With regards to the effect of angle of attack: It seems to me that there’s a simpler experiment, independent of one’s precise definition of angle of attack, which would nonetheless determine whether the angle of attack is the sole factor in lift. To wit: Is it possible to design a wing such that the front edge is symmetric with the back, and which would generate lift from an airflow travelling either way? By any reasonable definition of angle of attack, the AoA of such a wing would be zero. If the wing nonetheless generated lift, no matter how feeble, that would unambiguously prove that there are sources of lift other than angle of attack. In fact, I’m pretty sure I’ve seen working aircraft which had such wings (for instance, a VTOL prototype which could take off like a helicopter, then lock the rotor in place for use as a fixed wing), and it also seems to me that a common Frisbee would be another example of such a symmetric airfoil. So it seems that the experiment has actually been done, and it worked.
John: It can be difficult to see when your guard is raised.
Please take a deep breath and relax. Like you, I have become very frustrated by the Bernoulli believers that do not understand the true mechanism of flight; that have confused so many for so long. This has led to many a rant on my part.
You evidently did not see my real question of Reverse-Magnus. Which exactly is the red herring? It is not my comment that Reverse-Magnus is actually Bernoulli; it is your failure to recognize the question. Post #55 is my take on Reverse-Magnus. Please, no more red herrings.
You have asked us to read many, many more of your words than our in our thread. Please honor us by taking the time to read our full thread before you reply further.
You have done very well with your website. As I read more I will get back to you if I have comments and questions. Please no more self-cites. They tend toward pages and chapters when we are looking for paragraphs and summaries.
We see that.
Unfortunately the most important audience (children) cannot read your book. Even (especially?) many of us Dopers would have difficulty.
Please consider the process of distillation. During this process, extraneous detail is taken until only the essence, the truth remains.
How do airplanes fly?
Newton.
How is this not true?
rwj
P.S. Please consider us as friends.
The angle of attack is a red herring. The angle of attack only has to do with effeciency.
It is the angle of departure that defines lift.
rwj
In the sense that rocks don’t fly. 
Put your head up to the monitor. I wish to test Newton on flying rocks at rock heads.
Rocks fly by Newton.
Determine mass as per Newton.
Calculate deceleration to gravity and friction as per Newton.
Apply appropriate acceleration at appropriate vector as per Newton.
What is your theory?
rwj
See, that’s my point. If you’re saying that rocks fly in the same manner that planes fly, then you are answering a question that is different than the one that is usually asked, when people ask “How do airplanes fly?” It’s a matter of context. Nobody disagrees with Newtonian mechanics (except for, you know, it only being an approximation ).
I have now placed my head against my monitor. It is now safe for you to throw a rock at yours.
Newton is not limited to one manner. Newton acts in a different manner in fluids. That does not negate the truth of Newton.
Is the Harrier not a plane? Does it not fly by Newton when it is in a hover?
Rocks and planes both fly by Newton.
rwj
P.S. It might be safe for you and my head, but definitely not for the rock.
I’m virtually certain I never said that. What
I said was that there is no physical significance
to the choice of one definition over another, and
no physical significance to the fact that some
airfoils might or might not have nonzero lift at
zero angle of attack, depending on how you choose
to measure it.
Specifically, suppose I have two wings that work the
same, except one develops lift at zero angle of attack
while the other doesn’t. Then I just install them at
different incidence, and the overall airplane works
the same. As far as the lift-producing mechanism is
concerned, there is no physically-significant difference.
Wow, an appeal to scripture! That’s a poor substitute
for a logical argument. For details on why an appeal
to scripture is considered unscientific, see
Soft versus Hard Evidence; Appeal to Authority etc.
Saying it more emphatically doesn’t make it any
less wrong. Let me just point out that while choosing
to measure angle of attack relative to “the” chord is a
viable option for an airfoil section, it typically
doesn’t work for an actual wing because on a typical
wing the chord-direction varies considerably from place
to place, due to washout … not to mention movable
flaps, ailerons, et cetera. I thought this thread was
about wings (not airfoil sections).
Saying Coanda explains lift is not squishy … it’s just
wrong.
Agreed.
The same? Then why was Henri Coanda surprised?
Why was he granted patents on the Coanda effect,
starting in 1934? That seems kinda significant, because
to apply for a patent you have to claim that you’ve
invented something new. To put the dates in context,
a satisfactory practical and quantitative description of
how airfoil sections work dates back to 1912 and earlier
(Zhukovsky), and a solid understanding of lift and drag
in 3D dates to 1918-19 (Lanchester & Prandtl).
By the same token, air-hockey is due to the same
physical laws. But not everyone who plays air-hockey
is a rocket scientist.
Wings are special. If you are supposed to be explaining
why wings are special, you don’t get credit for citing
“Newton” or “physical laws” that apply to everything
(special or not) and you don’t get credit for citing
phenomena (Coanda) that do not occur near wings
under normal operating conditions.
Yeah, but that’s still wrong. If you want to say flow-turning,
say flow-turning. Don’t say Coanda, because that means
something else. And BTW it is not easy for a layperson
to look up reliable information on the Coanda effect.
=========================
People live their lives surrounded by fluids. Sometimes
they think that is enough to make them experts on
fluid dynamics. Well, it’s not. Fluid dynamics is really,
truly, complicated.
In response to several requests, I have started working
on a 25-words-or-less explanation of how wings work.
Here’s a rough draft (19 words):
free-stream velocity pressure density circulation
upwash downwash momentum area span trailing-edge
stagnation angle-of-attack lift drag Kutta Zhukovsky
Bernoulli Newton
If you want a version with verbs in it, you’ll have to
wait a while. First I have to finish my 25-words-or-less
explanation of how to play the piano, my 25-words-or-less
manual on neurosurgery, and my 25-words-or-less
explanation of quantum field theory.
In the meantime, you’ll have to make do with the
500-word summary of how wings work, at
http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary
and the longer, illustrated explanation at
http://www.av8n.com/how/htm/airfoils.html
I need a louder whistle.
That would be the Reverse-Magus Effect.
In Harry Potter and the Half-Blood Prince, Mr. Weasly admits his dearest ambition: “To find out how airplanes stay up”.
I was trying to engage ”grumpy John” in dialogue. I find it surprising that someone that enjoys Harry Potter, refuses to communicate with us muggles. He did say something about honor. Maybe he meant lack of.
I mistook this to be Dumbledore. I am now convinced it is just a Malfoy in disguise.
rwj
Scientific questions are not answered by seeing who argues the
loudest, or who has the biggest whistle. Instead they are answered
by a big lattice of observations and theories held together by logic.
Daniel Patrick Moynihan once said that everyone is entitled to their
own opinions, but they are not entitled to their own facts.
When Jo Rowling writes of flying hippogriffs and broomsticks, that’s
fantasy. When Lumpy writes about the Reverse Magus effect, that’s
good clever humor. When aerodave says that the Coanda effect
“explains” how wings works, that’s laughable … although non-experts
may not realize just how laughable it is.
There is a place for facts, and a place for opinions, and a place for
fantasy and comedy. You just have to know which is which.
Dwayne, down at the hair salon, wants me to respect his opinion.
In his opinion, the moon is made of green cheese. Well, too bad,
I’m not going to respect that. His so-called opinion is outweighed
by a huge collection of facts. He is entitled to his own opinion,
but not entitled to his own facts.
Aerodave has presented precisely no evidence in favor of the
Coanda “explanation”. Meanwhile, evidence to the contrary is
readily obtainable, even if you’re not a rocket scientist. Go down
to some nearby small airport. Go to the flight school, and ask an
instructor to show you the stall-warning sensor on some small
aircraft, and to demonstrate the operation thereof. There are
two types, as discussed at
3 Airfoils and Airflow
Both types are mounted on the front of the wing, slightly below
and very slightly aft of the leading edge. The vane type is
activated when the air at this location is flowing upward and
forward (noseward) at this location. The reed type is
activated when there is suction (low pressure) at this location.
In both cases, there is absolutely no way the observed behavior
could be explained in terms of the Coanda effect. Fortunately,
it is easily explained if you have a basic understanding of how
wings really work, as discussed at
3 Airfoils and Airflow
Pilots are supposed to check the stall-warning sensor before every
flight, so you might imagine it would be helpful to have some clue
about how they work. This is a question that commonly comes up
in the very first lesson when somebody starts taking flying lessons.
Many instructors don’t really understand it, which is sad and
somewhat scary. This is one of the reasons that motivated me to
write the book
See How It Flies
People sometimes expect wings to be simple. After all, you can make
a wing that has no moving parts. Well, the complexity isn’t in the wing;
it’s in the fluid … which has a great many moving “parts”. There will
never be a 25-words-or-less description of how wings work. The
question about stall-warning indicators is just one of several dozen
questions that people might reasonably ask, and which require a
nontrivial answer.
You can be reasonably sure that if somebody gives you a one-word
or two-word answer (“Coanda” or “Newton” or “green cheese”),
they don’t know what they’re talking about.
You can make life hard on yourself, or easy. One respectable but
very difficult option is to insist on 100% first-hand evidence. This
requires repeating all the calculations and all the experiments on
how a wing works. That requires a rigorous education plus years
of work. At the opposite extreme, it is easy but worthless to listen
to people who don’t know what they’re talking about. A sensible
middle ground is to listen to folks who appears to know what
they’re talking about, and then verify their story by doing
a few simple first-hand experiments … such as the stall-warning
sensor check mentioned above, among many others.
While we’re on the subject, it would be nice to answer Bill Rehm’s
original question about the relationship between the upward force
on the wing and the downward force on the air. The answer is that
they two phenomena are inseparable. You can’t have one without
the other. They are two sides of the same coin. This is a consequence
of Newton’s third law, or in more modern terms, the law of conservation
of momentum. The best way to think about it is as a transfer of momentum
from the wing to the air, as discussed at
3 Airfoils and Airflow
If anybody wants the next level of technical detail on this, see
Vortices: Momentum and Energy Near a Wing
And yes, it is mostly a pull on the top side of the wing, not a push
on the bottom side. This is illustrated at
3 Airfoils and Airflow
In a thread full of some of the more ludicrous statements made on this board in a while, all in the name of trying to boast who can describe the net effect of forces upon a wing the “best,” this has to be the topper. These two statements are, of course, inherently conflicting. If the wing is transfering momentum to the air, the air cannot be pulling on the wing, unless someone is using a definition for the word “pull” not generally known to speakers of the English language.
But more importantly, there simply is no way that the wing is being “pulled.” I defy the poster of this statement to draw a force diagram that shows the actual physical force being imparted upon the top layer of molecules in the wing by the layer of molecules of air directly above it. Or, for a more general thought experiment, take the same situation you have with the wing and the air, and remove all air from underneath the wing (i.e., replace it with vacuum). If you think there will be upward movement of the wing in that case, you probably believe in skyhooks as well.
No matter what terminology you wish to use, Newton, Bernoulli, etc., the wing goes up because there is a difference in pressure between the one side and the other. The reasons for this are complex; the attempt by the SDSAB to offer an easily understood explanation should not be condemned for the fact that it didn’t get into the intricacies involved to the satisfaction of someone who wishes even more detail; this is inherently true of ALL the answers given by the SDSAB. You might as well complain about the lack of thourough answer to the Taos Hum. For the vast majority of people the answer given on the site is sufficient to make us aware of the general forces involved, and to alert us that we may have been bamboozled by the more ridiculous efforts of our student science books to offer an explanation.
But if someone intends to be a nit-picker, or to thump his chest loudly and claim he explains things better than anyone else, it certainly helps to avoid really stupid statements such as highlighted above. Apparently, despite disclaimers to the contrary, hyperbole makes up as much of that poster’s argumentative efforts as anyone else’s.
Let’s see here. You have ridiculed the law of conservation of
momentum. Do you think your opinion will result in immediate
repeal of the laws of physics?
http://www.av8n.com/how/htm/airfoil...ec-air-momentum
You have ridiculed the diagrams that I computed from the
equations of motion. Do you think your opinion will change
the laws of motion? Do you think it will change the output
of the programs?
http://www.av8n.com/how/htm/airfoils.html#fig-3pv
Does that mean you will be giving us a simple explanation of how
wings work, directly using the molecular-dynamics description? Wow,
I can hardly wait.
Will this include a description of wingtip vortices, induced drag,
stall-warning sensors, and why an aileron at the back of the wing
is so effective at modulating the coefficient of lift … all in terms of
molecules hitting the wing? (These are phenomena of direct
interest and importance to pilots, the sort of thing that any
halfway decent explanation of how wings work ought to be
able to handle.)
Oh, and by the way, what is the evidence supporting your
assertions? Did you do any theory (calculus of complex variables
and all that)? Did you do any experiments? Any numerical simulations?
Big Whistle, (cute choice of name, btw), thanks for making a very good point so I didn’t have to. That is, Staff Reports are necessarily written and edited for “mass consumption.” There will always be subtleties and details that don’t make it through, even if the author would have wanted to express them. This is all especially true for a subject as potentially confusing and gnarly as aerodynamics.
I’ve been privileged to have worked with and studied under some of the most brilliant modern minds in all the fluid dynamic disciplines—experimental, computational, and theoretical. And none of them would be so bold as to say they have all of it (or even a good chunk of it) figured out. If anyone claims they have, they don’t know their own limitations, or they don’t really appreciate the nearly boundless complexity of fluid behavior.* The more you know, the more you realize that you don’t know.
So, my only goal in writing the report was to give the completely uninitiated reader a chance at getting a “feel” for what’s going on around that airplane wing, while at the same time being able to recognize the fallacies in all the “wrong” reasons they’ve heard. And even that’s a lofty goal for a two-page paper. The greatest hope one can have in writing such an article is that it will spark even one person’s interest enough to do some learning on his own. If that happens, then great; ignorance has been fought.
But, again, the report is a compromised piece of work. It contains the same implicit disclaimer that accompanies all the Staff Reports. Namely, that it’s not the last word, the whole story, or even an attempt at a complete summary. Nits will always appear for someone to pick. And in the case of my guest contribution, I welcome and enjoy the debate, as long as the people and the discussion remain civil.
(Denker likes to criticize my reference to the Coanda effect. While I have already conceded that airfoil flow is not precisely the same as the textbook definition of Coanda, I maintain that both penomena are created by the same pressure gradients, and result in the same flow attachment. The freestream-over-wing effect doesn’t explicity have a name, but acts similarly to and is caused by the same things as the Coanda effect. So, for the purposes of this report, it’s close enough to call it the same thing. If JD can definitively show me a non-arbitrary line that separates free jets from uniform freestream flow, I’ll be happy to help pioneer the naming of the “Denker effect.”)
*It’s especially poor form when one of those same people comes into this thread and questions my professional qualifications. But I’m better off leaving that for a pit thread.
You misstate even what I said. I ridiculed only one simple thing: “And yes, it is mostly a pull on the top side of the wing, not a push on the bottom side.” In doing so, I referenced your statement two paragraphs prior that was completely at odds with the statement ridiculed. I notice that you have not attempted to make the two statements compatable.
You made an assertion, a pretty simple one at that. You asserted that the wing is pulled. I challenge that assertion. By definition (at least, the definition commonly accepted for the word in the English language), to “pull” means to impart force from the direction towards which the force is imparted. This required an interaction between the object upon which the force is imparted and another object located in the direction of the force being imparted. So, I am challenging you to draw a force diagram in which you establish an interaction, physical, electro-magnetic, etc., between the wing and some object (presumably air molecules) located above the wing, and use that to explain in great measure the resulting lift on the wing. In so doing, you will, to show your assertion correct, necessarily have to explain said “pull” without reference to interaction between the wing and anything “below” the wing.
This thread has been littered with some pretty silly assertions by the participants, all of whom (other than aerodave, who seems to be sticking his neck out only on the “Coanda Effect” issue, which appears totally a nomenclature debate) seem to feel that they can better describe what makes an airplane go up than the others. rwjefferson has been doing that on this site for many years; his assertion that it is “all Newton” is only correct to the extent that you ascribe all physical or gravitational interaction between items as Newtonian, and even then it fails to describe what is going on at an atomic or sub-atomic level. Such a statement, therefore, serves absolutely no purpose, though I understand what he is trying to say with it. Similarly, I understand what you are trying to describe by saying that the wing is pulled up, but in actuality, it is a poor choice of words, for reasons I have asserted. I will be quite happy to have you prove me wrong.
The incompatibility is only in Big Whistle’s mind.
It turns out that the molecular-dynamics approach that he advocates is not the
only way of analyzing things … and certainly not the simplest or clearest way,
for reasons discussed at
http://www.av8n.com/how/htm/airfoils.html#sec-fluid
I am quite aware of the molecular-dynamics approach; I just choose, for
simplicity, not to use it directly … preferring the entirely conventional
description using the notion of a fluid.
Also, contrary to his assertions, my use of words like pull and suction
is not unknown “to speakers of the English language”. As one small example,
the American Heritage dictionary uses pull at one point in its definition of the
word suck. More to the point, anyone who bothered to read the reference
would have discovered that starting with the second sentence in this section
http://www.av8n.com/how/htm/airfoils.html#sec-pressure
it explains that in the diagrams,
All pressures will be measured relative to the ambient atmospheric
pressure in the free stream. The blue-shaded regions indicate suction, i.e.
negative pressure relative to ambient, while the red-shaded regions indicate
positive pressure relative to ambient.
This notion (also called gauge pressure) is familiar to most people, indeed
far more familiar than the molecular-dynamics description that Mr. Whistle
so shrilly advocates. Babies are born knowing how to pull a fluid by
sucking.
Indeed I asserted that, and indeed it is very simple.
The pressure change above the wing (relative to ambient) is vastly
more significant than the pressure change below the wing (relative
to ambient). In other words, generally speaking, the air pulls up on
the wing and the wing pulls down on the air.
The description of the air as a fluid is far simpler than the molecular
dynamics description. I will continue to use the fluid description.
If Mr. Whistle wants to work out the molecular dynamics description,
he is free to do so. I can hardly wait to see his results.
Once again, you simply dodge my questions. I have asked you to provide some VERY simple support for your use of the word “pull.” In the course of two posts, you refuse to do so.
There is a wing. It moves up. It moves up because of a force imparted to it. Draw the diagram showing the force, and the item or items creating that force. Until you have done that, you are not supporting your assertion that there is a “pull” on the wing.
Your pressure gradient diagram is interesting. And I don’t challenge your diagram. But you refuse to tackle my simple thought experiment: take your pressure diagram regarding the air above the wing, and assume a complete absence of air under the wing. Do you still think your wing moves “up?”
Without knowing the effect of the air molecules on the underside of the wing, you don’t have the proof of “lift.” Indeed, I can “suck” down on the wing harder than your moving air “sucks” up on the wing, and the wing will move down. Not too hard to understand. In either case, the thing that impels the movement is the relative pressure between the upper and lower parts of the wing. Since the lower cannot be ignored, you are not “pulling.”
For reference, btw: "1 a : to exert force upon so as to cause or tend to cause motion toward the force " From the definition at Merriam-Webster Online for the verb “pull.” Take a look at the transitive, heck, even the intransitive meanings listed. Notice the absence of the word “suck.”
Your statement is imprecise, nay, inaccurate. Withdraw it, and substitute for it your true meaning: There is a difference in pressure that forces the wing to go up, caused more by the lowering of pressure above the wing than by the force of the momentum of the air striking the bottom of the wing.
Or some such.
And if you intend to reply, I’d suggest the simple courtesy of actually answering the direct questions posed for you. 
(air) | (force) (air)
|
(wing: leading edge) ============= (trailing edge)
(air)
Happy now?
Specifically,
- The diagram shows the wing, indicated by “=” symbols. The wing
is moving toward the left through a more-or-less stationary air mass. - The diagram shows the force, indicated by “|” symbols. It acts
upward, pulling on the wing. The center of force is approximately
at the quarter-chord point, which is appropriate for a typical wing
under normal conditions. - The diagram “shows” the air in general terms, without symbols,
since air is everywhere around the wing. - The air is the item responsible for the upward force on the wing.
By the same token, the wing is responsible for a downward force
on the air (not shown).
Then I cannot imagine what the problem is. I have no idea what the “challenge” is or
where it is coming from.
Sorry, I am only interested in scenarios that are consistent with the
laws of motion. A scenario with air above the wing and no air below
the wing is irrelevant, impractical, well-nigh unphysical, and certainly
inconsistent with the notion that the wing is moving right-to-left through
a stationary air mass.
I have described how a wing behaves as it moves through stationary
ambient air. The fact is, the wing pulls down on the air and the air
pulls down on the wing under the stated conditions. Mr. Whistle has
repeatedly ridiculed this idea, even though it is well attested by
experiment and well understood theoretically, including the diagrams
of which he approves. His “disproof” apparently consists of observing
that under other conditions, other things might happen. Well, fine,
yes, yes, under other conditions other things might happen. Happy
now?
You can imagine sucking as hard as you like. But the pressure pattern
you imagine is inconsistent with the equations of motion for a wing in
normal flight.
I am quite sure I didn’t “ignore” the lower surface. The lower surface is
shown in all of my diagrams. The fact remains that most of the force
(somewhere more than 50%, possibly much greater than 100%) of
the upward force is developed on the upper surface, as anyone can
see by looking at the pressure diagram which Mr. Whistle “does not
challenge”. Again this pressure pattern (aka force pattern) is an
established fact, well attested by experiment and well understood
in terms of theory.
It wasn’t meant to be super-precise. That’s why my statement
included the word “mostly”. I trust most readers know what
“mostly” means.
Rude much.
I stand by what I wrote.