Precisely my point, one which you totally gloss over. Without the effect of the air under the wing, the air above the wing won’t “lift.”
All your words fail to accomplish your simple theme: demonstrating that it is something above the wing which pulls the wing upward. Those who are following understand this fallacy.
The thought experiments offered weren’t designed to do anything but emphasize that the existence and motion of the air under the wing is just as important as the existence and motion of the air above the wing. In the absence of an ability of the air above the wing to impart upward force to the wing, the definition of “pull,” you simply have an upwards force resulting from the combination of the effect of all the air molecules around the wing, above and below. That’s not “pulling;” and you have yet to show otherwise. Fancy attempts at talking your way past this point don’t prove diddly.
This discussion is rapidly becoming more tiresome than the “Light is a particle!” “No, it’s a wave!” discussions than physics undergraduates engage in upon a superficial reading of quantum mechanics, and threatens to become more opprobrious than the banal, “Tastes great!” “Less filling!” debate.
If we were able to model the behavior of every individual molecule in the air we could use Newton’s Laws directly to calculate the amount of lift any given configuration at a specified orientation (angle of attack) would produce in any particular regime. Because such a model would be computationally horrendous, we treat air as an aeroelastic fluid continuum. This treatment involves the applications of mathematical models which seek to simulate the gross mechanics of the fluid with only an acceptable amount of fidelity to describe the resultant behavior as it pertains to lift. In order to do this, we have to make certain assumptions relating to local discontinuities and boundary conditions which are highly variable and easily perturbed; as a result, these models only simulate the behavior in a very approximate manner over the entire range of conditions.
Whether you want to claim that the air “pushes” or “pulls” is essentially irrelevent to the essential property of lift; lift is the result of a net force owing to unbalanced pressures above and below the wing. Structurally and aeroelastically where the force “comes from” is of issue with regard to the behavior of the model, but in this case your selection of model is going to predetermine the answer you get to that question. Since there are so many secondary effects occuring at the boundaries that are unaccounted for by any one set of explicit models, any model is going to be necessarialy simplified, which is the reason than realistic computational modeling uses iterative approximation methods over explicit formal solutions.
As has already been explained repeatedly, aerodave’s Staff Report was intended for general consumption by a largely nontechnical audience; as such, it is written with a simplified and limited explanation and in somewhat humorous tone, as is typically for Staff Reports. The one significant point for potential debate–his use of the term “Coanda Effect” in a context that extends beyond the specific environment discussed in the technical literature–has been addressed in [post=6401209]this post[/post]. All other arguments, especially that of angle of attack (as being relative to the wing configuration versus relative to the angle of an aircraft to which the wing is attached) are just semantic discussions that are well past their “Use By” dates.
There has also been, if I may be permitted to observe, a rather nasty underlying and snarky tone added to the discussion, which has resulted in several seemingly apparent misrepresentations of statements, such as the alleged but undemonstrated request for an absurdly abbrieviated explanation (“25 words or less”) for lift. This and the scarcely concealed mudthrowing that has occured contributes nothing of value to the discussion.
And John Denker, your extensive cites would perhaps be more persuasive overall if every single one of them didn’t point back to your own electronic publication. While the “book” is well-illustrated and literately written, appealing to yourself as an ultimate authority isn’t particularly convincing and comes off as being merely egotistical and self-promotional. The bibliography of your work cites many authoritative primary sources, and it would be germane if you could, in addition to pointing back to your own work, make formal cites to the primary sources as well.
Or y’all could just agree to disagree. But it seems like there’s an ego or two in the way of that.
To continue the analogy: If one party says light is not a wave, it’s a particle,
and the other party says it’s not a particle, it’s a wave, it is OK to say both
are dead wrong.
It appears Stranger is trying to be impartial. But remember that Churchill
declined to be “impartial between the fire brigade and the fire”. When two
parties disagree, it is unwise to assume that both are wrong, and also
unwise to “split the difference”. As Arno Penzias once said, such a procedure
would give the biggest advantage to the biggest scoundrel.
Yes, this discussion is exceedingly tiresome and ill-mannered. But there is
a difference between right and wrong. All the namecalling in the world won’t
change that.
There are three main points under discussion.
A) There is air under the wing. Yes, it is true there is air under the
wing. Indeed it is important that there is air under the wing. Without
air under the wing, there wouldn’t be any air above the wing.
B) Most of the upward force developed by the wing is transmitted
through the top of the wing. In plain English, mostly the top of the
wing pulls down on the air and the air pulls up on the top of the
wing, for an ordinary wing in normal flight.
Big Whistle has espoused point (A). Nobody has disputed this.
Indeed, I think most of the readership already knew that there
was air under the wing. Let us stipulate that point (A) is true.
I mentioned point (B). Mr. Whistle has repeatedly ridiculed
this point. Nevertheless, point (B) remains true. Mr. Whistle
is wrong about this. I’m sorry to be so blunt about it,
but I don’t know how else to say it. Is there a politically
correct way to say it? Correctness-impaired? His best
argument against point (B) is to re-assert point (A), which
as far as I can tell does not in any way detract from point (B).
Yes, there is air below the wing. But be that as it may, most
of the force is transmitted through the top of the wing. Air is
not the same as force. I can measure air; I can measure force;
I am quite sure they are not the same.
My statement (B) is model-independent. It is easily verified by
measuring the forces on a real airplane in flight, or in a wind
tunnel.
Also: Any model that failed to predict this would have been
rejected out-of-hand, long ago. So in fact all the standard
models predict it just fine.
That brings us to the following point:
C) The Coanda effect does not occur near an ordinary wing in normal flight.
It is incorrect to explain the basics of lift in terms of the Coanda effect.
C1) Furthermore, the attempt to explain things in this way is pointless, because
the readership knows even less about the Coanda effect than they do
about lift itself. So even if it weren’t wrong, it would violate the proverbial
principle that “learning proceeds from the known to the unknown”.
C2) A sound explanation of lift provides a basis for understanding the
operation of stall-warning sensors of both types. In contrast, the Coanda
“explanation” is directly inconsistent with the observed behavior of
such devices.
C3) A sound explanation of lift provides a basis for understanding
soft-field takeoff procedure. The Coanda “explanation” does not.
C4) A sound explanation of lift provides a basis for understanding
wake-turbulence avoidance procedures. The Coanda “explanation”
does not.
C5) A sound explanation of lift provides a basis for understanding
the well-known fact that gliders tend to have long, skinny wings.
The Coanda “explanation” does not.
I respect the general audience enough to tell them the truth. I don’t
see what purpose is served by giving them an explanation that tells
them nothing of value, and is in fact incompatible with known facts.
I demonstrate it thus: the request was made by rwjefferson on 07-23-2005, 04:03 PM
Thanks for the compliment.
I would be delighted to discuss this topic with somebody who would actually
read the references.
The best use of your time would be to pick up a really good book such as Richard
von Mises, Theory of Flight, (1945; Dover reprint 1959) ISBN 0 486 60541 8,
and see what it says about the Coanda effect. This won’t take long, because it
says nothing. The Coanda effect is so obscure and so irrelevant that it is not
mentioned in standard references.
If anyone is still under the delusion that the Coanda effect is worth knowing
about, a good place to start would be Henri Coanda, US patent 2052869,
filed 19 April 1935, granted 1 September 1936. You might as well read the
whole thing; it’s only 2 and half pages long. That will give you some idea
what Coanda thought the Coanda effect is.
The next thing to read would be Peter Bradshaw, “Effects of Streamline Curvature
on Turbulent Flow”, NATO Advisory Group for Aerospace Research and Development
AGARDograph No. 169 (1973). I call particular attention to page 53, where he
discusses what should and should not be called the Coanda effect. I would
also like to discuss the buoyancy analogy and the expressions for the
Brunt-Väisälä frequency that appear on page 34. Of course none of this has
anything whatsoever to do with how ordinary wings work, but at least you
would understand why it doesn’t.
Okay, this non-technical reader would appreciate a link to this. I’m assuming that pressure sensors are placed on both the upper and lower sides of the wing and measurements taken thereof.
nitpick
Please stop putting in the extra line-breaks. It makes your posts very hard to read. The software will line-wrap for you based on the width of the veiwers monitor. Thanks!
/nitpick
Yes, that is one way of doing it. Specifically, you can get semiconductor strain gauges that are very light and thin, and use them to measure the force on selected areas of wing-skin. This is commonly done on prototype aircraft, to make sure the designers got everything right and there are no unexpected forces.
Another way of doing it is to measure the pressure in the air. This is done using a probe sticking up from the wing, with a whole array of pressure sensors along its side. The probe is sufficiently thin that it doesn’t disturb the airflow too much. Then you can calculate the force as pressure*area.
If you took enough such measurements, the result would look like what you see in diagrams such as 3 Airfoils and Airflow
Another way of doing it is to observe the stream lines, by injecting smoke, as shown in diagrams such as 3 Airfoils and Airflow
especially the middle panel (5 degrees angle of attack) which is typical of cruising flight. A given parcel of air exhibits streamline curvature if-and-only-if something is exerting a force on that parcel (and of course that parcel is exerting an equal-and-opposite force). The magnitude of the force depends on the amount of curvature and on the local airspeed. You can easily see that the air above the wing is more curved (and moving faster) than the air below.
Methods (2) and (3) give you the same information. Method (1) gives slightly less information, since it is confined to the wing surface … but often this tells you what you most need to know. You can use whichever method or combination of methods you find convenient.
Yes, that’s the right question. An ounce of data is worth more than a ton of “opinion”, ridicule, and bombast.
Such links are not very abundant. This is somewhat understandable, because almost the only folks who collect such data are aircraft manufacturers, and they’re not eager to publish their data, because it would give an advantage to their rivals (and would not be very interesting to most other folks).
Hints: (1) These plots represent pressure with negative numbers higher on the vertical axis (as you can see from the axis-labels). This representation is traditional in the fluid dynamics community, and it works quite well, once you’ve gotten used to it. (2) In each plot, the upper curve represents the the pressure on the upper surface of the wing, while the lower curve represents the pressure on the bottom surface. (3) Negative pressure on the lower surface corresponds to a downward pull, i.e. an unhelpful negative contribution to the overall lift.
You can see that nearly all of the lower surface is making an unhelpful downward contribution. This is extremely common in real aircraft when they are flying at a reasonably high CAS (calibrated airspeed). You can see that this data is consistent with what I’ve been saying all along.
Professor D;
Meaning and understanding are easily lost in the noise of argument.
It is clear that your thoughts were diverted when I first blew my :whistle:
[do over]
How would Prof. D explain lift to the least of his muggl…no, I mean: dopers…no, no, I mean “students.”…that’s right, I mean “students”
I am simply being curious. What is the best “short answer” to the first time question of “How do airplanes fly, really?”
I find the best single word to be: “Newton.” If understood, it is both simple and true.
The five words “Wing pushes, air pushes back” are not only simple and true, they clearly demonstrate Newton and lift, even to children.
“Airplanes fly by forcing (accelerating) mass of fluid air downward. The air reacts by equally pushing upward on the airplane.”
These twenty words seem to be a good place to start a conversation about lift. Would you accept them, debate them, or modify them? How much more clearly can lift be described by a first paragraph?
[hijack] - (note: this has nothing to do with either airplanes or fish)
A relatively slippery ball curves toward the side with the highest (fluid to surface) velocity differential. To the best of my understanding, this is the predicted effect of Bernoulli’s derivative of Newton. This experiment clearly demonstrates Bernoulli’s formula and principle.
Yet, this curve is called the “Reverse Magnus Effect”.
I wish to understand why is not rightly recognized as the Bernoulli Effect. [/hijack]
Peace
rwj
The problem, once again, is not Bernoulli. The problem is the inputs you try to apply to Bernoulli. It’s not the “velocity differential” that determines the pressure. Air going past one side of the ball faster (relative to the local surface of the ball) is not what changes the pressure. Think of it this way: turning on a tread mill at full speed doesn’t create a vacuum above the belt. Not even if the belt were moving at 200 mph. The difference in velocity between air and a surface near it has nothing to do with the pressure of that air.
You’re picking the wrong frame of reference. It’s almost as if you’re trying to gather the relative air velocity by picking a frame that rotates with the ball. For Bernoulli to apply, you have to talk about it in a steady reference frame. Bernoulli predicts behavior along streamlines. And streamlines only reflect the motion of actual fluid particles in a reference frame that makes the flow steady. Unsteady flow means no Bernoulli. Confusing, maybe. But just trust me…you need to do it this way.
So, think instead of a reference frame that is non-rotating, but which moves with the ball. In this frame the air flows past you. (That’s almost a Yakov Smirnoff joke). So now you basically have a spinning ball in a wind tunnel. Now, realize that the air in the boundary layer around the ball is affected differently on either side of the sphere. Let’s call Side A the side of the ball that has the higher velocity relative to the frestream (the side spinning into the air). Side A will change the air’s velocity more. In our reference frame, the air next to Side A is slowed down more by that higher velocity differential. An analogy would be walking onto the wrong direction of the airport moving walkway…that would slow your progress considerably
Since the air is slowed down more on the side rotating into the wind, it’s pressure increases as expected by Bernoulli. The air is “assisted” around the ball on the other side, increasing its velocity relative to the freestream. Or at least it is slowed down less. Either way, the pressure will be lower on that other side of the ball, Side B. So the ball moves toward the side spinning away from the flow…Bernoulli is not violated.*
The Reverse Magnus effect is a product of having a turbulent boundary layer on one side and a laminar BL on the other. It is a curious result of a very specific and narrow set of conditions. It has more to do with asymmetry and flow separation than straight up velocity differentials. But that doesn’t mean it violates Bernoulli’s principle. It’s just a more complex problem.
*But, since non of the above matters without viscous effects (boundary layers are crucial to the Magnus effect working), it’s not really right to talk about Bernoulli. The Bernoulli Principle applies to inviscid flows. The general idea that speeding up a flow will drop its total pressure remains, but the easy formula for predicting how much pressure changes vanishes. Energy losses become important, and Bernoulli is relegated to a qualitative descriptor. So Magnus and Bernoulli don’t get along once numbers are introduced. But the spirit of Bernoulli still applies.
Bernoulli’s Principle clearly states that all else equal, “higher velocity means lower pressure, and visa versa”. This describes velocity differential. As a matter of fact, Bernoulli describes only velocity differential (and its reciprocal effect on pressure).
I once read a text for children showing how balls curve according to Bernoulli. Arrows showed the forces on the ball causing it to curve in the “slippery ball” direction. The text was printed before Magnus. I believe the illustration was “corrected” in later editions.
Two Experiments:
Vary the airspeed over the top of a ball and observe the change in pressure.
Split the airflow and vary the airspeed between the top and bottom of a non-rotating ball.
I expect that even in viscous air, all else equal, “higher velocity means lower pressure”.
Bernoulli clearly predicts that a ball will curve toward the side with forward rotation.
Viscous effects are Newton. If even a small amount of fluid air mass is accelerated by the surface of a rotating ball, a significant reaction, equal and opposite occurs. Please understand that the acceleration of mass (MVV) carries a magnitude greater force than its Bernoulli derivative: velocity differential (delta V).
Indeed, it is not only velocity differential that can cause increase pressure; air compacting against a ball also increases pressure. As the surface of a ball rotates through the windward, the fluid air mass in the boundary layer increases in both density and size. As this layer crosses the lee, mass is ejected at the angle of rotation. This is the force that causes the ball to curve in the opposite direction, the cost is taken from rotation. Small acceleration overpowers great velocity differential. Newton trumps Bernoulli.
I would humbly suggest that the effects you describe as Magnus and Reverse Magnus are more accurately understood as Newton and Bernoulli…on (viscous) fluids.
Peace
rwj
I’m starting to feel that by the third page of this thread the arguement is getting towards semantics. To risk getting yelled at, I’d like to see if I understand the principles at work in my own words.
At normal air pressure, air pushes up on a wing as the wing pushes down on the air. This creates the initial lift force. However if pressure is equal on both sides of the air, the wing will then press up on the air and the air will press down with just as much force as the air pushing up.
Now if we reduce pressure above the wing but not below via the benoulli principle, air pushes up on the wing and the wing pushes down on the air. This causes the initial lift force. Now the wing pushes up on the air above it, but due to the lower pressure there is less air to push back, and the wing keeps some of the lift as the air pressing down < the air pressing up.
Is this a more correct general interpretation of nature? I’m reading this thread trying to filter out the arguements and understand both sides here to get a clear picture. It’s not easy.
Also I don’t see drag as a direct reduction in lift. It is simply the force slowing the plane down. Now if the plane is moving slower now then the air doesn’t have to move so fast and the air pressures are more equal resulting in less lift, but it’s a by-product of the reduction of airspeed, correct?
I hate double posting, but I notices an error, I should read “However if pressure is equal on both sides of the wing…” Not on both sides of the air, my error. :smack:
The topic is “How do airplanes fly, really?”. IOW, upon seeing a huge airliner pass slowly overhead at low altitude, the average person might say “what’s holding it up?”. The key is how best to explain it for the average person, or the common readership of this web site.
From this perspective it seems leading with or emphasizing Bernoulli (at least in detail) is often confusing.
To me the simplest explanation is air mass is redirected downward, the force of which equals the weight of the aircraft. The average person can readily grasp that, and it’s technically correct.
This also explains the downwash of a hovering helicopter, which the Bernoulli “suction lift” paradigm can make confusing. A helicopter’s main rotor is an airfoil, just like a fixed wing plane. In fact they’re called rotary wing aircraft. If the lift was exclusively from being sucked upward by lower pressure, that doesn’t intuitively – to the average person – explain why the tremendous downwash. A helicopter hovering above a huge truck scale in theory will produce downforce equal to the weight of the helicopter.
Likewise consider an aircraft flying above a theoretical scale of sufficient size which kept pace beneath it. The total force of the air diverted downward would equal the aircraft weight.
This view also clearly explains to the average person why an airplane propeller (which is also an airfoil) produces such wind blast. The airplane is propelled forward by the reaction of the air moved backward by the propeller airfoil.
Now, depending on the audience you can go into various degrees of detail about the underlying mechanisms producing that air mass movement – Bernoulli, recirculation, etc. However for the average person I think it’s clearer and more intuitive to lead with and emphasize action/reaction explanation, vs emphasizing the more arcane underlying mechanisms which produce that. It’s not a question of what’s right and wrong, but of emphasis and order in an explanation for a general audience.
This is commonly done in other areas of scientific education. When explaining atoms at the popular level, we don’t usually jump right into details of quantum indeterminacy and the strong nuclear force. We start off with a more simplified view.
When explaining “how airplanes fly” to the average readership, I’d suggest emphasizing the action/reaction concept. As the explanation unfolds, you can bolster that with progressive details about the underlying mechanisms.
To a degree I think the staff report http://www.straightdope.com/mailbag/mairplanesfly.html achieves that, however it could probably be improved. If there are valid questions about whether the Coanda effect is relevant, that aspect could be removed.
I’d also suggest early in the article you just simply state that planes fly because the wings divert air mass downward, the force of which equals the aircraft weight. Then you can progressively explain why. The article’s current wording can be confusing. Early on, it says “Newton’s Third Law…force generated by this deflection alone is far short of what we see in real life”. Then later it says “the Newton explanation is perfectly adequate”. This can seem contradictory to many readers and should be clarified.
The next step is realizing that when a fluid is made to curve, its pressure is “thrown” to the outside by centrifugal force (that’s the simplest way of describing it).
I heard at one point that centrifugal force does not exist, so I just did a google search and found several sites that also say it does not exist. The explanation, as I understand it is simply that it is part of Newton’s first law. Objects in motion stay tend to stay in motion. In the classic example of a passenger in a car, as the car turns, the passenger contines moving in a straight line. After a moment, the friction between the passenger and the seat turns the passenger, it is this force the passenger feels. Without the friction, the passenger would fly out of the car because he would contine in a stright line while the car turns. The simplest explanation I could find is here. http://regentsprep.org/Regents/physics/phys06/bcentrif/centrif.htm
It’s true…centrifugal force doesn’t exist as a real force if we’re talking about the forces applied to the rotating object. Instead, it’s an objects intertia that makes it want to move to the outside of a curved path. You’ve got no argument from me concerning the point you’ve raised.
In the statement you quoted, I took some liberty with the physics for the sake of getting the intuitive feel across, hence the disclaimer that it was “the simplest way to describe it.” Discussing rotating reference frames and Coriolis forces in that section would have caused an unnecessary burden on the reader, and would have got everyone hung up on details not critical to my point. And trust me, most aerodynamicists aren’t discussing those things either.
The principles of motion and fluid flow involved in the creation of lift have been very well understood for over 350 years. The application of those principles to real life practical applications has been understood, and practiced, for over 100 years. There is a small body of people who appear to have difficulty appreciating the simplicity of lift who publish complex and confusing explanations feeding on each other’s misunderstandings. The Internet has substantially assisted this greatly flawed process. These misunderstandings often have their root in misconceptions about various approaches adopted in order to build mathematical models used in the design of wings. These different mathematical approaches are represented as contradictory theories of flight, which they are not. Simple misunderstandings of some basic physical principles of force and motion often fuel the confusion further and ego lurks in the shadows.
The force accelerating (deflecting) the air is always equal to the force of lift. Whenever something exerts a force on something else it always experiences the same force in the opposite direction. We can be absolutely certain that, as the interaction of the wing and air imparts a force to the wing, there will be an equal and opposite force applied to the air. The only real question is what is the mechanism that delivers this?
The relationship between the two equal and opposite forces of lift and acceleration of the air is coincidental, not causational, they are both caused by the same thing and not by each other.
No force is created by the “deflection” of the air; the deflection is the result of a pressure force being applied to the air. An identical pressure force works in the reverse direction on the wing. The movement of the wing through the air causes the pressure forces, but how?
This is just confusion over the position of the reference line from which to measure the angle of attack. In the case of a flat plate wing the aerodynamic base line for measuring the angle of attack is clear, the flat surfaces. If the airflow is at zero degrees to this then no lift is created. Similarly, in the case of any symmetrical cross sectional shape the aerodynamic base line is clearly parallel to the centre line of the shape. But, for curved asymmetrical shapes there is no simple physical reference point and thus it is possible to draw an arbitrary line and the angle so created will also be arbitrary from an aerodynamic perspective. However, the true aerodynamic angle of attack will always be zero degrees at zero lift. Engineers may use a different reference point for pragmatic reasons but that has no role in understanding aerodynamics and appropriate adjustments have to be made.
Equal Transit Time is, of course, total bunkum. It is “lie told to children”; a gross over simplification mistakenly designed to give a little insight where the conditions for imparting a proper understanding do not exist. It has never been a theory outside this population. It was not traditionally linked to Bernoulli. The traditional explanation related the different distance between parcels or molecules along the longer rather than shorter length directly to pressure differences.
The linkage to Bernoulli that has materialised over the last 15 or so years appears to have its roots in a few particular authors exaggerating the significance of “equal transit times” using pseudoscience to wrongly elevate it to the status a “theory of flight”. The motivation appears to be to increase the kudos potential of their subsequent demolition of it. I find it a little ironic as Bernoulli’s principle simply and clearly illustrates that equal transit times cannot possibly apply to a wing generating lift.
It does, regrettably, appear in too many places although “respectable science books” may be an overstatement. There is a reason why the parcels of air should apparently realign behind the wing and that is that the alternative involves a significant discontinuity. That discontinuity is, of course, explained by the starting vortex but in the absence of that understanding the logic may reasonably appear compelling.
The model does totally miss the mark. It is a fundamental feature of lift that the greater the transit time difference the greater the lift; equal transit time theory could not be more wrong.
No comment on the existence or otherwise of any valuable, well defined effect.
This is just another way of describing the process of the air accelerating downwards. The nature of the “curve” is merely a feature of the frame of reference chosen and the time taken accelerating. Under no frame of reference is the flow ever at a constant radius and so, even if this particular visualisation were to have value, it would have to be expressed as a the combination of a circular (“centrifugal”) flow and a complex, varying flow. The “centrifugal/centripetal” consideration of forces and inertia is an alternative to considering them from a linear component perspective. To invite us to add the centrifugal force to the linear force identified in the earlier Angle of Attack section is to invite us to double count.
The notion that a force exists peculiar to the air at the “crest the hill” is erroneous. Accelerating force exists at all times the air is following a curved path. The “top” of the curve has no particular distinction. Of course, if the intention was to identify that additional lift balances the need to accelerate the air from an upward component to zero, then it is true. However this extra lift is absolutely offset by the negative lift created earlier during the air’s acceleration to give it the upward component in the first place.
The notion that some undefined “effect” causes the air to flow in a curve round the wing profile but centrifugal force throws it off the surface creating a partial vacuum does not work for me, not least because the stated cause and the stated effect are just different descriptions of exactly the same thing, it is a tautology. I suggest the following alternative: -
Air accelerates down a pressure gradient; the low-pressure region on the surface of the wing draws the air towards it causing it to follow the curve. If the air did not follow the curving path there would be no air at the surface of the wing – a vacuum. However, the air does flow and so it is not a vacuum. A steady state condition is achieved whereby the pressure at the wing surface and the rate of flow it produces are in balance.
No it is not wrong to invoke Bernoulli in the explanation, just unnecessary and possibly confusing. Bernoulli does not attempt to explain mechanisms; it gives some insight into numerate aspects that can aid the construction of mathematical models. Many people appear to believe that Bernoulli states (for example) that if air accelerates this will cause its pressure to reduce. This is wrong. Bernoulli merely points out that if a parcel of fluid goes through a process then the sum total of all forms of energy within that parcel of fluid will change by the amount of energy transferred in or out of it. For example Bernoulli is entirely content with the fact that as an aircraft accelerates from stationary to flying speed the air within the cabin accelerates but does not experience a pressure change. It is the mechanisms at work that defines what happens, not Bernoulli.
If one element changes then at least one other must also change in order to maintain the conservation of energy principle. However, despite many statements to the contrary, these two (or more) changes have a coincidental not a causational relationship. The same thing causes them; they are not caused by each other.
Newton is very similar. It dos not attempt to describe mechanisms it merely informs us that momentum is maintained. How this happens is not part of its remit, just that it will be. As I sit on my stool typing this, the reaction to the gravity on my body is nothing to do with any forces experienced by the stool. It is the equal yet opposite gravitational force that my body applies to the Earth. Neither of these two forces causes the other. I will leave others to explain what causes gravity. The “Newtonian Explanation of Lift”, while true, is not an explanation at all. Would explaining to a 5 year old that cars move forward because the road pushes them in that direction be an aid to their understanding or confuse it?
Neither Newton nor Bernoulli had theories of flight; neither of them describes mechanisms involved in flight. They have a use in the construction of mathematical models. Any theory of flight that does not comply with the Bernoulli’s conservation of energy principle or Newton’s conservation of momentum will be wrong.
The explanation that the air goes faster (for reasons we are can not be told about!) and Bernoulli decrees that means it has lower pressure (which it doesn’t, many other things could change instead) is nothing more than obfuscation. The pressure change is the primary effect and any speed change is a secondary effect. Every object moving through a fluid has a high-pressure region in front moving the fluid out of the way and a low-pressure region behind drawing the fluid into the void created. Speed changes within the pressure regions are incidental.
In reality it is as clear as a mountain stream. We can accurately design the most amazingly efficient aerofoils for use from gliders through power generating turbines to massive hypersonic aircraft. The challenge is not to understand the aerodynamics; it is to deal with the variability of real world operating environments and to find the material science solutions to be able to build the things that we can design.
The muddy understanding exists only within the lift Confuseniks. Once they were happy, innocent and interested people with a wish to learn. Often they were motivated by a realisation that their belief that lift was derived from shape cannot be correct (Dark thoughts enter the mind in the dead of night. How can aircraft fly inverted? How can aircraft fly level at different speeds?). And then these innocents, in their search for the truth unwittingly encounter the walking undead of aerodynamics, the lift Confusniks. Before these well-intentioned innocents appreciated the danger they were in they were infected and are destined to roam the Internet at night searching for fresh blood to indoctrinate and to grow the army of muddy thinkers further. Unfortunately it all happens without even the upside of the momentary excitement of an exchange of bodily fluids.
Guy
