

How Airplanes Fly: A Physical Description of Lift - hadronzoo
http://www.allstar.fiu.edu/aero/airflylvl3.htm

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tobinfricke
From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944:

The main fact of all heavier-than-air flight is this: the wing keeps the
airplane up by pushing the air down.

It shoves the air down with its bottom surface, and it pulls the air down with
its top surface; the latter action is the more important. But the really
important thing to understand is that the wing, in whatever fashion, makes the
air go down. In exerting a downward force upon the air, the wing receives an
upward counterforce--by the same principle, known as Newton's law of action
and reaction, which makes a gun recoil as it shoves the bullet out forward;
and which makes the nozzle of a fire hose press backward heavily against the
fireman as it shoots out a stream of water forward. Air is heavy; sea-level
air weights about 2 pounds per cubic yard; thus, as your wings give a downward
push to a cubic yard after cubic yard of that heavy stuff, they get upward
reactions that are equally hefty.

That's what keeps an airplane up. Newton's law says that, if the wing pushes
the air down, the air must push the wing up. It also puts the same thing the
other way 'round: if the wing is to hold the airplane up in the fluid, ever-
yielding air, it can do so only by pushing the air down. All the fancy physics
of Bernoulli's Theorem, all the highbrow math of the circulation theory, all
the diagrams showing the airflow on a wing--all that is only an elaboration
and more detailed description of just how Newton's law fulfills itself--for
instance, the rather interesting but (for the pilot) really quite useless
observation that the wing does most of its downwashing work by suction, with
its top surface. ...

Thus, if you will forget some of this excessive erudition, a wing becomes much
easier to understand; it is in the last analysis nothing but an air deflector.
It is an inclined plane, cleverly curved, to be sure, and elaborately
streamlined, but still essentially an inclined plane. That's, after all, why
that whole fascinating contraption of ours is called an air-plane.

~~~
vvpan
Great description. But then I read the XKCD mentioned in another commend, and
started wondering how plains fly upside down then? When inverted the "top" of
the wing would push the plane down, no?

~~~
Nick_C
Planes that often fly upside down typically have low camber wings. Some of
them have almost symmetrical wings. Those wings have less lift in upright
flight than a more highly cambered wing would, so (and for other reasons too)
the manufacturers compensate with bigger engines.

<http://en.wikipedia.org/wiki/Extra_EA-300>

and an engineer's question about wing design for aerobatic flight with some
good answers:

<http://www.eng-tips.com/viewthread.cfm?qid=183605>

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wbeaty
Don't miss their updated PDF version:
<http://www.allstar.fiu.edu/aero/Flightrevisited.pdf>

\---

Important but somewhat complicated point: flight in a 2D world is a venturi
effect, and regardless of altitude, a two-dimensional airfoil remains trapped
in "ground effect mode." The circulation extends outwards indefinitely, and so
an instant force-pair connects the wing with the surface of the Earth.

In 2D, the airfoil circulation creates lift upon the moving airfoil, but it
also creates an equal down-force against the ground. If the airfoil was flying
higher, the force pattern on the ground grows wider, but the net downforce
doesn't change. As a result, 2D airfoil diagrams do not describe normal
flight. They describe a sort of "Flatland flight" where no net work needs be
done to accelerate mass downwards. As an explanation of aircraft, they've been
simplified until they cross the line into actual error.

In the 3D world we can launch a vortex downward and experience a reaction
force upward. Or perhaps launch a vortex sideways (during turns.) No instant-
force upon the Earth is needed. The Newtonian force-pair arises between the
mass-bearing aircraft and the mass-bearing air entrained by the shed vortex.
Hovering rockets have an exhaust plume, and flying airplanes have a descending
vortex-pair, and both are essential to any explanation. Real world 3D flight
is "vortex-shedding flight," and the explanations based on 2D airfoil diagrams
leads to no end of confusion.

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po
See also, the awesome Amasci site discussion of this:

<http://amasci.com/wing/airfoil.html>

<http://amasci.com/wing/rotbal.html>

<http://amasci.com/wing/whyhard.html>

 _edit:_ Another interesting fact is that the flawed description of how
airplanes fly is so pervasive that Albert Einstein once proposed how to
improve the airfoil based on his understanding of it and it was a huge flop.

See the end of this article:

<http://www.mathpages.com/home/kmath258/kmath258.htm>

~~~
lutorm
I also highly recommend John S. Denker's "See How It Flies"
(<http://www.av8n.com/how/>), which along with more "aviating" tips has a good
discussion about circulation, the Kutta condition and the Kutta-Zhukovsky
theorem. The Wikipedia page on the Kutta condition
(<http://en.wikipedia.org/wiki/Kutta_condition>) is also quite informative.

I especially like this part from See How It Flies:

    
    
      We have seen that several physical principles are involved in producing lift.
      Each of the following statements is correct as far as it goes:
    
      * The wing produces lift “because” it is flying at an angle of attack.
      * The wing produces lift “because” of circulation.
      * The wing produces lift “because” of Bernoulli’s principle.
      * The wing produces lift “because” of Newton’s law of action and reaction.
    
      ... Do we get a little bit of lift because of Bernoulli, and a little bit more
      because of Newton? No, the laws of physics are not cumulative in this way.
      There is only one lift-producing process. Each of the explanations itemized
      above concentrates on a different aspect of this one process.
    

I think this is the crux of the matter. It's sort the same story as in the
recent HN post about the hazards of determining cause and effect in medical
research. The laws of physics set up a situation where the solution satisfying
all the constraints is that the wing produces lift. But just like you can't
pull out one equation out of a linear system and say that the solution is X=13
_because_ of this one equation, one can't say that a wing produces lift
_because_ of any one physical effect.

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MaxGabriel
<http://xkcd.com/803/>

(obligatory mention)

~~~
adolph
Is there one describing the truth?

Is there any way of searching xkcd by subject matter?

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bcardarella
A lot of this stuff also applies to sailing, a great website that I've been
preaching for years that focuses on the Coanda Effect is
<http://sailtheory.com> I highly recommend

~~~
smithian
Nice one, that I wasn't familiar with. Another great site for sailing theory
is Arvel Gentry's page. His Origins of Lift piece was the first reference that
made the process clear to me. <http://www.arvelgentry.com/origins_of_lift.htm>

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jcdreads
This is great.

I usually tell people that planes fly by pushing air down and backward: down
so the airplane doesn't fall and backward so the airplane moves forward. I
tell them this because otherwise they completely forget, and jump into
assertions about implementation details like laminar airflow.

Now I know to _first_ talk about air going backward and down and _then_ to
point them at the conclusions section of this article, referring them to the
body of the article for detail. A-and then I'll get back to my work so I can
get home at a decent hour.

~~~
onemoreact
Assuming the plane is not accelerating it's only pushing air back to offset
the air it's drag is pushing forward with the next effect being zero.

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DaniFong
This is a largely correct article debunking one of the main atrocities in
aerodynamics education today. I approve heartily!

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kcima
NASA's explanation of how the "Longer Path", or "Equal Transit Time" theory is
incorrect.

<http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html>

The page contains a simulator of a symmetrical airfoil that you can tweek,
angle of attack, particle flow, velocity, and pressure - as well as
instructions for experiments proving this theory incorrect.

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adolph
A couple of recent blog posts linked to the fiu.edu article. They may also be
interesting reading:

Kevin Drum: The Problem With Science: <http://motherjones.com/kevin-
drum/2011/12/problem-science>

Modeled Behavior: Of Science and Scientism:
[http://modeledbehavior.com/2011/12/21/of-science-and-
scienti...](http://modeledbehavior.com/2011/12/21/of-science-and-scientism/)

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nileshtrivedi
Can we not easily test the equal-transit-time explanation by a smoke test in
wind tunnel where the the smoke is not released continuously but only at
uniform time-intervals in a vertical profile? A picture of the smoke profile
(as opposed to streamlines) downstream of the wing would immediately capture
the fact that transit times are NOT equal.

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carlsednaoui
FIU on HN? Ha, go golden panthers!

