As I promised this morning (Baltimore, MD time) I am starting a thread to debate and how lift is actually created around an airfoil.

Here is where we left off over the weekend:

1) It seems that there is no one supporting the idea that Bernoulli's Principle creates a significant amount of lift to be a contributing factor (about 2% is what I read in the previous thread).

2) One source of the force called lift is the downward (leeweard) deflection of air hitting the airfoil. This results in a simple explaination of F=ma. This is a stance I took to explain how an airfoil works. I also said that this is only part of the explaination.

3) Myself and others proposed that the Coanda effect contributed the remaining force that makes up lift. The Coanda effect effect explains why airflow over an airfoil is important and why when a wing stalls it is so destructive to the creation of lift. The Coanda Effect again brings us to F=ma.

I think I have summarized where the old thread left off. Please let me know if I have misinterpreted/misrepresented anything so far.

These are the following papers that were cited in the previous discussion:

1) http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf

2) Anoterh post on this subject Another post on this subject

3) http://www.grc.nasa.gov/WWW/Wright/airplane/lift1.html

4) http://jef.raskincenter.org/published/coanda_effect.html

5) http://www.tspeer.com/Wingmasts/teardropPaper.htm

I am not saying these papers are correct, they are just papers submitted by others to cite some data and possinbly more creditable sources.

I think the debate is over exactly how is Newton working his magic on an airfoil. I think I've done alright summarizing the last thread on this. Any objections or new information to add?

Hang about for a minute. "How lift is created". Isn't that a bit of a "miss noma"? I have always understood that "energy can neither be CREATED nor destroyed, merely converted from one form to another", (unless we want to introduce “entropy” into the debate). If that premise is “kept in mind” then the way in which a foil “converts” one (or several) sources of energy/mass, into the “lift” that an aerofoil can use for the desired “work” should then become a little easier to understand? (If you are going to “get geeky” lets really get geeky??) he he he. Or if we want to talk "creation" do we introduce "God" into the debate???

You got me Any other thoughts on the subject?

"The Art of Paragliding" by Dennis Pagen gives a nice explanation of why paragliders don`t fall out of the sky until you put a pilot in one, easy enough for all of us to get to grips with without bringing Bernoulli, Botticelli, Ravioli or any other Italians into the explanation.
He also wrote "Understanding the Sky" - I`ve read it, and still don`t. But it`s good for a re-read, and explains weather in a fair amount of detail.
Me, I pull the string in, and the boat goes forward.

its my understanding that if the bernoulli principle was the sole contributor to our boats going forward, it would be possible to point the bows directly into the wind.

That Raskin page drives me insane. It is the most over-referenced, inane piece of "aerodynamic literature" from a pretentious poseur that I have seen on the Internet. First, check out his curriculum vitae. With the exception of being a partner in a model airplane company for five years, he has no training or experience in aerodynamics. A B.S. in math is enough to get him into trouble, and I suspect he wouldn't recognize the Navier-Stokes equations if they were hanging from his beard. Second, one would expect that from a paper entitled Coanda Effect: Understanding Why Wings Work that one would gain an understanding of the physics involved. We do not. Instead, we get these brilliant explanations.





Now, I don't know about y'all, but when I click on a link with "Understanding Why Wings Work" in the title, I kind of expect a "physical account" of an "experimental fact," not simplistic and incorrect explanations of a simple and applicable experiment. (blowing through a straw over shapes in a box) The Coanda Effect is an observed phenomena with physical underpinnings (pressure, shear stress, momentum) and if one is going to use it as an explanation of why wings generate lift, then one needs to explain why the effect works! Saying that wings lift because of Coanda is like saying that aircraft can fly faster than the speed of sound because of the sonic boom. It's an illogical cause and effect.

Nick, in the other thread and in your first summary point above, you seem to imply that the the pressure field around a wing is not what keeps it in the air. That is, integrating the pressure at each point over the wing panel does not keep the airplane in the air. Am I understanding your statement correctly?

The reason I ask is because that 2% number is another bit of "I read it somewhere" Raskin gibberish that gets tossed around as fact far too often. He determines the pressure differential between the upper and lower wing surfaces in an entirely erroneous manner (by using upper/lower surface length differentials and by imposing a pseudo-Kutta condition that isn't real) in order to show that Bernoulli is erroneous for calculating wing lift. I agree with Raskin in that Bernoulli is not directly applicable for calculating wing lift, but this whole 2% thing appears to lead people to discard not only Bernoulli, but everything else pressure related along with it, including the pressure field around a wing as the physical representation of lift acting on the wing.

The real reason why Bernoulli is not directly applicable (I'll get back to the directly part in a bit) is because the Bernoulli Equation is only applicable after certain simplifying conditions are met. These are,

1) Steady flow - Flow that does does not change with time. That is, flow at a particular point that does not change in speed or direction. Separated and turbulent flows are unsteady.
2) Incompressible flow - Fluid does not change density. This only arises in high-speed aerodynamics, not catamarans, general aviation aircraft or tabletop experiments.
3) Frictionless flow - No viscosity. This means that Bernoulli doesn't work with boundary layers.
4) Flow must be along a streamline A streamline is the familiar smoke trail that we see in car ads when they put the sporty car in a wind tunnel. It represents the path of a particle of fluid past the object.

As you can probably guess, the reason Bernoulli cannot be used to calculate wing lift as Raskin attempted to do is not because it drastically under-calculates the lift needed and therefore must be wrong, it is because it violates restriction number four. The upper and lower sides of the airfoil are not on the same streamline. This appears to be intuitively obvious, but instead of Raskin discarding Bernoulli for that simple reason alone, he goes on and on, confusing the issue with an erroneous calculation based on an erroneous assumption, the length difference. It's ok to simply say that a particular equation does not apply because the underlying assumptions for that equation are not met.

So where is Bernoulli applicable around a wing? Wouter gets into it a bit in the other thread, but it's when you follow a streamline and avoid areas where viscosity is a factor in determining the flow. That is, avoid the boundary layer and separated and turbulent flows. If you went into a wind tunnel with a wing, inserted a smoke trail that passed over the wing (again, staying out of the boundary layer and separated flow regions) and took pressure and velocity readings along the smoke trail, you would find that your measurements would be valid in Bernoulli.

In summary, Coanda explains nothing, 2% is good for milk and Raskin needs his site hacked.

I don't have time to provide a full response, but I'll hit the major points Steve mentioned.

Addressing my summary where I say Bernoulli does not keep a wing in the air. I am standing by this as I have done the actual experiment proposed at teh end of you last post. If you mesure the pressure differential above and below an airfoil as well as the velocity and plug the data into Bernoulli, it just doesn't cut it. Unfortuantly I have thrown away my report on teh subject from when I was running airfoil tests in a wind tunnel. I do remember though, I used a section of wing from a Cessna 172. I remember the portion of lift attributed to Bernoulli being less than 10%, but I do not remember exactly how much.

Also, you stated the four requirements for being able to use Bernoulli in you post. If you look at a wing in the real world none of the requirements you posted are met. The only one close is the steady flow requirement, but all airfoils have separation before the trailing edge. Therefore you would have to isolate only the portion of the wing that does not have spearation.

I will have more on Coanda later. I have to get back to work.

Nick ,

Thanks for this thread... In the most fundamental way; do we understand HOW lift is created ? If a comprehensive concensus cannot be reached here, maybe none exists...

PK

Nick,

If I'm understanding your test right, there's no need to plug anything into Bernoulli. Measure the pressure on the wing surface with a gridwork of surface pressure taps, multiply each pressure measurement times the incremental area that it acted on and total it up. Is this what you did? Even that won't give you the total lift of the airplane. The fuselage, horizontal stabilizer and propeller all contribute to total lift. As an aside, in addition to the inclined axial flow of the propeller contributing to lift, there is an additional lift component called the normal or radial lift. Curtis-Wright developed a wingless V/STOL aircraft around 1960 using the radial lift effect, so it's possible to get a lot of lift from this effect. NACA had a report on in back in the '40s.

The thing that gets me about Coanda from an intuitive level is the whole F=ma thing. That the wing throwing air downward somehow makes the wing lift. To me, this doesn't make sense on two levels. First, with F=ma, you have to have acceleration in order to generate a force. The highest accelerations in the flow are at the leading edge upwards. The lowest accelerations are at the trailing edge; nearly zero since the flow is almost straight and decelerating back to freestream velocity. (Does a decelerating flow result in negative lift?) Using a Coanda-based lift theory, this would result in large downward accelerations on the LE and small upwards accelerations on the TE. I don't know how that adds up to be lift, but an additional side-effect is that there now appears to be a downward pitching moment on the wing. Wing pitch moment curves have an upwards pitch with upwards lift, so thare's that discrepancy.

The second problem I have with a Coanda/F=ma argument is that there doesn't seem to be any connection between all this downward-pushed air and the wing. F=ma works beautifully for particles. If you sit in a wagon and throw a brick backwards, you go forward. Now imagine sitting on a swing resting just above the wagon. Throw the brick. You move, but does the wagon? No. So how does all that F=ma-ed air around the wing act on it and generate lift?

As for my statements on Bernoulli, I did say that it is not valid within a boundary layer or separated flow, so I don't understand the complaint. Strictly speaking, the Bernoulli equation isn't valid anywhere because none of those four conditions exist in the real world. However, if viscosity, compressibility and steadiness effects are small, then it's usable. Basically what this means is that the Bernoulli equation is not valid on the surface of the wing. However, on a streamline beyond the boundary layer or separated flow region, it's valid enough.

Edit: I appreciate the thread, too. Gives me a chance to rant about Raskin.

SAIL PERFORMANCE, C.A. Marchaj
SAILING THEORY AND PRACTICE, C.A. Marchaj
HIGH PERFORMANCE SAILING, Frank Bethwaite

Perhaps time better spent, yes?

Correct, a proper subject should be something like
"How foils take energy from a flow and provide lift".

The energy of the lift (plus heat and other losses) is equal to the energy taken from the flow, so there is no "creation of lift".

The exact mechanism is complex and involves deviation of the flow and creation of vortices.

I suggest the online book "See How It Flies" for more details in simple language.

Luiz

I finally read the Speer paper. It includes an excellent, clear, well illustrated and correct description of lift. I didn't get the graphics in the html page, but there is also a pdf of the same paper. His post (#6) in a boatdesign.net sail aerodynamics thread ties in the three conservation laws.

Luiz is correct about See How It Flies. It's also clear, correct and well illustrated. He addresses incorrect lift theories in sections 3.6 and 18.4.

Spot on, Just read C.A.Marchaj
His first book "Sailing Theory and Practice" is still the foremost and best work on Aerodynamics, Hydrodynamics, and their interaction at the boundary layer, in relation to sailing craft, that has ever gone to print.

Do what????

Bernouli works b/c of pressure differential produced by longer/shorter flow paths created by varying angles of attack. IF enough pressure differential is not created to provide enough forward thrust to over come the drag, you go backwards.

Tami beat me to the C.A. Marchaj reference, but his book "Sail Performance" is awesome! I've got a copy somewhere around here.... It explains everything from basic wing/sail theory to square head shape and why they are better. The section that really sells his research (IMHO) is when shows vector addition of the forces on a sail and graphs of manometer tube readings taken from sails. It's not light reading by any stretch of the imagination, but worth it if your pocket protector is big enough.

What really amazes me is that C.A.Marchaj wrote the "definitive" analysis on this subject (which went to print (in English) in 1964, but was actually published in Polish many years before), and has been reprinted, and in print, continuously since, and yet we have so many people STILL trying to "reinvent the wheel" as far as this subject is concerned?

Preach on brother, you're not the only one wondering!

IMHO, he puts it out there in such a way that you can't argue with the numbers.

From the reading recommended in this thread, Here's what I gather:

Bernoulli always works in the absolute sense as it is really just a reiteration of the laws of energy conservation. But the totality of the fluid moved by the foil would have to be considered, which as you know includes fluid in strata quite removed from the surface of the foil, which (probably simplistically) explains why a prediction of lift based solely on Bernoulli disagrees with pressure measurements on the foil surface. Bernoulli does then predict the velocity and pressure of the flow in totality, and at any specific point and strata given knowledge of the complementary parameters for that particular 'blob' of the fluid, but not necessarily how a foil behaves in that total flow. Two 'foils' could perturb the energy state of an airmass equally but produce vastly different amounts of lift by, for example, causing differing amounts of turbulence. So Bernoulli becomes for foils just a meaningless equality like 1 = 1; energy is conserved. We know that; it is always so.

Bernoulli does OTOH, predict well with tubes, where the airflow is constrained. Even in a Venturi with a nice foil shape, Coanda is meaningless as the airstream cannot separate from the surface. Therefore the concept of angle of attack becomes meaningless for Venturis, thus no Coanda.

Coanda effect is the reason the airflow stays attached to a foil's surface as it's pitch (angle of attack) is increased to a point where useful lift begins, but it cannot alone explain how the foil converts kinetic energy of the chordwise flow into lift. If you try to explain that conversion by Coanda alone, you will run into several problems as Steve mentioned, such as the major portion of the acceleration happening in the wrong place and direction.

Basically the flow has to bend as it travels around a foil that is set at a useful AOA. It stays adhered to the foil during this acceleration(change in velocity) because of boundary adhesion and Coanda effect. The change of momentum of it's original velocity causes the pressure drop observed on the lift side. As AOA is increased, more lift force is derived from the flow since the change in the direction of the airflow is greater; a change in direction of motion constitutes an acceleration. The speed of the flow in its original direction of motion remains unchanged, yet that flow has now acquired a new direction as it negotiates the lift side of the foil. Thus its speed increases. A simple vector diagram will illustrate this. The flow actually undergoes continuous change in velocity and speed as it follows the surface of the foil, although in this explanation it sounds as if happens as a singular event (This is where the proponents of the 'longer distance' explanation get sidetracked. It's not the differing distance but the change in velocity! Subtle but real.) More importantly, this creates a conflict between the adhesion of the fluid moving at the surface of the foil and the cohesion of the total fluid mass, with the fluid near the foil becoming rarified as a result. Essentially the opposite is happening on the other side of the foil, though not with equal reaction or lift.

At some AOA, the flow can no longer remain adhered to the surface of the foil, so cohesion overcomes adhesion, and a stall occurs. Lifting foils can only rarify the fluid so much before it breaks away as the flow's momentum overcomes its adhesion by centrifugal force. Interestingly, some Venturis can acheive very low pressures, even approaching absolute vacuum, since the fluid is constrained and cannot break away.

Feel free to correct (I know you will )

Jimbo

P.S.

In a conventional airplane, the lift of the horizontal stabilizer is subtracted from the total lift, not added as someone stated earlier. The higher the wing's AOA, the greater the subtraction. This is one of the attractions of the canard. This is why Saab favors canards for their jets; a canard adds to the total lift by lifting the nose while a conventional tail subtracts lift by pushing down on the tail to raise the nose. Thus canard jets can take off in shorter distances, a useful trait for a small country without giant air force bases with 3 mile runways.

Will, Bernoulli has nothing to do with the longer/shorter flow path. The length of the flow path over the leeward side of a fabric sail is the same as the windward. Bernoulli is simply an expression of the conservation of energy along a single, particular flowpath. It works for determining the conversion of potential energy (pressure) to and from kinetic energy (velocity) for a particular packet of air as it approaches, passes over and leaves the wing surface. The simplified form applicable for us is,

P1 + 1/2 ro V1^2 = P2 + 1/2 ro V2^2

P - pressure
ro - density
V - velocity

The two conditions, 1 and 2, are for one packet of air moved from one location to another at two points in time, not two packets of air in two different locations at the same time. Section 3.4 in See How It Flies goes over the Bernoulli Equation. In that section, he does mention comparing two different packets of air. However, it's dangerous to get into that without first accepting as true the single packet/two locations principle and how flowpath length differentials don't apply. The way it works is,

P1 + 1/2 ro V1^2 = P2 + 1/2 ro V2^2
and
P3 + 1/2 ro V3^2 = P4 + 1/2 ro V4^2

where 1 and 2 are on one flowpath and 3 and 4 another. Above, below the wing, doesn't matter. But, let's assume that states 1 and 3 are in undisturbed fluid well in front of the wing and therefore, equal. That makes the total energy states at 2 and 4 equal. What this means is that you can take a pressure probe and poke around anywhere in the flowfield and knowing the flow state at 1-3, you can calculate the velocity. Or, conversely, if you measure the velocity, you can get the pressure. But you do not get lift from this and as you can see, flowpath length doesn't enter into the measurements or calculations anywhere.

I apologize to all the people looking at algebra before having a cup of coffee.

All you guys seem to really need to know is that a lower pressure is generated on the leeward side of the sail than that on the windward side and the sail is “trapped” in between. Nature, hating such types of imbalances use her (nature is a woman by the way) little invisible “fairies” to continuously “push (or pull which ever the case may be –maybe both?)” the sail towards the lower pressure until equalisation occurs. Thank you Mother Nature for bringing such joy into my life as “a tall ship, and a star to guide me by”.
(shouldn't we look at the effect that "black matter" or at least "black energy" has on the applications of these "theories"???)

Bernie just re-iterates the conservation of energy rule: "All other things being equal, if the velocity increases, the pressure must decrease"
However, in our case the other things are not equal and the boundaries are badly defined.
So for solving nozzle problems bernie is tops, like why your boats bash together: when you approach one another and squeeze the water flow between, the pressure (water surface)must drop and so your hulls get sucked together. This also explains wave form drag and hull speed to some degree: as the water passes you it has to hurry up to squeeze past and the surface lowers (which makes the gap still smaller incidentally) forming the trough next to the boat.
Or why the jib helps the main - because it speeds up the flow in the slot, this air is lower pressure and fed to the back of the main.
However, a wing passing above the ground squeezes the air thu the gap, speeding it up and lowering its pressure, sucking the plane into the ground....huh...
Bernouli supports jetskis. Jetskis rely on the water being pumped thru a nozzle cone, and therefore having to speed up. The lowered pressure acts on the cones walls and pulls the cone forward. The cone is attached to the jetski, which in turn is typically attached loosely to a moron. Bernouli is responsible for the increase of velocity of morons. Tell your lawyer.