Do you really understand Wing lift?

I think anyone attempting a turn from base to final should read this thread and then be given a 50 question test (after we all agree which theory is correct) before being allowed to attempt such a maneuver.
 
NASA gets it wrong all the time......remember the Shuttle o-rings?
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
 
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

I would agree with all of that, as a broad statement, and I don't think I stated anything which directly conflicts with your link. In fact, the statement I like most at the grc.nasa.gov link is "The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface." Amen!

The problem with using Bernoulli to "estimate the same lift" is it's impossible to completely do so using a typical airplane airfoil... whereas it is possible to explain how a flat plate "flies" as a wing using a Newtonian explanation.

Bernoulli and Newton are both involved in understanding lift production... but Bernoulli's only involved in a small supporting role, really.
 
I mostly wanted to get those deeply interested read the Scientific American article and haven't been following the discussion much except for the two video explanations. But one thing has been verified for a long time is that two "partner" molecules DO NOT reach the trailing edge at the same time with curved top and flat bottom. The top gets there first.
 
The problem with using Bernoulli to "estimate the same lift" is it's impossible to completely do so using a typical airplane airfoil... whereas it is possible to explain how a flat plate "flies" as a wing using a Newtonian explanation.
It most certainly is possible to calculate lift on a conventional airfoil or a flat plate 'using Bernoulli' if you apply it correctly. If you look up a few posts you'll see a pressure distribution plot for a conventional airfoil and supercritical airfoil. You can calculate lift from those pressure distributions. You can do the same for a flat plate at an angle of attack (well, you can at 0 angle of attack too but there won't be any lift). The answer you'll get, if you do it correctly, is the same as you'd get if you instead considered the flow turning. Does the pressure distribution turn the flow or does turning the flow create the pressure distribution? THAT is the Bernoulli vs. Newton question, which is really one of point of view, not which one is right or wrong.

Equal transit time is a red herring in this thread - it is not required for a pressure differential to exist.

Nauga,
pressured to turn or turning to pressure?
 
It most certainly is possible to calculate lift on a conventional airfoil or a flat plate 'using Bernoulli' if you apply it correctly.

You misunderstand my statement. It is not possible to "estimate the same lift on a conventional airfoil using Bernoulli" as it is to "estimate the same lift on a conventional airfoil using Newtonian principles." I.e., it is not possible for the same wing to generate the same amount of lift using purely Bernoullian principles vs. Newtonian. I believe in one text there's a description of a hypothetical airfoil which would "develop lift" based purely on Bernoulli's principles and the camber was of absurd proportions.

Are these principles all at play in the generation of lift? Of course, and that is aerodynamically proven. The point, which remains unchanged throughout all the discussion, is that F=MxA and Newton's 3rd Law remain the primary workhorses of lift generation. Bernoulli's principles play a small supporting role and are certainly present, but not primarily responsible. The old playground explanation of lift which involves blowing over the top of a sheet of paper and watching it rise into the lower pressure created, for example, is simply not a good explanation of what is truly happening.
 
NASA gets it wrong all the time......remember the Shuttle o-rings?

Their engineers actually got that right, and said “Don’t fly.” They were overridden by management.

Management wanted a teacher to fly. She did, all the way to a 300G impact with the ocean.
 
bumblebees.png
 
Everybody takes a simple thing and tries to make it complicated. Well, the math is complex but the principle isn't. For the air to push up on an airplane wing the wing has to push the air down (Newton's third law). If you push air down it accelerated down (Newton's second law). If the force of the air pushing up on the plane equals the force of gravity pulling it down the plane remains in level flight (Newton's first law).

Bernoulli's principle only says that the static pressure in a fluid (such as air) decreases as its velocity increases. Nothing more. It doesn't get into the reasons why the flow is moving at any particular speed. As such, it's useful in describing the pressure distribution along an airfoil, and can help calculate how much lift will be produced, but it doesn't "explain" lift and no aerodynamicist will tell you so. The problem probably arose when somebody who never got beyond elementary school arithmetic came up with the ridiculous "equal transit time" concept to explain why the air moves faster on the top of the wing.
 
you must not understand the math....one must integrate the pressure distribution on all surfaces. .o_O
You misunderstand my statement. It is not possible to "estimate the same lift on a conventional airfoil using Bernoulli" as it is to "estimate the same lift on a conventional airfoil using Newtonian principles." I.e., it is not possible for the same wing to generate the same amount of lift using purely Bernoullian principles vs. Newtonian. I believe in one text there's a description of a hypothetical airfoil which would "develop lift" based purely on Bernoulli's principles and the camber was of absurd proportions.

Are these principles all at play in the generation of lift? Of course, and that is aerodynamically proven. The point, which remains unchanged throughout all the discussion, is that F=MxA and Newton's 3rd Law remain the primary workhorses of lift generation. Bernoulli's principles play a small supporting role and are certainly present, but not primarily responsible. The old playground explanation of lift which involves blowing over the top of a sheet of paper and watching it rise into the lower pressure created, for example, is simply not a good explanation of what is truly happening.
 
You guys are so full of it. Look, wings lift because of money. Need more lift? Spend more money.
Bunch of dead guys with weird European names babbling about vectors, coefficient of whatever? Baloney.
Money baby. Money makes wings lift.
 
Somehow it is better at redirecting the flow, thus better lift?
I mentioned Xfoil above, the attached has some results from playing with this - along with some pictures of a thin airfoil in a wind tunnel (NACA 1930s video ) which you may find interesting.
 

Attachments

  • Fundamentals of Lift V5.pdf
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Bernoullian principles vs. Newtonian
There is no vs. in real life. One can derive Bernoulli's equation directly from Newtons laws because Bernoulli's equation is simply a special case of conservation of energy. Below is an example of doing just that using nothing more than simple algebra (long boring video).
However, if by "Bernoullian principles" one is referring to attempts to explain the velocity differences between upper and lower surfaces because of curves and flats or whatever (which have nothing to do with Bernoulli's equation), then, yea. that stuff is just plain crap and doesn't work.

Pressure is, by definition, force per unit area, and the only way that air can apply a force to a wing is through pressure. If you integrate the pressure over the entire top and bottom surfaces of a wing you get the total net force applied by the air to the wing. That force will equal lift. If the lift is not at least equal to the mass of the aircraft times the acceleration of gravity, you ain't gonna be flying very long. 100% of lift is explained by pressure, 100% of lift is explained by Newton. The laws of physics prevail.

 
Their engineers actually got that right, and said “Don’t fly.” They were overridden by management.

Management wanted a teacher to fly. She did, all the way to a 300G impact with the ocean.
As I recall it was the engineers from the company that made the boosters that said don't fly, not NASA engineers.
 
Haven't read the previous two pages, but the explanation I was told that "the air on the top of the wing has to travel a longer distance than the air on the bottom of the wing, so it must go faster, and that creates low pressure and sucks the wing up" always felt too simplistic and "weak" of an explanation

I understand how "lift" works now, and it makes sense, but it took going down lots of NASA and stack overflow / stack exchange rabbit holes
 
As I recall it was the engineers from the company that made the boosters that said don't fly, not NASA engineers.

The NASA engineers were cut out of the chain via doing it all via a conference call. When they found out the decision was to fly, they tried to stop it and got rebuffed by the bureaucracy.

It’s all in the report.
 
As long as I might have the attention of those that like to go deeply into aerodynamics I have a question. Quite a while ago I was curious as to the drag caused by a super venturi so I decided to measure using my old 60 Ford pickup truck. But something unexpected happened when I increased the speed from 40 MPH to 80 MPH. The drag force did not quadruple but merely tripled. I tried this many times and even change the venturi mount from a sliding one to a hanging parallelogram to reduce any friction but the result was the same. Any ideas? Maybe due to a very low Reynoldslo venturi & truck.jpg number?
 
The equal transit time theory has been bunk for a long time. This video shows the upper airflow already moving faster when it reaches the leading edge. The difference in arrival times at the trailing edges is stark.

 
equal transit time theory has been bunk for a long time
It always bothered me, and was too simplistic. How does the molecule above the wing know, or care, what the molecule under the wing is doing. It doesn't "have to travel faster to go a longer distance" <- it's not a conscious entity.
 
As long as I might have the attention of those that like to go deeply into aerodynamics I have a question. Quite a while ago I was curious as to the drag caused by a super venturi so I decided to measure using my old 60 Ford pickup truck. But something unexpected happened when I increased the speed from 40 MPH to 80 MPH. The drag force did not quadruple but merely tripled. I tried this many times and even change the venturi mount from a sliding one to a hanging parallelogram to reduce any friction but the result was the same. Any ideas? Maybe due to a very low ReynoldsView attachment 82513 number?
You have a lot of stuff around the venturi that would affect the flow. To do a better test, you need the venturi far in front of the truck (nosebooms are generally 4 to 6' for a comparison), and you need to not have stuff in front of it for sure.
 
Do you really understand wing lift?

No, and I do not care to.

Bernoulli explains venturis, not wings.
Newton explains gravity, not lift.

Neither is correct, both are wrong (or at least; not 100% right(even combined)).

Flight is not possible, the Earth is flat.

Pilots are given "special dispensation" by FAAther GOD, for subjecting themselves to the rigors of flight instruction by flight instructors, who think that students are stupid, lumbering clods, until the Designated Penultimate Examiner (assigned by the FAAther) gives up and says, "Oh, posh! Give the guy his ticket. NEXT!"
 
You have a lot of stuff around the venturi that would affect the flow. To do a better test, you need the venturi far in front of the truck (nosebooms are generally 4 to 6' for a comparison), and you need to not have stuff in front of it for sure.
The stuff in front is the spring scale which was not in front when measuring. Maybe the board needs to be extended but I sort of doubt it since I am not looking for 1% accuracy which those nosebooms are probably looking for. When I repostioned the venturi with a long parallelogram mounting things did not change even though it was then below the board. Also a venturi on the side of a fuselage is going to have lots and lots of stuff in front.
 
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The drag force did not quadruple but merely tripled.
It's hard to say which direction the air will be flowing around it - but probably not from straight ahead given the flow diverging around the grill - can't give you a definitive answer...

Probably want to be a lot further from the truck body.
 
Airplanes, like bees, fly because they cannot read. Classical theory and texts says bees should not be able to fly because their wings are too small. Since neither can read they just fly!:D
 
It's hard to say which direction the air will be flowing around it - but probably not from straight ahead given the flow diverging around the grill - can't give you a definitive answer...

Probably want to be a lot further from the truck body.
Yup. The airflow ahead of a wing is disturbed as far as a wing chord ahead. The blunt nose of the truck will disturb the air reaching the venturi by a lot, probably causing significant lateral flow.
 

> "How does the molecule above the wing know, or care, what the molecule under the wing is doing?"

Entanglement of course.
 
There is no vs. in real life. One can derive Bernoulli's equation directly from Newtons laws because Bernoulli's equation is simply a special case of conservation of energy. Below is an example of doing just that using nothing more than simple algebra (long boring video).

Yes, we are again in violent agreement and despite what seems to be contradictory postures I can't think of anything you've said which I don't find eminently accurate. That said, my simplistic "vs." is not an attempt to untangle these two explanations of lift from each other, as they are positively related -- and I have said so from the beginning. The "vs." is the misunderstanding of Bernoulli and trying to explain lift using the simple grade school explanation of nonsense such as the wing being drawn into the lower pressure created by the upper camber of the airfoil, 'equal transit,' and etc., as opposed to the simple explanation of the wing being an air pump. F=MxA. Of course it's more complicated than one vs. the other; it's not possible to create lift without pressure differentials, after all; but at the end of the day, as you have stated, and I have absolutely supported, the laws of physics prevail.

I absolutely love the video you shared. I actually watched it last summer and although the math is simplified it's probably still a bit too complex for most pilots to be willing to watch and attempt to understand in its entirety. But that's okay, because understanding the math is not critical here. In a perfect world, the professor would make a 3 minute summary of just the concepts involved. There are a couple of moments in the video where he briefly summarizes the concepts, and they are worth listening to.
 
Ah! Another video from Light and Sporty Guy, and he has a graphic of the "equal transit airfoil" present, the one which I mentioned earlier.

He really does do a good job, just at the expense of taking far too long to make his point(s). It's worth being patient with him, however.


Screen Shot 2020-02-03 at 6.56.54 PM.png
 
It’s an interesting subject but in that this is an Internet forum many of the posts are falling into the TLDR category.
 
It’s all poppy cock. I was taught that the Bernoulli principle that keeps a plane in the air. No way! It’s money that does it.
 
As long as I might have the attention of those that like to go deeply into aerodynamics I have a question. Quite a while ago I was curious as to the drag caused by a super venturi so I decided to measure using my old 60 Ford pickup truck. But something unexpected happened when I increased the speed from 40 MPH to 80 MPH. The drag force did not quadruple but merely tripled. I tried this many times and even change the venturi mount from a sliding one to a hanging parallelogram to reduce any friction but the result was the same. Any ideas? Maybe due to a very low ReynoldsView attachment 82513 number?
q - aero-dynamic pressure

where

q = 1/2 (density * V^2)

https://en.wikipedia.org/wiki/Dynamic_pressure
 
It’s an interesting subject but in that this is an Internet forum many of the posts are falling into the TLDR category.

I know. Short attention spans. We all have them. Me included.

TLDR is why so many folks have difficulty with complex concepts. Me included (sometimes.)
 
I know. Short attention spans. We all have them. Me included.

TLDR is why so many folks have difficulty with complex concepts. Me included (sometimes.)

There’s also the practicality of getting this deep into it in true pilot training... we have time to hash this out to the Nth degree here, but it has to be condensed almost to the point of being inaccurate in an “FAA approved” 40 hour Private Pilot wonder pilot course of study.

It’s damn near impossible to go much further than “Read and understand the FAA description of how lift works, that’s what’s on the test, and make sure you know their explanation cold... but realize my trainee friend that there’s a lot of other more accurate mathematical ways to explain this that combine Newtonian physics and meld it with Bernoulli’s actual math... not these silly drawings in this book, that are technically more accurate. But if you’re being asked by the FAA, you’d better answer with what’s in the Airplane Flying Handbook, first. Anything after that may or may not score you brownie points with any particular examiner.”

And that sucks, because the real math, while difficult, is cool. But just a bit needless to teach at the initial training level. Even somewhat unnecessary for the higher ones. But any educator would hope folks look into it further on their own just out of natural curiosity later... but some folk have very little of that and never will. But they can still be taught airspeed and accelerated stalls and pitch plus power equals performance, and all the other direct applications of how aircraft behave, using pithy phrases like those... to safely do things from a purely operational standpoint.

It’s a shame that the depth of even this discussion just isn’t that practical to get to with most pilots in the training environment, but hard to justify, too.
 
And yet Newton works even in weightless space. Newton works with mass, not gravity.
And mass falls outta the sky, like apples, because of gravity, no matter how hard you blow on them to keep them airborne.
 
There’s also the practicality of getting this deep into it in true pilot training... we have time to hash this out to the Nth degree here, but it has to be condensed almost to the point of being inaccurate in an “FAA approved” 40 hour Private Pilot wonder pilot course of study.

I think that's 100% true and something I battle with as a practical challenge all the time in airman certification. Take a private pilot, especially one that is on an accelerated training track. Three months ago -- some times less -- this pilot had never flown an airplane. Now they're being asked to discuss, among other things, basic aerodynamics.

From the perspective of administering a practical test, the depth and breadth of the discussion on this message board would never occur during oral questioning. I find it challenging enough just to dig into a subject like induced drag and draw some reasonable but still very basic correlations (slow flight / region of reversed command, ground effect, soft field takeoff, etc.) and still have enough time left for the many other AOOs and associated Tasks. But a true understanding of how lift is actually generated is very helpful to a pilot not just in a cerebral way, but in application of skills to flying the aircraft.

Take a commercial pilot applicant. There it is in I.F., "Performance and Limitations," knowledge element CA.I.F.K3, "Aerodynamics." What is a reasonable depth now? In terms of pilot certificates and ratings, this ramps up to its highest level for the addition of a multi-engine rating to a commercial pilot certificate. That's due in part to the fact that the only tasks discussed during the ground portion of the practical test are I.E. and I.F. Aerodynamics are justifiably hit a lot harder for the multiengine rating as a thorough understanding of these concepts is absolutely necessary to operate a multiengine airplane safely. So this is a chance for me to sit down with an applicant and really dig into the aerodynamics. I have found generally good results here because by this point, the applicant has usually already completed three other practical tests at minimum. But at this level of certification if I ask an applicant about how an airfoil develops lift, and the answer includes a jumbled statement like "it takes the air longer to travel over the curved top part of the wing," I don't have high confidence that there's a sufficient level of knowledge there. We'll dig and probe and see what we come up with after that point, but's a a bad way to start because right away they're not recognizing how the wing is actually developing lift.

It’s a shame that the depth of even this discussion just isn’t that practical to get to with most pilots in the training environment, but hard to justify, too.

This discussion and others like it, when not acerbic (and this one doesn't seem to be so) are done outside the bounds of the training regimen and should be pursued by all pilots. I expect you'd agree, since you're here and participating. It's not a profession which is tolerant of punching a time clock or walking out of the airplane at the end of the day and closing off the mind to all matters aviation. There must be a constant interest in expanding knowledge and that happens all the time... which is why the successful professional pilots often have, at their core, a true passion and interest in aviation which goes beyond a nice paycheck.
 
Money ghosts.
The faster the plane moves, the more pressure the ghosts of your money exert on the bottom of your wing and push it up.
 
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