John Denker's "upwash contributes lift"

[William Beaty <billb@ESKIMO.COM>]

   http://www.monmouth.com/~jsd/fly/lift.htm
   http://www.aa.washington.edu/faculty/eberhardt/lift.htm

John, your critique of the Anderson/Eberhardt paper states:

>   The paper contains a number of significant errors. These include:
>
>  1) The paper contains the statements +This upwash actually contributes
>  to negative lift and more air must be diverted down to compensate for
>  it.; and +upwash is accelerating air in the wrong direction for lift.;
>
>  It is easy to see that these statements are wrong. Suppose you are
>  standing on a plank overhanging a trampoline. You throw a baseball
>  downward, rather hard. During the throwing motion, the recoil forces
>  you upward a little. Now suppose the baseball bounces off the
>  trampoline, comes back up, and is caught by you. Interestingly, when
>  you catch the ball you are forced upward again. The analogy to
>  airplanes is clear: both the incoming upwash and the outgoing downwash
>  make positive contributions to the lift.


I don't understand your reasoning.  In my opinion the "lift/upwash"
process probably violates Newton's laws.  Here's why I think so:

If an airplane flies into still air, then the parcels of air are not
moving initially.  The wing approaches, and upwash begins.  When upwash
first begins, a parcel of air must be accelerated upwards, and this can
only occur if there is a force-pair between that parcel of air and some
other object or parcel of air.

Why does this parcel of air initially start moving upwards to become
upwash?  That's what I need to know.  If the wing somehow causes the
initial upwards acceleration of the air-parcel, then Newton's laws require
that a force-pair exist between that parcel and the wing.  If such a
force-pair exists, then when the parcel of air is initially accelerated
upwards to become upwash, the wing must experience an equal downwards
force and accelerate downwards.

If the wing does NOT cause the initial upwards acceleration of the
oncoming parcel, then I need to know what does cause its upwards
acceleration, because I need to put the other end of my Newtonian
force-pair somewhere.  Parcels of air do not just magically begin
accelerating for no reason.  A force must cause their acceleration, and
the other end of this force must connect to matter somewhere, and it must
cause that distant matter to move in the opposite (downwards) direction.

OK, now the air-parcel is moving upwards as upwash (and something else
somewhere must now be moving downwards.)  The upwash climbs the
pressure-gradient towards the wing, the air-parcel is given a downwards
component of acceleration as it slows, and as a result the wing
experiences a lifting force.  This part of the story looks perfectly
correct.

So, has the upwash now contributed to the lifting force?  No, because we
don't know what caused the parcel of air to initially accelerate upwards
and become upwash in the first place.  If the wing somehow causes this,
then the wing is required to experience a downwards force at that time. If
the wing somehow caused the upwash to appear, then that downwards force on
the wing subtracts from the upwards force the wing experiences later, and
the sum of these forces is zero, not upwards.  Anything else would be a
"bootstrap effect" where the wing is creating the upwash without
experiencing a downwards force, and then being lifted by the upwash it
creates.

What is the solution?  Simple.  I need to know what causes the oncoming
parcel of air to initially accelerate upwards, and I need to know where
the two ends of the relevant force-pair may lie.  If the other end lies on
the wing, then "section #1" in your critique of the Anderson/Eberhardt
paper is wrong.  If the other end does not lie on the wing, then where
does it lie?  (If there is no answer, then your reasoning contains a large
hole, and it should not form the basis of a criticism of the
Anderson/Eberhardt paper.)


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