
A Physical Description of Flight, Revisited (2009) [pdf] - Tomte
http://www.allstar.fiu.edu/aero/Flightrevisited.pdf
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n00b101
_Many ask the simple question "what makes an airplane fly?" The answer one
frequently gets is misleading and often just plain wrong._

The exact same problem is found in sailing:

"Popular concepts as to how sails generate lift, and how two sails interact
with each other are discussed in light of modern aerodynamic research. Much of
the old sail theory in the sailing references is shown to be wrong." [1]

[1]
[http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...](http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf)

~~~
handedness
> Many ask the simple question "what makes an airplane fly?"

I'm reminded of perhaps the most excellent 30 seconds in all of ground school
instruction:

[https://www.youtube.com/watch?v=CK1UGWw15lc](https://www.youtube.com/watch?v=CK1UGWw15lc)

His joke aside, it illustrates an important point: there are a variety of ways
to produce lift. For example, when one observes a jet overfly the runway
during an airshow, holding its altitude while on its side, the engine is the
source of the majority of its lift in that scenario.

(Edited for clarity.)

~~~
6stringmerc
Excellent resource and augment to the conversation here, thank you for
posting! Totally love seeing the Harrier in that video - I saw one live at
Oshkosh and WOW that thing was LOUD.

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timeisapear
I think the easiest explanation to a lay person involves the observation that
all surfaces in a moving fluid (e.g. air) produce lift--whether a flat plate,
rotating cylinder, or extending your hand outside a car window. Airfoils are
merely highly optimized shapes to reduce the (often significant) drag the
lift-producing bodies have.

In an airplane, by reducing L/D enough you decrease the necessary power (which
indirectly produces its lift) needed to overcome the combined weight of the
aircraft (wings/body/powerplant) generated by gravity.

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fnj
You mean _increasing_ L/D?

~~~
timeisapear
Yes, you're correct. Increase* L/D by decreasing drag.

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mthiesen
Wow, Great paper. The most interesting (and counter-intuitive) part for me was
that the lift is generated by the _top_ of the wing bending air downwards. In
my head I always visualized lift as the bottom of the wing being pushed up,
but this paper claims lift is really more like the top of the wing pulling
upwards.

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WalterBright
The lift is the sum of all the pressure vectors around every point on the
wing. Since the pressure vectors are all normal to the wing surface, in order
to have a net upwards vector, the pressure on the bottom surface has to be
higher than that on the top.

I.e. air cannot "pull" anything anywhere.

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renox
If I remember well: in normal condition the lift come 2/3 from the top of the
wing and 1/3 from the bottom of wing.

So the depression above the wing is indeed "pulling up" the wing more than the
surpression below the wing is pushing up the wing.

~~~
WalterBright
No, a vacuum never "pulls". The pressure on the other side of the object
pushes it.

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hexane360
"Parasite drag of a Boeing 747 wing is only equivalent to that of a 1/2-inch
cable of the same length."

That's. . . impressive.

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mastre_
Via our favorite search engine, I found this [http://www.aviation-
history.com/theory/index-theory.html](http://www.aviation-
history.com/theory/index-theory.html)

OP PDF is link 2 ("Airfoils and Lift - Newton's Law"), but as text/html. The
other links look very informative, if they're anywhere near the quality of the
OP text, it will make for fascinating reading.

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6stringmerc
This is really helpful for my ParaWing invention. I still need a lot of work /
help on doing the math for the exact wing dimensions though; very
intimidating. However, using some sound research and an innovative design
technique I do know my path is sound versus conventional designs - more lift,
less aggressive stall, and smaller dimensions are all pretty necessary for
human-scale personal flight device.

