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

Alan Baker

In article .com,
wrote:

Alan Baker wrote:
In article .com,
wrote:

...

In level flight at constant speed the aircraft has constant horzontal
and zero vertical momentum.

True. But it is also experiencing a constant change in momentum in the
vertical direction. That's what a force is: a change in momentum over
time.

No. The airplane is in level flight at constant speed. Therefore there
is no change in momentum. The net force on an aircraft in level
flight at constant speed is zero.

When there is a constant change in momentum the vertical
direction the airplane climbs or dives.

Incorrect.

You're going to want to go back to your basic physics texts again. Check
the formal definition of "force".

http://www.rwc.uc.edu/koehler/biophys/2c.html

"Just as force is the time derivative of momentum, ..."

...

Yes, no question about weight being balanced by lift.

But the wings also exert a force on the air (Newton, remember: for every
force there is an equal and opposite, etc., etc.). That force is not
countered by *anything*. Hence, the air is accelerated downward; a
continuous stream of air receives an constant change in momentum.

If the air has a net increase in downward momentum, how is
momentum conserved.

By the increased upward momentum of the earth.

The earth pulls the plane downward, and the plane pulls the earth
upward. The movement of the plane towards the earth is transferred to
movement of the air towards the earth, which it does until it eventually
transfers its momentum back *to* the earth, leaving the system with the
same relative momentum with which it began.

It is the coilumn of air under the wing that supports the weight
of the plane. It does that, on the statistical mechanical level
via a series of minute momentum exchanges. It does that
because there are fewer air molecules transfering momentum
to it from above, that there are from below. But it does not
do that via a coherent stream of air.

A fluid can transmit force without flow in the conventional sense.
That is the basis for hydraulics.

The downflow observed from the wing initiates above the wing
and flows down behind the wing after the wing has passed.
It is not the air that suppors the wing.

...

I don't deny that downflow occurs. The point is that downflow is a
consequence of lift, 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.

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

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.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."