Shocked by the Newtonian explanation of lift.
Posted by olddoglearnsnewtrick@reddit | aviation | View on Reddit | 86 comments
Sailplane pilot here. I was raised on sound principles of which the most unquestionable is the Bernouilli effect as an explanation of lift generation around an asymmetrical wing profile. Never gace a second thought to anything else.
Now this very nice video https://www.youtube.com/shorts/QYrWfQYDUU4 tells me that is not the entire explanation and while it came as a shock, it makes a lot of sense for symmetrical profiles and high angles of attack.
I wonder if the pros in this subreddit all knew about this and it was just me ignoring the other side of the story.
joeballow@reddit
This interactive blog was a great way for me to build an understanding of what is going on, without assigning anything to a particular theory. Long read though!
https://ciechanow.ski/airfoil/
throwaway_4it4@reddit
Thank you for this!
olddoglearnsnewtrick@reddit (OP)
Supercool! Thanks a lot.
My initial knowledge of aerodynamics was based on an old 'Model flight' by Martin Simons way back when I was building my model planes https://medium.com/rc-soaring-digest/rediscovering-martin-simons-c4aca53b8fb5
die_liebe@reddit
You should treat this video as entertainment, not as something serious. The girl is nice.
Former_Farm_3618@reddit
You’re a science denier? There’s a huge debate among legit aeronautic engineers about wings actually create lift. I don’t think us basic pilots who cannot even do advanced calculus know any better.
I think it’s truly amazing there’s such a spirited debated about it.
die_liebe@reddit
It's just a social media post, with the aim of creating traffic.
Former_Farm_3618@reddit
Not really. That girl is a legit wizard at math. She’s not a insta/social media influencer. She’s the real deal, but a few here wanna deny her that.
Hareboi@reddit
And yet she repeated the popular internet myth about equal transit time which you can learn is false during second year aerospace engineering studies.
Don't assume people are infallible just because of their achievements.
die_liebe@reddit
If she understand what she is doing, she is being not honest.
Former_Farm_3618@reddit
Huh. Are you seriously saying someone with more math knowledge than this entire sub collectively doesn’t know what she’s talking about. Thats hysterical
die_liebe@reddit
She is just an influencer.
Former_Farm_3618@reddit
Haha. So all those hours and masters and PhD mean nothing, eh. Thats like saying pilots at the airliners and really pilots, they are just travelers.
die_liebe@reddit
I liked her video about the A4 format.
FarButterscotch4280@reddit
I would say there is NOT a huge debate at all among people in the airplane business. Almost all the debate is on the internet with people that haven't been schooled in the matter and are not willing to learn.
Former_Farm_3618@reddit
lol. K bud.
die_liebe@reddit
If a particle of air accelerates, there must be a force on it that causes its acceleration. That is Newton's law. Since the only forces that it feels are the forces from its neighbours, the pressure behind it must be higher than the pressure in front of it. Hence acceleration (in a stable flow), always means travelling from higher to lower pressure. That is Bernoulli's law.
Thaifeet@reddit
It’s newton. But the video omits what then happens at the top of the wing. Due to the curve, the air flows over and sticks to the curve (coanda effect) and thus is angled down when it releases at the trailing edge. Air goes down = wing goes up. This also explains what happens in a stall when the air releases too early from the top of the wing.
olddoglearnsnewtrick@reddit (OP)
Very interesting. So in this interpretation the stall is the cessation of the Coanda effect, right?
FZ_Milkshake@reddit
There is Bernoulli, Newton and Kutta-Joukowski, they are all correct simplifications of what is going on.
If you could calculate Newton for every air particle, it would give you the exact lift, if you could calculate Bernoulli pressures for every space, it would give you the exact lift, if you could spatially integrate the circulation according to Kutta, you would get the exact lift.
If you could solve the Navier Stokes equations, you would get the exact lift (and a Nobel price).
Newton lift is easy to imagine wrong, because it is not just the deflection of air downward, Bernoulli is somewhat easy to imagine wrong, because you could arrive at the failure of equal transit time.
Epiphany818@reddit
I'm totally splitting hairs and I know you already know this because you said it in your comment but I feel the theme of this post is pedantry 😆
We can put it cleanly into one set of equations, that's exactly what navier-stokes is, it's just those equations are almost always unsolvable unless simplified :)
Mission-Wasabi-7682@reddit
They are not unsolvable. Only numerically.
IM_REFUELING@reddit
That's why he said 'unless simplified.' There are tons of pared-down versions of N-S that have closed form solutions (like Bernoulli)
Mission-Wasabi-7682@reddit
Every good CFD solver can solve the full NSE numerically. And Bernoulli is not a simplification of NSE, at least not when it comes to calculating things like lift. This is an over-simplification that is often used in High-School, but is simply not correct (as OP just found out).
Blothorn@reddit
I’d note though that pressure differences do account for lift, but the equal-transit-time explanation that I most often see is still simply wrong. Transit time isn’t equal, and if it were it wouldn’t account for nearly enough lift.
GuaranteeUnhappy3342@reddit
I can recall years ago thinking the equal transit time idea was a bit weird…talking like all these molecules air were married (yeah a term used in Aero 101 back in 69) and had to meet after passing around an airfoil. Guess that is why I became a pilot rather than an engineer.
However I did work at Edwards (Performance and Flying Qualities) and we were having problems fitting some data we were getting in the 1970s with the original data from the 1950s flight test data. The engineer in charge talked to an engineer that had been on the original project. Turned out that the manual for flight testing had changed when they were first testing and they were having some problems…so some data was tossed out and curves were drawn and the airplane flew.
And years later we had problems making our data fit theirs. But back in those days we had photo panels and corrections to .25 of a knot on an airspeed indicator for it increasing or decreasing (but not steady?) I had the feeling we were trying to measure tenths of an inch with a wooden yardstick with 1/4 inch marks. And when making beautiful curves on a graph for the final reports…a few data points that did not fit got lost.
FZ_Milkshake@reddit
Bernoulli says only that the sum of pressures along a single streamline have to remain constant, if the velocity increases, the static pressure decreases, or vice versa if there is a pressure change along the streamline there will be a change in velocity.
Equal transit time theory/fallacy has nothing to do with Bernoulli at all, because Bernoulli can't make predictions for the different streamlines above and below the wing paper.
LordIffyBoatrace@reddit
This is the reading I didn't know I needed today.
Feisty_Donkey_5249@reddit
And the ever-popular Aerodynamics for Naval Avaitors. Bernoulli, Coanda, and angle of attack/water ski effect.
FoximaCentauri@reddit
Reminds me of my favorite science proverb: No model is accurate. But some are useful.
unhappytroll@reddit
exactly. this is why analogue computers are exists.
snakesign@reddit
Most of our science is just the first couple of terms of an approximating Taylor expansion.
Joatboy@reddit
Aka fluids are hard to model
olddoglearnsnewtrick@reddit (OP)
Wow thank you so much. I really love to feel ignorant, since it means there are new things I can l;earn :)
Bottoms_Up_Bob@reddit
If we are going to ask OP to do things can it please be solving the Navier Stokes form? That would be so useful.
Miserable_Bug_5671@reddit
Planes can fly upside down, which is a quick kick in the pants for the Bernoulli only model.
PilotBurner44@reddit
Fun fact, you can create a Bernoulli like low pressure above a flat plane that has a positive angle of attack, which will create lift. A piece of plywood that is angled up into the wind will create lift above it due to this. If you make that same piece of plywood into a box with a big flat front, so there is no air going over the top of it, but still has the same angle of attack, it will still generate lift from underneath, which is proof that the Newton theory is also correct, which is why NASA, the successor of NACA, which was the National Advisory Committee for Aeronautics, say that both are true.
Miserable_Bug_5671@reddit
Of course both are true, hence the word "only" above.
PilotBurner44@reddit
But the Bernoulli model also works on planes that fly upside down. Airfoils don't switch or pick and choose which method they use based on the orientation of the aircraft. An inverted airfoil will still generate positive lift so long as it has an appropriate positive angle of attack. Most airplanes actually have a a positive down force on their horizontal stabilizer (tail) that is generated by an asymmetric airfoil.
If the Bernoulli principle quit working when the plane was inverted, it would simply fall out of the sky because not enough lift is being generated by the Newton principle alone. The same goes for the Newton principle.
FarButterscotch4280@reddit
Nope. An aseymetrical airfoil can alsowork upside down to thanks to circulation and Bernoulli and a larger angle of attack. Find a Supercritical airfoil cross section in the internet and have a good look at the "Deck" on the top.
Miserable_Bug_5671@reddit
The larger angle of attack places it firmly in Newtonian territory.
KickFacemouth@reddit
Plus the fact that there are symmetrical airfoils... or even just sticking your hand out a car window at an angle.
olddoglearnsnewtrick@reddit (OP)
Absolutely and as I did aerobatic glider flying for sure I should have given a thought about it :)
Virian@reddit
If Bernoulli was the right answer, a paper airplane would never fly.
PilotBurner44@reddit
If Bernoulli was the wrong answer, every aircraft manufacturer wouldn't bother making the top of the wing curved and airfoil shaped.
Mid_Atlantic_Lad@reddit
Easily the best explanation I've ever seen on the topic that's still digestible for non-engineers.
https://m.youtube.com/watch?v=E3i_XHlVCeU
keenly_disinterested@reddit
Once you get it all figured out then go watch a bird fly.
The way I keep it straight in my head is there is a combination of physical effects occurring simultaneously, all of which result in the air moving around the wing deflecting and/or bending, depending on how you describe it.
olddoglearnsnewtrick@reddit (OP)
And then go see a bumblebee and despair :)
Whatsthathum@reddit
The bumblebee doesn’t worry about whether it can fly or not…
keenly_disinterested@reddit
🤣
AstuteCouch87@reddit
All I remember is having an older CFI getting very mad at me for suggesting that Bernoulli's was not the only way to explain lift..
FighterSkyhawk@reddit
Bernoullis is simply a simplification of the whole problem with a lot of assumptions that are mostly true. It’s kind of a shame that most pilots only learn that.
The real problem is unsolvable (at least currently) and that’s why we use supercomputer simulations for higher accuracy, and even then we need to do sub and full scale testing to verify the models.
There are several theories to help break it down into something we can understand, Bernoulli’s, Newton’s, Kutta-Joukowsky, Coanda effect, circulation, etc. but none of them get the full picture because all of them make assumptions.
The full problem are the Navier-Stokes equations and can perfectly describe the motion of a fluid, and thus can be used to solve for forces on a wing. But it can’t be solved currently because it’s too complicated. But it can tell you how fluids move. It basically uses conservation of mass, momentum, and energy, and equates that to viscous forces, body forces, and pressure.
Mission-Wasabi-7682@reddit
Yes, old news. Gets discussed here once a week. Everytime someone is willing to die on the hill of Bernoulli.
senorpoop@reddit
The traditional Bernoulli explanation of lift requires equal transit theory which 1: has no basis in modern science and 2: does not explain how airplanes with symmetric airfoils can fly or how airplanes with asymmetric airfoils can fly upside down.
If Bernoulli was the only way lift could be made, those little dollar store balsa gliders wouldn't fly as their wings are flat pieces of wood.
atomatoflame@reddit
I'd also posit that flat wing gliders are not as efficient as a proper wing. So they only explain a part of modern wing lift. If you look at the streamlines around a flat wing I'm sure there is a more direct path along the chord for air underneath compared to air above the flat wing.
Sage_Blue210@reddit
When considering theories and airfoil ms, remember that flat wings at an angle of attack glide well (balsa gliders can teach a lot).
olddoglearnsnewtrick@reddit (OP)
I did begin my love of flying with those, and then progressed to actually designing my country's F3B sailplane many years ago (piloted by Marco Lorenzoni) and then progressed through real saiplane piloting with a golf FAI badge with two diamonds :) so empirically I know something about flying and aerodynamic but just thought the theory of the Bernouilli effect explained enough without giving much more thought to it.
It is (an healthy) shock to have discovered I had glossed over many details.
Sage_Blue210@reddit
Observing balsa gliders as a kid taught me a great deal about wing placement, shifts of CG, effects of the vertical stabilizer, loss of the horizontal stabilizer, etc. So much fun.
olddoglearnsnewtrick@reddit (OP)
Yeah, fond memories.
My first model had a small spring loaded mechanism which triggered the horizontal stabiliser deflection and initiated the descent even if the glider was climbing in a thermal. Sometimes in the summer it went really high and you doubted it would ever come down :)
Efficient_Sky5173@reddit
There is no debate. Bernoulli is wrong.
Former_Farm_3618@reddit
Do I dare even ask how you feel about eating whale?
Efficient_Sky5173@reddit
Double dare.
sniper4273@reddit
I've always liked the NASA explanation at their k12 website.
olddoglearnsnewtrick@reddit (OP)
Thank you so much, very interesting.
Silver_River9296@reddit
As an instructor, I taught that the Bernoulli principle made wings fly but until I used the term ‘Sucked’ the wing into the air, NOBODY UNDERSTOOD IT. But after flying Ag Planes close to the edge I realized that there is the Barn Door Principle! A barn door with no airfoil will fly with enough wind ( or airspeed ). Only twenty years Rd s later did I hear Newtonian effect. Those guys get all the credit!
olddoglearnsnewtrick@reddit (OP)
Yes I do understand. Even though not my academic background I had read quite a lot about aerodynamic (especially low reynolds numbers for my F3B build with Selig-Donovan profiles) so thought I did understand quite well. Turns out I had a simplistic knowlege. Dunning-Kruger at its best :)
Zapatos-Grande@reddit
When I went through my CFI initial in the late 2000s, we had a discussion about this. If I remember correctly (almost 20 years ago), we found out the FAA materials (Pilots Handbook of Aeronautical Knowledge) were very Bernoulli heavy with some mention of Newton. NASA had a lot of material that was more Newton heavy when discussing lift. In reality, it's both.
wj9eh@reddit
Just to put my very valuable opinion in here, how a "wing works" is really the wrong question for what people are asking. It's obvious how a wing works, just stick your hand flat out of a car window and angle it a bit and it'll be pushed up and down bloody hard. That's where your "angle of attack" answer always comes in. The question people are asking is how does a wing work so well; how are they so small yet can lift up the plane? Well that's where all this science comes in but really it's by the by for what most laymen are asking, if you ask me.
The problem with developing powered flight was never how to generate lift from the air, that's always been pretty obvious. It was how to generate enough lift with little enough drag with enough thrust to counter it and take off.
Anyway, great video.
harryygee@reddit
To simplify this explanation of the “problem” with one word it is: control. This is why the Wright brothers were successfully credited with the first flight, they figured out how to control all of these concepts while airborne. It isn’t just take off, it’s the whole damn pie of flying.
Vegetable_Log_3837@reddit
Yeah, a brick can generate lift and fly.
The trick is to get it going fast enough and keep it oriented with a reasonable AoA. Lift isn’t the hard part, control is.
huehuehue1292@reddit
The video is just plain wrong. Most of her content is great, but this one video is not.
While there are some unknowns in fluid dynamics, that is not one of them.
The so-called Bernoulli and Newton explanations are just two ways of interpreting the same physics. There is no debate in the community to which is right, both are. Also, the way both explanations are shown in the video is also wrong.
The unknowns in aerodynamics are more related to turbulent flow. More specifically where it starts. But it has little to do with a broad explanation of how airplanes fly.
Hour_Tour@reddit
I have a lot of time for her, but a physicist and science communicator should not be caught out by the "two air molecules have to meet up again" misconception.
drdsyv@reddit
You might also be interested in the wikipedia article on "Lift (force)"
GrabtharsHumber@reddit
A wing creates lift by producing a pressure differential between its upper and lower surfaces. A flat plate will do this at any non-zero angle of attack. It will just do it with more induced drag than more RIFPIBy shape
Ok_Criticism6534@reddit
https://youtu.be/aFO4PBolwFg?si=P8Dg2c3TJDq5txLJ
The particles do not take equal time to reach the trailing edge. My understanding is that the wing causes the pressure differential due to its geometry relative to the oncoming air and that pressure differential creates the difference in velocity above and below….not the other way around.
unhappytroll@reddit
immediate misconception right at the start of "Bernoulli explanation" about two particles "meet" at the end of the aerofoil profile - no, they shouldn't, and in fact mostly don't. This video is a proper explanation of Bernoulli effect on wing, I think (albeit in horrendous quality, sorry).
ShonOfDawn@reddit
They are the same thing. The momentum conservation in Navier-Stokes is literally Newton’s second law. Bernoulli is a simplification of that, it’s the same thing
BGuy04@reddit
My two cents as an aerospace student: 1. There’s no reason the particles have to meet up, but the air does speed up on the upper side, as can be proven from experiments. 2. The pressure distribution over the two sides can be measured, and the air is always much lower pressure on the top compared to the ambient, and only slightly higher pressure on the bottom. This means the air mostly pushes up due to the low pressure, instead of the high pressure from the Newtonian explanation.
I personally follow John D. Anderson’s explanation, where the a wing/airfoil is simply and obstruction in the flow, and the way the air gets out of the way of an airfoil/angled shape speeds it up on the upper side and slows it down on the bottom. From the Bernoulli effect (which holds for both compressible and incompressible flows) this respectively decreases and increases the pressure on the two sides. And since the wing is pushed up due to the pressure difference, it also pushes down on the air, leading to the Newtonian explanation.
OkMarsupial9634@reddit
When I discovered the same my instant regret was the missed opportunity to fold a paper plane and send it straight down the classroom while the physics teacher was explaining the airfoil: “How does this fly, Sir?”
olddoglearnsnewtrick@reddit (OP)
lol
TommiHPunkt@reddit
Either "air go down, plane go up" is enough, or you need to go full on fluid dynamics simulation. Everything in between is unsatisfyingly incomplete. Paraphrasing this video
Nok1a_@reddit
I love her how she talks (accent) and tha many explantions of things she gives!
I would say newton one it´s more difficult to understand or picture, but if you think about simetrical airfoild then bernoulli should not work (in the explanation she gives)
FZ_Milkshake@reddit
A symmetric airfoil needs an angle relative to the airflow to generate lift and that causes the stagnation point in front of the wing to move downward relative to the centerline. From there on, the longer path is over the wing and the explanation works again (though it would have been nice to not see equal transit time).
Epiphany818@reddit
How do you explain a flat plate or rectangular airfoil using Bernoulli?
OriginalGoat1@reddit
Even for an assymmetric airfoil, Bernoulli could never work because airplanes don't generally fly inside pipes.
herrerarausaure@reddit
The real shock is that she received a PhD in fluid dynamics yet still presents the "equal transit time" explanation as valid.
post-explainer@reddit
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