Hmm. REALLY not understanding circulation

[xerj]

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?

[Larry Dighera]

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

[Stefan]

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

[Roger Long]

Discussion and animated graphic of circulation here:

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?
>

[xerj]

> 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.

[xerj]

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.

[xerj]

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 here:
>
> 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?
>>
>
>

[Roger Long]

> 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

[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

[Peter Duniho]

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

[xerj]

> 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.

[Roger Long]

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

[Tony]

Here's a little experiment that will be eye opening. Drive at 60 mph,
hold your hand out the window as though you were hand signaling for a
left turn (in the US, at least) with your palm facing downward.

OK, now the leading (thumb) edge up. What do you feel? Is your hand
being sucked upward, is the skin on the back of your hand being bowed
outward?

Or, are you feeling air ram into the palm of your hand, pushing it
upward?

If it feels as though it's sucked upward, the 'low pressure on top of
the wing' model makes a lot of sense. If it feels like it's being
pushed upward, you might consider conservation of momentum models,
where the stream of air is being diverted downward by your hand, and
that old saw about for ever action there's an equal but opposite
reaction comes into play.

OK, your hand doesn't have the contour of a wing. What do you think
you'd feel if you made the leading edge take on a wing like curve? I'd
bet, the same thing.

Finally, think about how some airplanes with 'correctly' shaped wings
fly upside down, and ask you that could be. But mainly, remember your
hand being pushed up, not being sucked up, when you put it in a fast
moving airstream.

[Roger Long]

> If it feels as though it's sucked upward, the 'low pressure on top
> of
> the wing' model makes a lot of sense. If it feels like it's being
> pushed upward, you might consider conservation of momentum models,
> where the stream of air is being diverted downward by your hand, and
> that old saw about for ever action there's an equal but opposite
> reaction comes into play.

Hogwash warning.

The contribution of the top of the wing to airflow deflection (which
is considerable) is subject to conservation of momentum (energy,
whatever) just as is diversion by the bottom. EVERYTHING and every
diversion of flow is subject to conservation of energy.

There is NOT a low pressure Bernoulli model for what is taking place
on top of the wing and Newton related equal and opposite reaction
model for what is happening on the bottom. There is NO controversy,
except in the minds of the uninformed in places like newsgroups, as to
which model better describes lift. There is only an overall flow
pattern set up around the airfoil, every aspect of which is governed
by conservation of energy. Newton's laws and Bernoulli's theorems are
each just tools for explaining and predicting various aspects of what
is happening.

The endless Newton vs Bernoulli threads are as pointless as arguing
about whether hammers or saws are more important to the construction
of a house.

--

Roger Long

[Tony]

Not quite. The stall warning simply is reporting that the stagnation
line has moved below the stall tab installation point. It's at some
greater angle of attack that most of the wing stalls. Otherwise, you
wouldn't need the warning, the stall would be warning enough.

[Peter Duniho]

"Tony" > wrote in message
ups.com...
> Not quite. The stall warning simply is reporting that the stagnation
> line has moved below the stall tab installation point. It's at some
> greater angle of attack that most of the wing stalls. Otherwise, you
> wouldn't need the warning, the stall would be warning enough.

How is this different from what Roger posted?

[Tony]

Roger said
At high angles of attack, it can get far enough
back that air does flow forward across the wing.

indicating that is what lifts the stall warning tab.

That's pretty deep into a stall. In airplanes I've flown the tab lifts
at high angles of attack, but I doubt very much the wing is very much
stalled. The horn sounds well ahead, and at lower angles of attack, a
stalled wing. For sure, though, the stagnation line had moved below the
tab, lifting it.

[Peter Duniho]

"Tony" > wrote in message
oups.com...
> Roger said
> At high angles of attack, it can get far enough
> back that air does flow forward across the wing.
>
> indicating that is what lifts the stall warning tab.

I must be missing something. I read Roger's post simply to mean that as the
stagnation line moves back and down, the flow above the stagnation line
heads away from the stagnation line, toward the stall warning tab, causing
it to move and turn on the stall warning device (buzzer, light...whatever).

I also read your post to say the same exact thing. I'm not getting where
you and Roger disagree.

Inasmuch as the tab is mounted slightly below the leading edge of the wing,
the air has to be moving from a position behind the stall warning tab,
toward the leading edge of the wing. It seems to me that this is what Roger
wrote, and is also what you wrote (essentially).

I suppose Roger could jump in and clarify the disagreement. :)

Pete

[Roger Long]

I said that air moves forward across the wing which was clumsy wording
since it could easily be misinterpreted. Air near the wing surface
above the stagnation line moves from front to back so, if the
stagnation line has moved back under the leading edge, there can
actually be a small bit of flow with a forward motion relative to the
wing. This is very localized and only occurs very near the wing and
only at high angles of attack.

A stall warning tab could sense the stagnation line in one of two
ways. It can be blown up and forward by the reversed flow or it can
simply be spring loaded so that the switch is engaged when air flow
drops below a certain point as the stagnation line approaches. I'll
confess that I haven't looked at one closely enough to know which way
they are set up. They may even be different on different aircraft.

When I sail, (which seems to be more than I fly now) I have lengths of
yarn taped near the leading edge of the jib. If I get to too high an
AOA, the one on the "bottom" of the sail will start to point straight
up and even forward. Even though the sail is still pulling hard, the
stagnation line has moved well around to the windward side. There is
a slight drop in efficiency but no dramatic stall.

Pop quiz class:

Sails don't stall and suddenly lose their lift causing the sailboat's
heel to suddenly decrease. Can anybody tell us why? (Hint: Assuming
you had long enough landing gear to get to stall AOA while rolling
along the ground, you couldn't create the same kind of sudden loss of
lift that you experience in the air.)

--

Roger Long

[cjcampbell]

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.

[Peter Duniho]

"Roger Long" > wrote in message
.. .
> [...] if the stagnation line has moved back under the leading edge, there
> can actually be a small bit of flow with a forward motion relative to the
> wing. This is very localized and only occurs very near the wing and only
> at high angles of attack.

That's what I thought you said. And I think it's the same thing Tony wrote.

> A stall warning tab could sense the stagnation line in one of two ways.
> It can be blown up and forward by the reversed flow or it can simply be
> spring loaded so that the switch is engaged when air flow drops below a
> certain point as the stagnation line approaches. I'll confess that I
> haven't looked at one closely enough to know which way they are set up.
> They may even be different on different aircraft.

I haven't gone around inspecting the electrically operated stall warning
tabs, but all of the ones I've seen (on my own airplane, and on the larger
Cessnas) have been the former: a switch held down simply by gravity (and,
while the airplane is in motion, by the relative wind), blown upward as the
stagnation line moves below it. No springs involved.

Pete

[George Patterson]

Peter Duniho wrote:
>
> I haven't gone around inspecting the electrically operated stall warning
> tabs, but all of the ones I've seen (on my own airplane, and on the larger
> Cessnas) have been the former: a switch held down simply by gravity (and,
> while the airplane is in motion, by the relative wind), blown upward as the
> stagnation line moves below it. No springs involved.

That's the way the Maule switch works.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

[xerj]

Wow, Todd.

Very, very comprehensive. Thanks a lot for taking the time to step me
through it, and everyone else who did the same. I'm working over the
material, and it is finally starting to make sense.

The lightbulbs are coming on!

Once again, many thanks.