Judgement on opposing airfoil views pt. 2

[William Beaty <billb@ESKIMO.COM>]

On Tue, 24 Aug 1999, Robert A Cohen wrote:

> On the other hand, attributing the lift to downward momentum of air can
> be misleading.  For example, as the vortex pair sheds off the wing tips
> and falls behind the plane, there is downward motion between the vortex
> centers.  As far as I can tell, this downward motion does not contribute
> to the lift of the plane because it occurs far away from the plane.

I think the answer is clear if we follow a single parcel of air as the
plane approaches.  The parcel is initially hanging in the atmosphere with
all the other parcels.  As the plane arrives and the parcel passes near
the wings (either above or below, it doesn't really matter), the parcel
moves up and down, and then it becomes part of the downwards-moving vortex
pattern in the aircraft wake.  Yes, this pattern is entirely behind the
plane.  However, its downwards motion is continuously being created by the
curving streamlines which are continuously bent by the wing's surfaces, so
the downwards motion of the downwash DOES connect to the wing after all.

My point?  Well, if the air-parcel has mass, and if it started out
NON-moving, and after the wing has passed, if the parcel is then moving
downwards... then that air-parcel has been given a net downwards
acceleration, right?  There must have been an F=MA force applied downwards
on the parcel in order to "create" its downwards motion.  The parcel ends
up with MV downwards momentum.  And there also must be a corresponding
upwards force somewhere else, because when the parcel was forced
downwards, something else had to be forced upwards (and also there must be
appear an -MV upwards momentum somewhere else as well.)

In other words, the region behind the wing where our parcel of air is
*already* moving downwards is actually ...unimportant!  What is relevant
to the lifting-force problem is the specific region where the parcel is
first given its net downwards motion.  This region is located where the
the air is in contact with the wing.

Something similar is true for a hovering rocket.  Suppose I argued that,
because the exhaust of a rocket is moving *outside* of the rocket, then
that exhaust can't exert an upwards force on the rocket, therefor the
"downwash" of a rocket-engine does not create lift.  This argument might
be correct in detail but is also incredibly misleading, because the
important part of the problem is actually located in the place where the
parcels of exhaust were initially unmoving, but then they were given a
downwards motion.

Same with the recoil of a gun.  The flight of the bullet is unimportant,
what's important for the "recoil" force is the region inside the barrel
where the bullet is being brought up to speed.

  (Aside: did you see on newsgroups the idea that "downwash exerts no
  forces because it is only BEHIND the wing"?  I saw it there earlier on
  sci.physics.  I rarely post to newsgroups, and was not moved to go start
  picking fights with the sci.physics flamers about it.  I didn't have the
  time back then.)

If an airplane wing flings the air downwards, and as a result a vortex
pattern is created which spins and also moves downwards, we *COULD* figure
out the lifting force experienced by the whole wing by watching the
individual parcels being accelerated vertically by the surface of the
wing, and then and applying F=MA and integrating across the wing's
surface.  This is the essence of the "Bernoulli" approach, although the
details differ from my description here.

Alternatively, we *COULD* do it differently: assume that the parcels were
initially unmoving before the wing approached them, then observe them
after the plane has passed. If they are moving with an average downwards
motion, we can measure their downwards velocity and use conservation-of-
momentum to calculate the lifting force experienced by the wing.  This is
the essence of the "Newton" approach.

The "Newton" approach says "downwash causes lift", but this is meant in
the same sense that "bullets cause recoil", or "rocket-exhaust causes
thrust."  The "thrust" isn't part of the exhaust-stream found outside the
rocket.  However, if we know the mass, speed, etc., of the hovering
rocket's exhaust stream, then we know the lifting force as well.

So you see, Newton and Bernoulli are perfectly compatible.  As a long time
"BernoulliNewtonist," I've included their inherent compatibility as a
funamental part of my "religious view".

In my considered opinion, it is a serious problem if Mr.  Denker goes
around trying to suppress the voice of the opposing "religion."  After
all, the goal is to crack that egg, not to use questionable tactics in
order to promote the "Only Right Way" to whack the spoon against it.


>    Verdict: both are right.

I'd have to disagree.  Yes, "Newton" and "Bernoulli" explanations of the
lifting force both are right, but the continuing problem between me,  Mr.
Denker, and Anderson/Eberhardt is something entirely different.

PS. I hope I'm not coming across as overly emotional or superior-sounding
in my writing above.

My stance has habitually been that of a "chastizing Moderator" who must
expose the transgressions of flamers to scrutiny and to shame. Johnathan
Swift, the author of Gulliver's Travels, wrote of the Lilliputians and
their bigendian/littlendian war, but he also apparantly cast himself in
the role of Gulliver, the "great one" who came in and rendered judgement.
I see this as a very serious problem.

Those of us who decide to leap into the fray and become the judges of
others must remain excruciatingly aware that only a short time ago we
ourselves had been willing members of the shameful armies of Lilliput.
Without humility, we will only make things far worse.  The only way we can
allow any retribution to occur against another person is if we temporarily
remove the "sword of retribution" from our own throats, and then put it
back there afterwards.  As a recent Liliputian myself, I am well aware
that this is the only proper long-term home for such a "sword:" aimed
squarely at me.  Mr. Denker is not the only one whose actions are open to
question here.  If I relax my vigilence for a moment, I certainly will
succumb to temptation and misbehave in just the same manner.  I've done so
in the past, and I've hurt people through my stupidity.  This is a very
difficult thing to forget.  I don't attempt to forget it.  When I see
someone else doing similar things, I want to take action, but my goal is
to "repair" things rather than to take it upon myself to punish.

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