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.