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PostPosted: Sat Jul 02, 2016 7:08 am 
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Yes, this is my take on the topic that is the best way to start a bar fight in aviation community. :mrgreen:




Kidding aside, it is astonishing how poorly the lift is most often explained. Or actually why it even needs all these explanations, maybe it is that it can mysteriously point upwards that makes it a difficult concept, we being used to things falling down primarily. If I asked someone if he understood how the rudder of a boat works, he likely answers "Of course!" Explaining the same force when it points upwards seemingly requires all kinds of fancy explanations, many of them being just wrong.

There are "Newton-based explanations" and "Bernoulli-based" explanations and so on and so forth. I dislike them all, because they hardly explain anything but are just names for some relationships that play their parts. We are very apt in naming things, and then learning those names so we can supposedly communicate ideas instead of understanding them. Very often these ideas tend to a pitfall in attempting to explain a phenomena over a few sentences where no such explanation exists.

So, to begin our coffee table discussion, have a deep breath and think about it: what is that stuff you actually breath in?

Air is made up of molecules, tiny groups of particles. These particles are so tiny that we don't even want to consider the number of them in that breath you took, it would be enormous! They are actually so tiny that if we looked close enough, the rules that seemingly govern our visible world start to break up with them. However, for a very good approximation, we can say that they are moving around like very tiny balls, moving into random directions and with various velocities, bouncing from each other and from the objects they collide into - just like any balls with some mass and velocity would if allowed for perpetual motion. There are billions of them everywhere, and they are constantly colliding with each other - actually, the mean free path an air molecule travels before colliding into another in room temperature at sea level is in order of 68 nanometers - that's about 2.7 millionths of an inch! Imagine all that going on all around you - and within you!

When these countless tiny particles constantly collide against a wall for instance, we feel that as pressure, the momentum of them. The faster they travel on average, harder they punch the molecules and atoms of the wall, making them jiggle too - we feel that as temperature. One can imagine from that the temperature and pressure of a gas are inherently related. And indeed they are.

The air is constantly exerting pressure on every object immersed in it - and importantly, to itself! Understanding this is the key to understanding lift, transforming it from a mysterious effect into a necessary result of the world around us. It is so important that we want to imagine the mechanism a bit better.

Imagine a solid object going very fast through the atmosphere, say, a cannon shell. If the air was something like sand, it would plow a void in it, which may or may not collapse. But due to the air being made up of these constantly jiggling tiny little particles colliding with each other and everything, something very interesting happens! When the shell passes, it indeed leaves a void behind it, for it went by very fast. Now, some of the air molecules are, by random chance, traveling into that direction. Some ricochet from the other particles into that direction. But in that void, there are no other molecules to collide with! Therefore, on average, the air molecules can travel farther into that direction. They tend to fill up the void, by mere randomness!

Essentially the pressure, as we saw, tells us how much of momentum the colliding particles throw against an object or the air itself. If we had an area of low pressure formed, on average the air molecules which were bounced into that direction were both less often and less forcefully bounced back into other directions. This creates this magnificent property of the air: it tends to flow towards lower pressures! And because of the sheer amount of the particles in it, and the tiny scale of their collisions, we feel the average movement as a flow of matter, behaving as if it was continuous fluid. But to understand the tricks the fluid does, it is necessary to understand what it truly is.

Why the void is not instantly, or extremely quickly at least, filled up? This is because the air, like any matter, has mass. Like Sir Isaac Newton described, the mass and a force interact in accelerating bodies: the mass resisting the change in momentum and the force...well, forcing it. In this ping ball game this is satisfied by the fact that only so-and-so many of the gas molecules can be bounced towards the lesser resistance in a given time, and the resulting flow towards the lower pressure actually reduces the availability of the molecules punching others towards it. So, the amount of the molecules traveling or punching others towards the lower pressure actually reduces when there is flow towards the lower pressure. Effectively the flow takes time to accelerate like any mass. A given drop in pressure results in molecules, on average, accelerating towards it at a predictable rate, the pressure differential pushing the molecules while the mass of the mass of them holding back the acceleration. If the resulting flow was directed towards some air that was stationary on average, the molecules would pile up, kicking back from the increased resistance, and on average, slowing down into that direction.

Sounds familiar, huh? Lower pressure - speeding up, and vice versa. That's the Bernoulli principle! Essentially it is just an extension of Newton's description on how forces accelerate masses applied to the structure of air (or any fluid). In that sense, the "complete explanations" based on "either" the Newton's or Bernoulli's principles are BS, for the principles themselves relate to each other.

Now a Piper Aerostar comes, plowing its way through the immense spectacle created by these molecules. The molecules actually feel its approach in advance. The situation is actually very complicated, but you can imagine how it tends to punch some molecules forward, like a baseball bat. These immediately collide with other molecules, passing the blow ahead at some definite speed dependent on the amount of jiggling and bouncing going on. That would be the speed of sound, and we ought to figure out it must be dependent of the temperature, which measured the average jiggling and bouncing going on, in a way that this blow is transferred ahead in the air slower if there was less jiggling and bouncing going on and vice versa. And indeed, that's exactly how it turns out to be! This also shows that the air has capability of being affected over distances: the information of something happening travels through it, transmitted by that jiggling and bouncing going on. It can really be used to transmit information, in a way of sound.

At around that Aerostar, these properties of air have some interesting and very useful consequences. The airplane has its wings, which are smoothly formed planes that are flown through the air at some angle of attack. The leading edge bounces the molecules away from it, them hitting each other, piling up the air. This puts some extra-pressure to the leading edge, felt as drag because it is un-countered by a similar piling effect on the trailing edge of course. But the air, due to all that jiggling and bouncing going on, wants to get somewhere! It starts to flow around the object, splitting up at the leading edge. Now, the surfaces of the wing...let us say the upper surface to pick an example, form a smooth curve from the stagnation point at the leading edge - the exact center around which the piling effect takes place, and where the molecules have some hard time "deciding" which way to go, eventually getting bounced into one or another. Very quickly the curve of the wing's upper surface starts to curve away from the direction where any molecules were directly bounced by the wing. O-oh.

But the air had this magnificent property! The air molecules very nearby, by their random chances are flying towards the wing and kick others towards it in millions of collisions every millimeter. The air turns to fill up the void which would form otherwise. The flow tends to stick to the surface that way, and there tends to be a smooth, constant deficiency of molecules hitting each other and the wing's upper surface for it being tilted away, and in this case, is nicely curved (it doesn't has to be, but it works much better if it is!). This is felt as a lower pressure on the lift-side of the wing. And because there is that deficiency of molecules the others travel to equalize - thereby the flow, the average movement we feel, speeds up to be there. On the other side of the wing, that is more directed against the flow than away from it in comparison, the molecules hit against it somewhat like before, or may even be piling up a bit. The difference of the molecules hitting the wing is felt as a difference in pressures.

That difference in pressures is called the lift.

By the time the flow hits the trailing edge, there is no more wing to bounce against - but there is the air from the other side! This equalization of the average momenta takes place in the flow having an average velocity downwards at the trailing edge, with both sides of the wings pointing the flow into a common direction, but with the higher pressure wanting to equalize where there is no this common direction of the flow - that would be around the wing tips! A molecule just outside the wing tip only sees the inrush to fill the lower pressure, so it joins that, by being bounced by the other molecules. That's how the wingtip vortices are formed.

The pressure difference along the wing in flight is actually sustained, that is, not allowed to fill up, by the mass of the air resisting the flow to equalize the pressure. If the air had no "weight", it would not be able to sustain that pressure differential, filling it up instantly, and there would be no flight that way (of course, it would have no pressure without weight in atmosphere neither!). The deficiency of the molecules - the low pressure - is therefore sustained by the air's slowness to accelerate as described by Newton for any object, the relationship of these two known as Bernoulli principle. By sheer imagination of the nature, these all, these tiny little "balls" jiggling and bouncing around, the total amount of them getting directed some amount against the lift direction (down if lift was up), them bouncing against the wing and each other when getting pushed there, creating flow acceleration, pressure differentials, even temperature changes as is suggested by those occasional fog clouds over the wings, effective means of transportation and immense amounts of fun, all work out in perfect balance where there would be no one effect without another. It works so perfectly to satisfy what we dare to call the rules of physics - the Newton's laws, the conservation of momentum, the conservation of energy, and most importantly, the numbers of the flight manual and the FAA certification requirements, that it is easy to forget what is behind the names of the phenomena.




So, quite a bit is left out of this, but I'll be back as someone once (or twice or more!) said! It is noteworthy that I could only add stuff, in my mind there is not much to take away. There are no short, easy explanations to much anything in the nature, for the very way she behaves is the tend for a balance, by definition involving all the aspects which are connected in a way or another.

Every so often that strikes me, and stretches my imagination: watching the flags to fly in the wind, and the birds and the airplanes in the air - all that actually happens by mere randomness in a scale so immense that it just has to work that way.

-Esa

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PostPosted: Sun Jul 03, 2016 8:00 am 
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I agree that the verbal explanation doesn't really matter a whole lot (including your explanation), as just by just sticking your hand out a window as a kid, you understand lift without even saying a word. However, Newton's and Bernoulli's theories are brilliant and do explain it very well. I'm not sure if you realize, but your explanation here uses both of those theories.

For me, it's more important to show to a pilot what is happening, visually, when a wing stalls.

You can actually guess pretty well how a plane will fly just by looking at the shape of the wing's cross section (camber). You see a bulbous, long wing and you know it will just leap off the runway at a slower speed but the air will not be able to follow it at a higher speed, eventually creating a wall of drag. While the thin smaller wing will require much more speed to be effective in creating lift, and create a load of drag when slow with it's required high angle of attack.

A variable swept wing, like on an F-14, is just so cool as it gets the best of both worlds.

Scott.

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PostPosted: Sun Jul 03, 2016 10:45 am 
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I also find the engineering that goes into the wings of airliners fascinating for the very same reason. They can shrink down and reduce camber for high speed cruise and extend out and significantly increase surface area to slow down for a landing. If you watch videos of large birds, you can see them doing the same things with their wings.

The F-14 is like a bird of prey, tucking the wings in to make the attack. As a pilot, or passenger, I enjoy watching birds of prey tuck in their wings and dive away at high speed as they manuever away from aircraft in their proximity. Clever little beasts! :)


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PostPosted: Sun Jul 03, 2016 1:17 pm 
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Oracle427 wrote:
I also find the engineering that goes into the wings of airliners fascinating for the very same reason.
Modern wings are quite aesthetically interesting too; the feather-like ailerons in A380, and those glider-like wings in A350 and 787. :) The controls in these planes hide some rather interesting technology also.

Scott - A2A wrote:
For me, it's more important to show to a pilot what is happening, visually, when a wing stalls.
Stall phenomenon brings up a very interesting question, which is not all that easy to answer straight away without getting deeper into the interactions. One may think it is clear that when the air is flowing around a surface that is further and further tilted away from it, it may not eventually be able to follow. (It is, of course, not instantly clear why it is so, even if the idea is intuitive.) However, shouldn't that mean there would be even less molecules pounding the lift-side surface, the flow getting ripped off from it - meaning even more lift? This isn't the case, obviously, and will get studied in "part 2". :)

-Esa

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PostPosted: Thu Sep 01, 2016 10:26 am 
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Scott - A2A wrote:
I agree that the verbal explanation doesn't really matter a whole lot (including your explanation), as just by just sticking your hand out a window as a kid, you understand lift without even saying a word.


Scott,

That's a misunderstanding! Sticking out your hand creates form-induced drag, not lift.

Frits

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PostPosted: Thu Sep 01, 2016 10:42 am 
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Corkscrew196 wrote:
Scott - A2A wrote:
I agree that the verbal explanation doesn't really matter a whole lot (including your explanation), as just by just sticking your hand out a window as a kid, you understand lift without even saying a word.


Scott,

That's a misunderstanding! Sticking out your hand creates form-induced drag, not lift.

Frits


No. It's not a misunderstanding at all. Lift will be created on a hand or a barn door if both are presented to a relative wind with a positive angle of attack. There is drag most certainly but there is also lift created by the usual means although admittedly not efficiently on either a hand or a barn door.

The "thing" about lift is simply in understanding that although Bernoulli and Newton had no idea what lift was and that their individual theories were presented dealing with other matters concerning physics, BOTH are complete explanations for how lift is created. It should be further completely understood that the two theories do NOT ADD to form a complete explanation but in fact are simply two methods of explaining the same thing. Both theories are in fact occurring at the same time as lift is being created.
I will add that of the two theories for explaining lift to a student pilot (or anyone for that matter), the Newton explanation is by far the easiest to comprehend and is the favored explanation used by most competent CFI's teaching in today's environment.
Dudley Henriques


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PostPosted: Thu Sep 01, 2016 1:46 pm 
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When discussing lift and drag, it must be realized that the distinction is nothing but man-made convention, and no two 'separate' aerodynamic forces fundamentally exist. Then again, it is good to return to the definitions: we call the component of the aerodynamic force that is into the flow direction (or against our motion) the drag; and we call any aerodynamic force component that is perpendicular to the motion the lift. Whenever there is an aerodynamic force that is into any other direction but against the motion there must be lift by definition, the hand-out-of-the-window case included.

-Esa

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PostPosted: Sat Sep 03, 2016 1:00 pm 
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DHenriquesA2A wrote:
Corkscrew196 wrote:
Scott - A2A wrote:
I agree that the verbal explanation doesn't really matter a whole lot (including your explanation), as just by just sticking your hand out a window as a kid, you understand lift without even saying a word.

Scott,

That's a misunderstanding! Sticking out your hand creates form-induced drag, not lift.

Frits

No. It's not a misunderstanding at all. Lift will be created on a hand or a barn door if both are presented to a relative wind with a positive angle of attack. There is drag most certainly but there is also lift created by the usual means although admittedly not efficiently on either a hand or a barn door.

Maybe Frits you are thinking in terms of the "skipping stone" theory? - from the NASA Glenn Research Center - The Beginner's Guide to Aeronautics website. To quote some texts:

The lift of a wing is proportional to the angle of attack. This is true for all wings, from a modern jet to a barn door.... In reality, the shape of the wing has little to do with how lift is generated, and any description that relies on the shape of the wing is misleading at best. This assertion will be discussed in detail in Chapter 1. It should be noted that the shape of the wing does has everything to do with the efficiency of the wing at cruise speeds and with stall characteristics. (Understanding Flight)

---
Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. (I'm sad to say, even from some of my older text books :? ) Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years. From: Bernoulli and Newton (NASA Glenn Research Center). That whole page worth a read.

One should be careful not to form the mental image of the air striking the bottom of the wing and being deflected down to produce lift. This is a fairly common misconception that also was held by Sir Isaac Newton himself. Since Newton was not familiar with the details of airflow over a wing, he thought that the air was diverted down by its impact with the bottom of a bird’s wings. It is true that there can be some lift owing to the diversion of air by the bottom of the wing, but most of the lift is due to the action over the top of the wing. As we will see later, the low pressure that is formed above the wing accelerates the air down. (Understanding Flight)

The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down. It shoves the air down with its bottom surface, and it pulls the air down with its top surface: the latter action is the more important. But the really important thing to understand is that the wing, in whatever fashion, makes the air go *down*. (Stick and Rudder)


DHenriquesA2A wrote:
The "thing" about lift is simply in understanding that although Bernoulli and Newton had no idea what lift was and that their individual theories were presented dealing with other matters concerning physics, BOTH are complete explanations for how lift is created. It should be further completely understood that the two theories do NOT ADD to form a complete explanation but in fact are simply two methods of explaining the same thing. Both theories are in fact occurring at the same time as lift is being created.
I will add that of the two theories for explaining lift to a student pilot (or anyone for that matter), the Newton explanation is by far the easiest to comprehend and is the favored explanation used by most competent CFI's teaching in today's environment.
Dudley Henriques

The more I understand, the more I agree with this Dudley. I think it best to leave Bernoulli to the engineering types, and rely on Newton and his laws of motion for those of us "less-endowed". :smile: In my defense tho, I will quote Langewiesche -- "forget Bernoulli's theorem" -- at least where Lift is concerned. :D

Understanding how lift is created has eluded me for many years -- only recently have I really begun to understand how a wing works - that it must divert a certain amount of (weighted) air down to make the airplane i.e. its weight "buoyant" (Langewiesche). In fact, it has been mainly Langewiesche's Stick and Rudder and Understanding Flight by Scott Eberhardt & David Anderson, where I have found this "enlightenment" regarding this process of the creation of Lift. Both books I find fascinating. 8)

-Rob

A fwiw... Let us do a back-of-the-envelope calculation to see how much air might divert by a wing. Take, for example, a Cessna 172 that weighs about 2300 lb (1045 kg). Traveling at a speed of 140 mi/ h (220 km/ h) and assuming an effective angle of attack of 5 degrees, we get a vertical velocity for the air of about 11.5 mi/ h (18 km/ h) right at the wing. If we assume that the average vertical velocity of the air diverted is half this value, we calculate from Newton’s second law that the amount of air diverted is on the order of 5 (English) tons per second. Thus a Cessna 172 at cruise is diverting about five times its own weight in air per second to produce lift. Think how much air a 600-ton Airbus A380 diverts. (Understanding Flight)

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PostPosted: Sat Sep 03, 2016 3:33 pm 
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Dudley, Rob,

Interesting stuff, guys; I guess I'll have to do some reading!

In my country, the Netherlands, at least private pilots are teached the Bernoulli way. So did I; I'm not familiar with Newtons' theorem.
In short: we believe lift is the result of dynamic and static pressure interacting where the upper side of the wing produces a static underpressure relative to the underside of the wing ( I know a chap who likes to say "I'm going to do a lilttle hanging", yokingly referencing Bernoulli :)
I'm greatly simplilfying stuff here but you guys know it and it's all in the books (the "Pilot's Handbook of Aeronautical Knowledge" explains Bernoulli).

So, back to the child sticking out it's hand. Assume that the child is capable of folding the hand into a "wing" (ignoring the fact that most of the times the hand is held plain flat vertically against the wind). Then yes I must agree with Dudley that there will be some lift but, in my Bernoulli view, the effects of drag in this particular case supersedes the effect of lift. I cannot imagine the child, or you and me, being capable of physically experiencing a relative very weak underpressure: we experience a pushing force, rather than a lifting force.

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PostPosted: Sat Sep 03, 2016 4:35 pm 
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Corkscrew196 wrote:
Dudley, Rob,

Interesting stuff, guys; I guess I'll have to do some reading!

In my country, the Netherlands, at least private pilots are teached the Bernoulli way. So did I; I'm not familiar with Newtons' theorem.
In short: we believe lift is the result of dynamic and static pressure interacting where the upper side of the wing produces a static underpressure relative to the underside of the wing ( I know a chap who likes to say "I'm going to do a lilttle hanging", yokingly referencing Bernoulli :)
I'm greatly simplilfying stuff here but you guys know it and it's all in the books (the "Pilot's Handbook of Aeronautical Knowledge" explains Bernoulli).

When you replied, it should have been like this:

Dudley,
(and then much further down)
Rob, (because I am just muddling thru this stuff...)

Most definitely some reading, just make sure it is the right stuff! :smile:

We too (U.S.) have been taught for decades the Bernoulli idea -- that the wing is half a venturi (along with the Newtonian idea of the lower wing deflecting the air - action/reaction). The Pilot's Handbook of Aeronautical Knowledge states (incorrectly, as I am starting to understand this) that As the wing moves through the air, the flow of air across the curved top surface increases in velocity creating a low-pressure area. Here's what "Understanding Flight" says:

It should be noted that the speed of the uniform flow over the top of the wing is faster than the free-stream velocity, which is the velocity of the undisturbed air some distance from the wing. The bending of the air causes a reduction in pressure above the wing. This reduction in pressure causes an acceleration of the air. It is often taught that the acceleration of the air causes a reduction in pressure. In fact, it is the reduction of pressure that accelerates the air, in agreement with Newton’s first law.

We now have the tools to understand why a wing has lift. In brief, the air bends around the wing, producing downwash. Newton’s first law says that the bending of the air requires a force on the air, and Newton’s third law says that there is an equal and opposite force on the wing. That is a description of lift. The pressure difference across the wing is the mechanism by which lift is transferred to the wing owing to the bending of the air.



Corkscrew196 wrote:
So, back to the child sticking out it's hand. Assume that the child is capable of folding the hand into a "wing" (ignoring the fact that most of the times the hand is held plain flat vertically against the wind). Then yes I must agree with Dudley that there will be some lift but, in my Bernoulli view, the effects of drag in this particular case supersedes the effect of lift. I cannot imagine the child, or you and me, being capable of physically experiencing a relative very weak underpressure: we experience a pushing force, rather than a lifting force.

I could concede that... that it could be very difficult to tell the difference between the two. But at the "right" AoA, there should be lift produced. Although I am thinking atm about Scott and the impending hurricane (who cares that it is post-tropical) soon to be in his neck of the woods... that if he went outside and leaned into it (enough) he would both "experience a pushing force" and proper "lift". :mrgreen:

The 2nd Edition of Understanding Flight has a whole section on "Misapplications of Bernoulli’s Principle." I've got the Kindle edition which I can recommend, as it was fairly cheap and makes it easy to search. Again, fascinating material.

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PostPosted: Sun Sep 04, 2016 12:01 am 
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Wow!! That's a lot of words.... I thought it just boiled down to pull back on the stick and houses get smaller and push forward they get bigger. Now I have to relearn it all......

Cheers, Chris

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PostPosted: Sun Sep 04, 2016 4:45 am 
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Great Ozzie wrote:
The more I understand, the more I agree with this Dudley. I think it best to leave Bernoulli to the engineering types, and rely on Newton and his laws of motion for those of us "less-endowed". :smile: In my defense tho, I will quote Langewiesche -- "forget Bernoulli's theorem" -- at least where Lift is concerned. :D
I think Newton's laws are actually something that each person dealing with any technical matter in any depth should understand by heart. This is because while as written they provide little explanation, they form a solid set of axioms, postulates that describe any kind of motion in a way they mostly can be taken as granted.

To recap,

    I. For each body, without a net force acting on it, its state of motion will remain unchanged.

    II. When there is a net force acting on a body, this results in proportional rate of change of its momentum (in practice, acceleration).

    III. For every force acting on one body, there must be exactly opposite force acting on another body.

Understanding these postulates, while not explaining why or how something happens, often gives a good idea of what must happen. Like so that:

    - The first law could tell us for instance that it is impossible that an airplane would need more lift during steady climb. By the fact that the climb is steady, the vertical forces must be in exactly same balance they would be in straight & level flight, otherwise the state of motion would not remain unchanged (steady).

    - Similarly, we can say by the third law that if airplane's lift is from aerodynamic origin, so that it is the airflow that pushes the plane up, there must be a downwash too, as the airplane must act on the air by exactly opposite force the air acts on the airplane, and by the second law we can see that this must result in an acceleration of the air into downwash.

Note that this last one does not explain its acceleration over the top surface in physical terms. None of the Newton's explain how or why the air interacts with the wing in the first place, but they tie the results of any interaction, stating we don't need to care what they are, as regardless they need to satisfy these postulates which were by then experimentally confirmed to closely match every known interaction.

For a given interaction, Newton's laws predict the results, while not considering the exact mechanism of that interaction.



I also agree that in general, the Bernoulli's principle should even be "forgotten" in training kind of discussion, because it is the easiest one to mis-apply. All these ideas of longer paths upstairs, venturi effects over the top surface and so on and so forth...tens of them likely...they make very nice ideas, in turn making them very dangerous - because they are wrong. In literature, one can find some of-interest calculations that run through these explanations, clearly proving them impossible even if the suggested phenomena did exist (they most often don't).

Bernoulli's principle is actually an extension of Newton's laws on gas flows. In quite non-rigorous setup, let us say we had a big bottle of air under some pressure. Then a short tube runs out of it, releasing the air into the atmosphere, the pressure of which is lower in this case. It follows that it must be the atmospheric pressure that is applied to the end of this tube, and the higher pressure of our big bottle to the other end. The Bernoulli's principle is actually an extended roller-coaster equation, not only considering the height (in gravitational field) and velocity, but also the pressure and velocity, and it brings a tool to understand this situation. Not completely, but enough to start with. As there is this difference in pressure in between the ends of our tube, there is an airflow - that is, the flow is accelerating in accordance with Newton's second along that pressure differential, exchanging it potential (pressure) energy into kinetic energy.

That's what Bernoulli's states. Now, add this to the basic Newton's laws, you'd have a reason to believe the lower pressure on the upper surface of the wing will accelerate the air "to fill it".

But there is still not much to tell us why there is this low pressure in the first place. This actually does result from the kinematic theory of gases Daniel Bernoulli worked very much by the time, but not from the Bernoulli's principle directly.

With both these theories supposedly understood, it is easy to mis-apply them. I've been in many enough embarrassing physics lectures where what is typically shown is some kind of a circular argument that because of low pressure, there is this accelerated flow, and because this accelerated flow there is this low pressure. It is a difficult trap as nearly such relationship does exist, but here the low pressure remains unexplained. The actual relationship is indeed related to the Bernoulli's principle, but ironically the (wrong) supposedly Newton's "small bullets hitting the wing" explanation brings us a best starting point, because that is what happens. We only need to add that these small bullets don't only come from flow direction but from everywhere, and they collide with each other too - and they are way too numerous to be counted like bullets. And that they are not bullets, but collide in a completely elastic way for our purposes. So not much of bullets at all...

The statistical results of kinematic gas theory do require this low pressure to form: essentially, the surface starts 'plowing' through the air, and the mass of air taking time to accelerate to 'fill it' by its internal collisions pushing molecules that way, result in sustained low (and high) pressure area(s) around any moving object traveling through the air. Some of it is felt as a drag and some of it is felt as a lift. Bernoulli's equation actually describes this "sustainability" of the low pressure rather directly, but it does not implicitly require there to be one resulting in net motion of air (and thereby lift&drag). Essentially, I could position the stagnation points arbitrarily over the wing and calculate a solution in complete agreement with Bernoulli's, but which would be incompatible with reality. Also, I'd quickly run into boundary layer issues...and so on!

As these "bullets" are so numerous and coming from all directions, it is often most feasible to describe air as a continuous fluid that "somehow" wants to equalize its pressure differentials ("fill the voids"), and also has a mass that resists this accelerating pressure on itself. Just leaving out the kinematic theory of gases which attempts to fill in the void of "somehow", and submitting this statistical concept of collisions with a concept of pressure within the fluid with no surface to act on, adding in the losses and other stuff, the clever minds ended up with Navier-Stokes equations, which to our knowledge describe a macroscopic behavior of the fluid to arbitrary accuracy.

Anyways, as said, the Newton's laws are likely the best starting point: they give a solid picture of what must happen: opposite forces on the air and on the airplane, the downwash and the lift. It is just that they a give little immediate reason on why and how this interaction takes place except insisting that it must take place, given our observations.

Great Ozzie wrote:
It should be noted that the speed of the uniform flow over the top of the wing is faster than the free-stream velocity, which is the velocity of the undisturbed air some distance from the wing. The bending of the air causes a reduction in pressure above the wing. This reduction in pressure causes an acceleration of the air.
....resulting in that "bending" in the first place! This relationship is indeed not A→B but A↔B, and is sustained. But the creation of this sustained pressure and velocity differential is a subject difficult enough that some have gone as far as stating it is the initial set of vortices formed when the wing starts moving that is essentially required....Jeez...maybe valid from conservation point of view, but my causality doesn't follow. :mrgreen: Got to love the subject! :D



Edit: Wrote "long" where I intended to write "short". That's corrected.

-Esa

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Last edited by AKar on Sun Sep 04, 2016 5:49 am, edited 1 time in total.

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PostPosted: Sun Sep 04, 2016 5:00 am 
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Akar,

Before I forget I'd like to thank you for your brilliant and illuminating article.
For now back to investigate and study :)

Frits

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PostPosted: Sun Sep 04, 2016 5:13 am 
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Frits,

If you are really interested in stuff, I might recommend that you loan or buy "Understanding Aerodynamics: Arguing from the Real Physics" by Doug McLean, Boeing's engineer in aerodynamics I understand. It is a great book in that instead of being a pure physics book, it is somewhat directed to bring your attention into certain issues that often leaves one wondering or left alone when studying the topic from common sources. It is somewhat mathematically intensive, as in having calculus staff, but readable even if skipping stuff, assuming you've studied your classical mechanics and some kinematic gas topics.

I've loved it because it kind of precisely studies the very points I've known to be important but have been felt like left in dark with when trying to learn aerodynamics. Of course, I've never studied them in-depth in any institution or school, so I don't know the state of current education.

-Esa

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PostPosted: Sun Sep 04, 2016 6:50 am 
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Best explanation of flying ever. https://www.amazon.com/Stick-Rudder-Exp ... 0070362408

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