Do you really understand Wing lift?

Snowmass

Snowmass

Line Up and Wait
Joined
Oct 31, 2019
Messages
740
Location
S.E. Arizona highlands
Display Name
Display name:
Arturo
The Feb 2020 issue of Scientific American p.45, has an article entitled "The Enigma of Aerodynamic Lift" which a small portion of you might find fascinating. You might be surprised why really understanding lift is not as simple as you may believe. If you do not relish deep thinking do not bother to read it.
 
This discussion has been going on for some time. Not being a math person, I couldn’t really follow it because it seems to be all about the inability to come up with a perfect mathematical model for aerodynamic lift. (Can’t see the SA piece due to the pay wall but would be interested).

I satisfied myself by first, memorizing the explanation recognized by the FAA.

Then second by applying some physics 101 and understanding that a parcel of air would be accelerated downward by the passage of a wing, plane, bird or whatever through it. The air mass x the rate of acceleration equals lift. I’m good.

It is an enigma, but I fly, therefore I do.


Sent from my iPad using Tapatalk Pro
 
This is how I try to explain it (̶c̶o̶r̶r̶e̶c̶t̶l̶y̶) in my classes:
lbWmN9V.png
 
Last edited:
Checkout_my_Six said:
shew me that ....with a super critical airfoil. :eek:
Click to expand...
Supercritical airfoils will look the same conceptually - low pressure on top, high pressure on the bottom, and freestream flow deflected downward. All a supercritical airfoil does is change the chordwise pressure distribution somewhat increasing critical Mach no, i.e. delaying shock formation. They also tend to produce less lift (call it differential pressure or deflected flow, chicken or egg) at a given AOA, but they have other advantages like less drag at transonic speeds due to the delayed shock formation.

images


Nauga,
who can be supercritical too
 
Last edited:
nauga said:
And they both result in the same estimate of lift when applied correctly. It's the "correctly" part that fouls up the simplistic answers like top vs.bottom.

Nauga,
and two paths to the same destination
Click to expand...
Top vs. bottom, blonde vs. brunette, Ginger vs. Maryann....the list goes on and on.

I guess we’re just naturally simplistic.
 
The problem with Bernoulli:
There is no problem with Bernoulli's equation. Given a flow pattern - faster over the top (on average) and typically slower under the bottom results in lower pressures above (on average) and higher on the bottom. The total differences in pressures time the wing area equal the force applied on the wing and equal the force of lift. Period. The problem, however, is explaining the flow pattern - why is flow over the top faster than flow along the bottom? That's when the wheels come off. An obvious (and correct) explanation is to talk about circulation and the Kutta condition (velocity at the trailing edge must be finite) - this gives you a good math model of the flow / pressures / lift. But, to explain circulation, at a minimum you need to show the vector sum of the undisturbed and circulation flow fields. And, the discussion quickly degenerates into vector calculus - I assume I'm not the only one that struggles with that. As a result, people have come up with semi-plausible sounding nonsense that they blame on Bernoulli such as the difference in distances, looks like a ventrui, pressure does not fully explain lift, air bounces off the bottom, Bernoulli 60% vs Newton 40%... All that is nothing less than 100% unadulterated male bovine excrement. So, while Bernoulli's equation is quite useful, explanations of lift that invoke Bernoulli tend to be nothing more than fairy tales for pilots.
If the fairy tales were correct, the "weight" that an airplane (or quad copter) exerts on the ground would be reduced due to the low pressure on top of the airfoil due to the shape as the speed of the airfoil increases. Right? Guess what:

The problem with Newton:
The simplistic but totally wrong fairy tales have been told and retold for so long by so many sources (including the FAA) that people have come to believe them and won't accept a factual explanation. (Which may include Bernoulli properly applied.) The other problem being that to describe the flow fields in detail, the math gets really messy and nowadays one resorts to computational fluid dynamics. And, the inner workings of CFD are somewhat on the less than intuitive side.

Bottom line:
Tradition.
We have taught fairy tales for so long, that we will accept nothing less than a fairy tale. We even believe that experts in fluid dynamics / aerodynamics don't understand how an airplane wing generates lift. One can (and I have) provide a reasonably intuitive non-mathematical explanation that is consistent with real life and the laws of physics, but no one is really interested.

But don't listen to me. How about this:
"AEROSPACE PROFESSOR SEMINAR SERIES
How does an aircraft wing generate lift?
This talk covers common misconceptions, including equal transit-time theory and the Venturi effect, and presents some explanations that appeal to physical intuition, including flow turning and streamline curvature.
Krzysztof Fidkowski is an associate professor in the Aerospace Engineering Department at the University of Michigan. His research interests include development of robust solution techniques for computational fluid dynamics, error estimation, computational geometry management, parallel computation, large-scale model reduction, and design under uncertainty. His teaching interests are in undergraduate aerodynamics and numerical methods, and in graduate computational fluid dynamics."

Geoff "Who should really proofread before clicking on the post button" Thorpe
 
Last edited:
I REALLY like your attitude Capt. Thorpe. So you might find the Scientific American article interesting since you realize that that. at a deeper level, the answer is not that simple. BTW you really need a new hat.

I just played the attached video, which is very good, and it is quite close to the Scientific American article which admits that even its explanations are still lacking. Knowing what happens is NOT the same as knowing why it happens. For instance the SA article brings in the the term "vacuum" or a negative pressure but we know that a negative pressure does not actually exist but is really a lower positive pressure. This is why suction can only lift water to to the limit of the positive atmospheric pressure pushing on the water surface. Try using a suction pump to lift water 40 feet.
 
Last edited:
I was able to pull it up, sans diagrams and color picture, in the online journal database for my school where I am registered in the professional pilot airplane program. Here is a full reference to the article mentioned in the OP:

Author: Regis, Ed
Source: Scientific American. Feb2020, Vol. 323 Issue 2, p44-51. 8p. 1 Color Photograph, 10 Diagrams.
Abstract: The article offers information on the enigma of the aerodynamic lift and also discusses why planes stay in air. Topics discussed include what generates the aerodynamic force known as lift; views of John D. Anderson, Junior curator of aerodynamics at the National Air and Space Museum, on issue; and also mentions that the Bernoulli's theorem attempts to explain lift as a consequence of the curved upper surface of an airfoil, the technical name for an airplane wing.
Word Count: 4503
ISSN: 0036-8733
 
In the flow redirection theory he presents, the redirecting of the flow generates lift through what effect? It sort of looks like Newton's 3rd but I missed the explanation.

And, in Bernoulli's he says that does not explain flat plate lift or upside-down wing lift.....but I did not see these explained in the flow redirection theory.


Capt. Geoffrey Thorpe said:
Krzysztof Fidkowski is an associate professor in the Aerospace Engineering Department at the University of Michigan.
Click to expand...
 
Capt. Geoffrey Thorpe said:
The problem with Bernoulli:
There is no problem with Bernoulli's equation. Given a flow pattern - faster over the top (on average) and typically slower under the bottom results in lower pressures above (on average) and higher on the bottom. The total differences in pressures time the wing area equal the force applied on the wing and equal the force of lift. Period. The problem, however, is explaining the flow pattern - why is flow over the top faster than flow along the bottom? That's when the wheels come off. An obvious (and correct) explanation is to talk about circulation and the Kutta condition (velocity at the trailing edge must be finite) - this gives you a good math model of the flow / pressures / lift. But, to explain circulation, at a minimum you need to show the vector sum of the undisturbed and circulation flow fields. And, the discussion quickly degenerates into vector calculus - I assume I'm not the only one that struggles with that. As a result, people have come up with semi-plausible sounding nonsense that they blame on Bernoulli such as the difference in distances, looks like a ventrui, pressure does not fully explain lift, air bounces off the bottom, Bernoulli 60% vs Newton 40%... All that is nothing less than 100% unadulterated male bovine excrement. So, while Bernoulli's equation is quite useful, explanations of lift that invoke Bernoulli tend to be nothing more than fairy tales for pilots.
If the fairy tales were correct, the "weight" that an airplane (or quad copter) exerts on the ground would be reduced due to the low pressure on top of the airfoil due to the shape as the speed of the airfoil increases. Right? Guess what:

The problem with Newton:
The simplistic but totally wrong fairy tales have been told and retold for so long by so many sources (including the FAA) that people have come to believe them and won't accept a factual explanation. (Which may include Bernoulli properly applied.) The other problem being that to describe the flow fields in detail, the math gets really messy and nowadays one resorts to computational fluid dynamics. And, the inner workings of CFD are somewhat on the less than intuitive side.

Bottom line:
Tradition.
We have taught fairy tales for so long, that we will accept nothing less than a fairy tale. We even believe that experts in fluid dynamics / aerodynamics don't understand how an airplane wing generates lift. One can (and I have) provide a reasonably intuitive non-mathematical explanation that is consistent with real life and the laws of physics, but no one is really interested.

But don't listen to me. How about this:
"AEROSPACE PROFESSOR SEMINAR SERIES
How does an aircraft wing generate lift?
This talk covers common misconceptions, including equal transit-time theory and the Venturi effect, and presents some explanations that appeal to physical intuition, including flow turning and streamline curvature.
Krzysztof Fidkowski is an associate professor in the Aerospace Engineering Department at the University of Michigan. His research interests include development of robust solution techniques for computational fluid dynamics, error estimation, computational geometry management, parallel computation, large-scale model reduction, and design under uncertainty. His teaching interests are in undergraduate aerodynamics and numerical methods, and in graduate computational fluid dynamics."

Geoff "Who should really proofread before clicking on the post button" Thorpe
Click to expand...
upload_2020-2-2_9-53-46.jpeg
 
Capt. Thorpe- thanks for the discussion, and the Fidowski video link. Very interesting. I did three semesters of calculus at university many years ago, but haven't used it much since. :)
 
Let'sgoflying! said:
In the flow redirection theory he presents, the redirecting of the flow generates lift through what effect? It sort of looks like Newton's 3rd but I missed the explanation.
Click to expand...
Short answer: conservation of momentum. Long answer: skip to 36:12 in the video below.

And, in Bernoulli's he says that does not explain flat plate lift or upside-down wing lift.....but I did not see these explained in the flow redirection theory.
Click to expand...
There is no difference between right side up and upside down, flat plate or "airfoil". The airflow turns in pretty much the same way in all cases (the major difference is how sharp it turns near the leading edge). It would have been more obvious if he had used a flat plate for his explanation rather than the stereotypical airfoil drawing which does not look like many airfoils that you find on airplanes.

The fundamental cause of lift has nothing to do with the shape of the airfoil. The idea that airfoils are curved on top and flat on the bottom is udder nonsense.

Probably about twice as long as it should have been, but ,none the less, total genius. :)

Why is "right side up" three words while "upside down" is only two?
 
Capt. Geoffrey Thorpe said:
The fundamental cause of lift has nothing to do with the shape of the airfoil.
Click to expand...

wow thanks for the explanations.

Now; considering your quoted comment, I am having a little difficulty understanding why we have all the airfoils that have been designed through the years (ie NACA)......or maybe you are saying that even though the fundamental cause of lift is unrelated to airfoil shape, their individual performance is?
 
Let'sgoflying! said:
even though the fundamental cause of lift is unrelated to airfoil shape, their individual performance is?
Click to expand...
Bingo.
A flat plate is a really bad airfoil, but it is an airfoil and generates lift the same way as any other airfoil.
Adding camber helps a lot. Improves lift/drag ratio, lets you get to higher angles of attack before it stalls.
Adding thickness also helps a lot - room for structure, ability to reach higher angles of attack, more manageable stalls.
Adding thickness or changing camber changes the flow on both sides of the airfoil. The effect of the thickness distribution tends to cancel out in terms of lift at any particular angle of attack.
Somewhere between many and most airfoils are NOT flat on the bottom and curved on the top like the typical cartoon.
 
Last edited:
Congratulations everyone . . . this is the first time I've seen a thread on this subject not get bogged down by those who drank the Bernouilli kool-aid so long ago that it's become part of their DNA . . . an epigenitic adaptation. Langewische made things so clear in S&R, a book I'd read before I ever started any actual flight training, that I never paid any serious attention to Bernoulli despite having it repeatedly thrown at me throughout the years with all the written tests and mainstream "FAA Approved" study materials. Even by professionals who should have leaned in their careers not to let fanciful and inconsequential issues dominate their common sense . . . but some do let that happen. Our societies are still infused with leftovers of what were at some earlier time common presumptions or, better said, common unquestioned acceptance of presumptive truths whose foundations turned out to be simply misinformation or ignorance. Look at how many steeples and minerats still obstruct the airspace? At least in Europe and other centers of civilization those cathedrals are now just tourist attractions. Only in medieval parts of the world - Timbuktu, the suburbs of Paris and small towns in Missouri, etc. - do people revere misinformation. In these very same places can also be found some who still pay homage to how the Wright Brothers learned to fly by examining the curvature of birds' wings! There are even some who still think the Wright Brothers invented the airplane! As obsolete beliefs slowly disappear, this aviation related leftover . . . this fuzzymoldy issue of what causes a wing to fly . . . seems to have finally been laid to rest by the informed and rational forum members of POA.

Despite this apparent clarity of thought however, I can't help suspect - based on many years of observation and futile arguments - that below the surface there still must lurk a sullen, resentful minority of Bernouilli lovers who will cling to what they "know" to be true to the very end. This gene . . .this meme really, will eventually disappear though. Even in Missouri.
 
I've been making the statement "stop abusing Bernoulli" for many years.

The wing is a pump. It moves a parcel of air from one place to another. This process generates lift. Justifiably, lift generation is thereby based largely on Newtonian principles.

A barn door can generate lift. Poorly and inefficiently, to be sure, but in the same fashion as any wing on any airplane.

I watched a bit of what was presented in this thread. It's true; Bernoulli is not fiction, but it's a small player in the production of lift from the typical airplane wing. F=MxA and Newton's 3rd law are the primary explanations for lift generation. It's possible to add a lot of extra layers, but the base concepts will remain the same. Always has been, always will be.
 
And your camber/thickness comments below, Capt...the reason we get better lift out that is? Somehow it is better at redirecting the flow, thus better lift?
My mind needs to connect it back to his explanation of why wings generate lift.
Thanks


Capt. Geoffrey Thorpe said:
Bingo.
A flat plate is a really bad airfoil, but it is an airfoil and generates lift the same way as any other airfoil.
Adding camber helps a lot. Improves lift/drag ratio, lets you get to higher angles of attack before it stalls.
Adding thickness also helps a lot - room for structure, ability to reach higher angles of attack, more manageable stalls.
Adding thickness or changing camber changes the flow on both sides of the airfoil. The effect of the thickness distribution tends to cancel out in terms of lift at any particular angle of attack.
Somewhere between many and most airfoils are NOT flat on the bottom and curved on the top like the typical cartoon.
Click to expand...
 
Oh my, Mike.
Wine?
(I know I achieve big-picture clarity and a better maneuvering of words after 2 glasses, lol)
I enjoyed it.

MikeS said:
Congratulations everyone . . . this is the first time I've seen a thread on this subject not get bogged down by those who drank the Bernouilli kool-aid so long ago that it's become part of their DNA . . . an epigenitic adaptation. Langewische made things so clear in S&R, a book I'd read before I ever started any actual flight training, that I never paid any serious attention to Bernoulli despite having it repeatedly thrown at me throughout the years with all the written tests and mainstream "FAA Approved" study materials. Even by professionals who should have leaned in their careers not to let fanciful and inconsequential issues dominate their common sense . . . but some do let that happen. Our societies are still infused with leftovers of what were at some earlier time common presumptions or, better said, common unquestioned acceptance of presumptive truths whose foundations turned out to be simply misinformation or ignorance. Look at how many steeples and minerats still obstruct the airspace? At least in Europe and other centers of civilization those cathedrals are now just tourist attractions. Only in medieval parts of the world - Timbuktu, the suburbs of Paris and small towns in Missouri, etc. - do people revere misinformation. In these very same places can also be found some who still pay homage to how the Wright Brothers learned to fly by examining the curvature of birds' wings! There are even some who still think the Wright Brothers invented the airplane! As obsolete beliefs slowly disappear, this aviation related leftover . . . this fuzzymoldy issue of what causes a wing to fly . . . seems to have finally been laid to rest by the informed and rational forum members of POA.

Despite this apparent clarity of thought however, I can't help suspect - based on many years of observation and futile arguments - that below the surface there still must lurk a sullen, resentful minority of Bernouilli lovers who will cling to what they "know" to be true to the very end. This gene . . .this meme really, will eventually disappear though. Even in Missouri.
Click to expand...
 
Ryan F. said:
It's true; Bernoulli is not fiction, but it's a small player in the production of lift from the typical airplane wing.
Click to expand...
Argh. From the NASA Glenn website that's been quoted many times when BvN comes up:

"So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force."

It is not "one or the other", when applied correctly they are two different ways of estimating the same lift.

ETA: https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

Nauga,
with different paths to the same answer
 
Let'sgoflying! said:
And your camber/thickness comments below, Capt...the reason we get better lift out that is? Somehow it is better at redirecting the flow, thus better lift?
My mind needs to connect it back to his explanation of why wings generate lift.
Thanks
Click to expand...
Broad brush concepts here. Not well organized.

Ok, we are "turning" the flow of air, right? Lettuce start with a thin flat plate at some angle of attack. For the air to follow the top surface, it has to make a pretty abrupt, sharp, turn right at the leading edge. And the higher the angle of attack, the more it has to turn - at some point, the air just doesn't make the turn and the airfoil abruptly stalls.
Now, suppose we put a bit of curve in our flat plate - now the air can make the turn more gradually and will hang with the upper surface longer - more lift - less radical changes in direction which mean less radical changes in pressure. We can get more lift. Not so great at low angles of attack because the air flowing under the wing is going to get tripped up by the sharp leading edge.
At this point, we can abandon the point and put a rounded leading edge with some kind of streamlined thickness distribution behind it. Now the turn around the leading edge becomes much more gradual across a range of angles of attack. This lets us get to higher angles of attack and have less drag at lower angles. Win win.
This gets us up to about the middle of World War 1 airfoil technology.
Prandtl then demonstrated that you could create quite thick (compared to what went before) airfoils with no more drag than the existing airfoils if you use the proper streamlined shape and this also let you get rid of the mess of wires used to carry the loads up until that time. The Fokker Triplane was the first widespread use of this thick airfoil with cantilevered wings. The even larger leading edge radius allows the stall to start more gradually and at the trailing edge as compared to small radius leading edges. Good for pilot longevity.
Now the game becomes one of optimizing camber and thickness combinations to get the best lift to drag ratio for particular applications - more camber means more maximum lift, but higher drag at low angle of attacks - great for STOL, sucks for cruise. Flatter wings are not as good at dragging you up out of a short muddy field, but get you to Disney World a lot faster. Thicker shapes give you more room for internal structure for cantilevered wings - this is why most cantilever wing aircraft (Cherokee, Mooney, King Air, etc.) tend to have wings that are rounded on the bottom as well as the top. Thinner shapes have just a little less drag and work well if you want to put up with struts - most of the strut braced Cessna singles use the NACA 2412 which was developed in the 1930's which gives a flatish bottom airfoil.
It should make sense that if you have a lot of airflow curvature at the leading edge, you have to have a pretty low pressure there (on the top surface). By the time you get to the trailing edge the pressure on the top surface is often even a bit higher than the static pressure - so you are asking the air to flow from a low pressure to a higher pressure area - stuff don't like to flow uphill. But by playing games with the curvatures you can keep the flow attached as much as possible.
Then came the idea of "laminar flow" where you try to delay the transition from a laminar (smooth, low drag) boundary layer to a turbulent (thicker, draggier, but better able to stick to a surface) boundary layer. This is done by playing more games with the thickness and moving the point of maximum thickness further aft.

You can download Xfoil and, with some effort, play games with different shapes - it kinda sucks at thin airfoils, but thin airfoils kinda suck to begin with, so not a great loss.
One of these days (God willing, and the creek don't rise) I will get into the wind tunnel at school and attempt to demonstrate this with the foil models that I made up over the last few months (it's a surprising amount of work to get a 3D print into good enough shape to go into a tunnel - 'specially for videos that earn $0.50 per month...).
 
nauga said:
when applied correctly they are two different ways of estimating the same lift.
Click to expand...
Prezactly.

The problem with "Bernoulli" is not Bernoulli, but the misuse of Bernoulli.

But I will say that I find a "Newtonian" approach to work better if you are trying to keep it to an intuitive level and avoid the vector math you need for circulation.
 
You must log in or register to reply here.
Back
Top