Coanda effect Vs Bernoulli
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I think you try to pigeon hole things too neatly.
Consider that, however you might want to explain things, an aeroplane doesn't fall down because, quite magically, it throws air downwards and, as a result, develops a force (lift) upwards.
Different explanations are useful for different purposes but all arise from the above observation.
If you want to get a handle on what really is going on, you need to read up on circulation theory. The wing generates a circulation and that is an observable thing experimentally. The easiest way to see what that does is to look at spinning cylinders in a tunnel or tank. A search on the Magnus Effect will bring up interesting sites for you to review .. you don't really want to play with the mathematics, I suggest, unless you have nothing better to do this evening ...
The pilot doesn't need to know anything much about this level of detail .. but, if you are interested, then go for it.
Consider that, however you might want to explain things, an aeroplane doesn't fall down because, quite magically, it throws air downwards and, as a result, develops a force (lift) upwards.
Different explanations are useful for different purposes but all arise from the above observation.
If you want to get a handle on what really is going on, you need to read up on circulation theory. The wing generates a circulation and that is an observable thing experimentally. The easiest way to see what that does is to look at spinning cylinders in a tunnel or tank. A search on the Magnus Effect will bring up interesting sites for you to review .. you don't really want to play with the mathematics, I suggest, unless you have nothing better to do this evening ...
The pilot doesn't need to know anything much about this level of detail .. but, if you are interested, then go for it.
Ahh - circulation theory at last, not to mention dimensional analysis.
So density and viscosity has nothing to do with it then!
Reynolds doesnt have anything to do with flight 
I couldn't agree more L.M. 
Reynolds Number
Otherwise I wasted several years at college.
(Actually, now I think about it, I did).
Reynolds Number
Otherwise I wasted several years at college.
(Actually, now I think about it, I did).
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Reply to Maverick
Lift is due to the change of momentum of the airflow over the wing. The wing can be on an A350, or a tea tray with an angle of attack, or a piece of smooth or rough balsa wood.
The effect of the wing is manifested in changes in the air flow's speed and direction that extend for at least about a chord's length above and below the wing, and can be approximated by the circulation theory description.
The total force on the wing is consistent with Newton's rate of change of momentum derived from the velocity field of the wake, and can be resolved into components both up and back, giving lift and drag respectively.
Along any streamline, Bernouilli's equation holds too, if you take care at shocks.
The ratio of inertial to shear forces, diagnosed by the Reynolds number, controls whether/how-quickly turbulence builds up in the flow near the wing, and whether you can model it in a wind tunnel of a specific size, operating at a certain speed and density.
The compressibility of the air affects how shocks (sharp changes in the air's speed, density, pressure and direction) form in the supersonic air above the wing at high speeds.
Individually, none of these elements gives a complete picture of how the air moves as the wing passes and the effect it has. Their interplay is required to describe and understand what happens.
Mr Tullamarine is absolutely right though - you probably need to know how it works, rather more than why it works, and to tell whatever suitable story keeps in your mind how it works.
The effect of the wing is manifested in changes in the air flow's speed and direction that extend for at least about a chord's length above and below the wing, and can be approximated by the circulation theory description.
The total force on the wing is consistent with Newton's rate of change of momentum derived from the velocity field of the wake, and can be resolved into components both up and back, giving lift and drag respectively.
Along any streamline, Bernouilli's equation holds too, if you take care at shocks.
The ratio of inertial to shear forces, diagnosed by the Reynolds number, controls whether/how-quickly turbulence builds up in the flow near the wing, and whether you can model it in a wind tunnel of a specific size, operating at a certain speed and density.
The compressibility of the air affects how shocks (sharp changes in the air's speed, density, pressure and direction) form in the supersonic air above the wing at high speeds.
Individually, none of these elements gives a complete picture of how the air moves as the wing passes and the effect it has. Their interplay is required to describe and understand what happens.
Mr Tullamarine is absolutely right though - you probably need to know how it works, rather more than why it works, and to tell whatever suitable story keeps in your mind how it works.
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Except for one statement I fully agree.
That statement holds true for Reynolds numbers of large enough magnitude, let´s say from 1000 up. That is where Airplanes do operate (far up from 1000...). Thats where wind tunnels do operate (still a fair bit above 1000) If we scale down to insects, the effect of friction is no longer limited to the boundary layer ("near the wing"). And the change of momentum of the surrounding air is no longer restricted to the effect of pressure (normal forces), but may also be an effect of friction (shear forces).
The Pilot just needs to fully understand what an aircraft does, not how and why. Unfortunately many do not even know what their aircraft may do in certain conditions... And don´t believe those who do 
The ratio of inertial to shear forces, diagnosed by the Reynolds number, controls whether/how-quickly turbulence builds up in the flow near the wing, and whether you can model it in a wind tunnel of a specific size, operating at a certain speed and density.
The pilot doesn't need to know anything much about this level of detail
Aaaah... This topic never fails to stir some back-n-forth, around here.
On previous occasions, my confidence in understanding lift has been stripped to pretty much nothing. By "understanding" I mean (for my interests) "being able to predict how changing shape will affect the lift (and L/D)". Without CFD modelling. Preferably without contour integrals either.
Yet today, I would like to ask a simple question:
Is there an airfoil shape that does produce (non-zero) lift and yet has the aiflow above its upper surface move at the same speed as the airflow below its bottom surface? A kind of "non-Bernoulli" airfoil.
I'm guessing there is not such thing, but I've never seen it stated categorically.
Thank you for the enjoyable reading, as always. Balsa
On previous occasions, my confidence in understanding lift has been stripped to pretty much nothing. By "understanding" I mean (for my interests) "being able to predict how changing shape will affect the lift (and L/D)". Without CFD modelling. Preferably without contour integrals either.
Yet today, I would like to ask a simple question:
Is there an airfoil shape that does produce (non-zero) lift and yet has the aiflow above its upper surface move at the same speed as the airflow below its bottom surface? A kind of "non-Bernoulli" airfoil.
I'm guessing there is not such thing, but I've never seen it stated categorically.
Thank you for the enjoyable reading, as always. Balsa
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CFD and Reynolds, while may be fine for subsurface vessels, are just a mere estimate of aerodynamics.
Fooling with Reynolds numbers to 'scale' aerodynamic flow is simply that, a model, with only relative meaning in actual aerodynamics. While fluids have a nominal ability to compress, this is not well understood, nor defined in any but the theoretical complex dynamic analysis models.
Air can be compressed, and has an infinite number of compression ratios due to temperature, altitude, and humidity, far too many variables for a CFD model.
If the mechanics of aerodynamic wing design were understood, the wing designs would generally tend to gravitate to a similar solution over time. This has not happened, as each manufacturer claims to have the most efficient design, yet the wings look completely different.
Look at wake turbulence, the rollup of the lift component of the aircraft, the entire aircraft including the center wing, and influenced by the flap configuration, landing gear, and engine location.
AND completely misunderstood in mechanics of creation, rollup, velocity, and advection.
Yes, with advances in technology, unfortunately, many, many people have wasted their time in school..then again, history is fraught with examples of this..unless you still think that you can drive off the edge, continents dont drift, and of course the heresy of evolution.
Fooling with Reynolds numbers to 'scale' aerodynamic flow is simply that, a model, with only relative meaning in actual aerodynamics. While fluids have a nominal ability to compress, this is not well understood, nor defined in any but the theoretical complex dynamic analysis models.
Air can be compressed, and has an infinite number of compression ratios due to temperature, altitude, and humidity, far too many variables for a CFD model.
If the mechanics of aerodynamic wing design were understood, the wing designs would generally tend to gravitate to a similar solution over time. This has not happened, as each manufacturer claims to have the most efficient design, yet the wings look completely different.
Look at wake turbulence, the rollup of the lift component of the aircraft, the entire aircraft including the center wing, and influenced by the flap configuration, landing gear, and engine location.
AND completely misunderstood in mechanics of creation, rollup, velocity, and advection.
Yes, with advances in technology, unfortunately, many, many people have wasted their time in school..then again, history is fraught with examples of this..unless you still think that you can drive off the edge, continents dont drift, and of course the heresy of evolution.
Last edited by FlightPathOBN; 20th Jun 2013 at 03:36.
Because at the same time the other theories will provide zero as well:
Same Speed above and beklow means => no circulation (obviously) and also no net vertical component behind the wing, because that would require more mass flow and thus flow velocity above the wing than below.
So, I give you a 'No'.
Upper Surface Blowing?
I know its cheating but it fits the criteria.
I know its cheating but it fits the criteria.
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Short-top, long-base airfoil?
A supercritical section looks rather like it fits your bill - although the airflow over the top travels relatively less far than the air under the bottom as compared with a classical airfoil section, it still gets to the back first, and leads to a net downwards flow in the wake.
Thank you all for the answers. (I hope the OP finds it all "on topic", as I do.)
Note that I am asking about existance of a shape with certain properties; not whether it is consistent with the theories. Not that I insist that the two are necessarily different; it's just that I'm looking here for some factual evidence.
You obviously saw that I was trying to do away with the circulation theory, for at least one airfoil case. But I don't get the reasoning about equal speeds leading to "no net vertical component".
In the simplified Newtonian theory, extremely easy to catch intuitively, the stagnation line (SL) is approximated to be level and passing close to the leading edge. Then the air below SL has to accelerate down (because of the approaching non-porous barrier = wing) and the upper air fills the vacuum behind the wing (which is its top side), also accelerating down. Simple and no contradictions. Except that I found out that the SL is not where it was postulated.
How about a flat plate, to keep it simple. Or your typical rudder section, to keep it realistic. Symmetrical by design, since Wright brothers.
What is this obsession of the air "above" with getting to the back first, at positive angles of attack? (Besides "so that it satisfies Mr. Such-n-Such equations".)
Originally Posted by henra
No, that doesn't exist.
Because at the same time the other theories will provide zero as well:
Same Speed above and beklow means => no circulation (obviously) and also no net vertical component behind the wing, because that would require more mass flow and thus flow velocity above the wing than below.
Because at the same time the other theories will provide zero as well:
Same Speed above and beklow means => no circulation (obviously) and also no net vertical component behind the wing, because that would require more mass flow and thus flow velocity above the wing than below.
You obviously saw that I was trying to do away with the circulation theory, for at least one airfoil case. But I don't get the reasoning about equal speeds leading to "no net vertical component".
In the simplified Newtonian theory, extremely easy to catch intuitively, the stagnation line (SL) is approximated to be level and passing close to the leading edge. Then the air below SL has to accelerate down (because of the approaching non-porous barrier = wing) and the upper air fills the vacuum behind the wing (which is its top side), also accelerating down. Simple and no contradictions. Except that I found out that the SL is not where it was postulated.
Originally Posted by awblain
supercritical section...
the airflow over the top...
gets to the back first...
What is this obsession of the air "above" with getting to the back first, at positive angles of attack? (Besides "so that it satisfies Mr. Such-n-Such equations".)
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What is this obsession of the air "above" with getting to the back first, at positive angles of attack? (Besides "so that it satisfies Mr. Such-n-Such equations".)
Keeping lift theory simple seems to be at the root of the misunderstanding.
Reading further on the NASA website, a fairly good synopses of the sources of misunderstanding regarding aerodynamic lift can be found HERE.
Reading further on the NASA website, a fairly good synopses of the sources of misunderstanding regarding aerodynamic lift can be found HERE.
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Originally Posted by Maverick2167
NASA denying Bernoulli, is what make me confused, So are the theory of Lift taught in flight schools all wrong then?? includes the Books Published by Mr Keith Williams,
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From a physicist's point of view, a wing is just a machine for producing downwash - a glorified wedge, if you like. The momentum change in deflecting the air in a given direction is what provides the "lift" force in the opposite direction - whether up, down, sideways or forwards.
Now the only way a gas can exert a force is through pressure - remember force over area? So, if there is a lift force there must be a pressure difference between the two sides of the wing.
If you then assume that the airflow is laminar and incompressible, and apply conservation of energy, you conclude that the air on the low-pressure side is travelling faster than the air on the other side - hey, you've derived Bernoulli's principle!
So much for the basic idea. In practice, most of the pressure change,and hence lift, occurs on the low-pressure side of the wing, and not, as you may expect, on the underside. This is where all the whizz-bang of streamlines, stagnation points and circulation comes in. It's not possible to explain this in simple terms, yet it is vitally important in keeping an aircraft in the air (a fully stalled wing, where virtually all the lift comes from the lower surface, can provide as much lift as an unstalled wing at a great enough angle of attack. But of course the drag is horrendous, the engines can't keep up and so gravity chips in to maintain equilibrium - AF447 in a nutshell). Hence the prevalence of Bernoulli in the teaching texts, and the resulting misconceptions from the pretty pictures.
You'll notice I haven't answered the question as to which shape provides the most lift. But then, that's just a detail
Now the only way a gas can exert a force is through pressure - remember force over area? So, if there is a lift force there must be a pressure difference between the two sides of the wing.
If you then assume that the airflow is laminar and incompressible, and apply conservation of energy, you conclude that the air on the low-pressure side is travelling faster than the air on the other side - hey, you've derived Bernoulli's principle!
So much for the basic idea. In practice, most of the pressure change,and hence lift, occurs on the low-pressure side of the wing, and not, as you may expect, on the underside. This is where all the whizz-bang of streamlines, stagnation points and circulation comes in. It's not possible to explain this in simple terms, yet it is vitally important in keeping an aircraft in the air (a fully stalled wing, where virtually all the lift comes from the lower surface, can provide as much lift as an unstalled wing at a great enough angle of attack. But of course the drag is horrendous, the engines can't keep up and so gravity chips in to maintain equilibrium - AF447 in a nutshell). Hence the prevalence of Bernoulli in the teaching texts, and the resulting misconceptions from the pretty pictures.
You'll notice I haven't answered the question as to which shape provides the most lift. But then, that's just a detail
Last edited by fizz57; 21st Jun 2013 at 10:47.
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Reply to balsa model
The air flows as the air flows, whether individuals have written down partial or complete explanations or not.
I don't know what the "obsession of the air with getting to the back first" is. That's just what the air does. Often this idea of "equal transit time" is mentioned, perhaps to try to "explain" why the top-surface air is faster than the bottom-surface air; however, it doesn't explain anything, and it's not true:
not for a balsa stick, a rudder, a tea tray or an A350.
Only Monsieur Navier and Mister Stokes' famous equation includes all the terms that are necessary to describe the flow. However, that doesn't lend itself to obvious intuitive understanding, has no analytic solution, and if there is turbulence present becomes very unwieldy.
I don't know what the "obsession of the air with getting to the back first" is. That's just what the air does. Often this idea of "equal transit time" is mentioned, perhaps to try to "explain" why the top-surface air is faster than the bottom-surface air; however, it doesn't explain anything, and it's not true:
not for a balsa stick, a rudder, a tea tray or an A350.
Only Monsieur Navier and Mister Stokes' famous equation includes all the terms that are necessary to describe the flow. However, that doesn't lend itself to obvious intuitive understanding, has no analytic solution, and if there is turbulence present becomes very unwieldy.
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"As I understand it, all Newtonian physical theories are only methods to describe the outcome of a given phenomenon. So, by definition they cannot be regarded as 'correct' or 'incorrect'. There value lies purely in how well their results match observed/measured data. So, NASA cannot legitimately say that Bernoulli is wrong. All they can say is that other theories more closely correlate with the experimental data."
Where do you want to get the (additional) vertical volume flow (the vertical component that obiously didn't exist before) from if we assume the horizontal component to remain constant? (Free stream air flow/speed behind the wing, no circulation). Air doesn't decellerate below free stream velocity without a good reason to do so.
If you want to add a vertical volume flow (Newton), you need more volume flow upstream over the wing. Considering air incompressible this requires higher velocity.
Bernoulli, here we come...
Last edited by henra; 21st Jun 2013 at 18:59.