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15 minute classroom version...

Hi Rick! Here's my spin on things...

(but with centralized vorticity, or uniformly distributed vorticty?
sorry... up too late... getting punchy... )


On Sun, 22 Aug 1999, Rick Tarara wrote:

From the 15 minute/intro class point of view let me offer/ask:

1) Does air really move faster over the top of the wing, and is there
(as Bernoulli suggests) a pressure difference?

Yes, it moves much faster over the top, and there is a significant
difference in pressure between the upper and lower surfaces.



a) If yes, then WHY?
i) The (apparently discredited) path length difference AND
continuity argument (if the air doesn't move faster it would
pile up on top of the wing--akin to why there are river rapids)?


Nope. With three-dimensional air flowing around a real-world wing, there
is no need for a "pile up", although in a purely 2D universe this might
seem difficult to imagine.


ii) The low pressure above the wing 'sucks' air into this
region--but how does the low pressure form initially--from air
deflected away from the top surface by the leading edge and/or
the angle of attack??


Something like that. The aerodynamicist K. Weltner describes it that way.

Me, I imagine that the tilted wing makes a sort of "hole" as it pushes
forward into the air, and therefor the air must rush over the top of the
wing in order to fill this "hole" above and behind it. In other words,
the lower pressure is always created by an effect which is similar to
attack angle, even if the wing is at zero angle of attack. See:

Physics of Flight - reviewed
http://www.rz.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

K. Weltner homepage
http://www.rz.uni-frankfurt.de/~weltner/


b) If yes then why isn't there any discussion of this portion of the
lift.

You mean discussion in the current threads, I presume? I think it's
because all parties agree on this point, therefor we haven't been debating
it. Only our *differences* cause strife. Identical twins with differing
tiny birthmarks who hate each other because of their differences.



If there is a pressure difference (lower on top) then
looking at the vertical components of the RANDOM motion
of the air molecules there would be more collisions with the bottom
of the wing (each molecular collision imparting some momentum to
the wing) than the top?
i) How big is this effect (this is what is generally calculated in
texts, but the velocity differences given may not be
realistic)?


It is large enough to support the entire weight of the plane. It should
also grow and shrink momentarily as the plane accelerates vertically to go
into a climb or a dive.


ii) What percentage of the lift can be attributed to this
effect--perhaps a lot for some craft and almost nil for others?


No. No, because 100% of the lift is explained by the pressure
difference. 100% of the lift is also explained by direct application of
Newtonian mechanics to the air-parcels which interact with the wing.
When both "Bernoulli" and "Newton" are done correctly and all their errors
are removed, either one can be used to explain exactly 100% of the lifting
force. There is no separate "bernoulli component" or "newtonian
component" in the lifting force.


2) Anyone who has flown commercially knows that in flight the nose of the
plane is up thus making
it easy to buy into the SIMPLE 'angle of attack' view that the bottom
of the wing deflects air
downwards and from N3 the wing must therefore be deflected upwards.

Plots in textbooks of the pressure distribution on wings show that a major
portion of the lifting force is caused by a lower pressure on the UPPER
surface of the wing. If we only look at the bottom surface of the wing,
we are ignoring the main part of the lift. On the other hand, the wing
doesn't know this, the wing only "feels" the pressure DIFFERENCE. In
terms of pressure difference, the upper and lower surface of the wing
contribute equally. In terms of comparison to static pressure of the
environment, the upper surface seems to be far more important. However,
if a wing is generally wedge-shaped like the very crude cross-section I've
drawn below, then note that only the upper surface has an "attack angle",
while the lower surface is NOT tilted. If the upper surface deflects air
downwards but the lower surface does not, we would expect upper/lower
absolute surface-pressures to be two different values, rather than an
equal positive and negative excursion from 1 atmosphere.

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I'm not clear if the Newtonists
are using this effect or go along with the Decker view that the
important downflow of air happens
behind the trailing edge of the wing. However, it is the direct
'bouncing' of the air off the bottom of the
wing that (I think) students can most easily see. How important?

The idea that lift occurs because air bounces off the bottom of the wing
is a typical misconception that children aquire (I myself figured it out
as a kid from experimenting with hands out the car window, also by analogy
with sprayers on hoses deflected from the hand.) In reality, most of the
lift appears because the flow of the air "attaches" to the upper surface
of the wing and is guided downwards because the trailing edge of the wing
is angled downwards. It's called "flow attachment", also "the Coanda
Effect." The air "bounces off" the upper surface of the wing, but the
"bounce" is reversed, it is like an attraction, and when the air is thrown
down, the wing must experience an upwards-directed force.

There was a very old article in Scientific American about Coanda
Effect. Fascinating. They had built a small flying saucer which flew by
blowing air radially outwards across the upper dome of the "saucer". Years
later I realized that this was just a fancy way to shoot a jet of air
downwards from the entire edge of the saucer. In other words, Bernoulli
and Newton get married and have "lightbulbs" for children. The first one
is named "AHA!"

:)



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