I'm afraid you have started your bar fight in my bar ))))))))))))AKar wrote: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.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". In my defense tho, I will quote Langewiesche -- "forget Bernoulli's theorem" -- at least where Lift is concerned.
To recap,
Understanding these postulates, while not explaining why or how something happens, often gives a good idea of what must happen. Like so that:
- 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.
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.
- - 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.
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.
....resulting in that "bending" in the first place! This relationship is indeed not A→B but Aâ†â€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. Got to love the subject!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.
Edit: Wrote "long" where I intended to write "short". That's corrected.
-Esa
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