Backwash Causes Lift?

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

BDS wrote:
"Bertie the Bunyip" ...
On 3 Oct, 13:27, "BDS" wrote:
"Mxsmanic" wrote

Le Chaud Lapin writes:
What is the definition of a stall anyway?
An abrupt loss of lift.
Son, for someone who continually chastises the pilots here for their
lack of
knowledge, you sure can come up with some doozies yourself!

Actually, it's correct, but only because he read it off wickepedia or
something.

Here's my take on it - a stall occurs at the angle of attack where the
coefficient of lift stops increasing with angle of attack and begins to
decrease. It continues to decrease beyond this point as angle of attack is
increased further. It is not necessarily an abrupt change - most lift
versus angle of attack curves that I've seen do not have a drastic (abrupt)
drop beyond the peak.

BDS



This is a good explanation.

--
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, 7:29 pm, 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.

The acceleration is indeed downward. If God declared that all air
molecules in the universe must remain still for the sake of USENET
explication, and the wing move forward, and you took a snapshot of
that picture, there would be a vacuum created above the wing. It would
be quite large (not laminar). The floor of this vacuum would be the
wing itself. The ceiling would be the underside of an air mass above
the entire wing, ready to move downward to fill the void. Now if God
said, "Let molecules move!", the air mass above would, indeed, push
downward. But they would not be allow to go completely downward.
Molecules accelerated from the leading edge of the wing would fly
backward, colliding with those coming from above, and the net-effect
would be a stream.

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.

The force is coming from the air mass above the wing, the air mass
that would be right above the vacuum created if no molecules were
allowed to move. That airmass pushes downward, toward the void. This
has nothing to do with the wing, except that the wing created the
void, and also created high-pressure area at tip of wing causing
acceleration of air backwards.

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.

I am not an expert either, but I know enough to know that the
explanations I am reading in books are, at best, misleading. Some of
them are plain wrong. Note: going to start a new thread so we can get
to the bottom of this.

And yes, I am certain.

-Le Chaud Lapin-

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

Tina wrote:
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.

Well.....as they say.....everything is relative; :-)))
I'm sitting here wondering if I could actually survive explaining
fairies and pixies at the bar at next reunion of the Naval Test Pilot
School :-))
With fighter pilots, you have to be REAL careful with the words you
throw out there, especially after a few rounds of "attitude adjustment".

Knew a guy once who got to his first fighter assignment and hadn't had
the honor of having a call sign given to him. To make an impression on
the squadron, on his first day of duty assignment he showed up at the
ready room with a tray full of the best damn muffins any of the guys had
ever tasted. His wife had baked them.
From that day on through his retirement, poor Ed was known as "Muffin".
Ya just have to pity a guy who flies formation with "Viper", "Ironman",
"and "Snake" with a callsign like "Muffin". He did get at least a
partial break. Most of the guys called him "Muff".
DH




--
Dudley Henriques

"We" are not in need of getting to the bottom of this. Most of us have
been there and done that.This particular writer, if she chooses to
analyse physics problems, tends to use the Newtonion approximations as
first principles. The good news is my profession doesn't demand those
skills often. I would, however, be interested, as I mentioned earlier,
how you derive conservation of mV from Newton's force/acceleration
relationship. I think you made that claim earlier in this thread.

Jim Logajan wrote:
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


I remember the lift demon stories from years ago. In fact, I believe the
origins go back to WW2 in my memory anyway.
I just liked the Mary put it together. :-)

--
Dudley Henriques

Morgans wrote:
The ONLY way to get rid of a know-nothing, know-it-all, obnoxious troll, is
to IGNORE him. ALL of him. EVERY time, not just when you feel like it.

Hush up and grab some popcorn with me n' Bertie. They're just announcing
the second act!

TheSmokingGnu