In article ,
Beryl wrote:

Alan Baker wrote:
In article , Beryl
wrote:
...

You've seen pics of it curling right back up.
http://www.efluids.com/efluids/gallery/gallery_pages/Morris_4.jsp
It hasn't bounced off the ground.

You've seen the edges curling back up.

That photo shows ALL of the flow curling back up. The bottom of the
vortex couldn't be any clearer, and there's nothing extending further
down underneath it.

Sorry, but you're wrong.

First of all, the downward motion of the vortex clearly carries right
out the bottom of the frame. Second, I've stated all along that as time
passes the momentum is diffused among more and more air.



Pressure waves can reach the ground, without the air in the column
descending to the ground.

I never said that the particular molecules that the aircraft touches
are the ones that have to reach the ground.

You said "The net flow is downward until it hits the ground and the
momentum is transfer to the earth."

And it is: the *net* flow.


The molecules that "reach" the ground are the ones that
were *already there* at ground level.

I never implied that the same molecules are the ones that eventually
strike the ground.


The fact is that if the aircraft and the Earth are to remain the
same distance apart, the plane must "push" against the Earth
with a force equal to the force of gravity. The air is the medium
by which the aircraft can transmit that push.

Right. And I can push against the hill across the road with my
voice. Not much, but enough to move the diaphragm in a microphone
over there, in a split second. The air expelled from lungs is never
going to make it across the road.

No, sorry. You don't push the hill with your voice.

Of course I do.

The pressure waves contain both positive and negative phases.

So you think that a positive won't push because a negative will be
coming along shortly?

I think their will be no net push, yes.

See the difference: sound waves, no net flow: no net push.


And to explain to you why your hypothetical craft with the radial
exhaust of air from a centrifugal fan won't work. The air that
enters downward gets turned to go sideways. That 90 degree turn
can only be accomplished by the a push upward from the aircraft
and thus the air must push down on the system with an equal but
opposite force.
But is that 90 degree turn *exactly* the same as a 180 degree turn
that directs incoming air back in the opposite direction? No, so
I'll just turn my squirrel cage upside down with 180 degree flow
redirection, and get lift with no net downwash.

No, you won't.

No downwash, no lift. No go learn something.

Let's learn here. From you. Is that 90 degree turn *exactly* the same as
a 180 degree turn that directs incoming air back in the opposite direction?

Read this:

"To determine [the angle represented by a greek letter in the original
text], we observe that no downwash is generated when the wing generates
no lift."

http://www.aoe.vt.edu/~cwoolsey/Cour...al/Aerodynamic
Properties.pdf

Read it over and over again until you get it.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg