AviationBanter - View Single Post - The Impossibility of Flying Heavy Aircraft Without Training

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

Alan Baker wrote:
In article .com,
wrote:

...

Well then if the downflow is NOT balanced by upflow why doesn't
the upper atmosphere run out of air?

Because the air contacts the earth and *stops* moving downward.

Could you define downflow?

Sure.

The aircraft passes through and air moves downward. As it moves its
motion is dissipated into more and more air moving less and less, but
eventually the momentum that was transferred to it is transferred back
to the earth.

I was hoping for a mathematical defintion, rather than a description
of the process. That would minimize my opportunity to draw an
incorrect inference. In this regaerd, a mathematical definiton
would be best.

I infer from your description the definition: "downflow is a flow
of air from the airplane toward the ground". That removes a
potential abiguity, whether downflow was a flow of momentum
through the air, (like a pressure wave) or a flow of mass.
Is that how you define downflow, as a flow of air molecules
(with mass) toward the ground?

Can you state a mathematical definition of downflow?

Fred Thomas' in _Fundamentals of Sailplane Design_ defines
the freestream velocity, then states the relationship between
that, the local velocity near the airfoil and the induced downwash
as a vector sum and goes on to show how this produces an
effective angle of attack less than the geometric angle of attack.
But he does not present a separate mathematical defintion of
induced downwash or the local velocity of the air near the wing
so the vector sum above does not serve (within the context of
his discussion) to define either term.

But it is clear that the induced downwash is a velocity, not a
a massflow. Yes, it is mass that has that velocity but the
parameter _induced downwash_ is a velocity.

So, can we agree to the definition of downflow as a flow
of air toward the ground and define the induced downwash
as the velocity of that air near the wing?

Meanwhile:

Earlier I wrote:

The point is that downflow is a consequence of,
not the cause of lift, and it is balanced by
upflow, (albeit a more diffuse flow) otherwise
the upper atmosphere would run out of air.
[and later corrected this to: downflow is a
consequence of the same phenomenum that
produces lift, not the cause of lift]

You replied:

No. It is balanced by the downflow eventually
transferring its momentum back to the earth.

So I asked:

Well then if the downflow is NOT balanced by
upflow why doesn't the upper atmosphere run
out of air?

Your response:

Because the air contacts the earth and *stops*
moving downward.

Earlier you corrected me regarding conservation of
momentum. Now, consider conservation of mass.

That the downflow stops upon contacting the Earth does
NOT explain why the upper atmosphere is not depleted
of air.

Plainly if air flows to the Earth and *stops* there as
you wrote, it has displaced other air which flowed
up to replace it, right?

--

FF