
How Airplanes Fly: A Physical Description of Lift (1999) - Tomte
http://www.allstar.fiu.edu/aero/airflylvl3.htm
======
jessaustin
Frankly I've never understood how the Bernoulli explanation became so
widespread. Anyone who has held her hand out a car window has learned
everything qualitative one can know about airfoils. It's almost as though
someone were trying so hard to find a non-intuitive model that he fooled
himself that his model actually explained anything.

~~~
burnte
It's fairly well accepted that the Bernoulli principle is not the only
mechanism of lift today, but it IS a real mechanism, and part of what creates
lift in most airfoils, is entirely responsible for technologies such as
Bernoulli disk drives, and a minuscule factor in things like kites and paper
planes (and your hand out the window).

~~~
nether
The thing is a lot of people have taken "Bernoulli" to mean "equal time of
passage" (a completely made-up, non-physical phenomenon) rather than the quite
valid Bernoulli equation:
[https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Incomp...](https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Incompressible_flow_equation).
In reality Bernoulli, Newton, and Kutta are just multiple correct abstractions
for turning the flow.

~~~
jessaustin
You're right, of course. However, the various explanations are suited to
different audiences. An aerospace engineer knows everything she needs to know
about all of these tools, so she just uses whatever is best suited to a
particular situation. For explanation to a general audience who know about
force and momentum because they've already seen Newton in other situations,
why even bring up "pressure"? I suspect this came about when some dumbass said
"well everyone knows about pressure because car tires".

------
wallstquant
I think John Denkar also provides a nice explanation here:

[https://www.av8n.com/how/](https://www.av8n.com/how/)

Specifically in ch 3 he talks about airfoils, airflow, and circulation:

[https://www.av8n.com/how/htm/airfoils.html](https://www.av8n.com/how/htm/airfoils.html)

~~~
Isamu
Thanks, this is an interesting and detailed explanation.

It should be pointed out that your link contradicts the op in places, in
particular Denkar says that the role of the Coanda effect in producing lift is
a "fairytale" where the op calls it out to explain the lift on the upper
airfoil surface (and I have seen this elsewhere as well.)

I hadn't known until recently just how religiously contentious this topic
could get.

------
WalterBright
The article is bunk. Everyone knows that lift is generated by gripping the
armrests. Proof you can try yourself: when you're in an airplane and it
suddenly drops, a good solid squeeze on the armrests will bring it back up.

~~~
TeMPOraL
Be careful when performing that experiment - if you grip the armrests
incorrectly, you may pull up the ground instead. Last time I tried it the
ground suddenly appeared and hit the plane. Fortunately, it was a low-speed
collision; the plane stopped, and the crew let us out like nothing ever
happened.

------
eddd
A bit off topic: this page is completely robust, the design is responsive and
it works on every browser. And yet it has been build on 1999 :)

~~~
pif
> this page is completely robust... And yet it has been build on 1999

Maybe it's robust _because_ it was built in 1999, where nobody felt an
obligation to add fancy scripts to a text document ;-)

------
sandworm101
I think part of the problem is that the classroom demonstrations give a false
impression of how quickly airplanes actually move. 200kph is not something
that can be visualized on a 20-inch wide TV screen, or even a 20-foot wide
projection. With slow-moving animations it is hard to imagine wings pushing
enough air downward to keep a 747 airborne. So observers latch onto quazi-
mystical explanations that sound good enough to be put on a test.

------
dang
A good discussion from 2011:
[https://news.ycombinator.com/item?id=3379928](https://news.ycombinator.com/item?id=3379928).

~~~
Steko
Another recent thread that touched on same subject:

[https://news.ycombinator.com/item?id=10218029](https://news.ycombinator.com/item?id=10218029)

------
zby
It is great atrticle - but one thing it has backwards - Bernoulli principle is
harder to understand than the second Newton law.

Also I don't buy that viscosity argument - viscosity diverts air not down -
but vertically. The move down is simple dynamics:

.\

the dot is th air particle - the \ is the wing - when the wing moves the dot
is forced down under the wing. The particle moves - and so it gets momentum
down. The particle touching the wing is moved by the wing (the wing pushes it
down - and the particle pushes it up according to the third law - generating
the lift), the other particles are pushed by the particles toughing the lift.

~~~
slavak
NASA calls this "The Skipping Stone" theory of lift[1], and it is incorrect.
What you're neglecting is that not only the bottom part of the wing diverts
air downwards, but indeed the top part does as well; this contributes a large
part of the lift generated by the wing.

[1]
[https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html](https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html)

~~~
Retric
Not quite. If you had a zero width wing you get a void behind the wing. Aka
you pushed air down so it's not there.

This creates a low pressure area behind the wing. But, that low pressure is
still pushing down. It just pushes less than the normal air pressure under a
wing.

You can see this by dropping a single sheet of paper vs a stack of paper. In
your model the low pressure above the wing pulls the paper up slowing it's
decent. But, if you drop a stack of loose printer paper the top piece falls as
fast as the bottom and is not pulled from the stack.

------
acchow
tl;dr The wings are angled so that air is forced slightly downwards when it
collides into the wing. This collision is sustained by engines which force the
plane into the air in front of it.

~~~
cft
If that was it, then the wings would have been symmetrical or flat. The angle
of attack probably accounts for less than 10% of the lift.

~~~
wysiwylwysiwyl
Consider how flaps are shaped. When extended they increase the angle of
attack; not the ratio of air travel distance.

~~~
rrrrrraul
Generally, when flaps and slats are extended, they increase the camber of the
airfoil, increasing the Coefficient of Lift for the same AoA. Also, the air
gaps introduced at the junctions between slats/flaps and the main wing re-
energize the boundary layer on the top surface of the wing to retard flow
separation on the upper surface (becomes a concern because of the increased
camber).

------
friendzis
For me the best 5 year old explanation of lift has always been such:

Body, gas including, temperature equals energy, total energy is the sum of
kinetic and potential energy and potential energy is what bumps into other
objects and pushes that away. The faster the gas flows, the more kinetic
energy there is, the less potential is left, thus the object is pushed with
lower force. If an object shaped such that gas around opposing faces flows in
different speeds moves through the gas, opposing faces are pushed with
different force and you get lift. The higher the speed, the more total energy
is converted to kinetic energy, the less potential energy is left, the higher
the force differential. Thus, the faster the object goes, the more lift it
generates. That explains minimal take-off speed of an airplane.

This is by no means scientifically rock solid, but gives really good layman
explanation.

~~~
arbitrage
5 year olds don't really have a good conceptualization of kinetic or potential
energy.

