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

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.

This is not true. it does not HAVE to.

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)

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.

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.

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

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

I haven't heard of this design ("S" shaped airfoils) in aerodymamics before
....

first semester aero eng books have had it for decades. It is not a practical
airfoil, but it shows why foils lift.

I haven't heard of this design ("S" shaped airfoils) in aerodymamics before
....

first semester aero eng books have had it for decades. It is not a practical
airfoil, but it shows why foils lift.

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.

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.

Wayne.B wrote in message
...
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.

It's OK Wayne. I understand the dynamics of lift reasonably well.
I'm trolling to check that Jax also understands it.

JimB

Wayne.B wrote in message
...
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.

It's OK Wayne. I understand the dynamics of lift reasonably well.
I'm trolling to check that Jax also understands it.

JimB