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