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Why Airplanes Fly - Voids Above A Planar Sheet



 
 
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  #1  
Old October 4th 07, 03:04 AM posted to rec.aviation.piloting
Le Chaud Lapin
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Posts: 291
Default Why Airplanes Fly - Voids Above A Planar Sheet

Hi,

The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.

Before I begin my exposition about what keeps the plane afloat, I
would like anyone who care to participate in this discussion to do a
couple of simple experiments. This will get us all on the same page
(no pun intended, heh):

We are all familiar with the "blow-over-sheet-of-paper" trick to
illustrate Bernouilli's principle. That trick is actually has more
going on than Bernouilli's princinple, but I am going to avoid talking
about it until we can all at least agree on the concepts of voids and
pressures.

Let us do an experiment that uses not one but two sheets of paper.

EXPERIMENT 1:

Take two sheets of paper. Superpose one on top of the other on a
desk, perfectly aligned. Then carefully grab the edges of the top
sheet with both hands, gripping the edges between your palms, but
making sure to keep the top sheet as close to the bottom sheet as
possible, including the edges. The closer, the better. It helps to
grab long-wise, not short-wise. Try to grab as much edge as possibe.
Now take a breath...

In one quick motion, yank up the top sheet. Watch what happens to the
bottom sheet. It follows the top sheet for a brief moment.

EXPERIMENT 2:

Do the same as EXPERIMENT 1, but be creative. Instead of simply
yanking upward, move in a nice fluid-but-fast motion all around the
room. If you are careful, you should be able to make some nice,
gracious curves, keeping the bottom sheet intact. Some of you might
find it hard to believe, but with the right contraption, you could
actually keep two pieces of cardboard stuck together like this,
dragging one with the other all over the room, even though there is no
glue or any other adhesive binding the two.

There is one important lesson to be learned from these experiments,
especially the 1st. Bernoulli's Principle has nothing to do with
this. Bernoulli's principle has to do with air flow that is
*coplanar* with the surface under discussion. Bernoulli's principle
has to do with gases that are flowing in a direction that is
perpendicular to the normal vector of the surface over which it flows.
When you yank the top paper to lure the bottom paper, you are moving
in a direction that is *colinear* to this normal vector. Simply
stated, if you do not move sideways *at all*, but only outward, away
from the paper, you will STILL cause the bottom sheet to follow.

I will leave it to the reader to explain why the bottom sheet follows
the top sheet.

-Le Chaud Lapin-

Ads
  #2  
Old October 4th 07, 06:38 AM posted to rec.aviation.piloting
Bertie the Bunyip[_19_]
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Posts: 3,851
Default Why Airplanes Fly - Voids Above A Planar Sheet

Le Chaud Lapin wrote in
ups.com:

Hi,

The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.

Before I begin my exposition about what keeps the plane afloat, I
would like anyone who care to participate in this discussion to do a
couple of simple experiments.



Good grief..

Bertie


  #3  
Old October 4th 07, 01:09 PM posted to rec.aviation.piloting
RandyL[_2_]
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Posts: 16
Default Why Airplanes Fly - Voids Above A Planar Sheet

Everyone knows that the only thing that keeps an airplane in the air
is.........
$$$$$$$

There is one important lesson to be learned from these experiments,

Yes there is. You are an idiot.

Randy L.
--
"When making an emergency off-field landing at night,
turn on your landing light just prior to touchdown.
If you don't like what you see, then turn off the landing lights."

"Le Chaud Lapin" wrote in message
ups.com...
Hi,

(snip)
-Le Chaud Lapin-

  #4  
Old October 4th 07, 02:06 PM posted to rec.aviation.piloting
VZ/res0zhra
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Posts: 8
Default Why Airplanes Fly - Voids Above A Planar Sheet


"RandyL" wrote in message
...
Everyone knows that the only thing that keeps an airplane in the air
is.........
$$$$$$$

There is one important lesson to be learned from these experiments,

Yes there is. You are an idiot.

Randy L.
--
"When making an emergency off-field landing at night,
turn on your landing light just prior to touchdown.
If you don't like what you see, then turn off the landing lights."

"Le Chaud Lapin" wrote in message
ups.com...
Hi,

(snip)
-Le Chaud Lapin-


OK -- everyone has been dancing all around this. The thing that keeps an
airplane
up is the WINGS! Once again, more slowly---------the
WWWIIIIIINNNNNGGGGSSSS!
No wings - no flying. With no wings you don't have an airPLANE, you have a
rocket (or a bomb,
depending on which way it's going). Any bee knows this. Most birds know this
and they have bird brains.


  #5  
Old October 4th 07, 03:13 PM posted to rec.aviation.piloting
Tina
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Posts: 500
Default Why Airplanes Fly - Voids Above A Planar Sheet

If "Getting on the same page" means learning some of the physics of
flying, I'd enjoy knowing how these 'experiments' are related.

Are you suggesting that a table top under the paper is in any way
representative of what goes on in a dynamic airfoil?

Are you really educated as an engineer?


  #6  
Old October 4th 07, 05:27 PM posted to rec.aviation.piloting
Le Chaud Lapin
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Posts: 291
Default Why Airplanes Fly - Voids Above A Planar Sheet

On Oct 4, 9:13 am, Tina wrote:
If "Getting on the same page" means learning some of the physics of
flying, I'd enjoy knowing how these 'experiments' are related.

Are you suggesting that a table top under the paper is in any way
representative of what goes on in a dynamic airfoil?


Well I was trying to illustrate what goes on between the two sheets of
paper, but I guess a table will do. Technically, if you place one
sheet of paper on top a table, and yank up hard on the paper, there
will be a tendency for the table to lift off the ground, but since the
mass of table is so great, the net pressure upward on table is not
enough to counteract gravity so the table remains at rest (actually,
at a quantum level it does not completely "not move", but for our
purpose we can say that it doesn't).

So I used two sheets of paper because the bottom paper will rise.

So yes, I believe this experiment illustrates an important phenomenon
in aerodynamics. It is not the only phenomenon that plays a role, but
it has one, nevertheless. The decriptions of lift that I read in
flight books seem to ignore it. This weekend I am going to download
material on aerodynamics and read what it says.

There is another experiment that could demonstrate this principle more
dramatically, using an actual airplane wing:

SMOKING CIGARETTES/AIRPLANE WING EXPERIMENT:

I would take an airplane wing, and mount it rails that can move in the
forward and aft directions along what would be the longitudinal axis
of the airplane if if the wing were so attached. Then I would take a
bunch of cigarettes, light them, and hang them up-side-down from a
high ceiling above the wing of the aircraft. The wing would have an
exaggerated AoA, say 30%, no flaps, displaced slightly so that it is
ahead of the hanging cigarettes, but so that the cigarettles cannot
touch. The cigarettes would be lit so that stream of smoke floats
upward.

Then I would use a tremendous force applied to move the wing forward
along the rails, say, by linear induction motor, or whatever, to move
the wing forward, being careful that the apparutus doing so is already
ahead of the wing and connected by steel wire to minimize interference
effects.

You would see that, if the impulse is great enough, not only would the
smoke be diverted from upward and moved in the direction that the wing
went (forward), but the hanging cigarettes themselves would move.

If flat pressure sensors were mounted above the wing, close to the
trailing edge, they would show a momentary decrease in pressure.

If flat pressure sensors were mounted below the wing, close to the
trailing edge, they would show a momentary increase in pressure.

After the force stops, there would be relaxation where the
rarefication above the wing and compression below the wing are
elminiated by flows due to the pressure gradient.

In a real airplane, this is what is happening, but because the the
wing is constantly moving foward, the rarefication above the wingg and
the compression below the wing are never quite normalized by to normal
atmosphere.

The downwash above wing is due to air rushing in to fill the void.

SMOKING CIGARETTE/HARD-COVER BOOK EXPERIMENT:

There is an similar, not-as-dramatic experiment you can do at home
that is closely related to experiment above. Let a piece of stiff
cardboard be your wing. Hold it from the side at an angle of attack,
as above, but don't rest your arm on top of a table. That would
create a boundary condition beneath the wing. Light a cigarette and
inverted so that it is the hot part is near the top of the wing, so
where in middle between leading and trailing edge. Get your arm out of
the way of the void that is about to be created. Now, in one quick
motion, move the cardboard forward. Notice the tremendous net impulse
force that is generated on the cardboard. The smoke will follow.

These things are happening in flight, along with Bernoulli.

Are you really educated as an engineer?


Yes, electrical/software.

-Le Chaud Lapin-

  #7  
Old October 4th 07, 05:32 PM posted to rec.aviation.piloting
BDS[_2_]
external usenet poster
 
Posts: 149
Default Why Airplanes Fly - Voids Above A Planar Sheet


"Le Chaud Lapin" wrote...
On Oct 4, 9:13 am, Tina wrote:
If "Getting on the same page" means learning some of the physics of
flying, I'd enjoy knowing how these 'experiments' are related.

Are you suggesting that a table top under the paper is in any way
representative of what goes on in a dynamic airfoil?


Well I was trying to illustrate what goes on between the two sheets of
paper, but I guess a table will do. Technically, if you place one
sheet of paper on top a table, and yank up hard on the paper, there
will be a tendency for the table to lift off the ground, but since the
mass of table is so great, the net pressure upward on table is not
enough to counteract gravity so the table remains at rest (actually,
at a quantum level it does not completely "not move", but for our
purpose we can say that it doesn't).

So I used two sheets of paper because the bottom paper will rise.

So yes, I believe this experiment illustrates an important phenomenon
in aerodynamics. It is not the only phenomenon that plays a role, but
it has one, nevertheless. The decriptions of lift that I read in
flight books seem to ignore it. This weekend I am going to download
material on aerodynamics and read what it says.

There is another experiment that could demonstrate this principle more
dramatically, using an actual airplane wing:

SMOKING CIGARETTES/AIRPLANE WING EXPERIMENT:

I would take an airplane wing, and mount it rails that can move in the
forward and aft directions along what would be the longitudinal axis
of the airplane if if the wing were so attached. Then I would take a
bunch of cigarettes, light them, and hang them up-side-down from a
high ceiling above the wing of the aircraft. The wing would have an
exaggerated AoA, say 30%, no flaps, displaced slightly so that it is
ahead of the hanging cigarettes, but so that the cigarettles cannot
touch. The cigarettes would be lit so that stream of smoke floats
upward.

Then I would use a tremendous force applied to move the wing forward
along the rails, say, by linear induction motor, or whatever, to move
the wing forward, being careful that the apparutus doing so is already
ahead of the wing and connected by steel wire to minimize interference
effects.

You would see that, if the impulse is great enough, not only would the
smoke be diverted from upward and moved in the direction that the wing
went (forward), but the hanging cigarettes themselves would move.

If flat pressure sensors were mounted above the wing, close to the
trailing edge, they would show a momentary decrease in pressure.

If flat pressure sensors were mounted below the wing, close to the
trailing edge, they would show a momentary increase in pressure.

After the force stops, there would be relaxation where the
rarefication above the wing and compression below the wing are
elminiated by flows due to the pressure gradient.

In a real airplane, this is what is happening, but because the the
wing is constantly moving foward, the rarefication above the wingg and
the compression below the wing are never quite normalized by to normal
atmosphere.

The downwash above wing is due to air rushing in to fill the void.

SMOKING CIGARETTE/HARD-COVER BOOK EXPERIMENT:

There is an similar, not-as-dramatic experiment you can do at home
that is closely related to experiment above. Let a piece of stiff
cardboard be your wing. Hold it from the side at an angle of attack,
as above, but don't rest your arm on top of a table. That would
create a boundary condition beneath the wing. Light a cigarette and
inverted so that it is the hot part is near the top of the wing, so
where in middle between leading and trailing edge. Get your arm out of
the way of the void that is about to be created. Now, in one quick
motion, move the cardboard forward. Notice the tremendous net impulse
force that is generated on the cardboard. The smoke will follow.

These things are happening in flight, along with Bernoulli.

Are you really educated as an engineer?


Yes, electrical/software.

-Le Chaud Lapin-



Are you crazy?! Do you know what cigarettes cost these days??!!

BDS


  #8  
Old October 4th 07, 06:48 PM posted to rec.aviation.piloting
Robert M. Gary
external usenet poster
 
Posts: 2,767
Default Why Airplanes Fly - Voids Above A Planar Sheet

On Oct 3, 7:04 pm, Le Chaud Lapin wrote:
Hi,

The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.


Or just buy the book "Aerodynamics for Naval Aviators" and look at the
pictures.

-Robert

  #9  
Old October 4th 07, 07:06 PM posted to rec.aviation.piloting
Le Chaud Lapin
external usenet poster
 
Posts: 291
Default Why Airplanes Fly - Voids Above A Planar Sheet

On Oct 4, 12:48 pm, "Robert M. Gary" wrote:
On Oct 3, 7:04 pm, Le Chaud Lapin wrote:
The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.


Or just buy the book "Aerodynamics for Naval Aviators" and look at the
pictures.

-Robert


What will I see?

-Le Chaud Lapin-

  #10  
Old October 4th 07, 07:19 PM posted to rec.aviation.piloting
Dudley Henriques[_2_]
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Posts: 2,546
Default Why Airplanes Fly - Voids Above A Planar Sheet

Le Chaud Lapin wrote:
On Oct 4, 12:48 pm, "Robert M. Gary" wrote:
On Oct 3, 7:04 pm, Le Chaud Lapin wrote:
The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.

Or just buy the book "Aerodynamics for Naval Aviators" and look at the
pictures.

-Robert


What will I see?

-Le Chaud Lapin-


What will I see????

The collective knowledge of the finest aerodynamics minds since the dawn
of aviation. ANA is the "bible" for anyone from Astronauts to Student
Pilots. It's not written for the casual user however.
I'm sure from what I've been reading of your posts that you might
perhaps have something you would like to see changed in the book to
reflect a more accurate text.
If you would like to do this, please email me and I'll put you in touch
with the right people at the Naval Test Pilot School at Patuxant NAS.


--
Dudley Henriques
 




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