Backwash Causes Lift?

On 3 Oct, 21:11, Le Chaud Lapin wrote:
On Oct 3, 1:33 pm, wrote:

On Oct 3, 10:56 am, Le Chaud Lapin wrote:

On Oct 3, 10:34 am, wrote:

Both Newton and Bernoulli are correct. Even inside a pipe the
static pressure drops as velocity increases. That's why your bottom
table jumps as you yank off the top one: you accelerated an airflow.
And in generating lift there's a displacement of air. Can't escape
that at all.

Also, if you don't mind, I would like to understand what you mean
here.

It's not clear to me.

[explanation of Bernoulli's principle clipped].

I did not mean that I did not understand Bernoulli's principle.

What I am saying is that I do not believe that the bottom table jumps
because of airflow acceleration. In fact, if I were to use tables
with circular faces, and put the entire apparatus in a giant
cylindrical tube, and pull up on the top table, assuming that the very
bottom of the tube were open-ended, the bottom table would follow the
top table upward, no matter how fast any air inside the tube were
moving. I could move the top table one molecular diameter every
10,000,000 years, and after the top table has moved, say, 0.5 meters,
the bottom table will follow. This assumes, of course, that the
appartus is airtight, that no air from outside the tube can squeeze in
between the walls of table and tube to fill the void that was created.

There are 14.7 lbs per square inch of pressure pressing upward against
the underside of the bottom table. The yanking of the top table
creates a vacuum between the two faces of the table. The lack of
pressure on the top of the bottom table leaves nothing to counteract
the pressure pressing upward on the underside of the bottom table.
Then the only thing holding the bottom table on the floor is gravity.
Assuming that the table is a typical table of typical weight and size,
one is guranteed that the impulse net pressure of 14.7lbs / in^2 is
enough to overcome gravity and lift the bottom table off the floor.

Note that this really has nothing to do with Bernoulli's principle or
dynamic pressures.

If it is still not clear, put the assembly in a tube again, anchor the
bottom table with a tie wire so it cannot move upward, and using a
hydraulic jack, pull the top table upward, then stop, wait a minute,
have a Coke (sipping with a straw of course), then take cutters and
snap the wire holding the bottom table to the floor.

At the precise moment that the wire is snapped, there is no movement
of anthing at all. There is no Bernoulli action.

The bottom table will rush up toward the top table, even slamming
against it quite hard if the coefficient of sliding friction between
table-side and tube wall is low enough.


Whoowh!
Zero point energy!

In my own kitchen!

I can tell the electric company to **** off now.


Bertie

On Oct 3, 4:14 pm, Bertie the Bunyip
wrote:
Whoowh!
Zero point energy!


Surely, you must be joking. The exposition I wrote above is nothing
more than high school physics.

Where do you see me implying zero point energy?

I know my physics. Do you? There is no "zero point" energy.

Plain and simple:

If a person sucks on a straw, the reason the fluid rises has *NOTHING*
to do with Bernoull's principle. It has to do with the balance in
force being eliminated. In particular, the air in the straw is
removed, so the 14.4lbs/square in will lift the fluid in the straw.

This should be familiar to you, since you are a pilot. Where do you
think 29.92 Hg comes from? It comes from the height that a column of
mercury will rise in a complete rarefied tube in STP, which just
happens to be 29.92.

Both you and Mxmanic are wrong.

-Le Chaud Lapin-

Le Chaud Lapin wrote:

Both you and Mxmanic are wrong.

-Le Chaud Lapin-

ooooo.....wait for it.....here it comes....

--
Message posted via http://www.aviationkb.com

On Oct 3, 4:04 pm, Le Chaud Lapin wrote:

If a person sucks on a straw, the reason the fluid rises has *NOTHING*
to do with Bernoull's principle. It has to do with the balance in
force being eliminated. In particular, the air in the straw is
removed, so the 14.4lbs/square in will lift the fluid in the straw.


So, if it has nothing to do with Bernoulli, what has it to do
with lift? With tables and straws and the like we're talking static,
not dynamic pressures. The airplane uses dynamic AND static
pressures.
In cruise flight (low AOA), I would expect a reduced pressure
on the bottom of the wing, though not as low as on the top. The
velocity of air across the bottom will drop its pressure, there, too.
Air has mass. Anytime you try to push it out of the way,
there will be some reaction. Newton says so. We know this as drag.
But we also know it as lift reaction. A flat plate flying
through the air at some tiny angle of attack doesn't have much faster
air over the top than the bottom, if any difference at all, yet it
will generate plenty of lift. Try this on, if you want to think
outside the box: The airfoil we know is just that: a foil (device to
deceive) to trick the air into flowing over it without breaking up at
much higher angles of attack than a flat plate would let us. So the
leading edge has to have some radius so the air can get around the
corner from the natural stagnation point under the LE at high AOA, and
that curve must gradually taper off toward the rear or the now-
disturbed air would want to separate and turbulate, and if it did that
it would then slow down dramatically, pressure would rise and lift
would decrease. But, happily, Newton is still at work underneath so
the airplane falls, but not as if the wings fell off. We're still
moving forward and the wing is still shoving air out of the way
downward, so lift is still generated.

Dan

On Oct 3, 5:56 pm, wrote:
So, if it has nothing to do with Bernoulli, what has it to do
with lift?

You can have lift of an object with no Bernoulli. It's simple vector
addition.

With tables and straws and the like we're talking static,
not dynamic pressures. The airplane uses dynamic AND static
pressures.

Right.

In cruise flight (low AOA), I would expect a reduced pressure
on the bottom of the wing, though not as low as on the top. The
velocity of air across the bottom will drop its pressure, there, too.

Right. The AOA matters here. Angle the wing up any significant amount,
and over-pressure will build under the wing.

Air has mass. Anytime you try to push it out of the way,
there will be some reaction. Newton says so. We know this as drag.

Hmm...ok, sure. I wouldn't call that drag necessarily. That's like
saying that a hydraulic piston assembly has drag. I more prefer to
think of drag as laminar fricitional forces of the fluid. Pushing out
of way implies that plane doing the pushing is perpendicular to the
direction in which pushing is being done (which is true at leading
edge of wing). But I guess this is acceptable.

But we also know it as lift reaction. A flat plate flying
through the air at some tiny angle of attack doesn't have much faster
air over the top than the bottom, if any difference at all, yet it
will generate plenty of lift. Try this on, if you want to think
outside the box:

Thinking outside the box is what lead me to refuse to accept hand-
waving explanations of aerodynamics from CFI's.

The airfoil we know is just that: a foil (device to
deceive) to trick the air into flowing over it without breaking up at
much higher angles of attack than a flat plate would let us. So the
leading edge has to have some radius so the air can get around the
corner from the natural stagnation point under the LE at high AOA, and
that curve must gradually taper off toward the rear or the now-
disturbed air would want to separate and turbulate, and if it did that
it would then slow down dramatically, pressure would rise and lift
would decrease. But, happily, Newton is still at work underneath so
the airplane falls, but not as if the wings fell off. We're still
moving forward and the wing is still shoving air out of the way
downward, so lift is still generated.

Yes this is true, but the explanation in the Jeppensen book is wrong.
it defines downwash:

downwash: - "the downward deflection of the airstream as it passes
over the wing and past the trailing edge"

It goes on to say:

"According to Bernoulli's principle, the increase in speed of air on
top of an airfoil profdues a drop in pressure and this lowered
pressure is a component of lift."

Ok, we really know that the lift results from what's under the wing no
longer being balanced out, but I won't nit-pick this explanation.

Next paragraph it says:

"In addtion to the lowered pressure, a downwar-backward flow of air is
also generated from the top surface of the wing. The reaction to this
downwash results in an upward force on the wing which demnstrates
Newton's third law of motion."

This is plane false. That is *NOT* what Newton said. Newton did not
say you could take any action and willy-nilly find what you think is
the reaction, and say, "Hey, this looks good, let's use this."

Newtons law, in fact, is better stated as reciprocity of force, IMO.
This says that, if you take two objects, one apply force to the other,
the other, by reciprocity, must appy an opposite force against the
first.

Newton's law, conbine with F=ma, also yields the notion of
conservation of momentum.

But getting back to Jeppensen, the downwash, if they mean what's
happening on the top surface of the wing, is *not* contributing to
lift. Note that they say "results in", but don't explain how. This
seems to be typical of books of flight dynamics.

I'd like to point something else out regarding Bernouilli's principle.

I haven't tried, but I suspect that I could build a contraption that
consists of surface where the velocity of air above the surface is
much higher than that below, but the pressure above the surface is
higher.

-Le Chaud Lapin-

In fact, if there is a downward component of the air's velocity that
had come from its passage over the upper surface of the airfoil, then
there had been an acceleration provided to that air -- acceleration in
this case being conventionally defined as the second time derivative
of position.

Now, if the air is accelerated downward, and it has mass, it means
there had been a force applied. The local prime mover is of course the
wing, so it must experience an upward force. Maybe you have a
different idea as to on what that equal and opposite force is
operating on -- I'd be interested in hearing about that.

There are a number of basic principles in operation here, be careful
not to paint yourself into too tight a corner unless you are quite
expert.

I am not claiming skill in this area -- physics was a minor a long
time ago -- but I remember some of the basics.

Tina wrote:
In fact, if there is a downward component of the air's velocity that
had come from its passage over the upper surface of the airfoil, then
there had been an acceleration provided to that air -- acceleration in
this case being conventionally defined as the second time derivative
of position.

Now, if the air is accelerated downward, and it has mass, it means
there had been a force applied. The local prime mover is of course the
wing, so it must experience an upward force. Maybe you have a
different idea as to on what that equal and opposite force is
operating on -- I'd be interested in hearing about that.

There are a number of basic principles in operation here, be careful
not to paint yourself into too tight a corner unless you are quite
expert.

I am not claiming skill in this area -- physics was a minor a long
time ago -- but I remember some of the basics.


The main thing about all this is that both Bernoulli and Newton are
complete explanations of lift and will stand alone. In fact they are
both explanations of the same thing really as they occur simultainously
as lift is being created.
The big rub about Bernoulli is that for years Ole' Daniel was raped by
text books stating several totally false applications of Bernoulli as
fact. The equal transit theory for example, often stated as an
explanation for Bernoulli is totally incorrect.
The real truth of it is that neither Newton or Bernoulli were dealing
with lift at all in their respective work that explains lift.
My fondest hope is that someday, pilots will collectively get it
together enough to realize that Bernoulli and Newton are not in
competition with each other and never have been.
Personally though.....I like my old friend Mary Shafer's explanation for
lift that blames it on the "lift demons". :-)
DH

--
Dudley Henriques

Mr Dudley sir, those are not lift demons, those are lift fairies or
lift pixies. If you call them demons they may take you high and then
let go.

Newton warned us that for every fairy there is an equal and opposite
demon.

I would be interested in having the OP tell us how to derive
conservation of momentum from F=MA though.

Dudley Henriques wrote:
Personally though.....I like my old friend Mary Shafer's explanation for
lift that blames it on the "lift demons". :-)

No science like lift demons is ever the work of just one person. See for
example the collection titled:
"The Emerging Science of Lift Demons":

at this site:

http://www.main.org/polycosmos/glxywest/lift_faq.htm

On Oct 3, 6:39 pm, Dudley Henriques wrote:

The main thing about all this is that both Bernoulli and Newton are
complete explanations of lift and will stand alone. In fact they are
both explanations of the same thing really as they occur simultanously
as lift is being created.
The big rub about Bernoulli is that for years Ole' Daniel was raped by
text books stating several totally false applications of Bernoulli as
fact. The equal transit theory for example, often stated as an
explanation for Bernoulli is totally incorrect.

Exactly. The equal-transit theory isn't correct. The air
over the top actually reaches the trailing edge *before* the bottom's
flow. Intuitive thinking would have it arriving later because the
distance is greater.
Our OP should see the diagrams he
http://www.av8n.com/how/htm/airfoils.html

This one shows the pressure distribution over the typical cambered
airfoil:
http://www.kemi.fi/kk019065/calculators/ClarkY.jpg

Note that there's pressure acting on the bottom. Where would
that come from, if not Newton? And note that Bernoulli runs out of
steam on the top near the trailing edge, and the pressure actually
goes above ambient there. I see this on the wing of my Jodel in
flight. It's a low wing, fabric covered, and the pressures are easily
visible by the way the fabric is pressed down or pulled up between the
ribs. Over about the last third of the chord, the fabric is pushed
below the ribs as the pressure there goes quite positive, while ahead
of that it's pulled up.
Look at that leading edge. Lots of lift over the first bit,
right where we'd expect a lot of drag (positive pressure) instead.
Not at all what you'd expect intuitively, is it? And that's
where the uninformed get into trouble: by using "experience' gained
from other, vastly different things, or from reasoning based on
inadequate information.

After all the years of reading this stuff and seeing wind-
tunnel demos and graphs and all such, I know there's an awful lot of
information out there on the generation of lift. Most of it is
available on the 'net. The strangest thing is the newbie who starts to
argue with his textbooks, very publicly (as on a newsgroup) without
Googling it for himself first. He knows better, he's sure.

Dan