Backwash Causes Lift?

Le Chaud Lapin wrote in
ups.com:

On Oct 3, 8:39 am, Tina wrote:
You might also want to think carefully about airfoil shapes, since
wings can provide lift when flying inverted. Any theory that does
not support inverted flight is obviously flawed.

Actually I did because every book I read about flying skimped on the
subject. I'm going to hop over to MIT's OpenCourseWare later this
week and download their most basic course on aero/astro. Benoulli's
principle is toss around as if it were facecloth, but I'm getting the
feeling that no one is really doing the physics.

Well, NASA have and they wholeheartedly support Bernoulli , as do an
Airbus engineeer and a Boeing Engineer of my acquantence. NASA used to
have a whole web page debunking the Bernoulli deniers, but it sems to be
gone now.
It's pretty simple in how it relates to how the airplane flies from one
standpoint and complex from another, but at the end of the day it's how
your knowledge makes the airplane perfomr that counts, and the only
thing tha's going to teach you that is flying an airplane guided by a
good instructor.
If you try to understand it at a molecular level, you've got a problem,
as nobody really undertands lift completely (Here's anthony's chance to
go all creationist on us now)


But the neat thing to do is to hold your hand out of a moving car's
window, and feel the impact pressure on its surfaces as you tilt it
in the airstream. It's not that the hand is being "sucked" up, you
don't feel suction on the top surface, you feel push on the bottom
one. Any theory you develop had better be consistant with those
observations. Someone with more time than I have might like to start
with the fact that air weighs about .08 pounds per square foot near
sea level, and crack some numbers to show how that deflecting that
mass can result in lift even if the lifting surface has some funny
shapes.- Hide quoted text -

Yes it is. In fact, I was having this discussion with someone who
claimed that it *was* Benoulli's principle only. I made the following
diagram to try to illustrate my point. View in fixed width


| inverted |
| table |
|--------------------|

|--------------------|
| upright |
| table |

The Bernoulli people often describe air flowing above the a table
being faster than air below a table, and therefore, pressure is
reduced. Hmmm... what happens if the horizontal velocities above and
below a table are both essentially 0?

If you place an inverted table on top of an upright table so that the
table tops are mated, then have a machine, with a tremendous amount of
force, on the order of 14.4lbs/in^2 of force, yank the inverted table
upward, in one quick jerk, I contend that the lower table will be
strongly inclined to follow by jumping updward, obviously due to
pressure beneath it. So any type of rarefication on one side of a
doubly-pressurized surface that is free to move in direction that is
perpendicular to the surface, will, indeed, move, if pressure is
reduced.

And this is why, I am pretty sure, that if I were to search the web,
one would find people who are fanatical about the leading edges of
wings, in the most minute detail, because it is not simply the length
of the top of the wing that matters, but the amount of pinching, and
the distribution of air as it flows backward from the pressure point.
IMO, that pinching results in displacement of the air above to make it
effective go backwares, causing rarefication.


That all sounds fairly sound, but it's too esoteric to relate to
handling an airplane well.

Don't forget, handling, and it's handling that is the aim of the
knowledge you seek, is primarily a right hand brain operation. If you
try to fly with too much of the left included, you are going to fly like
a chicken on crack.


Bertie

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.

It is the removal of air from above the table that causes the lift.

If a person sucks on a straw, thus removing air from the inside of the
straw, the fluid rises into the straw from the container do to the air/
fluid system outside the straw. This is the same phenomenon that is
occurring in my table scenario. In fact, I could enclose the entire
table scenario inside a tube, and cause the bottom table to rise up
off the ground. This has nothing to do with the velocity or
acceleration of air.

The stagnation point on a leading edge isn't right at the front.
It's slightly below the wing, and as AOA increases it moves back
underneath quite a bit. It's not all intuitive, you see, and that
intuitive understanding of some of this stuff is where people get all
messed up and think they have the answers that have escaped all the
other experts all these years. We run into this attitude rather
frequently in the flight training industry. It tends to make the
student unteachable.

I do rely on intuition to figure things out, but most importantly, I'd
rather actually understand, than except shallow explanations.

I never attempted to contradict either Bernoulii or Newton. What I
keep saying is that I have seen too many situations where someone will
rattle of "Bernoulli's Principle" and not really understand it
themselves.

For example, as I mentioned earlier, I am reading Jeppesens Private
Pilot manual, and there are clearly errors in concept the manual
(energy being created by engine, for example), even though Jeppensen
probably has access to as many Ph.D. laureates as they want.

-Le Chaud Lapin-

"Dan Luke" wrote in
:


"Le Chaud Lapin" wrote:

Obviously, any air above the wing can only result in a force downward
on top of the wing. The only force causing the plane to want to move
upward comes from beneath the wing. The effect of any air above the
wing is to cause rarefication above the wing, resulting in lower
pressure, thereby giving the 14.7lbs/in^2 (plus) to do its work. That
"reaction" coming from downward movement of air seems just plain
silly to me.


Then why does the wing stall and cease lifting when flow separates
from the upper surface?



Actually, that's not the definition of the stall, seperation occurs just
after the stall with most airfoil/planform combinations and in most
flight situations, ordinarily. Some wings will have seperation at the
stall, but I've never flown one.
The defintion is an abrupt loss of lift when the critical angle is
reached. Seperation usually occurs immidiatly after (*but not always,
for instance, deltas will continue to have smooth flow way below he
point they will actually keep flying) This is not to be confused with
the back siide of the drag curve, BTW.
Having said all hat, there are some reputable design texts that define
stall as the point at which the bubble breaks down and buffet occurs and
as far as I know, this doesn't disturb engineers (of which I am not one,
BTW, so take this all with a large grain of salt) any more than a "po-
tay-to, po-tah-to" argument would.
Bottom line is you're interested in keeping your airplane from going
down and the point at which the wing ceases to do what you would like it
to do is the point at which you're most interested.

Make sense? If it does I must not have explained it well.



Bertie

Bertie the Bunyip wrote:

like a chicken on crack.

Bertie

Funny...I suspect you know what that looks like. ; )

--
Message posted via AviationKB.com
http://www.aviationkb.com/Uwe/Forums...ation/200710/1

Le Chaud Lapin wrote in
oups.com:

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.

It is the removal of air from above the table that causes the lift.

If a person sucks on a straw, thus removing air from the inside of the
straw, the fluid rises into the straw from the container do to the
air/
fluid system outside the straw. This is the same phenomenon that is
occurring in my table scenario. In fact, I could enclose the entire
table scenario inside a tube, and cause the bottom table to rise up
off the ground. This has nothing to do with the velocity or
acceleration of air.

The stagnation point on a leading edge isn't right at the
front.
It's slightly below the wing, and as AOA increases it moves back
underneath quite a bit. It's not all intuitive, you see, and that
intuitive understanding of some of this stuff is where people get all
messed up and think they have the answers that have escaped all the
other experts all these years. We run into this attitude rather
frequently in the flight training industry. It tends to make the
student unteachable.

I do rely on intuition to figure things out, but most importantly, I'd
rather actually understand, than except shallow explanations.

I never attempted to contradict either Bernoulii or Newton. What I
keep saying is that I have seen too many situations where someone will
rattle of "Bernoulli's Principle" and not really understand it
themselves.

For example, as I mentioned earlier, I am reading Jeppesens Private
Pilot manual, and there are clearly errors in concept the manual
(energy being created by engine, for example), even though Jeppensen
probably has access to as many Ph.D. laureates as they want.



Yes, well, you obviously need to write a good old fashioned, angry,
frothng at the mouth letter to Jeppeson. I'm sure Elry will be suitably
rattled.


Bertie

"Kloudy via AviationKB.com" u33403@uwe wrote in news:79253d6018083@uwe:

Bertie the Bunyip wrote:

like a chicken on crack.

Bertie

Funny...I suspect you know what that looks like. ; )


Nah,I just have a good imagination.
Though if oyu send me some crack I do have some chickens I could try it on.

Bertie

wrote:
On Oct 3, 8:15 am, Le Chaud Lapin wrote:

Actually I did because every book I read about flying skimped on the
subject. I'm going to hop over to MIT's OpenCourseWare later this
week and download their most basic course on aero/astro. Benoulli's
principle is toss around as if it were facecloth, but I'm getting the
feeling that no one is really doing the physics.

Lemme see: People have been building flying machines since the late
1800's, about 125 years now, and none of them have been interested
enough in the phenomenon of lift to do the physics? How old are you,
anyway? Many of the contributors here have been flying much longer
than you have likely been alive and have studied this in detail, and
some of them might even have doctorates in the subject. The subject of
lift has been beaten to death on this forum and if you Googled it
you'd find some good information.
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.
The stagnation point on a leading edge isn't right at the front.
It's slightly below the wing, and as AOA increases it moves back
underneath quite a bit. It's not all intuitive, you see, and that
intuitive understanding of some of this stuff is where people get all
messed up and think they have the answers that have escaped all the
other experts all these years. We run into this attitude rather
frequently in the flight training industry. It tends to make the
student unteachable.

I think the main issue is that it doesn't require a Phd in physics to
fly an airplane and the explanations of lift, stall, drag, etc.
for pilots tend to be highly simplified, and rightfully so.

A full explanation that would satisfy a physisicist would likely
cause exterme eye glaze in the average pilot.

If one want's that level of insight, I would suggest they go read
a good aerodynamics text and not expect to find it in a couple of
paragraphs in a USENET posting.

--
Jim Pennino

Remove .spam.sux to reply.

On Oct 3, 11:28 am, Bertie the Bunyip wrote:
"Kloudy via AviationKB.com" u33403@uwe wrote innews:79253d6018083@uwe:

Funny...I suspect you know what that looks like. ; )

Nah,I just have a good imagination.
Though if oyu send me some crack I do have some chickens I could try it on.

Bertie

)))!

-Le Chaud Lapin-

On Oct 3, 10:34 am, wrote:
On Oct 3, 8:15 am, Le Chaud Lapin wrote:

Actually I did because every book I read about flying skimped on the
subject. I'm going to hop over to MIT's OpenCourseWare later this
week and download their most basic course on aero/astro. Benoulli's
principle is toss around as if it were facecloth, but I'm getting the
feeling that no one is really doing the physics.

Lemme see: People have been building flying machines since the late
1800's, about 125 years now, and none of them have been interested
enough in the phenomenon of lift to do the physics? How old are you,
anyway? Many of the contributors here have been flying much longer
than you have likely been alive and have studied this in detail, and
some of them might even have doctorates in the subject. The subject of
lift has been beaten to death on this forum and if you Googled it
you'd find some good information.

I want to be clear. I did not me to say "no one" is doing the
physics. Obviously there are aero/astro scientists all over the
world. What I mean to say is that there seems to be a lot of *pilots*
who are using Bernoulli's principle somewhat carelessly, IMO. Some of
these people are CFI's. Please don't ask me to name individuals, but
I know with certainty that there are at least 2 living, breathing
CFI's who do not understand where 29.92 Hg comes from, or does not
understand it well enough to make it make sense to a student. The
might have understood it at one point, but they don't now. I know
because I asked them. My feelings about teaching is that if you are
not very certain about something, you do more damage than talking
about it. Of course, this leads to the conundrum of having to explain
to a student why a plane stays in the air without providing erroneous
information. If I were a CFI, I would simply say that the aerodynamics
result in pressure below plane is sufficient to counteract pressure
above planes for force of gravity. If they wanted to know more, I'd
direct them to book on aerodynamics.

Can you fly without understanding many of these things? Certainly.
But personally, I would feel a lot better in a cockpit if I did. The
more I know, the more confident I am, and if something goes wrong, the
added perspective will allow me to quickly eliminate those things
which I am certain is not root of problem. This reminds me of
incident about year ago when I was fixing a neighbor's car, and
another neighbor kindly ask me if we needed to borrow jumper cables,
even though she had heard the engine turn over with no problem many
times. Someone who understands how automobile works would have known
that it is highly unlikely that there was problem with battery with
such robust cranking. [Turned out to be fuel line].

-Le Chaud Lapin-

writes:

Lemme see: People have been building flying machines since the late
1800's, about 125 years now, and none of them have been interested
enough in the phenomenon of lift to do the physics?

The physics seems simpler than it is, and the explanation of the physics
depends hugely on one's frame of reference.

However, the practical reality is simple: an airfoil with an angle of attack
greater than zero and less than the critical (stall) angle will produce lift.
This is completely reliable, and it's all a pilot needs to know (although,
oddly enough, many pilots don't know this).

Many of the contributors here have been flying much longer
than you have likely been alive and have studied this in detail, and
some of them might even have doctorates in the subject.

See above.

We run into this attitude rather
frequently in the flight training industry. It tends to make the
student unteachable.

Students only need to know about the angle of attack, if theory is required.
Or you can simply teach them by rote, which is even easier albeit somewhat
less safe.