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Lift %, Upper/lower wing sections

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Lift %, Upper/lower wing sections

Old 28th Jun 2022, 20:09
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Lift %, Upper/lower wing sections

How much lift is created by the upper surface and how much by the lower one?



Last edited by WING7; 29th Jun 2022 at 03:33.
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Old 29th Jun 2022, 06:45
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It's not some thing you can quantify because it depends both on the particular airfoil and its angle of attack. The diagram is of a representative example of the pressure field with the arrows pointing in the direction of "lift"..



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Old 29th Jun 2022, 10:03
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with the arrows pointing in the direction of "lift"

In essence. Strictly, the graphic shows a representation of pressure variation via pressure tapping measurements.

I've just done a few net searches to find some interesting stuff to illustrate what is going on.

this is a basic setup video for the pressure tappings

this gives a more serious story. Unfortunately the pressure variations are shown as a graph rather than as direct pressure tapping pressures which are the basis for the picture in megan's post.

I found this video showing the manometer height variation with alpha. Not as useful as I had hoped to find but it gives you the idea.
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Old 29th Jun 2022, 11:10
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I have always been confused about this. At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.

But when looking at a propellor in flight school and commenting that since it has a similar profile to a wing, does the propellor mostly 'suck' the aircraft along?, I was told oh no, a propellor works by pushing air behind it. And it manifestly does push air behind it.

And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.

So, which is it, or do wing and propellor do both?
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Old 29th Jun 2022, 17:09
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I've never liked the explanation that wings (or blades) "push" the air - at best it's a gross simplification, at worst it's badly misleading regarding the physics of what's going on (and I have a masters in this stuff).
IMHO, the best explanation of how a wing works is something like what megan posted. The physics of lift (and drag) can be explained by integrating those pressures over the entirety of the wing - or any other surface moving through the air. That's part of the beauty of it - it can be used on anything moving through the air (or even water) - integrate the pressures over the entire surface of the object, and you'll get lift, drag, as well as any rotational forces being exerted on the body.
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Old 29th Jun 2022, 17:38
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These links give some good explanations of the many factors contributing to lift rather than just the Bernoulli considerations:

How Airplanes Fly: A Physical Description of Lift

https://www.cam.ac.uk/research/news/...gs-really-work

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Old 30th Jun 2022, 00:53
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One of the best descriptions of how a wing works I've heard is that it throws air at the ground (by means of the pressure field about the wing). If you think in terms of a propeller or helicopter which throws air in a particular direction you can make sense of the description. A propeller produces thrust, so does the wing, the wings thrust being equal to the aircrafts weight, all things being equal, in straight and level flight.

A plot of the pressures pertaining to my previous post. Note that negative values of cp produce arrows pointing away from the airfoil, and positive values point towards the airfoil. In all cases, the formula for cp is cp=(p−p∞)/q∞
p is the local pressure, p∞ is the pressure far away from the airfoil, and q∞ is the dynamic pressure far away from the airfoil.

The dashed line is for inviscid flow, the solid line for pressure with boundary layer effects.



Last edited by megan; 1st Jul 2022 at 01:55. Reason: correct formula
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Old 30th Jun 2022, 01:26
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A few more thoughts.

How much lift is created by the upper surface and how much by the lower one?

It varies, as suggested by others. When I find and post a better manometer bank video showing the pressure variations with angle of attack, you will get a good intuitive picture of how the pressures (and lift) vary with wing alpha. Unfortunately, while we olde phartes grew up with wind tunnels hooked up to such banks (which were great for visualisation), these days we have you beaut electronic gadgets which measure the pressures directly. Great for engineering calculations but not so great for visualisation and intuitive understanding.

Megan's initial graphic is a direct picture of a snapshot of the manometer bank story at a particular alpha value in the wind tunnel and you will see lots of similar pictures in older texts. Keep in mind that it is talking about gauge pressure which is the difference between actual pressure and the background ambient pressure. So when we are talking about a "negative" pressure, we are talking about pressures a bit below ambient. When we are talking about "positive" pressures, we are talking about pressures a bit above ambient. His second graphic is how the first usually is shown in modern texts and is more useful for the engineering folk. The first (ie a time history of the manometer display) probably is better for the student pilot's understanding of what is going on ...

At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.

Proportions will depend on alpha and the particular wing section profile. In general, the "top" surface (ie whichever side is pointing to the heavens) will carry the greater burden on pressure delta from ambient. Better to think in terms of both surfaces providing pressure stuff and the forces which arise from those pressures. What is left when you average these out by doing some sums results in the net lift left over which is the useful bit for keeping the aircraft up, rather than going down.

It is very important to keep in mind that we can't generate forces directly in fluids - fine with solids - but doesn't work with fluids. For example, if you punch a solid brick wall, hard, it hurts real bad. The brick wall is a solid and you certainly can generate forces directly by interacting with (punching) it. However, if you try to punch a fluid, say, the water in a large bucket, or the air around you, the fluid just moves out of the way and doesn't do much anything. For fluids, to generate forces, you have to have pressure differences or gradients which then can apply forces to the bulk of the fluid - much the same level of difficulty as herding cats.

You can look at this from both sides of the table. If you see the effect of a force in a fluid, eg the fluid accelerates, turns or whatever, then there MUST be a pressure gradient somewhere there in the mix which is generating the force which, in turn, is causing that acceleration, turning, or whatever it is you are watching happen. This latter consideration is a very important part of explaining what air is doing as it meanders around an aerofoil. Because it is generally so poorly explained, most students are left confused with a thought that it is all a bit of magic going on. Not at all - for the speeds we are dealing with, it can be explained in terms of Newtonian mechanics - that discipline has stood the test of time for quite a while so it probably is pretty much OK for figuring out physical things. The only "difficulty" arises because so many people forget (or never knew) that we have to think pressure gradients, rather than directly in terms of forces, to figure out what is going on in fluids.


But when looking at a propellor in flight school
I was told oh no, a propellor works by pushing air behind it


Typical lack of understanding in the flight instructor fraternity. The propeller is much the same as a wing (hence, "fling wing" for helicopters - just ask Megan about that - apart from being a nice bloke, he has spent most of his adult life assisting helicopters to repel the earth in various rotary operations). Think more in terms that the propeller is accelerating a mass of air from one side to the other. Just like with a wing, it is not one side or the other doing the work - it is the mean, or net, of both sides each doing their bit to help. Again, Professor Newton had the basics of the story for the lower speed environment (after all, he was a Professor of Mathematics at the Univ of Cambridge).

And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.

That all sounds about right. Prof Newton, once again.

Tdracer, being another one of us engineers, tells the same sort of story.

Andycba's links are useful stuff for reading.

At the end of the day, Coanda really was talking about other stuff. His observation about fluid jets following surface profiles is fine and can be extended to the distributed flow over a lifting surface. But you can, probably more easily and with less appeal to black magic, explain that in terms of Newtonian mechanics, pressure gradients and the resulting forces generated on fluid bodies, allied with a consideration of boundary layer pressure gradient degradation leading to flow separation from the surface.

Bernoulli's theorem. Yep, works fine for the constraints inherent in the theorem, which are then conveniently ignored in the typical (wide of the mark) aviation student textbook. This is just a statement of conservation of energy and is very useful for figuring out the sums relating pressures and speeds. Has very little (directly) to do with lift stuff.

The engineering and physics story is that of the circulation model. This started off back in the mid-1800s, and was sorted out for wings independently by Lanchester (early 1900s in England) and Prandtl (war years in Germany). Kutta and Jukowski, again independently, fixed up a problem which caused some academic head scratching. The resulting model has been used over the past century to figure out values for lift by the aerodynamics fraternity. This model is able to be demonstrated experimentally and gives results which are so close to what is measured that the difference isn't really worth worrying about. Trailing vortices are a part of this story.

For me, I think the easiest way to explain lift is to start with the circulation model and bring in other stuff as necessary to fluff out the story as required along the way for the student .... Certainly, my theory students don't seem to have all that much difficulty getting their heads around the stuff.
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Old 30th Jun 2022, 22:16
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Originally Posted by WING7 View Post
How much lift is created by the upper surface and how much by the lower one?
From the AP3456 Manual of Flying:
Although most low speed aerofoils are similar in shape, each section is intended to give certain
specific aerodynamic characteristics. Therefore, there can be no such thing as a typical aerofoil
section or a typical aerofoil pressure distribution and it is only possible to discuss pressure
distributions around aerofoils in the broadest of general terms. So, in general, at conventional angles
of attack, compared with the free stream static pressure there is a pressure decrease over much of
the upper surface, a lesser decrease over much of the lower surface so that the greatest contribution
to overall lift comes from the upper surface
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Old 1st Jul 2022, 04:54
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My head starts to hurt if I go much beyond the basic idea that efficient lift requires both Newton and Bernoulli (with a dash of Coanda, Kutta and Jukowski, and others).

We could fly with the proverbial "barn door" for a wing, given sufficent power and speed and some AoA. All Newtonian: reaction, 3rd Law and all that. But Bernoulli camber and suction/reduced-pressure on top makes it far more efficient and effective.

The fact that catastrophic loss of lift (an AoA stall, or surface contamination stall) generally comes from disrupted flow over the top of the wing seems to me to be significant.

I'm not aware of disrupted flow on the lower surface of a wing (leaving out horizontal stabilizers) ever causing much in the way of "lift" problems. Drag, yes. Weight, sometimes. Not so much lift. But I'm willing to be educated.
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Old 1st Jul 2022, 10:17
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Thank you John and others.

So in simple terms; the pressure changes caused by an aerofoil section as it moves through the air 'turns' the "incoming" airstream through a significant angle, which produces a reaction according to Newton's 3rd law.

The majority of the pressure changing is done by the upper surface of a wing, but the wing is not 'sucked' upwards as such,

and

Both wing and propellor are doing the same thing by the same mechanism - one to produce lift, the other to produce thrust.
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Old 1st Jul 2022, 11:46
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That sounds fair enough. Now, if you want to do some sums and figure out loads, it gets a tad more involved. Fortunately, pilots don't need to do that, we have aerodynamicists for that sort of heavy stuff.

And we have a few of those good folk on PPRuNe to keep the rest of us on the straight and narrow paths of airflow.
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Old 2nd Jul 2022, 08:47
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Originally Posted by john_tullamarine View Post
That sounds fair enough. Now, if you want to do some sums and figure out loads, it gets a tad more involved. Fortunately, pilots don't need to do that, we have aerodynamicists for that sort of heavy stuff.
NASA makes it easy for ordinary folks "to do some sums and figure out loads."
https://www.grc.nasa.gov/www/k-12/airplane/foil3.html
"FoilSim Student JS is the latest (April 2019) version of the FoilSim family of interactive simulations. The different versions of FoilSim require different levels of knowledge of aerodynamics.... This web page contains the on-line student version of the FoilSim Student JS program."
Press some buttons to play around:
- select a shape - make up an aerofoil setion with a couple of clicks
- choose an angle of attack (you can vary camber on your simple aerofoil section as well)
- see the upper and lower surface pressures


Originally Posted by john_tullamarine View Post
And we have a few of those good folk on PPRuNe to keep the rest of us on the straight and narrow paths of airflow.
It has been a long while since I've been a working aerodynamicist - but two of my current aerobatic students are aeronautical/aerospace engineers and one of them was talking Kutta etc to me today.
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Old 2nd Jul 2022, 09:58
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er...'kay.

I've just skimmed the posts, and so will offer the following... feel free to move the response to jet blast if you want to.

we have had Bernoulli's theory rammed down our throats since time amoral, and immemorial... etc. like, forever!. The morality is that it still keeps being rammed and it just ain't so. We teach it, and we examine on it and it is a nice analogy but that is all it is.

The pressure distribution charts are valid, as are the pointy arrow thingies, but the problem is we have a massive disconfubulation (like a discombobulation but better!) as to the why. The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. Why? Does air have a mind? Does it have evil intent? is air sentient? Add a slot, and start doing the maths on what the little air mites are gonna do next, it's enough to confuse the poor wee things. For the wing/propeller comparison, if one does, and one doesn't, then we be needin' parallel universes in the same few feet, which just ain't gonna do it. No sir, it just ain't so.

We also read we gotta have deux Kutta points to get le lift de großer Flügel, which is curious, cuz the last 35 years of experimenting on propellers was all about getting rid of the trailing edge Kutta condition, which massively reduces drag (except at low AOA) and gives the CLs a happy smile. So that seems at odds with actuality.

Next time at the sink, bath, basin or pool, grab a flat/curved/airfoil shape or your hand or tea spoon... knife or another kitchen utensil sans tines and translate it across the fluid (water/beer/milk/cappuccino... etc) at no angle of attack to the vector of movement. The fluid doesn't object, and it doesn't make determinations as to going uppity or downy, it just don't, not a bit. Now do it again, and this time do it at a slight angle to the vector of motion and guess what.......

At the point where it started to move there is a vortex formed, and it rotates in a direction as... the forward flow of the vortex is in the direction of the leading edge of the blade to its trailing edge.... the whole blade tries to move in the direction that the leading edge is relative to the trailing edge (spoons, talking about mean camber lines)
when the blade gets to the other end of the pond, stop the blade, and the point where the blade stops will generate another vortex off the leading edge... both the start and stop vortexes are opposite directions to the bound vortex that was developed by the flow around the spoon/knife, foil, cheese slice, etc. So we have just proved over coffee, (tea works well too... in fact better, as the tea leaves are pretty to watch move around... and you can tell the future by reading them too). The bound vortex has a circulation that is rearwards over the "upper" surface and forwards on the lower surface... for a blade moving right to left with a positive AOA, the flow is clockwise, and start and stop vortex are anticlockwise... (Fig Newton be praised...) Add the translation velocity to the vortex and you get the velocity profile, and then go get the maffs from sitting on Boyles. And that is yon pressure distribution. join happy dots or arrows normal to the surface and you get your answers. The nice thing is, it gives the correct answer for flows in Slats, flap gaps and for Flettners thingey, and tennis balls with spin, or Donald J's slice through the heart of the constitution.

Aerohydrodynamic lift arises from bound vortex structures, the wing is just a vortex device, as is the propeller, rotor, fan blade, compressor blade etc. Now in turbines, (termites?) when there is lots of separation going along, then the flow gets more fun, and a simple approximation comes out of impingement/reaction-flat plate geometric lift but it is still in fact vortex flow. around the blades. the wake is just that, the flow in the wake of the blade. Ion or chemical rockets are simple reaction to the impulse, but then the flow in a throat or around an expansion body is pretty interesting where there is happiness to be found in maintaining laminarity of the flow near the surface, to minimize thermal transfer to the structure and to minimize erosion.

All good fun, just like Bernoulli thought it would be.

...Of course, that also explains why fan jet engines lose efficiency at altitude when considering TSFC. It's almost like they are a FANCY FIXED PITCH PROPELLER.... what a shame that you can't change the flow around a fixed-pitch propeller... wait on, as 44 said, yes you can! And it is a lot easier than transwarp beaming...

...That's what I'm talking about! How do you think I wound up here? Had a little debate with my instructor on relativistic physics and how it pertains to subspace travel. He seemed to think that the range of transporting something like a... like a grapefruit was limited to about 100 miles. I told him that I could not only beam a grapefruit from one planet to the adjacent planet in the same system - which is easy, by the way - I could do it with a life form. So, I tested it out on Admiral Archer's prized beagle...

Wait, I know that dog. What happened to it?

I'll tell you when it reappears. Ahem. I don't know, I do feel guilty about that.
What really puts the Bernoullis in the balus ay wok wok is playing with supercritical sections, or sonic flow, then the poor little mites get all collywobbly and confused. Vortex flow still werks gladly. kind of.










Maski long planti toktok!




On barn doors...
  • radius of the LE alters the shape of the CL/AOA curve around peak Cl...
  • Camber shifts the A-slope upwards...
  • TE thickness moves the slope like camber...
  • T/C alters Cd/AOA...
  • %MAC for max thickness alters Cd...
  • % chord for max camber alters Cm...
For helicopters, none of the above means much once the blade starts to turn, dynamic pitch changes make everythang much more interesting, there is no clean smooth line of coefficient for a rotor if the inflow is not perfectly axisymmetrical.... they all look like D,s P's and lopsided and inverted T's instead of pretty clean lines like you get to see in I'm an Abbott and a VonWanderedoff's [1] wonder book on Fairies of wing sections, which gets us back to Bernoulli.




PS: when you get to sit over the wing sometime, on yon jet transport Boeing/Bus/Embraer... etc and you have the sun aligned with the quarter chord, either away from the top or in the opposite direction, you will see the shockwave shadow, Herr August Toepler's schlieren image like. you will note that for most wings, it's nowhere near where it should be. It sits far forward of where the flow modeling of a section would suggest it should, for conventional sections such as BAC 450, 451, 452 etc, or for supercritical such as SC(1)-0710 and similar Whitcomb style sections, [2] [3][4] [5], Eppler's 403 etc... Flying is fun.


[1] Abbott, I. H. & Von Doenhoff, A.E., (1959) "Theory of Wing Sections Including a Summary of airfoil Data", Dover Publications, 1 Jan 1959
[2] Charles D. Harris, "NASA Supercritical Airfoils", NASA Technical Paper 2969, pp. 1-76.
[3] Richard T. Whitcomb, "REVIEW OF NASA SUPERCRITICAL AIRFOILS", National Aeronautics and Space Administration, Langley Research Center Hampton, Virginia pp. 1-17.
[4] K.Harish Kumar, CH.Kiran Kumar, T.Naveen Kumar, "CFD ANALYSIS OF RAE 2822 SUPERCRITICAL AIRFOIL AT TRANSONIC MACH SPEEDS", International Journal of Research in Engineering and Technology, Volume 04 Issue 09, September 2015, pp. 256-262
[5] Sana Kauser, Mr. Kumara Swamy, Dr. MSN Guptha "Aerodynamics Analysis Of Naca Sc (2) -0714 Supercritical Airfoil Using Computational Fluid Dynamics", International Journal of Scientific Research and Engineering Studies, 02 (06), June 2015, pp. 38-41.

Last edited by fdr; 2nd Jul 2022 at 11:02.
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Old 2nd Jul 2022, 13:22
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Devil

Hold a sheet of paper in front of your mouth; blow over it (on the top side only). The paper should rise, the pressure falls thus the paper rises. And in this instance how to you define the ‘bottom’ surface.

Similar effect blowing through a paper tube - a paper bag open at both ends. The sides move in due to air moving through it and reducing pressure - reduced with respect to ambient.

So for either the paper sheet or tube, if the ambient pressure has not changed then the surface over which the airflows faster must contribute all of the lift - reduced pressure with respect to ambient. cf JT # 8.

Re the question, the proportions of lift should to be considered and stated relative to a datum pressure (and AoA, wing section, etc).
An example would be to normalise the pressure plots in post #2 so that the lower surface is datum (zero), then all of the lift is from the top surface because the top surface ‘arrows’ have increased due to the added lower surface ‘arrows’ being reversed.

Over to the tech-science views who might wish explain otherwise.

Oh, then there is the rotating tube, Coandã effect, in still air …

And a wedge shape wing section in supersonic flow where one surface at the leading edge is parallel with the airflow …

And my simple view of the lower wing surface is so the top surface can be fixed on it to provide lift, and structural strength enabling mounting on the fuselage.
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Old 2nd Jul 2022, 14:54
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Originally Posted by fdr View Post
The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. Why? Does air have a mind? Does it have evil intent? is air sentient?
They don't teach this anymore, it's been corrected. But there are still stragglers, since it's so simple and elegant and ties everything together...

when the blade gets to the other end of the pond, stop the blade, and the point where the blade stops will generate another vortex off the leading edge... both the start and stop vortexes are opposite directions to the bound vortex that was developed by the flow around the spoon/knife, foil, cheese slice, etc.
The starting and stopping vortices are opposite to each other. The starting is CCW, stopping CW (stopping same as bound vortex). So all 4 vortices surrounding the rectangle swept by the wing's motion (starting, stopping, and 2 wingtip vortices), curl so the top goes inward, and the inside goes downward, which matches the downwash where the lift occurred.




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Old 2nd Jul 2022, 15:06
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Originally Posted by Uplinker View Post
I have always been confused about this. At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.

But when looking at a propellor in flight school and commenting that since it has a similar profile to a wing, does the propellor mostly 'suck' the aircraft along?, I was told oh no, a propellor works by pushing air behind it. And it manifestly does push air behind it.

And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.

So, which is it, or do wing and propellor do both?
This is two different scopes of the explanation. They don't contradict, but it can be confusing since they use some of the same terms.

The first one is a fine-grained detailed view of the action around the wing, where we can see the individual area contributions above and below. How much sucks from above vs. how much pushes from below? Here it is broken down by individual non-overlapping contributions. And as we've seen from the diagrams of all the little arrows or manometer tubes, the answer is that more sucks from above.

But there is also the larger-scale view where we're not concerned with the detailed view above. And in this view, the wind/blade simply moves air downward/backward, and this movement we tend to call pushing, and it accounts for 100% of the force. (When it comes to props and rotors this is called the actuator disk model, where we're not concerned with individual blades and all the complicated things that happen between and around them... it's just a magical disk that imparts momentum, and for a certain class of problems, this abstraction is still good enough to work with.)
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Old 2nd Jul 2022, 16:02
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"The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. "


That would explain things, just not aerodynamic things. Why would they teach that? Where do they teach that?
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Old 2nd Jul 2022, 18:13
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Thanks, vessbot I got it now, but my flight school instructor clearly hadn't all those years ago At the time, it did not seem likely to me that a wing and a propellor worked in completely different ways.

Regarding the air "wanting to get from A to B" and why it does this above and below; remember the air is static, and the aerofoil is moving through the static air.

So the air molecules are not trying to get anywhere, they are just sitting there, having a great time, when this hulking great wing or propellor blade suddenly whizzes past them. As it goes by, the molecules near the upper or forward surface are squeezed together more than the ones underneath/behind.

I guess this lower pressure makes the air above move downwards towards the wing, and then continue downwards over the back edge, resulting in a large mass of air being "pushed" down.
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Old 2nd Jul 2022, 21:51
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Originally Posted by Vessbot View Post
They don't teach this anymore, it's been corrected. But there are still stragglers, since it's so simple and elegant and ties everything together...



The starting and stopping vortices are opposite to each other. The starting is CCW, stopping CW (stopping same as bound vortex). So all 4 vortices surrounding the rectangle swept by the wing's motion (starting, stopping, and 2 wingtip vortices), curl so the top goes inward, and the inside goes downward, which matches the downwash where the lift occurred.

https://www.youtube.com/watch?v=VcggiVSf5F8

https://www.youtube.com/watch?v=bvV7-9wAXc0
My bad, late nite. A CW rotation around a section has a CCW start vortex, and the stop vortex is CW, same. as if the section is removed from the fluid, the flow continues CW..









Last edited by fdr; 2nd Jul 2022 at 22:07.
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