T o d d P a t t i s t wrote:

David CL Francis wrote:


The nature of things is such that ....


I've been following along (more or less :-) and chose
David's post to jump in again, since, from experience I have
great trust in any analysis by David.

This thread, however, seems to wander all over the place.
It looks like one participant will make one set of
assumptions, then another will assume something different.

I see the following discussions going on:

1) A pure thread related purely to lift and Bernoulli. In
this thread, the subject matter is maximally simplified by
a) using the standard Bernoulli assumptions, inviscid flow,
incompressible, subsonic, etc., b) ignoring parasitic drag
c) using 2-D flow (or equivalently infinite wing)
assumptions and looking at steady state conditions. This
gets to the heart of upwash and downwash.

2) The same as 1) above, but looking at 3-D flow. Now we
have induced drag and the wing/fan produces a net motion of
the air as it passes through. Much of this discussion seems
focused on issues relating to closed systems (rooms, earth
with ground, etc.) and what happens to the air, how big a
system should be looked at, etc.

3) The same as 2) above, but with viscosity added so that
the air ultimately stops its motion and heats up due to
viscous losses.

Quite honestly, for most of the posts here, I can't figure
out what assumptions lie behind the comments.

Todd,
Your #2 and #3 is where I wanted to go with thus mess,
Thankee.

The down wash, being transferred from air near the wing to air
far away from the wing...

Air is quite springy stuff.

The energy transfer is spread over an increasing area (or volume)
and quickly reduced in magnitude - to the point where it is no
longer detectable (without invoking Steven Hawking).

For all practical purposes, that would seem to indicate that the
"down flow" would not reach the ground before being dissipated into
the larger air mass. (not arguing against the eventual contact with
the entire surface of the planet. But that doesn't help us understand
basic aerodynamics!)

Only when the wing is close to the ground is the down wash detectable
because it hasn't had time (or room) to be absorbed/dissipated.

Now, while the above is obviously not true in the molecular sense,
it may help us understand the practical parts better.

Also, add #4?

While we have been concentrating on the pressure side (bottom) of the
wing, it's the upper surface that has the greater influence here.

The only way I see of increasing pressure on the bottom surface is to
increase speed (or density?).

But the top side is where the pressure is reduced.
And there are a lot of factors that effect that part.

Thickness of the camber line is a big one.
Deeper camber tends to cause a lower pressure on top - hence more lift
for a given surface and speed.

This is most often accomplished by deploying flaps.
True they go down into the stream on the bottom side - and probably do
to some degree - invoke some impact lift (pressure) on the bottom.

But the curvature of the airfoil has increased also - and the camber
line has deepened - and the apparent angle of attack has increased.

These factors further decrease the lowered pressure field on top of the
wing - WAY more than any useful increase in pressure below it.

Lastly (for now), if we are indeed pressing down on the air below the
wing, we are also Pulling Down on the air above it...

The air below presses against the earth. As I've said before, that one is
so obvious (that we stop looking?).

But I think the low pressure field Above the wing is also pulling down
on the atmosphere above it.

While air pressure decreases with altitude me may think that the field
above the wing dissipates quicker. Maybe true, BUT - the pressure field
Above the wing is of much higher magnitude - so maybe not.

Well, so much for my silly idea.
I don't know how to analyze that one mathematically.

I'd really like to see what the bigger brains can make of it.

Richard