Thrusting or Sucking (where's Howard Stern when we need him.)

Maybe someone more acquainted than me in nuclear physics
can shed some light on this, but my understanding is
that the Bohr model is false: The electrons revolving
around the atom would spin into the core, for one.
It is thus a model that could not sustain an atom in
life. After all, this is one of the main paradoxes
that lead to the creation of quantum mechanics.

But I'm digressing. The point I want to make is that,
just as the Bohr model is taught in school almost a
century after it has been found to be false (and not
just a bit inaccurate), there is an instructional role
for models that are false, but useful as a stepping
stone to the understanding of very complex phenomena.


One has to state the model, make people understand
it AND THEN exlpain why it is not correct.

Otherwise, it is most often the case that complex phenomena
tend to remain within the purview of the very few that
will come to understand the very complex mathematics
that describe the phenomenon in its full details.

There is plenty of good understanding that can be gained
by explanations that contain false premices. Should
we be as purist as this to reject even a cursory mention
of things that are not 100% accurate in the educational
process?

So, when trying to explain the world of physical matter,
do we bring out the Bohr model, or should we skip this
and directly introduce the fuzzy world of QED.

In the same principle, when trying to explain the very
complex phenomenon of wing lift, do we go from stating
the Bernulli effect and Newton's 2nd law to diferential
equations of fluid dynamics -- or possibly stop there,
explaining that the actual phenomenon is too complex
for 'common people' to understand?

Or can we bring up a series of models, each more complicated,
to ease in an understanding of the phenomenon by helping
people visualize what is taking place. Of course this
can only happen up to a point and then we are faced
with the pure mathematics.

I would like to know what this forum thinks in terms
of this? In my experience, quite a few gliding instructors
are either very much missinformed about the aerodynamics
of lift (and keep on missinforming students), or offer
explanations that while accurate fail to answer a student's
question by advancing the understanding of what take
places. This is very discouraging for most people and
it is only the few who will then go back into the literature
and try to sort things out.

The funny thing about this is that, while it is not
very important to know about aerodynamics to fly a
glider, all sorts of misinformation eventually trickles
into discussions and advice about such things as the
best bank to turn in, or when a stall occurs etc.

So what should we do about this?



At 10:30 11 January 2006, Bert Willing wrote:
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
(Remove DONTSPAMME from address to email reply.)

Alexander Georgas schreef:
There is plenty of good understanding that can be gained
by explanations that contain false premices. Should
we be as purist as this to reject even a cursory mention
of things that are not 100% accurate in the educational
process?
Being an aerospace student myself I`m probally a bit of a purist as
well. Nevertheless I think that the final goal of teaching aerodynamics
(anything actually) is understanding, not perfectionism.


In the same principle, when trying to explain the very
complex phenomenon of wing lift, do we go from stating
the Bernulli effect and Newton's 2nd law to diferential
equations of fluid dynamics -- or possibly stop there,
explaining that the actual phenomenon is too complex
for 'common people' to understand?
What`s wrong about only telling the short version of the theory,
without complicated stuff:
"An airfoil creates an flow which accelerates air downwards. This
results in a pressure (and shear) field on you`re wing which causes
both lift and drag. Nothing really complicated about that I guess, and
no problem with applying it to reality at all.


I would like to know what this forum thinks in terms
of this? In my experience, quite a few gliding instructors
are either very much missinformed about the aerodynamics
of lift (and keep on missinforming students), or offer
explanations that while accurate fail to answer a student's
question by advancing the understanding of what take
places. This is very discouraging for most people and
it is only the few who will then go back into the literature
and try to sort things out.
About a year ago I was having a discussion with a couple of gliding
instructors. None of them understood "anything" about aerodynamics and
I heard the weirdest theories. Nevertheless they seem to teach flying
quite well without any problems for their student. Not understanding
flow completely doesn`t seems to be such a problem in flying a glider.


The funny thing about this is that, while it is not
very important to know about aerodynamics to fly a
glider, all sorts of misinformation eventually trickles
into discussions and advice about such things as the
best bank to turn in, or when a stall occurs etc.

So what should we do about this?

Just skip the complicated theory?
Real world aerodynamics is extremely complicated with a lot of issues
(especially vortices) still beyond oure understanding. You can`t expect
every glider pilot to understand complex differential equations or the
navier-stokes equations.
Instead of putting a lot of effort in explaining lift we maybe should
concentrate a bit more in the relationship of weight, surface and so
on, which is very usefull in normal flight operations. The square and
squareroot equations seems to be complicated enough for most pilots.

Jarno Nieuwenhuize,
The Netherlands.

I would like to know what this forum thinks in terms
of this? In my experience, quite a few gliding instructors
are either very much missinformed about the aerodynamics
of lift (and keep on missinforming students), or offer
explanations that while accurate fail to answer a student's
question by advancing the understanding of what take
places. This is very discouraging for most people and
it is only the few who will then go back into the literature
and try to sort things out.

The funny thing about this is that, while it is not
very important to know about aerodynamics to fly a
glider, all sorts of misinformation eventually trickles
into discussions and advice about such things as the
best bank to turn in, or when a stall occurs etc.

So what should we do about this?

I agree with your statements.
When do your instructions begin?
I regularly abandon wrong assumption. Through this process
I am learning. For example the lift equation
CL * V^2 * area / density , how does it fit into the circulation
equation? I assume it was a stripped down version that I have
seen in an earlier post. I view the circulation as a 3D flow
about the wing and glider structure. As this flow must effect
every thing in one continues interaction, to express this in
a formula and still have a visual understanding of the process
must be nearly impossible. As a person interested in
aerodynamics I find the incremental approach much more to
my liking and I get results that are still within what I experience.
Regards
Udo

Do I have to go to the back of the class? I thought a propeller was a
screw.

"J. N." wrote in message
oups.com...
You`re pushing air, water or whatever downwards. The result is a
pressure and shear distribution on a plane which creates both lift and
drag.
In reality you can only apply bernoulli on a slow (200 kts), airfoil
shaped device because of compressibility and possible detatchment of
the boundary layer. When you start adding slats, fowler flaps or start
flying fast or very slowly (model airplane`s) Bernoulli is far beside
the thruth.

A propellor or rotor is simply a rotating wing, and works exactly the
same. It both sucks and pushes the air afterwards.

The Blackbird would probally not fly at all if you use Bernoulli.

That`s it...

Jarno Nieuwenhuize,
The Netherlands.

Schroedinger says yes! and no!
"Shawn" sdotherecurry@bresnannextdotnet wrote in message
...
Don Johnstone wrote:
I think you can also obtain books that prove the earth
is flat and the Holocaust never happened. Oh and I
forgot there are several that prove global warming
:-)

"Climate Change" C'mon don, get with the times. "Global Warming" is so
1997.
;-)
BTW, was the Earth flat before Magellan sailed around it?

Shawn

Ian Johnston a écrit :

What the unititaited don't realise is that forces can be transmitted
by pressure or by momentum flux. At the surface of a wing, all the
force comes from pressure. A long way away from the wing, all the
force comes from momemntum transfer (there is effectively no pressure
difference). As you go from one to the other the balance changes. Most
people who "prove" theories of lift wrong simply forget to take
account of momentum flux.

Please explain us how momentum flux can be transmitted in a gas without pressure change. I think you will get at least a Nobel Price for such a revolutionary theory ;-)

On Fri, 13 Jan 2006 16:11:29 UTC, Denis
wrote:

: Ian Johnston a écrit :
:
: What the unititaited don't realise is that forces can be transmitted
: by pressure or by momentum flux.

: Please explain us how momentum flux can be transmitted in a gas without pressure change. I think you will get at least a Nobel Price for such a revolutionary theory ;-)

Eh? Any fluid crossing the boundary of a control volume takes momentum
with it. That's a momentum flux. Elementary fluid mechanics.

Ian

On Fri, 13 Jan 2006 17:14:46 UTC, T o d d P a t t i s t
wrote:

: The point is that the sum (pressure plus MF) is always the
: same, equal to the lift on the airfoil. The point of the
: original post was that you have to account for *both*
: pressure and momentum flux to figure out the force.

Thanks - that was exactly my point. Peraps I was not terribly clear.

Ian