How Lift Is Created... If you are interested

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.

Let's go with the Fairies theory. The rest of this seems too much like work.

I curse all of you to a 3 hour presentation by a CFD new grad on his/her wonderful discovery ( a change in the 3 decimal place of a 2nd order coefficient) and how this will make fantastic improvements in the accuracy of their predictions. This meeting will end like all such meetings when the person next to you, who was asleep and drooling on your shoulder, wakes up a points out to the presenter that their changes are still well within the margins of error.

Most of the equations cited in this thread are best used for illustrating the physical phenomena that produces lift. Except for some limited cases they are not very good for predicting it.

Given some good test data I think I could write some equations to predict lift in terms of Fairies.

EEERRRR, wrong !

This one of the other persistant myths that have been disproven time and time again but refuses to die.

Wouter

Please understand that Mr Tilley was indulging in sarcasm, which may well be lost to you as English isn't your first language. Don't take my comment negatively, it's not many in America who can speak their OWN language well, much less two languages.

On the other hand. Tilley's sarcasm gets lost to those of us who have known him for years.

Personally, I think all of y'all are wrong.

There's no such thing as lift. Government sucks, and the drafts thereof drive all things windy. Any perceived deviations thereof are driven by fairies, who are angry because of persecution and discrimination.

sea ya
tami

Dave, you are bringing Donna and da Woo in for our anniversary, huh? We have apparently quite a few folks coming in, so do come visit and sail!

Wout. You are right.
I was just a little scared of dissapearing in a flash of bright white light, though everyone tells me not to worry?... ( the other thread got me shook up)

The most readable, logical, intuitive and understandable article I have read on lift is How Airplanes Fly: A Physical Description of Lift. The credentials of Scott Eberhardt, one of the co-authors, suggest that the concepts it presents should be reasonably accurate.

As its name makes clear, "A Physical Description of Lift" presents a Newtonian explanation for lift. "See How It Flies" on the other hand presents a pressure based explanation. These two approaches may not be incompatible. According to the Wikipedia article and discussion on lift they may be two methods of describing the same thing. This is also addressed on this NASA page.

There is one major disagreement. "A Physical Description of Lift" claims that the Coanda effect has a significant role in producing lift. "See How It Flies" goes to considerable lengths (some of them a little dubious) to claim that the Coanda effect is not involved in producing lift. One of these claims is wrong.

Another article that supports Coanda is How Planes Don't Fly: Debunking a standard explanation of lift.


After contact with the inclined wing, the airstream has changed direction. Therefore, acceleration has occurred:

Personally, I propose we take an initiative from intelligent design proponents and declare that lift is too complex and therefore a supernatural being must be responsible.

Can't we wait till winter for these discussions. Right now I'm on my way to go sailing. It's Summer guys.

Mike Hill

True, although you might have to resort to thermodynamics and entropy to measure "disturbance." However, a symmetric section and a cambered section both at zero angle of attack could presumably have equal disturbances with different lifts. As I mentioned above, some of the reading should be discarded. Also, I mentioned this far too forcefully and stridently. If anyone took offense to this, my apologies.



Partially true. The reason Bernoulli is so easy and useful in a tube is that if you assume no boundary layer, the flow is uniform across the diameter of the tube. Also, a poorly designed venturi with the downstream expansion portion at too steep an angle can have separation. The Coanda effect could be used to keep the flow attached, but I'll get to that below.



Yes and no. Coanda really doesn't have anything to do with it. Please read See How It Flies Section 18.4 It describes the situation in detail.



Uniform flow over a wing is not a jet. The jet of fluid has more energy (from the increased velocity) than the surrounding fluid and can be used to an advantage. Later in that section, he describes how the Coanda effect can be used on a wing to delay flow separation by blowing high velocity air into a boundary layer that is near separation and allow the wing to operate at a higher angle of attack. This Coanda effect air injection could also be used to keep flow attached on a too-steep venturi (if for some reason you were forced to have one) and has been used to remove rotars from helicopters. A key quote.



Generally speaking, the reason the flow stays attached to the wing and turns is pressure. The reason it stops turning and separates when the angle of attack gets too great is a lack of pressure. If you look at a packet of air and three adjacent packets (ahead, behind and above) and qualitatively work out what's happening using Bernoulli, you can work it out. No need for math or exact numbers, just sketch out a few places over the wing and work out what's happening with each term in the equation. If anyone wants me to go through it, I will, but I've probably typed too much already and I'm hoping to go sailing this afternoon.

Also, the connection between all that downward accelerated air that the Coanda proponents point to and the wing is pressure. You can qualitatively work that out with four packets of air and Bernoulli as well, but you'll need some vector force diagrams, too.



That's only in cases where the designer hangs the center of gravity well forward of the wing's center of lift for highly stable stall recovery. Check out Section 6.



And Marchaj was late to the party. If you really want your head to explode, check out http://www.navier-stokes.net/nsfield.htm Navier derived these equations in 1822, so there really hasn't been an excuse for anyone to misunderstand lift for the past 183 years.

Summer schmummer, the wind lightens up here in Summer, so we wait for winter to sail

well, the wind lightens up with notable storm related exceptions...

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42.

This only changes the situation by degree. All properly designed conventional airplanes will have the Cg forward of Cl, just some more than others (some MUCH more!). So in cases where the Cg and Cl are close, the horizontal stab does not need to pull the tail down with much force to change the pitch. But it still pulls down some, and still subtracts some lift. The amount of down force needed could be calculated as the ratio of the distance between the Cg and Cl(wing) and Cg and Cl(stab), with smaller Cg/Cl(wing) and larger Cg/Cl(stab)trending the downforce lower. So if the tail is long or Cg/Cl close the airplane is more efficient.

Jimbo

As it feels like this thread is approaching closure , and grappling for a practical value :
I'm buying a mainsail for my uni-rig F16. All the sails from the various manufacturers represent an invisible hierarchy of "best" to "worst" at hastening the force needed to move the boat forward in the wind conditions in which I intend to race.From the standpoint of pure consumerism and given the abundance of misconception about how sailboats move - how does one distinguish a smart (fast) sail from one cut by a minion of the church of equal transits (buy what's winning ?) ?

I'm pretty new to sailing , has the misinformation about how lift occurs manifest in wide spread purchase of bad sail designs in the past or am I safe assuming most sails offered are "smart"?

Regardless , loved this thread...

PK

scott@aa.washington.edu
Office: 101 Kirsten Wind Tunnel Building

He certainly has a credible location. Means I'm going to have to be reeeeeal certain about the email I'm going to send him about the Coanda effect.

The two approaches, Newtonian and pressure must be compatible. They're both based on physical laws. That's the fallacy that I was getting at with the Coanda/F=ma argument. In all the presentations of that argument that I've seen, there is a lack of connection between all the downward deflected air and the wing itself. Then viscosity and "stickiness" gets brought into it as to how flow stays attached and, personally, I don't think that pans out, either. First, viscosity leads to boundary layers and boundary layers lead to separation. As for "stickiness," as opposed to a solid, a fluid, in and of itself -- the special case of surface tension excepted -- cannot support a load in tension or compression. It's like pushing a rope cut into 10 pieces. But surround that fluid that you are trying to apply a force to with a pressurized fluid of a same or different type and now it can support a load.

Worse, I'm going to go out on a limb, although I believe it's a short, sturdy limb and say that Eberhardt and the How Planes Don't Fly article are wrong in their description of the Coanda effect. If Coanda is viscosity/boundary layer driven, then the flow slows down, the bondary layer thickens and the flow separates and it sure doesn't seem like it would help in keeping flow attached on a nearly stalled wing! I'm going to have to ponder this one, but I suspect that in the water over the glass scenario, the flow leaves the faucet at atmospheric pressure, continues to accelerate down the glass due to gravity, decreasing its pressure to below atmospheric and the surrounding air pressure holds the stream against the glass until the boundary layer slows the flow too much and the stream separates from the glass. The water's surface tension would help keep the water attached to the glass, but surface tension sure doesn't have a role in the rotarless helicopter or the blown boundary layer referenced above. I'm not 100% percent on it, but I'd be willing to put money down on a claim that the water-jet-on-a-glass Coanda effect would not work nearly as well, if at all, on the space station.

Supernatural beings like fairies?

And since I'm disparaging other people's credentials and criticizing credentialed experts, I suppose I should toss my credentials into the mix. I have an MS in mechanical engineering, have been working in hydrodynamics for about 15 years and have designed and analyzed about 15 underwater vehicles of various types and speeds.

I haven't had time to read this whole thread but there are a few things that I have to through in after reading a few of steveh's posts. The reason the Bernoulli equasions don't seem to be accurate for a wing is because most people seem to think that if 2 paricles of air side by side strike the leading edge of a wing and one goes over the top of the wing and the other goes under the wing, the two particles must both meet back up at the exact same time at the trailing edge of the wing. This is an insane assumption. There is no law of physics that says this will happen. The equasions aren't wrong, the assumptions people are making are wrong.
Next, the relation between the Coanda effect and lift. The Coanda effect simply shows how air is accelerated by the wing. I don't mean slowed down or speed up, I mean that the direction of travel is changed. Remember that when you have a wing creating lift, there are only 2 elements interacting, the air and the wing. If the wing is creating lift it must be doing it by interacting with the air in some way. Lift is a force normal to the direction of air flow. The only possible way for a wing to create lift is if it creates an equal but opposite force on the air. That force is accelerating a mass of air downward. Contrary to some of the bizzare circulation theories, air must be deflected downward in order to create a force on the wing in the opposite direction.
Circulation theories, CRACK SMOKING BS. Just a little bit of common sense will show that this idea is totally wrong. If there is a circulation around the wing that produces the lift then I should be able to destroy all lift by destroying the circulation. Since the "circulation" is from the trailing edge toward the leading edge on the bottom of the wing, all I should have to do is greatly accelerate the air under the wing from the leading edge toward the trailing edge to distroy the circulation. What if we bolt a jet engine under the wing to accelerate the air flow. By the reasoning of the circulation theory, every jet that has an engine mounted under the wing should never be able to get off the runway. By use of simple vector math you can show that a circulation plus a flow does not create lift. Some of the assumptions and leaps made by Mr. Gentry are soo wild I can't believe anyone would take him seriously.
I'm sure that many people will disagree with me on the circulation theories and I welcome your arguments but first I ask you to think for your self rather than blindly following the latest craze in lift theories. Does it make sense?

I never appreciated the circulation theories, but I do recognize that they can be used as mathematical describtions that will give accurate results. In a way it is not a explanation but a different type of model, one that can be useful in certain circumstances.

For the educated people among us. Complex numbers or imaginairy numbers are just plain BS in the real world but with this math structure we humans are able to solve many complicated problems. I feel circulation theories are comparable to that.

In all other aspects I'm with Steveh

Wouter

Our current summer feels much like winter. Cloudy and plenty of rain with regulary a storm coming through. Another wasted season.

Wouter

To clarify a little about pressure. Total pressure is static pressure plus velocity pressure. Velocity pressure acts in the direction of velocity while static pressure acts in all directions. A pitot tube measures total pressure at the tip (pointed into the air stream) and subtracts static pressure at right angles to velocity (through the little holes around the perimeter of the tube) to give you a reading of velocity pressure which is then allows calculation of velocity. That is assuming you connect both hoses to a manometer. If you increase velocity as in a venturi, there is a reduction in static pressure as it converts to velocity pressure where the velocity is higher (at the restriction). Reducing velocity, the pressure is reconverted to static pressure with some efficiency loss.

In a sail, increasing the velocity on lee side reduces the static pressure on that side compared to the static on the windward. That results in a force or lift which is the difference in the static pressures. I would believe there are other forces involved as well such as a portion of the velocity pressure on the windward side.

Howard

deseely,

Most of this has been covered and to my mind, the Coanda effect as a "how" or cause for the downward deflection is not only DOA, it's a misnomer. Effects can't cause things! Try this.

Bob: How does the air get curved downward going over a wing?
Joe: The Coanda effect.
Bob: How does the Coanda effect curve the air?
Joe: It's like water from a faucet flowing over a glass.
Bob: Ahhhh... But how does the air or water curve?
Joe: (head explodes)

That's pretty much Raskin's argument in a nutshell, but instead of his head exploding, he waves his arms around and says we don't need to know how it works, leaving us

As for the crack smoking circulation theory, as Wouter said, circulation is just a mathematical and visualization tool, not a conspiracy. Check out Section 3.10 of See How It Flies for a very clear description of circulation. Circulation doesn't state that air moves from the trailing edge to the leading edge while an airplane is flying, it states that this kind of flow plus that kind of flow produces the same flowfield as a wing. In fact, circulation assumes that the forward flow under the wing from the TE to the LE is exactly cancelled out by the linear flow passing under the wing from the LE to the TE. You don't need a jet engine to cancel that forward flow, it's already done.

Ironically, a jetstream improperly placed under a wing could decrease lift in that region because of the...



...Coanda effect.

I wish I was kidding.

Wouter,

One could say that the jib works in the mainsail's updraft and the mainsail in the jib's downdraft, but I miss the advantage of the combination.

If I look at the jib, it can be set in a more opened angle due to the main, but if I look at the mainsail, it needs to be moved closer to the centerline with a jib, compared to without it. Doesn't seem to work this way.

Luiz

There is an AWFUL lot of bull sheeeet being thrown about here guys!! The best, truest and nicest answer is that the fairies were the ones what dun it!! And very nice little folk they are too, so DONT GET THEM ANGRY OR THEY WILL TAKE THEIR "LIFT" AWAY - SELECTIVELY, which means that when you are gliding in, quite nicely placed, towards the finish line and you think that you have a top place all wrapped up, it is then that the "lift fairies" will strike!!! And instead of having a favourable wind to cross the finish line with, you will be suddenly "lifted" almost straight up and thrown aback as you watch several other "believers in the powers of fairies" cross in front of you. So take care, you have been warned. (I have seen it happen many times – and that’s the truth -)

There are actual real world physical phenomena described well by complex numbers such as electromagnetic wave theory, where electrical and magnetic sine wave constituents are said to be operating in planes 90* apart, thus creating a 'Z' axis. The X, Y and Z axis relate exactly as the plane of the real numbers intersects the imaginary.

Agree with you on the circulation theory, though. Many people completely misapprehend the intent of this theory thinking that it supposes the fluid is actually flowing backwards on the underside(??!!) instead of simply returning to the same energy state as before negotiating the foil.

Jimbo

First, Tami, that kills me, glad somebody said it (42..love it)

Second, I didn't read more than 30% of those long posts, I don't have the time (or attention span). Like my boss says, "get to the point".

Tami said it once, I said it once, read a Marchaj book, he's not written the "definitive" books on the subject for no reason.

I deleted my post explanation here, b/c.... I'm tired and don't feel like finishing it now.

In short, the circulation effect accelerates the air on the lee side in comparison to the windward side. SINCE THE LEEWARD FLOW IS FASTER, THE PRESSURE EXERTED (BERNI) IS LOWER ON THE LEE SIDE THAN THE WINDWARD SIDE. Berni is able to tell us the pressure differential b/c of the differing velocities. Berni, then tells us part of the lift coeff.

This guy uses the same type of information as Marchaj.
http://www.av8n.com/how/htm/airfoils.html#fig-flow-past

I can't find my Marchaj book otherwise I'd post a pic of the graphs of pressures and resultant vectors.

He did wind tunnel testing with pressure readings. It's hard to argue with numbers.

As he explains it, the jib pulls 4-5x harder per in^2 than the main does. The jibs main purpose is, however to aid in keeping flow attached to the lee side of the main thus reducing drag and increasing efficiency. It does decrease pointing ability, however provides quite a bit of driving force on reaches.

Was I pissy and terse, possibly.... am I about to get done with a week of AF inspections w/sh*tty inspectors and bogus findings... yes. Have I worked 50+hrs in 4 days and only had ~4-5 hrs of sleep/night... yes. I'm going to bed.

p.s. Under the "fallacies" section, he kills the Coanda argument.
http://www.av8n.com/how/htm/airfoils.html#sec-other-fallacies

Alright "smarty pants" TAMI, (who's a pretty boy then), we all know the answer (42) but what is THE question???????
I can only reiterate what has been said earlier (if you REALLY want to know about "lift" relative to sailing)

READ C.A.MARCHAJ !!!!!!!!!!!!

Jimbo,




You misunderstood my statement. I myself wrote :"this math structure we humans are able to solve many complicated problems". Sound like "... actual real world physical phenomena described well by complex numbers ... " to me.

What I was referring to is that an imaginairy number has no meaning in the real world. Example. If a wave has a frequency of say 10 Hertz that we know what that physically means. If however a wave has a frequency of say (10 + 4J) (= imaginairy number, with the J identifying the imaginairy part and the 10 being the real part) than nobody knows what that means in physical terms. Nor does such a wave form exist in the real world or can ever be made to exist. Such a construction that is very much possible in mathematical terms, is simply non-existant (impossible) in the real world where ONLY REAL numbers exist. That is why I say that an imaginairy number is just plain BS in the real world. It is as concrete in the real world as little wood elfs.

Did I loose any of the other guys yet ? Yep we are talking about imaginairy numbers, no kidding they really do exist in mathematics and are quite helpful, especially in the analysis of oscillating systems.

So for you guys the answer to how can imaginairy numbers be helpful in real life problems ? Because any system that initially is worded in real valued equations will produce real valued answer even though the path in between is using imaginairy numbers. In such a case the imaginairy numbers occur in what is called complex conjugate pairs. Example : whenever the number 10 + 4j arises than somewhere else its mirror image 10 - 4j arises as well; that is when the original problem is real valued. The math structure around these numbers keeps the pair linked to one-another so they constantly are added/subtracted or multiplied with eachother. Often right before the final sought after answer, which always is a value with meaning in the real world, the two mirror images are added or multiplied by one another resulting in a real valued number.

example : (10 + 4j) + (10 - 4j) = (10 + 10) + (4j - 4j) = 20 + 0J = 20 = real value.

Similar thing happens when multiplying to mirror complex numbers.

The structure that enforces this behaviour is linked to wether a problem exist in the real world. If the final answer is NOT a real value then either you have made a calculation error or the original problem you were analysing simply does NOT exist in the real world.

Funny stuff this is, right ?

Who said Mathematics are boring ? The further you get into studying mathematics to more wild the phenomenons get. You just need a good teacher to get through the basics after that it gets mind blowing

Wouter

I`ll go back to my reference to Dennis Pagen, who wrote "Understanding the sky", wherein he explains how thermals are formed, which is kinda important to free-flying people, and therefore relevant in this thread.
I won`t get into it in detail, suffice to say that thermals are really just a whole lot of hot air.
Which brings me to my point : If this thread was on a paragliding forum, we`d all be at cloudbase by now.

Complex numbers, real numbers and even integers are all human conceptual/modelling devices; none of them "exist" in the real world...

Will
Again I only ask that people think for them self. We have all read Marchaj's books. I have every book he wrote. Marchaj didn't come up with these theories, he only rewrote them. Secondly your referance to John Denker's page does not in any way "kill" the Coanda arguement. I have read all of Denker's theories and again he did not create them, he simply restates them. Denker's barn door analogy is an excellant example of how wrong he is. The air flow "or water" will adhear to a flat plate at low angles of attack. If you don't believe it, try it in the sink with a sheet of plastic or metal. One of the mistakes he makes is using a fast stream that is broken and full of air. Just like a cavitating prop, the water will not adhere as well when mixed with air. Again the coanda effect doesn't create lift. Changing the direction of the air flow does. You can't just say that Newtonian physics does't apply to wings. It does, it always has and always will. The ONLY way to create lift is by changing the direction of the fluid flowing past the wing. The forces must be equal and opposite each other. Another on of the completely ridicules arguments by the "circulation beleivers" is that air is like "be bees" shot from a gun and bouncing off the bottom of the wing. I think we have all seen the diagrams of the wing with the be bees flying at it. There explination is that the only the air that strikes the bottom of the wings would change direction and therefore Newtonian physics can only account for a fraction of the lift. Use a little bit if common sense here and look at there diagram. If air were like be bees, there would be a perfect vacuum behind the wing because the wing has deflected those be bees away. Air is not like be bees, it's a fluid. Take a look at any of the diagrams in Marchaj's books of the fluid flow around a wing or sail. Look at the direction of the free fluid flow well ahead of the wing and look at the direction of flow behind the wing or sail. If there is lift, there is a directional change in the fluid flow. This whole circulation theory comes out of a misunderstanding of the Kutta-Joukowski lift on a rotating cylinder. The rotation of the cylinder is causing a change in the directional flow of the fluid flowing past the cylinder. See this link for a complete explination. http://www.lerc.nasa.gov/WWW/K-12/airplane/beach.html .

Will R wrote:


and


I haven't picked up a Marchaj book in about 15 years, but I doubt he uses the unequal flowpath theory. If he does, I think I'll take a pass.

I'm sorry about being longwinded, but it takes a bit of explanation and detail to dispell myths and present (I hope) a clear and correct explanation. The true situation is a bit more complicated than just saying, "Unequal flowpaths!" or "Coanda!" or "Bernoulli!" and not nearly as sexy as those East European or Italian names.

You think it's BS? Fine, present your case. But referencing this or that expert does not necessarily mean that there's understanding. I've read Rick's cat racing book, but until I can successfully apply it, I sure won't feel like I understand it. Don't like it, find it boring, think it's all hot air, well, there's a thread full of pithy rejoinders on light air racing to enjoy.

deseely,

That rotating ball example you linked IS a real world example of circulation. If that Java simulator could be set to a straight line flow of zero when you spun the ball, you would have a rotating, circular flow in the direction of rotation, i.e. circulation. Add the straight flow and you get lift. Straight flow plus circulation generates lift is the circulation theory. What's the issue with this?

No problem with it Steve, in the real world a spinning cylinder or ball in a flow will produce lift but that is becauce it alters the direction of the fliud flow. The vector addition of a circulation plus a flow does not. The key here is that the fluid produced a force on the cylinder or ball and the ball or cylinder provided an equal but opposite force on the fluid flowing past it.

10 Steps to Circulation Enlightenment

1. Obtain CAD program
2. Draw circle of radius 5
3. Draw rotation vectors tangent to circle of length 0.5 pointing in clockwise direction
4. Draw horizontal linear flow vectors of length 1.41 at the tip of each rotation vector pointing right (the length makes it easy later)
5. Draw the resultant flow vectors
6. Measure the length of each resultant flow vector
7. Assume upstream pressure of 1 and density of 1
8. Using the upstream pressure and velocity and the velocity vector measurements in the Bernoulli equation, calculate the pressure at each point
9. Plot results on corresponding points
10. Analyze pressure field and determine that the lower pressure values above and the higher pressure values below will result in lift

Almost as hot as the Nacra 580.

That's it, I'm goin' to Disney World.

Altering the direction of flow is the function of a foil. By altering the direction (velocity) of the flow, lift is converted from the kinetic energy of the flow. The spinning cylinder or ball is a type of foil.



The spinning ball alters the flow because the flow adheres to the ball on the retreating side more than on the advancing side or more than it would on a stationary example. This increased adhesion prevents the flow from separating from the cylinder, thus it follows the cylinder's surface and it undergoes a continuous change in velocity (acceleration) as it does so. Lift is a reaction to this acceleration and the boundary adhesion.

Jimbo

I see you went to visit the big rat. Hope he didn’t gnaw too big a hole in your wallet. Looks like Jimbo is among the ever growing number of people who have realized that lift is just simple Newtonian physics. My sail puts a force on the air that passes by it and the air puts an equal and opposite force on my sail. That’s all, nothing more. I’ve been struggling with this concept of lift for about 15 years (since college) and I would have to say that I didn’t come to this conclusion until about a year ago. Sometimes the simplest answer is the correct one.
I’m so glad you have discovered 10 OF THE 12 steps to enlightenment. I hate to tell you Steve but once you start drawing diagrams, you can’t help but see the flaw in the circulation theory. To start out let’s place your diagram in front of you with the flow from left to right and the circulation clockwise. Now to the right of your diagram lets put a velocity vector to represent the flow after interacting with the circulation. If you add your 4 resulting vectors together and divide by 4 to get an average velocity vector after circulation, you will notice that the circulation cancels itself out. The flow before is exactly the same as the flow after and the circulation has no net effect on the flow. This was probably obvious to you before you started since circulation has no net direction of flow. If you sit down with the diagram you just drew and the flow diagrams from the actual experiment and ask yourself 1. Why is the flow in my diagram different from the actual flow? 2 If there is a pressure difference then why don’t I see a net change in my velocity vectors indicating an equal and opposite force being placed on the air? Why don’t my forces balance out?
Let’s start with the flow diagrams. First you are using a pure circulation with no physical object in the middle, where as a spinning rod is used in the actual experiment. Not a big problem. It’s not the flow around a stationary rod that is important, it’s the circulation created by spinning the rod and how it reacts with the flow that we want to look at. Look at the velocity vector on the left and right of your diagram then look at the flow lines from the actual experiment. Do you notice anything? In your diagram the flow has no net change in velocity after interacting with the circulation. In the actual experiment you will notice that the flow lines have changed directions after interacting with the circulation. How could we make your diagram more accurately depict the true flow? Go back in to your cad drawing and let’s make a change to your circulation. On the left side of your circle your circulation vector has a length of 0.5 pointing up. Let’s change it to 0.2. Now let’s change the circulation vector on the right to 0.8. Notice that I didn’t change the vector on top or bottom that gave you the pressure difference. Now redraw the resulting vectors and the vector that represents the flow after interacting with the circulation. Now you will notice that there is a change in the direction of the velocity vector of the flow after interacting with the “circulation” and it is in the same direction of the flow line depicted from the actual experiment. Now your diagram agrees much more accurately with actual flow lines. Now let’s look at the force balance. Sum the resultant vectors around your circulation. You will notice that there is a component that is now pointing down. The change in the direction of the velocity vector was caused by a force. You will notice that the direction of force on the flow is down and an equal but opposite force is applied to the circulation. The direction of the force on the circulation coincides with the difference in pressure. Now your diagram agrees with the actual flow lines and the forces balance out correctly not to mention that the pressure difference above and below the “circulation” agrees with the forces indicated by the velocity vectors. Everything agrees except you no longer have pure circulation. In fact you will notice that there can’t be any forces on the wing or rod if there is pure circulation because no forces can be transmitted to the flow. The original diagram that you drew with pure circulation won’t ever exist without another component introduced into the flow to force that condition but we can go into that in the future. It may take a few days, a few beers and lots of paper to digest all of this.

Pardon my amusement here but you don't need any knowlegde of physics to come to that conclusion. This is like saying that when you put your hand in the fire your hand will be heated up and the fire will get cooled. Leaving us no further in understanding WHY this is the end result of such an action. In effect everybody knows that a car will accellerate when you press on the gas pedal but doesn't mean that everybody understands what happens between pressing the peddle down and the car accellerating.

In short, you haven't provided much clarity to the issue apart form something that is almost obvious.

Also I think that a quatum physicist will have a problem or two with your statement that it is all JUST Newtonian physics.

The point you are still trying to make about circulation theory is still a mute point as it is nothing more than a different mathematical model of the same lift producing proces. It is not an explanation but rather a way of looking at that proces from a modelling (mathematical) viewpoint with certain characteristics. In some applications it is handy, in most however it is not. There is not much point in disproving as it very much is mathematically correct. Meaning that the results acquired by using this model are accurate. It is just like prefering to say that 2 = (2*7 - 4) / 5 instead of 2 = 1 + 1, both are correct expressions although the first isn't very practical at first sight but never the less could be very handy in certain special cases.


Wouter

"That's it, I'm goin' to Disney World" was rhetorical. I did the vector math (with 23 points, not 4), worked up an Excel sheet and I got lift. In addition to no net flow deflection, I also got no drag, so what good is this circulation nonsense?

Oh yeah, I got lift. And without all that mess of viscosity, Reynolds number, boundary layers, separation points and 3D, Navier-Stokes CFD codes. The spinning cylinder example you posted earlier doesn't have any net deflection, either, nor any drag.

deseely, my question for you is, how does all that downward deflected air from simple Newtonian physics hold up the airplane? In F=ma, how do you measure or estimate mathematically the mass of air moved and its acceleration in order to calculate your lift force? Also, how and where is that lift force applied to the wing?

Crack smoking circulationists have used circulation and idea of superposition of flows to develop a very successful aerodynamic modeling tool called panel methods** for estimating forces on a body. Including in three dimensions. If they can do it, I would hope that simple Newtonian physics could do the same.

** - PMARC is just an example. I liked it because in the description, they use the term "ficticious flow." So what comes from circulation really is cracksmoking bs.