

What You Learned About How Planes Fly Was Probably Wrong - danielrm26
http://danielmiessler.com/blog/why-planes-fly-what-they-taught-you-in-school-was-wrong

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arakyd
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.

~~~
kurtosis
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/...](http://www.amazon.com/What-Makes-Airplanes-Peter-
Wegener/dp/0387947841)

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

~~~
kurtosis
Here's more info on the author

<http://opa.yale.edu/news/article.aspx?id=6066>

Apparently he worked on the V2 rocket. But there are lots of scientists that
can write technical books targeted to experts. This book is worth it because
this guy is devoted a lot of his teaching career to explaining difficult
topics to non-experts. I think he did a marvelous job, of mixing technical
material and history. I think this book was part of a course he taught at yale
univ.

It is an expensive book, but I'll go in and say it's worth it. Check your univ
library.

------
jacquesm
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...](http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.png.html)

~~~
vaksel
do you have pictures of the windmill?

~~~
jacquesm
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...](http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jpg.html)

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.

------
vinutheraj
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...](http://www.iop.org/EJ/article/0031-9120/38/6/001/pe3_6_001.pdf)

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

~~~
danielrm26
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.

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

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cesare
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.

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jussij
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
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.

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dattaway
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.

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randallsquared
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.

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

[http://podcast.geekcruises.com/index.php?search_string=airpl...](http://podcast.geekcruises.com/index.php?search_string=airplane&Submit=Search&search=1)

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danbmil99
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.

