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Re: but does the *earth* push up on the *wing*?



Let me jump into a field in which I know no more than has been discussed
here and make two points.

Does the aircraft push down upon the earth and as a consequence the
earth lifts it up, or does the aircraft accelerate air downwards and
is lifted upwards as a consequence?

1. An equivalent question might be "if I pull on one end of a rope or
spring, does the person at the other end of the rope/spring feel the third
law reaction force? At a fundamental level the answer has to be NO, but at
the practical level the answer is usually YES! I apply a force to the end
of the rope, and the END OF THE ROPE applies an equal and opposite force to
me. The end of the rope in turn applies a force to the next section and so
on down the line to the other person. Other than transmitting the force
and introducing a negligible time delay, the rope can mostly be ignored and
the force pair can be considered acting between the two people. With a
spring it is a bit more problematic, since the time delay can be much
greater, but in equilibrium, it is clear that the two people feel equal and
opposite forces.

I would say the air is like a giant spring. The airplane produces the
immediate lift by pushing against the spring, but eventually (since we have
a closed system with the earth at the other end of the spring) the force
must be transmitted to the earth.

So at a fundamental level the answer has to be NO - the earth and airplane
don't form an action-reaction pair, but at the practical level my answer is
that they effectively are such a pair.


2. Now I'll go further out on a limb and take a shot at the machine
gun/circulation debate. Consider a large column of calm air. An airplane,
trimmed for level flight, is thrown into the column from a region which is
a VACUUM. What course will the plane take???

I think (based mostly on intuition) that the plane will not immediately
commence to level flight, but will sink for a moment until a "cushion of
air" has built up under the wing. (Much like a car "thrown" along a road
with the wheels just barely touching the ground would settle until the
shocks had compressed enough to carry the weight.) The initial downward
acceleration would be considerably less than g due to the collisions, but
more than zero. Once the cushion builds up, the acceleration falls to
zero. To get this cushion ahead of the wing (which is the only place it
does any good) the air must be moving forward along the bottom. This
sounds like John Denker's "circulation" although I see it as an effect, not
a cause, of lift.

If my intuition about the initial dip in elevation is correct, this would
suggest that the "machine gun" model is a bit simplistic. While it
explains much of the lift, it can't explain all of it. The compression
(i.e. increased pressure) of the air under the will also contribute a
significant amount to the lift.

Or put it another way. If "Maxwell's demon" could catch and remove the
deflected air molecules right after they hit the wing, there would still be
considerable lift. But if they are not caught, they will to some extent
bounce back up from air molecules below them, hit the wing a second time,
and provide extra lift.

Well, I've wasted too much of my afternoon on this, so I better get ready
for classes, which start on Monday. I'll try to find time to defend my
views if people find this though provoking.

Tim Folkerts
tfolkert@fhsu.edu