Hmm. REALLY not understanding circulation

Woops. This paragraph was supposed to read:-

The gap in understanding is now exactly *why* air pressure is lower and
speed is faster above the wing. I accept that air that goes over the top of
a wing with a positive angle of attack is sped up, and can grasp how this
creates pressure differences that results in lift.

Bernoulli

Saying that lift is caused by Bernoulli is like saying that moving a
stack of boxes is caused by the compression of flesh against
cardboard. It's an accurate description of a physical aspect of the
process, it's essentially to fully understanding what is going on, it's
a true in a very limited way, but, when you put that "cause" concept
in there, it is very, very, misleading.

Think of air as a spring. Its pressure is a form of stored energy.
(Remember that all the air we experience is kept in that potential
energy state by gravity.) Fill a container with air on the ground and
take it up to 18,000 feet and open a valve. Air will rush out and can
spin a little fan. It also takes energy to compress air.

Bernoulli's principle is just a subset of the law of conservation of
energy. Start a flow of fluid over a obstacle that changes the
direction and speed of the flow. For the principle to remain valid,
there must be no other energy inputs or drains from the system. This
is a key and seldom recognized point. It is also never true in the
real world.

The mass and velocity of the fluid at the beginning of the region in
which you are going to measure speed and pressure changes represents a
quantifiable amount of energy. It takes energy to make any part of the
flow speed up. For the total energy to remain constant, there has to
be a corresponding reduction in energy somewhere else in the system.
That reduction comes from pressure. Pressure and velocity remain in
balance.

If energy is added or subtracted from the system locally, the balance
predicted by Bernoulli does not need to be maintained. For example, it
the air is speeded up by a row of little engines and propellers,
pressure will not fall. Conversely, and here is where it falls apart
in the real world, if energy is drained out by the fluid being warmed
up due to friction, the energy taken out in the form of heat will not
need to be balanced by a corresponding rise in pressure.

The flow around an airfoil which results in lift creates a condition
in which flow increases above the wing and slows below it. Bernoulli
predicts that this will result in a pressure differential. Because of
the symmetries required by conservation of energy, the pressure
differential will be equal to the weight of the aircraft. To say that
this is the lift is where it is usually explained in a misleading way.
It is just a true (or false depending on your point of view) to say
that the pressure differential is the result of the lifting process.


--

Roger Long

Thanks Roger...good post. I do have to nitpick one little thing:

"Roger Long" wrote in message
...
[...] Because of the symmetries required by conservation of energy, the
pressure differential will be equal to the weight of the aircraft.

Two problems:

1) Pressure is measured as a force over area. Pounds per square inch, for
example. Lift is measured as a force. The two cannot be "equal"...they
aren't the same kind of measurement.

2) Perhaps you meant to say that the pressure differential will be equal to
the weight of the aircraft divided by the area of the wing. However, that's
only true during unaccelerated flight. The airplane regularly is flown so
as to create more or less lift than the weight of the airplane; it's an
essential component of maneuvering.

None of this, however, takes away from your nice way of presenting in a
simplified way, why it is that the air speeds up over the top of a wing.

Pete

None of this, however, takes away from your nice way of presenting in a
simplified way, why it is that the air speeds up over the top of a wing.

I'm probably driving everyone who understands this crazy by now, but I'm
still not getting it. Every time I think I am, I challenge myself to explain
it to myself, and I fall down at the same point. And the point is ~still~
why the air speeds up over the wing at a positive angle of attack, and gets
faster as the angle of attack gets greater.

I grasp the concept that higher speed leads to lower pressure. That is
settled.

What is eluding me is the reason why the pressure is lower above the wing. I
answer it by saying "the air is faster", but that brings me back to the
question: "why does a wing oriented at an angle of attack make the air go
faster?". Obviously I can't answer it with "because it lowers the pressure",
because that would just cause me to ask "why does it lower the pressure?"
which we would be answered by "because the air is going faster", which gets
me back to the start. This is the short-circuit in my understanding at the
moment.

Roger's description here seems to make sense to me:-

http://www.avweb.com/news/airman/183261-1.html

"Since the wing is at an angle, its movement also tries to sweep out a space
behind the top. The inertia of the air that goes over the top of the wing
tries to keep it moving in a straight line, while the pressure of the
atmosphere tries to push it down towards the wing's surface. The inertia
prevents the atmospheric pressure from packing the space as firmly as it
would if the wing were standing still. The result is a low-pressure region
above the wing. Air rushes from high- to low-pressure regions, from the
high-pressure area ahead of and below the wing into the low-pressure space
being swept out above and behind it."

...... as does this:-

http://www.allstar.fiu.edu/aero/airflylvl3.htm

"So how does a thin wing divert so much air? When the air is bent around the
top of the wing, it pulls on the air above it accelerating that air down,
otherwise there would be voids in the air left above the wing. Air is pulled
from above to prevent voids. This pulling causes the pressure to become
lower above the wing. It is the acceleration of the air above the wing in
the downward direction that gives lift."

Are these enough to rely on to give a broad overview of what is going on
above the wing?

I actually think I do understand a fair bit of the stuff "downstream" of
this point. I'm pretty sure that once the above concept clicks into place.

Thanks in advance. I hope I'm not making anyone head butt their keyboards in
frustration.

The problem is that you are beating your self up with
which-came-first-the-chicken-or-the-egg type thinking. The answer to
that conundrum lies in being able to step back and look at the larger
picture of evolution.

You can't figure this out going "ta duh tee dee da dum" either. All
the aspects of flow adjust simultaneously to satisfy the requirement
for energy conservation. Whenever the idea of "cause" intrudes into
your thinking squash it. Instead look for the symmetries that create
the balance that conserves energy and follow the energy flow.

You can't have a change in flow with out changes in pressure. You
can't have pressure differentials without flow (in an open system).
All of these things being discussed are just aspects of what happens
when a flow is set up in such a way that energy conservation requires
an upwards force (lift) equal to the downward pull of gravity. None
of them are "causing" any of the others. They are all caused by need
for energy to be conserved.

It is very simple and elegant once you can free your mind up from
plodding point to point and can just see the whole picture.

Why does the air speed up on top of an airfoil? Because none of the
other requirements for lift to be produced could be happening together
unless it was.

--

Roger Long

xerj wrote:
None of this, however, takes away from your nice way of presenting in a
simplified way, why it is that the air speeds up over the top of a wing.

What is eluding me is the reason why the pressure is lower above the wing. I
answer it by saying "the air is faster", but that brings me back to the
question: "why does a wing oriented at an angle of attack make the air go
faster?".

There are three types of air pressure. Total pressure is the total
force exerted by all the air molecules in all directions. It is
measured by a barometer. Dynamic pressure is the force exerted by
molecules that have been accelerated in a particular direction. Blow
air against your hand and you will feel dynamic pressure. Dynamic
pressure is measured by a pitot tube. Static pressure is measured by
the Bernoulli principle. It is the pressure of the remaining molecules
that have not been accelerated in a particular direction. Static
pressure is measured by your static port.

Total pressure at any altitude must remain constant and it must equal
the total of dynamic pressure plus static pressure. If you increase
dynamic pressure in one direction by moving air over a wing, blowing it
through a tube, against your hand, whatever, then static pressure must
be reduced in order to keep total pressure constant. A wing increases
dynamic pressure from front to back over the top, so static pressure is
reduced. At the same time, the positive angle of attack on the bottom
of the wing reduces dynamic pressure from front to back, so static
pressure on the bottom is increased. This increase in pressure
differential is not enough to account for total lift, however.

The reduced static pressure above the wing also causes air to move down
toward the wing, much as the reduced static pressure of water flowing
through the nozzle of a garden hose sprayer forces fluid up and out of
the bottle and into the nozzle. This air is caught up in the flow of
the boundary layer and is also forced off the trailing edge of the wing
and down. This actually generates most of the lift -- as air is forced
down toward the wing and then down off the trailing edge, there must be
an opposite and equal reaction and the wing is forced upward. You can
see the air is forced down by the wing by watching an airplane fly over
a cloud or smoke, or low over water. The downward moving air creates a
canyon in the cloud or ripples on the water almost directly under the
airplane.

Air is accelerated over the top of the wing because the top is curved.
Air molecules are forced upward by the leading edge, but because air is
slightly sticky (viscous), it sticks to the wing rather than just
continuing up. It is like holding a water glass sideways under a
faucet; the water instead of just dropping straight down off the side
of a glass instead follows the glass all the way around to the bottom
and then falls off.

Really, an airplane is nothing more than a fan blade. Instead of
whirling around, it moves straight through the air. Air is drawn from
above the wing and forced down behind it, just like a fan blade forces
air through it. People get a little confused by watching wind tunnel
streams, because in a wind tunnel the air is moving and the wing is
stationary, so instead of moving down off the trailing edge the air in
a wind tunnel is blown straight behind the wing.

Instructors like to demonstrate the Bernoulli principle by blowing over
the top of a sheet of paper. The paper rises, demonstrating lift. What
instructors don't usually do, though, is blow under the paper. The
paper still rises, demonstrating lift.

As the angle of attack increases the boundary layer begins to separate
from the wing, causing turbulence instead of lift, but lift continues
to increase because of the greater lift coming from the area towards
the leading edge. At some point, though, the angle of attack becomes so
great that there is not enough lift to overcome the turbulence from the
separating boundary layer and the wing stalls.