Backwash Causes Lift?

On Oct 7, 5:54 pm, flightoffancy wrote:
In article ,
says...

The downwash thing is wrong. Yes, there is some dispacemtn of air that
causes lift, but it' only a minor contribution in the bigger scheme of
things.

I admit to being a relative retard on this issue (not as retarded as a
non-pilot probably is, but not as educated as an aeronautical engineer).

I thought I had read in numerous books during training that the primary
component of lift is the air that gets knocked downward by the wing. I
was calling that "downwash". Maybe my concept of downwash is wrong,
maybe it's a separate consideration from the air that gets knocked
downward by the airfoil. Hell I might not be remembering any of that
correctly.

Just wanted to reiterate what I said in my OP and each subsequent post
for you benefit since you just joined the discussion.

If you have an aifoil, and you move it forward, there will be
compression beneath the wing. Newton's law will be at play here, and
there will be downwash. This downwash results from the induced
pressure gradient.

That is not what I was talking about. The books that I have been
reading are talking about downwash that is _on top of_ the wing. The
pictures show air moving at an angle, backward and downward near the
trailing edge of the wing.

Note that these are two "downwashes".

I am saying that downwash on top of the wing does not generate a force
on the wing that causes the wing to move upward.

Anyway you say downwash is minor.

Well okay. But then what are the major contributions that cause lift in
the bigger scheme of things?

-Le Chaud Lapin-

On Oct 7, 7:10 pm, Le Chaud Lapin wrote:
On Oct 7, 5:54 pm, flightoffancy wrote:





In article ,
says...

The downwash thing is wrong. Yes, there is some dispacemtn of air that
causes lift, but it' only a minor contribution in the bigger scheme of
things.

I admit to being a relative retard on this issue (not as retarded as a
non-pilot probably is, but not as educated as an aeronautical engineer).

I thought I had read in numerous books during training that the primary
component of lift is the air that gets knocked downward by the wing. I
was calling that "downwash". Maybe my concept of downwash is wrong,
maybe it's a separate consideration from the air that gets knocked
downward by the airfoil. Hell I might not be remembering any of that
correctly.

Just wanted to reiterate what I said in my OP and each subsequent post
for you benefit since you just joined the discussion.

If you have an aifoil, and you move it forward, there will be
compression beneath the wing. Newton's law will be at play here, and
there will be downwash. This downwash results from the induced
pressure gradient.

That is not what I was talking about. The books that I have been
reading are talking about downwash that is _on top of_ the wing. The
pictures show air moving at an angle, backward and downward near the
trailing edge of the wing.

Note that these are two "downwashes".

I am saying that downwash on top of the wing does not generate a force
on the wing that causes the wing to move upward.

Anyway you say downwash is minor.

Well okay. But then what are the major contributions that cause lift in
the bigger scheme of things?

-Le Chaud Lapin-- Hide quoted text -

- Show quoted text -

If the airflow on top of the wing doesn't contribute to lift, then how
can we explain the phenomenon of the wing stalling? When the wing
stalls, it is the airflow over the top of the wing that detaches from
the curve of the wing and becomes turbulent. This causes a radical
loss of lift. To me, this indicates that the airflow over the top of
the wing plays an essential role in providing lift.

I know the Bernoulli effect has been invoked historically to (at least
partially) explain the lift produced by the top surface of a wing. I
think another way to look at it is the Coanda effect (
http://en.wikipedia.org/wiki/Coand%C4%83_effect ). The airflow tends
to follow the curve of the top of the wing, and is displaced
downward. As long as the air flow follows the curve faithfully, you
have good lift. When the airflow detaches in a stall, you lose most
of your lift. This top surface lift is combined with the downward
displacement of air by the bottom of the wing. The wing is
essentially throwing air downward using both the top and bottom
surfaces. This is why a curved wing is a better lift producer than a
simple flat wing. The top surface curve helps contribute to the lift.

Now, how does the wing feel the lift? It feels high pressure on its
bottom surface, and it feels low pressure on its upper surface. It is
pushed up from below, and sucked up from above. That is how the
airplane experiences the effects of the downward displacement of air.

Phil

On Oct 8, 11:38 am, Phil wrote:

First, I would like to point out that your post is interesting because
it implies at first something which I disagree with, but then at the
very end of the post, what you say is exactly true.

Let me try to explain:

If the airflow on top of the wing doesn't contribute to lift, then how
can we explain the phenomenon of the wing stalling? When the wing
stalls, it is the airflow over the top of the wing that detaches from
the curve of the wing and becomes turbulent. This causes a radical
loss of lift. To me, this indicates that the airflow over the top of
the wing plays an essential role in providing lift.

What I am saying is that Newton's law is not at play with downwash,
not in the "uppper surface of wing pull down on molecules" sense. Yes,
there is downwash. Yes, the camber of the wing will influence the net
force exerted on the wing. Yes, there will be stalling, turbulence,
etc. all these things will happen.

The key here is that the air molecules that are above the wing cannot
be pulled down by the wing more can they pull up on the wing. Those
air molecules can only causes the lateral forces of friction (laminar
drag), and a perpendicular downward force on the wing which aircraft
designers obviously want to keep from happening.

I know the Bernoulli effect has been invoked historically to (at least
partially) explain the lift produced by the top surface of a wing. I
think another way to look at it is the Coanda effect (http://en.wikipedia.org/wiki/Coand%C4%83_effect). The airflow tends
to follow the curve of the top of the wing, and is displaced
downward. As long as the air flow follows the curve faithfully, you
have good lift. When the airflow detaches in a stall, you lose most
of your lift. This top surface lift is combined with the downward
displacement of air by the bottom of the wing. The wing is
essentially throwing air downward using both the top and bottom
surfaces. This is why a curved wing is a better lift producer than a
simple flat wing. The top surface curve helps contribute to the lift.

I agree that air is being thrown downward by the bottom surface. I do
not think a top surfaces throws air downward. Even this Coanda effect
says that contact, at least initially, is caused by a pressure
differential. From your link above:

"As a gas flows over an airfoil, the gas is drawn down to adhere to
the airfoil by a combination of the greater pressure above the gas
flow and the lower pressure below the flow caused by an evacuating
effect of the flow itself, which as a result of shear, entrains the
slow-moving fluid trapped between the flow and the down-stream end of
the upper surface of the airfoil. The effect of a spoon apparently
attracting a flow of water is caused by this effect as well, since the
flow of water entrains gases to flow down along the stream, and these
gases are then pulled, along with the flow of water, in towards the
spoon, as a result of the pressure differential. Supersonic flows have
a different response."

"greater pressure above the gas flow and the lower pressure below the
flow caused by an evacuating effect..."

This is what I keep saying. I have been using the words "rarefication
and rarefaction" and instead of "evacuating effect", but this is
essentially what I mean.

Now, how does the wing feel the lift? It feels high pressure on its
bottom surface, and it feels low pressure on its upper surface. It is
pushed up from below, and sucked up from above. That is how the
airplane experiences the effects of the downward displacement of air.

I agree with the downward force. I do not agree that there is a
sucking force above, any more than I agree that there is a sucking
force when a purpose sucks on a straw.

Given that the bottom surfaces of the wing is already 14.7lbs/in^2,
one simply needs to reduce the pressure above the wing to cause lift.
This is what I tried to illustrate with my two-pieces-of-paper-
superposed demonstration.

But in many cases the bottom surface has even more than 14.7lbs/^2.

-Le Chaud Lapin-

Le Chaud Lapin wrote in
ups.com:

On Oct 8, 11:38 am, Phil wrote:

First, I would like to point out that your post is interesting because
it implies at first something which I disagree with, but then at the
very end of the post, what you say is exactly true.

Let me try to explain:

If the airflow on top of the wing doesn't contribute to lift, then
how can we explain the phenomenon of the wing stalling? When the
wing stalls, it is the airflow over the top of the wing that detaches
from the curve of the wing and becomes turbulent. This causes a
radical loss of lift. To me, this indicates that the airflow over
the top of the wing plays an essential role in providing lift.

What I am saying is that Newton's law is not at play with downwash,
not in the "uppper surface of wing pull down on molecules" sense. Yes,
there is downwash. Yes, the camber of the wing will influence the net
force exerted on the wing. Yes, there will be stalling, turbulence,
etc. all these things will happen.

The key here is that the air molecules that are above the wing cannot
be pulled down by the wing more can they pull up on the wing. Those
air molecules can only causes the lateral forces of friction (laminar
drag), and a perpendicular downward force on the wing which aircraft
designers obviously want to keep from happening.

I know the Bernoulli effect has been invoked historically to (at
least partially) explain the lift produced by the top surface of a
wing. I think another way to look at it is the Coanda effect
(http://en.wikipedia.org/wiki/Coand%C4%83_effect). The airflow tends
to follow the curve of the top of the wing, and is displaced
downward. As long as the air flow follows the curve faithfully, you
have good lift. When the airflow detaches in a stall, you lose most
of your lift. This top surface lift is combined with the downward
displacement of air by the bottom of the wing. The wing is
essentially throwing air downward using both the top and bottom
surfaces. This is why a curved wing is a better lift producer than a
simple flat wing. The top surface curve helps contribute to the
lift.

I agree that air is being thrown downward by the bottom surface. I do
not think a top surfaces throws air downward. Even this Coanda effect
says that contact, at least initially, is caused by a pressure
differential. From your link above:

"As a gas flows over an airfoil, the gas is drawn down to adhere to
the airfoil by a combination of the greater pressure above the gas
flow and the lower pressure below the flow caused by an evacuating
effect of the flow itself, which as a result of shear, entrains the
slow-moving fluid trapped between the flow and the down-stream end of
the upper surface of the airfoil. The effect of a spoon apparently
attracting a flow of water is caused by this effect as well, since the
flow of water entrains gases to flow down along the stream, and these
gases are then pulled, along with the flow of water, in towards the
spoon, as a result of the pressure differential. Supersonic flows have
a different response."

"greater pressure above the gas flow and the lower pressure below the
flow caused by an evacuating effect..."

This is what I keep saying. I have been using the words "rarefication
and rarefaction" and instead of "evacuating effect", but this is
essentially what I mean.

Now, how does the wing feel the lift? It feels high pressure on its
bottom surface, and it feels low pressure on its upper surface. It
is pushed up from below, and sucked up from above. That is how the
airplane experiences the effects of the downward displacement of air.

I agree with the downward force. I do not agree that there is a
sucking force above, any more than I agree that there is a sucking
force when a purpose sucks on a straw.

Given that the bottom surfaces of the wing is already 14.7lbs/in^2,
one simply needs to reduce the pressure above the wing to cause lift.
This is what I tried to illustrate with my two-pieces-of-paper-
superposed demonstration.

But in many cases the bottom surface has even more than 14.7lbs/^2.



Meanwhile your airplane is tearing along and you don't have a notion
what to do with it.



Bertie

On Oct 8, 2:34 pm, Le Chaud Lapin wrote:
On Oct 8, 11:38 am, Phil wrote:

First, I would like to point out that your post is interesting because
it implies at first something which I disagree with, but then at the
very end of the post, what you say is exactly true.

Let me try to explain:

If the airflow on top of the wing doesn't contribute to lift, then how
can we explain the phenomenon of the wing stalling? When the wing
stalls, it is the airflow over the top of the wing that detaches from
the curve of the wing and becomes turbulent. This causes a radical
loss of lift. To me, this indicates that the airflow over the top of
the wing plays an essential role in providing lift.

What I am saying is that Newton's law is not at play with downwash,
not in the "uppper surface of wing pull down on molecules" sense. Yes,
there is downwash. Yes, the camber of the wing will influence the net
force exerted on the wing. Yes, there will be stalling, turbulence,
etc. all these things will happen.

The key here is that the air molecules that are above the wing cannot
be pulled down by the wing more can they pull up on the wing. Those
air molecules can only causes the lateral forces of friction (laminar
drag), and a perpendicular downward force on the wing which aircraft
designers obviously want to keep from happening.

I know the Bernoulli effect has been invoked historically to (at least
partially) explain the lift produced by the top surface of a wing. I
think another way to look at it is the Coanda effect (http://en.wikipedia.org/wiki/Coand%C4%83_effect). The airflow tends
to follow the curve of the top of the wing, and is displaced
downward. As long as the air flow follows the curve faithfully, you
have good lift. When the airflow detaches in a stall, you lose most
of your lift. This top surface lift is combined with the downward
displacement of air by the bottom of the wing. The wing is
essentially throwing air downward using both the top and bottom
surfaces. This is why a curved wing is a better lift producer than a
simple flat wing. The top surface curve helps contribute to the lift.

I agree that air is being thrown downward by the bottom surface. I do
not think a top surfaces throws air downward. Even this Coanda effect
says that contact, at least initially, is caused by a pressure
differential. From your link above:

"As a gas flows over an airfoil, the gas is drawn down to adhere to
the airfoil by a combination of the greater pressure above the gas
flow and the lower pressure below the flow caused by an evacuating
effect of the flow itself, which as a result of shear, entrains the
slow-moving fluid trapped between the flow and the down-stream end of
the upper surface of the airfoil. The effect of a spoon apparently
attracting a flow of water is caused by this effect as well, since the
flow of water entrains gases to flow down along the stream, and these
gases are then pulled, along with the flow of water, in towards the
spoon, as a result of the pressure differential. Supersonic flows have
a different response."

"greater pressure above the gas flow and the lower pressure below the
flow caused by an evacuating effect..."

This is what I keep saying. I have been using the words "rarefication
and rarefaction" and instead of "evacuating effect", but this is
essentially what I mean.

Now, how does the wing feel the lift? It feels high pressure on its
bottom surface, and it feels low pressure on its upper surface. It is
pushed up from below, and sucked up from above. That is how the
airplane experiences the effects of the downward displacement of air.

I agree with the downward force. I do not agree that there is a
sucking force above, any more than I agree that there is a sucking
force when a purpose sucks on a straw.

Given that the bottom surfaces of the wing is already 14.7lbs/in^2,
one simply needs to reduce the pressure above the wing to cause lift.
This is what I tried to illustrate with my two-pieces-of-paper-
superposed demonstration.

But in many cases the bottom surface has even more than 14.7lbs/^2.

-Le Chaud Lapin-

Then how do you explain what happens when a wing stalls? When a wing
reaches a high enough angle of attack to stall, the bottom surface is
still deflecting air downward. Yet when the airflow over the top of
the wing detaches and becomes turbulent, most of the lift of the wing
is destroyed. If the attached airflow over the top of the wing is not
generating lift, then why does the lift disappear when that airflow
detaches?

Phil

On Oct 8, 2:45 pm, Phil wrote:
Then how do you explain what happens when a wing stalls? When a wing
reaches a high enough angle of attack to stall, the bottom surface is
still deflecting air downward. Yet when the airflow over the top of
the wing detaches and becomes turbulent, most of the lift of the wing
is destroyed. If the attached airflow over the top of the wing is not
generating lift, then why does the lift disappear when that airflow
detaches?

Because the turbulent air on top of a wing during a stall pushes down
on the wing harder than does when the airflow non-turbulent.

-Le Chaud Lapin-

Le Chaud Lapin wrote in news:1191876409.965861.63860
@r29g2000hsg.googlegroups.com:

On Oct 8, 2:45 pm, Phil wrote:
Then how do you explain what happens when a wing stalls? When a wing
reaches a high enough angle of attack to stall, the bottom surface is
still deflecting air downward. Yet when the airflow over the top of
the wing detaches and becomes turbulent, most of the lift of the wing
is destroyed. If the attached airflow over the top of the wing is not
generating lift, then why does the lift disappear when that airflow
detaches?

Because the turbulent air on top of a wing during a stall pushes down
on the wing harder than does when the airflow non-turbulent.



Wow, easy to see your=re conversant with physics.


Bertie

On Oct 8, 3:46 pm, Le Chaud Lapin wrote:
On Oct 8, 2:45 pm, Phil wrote:

Then how do you explain what happens when a wing stalls? When a wing
reaches a high enough angle of attack to stall, the bottom surface is
still deflecting air downward. Yet when the airflow over the top of
the wing detaches and becomes turbulent, most of the lift of the wing
is destroyed. If the attached airflow over the top of the wing is not
generating lift, then why does the lift disappear when that airflow
detaches?

Because the turbulent air on top of a wing during a stall pushes down
on the wing harder than does when the airflow non-turbulent.

-Le Chaud Lapin-

Do you know of any research that supports that theory?

Le Chaud Lapin wrote:

Because the turbulent air on top of a wing during a stall pushes down
on the wing harder than does when the airflow non-turbulent.


You really need to look at some video of Tuft testing.

Here's one to start with.

http://www.youtube.com/watch?v=zrwlpHE7P8Q