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Re: [Phys-l] Bernoulli Effect



On 12/15/2007 01:59 PM, David Abineri wrote:

When looking at the cross section of a wing, I have always said that the
air flow under the wing (which is a relatively flat surface) is slower
than the air flow over the top of the wing which is curved. The
question from them is "why isn't it just as likely that he air over the
top of the wing does not speed up and simply takes a longer time to
travel the longer distance over the curved section of the wing?".

1) There is *no* reason why the air flowing over the top
should "line up" with the air flowing under the bottom.

2) In fact, for any wing that is producing lift, the air
that flows over the top arrives at the back *early*
compared to the corresponding air the flows under the
bottom. This is despite having followed a longer path.
This is required by the Kutta-Zhukovski theorem. See
http://www.av8n.com/how/htm/airfoils.html#fig-flow-past
http://www.av8n.com/how/htm/airfoils.html#fig-circ-offset
and associated discussion.

3) There is no reason to assume that the bottom of the
wing is flatter than the top of the wing. Sometimes it
is, but sometimes it isn't. Many general-aviation
aircraft use symmetric airfoils (zero camber). See
http://www.av8n.com/how/htm/airfoils.html#sec-inverted-camber

On 12/15/2007 03:46 PM, Peter Schoch wrote:

Let's look at 2 particles, one goes over the wing, and one goes
under. If the one going under the wing goes faster (shorter distance
and shorter time) it gets to the end of the wing first, while the
other particle is still over the top of the wing.

This makes the next 2 particles do the same. After a while the
particles will 'back up' on top of the wing. If that were the case,
the density would have to become greater over the wing, pushing down
more on the wing, and it would not fly!

100% baloney.
Diametrically inconsistent with theory.
Diametrically inconsistent with wind-tunnel data.
For details, start with:
http://www.av8n.com/how/htm/airfoils.html#fig-delay

See also item (2) above.

On 12/15/2007 06:26 PM, Anthony Lapinski wrote:
This sounds reasonable, but then I always wondered how certain planes can
fly upside-down. Are the wings on these planes shaped differently?

Ah, it's nice to see somebody is paying attention. Yes, this
makes a nice one-line disproof of the idea that being flat on
the bottom has anything to do with the basic lift-producing
mechanism.

BTW we're not talking about "certain" planes. They can _all_
fly upside down.

While I truly appreciate the website/book given as a reference -- I think
it would be a bit much for my Algebra-based students or High School
students. :^)

Algebra-based physics? That sounds like a luxury. I am called
upon to explain how a wing works to student pilots who don't know
any algebra or any physics. The presentation at
http://www.av8n.com/how/htm/airfoils.html
has been carefully tuned over the years to suit a no-physics
no-algebra audience.

OTOH I have the advantage of highly motivated students.

If you want a good website for those target audiences, that also have good
dynamic simulations for use on a SmartBoard or other projection system,
take a look at:

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

Well, that particular page doesn't answer any of the questions
people on this list just asked, and if you go to
http://www.grc.nasa.gov/WWW/K-12/airplane/factors.html
the figure is just completely wrong, and
http://www.grc.nasa.gov/WWW/K-12/airplane/presar.html
has a whole bunch of math, not just algebra but integrals.
Also at
http://www.grc.nasa.gov/WWW/K-12/airplane/lift2.html
NASA finally got around to using the colored pulsed streamers
that have been at
http://www.av8n.com/how/htm/airfoils.html
for 15 years. If you want the animated versions, at various
angles of attack, check out
http://www.av8n.com/irro/profilo1a_e.html
which have been there for >10 years and are smoother than
the NASA animations, and easier to interpret because they
use more colors. (Credit: Prof. Marco Colombini.)

When I lecture on this subject, I use two projectors: One
is for the main line of discussion, while the other runs
http://www.av8n.com/irro/animation/prosmo015.gif nonstop
the whole time.

Final note: If you're pressed for time and have to skip
some material, DO NOT skip the fluttering card demo:
http://www.av8n.com/how/htm/airfoils.html#fig-flutter
This makes the point about circulation in a way that no
words or pictures possibly can. And if you don't understand
circulation, you don't understand anything about how wings
work.