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Backwash Causes Lift?



 
 
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  #1  
Old October 3rd 07, 02:57 AM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Backwash Causes Lift?

Hi,

Student pilot here, self-teaching using the Jeppensen Private Pilot
Kit after taking ground school.

I read in the book that combustion "creates" energy, which is
technically not true, but I decided to ignore it since the pictures
are sooo pretty.

Now, in Chapter 3, section about airfoils, it actually says:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."

IMHO, the latter part of this paragraph is correct, but the former
part is wrong.

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.

I am also inclined to take issue with the explanations of Bernouilli's
Principle which I see often in the literature, but that's a different
subject. [Note, I don't doubt Bernouilli's Principle, I just think
there is more to it than the way it is being described in context of
flying.]

-Le Chaud Lapin-

  #2  
Old October 3rd 07, 03:06 AM posted to rec.aviation.piloting
Dan Luke[_2_]
external usenet poster
 
Posts: 713
Default Backwash Causes Lift?


"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?


--
Dan
T-182T at BFM


  #3  
Old October 3rd 07, 03:23 AM posted to rec.aviation.piloting
Dudley Henriques[_2_]
external usenet poster
 
Posts: 2,546
Default Backwash Causes Lift?

Le Chaud Lapin wrote:
Hi,

Student pilot here, self-teaching using the Jeppensen Private Pilot
Kit after taking ground school.

I read in the book that combustion "creates" energy, which is
technically not true, but I decided to ignore it since the pictures
are sooo pretty.

Now, in Chapter 3, section about airfoils, it actually says:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."

IMHO, the latter part of this paragraph is correct, but the former
part is wrong.

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.

I am also inclined to take issue with the explanations of Bernouilli's
Principle which I see often in the literature, but that's a different
subject. [Note, I don't doubt Bernouilli's Principle, I just think
there is more to it than the way it is being described in context of
flying.]

-Le Chaud Lapin-


Please don't take this the wrong way but I'm sure you would have passed
me up as a potential flight instructor.
:-))))


--
Dudley Henriques
  #4  
Old October 3rd 07, 03:37 AM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Backwash Causes Lift?

On Oct 2, 9:06 pm, "Dan Luke" wrote:
"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?


Because the situation of rarefication no longer exists during a stall,
or is significantly abated.

When plane is on the ramp, pressure above and below the wing are
equivalent.

When plane is flying certain critical speed, there is, IMO, pinching
that occurs at the leading edge of wing. This area of high pressure
results in a tendency for air to flow away from that pressure point in
all directions.

1. Flowing forward is not an option - that would make pressure
situation worse.
2. Flowing backward, toward the empennage, is not an option. The
leading edge of wing is there.
3. Flowing upward is possible, since above-the-pressure-point pressure
is less than that induced at pressure point.
4. Flowing backward is possible, since below-the-pressure-point
pressure is less than that induced at pressure point.

But here is the catch. If you take an umbrella, open it, find a friend
with extremely long arms, and ask him to yank the umbrella toward his
torso in one, quick, abrupt motion, he will feel a force immediately.
The umbrella might even invert if the impulse is strong enough.
[Sidenote: In the 1970's, I convinced small children that they could
fly if they jumped of 7ft brick wall with umbrella. Very amusing to
see their faces when they hit ground going just about as fast as they
would have without umbrella.] The force that is felt is due to
pressure building under the curved part of umbrella. But even if the
pressure did not build from compression, a force would still be felt,
becaue the force that was equalizing the pressure under the curved
part will have been removed.

And now the $1,000,000 point:

The air on the "outside" of the umbrella does *NOT* instantaneously
fill the void that is created by yanking the umbrella. A finite
amount of time is required for such air to rush in. If the unbrella
is pulled at even a low speed, the net effect can be felt. Pull it
fast enough, and it will invert or snap.

This is, IMO, a more illustrative way of looking at aerodynamics above
the wing than the canned Bernouilli speech.

1. The pinchage creates pressure.
2. A void is created over the wing, provided that plane is moving fast
enough that air high above win cannot rush in.
3. Air at back of wing participates in futile effort to fill the void.

But the most important thing is the pinchage. That pinching results
in high net speed of air molecules backward. Any air above wing that
tries to rush in and fill void is bombarded backward before it can
"touch" the upper surface of wing. I speculated that, if this point a
view were correct, gliders should have short chords with very long
spans, which, of course, is true.

About stalling:

When the angle of attack is too great, the pinchage is still present,
and depending on the shape of the leading edge, the backward flow is
still pressent, but not at the right angle relative to wing, and
certainly not flowing backward enough to stop the onrush of air coming
from above at back of wing. In the air comes, rushing in, and
pressure builds on top of wing.

But there is an ace in hole. Some books say that a plane will stall
if AOA is above critical angle. I do not think this is quite true. It
would seem that, worst-case-scenario, the dynamics above the wing
become royal mess at huge AOA. However, that mess will be less than
static pressure, and there will still be compression beneath the wing.
So if thrust is great enough, airplane should be able to do whatever
it wants.

-Le Chaud Lapin-

  #5  
Old October 3rd 07, 04:23 AM posted to rec.aviation.piloting
Bertie the Bunyip[_19_]
external usenet poster
 
Posts: 3,851
Default Backwash Causes Lift?

Le Chaud Lapin wrote in
oups.com:

On Oct 2, 9:06 pm, "Dan Luke" wrote:
"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?


Because the situation of rarefication no longer exists during a stall,
or is significantly abated.

When plane is on the ramp, pressure above and below the wing are
equivalent.

When plane is flying certain critical speed, there is, IMO, pinching
that occurs at the leading edge of wing. This area of high pressure
results in a tendency for air to flow away from that pressure point in
all directions.

1. Flowing forward is not an option - that would make pressure
situation worse.
2. Flowing backward, toward the empennage, is not an option. The
leading edge of wing is there.
3. Flowing upward is possible, since above-the-pressure-point pressure
is less than that induced at pressure point.
4. Flowing backward is possible, since below-the-pressure-point
pressure is less than that induced at pressure point.

But here is the catch. If you take an umbrella, open it, find a friend
with extremely long arms, and ask him to yank the umbrella toward his
torso in one, quick, abrupt motion, he will feel a force immediately.
The umbrella might even invert if the impulse is strong enough.
[Sidenote: In the 1970's, I convinced small children that they could
fly if they jumped of 7ft brick wall with umbrella. Very amusing to
see their faces when they hit ground going just about as fast as they
would have without umbrella.] The force that is felt is due to
pressure building under the curved part of umbrella. But even if the
pressure did not build from compression, a force would still be felt,
becaue the force that was equalizing the pressure under the curved
part will have been removed.

And now the $1,000,000 point:

The air on the "outside" of the umbrella does *NOT* instantaneously
fill the void that is created by yanking the umbrella. A finite
amount of time is required for such air to rush in. If the unbrella
is pulled at even a low speed, the net effect can be felt. Pull it
fast enough, and it will invert or snap.

This is, IMO, a more illustrative way of looking at aerodynamics above
the wing than the canned Bernouilli speech.




Nope, it's not the same at all.

1. The pinchage creates pressure.
2. A void is created over the wing, provided that plane is moving fast
enough that air high above win cannot rush in.
3. Air at back of wing participates in futile effort to fill the void.

But the most important thing is the pinchage. That pinching results
in high net speed of air molecules backward. Any air above wing that
tries to rush in and fill void is bombarded backward before it can
"touch" the upper surface of wing. I speculated that, if this point a
view were correct, gliders should have short chords with very long
spans, which, of course, is true.

About stalling:

When the angle of attack is too great, the pinchage is still present,
and depending on the shape of the leading edge, the backward flow is
still pressent, but not at the right angle relative to wing, and
certainly not flowing backward enough to stop the onrush of air coming
from above at back of wing. In the air comes, rushing in, and
pressure builds on top of wing.

But there is an ace in hole. Some books say that a plane will stall
if AOA is above critical angle. I do not think this is quite true.



It's precisely true since th ecritical angle is defined by the stall.





Bertie
  #6  
Old October 3rd 07, 04:24 AM posted to rec.aviation.piloting
Bertie the Bunyip[_19_]
external usenet poster
 
Posts: 3,851
Default Backwash Causes Lift?

Dudley Henriques wrote in newsaednT-q-
:

Le Chaud Lapin wrote:
Hi,

Student pilot here, self-teaching using the Jeppensen Private Pilot
Kit after taking ground school.

I read in the book that combustion "creates" energy, which is
technically not true, but I decided to ignore it since the pictures
are sooo pretty.

Now, in Chapter 3, section about airfoils, it actually says:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which

demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."

IMHO, the latter part of this paragraph is correct, but the former
part is wrong.

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.

I am also inclined to take issue with the explanations of

Bernouilli's
Principle which I see often in the literature, but that's a different
subject. [Note, I don't doubt Bernouilli's Principle, I just think
there is more to it than the way it is being described in context of
flying.]

-Le Chaud Lapin-


Please don't take this the wrong way but I'm sure you would have

passed
me up as a potential flight instructor.
:-))))



I nominate Anthony!

Bertie

  #7  
Old October 3rd 07, 04:33 AM posted to rec.aviation.piloting
george
external usenet poster
 
Posts: 803
Default Backwash Causes Lift?

On Oct 3, 1:57 pm, Le Chaud Lapin wrote:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."



I blame the lift pixies myself

  #8  
Old October 3rd 07, 05:20 AM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Backwash Causes Lift?

On Oct 2, 10:23 pm, Bertie the Bunyip wrote:
But there is an ace in hole. Some books say that a plane will stall
if AOA is above critical angle. I do not think this is quite true.


It's precisely true since th ecritical angle is defined by the stall.


What is the definition of a stall anyway?

I'm saying that, if you take a plane with certain critical angle,
throw away engine, put on an engine that can generate 10x the thrust,
the plane should still fly, even if you exceed critical angle.

These books imply that the critical angle is angle at with bad things
happen above the wing, and because of that, the plane will fall. I'm
saying that, you can have all the bad things happen above the wing and
still be able to keep the plane aloft due to compression that occurs
beneath the wing.

Of course, I have only been doing this officially 7 weeks, so I might
be wrong.

-Le Chaud Lapin-

  #9  
Old October 3rd 07, 05:23 AM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Backwash Causes Lift?

On Oct 2, 10:33 pm, george wrote:
On Oct 3, 1:57 pm, Le Chaud Lapin wrote:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."


I blame the lift pixies myself


LOL.

This flying business is a bit too fascinating. I'm having trouble
concentrating on my day job. In no other hobby has the opportunity
arisen to apply essentially everything technical I have ever learned.
Physics, chemistry, mathematics, electronics, computation...it's all
there.

-Le Chaud Lapin-

  #10  
Old October 3rd 07, 05:28 AM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Backwash Causes Lift?

On Oct 2, 10:24 pm, Bertie the Bunyip wrote:
Please don't take this the wrong way but I'm sure you would have

passed
me up as a potential flight instructor.
:-))))


I nominate Anthony!


Heh.

I knew going into ground school that the focus would be flying, not
aero/astro, so I was not disappointed with the course. By rushing us,
the instructor gave us a broad overview of what we should know. This
has been hugely beneficial for my learning.

Makes going through it again, slowly, with physics book nearby very
pleasurable.

-Le Chaud Lapin-

 




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