Theory on lift
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Lyman..
Zero lift AoA is a geometric AoA measurement, meaning it's the angle between the chord line and the relative wind. Zero lift means that the airfoil is producing no net lift force in either the positive or negative direction - ie: there is not net deflection of the air either up or down.
The zero lift AoA for an asymmetric airfoil will be negative (ie: -3 degrees), the zero lift AoA for a symmetrical airfoil will be zero degrees.
Italia: "The zero lift AoA of an asymmetric airfoil will be negative. The zero lift AoA of an asymmetric airfoil will be zero."
The zero lift AoA for an asymmetric airfoil will be negative (ie: -3 degrees), the zero lift AoA for a symmetrical airfoil will be zero degrees.
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Peter...
I briefly read through that paper and here are my thoughts on it. That paper seems to be written to just talk about false theories of lift. I can't speak for what the author's thoughts were but it seems he is passionate about correcting errors in popular theories of lift and he might have given the impression that Bernoulli isn't really responsible for lift... which isn't exactly correct.
This is one that he mentioned: Incorrect Lift Theory
I'd recommend just going through the other wrong theories on that NASA site.
I find people, naturally, want to find THE responsible thing for an event. If someone was killed, they want THE murderer. But in both cases, it isn't just one thing or person that is responsible for an event. Lift happens. We can see its effects, and the science of aerodynamics and physics studies it to try to understand how to manipulate lift forces favourably (for people to get to England in less than 2 weeks!). The thing is, no body really knows why lift happens. It can be explained why up to a certain point but then it breaks down pretty quickly, however, we do know quite a bit about how lift is created and to explain different aspects of it we have different laws and theories. There is no ONE theory that explains it all. Newton can describe how the forces that are created by the wing will actually lift it, Bernoulli helps explains the pressure differences around the wing, coanda/reynolds numbers/friction help explain how air is bent and how it reacts to different inertial and viscous forces, Kelvin's circulation theorem explains the circulatory flow around the airfoil in flight, etc. They all explain a little piece of the puzzle.
What this paper's goal seems to be is to let people know that Bernoulli doesn't explain EVERYTHING about lift, etc. I was taught the equal transit time theory (which is incorrect) when I was first learning and I notice it in books and have seen other instructors teach it. I think all these incorrect theories come about because people who aren't qualified to do so are 'simplifying' these theories so that it's intuitive and easy to understand for themselves and their students. The problem is that a lot of things aren't intuitive unless you have an understanding of physics and have studied these aerodynamics problems. Other instructors believe that EVERYTHING can be simplified and explained easily, which is not true. There is only so much simplifying that can be done before you end up with an explanation that is partially or completely wrong.
Does that provide any clarity to the paper?
I briefly read through that paper and here are my thoughts on it. That paper seems to be written to just talk about false theories of lift. I can't speak for what the author's thoughts were but it seems he is passionate about correcting errors in popular theories of lift and he might have given the impression that Bernoulli isn't really responsible for lift... which isn't exactly correct.
This is one that he mentioned: Incorrect Lift Theory
I'd recommend just going through the other wrong theories on that NASA site.
I find people, naturally, want to find THE responsible thing for an event. If someone was killed, they want THE murderer. But in both cases, it isn't just one thing or person that is responsible for an event. Lift happens. We can see its effects, and the science of aerodynamics and physics studies it to try to understand how to manipulate lift forces favourably (for people to get to England in less than 2 weeks!). The thing is, no body really knows why lift happens. It can be explained why up to a certain point but then it breaks down pretty quickly, however, we do know quite a bit about how lift is created and to explain different aspects of it we have different laws and theories. There is no ONE theory that explains it all. Newton can describe how the forces that are created by the wing will actually lift it, Bernoulli helps explains the pressure differences around the wing, coanda/reynolds numbers/friction help explain how air is bent and how it reacts to different inertial and viscous forces, Kelvin's circulation theorem explains the circulatory flow around the airfoil in flight, etc. They all explain a little piece of the puzzle.
What this paper's goal seems to be is to let people know that Bernoulli doesn't explain EVERYTHING about lift, etc. I was taught the equal transit time theory (which is incorrect) when I was first learning and I notice it in books and have seen other instructors teach it. I think all these incorrect theories come about because people who aren't qualified to do so are 'simplifying' these theories so that it's intuitive and easy to understand for themselves and their students. The problem is that a lot of things aren't intuitive unless you have an understanding of physics and have studied these aerodynamics problems. Other instructors believe that EVERYTHING can be simplified and explained easily, which is not true. There is only so much simplifying that can be done before you end up with an explanation that is partially or completely wrong.
Does that provide any clarity to the paper?
Last edited by italia458; 15th Oct 2012 at 21:52.
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If the wing section at AoA of zero degrees produces no lift, then the lift associated with a wing section must be associated with Newtonian physics rather than Bernoulli gas laws...
Actually, IF the wing section did produce lift, with the current thrust provided, the wing section would be near a negative AoA, attempting to contain lift... (ie even at a negative AoA, the top surface is still longer than the bottom, hence a negative AOA would push the ac forward...)
Actually, IF the wing section did produce lift, with the current thrust provided, the wing section would be near a negative AoA, attempting to contain lift... (ie even at a negative AoA, the top surface is still longer than the bottom, hence a negative AOA would push the ac forward...)
Last edited by FlightPathOBN; 15th Oct 2012 at 22:51.
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Italia. You realize the shaded box in your post is me quoting you? You claim two LIFT values for one AoA. The cross section for any chord is not relevant.
Hence the confusion. Zero Lift cannot be other than 0 AoA. Regardless of shape.
Asymmetric, Symmetric, flat plate, etc. It is not related to cross section.
Also reread your posit, AoA cannot be both negative and zero at zero lift. Any AoA other than zero produces lift, though in one case it would be inverted.
Do you not see this? Angle of Attack describes an "attack" angle, eg "other than "ZERO" Whether the lift is up or down, there is lift, hence other than zero AoA. To say otherwise draws attention to a deliberately arbitrary Incidence.
Zero AoA can be replaced with "LIFT NEUTRAL" if you wish.....
Please assume velocity sufficient to create lift....initiating....
FlightPathOBN.....
'If the wing section at AoA of zero degrees produces no lift, then the lift associated with a wing section must be associated with Newtonian physics rather than Bernoulli gas laws...'
muchas gracias.....
To validate Bernoulli requires something for nothing.... 0 degree angle of attack producing LIFT, in other words. Bernoulli's "accelerated upper flow" violates Newton's first LAW. Failing "accelerated", the argument defaults to "it travels further". No, it does not..... separated airflow needn't (and does not) arrive to meet at the trailing edge......
Hence the confusion. Zero Lift cannot be other than 0 AoA. Regardless of shape.
Asymmetric, Symmetric, flat plate, etc. It is not related to cross section.
Also reread your posit, AoA cannot be both negative and zero at zero lift. Any AoA other than zero produces lift, though in one case it would be inverted.
Do you not see this? Angle of Attack describes an "attack" angle, eg "other than "ZERO" Whether the lift is up or down, there is lift, hence other than zero AoA. To say otherwise draws attention to a deliberately arbitrary Incidence.
Zero AoA can be replaced with "LIFT NEUTRAL" if you wish.....
Please assume velocity sufficient to create lift....initiating....
FlightPathOBN.....
'If the wing section at AoA of zero degrees produces no lift, then the lift associated with a wing section must be associated with Newtonian physics rather than Bernoulli gas laws...'
muchas gracias.....
To validate Bernoulli requires something for nothing.... 0 degree angle of attack producing LIFT, in other words. Bernoulli's "accelerated upper flow" violates Newton's first LAW. Failing "accelerated", the argument defaults to "it travels further". No, it does not..... separated airflow needn't (and does not) arrive to meet at the trailing edge......
Last edited by Lyman; 15th Oct 2012 at 23:55.
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Originally Posted by lyman
At the point where an airfoil begins to develop lift becomes my datum. Zero means zero, or should. Just a quirk of mine, but not just me. It is an accepted datum in the industry,
OKay, so attempting to decipher your gibberish, apparently you are trying to claim that the only true AoA is that measured relative to the zero lift axis of the airfoil. Certainly there are occasions when that is convenient. But there in other contexts in which is is more convenient to express AoA as relative to the physical chordline of the airfoil, which can be seen and measured using ordinary measuring instruments. Doesn't really mater as long as the terms are defined clearly, but you'll find that the latter is a lot more common than the former, regardless of how firmly you you are convinced of it's superiority. Oddly, you seem unable to wrap your own mind around the two, as evidenced by your own words:
Originally Posted by lyman
Even an asymmetrical profile does nothing without AOA. At zero AoA its lifting force is downward....
Originally Posted by lyman
I rebel against illogic, and squishy nomenclature.
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Originally Posted by Originally Posted by FlightpathOBN and subsequently agreed with by lyman
If the wing section at AoA of zero degrees produces no lift, then the lift associated with a wing section must be associated with Newtonian physics rather than Bernoulli gas laws...
Ummm, no. Beginning by defining angle of attack as the angle relative to the zero lift axis, then attempting to draw conclusions from the fact that the lift at a zero angle of attack is relative to the zero lift axis is ........wait for it............... zero (surprise!!!!!) is nothing more than a very obvious exercise in circular reasoning. Illogic, if you will.
Last edited by A Squared; 16th Oct 2012 at 00:20.
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To clear it up partially, The second quote is true only if 0 angle of attack produces Lift, as per Italia's posit. No need to get hostile...
Look, the quote above 149 is from FlightPath OBN, please keep your quotes accurate. Your response to it is unintelligible.
You are completely disorganized, quoting one person as though someone else, etc.
I would ask you to correct your post, please, and utilize correct attribution.
Thank you.
Look, the quote above 149 is from FlightPath OBN, please keep your quotes accurate. Your response to it is unintelligible.
You are completely disorganized, quoting one person as though someone else, etc.
I would ask you to correct your post, please, and utilize correct attribution.
Thank you.
Last edited by Lyman; 16th Oct 2012 at 00:03.
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Originally Posted by lyman
Look, the quote above is from FlightPath OBN, please keep your quotes accurate.
Don't try to distance yourself from that thought now.
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I do agree with FPOBN,
would you like reference?
No thanks, I think I can work though that on my own.
Incidentally, I've edited the quote attribution, as you requested
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It is not circular, how so? Bernoulli requires Lift at 0 AoA to sustain, if there is none, Lift is explained via Newton. There is a reference in the paper posted by peter kent. Did you catch it? The paper also proves that upper surface airflow velocity is less than the lower airstream. substantially less. Additionally, if Bernoulli applies on a real wing, the upper surface would be shaped like half a circle.
Thanks for the re-do.....
Thanks for the re-do.....
Last edited by Lyman; 16th Oct 2012 at 00:32.
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Originally Posted by lyman
There is a reference in the paper posted by peter kent. Did you catch it? The paper also proves that upper surface airflow velocity is less than the lower airstream. substantially less.
Yeah, I caught it. the trouble is, it says exactly the *opposite* of what you claim it says.
Here's what the paper says (verbatim quote)
The greater the lift the greater the different (sic) in arrival times at the trailing edge with the air going over the top of the wing arriving considerably before the air below the wing.
the author is merely addressing the "equal transit time" myth, which pretty much everyone understands is a myth.
Last edited by A Squared; 16th Oct 2012 at 00:52.
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My recall is the upper air arrives later, I'll look.
On a cambered wing that is producing upward lift the airflow over the top is faster than the airflow over the bottom. This is one of the basic facts that is not in question and is demonstrated unambiguousky by more than a century of wind tunnel data.
You don't have even this most basic piece of information correct. yet you presume to lecture others on the *truth* of aerodynamics.
Last edited by A Squared; 16th Oct 2012 at 01:00.
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Originally Posted by lyman
It is not circular, how so?
There are several valid ways to define angle of attack.
You chose to define angle of attack as the angle relative to angle where it produces zero lift.
There's nothing wrong with that definition as far as it goes.
But.......
Inevitably, if you define angle of attack in this manner, the lift at zero angle of attack is by definition, zero. This is a result of your definition of angle of attack, not a demonstration of some overriding physical principle, yet you attempt to use it as "proof" of some principle.
If you can't follow how this is nothing more than circular reasoning, I can't help you.
Last edited by A Squared; 16th Oct 2012 at 01:15.
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It seems I got well ahead of myself, I read it wrong, and didn!t stop to think about it, my apologies. Also for my tone, it was insufferable.
The paper makes reference to "effective angle of attack", which I mean to take as the zero lift position for any lifting surface, regardless of shape, is this how you read it? I'll leave some room here for others and will read for now.
Thanks.
The paper makes reference to "effective angle of attack", which I mean to take as the zero lift position for any lifting surface, regardless of shape, is this how you read it? I'll leave some room here for others and will read for now.
Thanks.
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This looks like a whole bunch of non-sense going on here!
Lyman...
Of course I realize that's you quoting me...
Re-read my post where I defined the AoA terms that I was using and where I provided references to further expand on my definition. It might take time to sink in but what I said makes sense.
Lyman...
Italia. You realize the shaded box in your post is me quoting you? You claim two LIFT values for one AoA. The cross section for any chord is not relevant.
Of course I realize that's you quoting me...Re-read my post where I defined the AoA terms that I was using and where I provided references to further expand on my definition. It might take time to sink in but what I said makes sense.
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FlightPath...
That's a false premise.
In earlier posts I stated a few definitions of AoA... I'm not sure you have the same understanding of those definitions as I do.
If the wing section at AoA of zero degrees produces no lift, then the lift associated with a wing section must be associated with Newtonian physics rather than Bernoulli gas laws...
In earlier posts I stated a few definitions of AoA... I'm not sure you have the same understanding of those definitions as I do.
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Prandtl
I learned a lot seeing the old "polaires de Prandtl"...
they are the Cz(Cx) closed graphs at a definite airspeed.
Would anybody know where we could find them on the Web ? Thanks
they are the Cz(Cx) closed graphs at a definite airspeed.
Would anybody know where we could find them on the Web ? Thanks
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Lyman...
I have no idea what you're trying to say... I don't think you understand the definitions that I've already mentioned: geometric angle of attack and absolute angle of attack. It's generally accepted that, unless otherwise specified, when ever you mention AoA you are talking about the geometric angle of attack. If you did understand them you'd either understand what I've already said or you'd be able to specifically point out an error I might have made.
Please explain why you believe this definition dismisses Bernoulli. Effective angle of attack is yet another definition that is completely different from geometric and absolute. Do you know what effective angle of attack means?
I have no idea what you're trying to say... I don't think you understand the definitions that I've already mentioned: geometric angle of attack and absolute angle of attack. It's generally accepted that, unless otherwise specified, when ever you mention AoA you are talking about the geometric angle of attack. If you did understand them you'd either understand what I've already said or you'd be able to specifically point out an error I might have made.
From the paper in discussion, the notion of "Zero lift defines the effective angle of attack" would appear to dismiss Bernoulli.