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Re: Judgement on opposing airfoil views pt. 2



On Thu, 26 Aug 1999, William Beaty wrote:

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.

There is no disagreement (I think) that what is relevant is the specific
region where the parcel is first given a vertical acceleration. My point
was that a naive person might infer that *any* downward motion must be
related to an upward force on the wing. I was not trying to imply that
anyone was actually stating such a thing, though. Sorry about that.

In my previous post I mentioned the possibility of examining "flight" in
water. Air is assumed to be incompressible (to first order) but that
is pretty hard to picture for most people. It is easier to consider
water as being incompressible. Still, there are pressure differences in
water and those pressure differences can produce lift. To me, I have a
difficult time seeing a *net* downwash for something under water, since I
don't expect the upper surface to go down with time. Does it matter how
deep the water is or how close the surface the object is? Again, if
someone can explain lift under water, it might shed some light on the
subject.

[snip]

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.

See my previous post regarding cause and effect.

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.
[snip]

See my previous post. I am not saying that the tactics were right. I am
trying to address the physics only.

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
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