A few planes have been built with the Coanda effect in mind (http://en.wikipedia.org/wiki/Boeing_YC-14), but in general it isn't very important.

raquo• 17 years ago

Hm. a hundred years after first plane took off, we still argue how exactly do they fly...

pg• 17 years 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.

kurtosis2• 17 years ago

For anyone interested in this I recommend the really great "What makes airplanes fly" by Peter P Wegener.

http://www.amazon.com/What-Makes-Airplanes-Peter-Wegener/dp/...

This book was part of a course he taught for non aerospace engineers. His explanations of tough topics like boundary layer theory and airfoils are clear

adamc• 17 years ago

Kind of expensive for anything short of consuming interest, though.

lutorm6• 17 years ago

Another good discussion about this is in the online pilot textbook "See How It Flies" at http://www.av8n.com/how/ (section 3). That author also disagrees with the Coanda effect conclusion.

The discussion in there is the best I've seen, it centers on the concept or circulation, the vorticity of the flow field around the wing. One example that's used is the fact that if you just take a wing by itself (try it with a long, flat piece of cardboard) it will "fly" while rotating in the air. It doesn't fly very well, admittedly, but it certainly exhibits a glide ratio a lot better than something with the same cross-sectional area that's not a wing. In that case, there's no top or bottom of the wing at all.

arakyd2• 17 years ago

Actually the whole av8n.com site is a great resource for anyone interested in learning more about physics. One of the reasons why I linked it ;).

baddox• 17 years ago

From everything I've read, it seems there is debate on exactly how to describe/explain what generates lift. Most of the explanations are correct, but insufficient by themselves.

dattaway• 17 years 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• 17 years 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• 17 years 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.

yardie2• 17 years 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.

danbmil99• 17 years 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.

alexvasi• 17 years ago

Geek Cruise lecture "Why Airplanes Fly - A Modern Myth" (audio only, 3 parts):

http://podcast.geekcruises.com/index.php?search_string=airpl...

What You Learned About How Planes Fly Was Probably Wrong

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