Richard Lamb wrote in
nk.net:
TRUTH wrote:
"Jim Macklin" wrote in
news:uX8Lf.104268$4l5.39451@dukeread05:
It was sad and never should have been allowed to happen.
Some people knew what was planned, many escaped, but the
world was unwilling to stop Hitler. Hope we don't make the
same error again.
Anyone here familiar with the Bush family/ Nazi connection???
"How does a wing generate lift?"
Though this seems like a simple enough question, the general public
would probably be amazed to find out that engineers and scientists
still debate just how lift is produced even 100 years after flight
became a reality.
In fact, it is quite easy to be drawn into charged debates on the
subject, as I was when trying to answer this question. So, to be fair
to the proponents of each theory, I will discuss each in turn. But
first, let us simplify our discussion slightly by thinking of the wing
as only a two-dimensional shape.
Consider the cross-section of a wing created by a plane cutting
through the wing. This two-dimensional cross-sectional shape is called
an airfoil (or aerofoil to our British friends). An example of a
common airfoil shape is the Clark Y.
Bernoulli theory:
The most common explanation of the concept of lift is based upon the
Bernoulli equation, an equation that relates the pressures and
velocites acting along the surface of a wing. What this equation says,
in simple terms, is that the sum of the pressures acting on a body is
a constant. This sum consists of two types of pressures: 1) the static
pressure, or the atmospheric pressure at any point in a flowfield, and
2) the dynamic pressure, or the pressure created by the motion of a
body through the air. Since dynamic pressure is a function of the
velocity of the flow, the Bernoulli equation relates the sum of
pressures to the velocity of the flow past the body. So what this
equation tells us is that as velocity increases, pressure decreases
and vice versa.
To understand why the flow velocity changes, we introduce a second
relation called the Continuity equation. What this relationship tells
us is that the velocity at which a flow passes through an area is
directly related to the size of that area. For example, if you blow
through a straw, the air will come out at a certain speed. If you then
blow in with the same strength but now squeeze the end of the straw,
the air will come out faster.
So how do these equations relate to our two-dimensional airfoil? Look
again at the Clark Y and notice that an airfoil is a curved shape.
While the bottom is relatively flat, the top surface is thicker and
more curved. Thus, when air passes over an airfoil, that flow over the
top is squeezed into a smaller area than that airflow passing the
lower surface. The Continuity equation tells us that a flow squeezed
into a smaller area must go faster, and the Bernoulli equation tells
us that when a flow moves faster, it creates a lower pressure.
Thus, a higher pressure exists on the lower surface of an airfoil and
a lower pressure on the upper surface. Whenever such a pressure
difference exists in nature, a force is created in the direction of
the lower pressure (since pressure is defined as force per unit area).
Think of it as the upper surface being sucked upward. This upward
force, of course, is lift. It is this theory that appears in most
aerodynamic textbooks, albeit sometimes with incorrect assumptions
applied and conclusions drawn.
Newtonian theory:
A theory currently gaining in popularity and arguably more
"fundamental" in origin is the Newtonian theory, so named because it
is said to follow from Newton's third law of motion (for every action
there is an equal and opposite reaction). First, one most realize that
any airfoil generating lift deflects the air flow behind it. Positive
lift deflects the air downward, towards the ground. Thus, the motion
of any lifting surface through a flow accelerates that flow in a new
direction. Newton's second law tells us that force is directly
proportional to acceleration (F=ma). Therefore, we must conclude from
Newton's third law that the force accelerating the air downward must
be accompanied by an equal and opposite force pushing the airfoil
upward. This upward force is lift.
Circulation theory:
The most mathematical explanation for lift is the circulation theory.
Circulation can be thought of as a component of velocity that rotates
or swirls around an airfoil or any other shape. In a branch of
aerodynamics called incompressible flow, we can use potential flow
relationships to solve for this circulation for a desired shape. Once
this quantity is known, the force of lift can be solved for using the
Kutta-Joukowski theorem that directly relates lift and circulation.
This approach tends to be more mathematically intense than I wish to
get into here, and it's really more of a method of calculating lift in
an ideal flowfield than an explanation of the physical origins of
lift.
Conclusion:
So the reader may be asking which of these theories is correct?
In TRUTH, each is valid in some respect and useful for certain
applications, but the ultimate question is which is the most
fundamental explanation.
Mathematicians would surely prefer the circulation theory, which is
certainly a very elegant approach firmly based on mathematical
principles, but it fails to explain what force of nature creates
circulation or lift. Many would argue that the Newtonian explanation
is most fundamental since it is "derived" from Newtonian laws of
motion. While this is true to some degree, the theory lacks an
explanation as to why an airfoil deflects the flow downward in the
first place. Even accepting this principle, the idea that an airfoil
deflects the flow and therefore experiences lift also fails to capture
the fundamental tools of nature (pressure and friction) that create
and exert that force on the body.
Proponents of this explanation generally deride the Bernoulli theory
because it relies on less fundamental concepts, like the Bernoulli and
Continuity equations. There is some truth to this complaint, and the
theory may be more difficult for the novice to understand as a result.
However, both equations are derived from Newtonian physics, and I
would argue from more fundamental and more mathematically sound
premises than the Newtonian theory.
In the end, I leave it up to the reader to decide.
Attrib:
http://www.aerospaceweb.org/question...cs/q0005.shtml
But those statements do not apply to controlled demolitions at the WTC
from Jones paper:
Those who wish to preserve fundamental physical laws as inviolate may
wish to take a closer look. Consider the collapse of the South WTC Tower
on 9-11:
http://www.911research.com/wtc/evide..._collapse.mpeg
We observe that approximately 30 upper floors begin to rotate as a block,
to the south and east. They begin to topple over, as favored by the Law
of Increasing Entropy. The torque due to gravity on this block is
enormous, as is its angular momentum. But then – and this I’m still
puzzling over – this block turned mostly to powder in mid-air! How can we
understand this strange behavior, without explosives? Remarkable,
amazing – and demanding scrutiny since the US government-funded reports
failed to analyze this phenomenon. But, of course, the Final NIST 9-11
report “does not actually include the structural behavior of the tower
after the conditions for collapse initiation were reached.” (NIST, 2005,
p. 80, fn. 1; emphasis added.)