No, no, rwjefferson was making the distinction between theories of lift, and causes–Bernoulli was said to be a calculation, whereas Newton’s laws were causes.
Close. I am thinking of total pressure. You are correct. Dynamic pressure is 100% of the total pressure along to the streamline. I believe it is also true that static pressure is 100% of the total pressure perpendicular to that streamline. I was using tangential to mean “not along streamline”.
I seem to be having a problem understanding Bernoulli’s Equation (the one in the illustration). I am not sure which p or r or V in this equation accounts for the change in the Vector of the airflow.
Also, please tell me again: How much does dynamic pressure increase for each decrease of ½ of the static pressure?
rwj
My money’s on the Quantum Bernoulli Theory.
rwj
:smack: Doh! Wrong question.
Velocity.
How much does the Velocity increase when the static pressure decreases by ½?
rwj
It’s not wierd at all - besides for the fact that the traditional “hump theory” is wrong in its axioms, the fact that air deflection is intuitively reasonable to laymen is the very reason that it’s the preferred explanation of wing lift to a layman. Why explain lift to flight school students as some esoteric conservation of energy artifact, which happens to be technically wrong, when it’s perfectly correct and very understandable to teach it as air deflection?
Myabe because, strictly, air deflection is technically wrong in the sense that it seems to be understood by the layman. aerodave says in the staff report that it is not just the underside that is doing the deflection.
Wait, someone said “axioms”? 
No, I don’t think it is wrong in its axioms, either. The premise is that the far field is unperturbed–in other words, not far from the wing, the atmosphere has recovered or is undisturbed. To a reasonable approximation, that is true, although we’ve all seen the pictures of vortex streets. Also assumed is a lack of friction–the upper flow beats the lower flow partly because of an increase in friction on the lower surface, I imagine.
Do you happen to have access to a wing with an underside but no topside? Or a top without a bottom? I’d love to do some wind-tunnel tests on one. But theoretically, the only forces which can be acting on the wing would be pressure forces, which always act normal to a surface and in a direction pointing from the fluid to the surface. Which would mean that any force pushing upwards on the airplane must necessarily be acting on the lower surface of the wing.
Now, it may well be that the shape of the upper surface of the wing has some effect on how much force is exerted on the lower part. And it’s almost certainly the case that the shape of the upper wing surface effects how much pressure force is pushing down on the wing (this pressure force is, for most aircraft, nearly as large as the force pushing up from below). But all of the actual upwards force comes from below.
Well, I said “axioms,” but maybe it was the wrong word. What I was talking about was the equal-transit-times assumption which is always mentioned but no attempt is ever made to justify it.
Hey, that wasn’t me…
Discounting friction.
Yabbut, we’re not counting static pressure forces. If I hold a wing stationary in a wind tunnel, with no wind, there will be a force on the top and on the bottom, but no lift force at all. In that sense, there are no lift forces. If we suddenly evacuate the air from above the wing, it will rise–and the entire effect has to do with the top of the wing, not the bottom. If someone had watched only the changes in bottom pressure, they would have predicted no lift.
OK, but like you said before, that doesn’t really give us any useful detail.
I’ve always seen it justificed in the way I did above–true, I haven’t seen it too many times.
You are conflating the notion of ambient and static pressure. Static (or sometimes referred to as quasistatic) pressures are the instantaneous pressures measured at any point alnog the flow; while the air may be flowing parallel to the wing surface, the static pressure is the same in any direction. When the wing deflects air, pushing it down, it creates a change in momentum that results in a higher static pressure. (Stagnation pressure is the pressure sufficient to completely arrest the motion of the fluid, as in at the leading edge of the wing.)
Lift forces come from a differential in the pressures between the top and bottom sides of the wing; reducing pressure on the top side of the wing isn’t sufficient to provide lift; there must be a pressure on the bottom side that creates a positive net force upward. Although Bernoulli’s Equation accounts for the changes in pressure (if you were able to maintain measurements on a bounded mass of air), lift is the result of unbalanced forces; i.e. Newton’s First Law.
Stranger
As aerodave said, the leading area raises the airflow slightly away from the upper surface. This leads to low pressure and low friction and higher velocity.
Raising the angle of attack leads to greater deflection under the wing,but leads to great increase in induced drag (drag that does not increase lift).
Over the wing air deflection: free lift,
Under the wing air deflection: what a drag.
rwj.
dave:
Using Newton’s Calculus, Bernoulli removed the Vector dimension from Newton’s F=MassVelocityVector. In this way Bernoulli identified the tangent force in chaotic Newton (fluids). Bernoulli speaks only of a closed system: Vector (of airstream) = 0. If you open the system, if you put the Vector back into airstream, you no longer have just Bernoulli. You do, however, still have Newton.
I believe this gives the Newton mechanism precedence over the Bernoulli mechanism.
Bernoulli can only describe the effects of tangential air pressure. It may predict, but it cannot describe mass acceleration.
Newton contains a dimension that Bernoulli lacks. Until you discover the Quantum Bernoulli formula, Bernoulli will be an incomplete Newton.
Only Newton will fully account for lift.
Peace
rwj
P.S. If Quantum Bernoulli would predict Newton, what would Quantum Newton predict?
I suppose so, it’s been a while. (I just noticed that NASA does it too, so maybe it’s just a matter of being clear, in the context.)
I don’t think even Bernoulli believers argue against that, though.
In a report dated 12-Jul-2005 aerodave wrote:
> You’d think that after a century of powered flight we’d have
> this lift thing figured out. Unfortunately, it’s not as clear
> as we’d like.
What do you mean by “we”, Kemosabe? Sure, there are plenty
of people who don’t understand it … but there are plenty of
people who do. The right answer has been known to experts
for many decades. The right answer has been available in
non-technical terms for many years.
> A lot of half-baked theories attempt to explain why airplanes fly.
Yes, we can agree on that. By way of analogy, there are also a
lot of half-baked theories about where babies come from, but that
doesn’t make the right answer any less right.
> To hear some tell it, wings work due to angle of attack and
> little else.
That sentence takes two perfectly good nouns (wings and angle of
attack) and joins them with an inappropriate verb, thereby creating a
sentence that is somewhere between meaningless and absurd.
The fact is that you can’t understand how wings work without
(among other things) knowing about angle of attack … but the
idea that lift is due to “angle of attack and not much else” is
absurd. It’s pointless to bring it up.
In the same paragraph:
> Newton’s Third Law–every action has an equal and opposite
> reaction–says this downward push on the air must result in an
> upward push on the wing. But there’s more to it, since the force
> generated by this deflection alone is far short of what we see in
> real life.
-
The paragraph needlessly conflates the idea of angle of attack
with the third law of notion. Each of those is a valuable idea, but
they are not the same idea. -
This passage is open to misinterpretation, because of a dangling
pronoun (“it”). The naive reader might take it as claiming a violation
of the third law of motion, which (I earnestly hope) was not intended.
> it suggests … that a thin flat plate would do just fine.
Actually, a thin flat plate does work OK. You can verify this
by buying a balsa-wood glider at the dollar store. The
behavior of flat wings can be explained using the same terms
that apply to more sophisticated airfoil sections.
For details on this, see
http://www.av8n.com/how/htm/airfoils.html#sec-circulation
> Cambered airfoils … can generate lift without any positive AOA
> at all, or even a slightly negative angle!
That’s not necessarily true, and even if/when true, it has no
physical significance. It depends on what you choose as your
reference for measuring angle of attack.
For details on this, see
http://www.av8n.com/how/htm/aoa.html#sec-def-aoa
and
http://www.av8n.com/how/htm/aoa.html#sec-raoa-aaoa
> Of all the flawed explanations of why planes fly, the one most
> people have heard involves the Bernoulli principle and “equal
> transit times.”
That needlessly conflates a good idea with a bad idea.
A) We can agree that “equal transit times” is a completely
bogus notion.
B) Bernoulli’s principle is a consequence of the laws of
physics, and (when properly applied) is really not open
to question. Sure, it can be misapplied, but so can
every other law of nature.
Prof. Bernoulli was a very smart guy. He was not even
remotely responsible for the "equal transit times" notion.
> Where does lift come from? One important fact is that fluids
> have a tendency to adhere to curved surfaces as they flow
> past. This is called the Coanda effect.
The Coanda effect does not explain how wings work. The
conditions under which the Coanda effect occurs are not
present near ordinary wings under ordinary operating
conditions.
For details on this, see
http://www.av8n.com/how/htm/spins.html#sec-coanda
> This illustration makes the notion of downward deflection a little
> clearer.
For a sequence of vastly clearer illustrations (including correct
timing information as well as streamline information), see
http://www.av8n.com/how/htm/airfoils.html
> The rest we chalk up to magic.
That is a terribly unscientific attitude.
The problem with “magical” explanations is that they can explain
false things just as easily as true things.
=================
Bottom line: It is quite possible for a pilot or a layperson to have
a solid understanding of how wings work.
For details, see
http://www.av8n.com/how/htm/airfoils.html
I really don’t have the time or energy to debate my entire article point by point. Let’s just start by saying that it was vital to bring up the flawed notion of equal transit times, debunk it, and provide the basics for a physical understanding of lift. All in a 1000-word article digestible by the masses. Also add to that the fact that what makes it to the web site is only vaguely reminiscent of what gets submitted for publication. Editing is a beautiful thing.
I will, however, tackle most of your points:
Yet in the entire rant, you don’t provide a clear-cut answer yourself. Despite promising in the post title. And again, I had to present a basic understanding in a way that everyone could get. If I were writing to an audience of scientists and engineers, it would have looked 110% different. But when writing for that average person in a confined publication space, you can’t bring certain things up.
This was a victim of editing, pure and simple. My original wording attributed the notion of AOA being solely responsible for lift to Langewiesche in Stick and Rudder. The pre-edit version contained the sentence: “Even Langewiesche makes too big a deal out of AOA, saying it’s ‘almost literally all there is to flight.’”
So, I didn’t pull that one out of thin air.
I already addressed this in the first page of this thread. I recognize that flat plate wings are functional. By “would do just fine”, I meant that ignoring the upper surface implies that a thin flat plate would make a wing just as efficient and effective as a properly-shaped airfoil. It does imply this, and that is wrong. That’s all I meant.
If I had elaborated to that extent, I’d have used up too many of my 1000 words on a point that didn’t deserve it. Sometimes, you have to take literary shortcuts. Sorry.
But it’s still a valid point. Because if you thought that a flat plate “would do just fine,” in the sense that it’s an acceptable design for a passenger jet’s wing…you’d be wrong. There’s no argument there.
That is necessarily true. There are arifoils (a lot of them) that have negative zero-lift angles of attack. That is, they are still producing lift as alpha goes below zero…if only a little bit of lift and only for a few degrees. And by AOA here, I mean the same AOA that aerodynamicists all agree on for airfoil analysis. That is, relative to the leading edge-trailing edge chord line.
And how does it not have physical significance? It shows perfectly well that you don’t need any flat-plate style deflection to generate lift. It’s goes right to my point that lift isn’t just from air bouncing off the bottom of the wing.
If you were paying attention to any of this thread, (or even to my article) you’d know that this has peen the point I’ve been trying to hammer into everyone’s head. That is, Bernoulli is correct, but shouldn’t get mixed up with such unsavory ideas as equal transit times.
But that doesn’t mean that many people don’t incorrectly put them together. And that was why I brought up the notion. In the article, and in reference to the flawed combining of equal transit times with Bernoulli (which combination is the most popular wrong theory of lift), I said
On the surface it’s a pretty satisfying theory. … Its only problem is its complete lack of resemblance to reality.
So don’t go accusing me of combining them inapproriately. Sometimes it’s necessary to describe the ignorance so everyone knows what to look for when they fight it. So the conflation of the good idea and the bad idea isn’t my doing…it’s only me exposing the lies.
People treat “Bernoulli” as a bad word once they realize the “Hump Theory” is incorrect. Tossing out the baby with the bathwater, as it were. My point is that Bernoulli isn’t wrong, it’s the other assumptions people tack onto it.
Cite? Under what conditions do you suggest they do and do not occur? The Coanda effect happens at a huge range of Reynolds numbers and scales. It’s not just bending the stream of water from your kitchen sink with a spoon.
It’s called a joke. You know…tongue-in-cheek? Where’s the rolling eyes smiley when you need it?
There’s nothing to debate. Fluid dynamics is not a matter of opinion.
Au contraire, I do provide a clear-cut answer, in detail, in terms a
non-expert should be able to understand. My explanation of how
a wing works can be found at
3 Airfoils and Airflow
I provided links to this from several places in my previous “rant”.
Server logs indicate many people have followed these links. Perhaps
aerodave should do the same.
The original cited remark is still not necessarily true. What I said
is still true. Anybody who thinks otherwise is just showing their
lack of expertise. As I previously explained, the reason that “zero
angle of attack” cannot have physical significance is because the
choice of reference against which the angle is measured is arbitrary.
If you want to understand this point, please read the references I
cited previously:
2 Angle of Attack Awareness and Angle of Attack Management
and
2 Angle of Attack Awareness and Angle of Attack Management
Another readily-available reference that makes the same point is
http://scienceworld.wolfram.com/physics/AngleofAttack.html
Defining angle of attack in such a way that when the angle of
attack goes to zero then by definition, by construction the
lift goes to zero is among the common and sensible choices, and
is particularly convenient from a pilot’s point of view. For details
on how this works, see
2 Angle of Attack Awareness and Angle of Attack Management
Please read my post before replying to it. Please do not
rip my statements out of context and then complain that
they lack context. In my original post, the very next
sentence provided an appropriate citation, to wit:
> The Coanda effect does not explain how wings work. The
> conditions under which the Coanda effect occurs are not
> present near ordinary wings under ordinary operating
> conditions.
>
> For details on this, see
> 18 Stalls and Spins
As explained in that cited reference, a sufficient condition for
observing the Coanda effect is to have a narrow, high-speed
jet of fluid impinging on a curved surface. More generally, you
need to have a velocity field that falls off rapidly as a function
of distance, faster than the field near an ordinary wing does.
If you want still more detail, please read the NATO AGARDograph
by Peter Bradshaw that I cite in my bibliography.
22 Bibliography
I realize that most readers are not interested in highly
technical NATO research reports. However, reliable info
on the Coanda effect is hard to come by otherwise …
which should tell you something about how unimportant
Coanda is for understanding the behavior of ordinary wings.
Even people who have spent years in graduate school followed
by years working as airplane designers don’t know (and don’t
need to know) much about the Coanda effect.
The fact remains that contrary to what aerodave said in
his article, using the Coanda effect to explain the operation
of a normal wing makes about as much sense as using
bowling to explain walking. To be sure, bowling and walking
use some of the same muscle groups, and both at some level
depend on Newton’s laws, but if you don’t already know how
to walk you won’t learn much by considering the additional
complexity of the bowling situation. Key elements of the
bowling scenario are not present during ordinary walking.
John:
Welcome aboard this flight.
:whistle: May I call a truce for the time being?
How would you sum up the answer to How do Airplanes fly?
In one word:
In a few words:
In a sentence (or two):
I am also curious on your take of this previous post:
rwj
P.S. Are you sure magic has nothing to do with flight? I remember seeing Professor Dumbeldore on a website devoted to flight.
Let’s see. The professional know-it-alls at straightdope.com
wrote 1000 words without correctly answering
the question, and you want me to do it correctly
in a sentence or two?
I’m flattered. Really. But I must decline the honor.
If I could explain it in a sentence or two, I wouldn’t
have written a book chapter with 12,000 words and
28 color illustrations.
3 Airfoils and Airflow
A 16-point summary can be found at
3 Airfoils and Airflow
That’s a red herring if ever I saw one.
-
The reverse magnus effect occurs only in a small
sliver of parameter space. An ordinary wing, under
normal operating conditions, does not exhibit the
reverse magnus effect or anything like it. -
Among teachers there’s a proverb that says
“Learning proceeds from the known to the unknown.”
In the present case, it would be madness to use one
thing the readers don’t understand (the reverse
magnus effect) to explain something else they don’t
yet understand (wings). There are much simpler and
more direct paths leading to an understanding of
wings. Again:
3 Airfoils and Airflow
Professional?! How much are you guys getting paid?
And is any of the money coming from the Democratic Party?
We’re supposed to get paid? 
Looks like I need to talk to Dex.
As for Dr. Denker, reviewing the last couple day’s worth of threads reveals that, of all the nit-picks he brought out in his first post, we’ve whittled it down to two remaining differences in our points of view:
[ul]
[li]The definition of angle of attack[/li][li]The role of the Coanda effect in lift[/li][/ul]
I brought up the reality of negative values of zero-lift angle of attack. Denker says I’m using the wrong definition of AOA, and gets fairly close to Pit territory in doing so. (Know-it-all? This woudl be more pleasant if you would learn how to differ in opinion like a professional.)
Look up any set of airfoil data, particularly the de facto Bible of airfoil analysis: the NACA airfoil data. Cambered airfoils do have negative values of alpha when lift is zero. Period. There is no debating that, it is a hard fact. Engineers almost exclusively define AOA based on the airfoil chord. This is the definition that allows for negative zero-lift alpha, and is the definition I employed in the article.
So you’re a pilot. Good for you. Pilots use different definitons of alpha, sometimes depending on the application in question. But you can’t argue that it is incorrect to analyze airfoil behavior based on the definiton used by the thousands of engineers responsible for designing the very planes you fly.
And as for the Coanda effect, I’ll concede it’s a squishy argument. It’s true the Coanda effect typically refers to the behavior of impinging jets, and not to whole flowfield behavior over wings. But I counter that it’s mainly a question of semantics. And that’s mainly because of the circumstances of its discovery, when Henri Coanda discovered that his early jet prototype kept setting itself on fire because of the effect that now bears his name. But the fundamental reasons for why either type of flow sticks to a convex surface are the same. While it may give an engineer the creeps a little bit to attribute the flow turning to Coanda, I still say that’s only because of the traditional use of the term. Either behavior (airfoils or impinging jets) is due to the same physical laws.
So, I took the flow turning behavior, and attributed it to a related and not-inaccurate effect that would be easy for the lay person to read up on (and perhaps they’ve even heard of it already). I don’t agree with your bowling/walking analogy. It’s more like using a treadmill to explain walking to someone. It’s the same thing, but a little diefferent at the same time.