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You thought you understood lift?

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You thought you understood lift?

Old 16th Jan 2020, 07:14
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You thought you understood lift?

Good article here, it thought this was squared away, apparently not!

https://apple.news/A0Zc9zaiHRXqIyH2c_aHCtQ
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Old 16th Jan 2020, 08:15
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Unable to read the story in that link because I'm not using a device with Apple News installed.

Got a link that can be used with any browser?
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Old 16th Jan 2020, 08:28
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There appears to be a precis of it here (post #6). Nothing we haven't heard before.
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Old 16th Jan 2020, 08:47
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Originally Posted by Jonty View Post
Good article here, it thought this was squared away, apparently not!

https://apple.news/A0Zc9zaiHRXqIyH2c_aHCtQ
Yet still, the bernoulli theory of lift is being taught in schools. Were it true, symetric profiles and upside down flight would not be possible. Not to mention flat wings, which - while inefficient - also work.

If it had nothing to do with newton, then choppers would not have a problem with downwash. I find choppers particularly interesting elephant in the room, as they are nothing but planes with rotating wings. Yet, somehow, we accept chopper downwash as normal, and keep saying that downwash from non-rotating wings has nothing to do with lift. It just shows how dogmatic "science" is.

Personally, I think coanda effect and newton explain lift quite well, at least on the macro level.

PS: nasa has a list of incorrect theory of flight published for quite some time: https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
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Old 16th Jan 2020, 10:55
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I always thought Bernouilly's Law was a law on pressure, not lift. It says that the sum of static and dynamic pressure (and not lift) within the same large body of air (gas of course) is constant. Lift only occurs if there is a pressure differential over a body (eg a wing). If the velocity differs in two situations (eg the top and the bottom of a wing) within the same overall body of air, then according to Bernouilly's Law there will be two different pressures acting on that body, resulting in a force. That is lift. Maybe I'm wrong, but that's because it's been long a time since I studied these fascinating phenoma.
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Old 16th Jan 2020, 11:19
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Actually, Bernoulli's laws are still true. But with a grain of salt.
It is the idea that that air particles meeting the leading edge together do meet again at the trailing edge that is in error.
Actually they don't.




The theory of lift involves the notion of circulation of the flow around and airfoil;

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Old 16th Jan 2020, 12:56
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Never!


Making pilots:

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Old 16th Jan 2020, 13:24
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All the annotations of "magic" in the above diagram can be replaced by the word "money"
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Old 16th Jan 2020, 14:11
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No viscosity, no lift I seem to recall ( from the last time this was discussed here).
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Old 16th Jan 2020, 16:27
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With regard to a flat wing as in many military fighters, an aircraft flying inverted must have a higher AOA. The air flowing over the upper surface will still have further to travel than that flowing underneath due to the AOA, so a low pressure area above the (inverted) wing will still form, resulting in upwards lift.
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Old 16th Jan 2020, 16:44
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Tip: ask Owain Glyndwr...
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Old 16th Jan 2020, 21:27
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And Owain, in all probability, will refer you to a read of the late Arvel Gentry's articles - Gentry Sailing | Theory and Practice - as he has previously.

The problem with flying training is that we dumb down the stuff to suit what the new chum needs to get started ... you don't need to be a brain surgeon to fix up a minor cut on the finger. Unfortunately, the simplifications have become gospel over the years rather than being viewed as contributory discussion. The pilot doesn't need to be up to speed with the engineer's approach to things - rather, he/she needs to have an appreciation of the practical application to stick and rudder stuff.

An appreciation of circulation theory tells the story but one needs to keep it conceptual, lest we frighten off the new punters. The theory is nothing new - it started off a hundred years or more ago in the engineering world.

Another useful read is https://web.stanford.edu/~cantwell/A...mic_Theory.pdf
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Old 17th Jan 2020, 01:14
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Interesting conversation...

I have first hand knowledge of the question...

I was the lead on wake turbulence studies to reduce distances between aircraft...

Both of the wing designers were there from BA and AB..

They each gave radically different views on lift...(much to my surprise)
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Old 17th Jan 2020, 06:25
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Anyone here who participates in sailboat racing will know that technically-oriented racing sailors love to debate this question almost as much as pilots do. In fact, the second-fastest way to provoke a shouting match at the yacht club bar after racing, is to bring up the pressure difference (Bernoulli) vs flow deflection (Newton) theories of lift.

The two theories are not competing explanations at all -- they represent two different ways of talking about exactly the same behavior by the air molecules involved. The two explanations are inexorably linked together. The only way for the pressure above the wing to be lower than the pressure below the wing is if air is being shifted downward. So it makes no sense to argue about whether it is the pressure difference or the downward deflection of the air that creates the lift, because one cannot have one without the other.

(as an aside, this is the second fastest way to provoke a yacht club brawl because it'll never surpass asking, with feigned innocence, "Say, which do you think has lower drag, a hull with a mirror finish on which water would bead, or the same hull with a matte finish on which water would form sheets?)
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Old 17th Jan 2020, 09:35
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I am fascinated by the prevalence of the Bernoulli "explanation" and the fervour it arouses among the uninitiated, when as G&D has stated they are equivalent descriptions of the same phenomenon: one is a force approach and the other an energy approach. As anyone who has had to puzzle out the speed of a roller-coaster in school will know, the energy approach makes the calculation easier - but does "changing potential energy into kinetic energy" really provide an explanation of what is happening? Is this the source of the notion, I wonder, that aerodynamicists make their calculations in this way ("circulation") and when asked what causes the lift obviously reply "Bernoulli"? As has been pointed out above, Bernoulli has a "magical" overtone which I'm sure pilots and aerodynamicists like to have associated with their profession.

On the other hand, could there be another explanation, that the "wedge" explanation of lift does not make immediately obvious the necessity of keeping the upper-surface airflow attached, literally a matter of life and death in aviation (as well as in sailboat racing, though not so immediate). So for pilot training an explanation that emphasises this may be more useful, hand-waving and all.
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Old 17th Jan 2020, 22:04
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Originally Posted by fizz57 View Post
I am fascinated by the prevalence of the Bernoulli "explanation" and the fervour it arouses among the uninitiated, when as G&D has stated they are equivalent descriptions of the same phenomenon: one is a force approach and the other an energy approach. As anyone who has had to puzzle out the speed of a roller-coaster in school will know, the energy approach makes the calculation easier - but does "changing potential energy into kinetic energy" really provide an explanation of what is happening? Is this the source of the notion, I wonder, that aerodynamicists make their calculations in this way ("circulation") and when asked what causes the lift obviously reply "Bernoulli"? As has been pointed out above, Bernoulli has a "magical" overtone which I'm sure pilots and aerodynamicists like to have associated with their profession.

On the other hand, could there be another explanation, that the "wedge" explanation of lift does not make immediately obvious the necessity of keeping the upper-surface airflow attached, literally a matter of life and death in aviation (as well as in sailboat racing, though not so immediate). So for pilot training an explanation that emphasises this may be more useful, hand-waving and all.

The reason Bernoulli is prevalent is that its effects are directly measurable at every point on or near the aircraft. One can integrate those measurements to determine the overall forces involved. It also explains stall conditions; not so good at the turbulence. Where people fail in using it is they don't look at the entire flow-field where the air under the lift-producing wing of the typical plane is being shoved forward, decreasing the relative speed while the air above is accelerating, increasing the speed; this effect prevents the "same transit time" myth from being true. If one subtracts the airspeed distant from the aircraft it results in air moving forward under the wing, up in front, back, and then down.

Mathematically, Bernoulli is also friendly. One can set up functions where one kind of function acts as a source of fluid and another acts as a sink. Combining these can duplicate the observed flow field around aerodynamic bodies, allowing decent predictions of performance. See https://courses.maths.ox.ac.uk/node/..._material/1967 for part of the mathematical treatment of incompressible flow.

This is the biggest advantage and avoids having to do all the Newtonian book keeping of molecular level momentum transfer in order to predict what a wing might do.
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Old 17th Jan 2020, 22:36
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(as an aside, this is the second fastest way to provoke a yacht club brawl because it'll never surpass asking, with feigned innocence, "Say, which do you think has lower drag, a hull with a mirror finish on which water would bead, or the same hull with a matte finish on which water would form sheets?)[/QUOTE]

I always though it was when they announced they had run out of Rum😂😂
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Old 18th Jan 2020, 03:12
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To elaborate a bit on what Gauges and MechEngr wrote, what creates lift is the difference in air pressure between the top and the bottom of the wing. If you could magically measure the air pressure at every point on the wing, and then integrate it over the entire wing surface, the net result with be the lift and drag vectors that the wing creates. Those differences in air pressure are a direct result of the local velocity of the air - when the air speeds up, the local static pressure drops, when it slows down the local static pressure increases. The net result is lift and drag. Thinking about those pressure differences in terms of Bernoulli or Newton is largely a matter of preference.
Personally I think of it in terms of Bernoulli - it works for thrust as well as lift and drag - and as MechEngr notes it's (relatively) easy to model in computer airflow simulations. But I've found the easiest way to explain it to the initiated is to simply describe the pressure difference between the top and bottom of the wing.
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Old 19th Jan 2020, 16:05
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Same logic for jet engines and rocket engines

The same double explanation also applies to rocket engines and jet engines. You can say that thrust is created by accelerating the exhaust flow aftward, which creates an equal and opposite force in the forward direction: the mass of exhaust being accelerated times the acceleration equals the force we call thrust. ... or you can say that thrust is created by the pressure difference across the nozzle: the pressure difference times the area of the nozzle equals the force we call thrust. (slight oversimplification, intake must be considered, etc.) As with Bernoulli vs Newton explanations of airfoil lift, these are one and the same explanation. For any given density and viscosity of air, the pressure difference times nozzle area and the mass acceleration are mathematically related -- you can't have one without having the other.
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Old 20th Jan 2020, 02:15
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you can say that thrust is created by the pressure difference across the nozzle
How Rolls Royce explains thrust generation, the nozzle is only part of the story.

Although the principles of jet propulsion will be familiar to the reader, the distribution of the thrust forces within the engine may appear somewhat obscure- These forces are in effect gas loads resulting from the pressure and momentum changes of the gas stream reacting on the engine structure and on the rotating components. They are in some locations forward propelling forces and in others opposing or rearward forces. The amount that the sum of the forward forces exceeds the sum of the rearward forces is normally known as the rated thrust of the engine.

The diagram is of a typical singlespool axial flow turbo-jet engine and illustrates where the main forward and rearward forces act. The majority of the thrust in the example is provided by the combustion chamber (34,182 lbs) and secondly by the compressor (19,049 lbs).





At the start of the cycle, air is induced into the engine and is compressed. The rearward accelerations through the compressor stages and the resultant pressure rise produces a large reactive force in a forward direction. On the next stage of its journey the air passes through the diffuser where it exerts a small reactive force, also in a forward direction,From the diffuser the air passes into the combustion chambers where it is heated, and in the consequent expansion and acceleration of the gas large forward forces are exerted on the chamber walls.

When the expanding gases leave the combustion chambers and flow through the nozzle guide vanes they are accelerated and deflected on to the blades of the turbine. Due to the acceleration and deflection, together with the subsequent straightening of the gas flow as it enters the jet pipe, considerable ’drag’ results; thus the vanes and blades are subjected to large rearward forces, the magnitude of which may be seen on the diagram. As the gas flow passes through the exhaust system, small forward forces may act on the inner cone or bullet, but generally only rearward forces are produced and these are due to the ’drag’ of the gas flow at the propelling nozzle.
It will be seen that during the passage of the air through the engine, changes in its velocity and pressure occur. For instance, where a conversion from velocity (kinetic) energy to pressure energy is required the passages are divergent in shape, similar to that used in the compressor diffuser. Conversely, where it is required to convert the energy stored in the combustion gases to velocity, a convergent passage or nozzle, similar to that used in the turbine, is employed. Where the conversion is to velocity energy, ’drag’ loads or rearward forces are produced; where the conversion is to pressure energy, forward forces are produced.
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