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

On Sat, 21 Aug 1999, Timothy Folkerts wrote:

2. Now to the third law. I guess my point here is how immediate do the
cause and effect have to be in order to be considered an action-reaction
pair? If I pull on your hands, is that an action-reaction pair? What if
we are wearing gloves? What if we pull on opposite ends of a rod? What if
we pull on opposite ends of a long, stretchy spring? It's mostly a matter
of personal preferrence as tio where you draw the line, but in every case,
the force at one end is transmitted to the other.

Easy answer! If there is a force upon an object in one direction, and an
equal force on a second object directed in an opposite direction, then
that is an action/reaction force-pair. If the wing pushes (indirectly)
upon the earth, and as a result the earth much later pushes (indirectly)
upon the wing with equal force, then that is an action-reaction
force-pair. If the earth does not push equally upwards upon the wing,
then there is no force-pair between the earth and the wing.

Perhaps things are confusing because here we have two SEPARATE
action/reaction pairs of identical magnitude. One force-pair is at the
wing. The other force-pair arises later at the surface of the earth. If
the two are independant, then it doesn't matter that they are equal, there
is still no force-pair between the wing and the earth. A force-piar
certainly exists between the wing and the air, and a totally separate
force-pair exists between the earth and the air which later hits the
ground. These two force-pairs are equal, and that might be a very
confusing thing. It makes us think that perhaps there is just one
force-pair here, a force-pair between the wing and the earth. But there
is not.

That's why I keep harping on the machine gun. It demonstrates that there
is no stretchy-spring effect. Yes, if the bullets bounced off the earth
and were caught by the machine-gun (and caused it to rise), then the earth
would be pushing upwards on the gun, and there would be a force-pair
between the earth and the gun. Without catching some upwards-moving
bullets, the gun cannot experience a force exerted upwards by the earth.
The same is true of wings. Only if the air should reflect from the earth,
fly upwards, and then zero in on that tiny airplane in that vast sky and
strike it, could there be a significant force-pair between earth and wing.

This stuff is not really part of the Newton/Bernoulli thing it all,
because it is a simple error in physics. In other words it is WRONG to
say that the wing is held aloft by a force-pair between the earth and the
wing. A *correct* analysis of "Bernoulli-ist" physics would not find any
need for a force-pair between the earth and the wing. Done right, the
"Bernoulli-ist" and "Newton-ist" positions must agree with each other.


Now, the airplane is admittly at the "stretchy spring" end of the spectrum.
But I bet for a plane flying near the ground at constant speed, you could
measure the the pressure distribution and if you integrate
delta(Pressure)dA at any given instant, and you would get mg.

Ah, that's entirely different. I think everyone here agrees that when an
airplane flys near the ground, it is in "ground effect flight" and the
rules of the game become very different. In that case there is a large
force between the earth and the air. In that case the "machine gun"
really is being "hit by reflected bullets."


In
principle, if the plane was flying higher, the pressure changes would be
less and the area it is spread over larger, but it would still be mg.

Nope. As the plane flys higher, the ground-effect forces drop rapidly to
insignificance. The math is similar to the description of forces between
two short bar magnets , or the force between a bar magnet and a horizontal
plate of superconductor metal. When they are near each other, the forces
are large, but if they are farther than a particular distance, the forces
become insignificant. Obviously the forces never become zero, but that's
not the point. If they become so incredibly small that they have no
bearing upon the explanation of the lifting force, then we need not
mention them in our explanation. We DO need to mention them when the
airplane is within a wingspan or two of the earth. But the force-pair
between earth and wing drops off so quickly above that altitude that it no
longer contributes significantly to the total lifting force.


In
principle, plane flying loop-the-loops at +- 6g would cause pressure
changes which would vary at the same frequency as the loop-the-loops.
There may be a considerable delay, and there may be considerable damping,
but the average of the integral delta(P)dA would still be mg.

Or perhaps these pressure-changes are just a form of weak, low frequency
sound, and any forces they produce are insignificant compared to the
forces produced when the wings fling massive parcels of air around. Yes,
a looping airplane must send out sound waves, but so does a pair of kids
throwing a baseball back and forth, yet we don't need to look at these
sound waves to explain the forces upon the kids and upon the baseball.



Clearly the wings are pulling the plane down. They
cannot be pushing on the earth.
3. Part 2 doesn't say that pushing on the earth is a necessary condition
for flight, anymore than it says that a rocket needs to push on the earth
to fly.

I think the author is referring to John Denker's long-held assertion that
airplanes fly because there is a force between the wings and the earth's
surface. That's what this whole debate is really about. (Although I
haven't heard what Mr. Denker's explanation of high-level flight has
become currently, so perhaps he has changed his position.)


But in either case, as long as particles shot downward are
"caught" by the earth, the ultimate fact is that they DO apply a time
averaged force to the earth exactly equal to the time averaged force
applied to the plane and/or rocket.

Yes! No disagreement here. And that second force causes confusion. It
has the same magnitude as the lifting force, but it is a totally separate
force-pair which only exists between the earth and the downwards-flung
air.

Just because the two independant, spatially and temporally separate
force-pairs are equal in magnitude and direction does not mean there is a
back-and-forth communication going on between the wings and the distant
surface of the earth.


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