Well, trying to explain something by starting with a statement which is
completely wrong is a very strange educational twist...
The Bohr model is not accurate enough to explain all details, but it is not
wrong.

Bert
ASW20 "TW"

"Alexander Georgas" wrote in message
...
OK OK,

I said it was inaccurate to begin with!

Good to see this has stirred up some debate, thought.

While I agree this is more urban myth than fact, I
do feel that it helps more in visualization than statements
such as 'pressure builds below the wing' or 'it is
the Bernulli effect above the wing' (some even refer
to the Venturi effect).

I feel that the equal transit time proposition is a
good first step in helping people visualize what happens
when lift is generated, i.e. people start to think
about the flow fields over the entite wing section
in a more visual manner.

It is quite intuitive and easy to visualize. Once one
has actually understood this, he can now accept that
it is not true and move to the next level of understanding.

So if you drop the wing geometry idea (which I do not
aspise to begin with) and replace it with the concept
of angle of attack and also do not get too stuck with
the 'particles exit at exactly the same time' proposition,
you are starting to visualize things more properly.

Once you understand this, and the fact that the flow
over the wing is much faster than what the equal transit
time proposition must suggest and that the angle of
attack has created a low pressure area which has accelerated
the top flow field quite considerably, you can start
visualizing things like the vortices that are generated
once the two flows meet up behind the trailing edge
etc.

Now you are getting somewhere!

Otherwise, jumping from the 'there must be a Bernulli
effect somewhere' to the Euler equations is just too
much of a leap for the imagination (at least my imagination).

Now, concerning the Newtonian explanation, I consider
this too much of a cop out. Ok we all know it is correct,
and it is plain simple to understand. It just doesn't
explain anything about the mechanics of flight. It
is as saying (it is, in fact, saying) that the laws
of energy conservation will be upheld and that the
laws of thermodynamics will not be broken and that
the universe will keep on working the same way as it
was before. We sort knew this already. So where is
the understanding that comes from this explanation?

So I feel that while the equal transit time paradigm,
while false, is a good starting point for introducing
people to the aerodynamics of wing sections. They just
need to be told, once they understand the concept,
why it is false.

It is as useful as the Bohr model of the Atom: extremelly
inaccurate, but easy to visualize as a starting point
to understanding a very complex phenonenon. Would you
ever introduce particle physics to a student by writing
down the Schroedinger equation and solving a few Hamiltonians?

Alexander Georgas

At 16:24 10 January 2006, T O D D P A T T I S T wrote:
Alexander Georgas
wrote:

Ok, here is another way this can be explained (if just
a bit inaccurate):

Sorry, but this is a lot inaccurate (a.k.a 'wrong')

The critical point to consider is angle of attack.
You just need to think of the airflow meeting the wing
at a specific angle. If you now imagine the airflow
separating to move above and below the wing section
as two particles, you have the following explanation:

-The particles are going to separate at the leading
edge and meet up together approximatelly at the wing's
trailing edge.

The particles do *not* meet up at the trailing edge.
This
is often referred to as the 'equal transit times'
explanation of Bernoulli, which is thoroughly discredited.

-Because of the angle of attack (and of the wing's
thinkness), the air particle that will travel above
the wing section will have a greater discance to cover,
so it will have to run faster if it is to meet up with
the other particle which is traveling below the wing
section

The particles do not have to meet up.

-Particles traveling at greater speeds (compared to
particles in neighboring areas) create areas of low
pressure -- just think: because the particles are running
faster over the wing compared to under the wing, there
are fewer of them in a specific area. Alternativelly,
you can take Bernoulli's word on this one.

Bernoulli does not explain the flow pattern. Bernoulli
explains the lift (pressure differentials given a specific
flow pattern. You have to call in some other physics,
namely that the air flows smoothly off the trailing
edge, to
establish the flow pattern. Once you have that, you
apply
Bernoulli.

-Now let's examine the situation from the perspective
of the wing. There is a low pressure above and a high
pressure bellow. The resulting force is thus upwards
(in relation to the angle of attack).

The only presumption which remains to be explained
is why the two particles have to meet at the back of
the wing section. The easiest way to gloss over this

Take a look at John Denker's start page:

http://www.av8n.com/how/

See the graphic and the bands of red, orange green
blue?
The blue band air over the top hits the trailing edge
before
the blue band air at the bottom, even though they started
out together.

is to consider that if this is not approximatelly the
case, an imbalance will be created whereby there will
be more air crossing below the wing section and a low
pressure area buildup at the top back end of the wing,
created by the lack of air (teh air has just not had
the time to reach there). This would probably invite
air from below the wing to bleed back up towards the
back top.

Guess what, that 'low pressure buildup' causes the
air on
top to accelerate, and it does tend to cause the high
pressure to flow around to the top (resisted by air's
finite
viscosity and the sharpness of the trailing edge).

In this case, the airflow above the wing
would separate from the wing section before the trailing
edge (does this sound a bit like a stalled wing?)

In a stalled wing, this does happen, but we're talking
about
a non-stalled wing and in that case the air flows faster
over the top.

I don't know how much science there is behind this
explanation. I just find it a bit easier to swallow
than the Newtonian explanation. In my view the action-reaction
way of putting it (air is deflected downwards) is true,
but unable to shed any light on the mechanism.

I agree that the Newtonian explanation is hard, but
so is
the Bernoulli. That's the nature of lift.

I think of it like this: When the wing starts forward
at a
positive angle of attack, the space behind the wing's
upper
surface is being swept clear of air molecules - at
least
that's what the wing is trying to do. However, in
a fluid
(air), sweeping the molecules clear would produce a
vacuum
behind the wing, so the instant that the pressure begins
to
fall behind the wing, air from on top and ahead of
the wing
and air above the wing begins to rush towards this
region.

The air from ahead and on top of the wing that rushes
back
towards this low pressure region is being accelerated
-
exactly what the Bernoulli explanation needs for its
lower
pressure due to faster flow. The air above the wing
moving
down, coupled with higher pressure air below the wing
(also
moving down to escape that higher pressure) produces
downwash - exactly as the Newtonian explanation needs.


In reality, these are just two faces of the same coin.
There's no need to use the false 'equal transit times'
explanation of Bernoulli.

It is
just an energy checks-and-balances way of explaining
lift, which we already know occurs.

Alexander Georgas


--
T o d d P a t t i s t - 'WH' Ventus C
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