Hmm. REALLY not understanding circulation

Every time I think I have it sort of worked out, I fall in a heap. I've read
through "See How It Flies", but it's still a bit beyond me.

I can understand why a spinning ball would create a circular flow around
itself, but I can't really grasp why a wing would.

Does anyone have a lay explanation as to why?

On Thu, 15 Sep 2005 12:23:38 GMT, "xerj" wrote in
::

Every time I think I have it sort of worked out, I fall in a heap. I've read
through "See How It Flies", but it's still a bit beyond me.

I can understand why a spinning ball would create a circular flow around
itself, but I can't really grasp why a wing would.

Does anyone have a lay explanation as to why?


Perhaps this will help: http://aerodyn.org/summary.html

xerj wrote:

I can understand why a spinning ball would create a circular flow around
itself, but I can't really grasp why a wing would.

There's nothing to grasp, because there is no circular flow.

However, there is the fact that the air flows faster on the upper side
of the wing than below, and it flows even so much faster that it arrives
earlier at the trailing end of the wing. Mathematically, really only
mathematically, this can be described by the overlay of two independant
flows, a straight one and a circular one. But this is strictly a
mathematical model, there really is no circular flow.

I think this "circular flow" thing is a good example of a inadequate
explanation to laypersons. It's a mathematical method and doesn't help
the least bit for a layperson to understand what's going on. It simply
shouldn't appear in basic, purely qualitative explanations.

Stefan

Discussion and animated graphic of circulation he

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

--

Roger Long



"xerj" wrote in message
...
Every time I think I have it sort of worked out, I fall in a heap.
I've read through "See How It Flies", but it's still a bit beyond
me.

I can understand why a spinning ball would create a circular flow
around itself, but I can't really grasp why a wing would.

Does anyone have a lay explanation as to why?

The simplest "lay explanation" answer is that a wing does
*not* create a circular flow around itself, at least not in
exactly the same sense as the spinning ball. The air
molecules approach the wing, and flow over either the top
or the bottom. No air molecules ever actually go over the
trailing edge and forward along the underside of the wing
and then back up over the leading edge. The closest thing
to this is: Since air pressure is quite low on top of the
wing, and high below the wing, some air molecules will be
sucked from near the front bottom of the wing and up over
the top (that's what triggers the stall warning vane).
However, air never really spins completely around the wing.

OK, that last bit makes sense to me.

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 positive angle of attack wing with a positive angle of attack is sped up
and, and can grasp how this creates pressure differences that results in
lift.

At first I tried to explain it to myself that the air isn't actually sped up
above, that what has happened is that air below is slowed down due to an
obstacle effect so the air above is just faster in relation to the air
below, but this can't be right because the pressure patterns of a wing
generating lift show a decreased pressure in relation to ambient pressure
rather than just an increased pressure below.

So I arrive at my current quandry. What influences the air molecules that do
go above the wing to speed up? What forces are acting on it? Is it a case of
"speeding up first, reducing pressure, therefore pulling air over the front
of the wing above the stagnation line" or "reduced pressure, increasing
speed of the air above the stagnation line"?

I'm sure this all seems so ridiculously easy to those of you who understand
it, but there's just a lightbulb that hasn't gone off in my head yet.

Thanks in advance.

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.

Thanks, Roger.

I've seen your page before, but the thing that has confused me about it and
especially that circulation diagram is what would happen to a tab-style
stall warning device installed at the front of the wing in that diagram.
Wouldn't it be going off all the time?

"Roger Long" wrote in message
...
Discussion and animated graphic of circulation he

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

--

Roger Long



"xerj" wrote in message
...
Every time I think I have it sort of worked out, I fall in a heap. I've
read through "See How It Flies", but it's still a bit beyond me.

I can understand why a spinning ball would create a circular flow around
itself, but I can't really grasp why a wing would.

Does anyone have a lay explanation as to why?

I've seen your page before, but the thing that has confused me about
it and especially that circulation diagram is what would happen to a
tab-style stall warning device installed at the front of the wing in
that diagram. Wouldn't it be going off all the time?

The diagram doesn't show the flow at fine enough scale to infer that
kind of thing. Even though there is a general upward movement of air
ahead of the wing it isn't a clockwise flow relative to the wing.
There is a line at the leading edge called the stagnation line. The
air that hits the wing above this line goes over and the air below the
line goes under. As angle of attack increases, the stagnation line
moves back and down. At high angles of attack, it can get far enough
back that air does flow forward across the wing. This lifts the stall
warning tab and sets of the horn. This is related to but is not the
same thing as circulation.

--

Roger Long

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