View Single Post
  #86  
Old October 4th 07, 02:29 AM posted to rec.aviation.piloting
[email protected]
external usenet poster
 
Posts: 1,130
Default Backwash Causes Lift?

On Oct 3, 6:39 pm, Dudley Henriques wrote:

The main thing about all this is that both Bernoulli and Newton are
complete explanations of lift and will stand alone. In fact they are
both explanations of the same thing really as they occur simultanously
as lift is being created.
The big rub about Bernoulli is that for years Ole' Daniel was raped by
text books stating several totally false applications of Bernoulli as
fact. The equal transit theory for example, often stated as an
explanation for Bernoulli is totally incorrect.


Exactly. The equal-transit theory isn't correct. The air
over the top actually reaches the trailing edge *before* the bottom's
flow. Intuitive thinking would have it arriving later because the
distance is greater.
Our OP should see the diagrams he
http://www.av8n.com/how/htm/airfoils.html

This one shows the pressure distribution over the typical cambered
airfoil:
http://www.kemi.fi/kk019065/calculators/ClarkY.jpg

Note that there's pressure acting on the bottom. Where would
that come from, if not Newton? And note that Bernoulli runs out of
steam on the top near the trailing edge, and the pressure actually
goes above ambient there. I see this on the wing of my Jodel in
flight. It's a low wing, fabric covered, and the pressures are easily
visible by the way the fabric is pressed down or pulled up between the
ribs. Over about the last third of the chord, the fabric is pushed
below the ribs as the pressure there goes quite positive, while ahead
of that it's pulled up.
Look at that leading edge. Lots of lift over the first bit,
right where we'd expect a lot of drag (positive pressure) instead.
Not at all what you'd expect intuitively, is it? And that's
where the uninformed get into trouble: by using "experience' gained
from other, vastly different things, or from reasoning based on
inadequate information.

After all the years of reading this stuff and seeing wind-
tunnel demos and graphs and all such, I know there's an awful lot of
information out there on the generation of lift. Most of it is
available on the 'net. The strangest thing is the newbie who starts to
argue with his textbooks, very publicly (as on a newsgroup) without
Googling it for himself first. He knows better, he's sure.

Dan