RE: Hi Hal
 

This is an aerodynamics forum, and this thread is titled "Bernoulli's Equation".

I'm sorry you don't enjoy this particular thread - but it's not like this is the only thread in the aerodynamics forum. You might want to consider one of those, rather than suggesting that those of us enjoying this one are out of place. I actually find your postings more disquieting than the ones by those who disagree with my theories on what causes lift. At least they are on topic, and they usually help me to elevate my knowledge - whether they agree with me or not. Your posting though, haven't increased my knowledge. At least not yet. Why not join in with your thoughts on lift - you certainly can't be any crazier than the rest of us.

RE: Hi Hal
 

sorry about my post. get alittle carried away when things drift from balsa shaving. have tried to forget the all the mathematics when I retired from it. new ventures and models have taken its place. Retirement is nice. dick

RE: Comments
 

(Dick, consider yourself forewarned. This is a long ramble on the esoteric field of aerodynamics, and if you have no interest, just don’t read it.)

Shoe, I can identify with Ken who is tired of this thread, however you raise some good questions and they merit a thoughtful response. We may be on the verge of writing a sequel to “The Never-Ending Story” but here goes.

I certainly hope that that is not the case. It’s not my purpose to “win” some argument or push some point of view. Rather it’s my intent to help expand our understanding of what goes on in the physical world in the fascinating area of heavier-than-air flight. Before getting into my response, I will revisit the term “significant” that we discussed before.

Many people will stoop to pick up a penny from the sidewalk. Even if that person has an income well below the poverty line most would not consider that penny as having a significant effect on their financial condition. The fact that something is real and measurable does not necessarily mean that it is significant. To be significant something should be important, or consequential. In practical engineering, it is perfectly legitimate to ignore the “penny” stuff.

As a wing moves (at subsonic velocities) through an unbounded volume of air it affects the air in its vicinity in several ways. It creates disturbances that move before it telegraphing its approach. Like the heralds before the king’s carriage, it says, “Get out of the way. A wing is coming through.” As these signals press ahead, some of the air molecules panic and pile up at the stagnation point (resulting in a rise in pressure) before being pressed on around. Others are more cooperative moving up or down, following paths that have been abundantly described in previous posts. Near the wing tips still others sweep sideways and up to get out of the way. How far these signals go before they are no longer effective depends on several factors such as the speed of the wing and how streamlined or blunt it is. Recognizing that theory may see some “pennies” to the limit of the volume, the significant effect is limited to the immediate vicinity of the wing. Several chord lengths forward and about a semi-span above, below and sideways are the generally observed limits.

Now to address your first comment regarding our last exchange, you said,

It is only true that some of the air is deflected upwards and that only within the immediate vicinity of the wing. That upward deflection is the beginning of the trailing vortex and it is in the moment of the wings’ passage that the swirl is begun. Effect on the air beyond that is “pennies”, if it exists at all. Within the wings’ area of influence, there is a net downward deflection as described before.

True, the theoretician is free to choose any control volume he may desire, but if it’s other than the volume actually affected, the results are not meaningful.

Now let’s consider the volume of air after the wing has past. The wing sent ahead disturbances warning of its imminent arrival. As the wing passes it splits the air by its physical presence pulling and twisting it, starting motions that remain well after the wing has past. However once the wing has past, there remains no mechanism wherewith it can still move the air around the wing. The air simply proceeds to move toward the equilibrium it had before the wing so rudely interrupted its peace. The motion behind the wing, though it may be furious just after the wing has past, is in the process of dissipating, and is no longer capable of interacting with the wing.

The photos clearly show the extent of the area affected by a wings passage, and shows the air behind already moving back into the space it originally occupied. (By the way, the picture was not just a fortunate opportunity, but was part of a systematic study of downwash by Cessna, and used a B-25 for a camera plane.)

Now I will address your comments concerning ground effect. When a fixed boundary is introduced within the vicinity of the wing, it greatly alters the way in which the wing disturbs the air. Other than the flight of the droid fighters in the crevices of the death star, I can’t imagine a situation where a solid surface intrudes from the sides, and only in some wind tunnel setups could one be above the wing. But in the process of take off and landing, the earth forms a solid boundary well within the area of influence of the wing. Before going into a discussion of ground effect, consider an aircraft toward the end of a takeoff roll. There is a point where much of the weight is supported by the wing, but a little is still being supported by the landing gear. I’ll come back to this concept.

Theoreticians postulate a “mirror image” of the horseshoe vortex bound to the wing. Combining the actual velocities with the reflected velocities from the mirror vortex gives good correlation with actual data. However it does little to describe the physical changes in motion that cause the differences. It is obvious that the mirror vortex is just a mathematical fiction since it can’t really exist under the ground. I will attempt to describe how it actually takes place.

When operating in ground effect, there are two primary changes in the wings effect on the air. The motion at the tips is affected such that the air that would be moving mostly downward in an unbounded volume is forced to move more outboard. This has the effect of moving the tip vortices outboard increasing the effective span of the wing. The wider effective span effects a larger mass of air so it doesn’t have to deflect it as much to produce the same lift.

The disturbances sent out from below the wing, announcing its arrival, instead of dissipating as usual are reflected back by the surface of the ground causing the pressure to increase between the lower surface of the wing and the ground. The air is “squeezed” in this area forming a “cushion”. Since the ground is a solid unmovable surface against which the pressure can react, this cushion supports a little of the weight much like the landing gear above. These two phenomena describe the physical basis for ground effect.

Significantly it all takes place before and during the wings passage. Once the wing has past it no longer effects the air. Of course the air continues to move as it returns to a state of equilibrium, much like a windup toy runs down after it is set in motion.

This is a little misleading. In a wing centric view (during steady state flow) it may appear that it is true. And for steady state flow it is useful to assume that it is so for ease of handling computations. However it takes a finite time for a wing to pass, and different points in the “flow” are separated by time. What is seen as flow ahead of the wing is happening before the wing arrives, and what is seen as flow after the wing passes is the result of forces applied earlier and is not what is happening now. Subsonic or sonic, there is just no mechanism by which the motion of air after a wing has past can influence the forces on the now gone wing.

If Harry Potter could work some of his wizard stuff and point his wand and cause the Citation to suddenly vanish, the path through the clouds would remain and the air would still move toward the stillness that was its original condition. It’s merely fulfilling the destiny that was established with the passing of the wing. In order for the wake to affect the wing, it would somehow have to travel back in time and change what the wing has already done. (The gun analogy is still valid.)

RE: Comments
 

I'll say it again... the gun analogy just plain does not hold in subsonic flow. Although I don't necessarily agree with the mechanisms you describe, you say:

I think you are suggesting that the ground modifies the shape of the wake (vortices), and therefore the forces on the wing. How can moving the tip vortices outboard affect the the forces on the wing if the gun-bullet analogy holds? The tip vortices don't form until the air has left the wing.

Can you provide any analysis (other than qualitative) to show that the lift on the wing is balanced by momentum transfer? I'll make the same request you made of me a few posts back: "Do it."

Actually calculate the rate at which a wing is transferring momentum to the air and show that is equal to the lift.

RE: Bernoulli's equation
 

OK this is fun stuff -
I am really surprised my own example of CG relevance on a weightless airframe was dismissed as idle chatter.
keep going --

RE: Comments
 

I am not sure if this has been mentioned before but the wing tip vortices will cease to exist when an airplane touches down on the runway or will not form until the a/c has left the runway.

RE: Comments
 

Lou, you once said, "That doesn’t alter the fact that the inertia of the mass of air above the wing resisting being pulled downward is as much the source of lift as the pressure pulling upward on the wing. Neither can exist without the other. "

Perhaps this is where I come to a halt when I read what you write. You say the pressure is pulling upward on the wing. It doesn't pull. The summation of pressures gives a net positive on the underside of the wing which is pushing up on the wing. The negative pressure on top of the wing does not pull. It is an important difference I would think.

When considering air, pressure can only push, not pull or suck. The mass of air above the wing is not resisting being pulled downward. It is resisting being pushed downward by the higher pressure in the far distance above the wing. The push downward on the air from far above is what is causing the downwash flow. Not the suction of the negative pressure above the wing.

If the air can only push then the only action on the aircraft wing or body is a push. Downwash is not causing a push on the wing or body.

I truly believe your thinking about the negative air space sucking the air downward and sucking the wing upward is biasing your outlook on this issue.


Dick back when you were talking about wind tunnels being different whether being pushed or pulled - in a continuous flow tunnel the fans are turned on. The inertia of the air makes the increase in speed of the circulating air slowly increase. Eventually you get the whole mass moving at the velocity you desire. The fans are adjusted to maintain this velocity. Typically you would like the fans downstream of the test section with enough tunnel distance in the remainder of the big rectangle of duct to have time to remove rotation and cross section velocities to the point that for all useful purposes the flow is smooth and steady. When the test section is finally reached you hope to have the flow very nice. At that point in time there is basically no difference in pushing or pulling on the air as it were. Actually pulling on the air is a misnamed action. You create a negative pressure with the fans and the air flows into the section as a result of the pressure differential.

Even in this case the high pressure upstream is pushing on the air into the low pressure regions.


Hal, are you sure about this? A wing tip vortex is caused by high pressure air flowing into the low pressure air on top of the wing (assuming some lifting action is taking place). As such it wouldn't matter if the airplane was on the ground or not, just whether or not the wing was producing lift. Now generally the wing is producing very little lift as it starts down the runway and is rotated to an angle of attack just prior to takeoff. That would cause the vortex formation to really take place at the rotation point which just happens to coincide with leaving the runway. Think about it.

RE: Bernoulli's equation
 

Ben - I really did not state my "what if" clearly ,re: pushing or pulling air over the wing.
I was not assuming a proper wind tunnel-
Rather, simply an open tube -fan at one end - atmosphere at the other.
It seems to me that creating a low pressure AFT the test section, would creat a more coherant flow.
On an unrelated matter --I was always very impressed with the design of the jet engine used in the V-1 and then modeled as the Dynajet.
This always seems to be genious at its
best .
Who came up with the concept?

RE: Comments
 

Ben,

To echo what you are saying, I think it's important to clarify some terminology. There's really no such thing as "negative pressure" - gas can only push against a surface. In the limit where the density of the gas is reduced to vacuum, the pushing goes to zero, but it never pulls. If you define a pressure coefficient: (local static pressure - free stream static pressure)/(dynamic pressure), then you can have "negative pressure coefficients", but not "negative pressures". Even pressure coefficients reach a lower limit when the local static pressure is zero. Under certain conditions, old-fashioned high altitude airplanes like the SR-71 actually relied on compression on the lower surface of the wing for lift.

RE: Comments
 

You and shoe are right of course. Everyone knows that air can't "pull" and The only "scientific" reference datum for pressure is the hard vacuum of interstellar space. It is obvious that since I choose to write in common English that anyone can understand, I couldn't possibly have anything important to say. There are religious persons that will only discuss religion with a lot of "thees" and "thous", lawyers that sprinkle "wherefores" and "party of the first part" throughout their writing, and engineers who insist on the stilted technical report writing style. In each case such jargon makes one appear above the great unwashed masses. I happen to think that most ideas can be expressed using standard English that is well understood by most English speaking people.

By the way, since we are addressing imprecise language you might consider dropping the description of flow, as in air flowing over a wing. Everyone knows that except in a wind tunnel air doesn't flow at all. It is stationary and the wing passes through it. But of course that's different............really?

You guys tickle me.

RE: Comments
 

LouW,

No offense was intended, but I think the terminology issue here goes beyond mere semantics. In common English, pressure is understood to mean force per unit area. The term "negative pressure" implies force in the direction of the surface normal (a pull force). I was only trying to reinforce Ben's suggestion that most gasses cannot support a force in the direction of the surface normal. The fact that engineers use pressure coefficient (which can be negative under normal conditions) clouds the issue. You are welcome to the opinion that the clarification was offered for the purpose of assuming a superior appearance. I give everyone who cares to read these ramblings credit for being able to separate meritorious content from superfluous vocabulary. I apologize if using the term "flow" was causing confusion.

RE: Comments
 

"Everyone knows that except in a wind tunnel air doesn't flow at all."
.
????
Local wind.... NE @ 12, Gust NNW @33

RE: Comments
 

P.S. Today I walked past some parked airplanes and the air was moving past them at about 25 kts. Apparently the air didn't get the memo that "except in a wind tunnel air doesn't flow at all". ;)

Fundamental Concepts and Misunderstanding
 

Lou - getting the terms right is important. You are erroneously thinking of the low pressure above the wing pulling the air downward and causing the downwash. This leads to the "downwash" theory of flight where downwash is emphasized.

Thinking of pressure always pushing where the resultant summation of the pressure field pushes the wing up leads to the "pressures on the wing is causing lift" theory of flight where the wing is emphasized.

It is not intellectual snobbery. It is fundamental and easy to understand by anyone. What causes lift? The air is moving faster over the top of the wing than the bottom. This action is creating a lower pressure on the top of the wing (Bernoulli effect). The lower pressure simply allows the high pressure under the wing to push in an upward direction giving lift. The air above the wing also rushes in to fill the negative pressure. The inertial characteristics of that moving air causes it to keep moving even when the wing has gone by and that is what we call downwash.

I fail to understand why the above is not easy to understand since it is the truth. There are no strange engineering snobbery stuff going on.

To take a non aero type who asks, "what causes lift", and give the answer that the downwash is causing the lift is simply not right.

To cling to the downwash explanation is simply bad engineering.


Dick - I am more impressed with the big tube bent pipes with no moving parts that act as pulse jets. You just squirt fuel into them and the characteristic length causes a pulsing thrust out the open end. I have forgotten what their name is but I saw them used on Junkyard Wars to get propulsion. Of course they turned red hot and were scary but they produced thrust.

RE: Fundamental Concepts and Misunderstanding
 

So close yet not quite there……’tis a shame’

When a jeweler is examining a precious stone with his glass, he moves it around to catch the light on different facets. The different perspectives reveal the ultimate beauty of the stone (and an occasional flaw). There are benefits to looking at common things from differing perspectives. However most of you all seem much more interested in a little game of trivial pursuit or getting an occasional “gotcha” than in a serious exchange of ideas.

Go ahead and play. I’m out of here.

RE: Fundamental Concepts and Misunderstanding
 

LouW,

Before you go, could you provide a quantitative analysis showing that the rate at which a wing transfers momentum to the air is equal to the lift on the wing? I think that is the ONLY way to make a convincing argument here, and it would help greatly in the exchange of ideas. Thanks.

I am ....
 

not playing games, or trivial pursuit or anything, not looking at a gem stone, not sniping out a sentence and going tsk, tsk.

What do you say to a beginner at the flying field that comes up to you and asks, "What keeps the airplane flying", "or what causes the wing to lift" or something similar?

Way back when, you said --

"The air going above is first pushed up sharply, then due to inertia tries to pull away from the upper surface and the local static pressure is decreased. The sum of all these changes to the local static pressure results in a net force on the wing. The vertical component is called lift and the horizontal component is called drag.

The resulting pressure changes not only act on the wing, but also act on the air surrounding this pressure field. The air a little more distant from the wing, flows toward negative pressure on top of the wing, and below the wing it is pushed a little further away. The overall effect of the passage of the wing and its surrounding pressure field is to accelerate a mass of air downward, and forward. The reaction to the downward acceleration of the mass is called lift and the reaction to the forward acceleration is called drag.

The pressure distribution around the wing is the proximate cause of lift (the Bernoulli side), but the ultimate source of lift is the acceleration of a mass of air (the Newton side). "

In the first paragraph you say, "The vertical component is called lift." referring to the sum of the local static forces.

That is exactly what I have been saying since day one. However the low pressure is not a result of the inertia of the air pulling away from the surface. Smoke tunnels clearly show the air follows the surfaces. The Canola effect I believe. The resulting flow, working like the Bernoulli effect, creates lower pressures.

Then you explain downwash properly and it is well done.

Then you go on to say the ultimate source of lift is the acceleration of a mass of air". But you and I both have just previously agreed - "The vertical component (of static pressures) is called lift."

Make up your mind. Everyone else has defined the vertical component of the pressure distribution as lift.

Most of us have not defined downwash or Newtonian reaction as lift. We call downwash --- downwash. We call the Newtonian reaction -- Newtonian reaction. We don't call them -- lift.

We call the vertical component of the static pressures around a wing -- lift.

Is that nit picking at words? Perhaps, but a lot of other writers of text books, profs, and philosophers have decided to define lift the same way - the vertical component of the static pressures around a wing. Isn't the resulting pressure field the ultimate source of lift?

RE: Fundamental Concepts and Misunderstanding
 

When beginners ask me that question, I usually respond with "money". ;)

RE: I am ....
 

The component of force on a body perpendicular to the free stream is by definition the lift (as long as you don't distinguish between lift and side force). If you prefer to work in an Earth-fixed coordinate system, lift is the component of force on a body perpendicular to its velocity (if there's no wind). Lift can ALWAYS be calculated by summing the pressures and shear stresses over a body and taking the "perpendicular" component. That's just Statics, not Aerodynamics, and I don't think anybody here disagrees.

If the air exerts an unbalanced force on the body then Newton's third law says that the body exerts an equal and opposite force on the air (call it F1). If this is the case, then Newton's second law says one of two things must happen to the air: either it must experience a rate of change of momentum, or it must experience a balancing force on its outer boundary. As Drela succinctly put it back on page 4: "Newton sez: F1 + F2 = Delta(M)". For a given F1, control volume size and shape determine the balance between F2 and Delta(M). That makes it kind of tough to say with any generality how much momentum the body does or doesn't transfer to the air. I think Ben's point is: Who cares? If you can always calculate the lift by summing the pressures and shear stresses on the body, why worry about control volumes and momentum changes? After all, the lift is the lift, and we just agreed that we can always calculate the lift.

I think I can make a case for why one might also care (or at least for why I care) about what happens to the air. If you're interested in the drag force on a body (the force parallel to the free stream), you can learn quite a bit by studying the body's wake. Roughly speaking, profile drag manifests itself in the wake as a combination of axial momentum and total pressure loss. Induced drag shows up quite differently: as a static pressure drop associated with large-scale kinetic energy transfer (again roughly speaking). I would argue that wake measurements can actually yield more insight into the origin of drag on a body than even very precise measurement of pressures and shear stresses over its entire surface. Even though it's tough to argue against the pressures and shear stresses as the "cause" of drag, you might actually learn more by studying their "effect" (and not just foofy qualitative stuff, useful stuff like drag components that can be accurately scaled). What does this have to do with lift and momentum transfer? The same basic analysis used to calculate drag components can be used to calculate lift (you just do the same analysis 90 degrees out). If you are very careful, you can connect the power a body supplies to the air through induced drag to the momentum it supplies through lift (LouW pointed this out several posts back). I think this gives quite a bit of insight that you can't get by looking just at surface pressures. My only disagreement with LouW is over the connection between control volume shape and net momentum transfer to the air. He considers it insignificant, I consider it inescapable for a lifting wing.

I apologize if the tone of this post is inappropriate or grating. I just wanted to wrap my thoughts up as this thread appears to be (finally) winding down. The bottom line is I think it's nearly impossible to explain the "essence" of lift to a smart high school student without glossing over subtle but important details.

RE: closure
 

Since several are making kind of a closing summary I just might as well too.

Observations:

When a real wing, producing lift, passes through air:

1. It always deflects some air downward.
2. The downward deflection is always proportional to the lift being produced.
3. After the wing passes there is no physical indication, other than its remaining motion , that the wing has passed.

Conclusions:

1. The wing must have somehow produced a force (acting on the air) to cause the deflection.
2. That force must have somehow produced an upward reaction acting on the wing (Newton’s second law).
3. If the path of the wing is level, that upward reaction force (lift) must be equal to the weight being supported by the wing.

At its most basic, a wing is an air deflector. This explanation is simple, elegant, and true.

[Of course since the wing also produces drag, the motion imparted to the air is also slightly forward]

I already see hands going up all around the room waving to challenge me on this. “If it’s really that simple why the thousands of words written just in this forum, and how about the thousands of aerodynamic textbooks in hundreds of engineering libraries?” “And why then did I have to sit through four or more years of theory to earn an engineering degree?”

It’s because all of that stuff is necessary to deal with the “somehow” in the first two sentences in the conclusions above. In fact the behavior of air in the vicinity of a lifting wing is quite complex. Even the most comprehensive equations dealing with it involve some degree of simplifying assumptions just to make the math manageable. Although thanks to countless researchers, a lot more is understood today than was, even by the middle of the last century, there is still work to be done.

I sat through many of the same courses as most of you. The first couple of decades of a forty-five year career were spent as a working engineer in aircraft design, development and flight testing. With all of that, I have a deep appreciation of aerodynamic theory and it’s importance as the only route to more efficient design. I completely agree with Ken’s pressure explanation of lift. I have never said that Bernoulli was wrong (just that it is not necessarily complete). When I have tweaked you a little it was to try to get you to think out of a rut.

What I have said is that to satisfy the laws of physics, there must be a transfer of momentum for any heavier-than-air machine to fly, supported by air alone. Such a transfer defies easy analysis, and it isn’t of much use in design, but it is real none the less.


To quote Forrest Gump, “that’s all I’ve got to say.”

RE: closure
 

Not a summary, just some final nits.

True. The key word here is "some". There is also "some" air that is deflected upwards.

You can make just about anything "proportional". My challenge to you is to make it equal. I argue that "deflection" is what we disagree on. This is where your control volume becomes important... and the analysis becomes very very fuzzy.

I don't understand why that is relevant. If its to show that air not in the vicinity of the wing doesnt affect its flight, i think that others have argued better that it does in fact affect its flight. Especially since this is subsonic flow. Pressure waves (sound waves) in front of, and behind the wing, will affect its flight. Likewise... pressure waves emanating from the wing itself, will affect the flow all around it.

True.

True... simple accounting... but it neglects other flow patterns that would cause the NET force to balance out to zip.

True. Thats just the result of a free body diagram.

I would better characterize a wing as a device that uses energy (thrust) to create a force on a solid object (the wing itself) by altering the flow pattern of a fluid.... specifically by circulating it... but not by deflecting it. Fluids are better deflected by using pipes and nozzles, not wings.

Wings do deflect air... but the "simple" explanation neglects a deeper elegance in how it is deflected. If all we wanted to do was deflect air, there are better devices for doing this.

This is a misapplication of Newton's laws.... and has been PERVASIVE in this thread. If a force exists, an acceleration is involved, but it does not mean that anything is moving.... such as moving downward or forward. If i set a rock on the earth, there exists a force... up on the rock, down on the earth. Equal and opposite. There exists an acceleration... 'g' ...but it does not mean that there is any momentum involved... or any net motion.

The trouble with a fluid is that it is squishy and difficult to draw a free body diagram around. I plead with you to examine previous points regarding such things as the rocket equation, or an example i posted earlier of flow in pipes. Or the very early explanation of an ICE... in which case we are actually neglecting momentum and simply looking at pressure (looking at it as a quasi-steady-state process).

I don't recall anyone disagreeing with me about the difference between pressure thrust and momentum thrust in the rocket equation (similar to a wing because it involves fluids). It sends chills down my spine when you equate these explanations (pressure versus momentum). They are two different things! A wing can produce lift by both of these methods.... pressure -and/or- momentum. BUT... when we are talking about conventional subsonic flow there is no momentum involved... just pressure. Because the density of the working fluid cannot change (subsonic flow), conservation of momentum states that momentum cannot change.... meaning that it cannot be greater than zero.

Sorry... this is a little ranty now...

Nobody has even mentioned the pitching moment of the wing in flight... that's a whole other force we havent even looked at that would be very difficult to explain as arising from momentum.

I never promoted the popular "bernoulli" explanation. My position has always been "circulation".

Your more "complete" explanation involving momentum violates conservation laws... this is what bothers me (and a few other i think). The only way we can get momentum involved is if we start compressing and expanding our working fluid.

This is the violation of conservation and the pervasive misunderstanding of Newton's laws. See above example involving the rock.

Aww c'mon Lou... i never said Canola... that was Ben... and yes i used it to make some pancakes this morning. LOL

RE: closure
 

Try the tests indicated in my attached pic. I used really light soft balsa blocks, about 6" long, 3/8" thick, and 1.5" wide. I left one untouched, the other two I made into typical airfoils, except one of those had way too much reflex at the trailing edge. Then I directed air across the top surfaces of each one via a straw (no air was directed below them). Only the normal airfoil exhibited lift. But they all experienced the Bernoulli effect due the the fast air going across the top.

This leads me to wonder if the lift is due to the Coanda effect, rather than Bernoulli, as Coanda effect on the normal airfoil would have directed the originally horizontal air in a downward direction.

Please try this before commenting.

You can try it by blowing across top and bottom - same results - but I didn't do that because, wanting as much Bernoulli effect as possible, I blew only across the top.

RE: closure
 

Tim,

Your demonstration does not really illustrate the "Bernoulli effect". When you blow through a straw, you put the air in motion by increasing its total pressure. The air exits the straw at a rate such that the static pressure in the jet matches atmospheric pressure. The static pressure in the jet of air is actually no lower than in the surrounding air.

Only when the jet is subjected to curvature (via interaction with the upper airfoil surface) you will begin to see static pressure variation. I don't believe that your setup makes a good demonstration because it introduces phenomena that aren't present in the "real thing", such as viscous entrainment into the shear layer at the edges of the jet. The forces on your "wing" from this "induced flow" can mask the effects you are trying to observe.

RE: closure
 

Exactly why I tried the experiment with a normal airfoil that was reflexed on the trailing edge. The curvature's there, the Bernoulli effect's then got to be there, but the lift isn't - because the reflex throws the air in the wrong direction.

RE: closure
 

That's not really a reflexed airfoil. Thats a blunt or divergent trailing edge (DTE). We had a discussion on this in another thread.