On Thu, 1 Apr 2004 10:33:19 +0100, "JimB"
wrote:

I'm not proposing that the air 'has to catch up'. I'm just saying
that if it loses pressure, it's got to gain speed (or disperse
energy in some other way).

==================================

Let's try for an intuitive approach using a flat plate (your hand, for
example). Imagine sticking your hand out the window of a moving car
and "flying" it through the air as most of us have probably done as a
kid until our parents yelled at us.

If you hand is more or less parallel to the ground, you have wind
resistance (drag), but no lift. Tilt you hand slightly upwards and
now the wind strikes the bottom of your palm and forces it upwards
(lift). The reason lift is created is that your hand is deflecting
molecules of air downwards (change in momentum), and the resultant
force is upwards. It's simple Newtonian mechanics.

JAXAshby wrote in message
...
jim, the 'bernouli's" say the air is one unit at the leading
edge of the wing,
and because it travels a shorter distance along the straight
bottom surface
than the longer top surface it therefore means the air along
the top *had to*
speed up to "catch up" with the air on the bottom, therefore
less pressure.

Bernouilli may have said that. I didn't. And I like symmetrical
aerofoils, like rudders and keels, or ones that work well upside
down as well as the right way up.

This is not true. it does not HAVE to.

I'm not proposing that the air 'has to catch up'. I'm just saying
that if it loses pressure, it's got to gain speed (or disperse
energy in some other way).

The most efficient wing surface out there is a thin sheet
shaped into an "S",
with the top forward part of the "S" the leading edge and the
bottom rear part
the trailing edge.

Indentical distances for air to travel and the most lift
available for the
drag. (problem: andy particular "S" only works for one
airspeed)

Now I think the device you're proposing is designed to add a
downward speed to the air, then subtract that speed, leaving no
net change in downward speed. Is that correct? If so, you're
proposing that a net force can be generated by displacing air
through a distance, rather than adding momentum to it.
Interesting.

I haven't heard of this design in aerodymamics before except in
the context of windmills. I can see how such a device would
generate a magnificent torque (lift at the front, cancelled by
'anti-lift' at the rear). This torque would have to be stabilised
by a net lifting body on a lever arm for any practical
application. But I am very out of date in this subject, so a few
questions first:

Has it been written up anywhere?
Do you know any practical applications?

For sure, I wouldn't want to fly upside down with this one. Nor
use it for a keel.

Instead of thinking of "low" pressure and "high" pressure,
think of what low
and high means. high has more air molecules pressing against
the surface at
higher molecular speeds. low has fewer molecules and/or lower
molecular speed
of those molecules.

Yup. Got that. Brownian motion or some similar name and all that.
But I don't see what difference it makes.

To create lift (by changing the momentum of the passing air)
there must be low pressure above the wing compared to the
pressure below the wing.

reword this using impact of molecules instead of low/high
pressure and see what
happens.

Right:

'to create lift there must be fewer molecules at lower molecular
speeds above the wing compared to the higher number of molecules
at higher molecular speeds below the wing'.

So?

Jax, I hope you're not making the mistake of confusing the
temperature/pressure linked random motion of molecules with the
mean speed of a flow pattern!

My point remains: 'there are fewer molecules at lower [random]
molecular speeds above the wing' - so what did they sacrifice
their energy to? My argument is that it could only go to an
increased mean stream speed. Are you denying this? or am I out of
date here as well?

JimB

JimB wrote:

JAXAshby wrote in message
...

jim, airspeed over a wing does not have to faster than airspeed
below a wing
for a wing to have lift. "bernoulli" sounds conventiently
scientific to
explain lift, but it ain't real.

My assumptions were that we're talking subsonic, and
substantially laminar flow. I made that clear. These assumptions
are relevant to this group, since sails and keels aren't
supersonic, and try to minimise flow breakaway in the interests
of efficiency.

To create lift (by changing the momentum of the passing air)
there must be low pressure above the wing compared to the
pressure below the wing. There will be tip vortices proving this
point. I'm sure you accept this.

Within my assumptions, to accept your flat statement: 'airspeed
over a wing does not *have to [be]* faster than airspeed below a
wing for a wing to have lift', I would need to understand where
the energy due to this pressure drop goes. My assumption (perhaps
incorrect) was that it goes into a temporary increase in kinetic
energy - ie, an increase in local fluid speed. Whether this is or
isn't Bernouilli is irrelevant.

So, what assumptions do you make that allow this pressure drop
not to be accompanied by a speed increase? Where does your energy
go?

Incidentally, I'm enjoying this revision of basic aerodynamics,
and intrigued to learn what's changed since the 1970's, so keep
going. It helps if you answer my questions directly.

JimB


How airfoils generate lift is one of my favorite subjects, so I can't resist
bothering you folks with my (mostly uneducated) opinion.

This is my currently favorite explanation of lift, which AFAIK is partly my
own. And no, I'm not an expert. Possibly, some experts will get
around to adding enough to this discussion to generate massive confusion in
everyone, including themselves. Please keep in mind that I'm attempting to
describe Reality, not theories.

I think the key is in understanding that if air is given a place to flow to,
it does not instantaneously fill the "void" to ambient pressure, but
instead fills it at its own rate, according to local conditions. It also
helps to remember that air is *not* a single entity (it's a collection of
atoms and molecules) and that attempting to treat it as such is not always
correct.

Note: 'up' and 'down' are meant here as they would relate to normal flight.

Lift generated by the interaction between the airflow and the bottom of the
wing is easy to explain. It's a simple action-reaction event. If there is a
positive angle of attack, the air is accelerated in a downward direction,
causing an increase in pressure at the wing's lower surface.

Less easily explained is how the wing's upper surface generates lift. If you
are willing to keep it simple, it's easy to understand.

As the wing moves forward through the air (or vice versa), it creates a
"hole" (by that I mean an area of reduced pressure) in the current(as in
time, not flow) air column that must again be filled by air until ambient
pressure is reached. Because air molecules have mass and inertia, they
don't all immediately rush to fill the hole. Some molecules are already
there, but more arrive, then more, until the hole is finally filled to
ambient pressure.

Another way to put it: When the wing moves forward, its curved or sloped
surface "yanks the floor out" from under the current air column, leaving
the air to refill the space in its own way and its own time, which creates
a temporary decrease in pressure. Because the wing's upper surface is in
the area of reduced pressure, a lifting force occurs.

"Yanking the floor out" can be further illustrated by what happens if you
are in an elevator at the top of a tall building. As the elevator starts
its downward movement, you can feel your weight temporarily decrease. That
weight decrease would correspond to the pressure decrease on the surface of
a wing.

Because the wing is moving in relation to the air, it is continuously
"yanking the floor out" from under its current air column, thereby
generating a continuous lifting force.

I'm sure this could have been written better, but hopefully you will get the
meaning.

nonameneeded wrote in message
. com...
JimB wrote:

JAXAshby wrote in message
...

jim, airspeed over a wing does not have to faster than
airspeed
below a wing
for a wing to have lift. "bernoulli" sounds conventiently
scientific to
explain lift, but it ain't real.

Because the wing is moving in relation to the air, it is
continuously
"yanking the floor out" from under its current air column,
thereby
generating a continuous lifting force.

Each person can visualise the causes of lift the best way it
works for themselves. I can understand completely what you're
saying, and it's a nice simile. Like all similes, it's
incomplete, but that's irrelevant for sailors and pilots.

Brian Walcott in his 1 Apr post gave an excellent, more thorough
and technical description of the various elements of cause and
effect when creating lift. I would guess he teaches fluid
dynamics.

Jax has been trying to say that you can create lift (low pressure
over a wing) without causing air (in that low pressure) to speed
up, and later, without causing air to move downwards. Rather than
just telling him he's wrong, I've been trying to show him that
his statements are inconsistent with with the laws of physics. So
far he hasn't responded, apart from saying 'nah'.

JimB

nonameneeded wrote in message
. com...
JimB wrote:

JAXAshby wrote in message
...

jim, airspeed over a wing does not have to faster than
airspeed
below a wing
for a wing to have lift. "bernoulli" sounds conventiently
scientific to
explain lift, but it ain't real.

Because the wing is moving in relation to the air, it is
continuously
"yanking the floor out" from under its current air column,
thereby
generating a continuous lifting force.

Each person can visualise the causes of lift the best way it
works for themselves. I can understand completely what you're
saying, and it's a nice simile. Like all similes, it's
incomplete, but that's irrelevant for sailors and pilots.

Brian Walcott in his 1 Apr post gave an excellent, more thorough
and technical description of the various elements of cause and
effect when creating lift. I would guess he teaches fluid
dynamics.

Jax has been trying to say that you can create lift (low pressure
over a wing) without causing air (in that low pressure) to speed
up, and later, without causing air to move downwards. Rather than
just telling him he's wrong, I've been trying to show him that
his statements are inconsistent with with the laws of physics. So
far he hasn't responded, apart from saying 'nah'.

JimB

JimB wrote:

JAXAshby wrote in message
...

jim, airspeed over a wing does not have to faster than airspeed
below a wing
for a wing to have lift. "bernoulli" sounds conventiently
scientific to
explain lift, but it ain't real.

My assumptions were that we're talking subsonic, and
substantially laminar flow. I made that clear. These assumptions
are relevant to this group, since sails and keels aren't
supersonic, and try to minimise flow breakaway in the interests
of efficiency.

To create lift (by changing the momentum of the passing air)
there must be low pressure above the wing compared to the
pressure below the wing. There will be tip vortices proving this
point. I'm sure you accept this.

Within my assumptions, to accept your flat statement: 'airspeed
over a wing does not *have to [be]* faster than airspeed below a
wing for a wing to have lift', I would need to understand where
the energy due to this pressure drop goes. My assumption (perhaps
incorrect) was that it goes into a temporary increase in kinetic
energy - ie, an increase in local fluid speed. Whether this is or
isn't Bernouilli is irrelevant.

So, what assumptions do you make that allow this pressure drop
not to be accompanied by a speed increase? Where does your energy
go?

Incidentally, I'm enjoying this revision of basic aerodynamics,
and intrigued to learn what's changed since the 1970's, so keep
going. It helps if you answer my questions directly.

JimB


How airfoils generate lift is one of my favorite subjects, so I can't resist
bothering you folks with my (mostly uneducated) opinion.

This is my currently favorite explanation of lift, which AFAIK is partly my
own. And no, I'm not an expert. Possibly, some experts will get
around to adding enough to this discussion to generate massive confusion in
everyone, including themselves. Please keep in mind that I'm attempting to
describe Reality, not theories.

I think the key is in understanding that if air is given a place to flow to,
it does not instantaneously fill the "void" to ambient pressure, but
instead fills it at its own rate, according to local conditions. It also
helps to remember that air is *not* a single entity (it's a collection of
atoms and molecules) and that attempting to treat it as such is not always
correct.

Note: 'up' and 'down' are meant here as they would relate to normal flight.

Lift generated by the interaction between the airflow and the bottom of the
wing is easy to explain. It's a simple action-reaction event. If there is a
positive angle of attack, the air is accelerated in a downward direction,
causing an increase in pressure at the wing's lower surface.

Less easily explained is how the wing's upper surface generates lift. If you
are willing to keep it simple, it's easy to understand.

As the wing moves forward through the air (or vice versa), it creates a
"hole" (by that I mean an area of reduced pressure) in the current(as in
time, not flow) air column that must again be filled by air until ambient
pressure is reached. Because air molecules have mass and inertia, they
don't all immediately rush to fill the hole. Some molecules are already
there, but more arrive, then more, until the hole is finally filled to
ambient pressure.

Another way to put it: When the wing moves forward, its curved or sloped
surface "yanks the floor out" from under the current air column, leaving
the air to refill the space in its own way and its own time, which creates
a temporary decrease in pressure. Because the wing's upper surface is in
the area of reduced pressure, a lifting force occurs.

"Yanking the floor out" can be further illustrated by what happens if you
are in an elevator at the top of a tall building. As the elevator starts
its downward movement, you can feel your weight temporarily decrease. That
weight decrease would correspond to the pressure decrease on the surface of
a wing.

Because the wing is moving in relation to the air, it is continuously
"yanking the floor out" from under its current air column, thereby
generating a continuous lifting force.

I'm sure this could have been written better, but hopefully you will get the
meaning.