Understanding Lift - As Non-technical as possible, Bottom-up

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DHenriques_
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Re: Understanding Lift - As Non-technical as possible, Botto

Post by DHenriques_ »

AKar wrote:
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
I'm afraid you have started your bar fight in my bar :-)))))))))))))

I am in total disagreement with you that Bernoulli should be disregarded in training. Bernoulli is TOTALLY relevant in any discussion concerning lift.
The proper way to teach the issues concerning lift is exactly the way I explained it above; that BOTH Bernoulli and Newton are complete explanations and both should be understood as relevant.
As I said, Newton is the easier to understand and the easier to teach, but by NO MEANS should Newton EVER be taught by any CFI to a student at the expense of Bernoulli.
Dudley Henriques

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AKar
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Re: Understanding Lift - As Non-technical as possible, Botto

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I don't know, Dudley. I'd figure it kind of depends on the training.

Back when I went through the tech studies in mil and GPL/PPL theories, the Bernoulli theory most often led into gross mis-applications and in resulting confusion by the time when eventual questions came out. This mostly was repeated in other physics studies, where the Bernoulli just had to be mentioned in this context...I'd figure there could have been better examples of his achievements in those lectures!

This is because the Newton's stuff is something that almost every secondary school student in the fields related should know by heart, being familiar part of view-of-the-world, and thereby 'confining' the discussion from outside, from the results point-of-view. To me, as the Newton's-based explanation is very good in explaining the results where the mechanism is most often irrelevant, it brings a little added value to bring in a theorem by Bernoulli that is so easy to take for more than what it is and to miss-apply it to describe phenomena it doesn't actually consider. I don't mean it shouldn't be known or introduced, of course not, as it is a very important part of understanding gas dynamics, what I mean that its connection to lift explanations is maybe more futile than it is of any good. Obviously, as long as any teacher can add stuff up without adding undue confusion, the more the merrier!

Edit: The bar fight came after a long silence, not my fault therefore! :mrgreen: No, as always, discussion - even in high voice! - is good, but throwing glasses must be avoided! :mrgreen:

-Esa

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DHenriques_
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Re: Understanding Lift - As Non-technical as possible, Botto

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AKar wrote:I don't know, Dudley. I'd figure it kind of depends on the training.

Back when I went through the tech studies in mil and GPL/PPL theories, the Bernoulli theory most often led into gross mis-applications and in resulting confusion by the time when eventual questions came out. This mostly was repeated in other physics studies, where the Bernoulli just had to be mentioned in this context...I'd figure there could have been better examples of his achievements in those lectures!

This is because the Newton's stuff is something that almost every secondary school student in the fields related should know by heart, being familiar part of view-of-the-world, and thereby 'confining' the discussion from outside, from the results point-of-view. To me, as the Newton's-based explanation is very good in explaining the results where the mechanism is most often irrelevant, it brings a little added value to bring in a theorem by Bernoulli that is so easy to take for more than what it is and to miss-apply it to describe phenomena it doesn't actually consider. I don't mean it shouldn't be known or introduced, of course not, as it is a very important part of understanding gas dynamics, what I mean that its connection to lift explanations is maybe more futile than it is of any good. Obviously, as long as any teacher can add stuff up without adding undue confusion, the more the merrier!

-Esa
Absolutely not! It doesn't "have to do with the training", unless of course the training is flawed.
As the man once said, "You can bend the rules but you can't change the laws." The simple fact is that Bernoulli is relevant because it's TRUE and what is true is totally relevant.
What you will find as you research aviation sources is that every pertinent and accepted INDUSTRY SOURCE properly explains BOTH Bernoulli and Newton as I have done here.
You will also discover white paper from individual sources strongly in favor of the Newton explanation.
This has occurred over time due to the simplicity of the Newton approach which is fine.....up to a point.
As I said before, there is no doubt that Newton theory deals with lift just fine and in effect Newton CAN be explained and taught without going into Bernoulli, but conversely, the same can be said for Bernoulli.
The ultimate answer is that any flight instructor worth the certificate should never teach one explanation without teaching that both are relevant. The problem doing it that way is that the student is taught only a partial explanation where more explanation is necessary to fully comprehend the issue.
I myself favor the Newton explanation but I would never even consider barring Bernoulli from any answer I would give or course I would teach on the subject of lift.
The only problem a CFI has when explaining Bernoulli is in including the improper applications of Bernoulli that unfortunately have been taught to pilots and included in texts over time.
To emphasize how simple it is to do this, I usually in lecture, can cover equal transit and the rest of the BS concerning Bernoulli in about 15 minutes.
In a way, what you said "It depends on the training" is correct. It DOES depend on the training. If the training excludes Bernoulli, the training should be corrected immediately to reflect the inclusion of BOTH Bernoulli and Newton.
Dudley Henriques

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Re: Understanding Lift - As Non-technical as possible, Botto

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I kind of agree - however, I'd argue it will depend on the training, and training "being flawed" from my point of view seems to occur more often when they start from Bernoulli stuff. I don't say one can't put up a good, solid-enough explanation that 'favors' Bernoulli's principle instead of direct Newton's law explanation, it only appears to be very difficult to do without slipping into a trap of describing something the Bernoulli's principle doesn't describe. That route is mined with pitfalls, as one must be quite precise: one of my favorite idiocies is to use Bernoulli to explain that if one has a paper sheet flat on the table, and blows fast jet of air over it using a hose or something...how this supposedly reduced pressure due to faster velocity air lifts the paper off. That kind of stuff, showing utmost ignorance over what Bernoulli's theorem is all about, but shown all over.

Of course, as long as any good explanation is created, one wouldn't care from who's theorem the explanation is started - those are just names after all. The distance to the nearest mine field appears to be shorter from using Bernoulli's as starting point, in my experience however. :)

-Esa

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Re: Understanding Lift - As Non-technical as possible, Botto

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AKar wrote:I kind of agree - however, I'd argue it will depend on the training, and training "being flawed" from my point of view seems to occur more often when they start from Bernoulli stuff. I don't say one can't put up a good, solid-enough explanation that 'favors' Bernoulli's principle instead of direct Newton's law explanation, it only appears to be very difficult to do without slipping into a trap of describing something the Bernoulli's principle doesn't describe. That route is mined with pitfalls, as one must be quite precise: one of my favorite idiocies is to use Bernoulli to explain that if one has a paper sheet flat on the table, and blows fast jet of air over it using a hose or something...how this supposedly reduced pressure due to faster velocity air lifts the paper off. That kind of stuff, showing utmost ignorance over what Bernoulli's theorem is all about, but shown all over.

Of course, as long as any good explanation is created, one wouldn't care from who's theorem the explanation is started - those are just names after all. The distance to the nearest mine field appears to be shorter from using Bernoulli's as starting point, in my experience however. :)

-Esa
I can see you and I will simply have to agree to disagree on this issue. No problem.
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Re: Understanding Lift - As Non-technical as possible, Botto

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That happens every now and then! :D No problem there, and it could just be that you haven't ran into as many bad lectures misapplying Bernoulli's as I have. It can be just a matter of my perspective why I dislike the starting point.

-Esa

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Re: Understanding Lift - As Non-technical as possible, Botto

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AKar wrote:That happens every now and then! :D No problem there, and it could just be that you haven't ran into as many bad lectures misapplying Bernoulli's as I have. It can be just a matter of my perspective why I dislike the starting point.

-Esa
On the contrary after teaching these issues for over 50 plus years and having written and lectured on them all over the world I think I can say with some degree of authority that I've heard "as many bad lectures as you have".
Anyway, as I have already said, no problem.
DH

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Re: Understanding Lift - As Non-technical as possible, Botto

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:mrgreen: Once upon time, I think it was a course of "elementary electrostatics" or something in polytechnic, and the topic was about concept of voltage, I remember how someone who had always found the electrics stuff a bit difficult sincerely asked the lecturer something in the lines of following:

— You said the voltages are always relative, that is, the volts are from point A to point B, right?

— That's correct.

— Okay, and they were also additive, in that I could make a circuit of batteries in series, and the voltage in between the ends of the battery line would add up from all the voltages in series?

— Yes, correct. Voltages are referred only in between two points, and by adding up voltages in between, they are additive, yes.

Then came the kicker (one may try this one out oneself!):

— But why then, as I measure the voltage with a multimeter in between the protective earth (PE) connection of my house and the negative terminal of a battery, it reads zero - but also, when measuring the voltage in between the PE connector and the positive terminal of the battery, it also reads zero! How can there exist a potential difference, or voltage, in between the terminals if both are in same potential in relation to the earth!?

We had some seriously hot-shot folks in electrodynamics shouting to each other when figuring out the "correct" explanation. I think I know my answer now to this very fundamental question when it comes to understanding voltage and electronics, but I think many other folks know theirs as well! :mrgreen:

In similar sense, I guess the aerodynamics (or any-dynamics) are topics where there is hard if not impossible to find answers that all find satisfactory! :D

-Esa

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Re: Understanding Lift - As Non-technical as possible, Botto

Post by Corkscrew196 »

Quess what in quantum mechanics molecules don't even exist...
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Re: Understanding Lift - As Non-technical as possible, Botto

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They do, or can be defined to exist, albeit being more...abstract beings due to the electrons and stuff being mostly re-defined. Or as far as I've ever went into it. Not much into quantum stuff, perhaps if I ever went into university. :mrgreen:

But even in macroscopic world, one could of course always argue how the molecules (or any given particles) do collide so readily with our wings or stuff, giving the pressure with no appreciable leaks, given how sparse the "grid" of atoms actually is for this kinetic gas theory stuff to work out.

And it is true, it won't work out that far as a perpetual ping-ball game. As it turns out, the electrical effects normally cancelled out over any significant distance, are what make the "touch" possible, due to the exceptionally close proximity of groups of particles (not anymore such regular bouncing balls when looked this close!) that are regularly neutral outside electrically, but not anymore when approached close enough. Of course, things like "touch" and "position" get a bit iffy in that curious world I've never went too much into.

As they say, the difficulty in discussing any question thoroughly mostly lies in where to put the end to the eternal but-why's, and when to call that a starting point. :)

-Esa

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Re: Understanding Lift - As Non-technical as possible, Botto

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I've removed this post myself.
DH
Last edited by DHenriques_ on 04 Sep 2016, 22:13, edited 1 time in total.

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Nick - A2A
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Re: Understanding Lift - As Non-technical as possible, Botto

Post by Nick - A2A »

Just make sure you don't spill my drink guys, when you start hurling each other along the top of the bar! :wink:

Seriously though - very interesting (if somewhat confusing for me) topic. It makes me feel a bit like Penny from TBBT in fact! :mrgreen:

Nick

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Re: Understanding Lift - As Non-technical as possible, Botto

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Nick M wrote:Just make sure you don't spill my drink guys, when you start hurling each other along the top of the bar! :wink:

Seriously though - very interesting (if somewhat confusing for me) topic. It makes me feel a bit like Penny from TBBT in fact! :mrgreen:

Nick
No problem. Drink on! :-)
DH

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AKar
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Re: Understanding Lift - As Non-technical as possible, Botto

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Indented for light discussion only, of course not trying to tell any "truths" to anyone - here or elsewhere! :) Just for clarification!

I'm feeling too old for bar fights and I'm lousy with darts anyway! :mrgreen:

-Esa

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Re: Understanding Lift - As Non-technical as possible, Botto

Post by DHenriques_ »

AKar wrote:Indented for light discussion only, of course not trying to tell any "truths" to anyone - here or elsewhere! :) Just for clarification!

I'm feeling too old for bar fights and I'm lousy with darts anyway! :mrgreen:

-Esa
I am enclosing for your edification and for additional interested parties an excellent white paper from NASA that deals directly with the Newton vs Bernoulli question.
You will note in the last paragraph summation that both Newton and Bernoulli are considered THE SAME when discussing the generation of lift.
Now assuming NASA is correct what remains between us is the question relating to whether it is proper or improper for a flight instructor to teach Bernoulli as an equal and complete explanation for lift.
I believe your position in your initial posting is that you believe Bernoulli should be left out of the teaching equation. I on the other hand am in total disagreement with your argument.
There is no "bar fight here" unless you question my expertise as a flight instructor. In that case I would be interested in learning of your expertise in my field.
The NASA link as follows;
https://drive.google.com/file/d/0B8ZDKc ... sp=sharing
Dudley Henriques

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