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- เผยแพร่เมื่อ 8 พ.ค. 2024
- Explore the physics of flight, and discover how aerodynamic lift generates the force needed for planes to fly.
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By 1917, Albert Einstein had explained the relationship between space and time. But, that year, he designed a flawed airplane wing. His attempt was based on an incomplete theory of how flight works. Indeed, insufficient and inaccurate explanations still circulate today. So, where did Einstein go wrong? And how exactly do planes fly? Raymond Adkins explains the concept of aerodynamic lift.
Lesson by Raymond Adkins, directed by Michael Kalopaidis, Zedem Media.
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what is reality?
History vs Alexander the Great
Also, could you guys also make video about Persian Heroes Rostam & Esfandiar somewhere in future too? These two are not as know as say, King Arthur, Bewoulf or Heracles, but need more public regonition if you ask me.
Best I've seen so far.
2023(Gregorian) “Respect and dignity.” Furthermore:
Santos Dumont is the true father of aviation and not the lying brothers 👍
As an aerospace engineering graduate, this pretty much sums up what you learn in basic aerodynamics. And because of we still don’t know how it happened, we used a lot of coefficients (which is derived from experimental methods) to account for them
How does a plane fly upside down? Now the curve of the wing is upside down too.
Not quite, we actually know how it works and how it happens... The secret is on the boundary layer detachment on the trailing edge... Even without that insight, we have very good mathematical models, developed by engineers and math geniuses like Ludwig Prandtl and Nikolay Zhukovsky.
I recommend you to check the beautiful book:
Fundamentals of Aerodynamics by John Anderson
@@gsilcoful because of angle of attack... Typically with a symmetric airfoil... But even if it's very asymmetric, you put enough angle of attack to curve the flow field the way it fits you (upward force)
@@abrahamvivas9540 are u saying it only possible with plane wing that move? A Boeing can't, right?
@@Watch-0w1No moving wing needed, just pitching up the nose of the plane (even if it's upsidedown) ... In theory a Boeing could... In practice I don't know if it could resist the stress of such a manoeuvre, and even if it could, it probably wouldn't be very stable...
I will continue to believe that it is magic.
Yes
And this is how earth can be flat...
@@Miguel_Noether what?? son what school u go to 🤦♂️🤦♂️ the earth is a donut
@@ravelnavarro9625 torus*
@@ravelnavarro9625 Wait, hold on. When did the earth dinosaur theory got disproven?
To summarise: "A wing doesn't work by accelerating the air over the top because a wing works by accelerating the air over the top. And engineers use equations that are unsolvable to solve their problems."
Exactly what I thought. But the most surprising thing was the "pregnant duck". Learn something new every day.
Navier Stokes equations are general equations for fluid flow. This was only practical with computers invented well after most aircraft designs. Before then most airfoils are calculated and predicted using the Kutta-Joukowski theorem, which holds true for most airfoils that don't separate i.e. stalling.
@@jonathansoto5480 They can be approximated to a reasonable amount by a good PC, the processing time grows exponentially with the precision.
@@jonathansoto5480 It's a huge trade-off, a project made by a team could easily take days to simulate on a high end pc. Why are you talking like it's a minor inconvenience? XD
sounds perfect
wow i'm absolutely amazed, i'm not really an engineering student or anything but i've had a really big interest in aircraft and the engineering behind them for years, the fact that i didn't even properly know how a wing worked is kinda crazy to me
Same 👍
Nobody knows. That's what's so intriguing.
Amazing that the low pressure pocket is false and it's really faster air creating lower pressure over the wing...
@@goofyfoot2001 well the low pressure pocket isn't really false it's just that it's created by the faster air no?
As an aerospace student, I adapted the policy of, if it works it works
As a pilot I can confirm that it works
As another student..if it works
We programmers are also very adaptive to this situation. CS and aeronautical branches of Engineering are not so different afterall😂
@Sumi E. Eweits considering the fact I had to take so many coding class already I would say the aerospace department think we're cs majors
I like your way of thinking
Dang this ISN’T Wendover Productions?
OMG , I thought the same thing!
No this is wenunder
No this is Wend ever productions
Yeah it’s less than 12 minutes long
@@roryhewson695Then it would be Half as Interesting!
I remember during my aerospace engg degree- a senior professor was teaching us aerodynamics - and he asked the class why is there high velocity on the top of the wing- I and many of classmates quotes that because the air particle have to cover a longer distance- after hearing this, prof corrected us and gave the above-mentioned reason
Yup, it is not the high velocity that creates lift
High velocity air is created because of vaccuum generated at the top back end of the wing
Plane wing are the same design as flat wing angle up, with some smoothing to reduce turbulence
@rifa.3307 okay then how is the vacuum created?
@@brlyjo because the air are being forced to move downward by the wing
@@rifa.3307 yup
Where did u do ur aerospace engineering?
There is so much more to this. I hope videos like these push people into digging deeper. TEDs are the best
Crazy to think that we actually have no definite answer to the question „how do planes fly“ and yet we just trust that it’s gonna happen every time we take off in one
Actually, there is a very recent paper called: A variational theory of lift by Gonzalez and Taha. And it pretty much answers that question.
bro you just watched a video on the definitive answer of it 😂😂 4:11
@@LeprosuGnome no, as the video said, we have multiple theories (not mathematically demonstrated) that try to explain the phenomenon, we have a general basic understanding of the principle (lift = difference of pressure) and we have an equation that describes the mathematics behind the fluidodynamics of flight, but there is no definite solution to this equation (it's one of the famous millennium problems), only to simplified versions that assume certain parameters, plus lots of approximate equations based on experimentation
@@JohnnoNonno"... the explanations may vary ... But when it comes to math, there is no controversy..." - the video
@@LeprosuGnome ...
Yeah because there's an equation that explains it, but the equation has no general solution, it has been calculated that it works in many applications and specific cases.
I had heard the common "wrong" explanation so many times, so I accepted even though it didn't completely make sense to me! This was a fantastic video, thank you!
So did my physics teacher! She explained the other way around, that air below the wings has lower air pressure
@@JunWongs OMG ! That explaination sinks our Aeroplane 😂😆
@@JunWongswatching too much F1
Same
I had never learned the wrong explanation, I just memorised that curved surface is aerodynamic.
But I learned something today.
As a student going through Aerospace Engineering, this is a very good introduction to lift. We learn about a bunch of models and methods to determine lift. I believe it is the circulation model that relates to the Navier-Stokes equations. Its a very interesting model and I encourage anyone interested to look into it!
Note that if we compare the circulation model to the transit-time fallacy, we notice that, if the divided parcels arriving at the airfoil trailing edge REALLY DID arrive at the same time, then the circulation must be zero, and the lifting force is then zero.
So, the transit-time fallacy is far more than simply wrong, but also it's a recipe for guaranteeing that the lift is exactly zero. Amazing, no?
We can adjust the attack-angle until the divided parcels do rejoin behind the wing. That's the angle where the lift is zero!
@@wbeaty you seem smart how is it we haven't figured this one out yet?!
@@ericcotter1984 Major textbooks get it wrong. Not just oversimplified, but actually incorrect. And I think their authors would rather die than to admit this in public. They never will give an improved corrected explanation, because that would suddenly spotlight their years of flawed verisons. Not good.
To my mind, this marks them as classic examples of Feynman's "Cargo-cult Scientists." The aero community is insufficiently honest to be able to do real science. They don't get it: wherever truth and simple reality is involved, we're not allowed to distort things. In that case, being publicly embarrassed, even destroying our academic reputations, even destroying our careers, that's nothing if it cuts through all the distortions surrounding lifting-force explanations; solving the entire problem while also getting us all fired from our jobs as college professors, with our aero textbooks suddenly banned from general use. But that would be worth it. The truth is that important.
Strong words? A bit IMPOLITIC?
Well, yeah. They're needed here. The problem really is that bad. Lifting-force explanations, and the problem of "equal transit-time fallacy," that's just the tip of the iceberg.
The problem is exposed when we ask why students have such problems understanding this topic. It's because the explanations are basically un-physical, directly violating Newton's laws. (So we might do like NASA GRC did, and analyze the problem from the aspect of common misconceptions which interfere with our understanding. Perhaps even point out the origins of these misconceptions, mistakes which are being taught to everyone by particular textbooks.)
Major example: if we draw an airfoil diagram with streamlines and Kutta condition, but we don't draw the ground surface, nor explain its role, then we're directly violating Newton's 3rd law. (But aero textbooks have been doing this since day one, so that makes it OK, right? Actual physicists might disagree, I think. And if students mysteriously remain forever confused, they'll never be able to figure out why this happened. Built-in Newton-violations could be one major cause. They tend to come back to bite us. )
Another: if Prandtl sets the horseshoe diagram to horizontal, but doesn't admit that the vorticity then becomes exactly zero, that's profoundly unphysical: a "reactionless engine" producing forces wo/exhaust-plumes. Another: if Prandlt sets the wingspan to infinity, or sets the flight velocity to infinity, that's unphysical, and has converted wings into ground-effect machines, removing all the "reaction motor physics" from a system which is inherently a reaction motor. Finite-span wings are examples of propulsion, where Bernoulli Eqns are forbidden. But in aero, we get rid of all that, and instead analyze them as if they were laminar venturies. But lift is actually based on 3-dimensional wings of finite span, and vortex-shedding. No need to push from a distant surface, because real wings push against the air alone, via vortex-shedding. Movable vorticity is just too hard, so let's pretend that all airfoils instead are flying by instant contact-forces and ground-effect. Pushing off from a distant surface. And then, never mention this to students, and worse, erase the ground from the airfoil diagrams. (Note that L. Prandtl is the very guy who originated the whole Equal Transit-Time fallacy. Klaus Weltner of U. Frankfurt was the one who tracked down that particular bit. Thanks, Dr. Klaus!)
The following physics-teaching ditty could be modified to apply to lifting-force explanations... "Teaching thermodynamics is easy as a song! We make it so much simpler if we make it completely wrong."
Aerospace engineer here. The explanation provided at 2:12 is certainly faulty. Saying that velocity on top increases so the pressure decreases assumes Bernoulli's principle to apply here, which doesn't in this case. For the Bernoulli's principle to be true the following three conditions have to be met:
1. The flow has to be subsonic (going below the speed of sound). The video doesn't mention anything about the regime but ok. ☑
2. The fluid has to be incompressible. Air IS a compressible fluid. ❌
3. Most importantly, the process has to be Adiabatic (there has to be no transfer of energy between air and the wing) and reversible which in here it is certainly not. In other words the air flowing around an airfoil is not a closed system! You cannot use a simple Bernoulli's to explain lift. ❌
Another mistake in this video happens at 2:05. The acceleration in this case is due to the change in the direction of the velocity vector and not its magnitude! If the magnitude of velocity hasn't changed, why should it affect pressure at all?! This means pressure would not be affected even if Bernoulli's principle were hold true.
True answer: We currently have to institutive understanding of aerodynamics and specially the lift and drag forces. You'd have to use three laws of Newton in conjunction with three laws of Euler for fluid dynamics (conservation of Mass, Energy) to explain these forces. And yes, it's harder than rocket science.
Explanation using stream tube of air given by JD Anderson in Introduction to flight book is still most convincing explanation of how lift is created.
Interested people should read this. 😊
This is a very impressive and informative video. The visual support along with the organization of the video makes the topic easy to understand. I liked how you bring up Bernoulli’s principle and many other theories that help explain how lift is generated. The only thing that I would love to see added is how the angle of attack plays a role in lift.
Ted-ed makes complicated subjects so easy to understand and fun to learn. The top-notch animation and narration is one of the best in TH-cam.I am just so grateful there are channels like this where I can both learn new things and have a fun time😂👍👍.
Reminds us the forgotten fact, truth in a child's mind : "Learning is Fun"
I didn’t find this easy to understand! “Wings really don’t work the way they really work”? Huh?
Okay, then we’ll solve it with unsolvable formulas. This is poorly constructed, poorly explained, and poorly laid out.
Sorry mate but for those in the know, this was a terrible explanation.
@@ndvorsky well then, how would YOU explain it?
@@alexanderstar8360 I'd say you should go to college. There is a reason it takes people a 4-year degree to do this stuff with any skill beyond winging it (pun intended). Not everything is able to be accurately described in five minutes.
For the layman's explanation, I would just say that the wing is able to direct air down and therefore the plane up (not to be confused with the "Newton's law explanation" of aerodynamics which is wrong) due to the conservation of momentum. Add on that different shapes are more efficient or have different characteristics and that they behave differently at different scales. That is a fully accurate explanation that doesn't get into the very complicated nature of quantifying the interactions.
Top notch animation as always.
As a physicist and a practical person I have always believed in experiment. On a sail we use pieces of wool to determine the air flow. You pull the sail in until the downwind wool starts to flutter ( aerodynamic stall) then back off just a little. Keeping at that point gives the maximum "lift" ( in this case forward force) which helps us to win races.
On my sailboard the centreboard is symmetric. In a straight line it has no lift. But as the wind blows it effectively moves the boat at a slight angle to the forward direction. Now the centreboard generates great lift and opposes the motion. Meaning I don't get wet. So the angle of attack is the key component in this application not the curve of the board.
I luff your explanation.
Yeah this video is awful. You can fly with completely flat wings as long as they have a positive angle of attack. The effects described in this video are only an optimization of aerodynamic efficiency. They just want to look like they're explaining something surprising so they neglect the more intuitive and important aspect of lift.
But the angle of attack is not everything though... The key element was mentionned in the video: it is the pressure distribution around the airfoil. The problem is that in reality only a solution of the navier-stockes equations for a given problem or configuration of flight will explain the pressure distribution and therefore the lift. The only dilemna is that these are equations noone can solve analytically for arbitrary configurations of flight. The effect of the angle of attack which you mention is only predictable as long as it is not to high so that stall does not occur - also the wing has to be very slim.
It was very intriguing to learn about the misconceptions first before diving into the actual definition of the generation of lift. The video was very creative and helped strengthen my understanding by listening to the topics of velocity, pressure difference and curved airflow.
Wow! TED-Ed's animation of the Sky scene gives a Fresh feeling and is just soothing to the mind!
4:18 - The momentum equation should use different symbols for density and pressure. The first term on the LHS and second term on the RHS should use the Greek letter "rho", customarily used to denote density.
This is a good introduction to the corrections of the misconceptions of lift. I encourage everyone to keep diving into the subject.
A very good presentation of the basics of lift. If you add angle of attack to the equation, it will be perfect.
Ultimately it all serves to take air that had been stationary before the wing arrived, and accelerate it downward. A propeller is also a wing. When it generates lift (thrust) it also produces propwash (air that is accelerated rewards). A helicopter rotor is also a wing (a rotary wing). Helicopters produce downdraft (air that is accelerated downwards. Airplane wings do the same thing.
That's just your _OPINION_ Man!!
😜
As an Aerospace Engineering student, this was fulfilling to watch.
I just love watching Ted-Ed videos. Keep them coming forever. I love to learn.
Good work in making it brief and removing a lot of jargons. I was expecting the another wrong explanation, involving boundary layer theory but I guess it would complicate it more.
Love the quote at the beginning lol
Great video! Would be great to see these animations for non solid air-foils like sails on sail boats, going against the wind for example.
Thank you very much. I always had it in the back of my mind that the 'air above moves faster to meet with the one below' theory never felt true. I just couldn't counter it so I just stayed quiet and accepted it as such.
Hi TED-ED! I'm in middle school and I love writing down and watching one of your videos every day! They are so knowledgeable, I wonder why people my age don't take advantage of the unlimited source of information given to us on the world wide web!
they DO, dumbo.
@@alexanderstar8360 Sorry, I just like watching their videos, I don't know anyone who watches their videos. I didn't mean to offend you. I should've thought about it. I'm thankful for that :D
@@syrup- you did not offend me. i was just saying that a lot of middle schoolers watch their videos because ted-ed helps us learn in such a fun and interesting way ( i should know since i am a middle-schooler myself😜😜).I watch it along with my sister almost every day.
@@alexanderstar8360I’m reading this as a aircraft technician student 🤣 I also enjoy the simple and still entertaining content. Just found it funny how y’all made it seem like middle school stuff 😅
@@TeamOdiffat Apparently you did not understand what I was saying. Nowhere did I say that Ted-ed is only for middle-schoolers. It can be helpful for people from all age groups all over the world. Maybe you should have read my comment more carefully before replying.
How does centripital accelaration increase the wind velocity(2:16)?
That was a bogus use of centripetal acceleration. Centripetal accelerations do NOT increase speed, they only change direction. Also, their arrows are in the wrong direction for centripetal force. Maybe they are representing the Newton's third law reaction force on the top of the wing in order to pull inwards on the air flow to make it curve? Not sure - but I don't think this video clarified much except that the equal time explanation is wrong.
"The force you feel in a sharply turning car [arrows showing movement towards the outside of the curve] " is centrifugal force. Centripetal force is the force directed towards the inside of the turn, causing the acceleration of the car. Also, this video confuses turbulence with flow separation. They are very different things and aeroplanes often have vortex generators to induce turbulent flow as it delays flow separation.
True about the arrows (and turbulence). But the forces you _feel_ in a cornering car are the door or seatbelt pushing against you, the friction between you and the seat etc.
(in the non-inertial reference frame of the car you'd then _explain_ feeling these forces by saying "Well, I must be being pushed out, away from the centre of the turn" - centrifugal force - but in the inertial reference frame of e.g. looking down from a stationary point above, your explanation doesn't need that extra "force" because Newton's laws in their simplest form suffice. What you actually _feel_ though - what you _measure_ in other words - is, crucially, the same in _both_ reference frames)
Regardless of airfoil, if the air is deflected downwards, you have lift. A flat board at the proper angle can produce lift just fine.
I've always thought that airplane wings were slightly tilted upward, so the wind hit the wings and got pushed down, thus pushing the plane up
They actually are tilted upward a bit, this is called angle of attack (AoA) and indeed deflects the wind downward (pushing the plane up as a reaction force). The lift increases with the AoA up until a certain limit where the airflow cannot follow the top surface anymore and stops generating lift. The AoA is usually already present when the aircraft still has all wheels on the ground, although it wasn’t animated as such.
@@HorrorMotion Marine aircraft mechanic here, never saw that. Every wing I worked on or saw was parallel to the line of the fuselage and, as a matter of fact, some few fighters were nose down attitude sitting on the gear and they looked strange taking off with the nose down. Then, at lift speed the attitude changed and rotation followed soon afterwards. Why would a modern aircraft wing need impact/reaction features in the wings when they all have flaps?
That is exactly what Newtonian lift is. For every action, there is an opposite and equal reaction.
Well, an actual airplane has flaps and slits so saying they are tile upwards is a simplification.
But the main point is that the action reaction between air molecules and the wings lower surface is part of the lift. However if you look any wind tunnel demo about wings, you will see that air ABOVE the wing is being 'pulled' down which you cannot explain with simple newtonian principles. This I think is caused the low pressure above the wing which causes atmospheric pressure to push down the air above the wing.
If you want to use the newtonian principle I guess you have to explain it in terms of all the mass of air being accelerated downwards by the complex flow of air around the air foil.
I guess if you mount a large enough motor to a barn door as they famously claim is possible to make it fly then you can put that down to the air molecules hitting the door ;)
@@Axel_Andersen The air on top of the airfoil adheres to the wing due to the Coanda effect. All moving fluids behave this way, and it is the reason why you can do that water bender trick in the shower ;)
This explained everything so nicely I'm glad I could find this video I needed to study quick for air cadets and this made it so much easier
This is a great video with a clear and concise explanation of the general way in which lift works. As an engineer currently studying the theory surrounding lift, I’m very appreciative that you took the time to explain the equal transit time fallacy as that is certainly a misconception that I have had before in my understanding of how lift works. It was also great that you mentioned the importance of airfoil geometry, angle of attack, and touched on the fact that there can be other factors that affect lift e.g. vorticity. Overall a great explanation of the cause of the simultaneous differences between velocities and pressures, and how they work to produce lift!
since I was young, I was never completely satisfied with the equal transit time description, because I could never really get the hang of it. Then I started thinking about it thoroughly, as to WHY the pressure is lower above the wing, and after a while I came to the same conclusion, that the air above the wing has to have such high inertia, that it "pushes less" on the wing, than the air below it. It really just comes down to mechanics if you think about it, and I think this sort of explanation is much more intuitive
Arthur Weasley finally got his answer
PS: Remarkable animation as always ♥️
Very nice explanation. The centripetal force you were talking about is also known as the Coanda effect: when air flows over a curved surface, a low pressure is created which provides the centripetal force that makes the airflow "stick" to the wing. That same low pressure, apart from bending the flow, also accelerates the air due to Bernoulli's law, which further strengthens the effect since the Coanda effect now has to create an even lower pressure to keep the faster airflow attached to the wing. This feedback effect continues until both equations for Coanda and Bernoulli are satisfied, they are basically two components of a single vector equation (which is a part of the Navier Stokes momentum equation).
This is not the coanda effect, as one aerospace engineer told me. That is something completely different, and only applies to a jet moving through static fluid and entraining the fluid around it.
Air is going to follow the curved surface because it’s the only thing it can reasonably do, because of air’s low viscosity and we live in a pressurized atmosphere.
The Coanda effect is something that sounds similar but is an entirely separate concept, and is usually only in play when you have something like slotted flaps that are extended. The important distinction is that the Coanda effect requires a "jet" of air, something that cannot be achieved if all you have is freestream.
@@anongos I don't think that's correct. The source of the airflow does not matter, just the fact that the flow of air follows the curvature of the wing. When you increase the angle of attack to the point where the airflow no longer sticks to the wing, lift is reduced and this eventually leads to a stall. That means the coanda effect is pretty important for lift generation.
It is true that the coanda effect is often enhanced by blowing air over flaps (slotted flaps, engine exhaust over flaps,...) but that's not the only situation where it occurs. Every wing uses it, and wings would generate drastically less lift without it.
@@michelcolman314 It's an important part of the definition of the Coanda effect, that the flow is a "jet", or in other words, originate from a nozzle or orifice of some sort. Again, you cannot achieve that effect with freestream alone as you don't satisfy the condition for the Coanda effect in the first place.
@@anongos And what exactly makes the difference between an airflow from a jet and a really big airflow? In both cases the air follows the surface. Perhaps we may disagree on the exact definition of the coanda effect (which seems to change depending on where and when you look, I remember it being described differently on Wikipedia a few years ago) but the principle seems to be exactly the same: air flowing over a curved surface tends to follow that surface, and this is caused by a pressure gradient.
Received my pilot certification in 1967. In ground school the explanation was the Bernoulli principle. Nice and simple. But when flying the focus was on angle of attack to the relative wind. Even today more intuitive equipment is being invented to let the pilot know when a stall is imminent. So few explanations describe the pressure on the bottom of the wing as a major factor for flight. This is why you can high speed taxi an airplane without lift off if the angle of attack is zero or negative. Just my experience.
Thank you, you're so right pointing out lack of discussion about the effect of wind on the underside of the wing. When does a plane lift off of the ground and actually start to fly ? When you rotate and get wind on the UNDERSIDE of the wing. Stick your hand out of a car window at 60 mph, right ? Lot of pressure.
Of course the low pressure on top is part of it, but waiting on only the Bernoulli effect to get you off of the ground....well, good luck !
As you said, fast taxing with no flying !
I am an aircraft maintenance engineer by profession so I have a good foundation in theory of flight and aerodynamics. Recognizing the over simplification for brevity's sake, I think your description should specify that the air pressure under discussion is atmospheric or static air pressure. Also missing is that a wing by itself does not generate the necessary relative wind over the wing surface. The wing profile and surface must be acted upon by an outside force to push it through the medium, e.g. power plants for powered aircraft and gravitational and thermal forces for sailplanes etc. Although you touch on it briefly, it is important to consider the effect how the angle of attack of the of the wing affects the delta P on the underside of the wing.
Hi Ted-Ed I was wondering,
On timestamp 2:07 you present a diagram of centripetal acceleration on the nose of the wing. It shows force arrows pointing away from the center of the curvature. Doesn't centripetal acceleration have force directed towards the center of the curvature? I was confused since centrifugal force is directed outwards. Is this a typo, am I overcomplicating things, or am I completely wrong?
@@bradyeastridge3592 But what about the car?
The explanation is probably wrong, if not, it's at least confusing. Centripetal/centrifugal force isn't real, it can't be the cause of anything. It is an artifact of viewing a problem from a certain point of view. This means that the actual cause of the acceleration is not explained by the video leaving it as empty and confusing as all other videos claiming to explain how wings REALLY work. Further confusing the topic, the explanation in the video is counter to the reality of cambered wings (Both bottom and top surfaces curve up then down rather than just the top) which have a higher lift. People who don't know anything about aeronautics need to stop making these videos.
While I'm here I'll add that the Bernoulii vs Newton debate is wrong on all fronts. It's not one or the other nor is it a "bit of both". All laws of physics exist all the time. That's why they are called laws. It's 100% Bernoulli, and 100% Newton and 100% others. It's wrong to try to separate x% for one law or another especially when most laws of physics/phenomena are dependent on each other. Bernoulii's equation is derived from newtons laws among others.
I noticed the same thing!
Also centripetal acceleration doesn't change the magnitude of velocity, only its direction
Literally just now looking out the window watching a propeller powered plane take off from the local private airport wondering exactly the question that makes the title of this video.
I knew an F-15 Instructor who said his aircraft flew because of money. (I believe this is related to why boats float.) He said when he was flying an F-4, that aircraft flew because of kerosene.
I learnt that the reason behind the higher velocity at the top of the wing is the conservation of angular momentum. You have to make sure to get a swirl at the end of your wing. Thats why it is also possible to fly with flat wings. There is no need for a curve as long as the wings are tilted in right to create this swirl.
I remember this similar question was asked to one of the Mercedes formula one engineer that came to our school for science week where he explained the concept of aerodynamics and the use of wind tunnels.
Great video! Two questions though.
1: From my understanding of physics, centripetal acceleration occurs because when something changes direction, it is "Constantly accelerating" because the direction is always changing, but the speed is not increasing.
2: What causes the low pressure? The centripetal acceleration or just the shape of the wing?
Thanks!
2: the pressure drop can be explained by Bernoulli's equation. The total energy of the fluid is expressed by its kinetic energy, its gravitational potential energy, and its pressure combined; the total energy must be constant (you can't create energy out of nothing nor exclude it from existence). Assuming we don't change planets, the potential energy from gravity won't change. Therefore, if you increase the speed (thus increasing the kinetic energy), the pressure MUST decrease.
@@PedroFerreira-fh3dk Thanks so much. So when the air accelerates around the surface of the wing it changes the pressure right?
@@rosstheboss8633 that's right.
Centripetal Force is radially inwards while Centrifugal force is radially outwards. The example figure at 2:09 is inaccurate
Yeah I noticed the same thing. Also, is t centrifugal force an "imaginary" force. At least that is how I learnt it
My boyfriend picked up on it as well and we thought somebody else must have noticed it. 👏
It also says it experiences centripetal acceleration, then equates that to a force, which is not really technically correct. What you experience around a corner in a car is a result of inertia. Also the point of centripetal acceleration is that it changes direction, not speed of an object / particle, so the notion that the centripetal acceleration causes the air to speed up doesn’t make much sense. There is also something weird about how they just say that the air experiences a centripetal acceleration but don’t elaborate as to what force is causing the acceleration. Centripetal isn’t really a good enough description, it just means that it is pointing towards the centre of an arc / perpendicular to a direction of travel. So yeah, I’m not pretending I know the real explanation, but the one they give here is a bit rusty
Yeah, i wasnt fully satisfied with this explanation as well. The one explanation that makes sense is the one i found from Wikipedia, quoting, "The lift on an airfoil is primarily the result of its angle of attack. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack" (the one in the video), "The air deflected by a aerofoil causes the airfoil to generate behind a lower-pressure "shadow" above and behind itself. "
This is a great explanation! All the theories discussed here were explained pretty much perfectly. Would have liked if they'd touched a bit more on some of the other incorrect theories though.
According to a novel approach, based on Potential Flow Theory, the lift (and drag force) generation mechanism is due to the additional circulation/detached vorticity from the entire surface, not only along its edges (e.g., wing tip vortices). In fact, such approach is very intuitive since the detached vorticity should not be confined only to certain regions due to a continuous medium assumption of the surrounding flow. You can find the recently published article (The Full Multi-wake...) in "Advances in Aerodynamics" Journal.
Lift comes from turning the flow of air. If the air is flowing downward after negotiating the wing, you've generated lift. If it's flowing up, you've generated downforce. Any solid object can turn a flow. Wings are extremely efficient at it. I've made this comment half a dozen times trying to link to a NASA site that explains it but TH-cam doesn't like links it seems. You'll have to look for yourselves. Glenn Research Center Guide to Aerodynamics.
Exactly! I'm sooo tired of people who get busy saying "The texbooks are wrong" and then get busy proving that they don't understand why a wing provides lift. This video isn't complete nonsense, just mostly so.
Yup, in plane model wing, the splitting point of air in front of the wing are higher than the meeting point of air in the rear, so this forces the air to go down
True. If I remember correctly only 1/3 of the lift comes from the phenomenon in the video
Good explanation
You know something is tricky when Eintstein himself couldn't get a hang of it.
i was struggling to understand this phenomina from the equal time arguement. very helpfull video
This reminds me of the two theories of electricity. One is conventional current flow where positive charge carriers (whatever they are) flow from positive to negative. The other is electron flow where negatively charged electrons flow from negative to positive. As it turns out, the circuit works whichever way you explain it. So we use whichever theory best fits what we are trying to explain.
2:10 Centripetal acceleration doesn't change speed, it just changes the direction.
yea that was just wrong
Yep. It changes velocity, not speed.
So what causes the air to move faster above the wing?
I think best explanation to the lift is still the stream tube theory.
This was frankly a "meh" explanation, I expected a better comparison and contrast between Bernoulli pressure and and Newtonian 3rd law equations that both are capable of resulting in nearly identical/interchangeable results for "simple" wings. The interesting properties of lift at low speed for gliders and at high speeds where drag-overcoming thrust vectoring reduces wings to the scantest of control surfaces were all missed opportunities here. Keep up the good work in general TEDEd, but do more homework on flight, it deserves better coverage than this.
My late uncle, William Elmore, was a famous areospace engineer for McDonnall Aircraft. He was convinced that the air below a wing was much warmer than air above the wing. The air temp difference was crucial for lift. Warm air rises and cold air drops. Consequently, advanced jet fighter aircraft can usually fly upside down, pretty well. Surface temperature differential was believed to be critical by most engineers of his era.
This is how I see it: before the air meets the wing, we have horizontal flow with equal flux throughout the height. At the wing, the bottom part has little curvature, so the flow below moves more or less with the original flow. The flow at the top now has to travel more vertically due to the shape of the wing. However, the air that was trailing behind (still in the horizontal direction) now squeezes the air at the wing, so we end up with a "hose effect", where the flow of the initial air is now squeezed between the wing and the trailing air, thus decreasing easily accessible cross-section and increasing horizontal speed. And then Bernoulli's law gives the change of pressures. Notice no mention of meeting of flows, only inertia of air flow and hose effect.
I was taught the 'false' longer path idea of the same quantity (n) of air molecules traveling above as below the surface but at different speeds creates a lower pressure. And yet, as I watch this video three times now, I see just about the same 'corrected' explaination, using the centripetal acceleration (faster), so the air traveling above the wing goes faster than the air below. This increase in speed decreases the pressure above the wing.
I take it that the part I was taught incorrectly was that if we have a superset of N fluid molecules that split along the leading edge of the wing, n-above and n-below don't necessarily transit past the trailing edge of the wing at approximately the same time? This 'same time' idea is the part that is wrong -- as the n-above mass exits behind the wing sooner. It's the additional pressure exerted on the bottom surface (n-below) as much (or more) than the lower pressure of n-above? If this can be modeled mathematically, what are the equations that would allow us to predict the necessary speed (>stall speed) of a wing structure for a given mass (plane+cargo)? Is this akin to calculating the tonnage capacity of a ship via bouyancy?
I think there is some small portion of a camming or kiting effect too. Kind of like a watercraft or ski getting on plane in the water. Yet we know when the angle of attack becomes to great the air above a wing becomes turbulent and the wing quits proving enough lift and stalls.
Whenever somebody tells me about the half Venturi tunnel or equal transit time explanation I always ask them if wings need to be that special shape to create lift. They always say yes, upon which I ask them how paper airplanes fly.
All that without mentioning the Coanda Effect?
Also a flat wing with a positive angle of attack not only gets lift from the air above the wing but the air underneath it is forced to change direction downwards which produces (arguably more) lift in the upward direction through F=MA. Just a tweak to a great article. 🤗
I still think Newton should take a lot of the credit. A plank of wood will act as a wing if it has the right angle of attack and airspeed. When a jet aircraft is taking off, it doesn’t gently float off the ground. The pilot rotates(pitches) the aircraft, which presents the lower surface of the wing to the airflow. This causes a dramatic increase in lift due to Sir Isaac’s third law. The smooth shape of an Aerofoil pretties up the airflow, and improves the efficiency and stall characteristics of the “plank”. It also plays an important part in strengthening the wing, although with modern materials, wings can be made much thinner.
Exactly, stick your hand out of a moving car window at an angle (of attack) and it will lift. Though the aerofoil is required for controllable accurate flight controls.
@@jimbo4375 That's exactly the suggestion I always give. It is much more tangible and easy for anyone to try for themselves. Newton's third law describes it well and you can feel the lift force.
Isn't it centrifugal instead of centripetal if the force is pointing away from the center of the curve?
Centrifugal force doesn't exist
My understanding is that centrifugal force isn't a real force and is just how we perceive inertia when moving along a curved path. In order to move along a curved path, there has to be a centripetal force toward the center of the circle you're moving around, which is why you move/stay closer to the center than you would going straight.
I always thought the 'air has to go faster' idea was dubious when I studied principles of flight 40 odd years ago but there's still one question I've never had answered so serious request: could you do one on how planes manage to fly upside down please? Thanks.
It’s pretty much answered in the video after 3 minutes: the shape of the wing helps optimize airflow but a perfectly flat wing can also cause lift if tilted correctly. A flat profiled wing thus can fly upside down as well if it is tilted upward (the nose of the plane is pointed above the horizon)
@@johnsongetdown The video doesn't really give any physics principles to back that, though. I recommend watching "Lift and Wings - Sixty Symbols" for a more nuanced explanation.
Planes that properly fly upside down (like aerobatics ones) use symmetrical wing profiles, where the angle of attack determines where that lift will point towards, thus their wings are always "tilted". The same symmetrical profile is true for the vertical stabilizer at the back, any yaw motion (on a plane that would be like it's going level, but your cabin points to the left or right relative to going straight) increases lift on one side, pushing it back towards the middle.
EDIT: Adding some extra stuff, not all wings can generate lift both sides, and if I'm not mistaken, current airliners cannot fly upside down because of this (and several other factors like fuel delivery). The wings will stall and it will just fall instead, until it recovers.
AoA
@@Kalvinjj Hmm, they can't fly efficiently upsidedown but even jetliners can generate lift when upside-down, just not for sustained periods due to other factors. I've flown many Radio controlled model gliders that have a completely flat wing bottom and conventionally curved top and they will still generate lift inverted with sufficient angle of attack but drag increases significantly and lift is reduced. They are gliders so there is no reliance on engine thrust holding them up in the air. With sufficient initial speed they could loop inverted .
Your graphic has 1 major flaw; the longitudinal axis of the wing is Not parallel to the longitudinal axis of the fuselage as depicted at 1:43.
The wings “angle of incidence” is actually tilted 4 or 5 degrees in order to produce downwash, and we know from newtons 3rd law of motion, pushing air downwards will result in the wing reacting upwards.
I am most comfortable with Newton's third law for producing lift as you have described. A helicopter's blade/wing shows this very well.
@04:24 in the momentum equation, only the gradient of p has pressure, rest are rho (density), not p.
Pretty good explanation. Professional pilots are taught about the concepts you covered in relation to Bernoulli's Principle and the venturi effect. There are many theories as to how lift is truly generated, the answer is likely a combination of them
There is a combination of effects. But they all act to turn the airflow, air having mass, mass being accelerated, generates an elementary newtonian physics reaction.
The primary control a pilot has at his disposal to modulate lift is angle of attack.
@@jj4791
No. As a professional pilot for over twelve years now n military and civilian service, and holding a Bachelor’s and Master’s degrees in aeronautical engineering, the answer is pressure distribution over the surface of the aircraft.
Newtonian lift is theoretical.
@@montithered4741 But pressure but the force generate by a fluid over a surface
@@montithered4741 Newtonian lift is a perfectly valid explanation. If you take a large control volume around an aircraft and calculate momentum balances, you will find that change in downward momentum of the air is indeed equal to lift. We as engineers just focus on pressures, because it is far easier to measure and integrate surface pressures than to measure airflow away from the aircraft, and because the local detail in surface pressures is far more useful to evaluate and refine an aircraft design. But both are valid ways to explain lift.
Where is description about AoA? It’s counterintuitive to explain lift without angle of attack i think
If you look at the lines you used to see the flow, it can be seen as a half of a Venturi , convergent divergent nozzle which produces a low pressure region in the nozzle which acts on the upper surface of the wing generating lift.
And the debate goes on. Amazing. The whole aircraft industry is magic, and the complexity of airliner-class aircraft is mind blowing.
When you try to correct everyone but believe that the centripetal acceleration is actually a force -_-
In my eyes it's still a goddamn miracle. Just because we can explain it doesn't take away from it's magnificence. Just saying...
FYI - Miracles are occurences beyond rational explanations.
I think things are even more interesting when they have explanations. "It's just magic" is to me a boring non-explanation, while something that can be explained but seems like magic if you don't understand it is fascinating.
@@abdel4455 this can also be said when people say someone is talented
@@ISS_AM Agreed. I love to see people's hard work pay off to look like they're just naturally good at something. It's just unfortunate that some people actually think they didn't put the work to reach that point.
It still seems like a miracle because its still a bad explanation. Pretty common with all things that look like miracles.
I remembered an explanation from NASA’s website “principle of flight” where Coanda Effect plays its role to generate lift. This effect explains that a wing's trailing edge must be sharp, and it must be aimed diagonally downward if it is to create lift. I’m not quite certain that this effect is really the reason of generating lift until I try my self to attach the spoon into flowing water from a water tap (as it’s shown in the experiment in the webpage).
The “longer path” explanation has bothered me since I first heard it. This was great to learn!
Great vid! Not an engineer, but if the angle of attack contributes to lift, (in the case of the flat wing) could that also be the main reason for lift as opposed to the 'curved wing'?
For example, the angle of attack accounts for 80% of the lift and the curvature of the wing accounts for 20%
Hi, I'm an aerospace student, and although I'm not particularly an expert in aerodynamics, I do understand its basic concepts from my aerodynamics course.
The flat wing ("symmetric airfoil") cannot produce lift if placed at 0° angle of attack (completely horizontal), because there is no pressure difference between the upper and lower surface. Thus, if you want to produce lift on a symmetric airfoil, you'll need a non-zero angle of attack to create that pressure difference.
In general, the larger the angle of attack, the larger is the lift produced. Also, the larger the camber ("more curved"), the lift produced is also larger. Cambered airfoils enable the wing to produce lift at 0° angle of attack.
However, I've never heard anyone studied how many percent the camber and angle of attack affects the lift. I think those two cannot be compared directly. In my experience, we should take account of both of them when designing an aircraft so that the optimal configuration of wing camber and angle of attack can be chosen. Hope it helps!
As a mechanical engineer I can confirm that @@josephbernard6802 is correct. I would add that the angle of attack phenomenon, while not strictly necessary to produce lift, is strictly necessary to have flight. The increase in lift from changing the angle of attack is so tremendous compared to modifying the shape of the airfoil that it is the primary method of control of both orientation and thrust for propeller driven aircraft. Changing the angle of attack also tends to work in conjunction with angle of thrust for fixed wing aircraft and gyrocopters, thus producing increased aerodynamic efficiency. To clarify, the "flaps" on an airplane change the curvature of the wing and increase lift significantly, but at the cost of fuel efficiency and speed, which is why they are only used for take-off and landing. Whereas the elevator pitches the entire airplane up and down, changing the angle of attack and thrust by the same degree, ensuring that any loses in efficiency are due to countering gravity, and not aerodynamic forces.
Can you guys make a video on how a plane lands without crashing too? 👀
Thank you!!
Ground effect and flaps
As stated flat plates generate lift if given a few degrees angle of attack (AoA. ie nose up). Wings even work if fitted upside down, though not as well and with more angle up needed. Fully symmetrical cambers also work..effectively like a bulged flat plate, but with gentler stalls. Flat bottommed airfoils, where the bottom is parallel to the airstream are in fact operating at a positive AoA. In all cases where lift is produced, air leaving the trailing edge descends. A cambered airfoil is half of a venturi and you might expect all the upward lift to be produced in the low pressure region all down its sloping back…but in fact the lift acts at a point much nearer the thickest point of the bulge where the the air moves fastest and the pressure drop is higher. As the AoA increases the centre of upward pull moves forward. When airflow becomes turbulent over the top due to excessive AoA (typically 15 to 19 degrees), a stall develops and a lot of lift is lost, but note a point seldom raised is only about a third of the lift is lost…enough to drop an aircraft, but the rise in drag is the more serious issue, since that slows the aircraft (a lot) further reducing lift and thus turning a flying machine into a brick. Why I raise this is, highly overpowered aircraft with continue to fly even with a fully stall and AoA pushed to 90 deg vertical.
It wouldve been nice if you mentioned the founder / most important scientist who we base all this on: Bernoulli. What you describe is called Bernoulli's principle and its the foundation of what makes planes fly
2:15 is a bit weird to me, centripetal acceleration does not necessarily generate more speed, with your car, it just changes direction. So as an explanation it feels unsatisfying
Centripetal acceleration however is acceleration that changes direction and not speed, so how does it speed up the airflow? This needs more explanation
Was waiting for someone to explain this exact thing. Thanks a lot
Edit: from months
A good, clear video. It would help if we look at the word "lift" which is not quite appropriate. Some get the impression that "lift" is due to the thinner air at lower pressure above the wing pulling the aircraft up. Actually, the lift is created by the greater pressure of the air under the wing pushing the airplane up. Think of the wing as being like a waiters tray. The waiter lifting the tray is actually pushing from below, not pulling up as if the tray had strings on pulleys above it. The math works of lift work out the same either way because the net lift is the differential between the push up from below and the lesser push down from below. But it is much easier to explain thing like stall (when the wing loses lift at extreme angles) and the effects of flaps and ailerons if we see lift as a push up from below that is not being fully cancelled out by a push down from below.
2:12
There's no immediate relation between centripetal acceleration and speed increase. Non sequitur.
I’m now dumber for having watched this. Air above the wing doesn’t magically pull up on the top of the wing.
It's the air from below thst pushes up (because of higher pressure).
But as the video says, even now we don't really know how it works exactly (or what is actually creating the pressure difference), we omly have good guesses.
Nope, but the top of the wing does pull upwards thanks to the reduced pressure caused by the angle of attack. Basically, thanks to the angle of attack deflecting air downwards, the bottom of the wing pushes the air beneath it while the top of the wing pulls on the air above it.
When you tie weight on a end of a string and spin it, you can feel a tug on the string
The car, going around a turn has the same effect as also the air going around the curve of the wing
That pull creates a lowered air pressure
As long as keeps a parallel path, tracing the wing, that lowered air pressure will create lift
Interesting, in school I learned about the Bernoulli/Venturi/ effect as being the result of as air speed increases, pressure drops. We can use that for lift on a wing, draw fuel into a carburetor, accounts for sediment transport (sand in deserts or sediment in flowing water seeds or pollen distribution etc), increase air velocity to create a breeze for comfort in outdoor spaces, pressure drops result in lower temperatures. Part of my education also included a fundamental understanding that we don’t have to understand why something happens in order to use it. If observation yields a set of conditions results in an outcome, ie air movement over an inclined surface or curved surface results in movement in the direction of the curve or incline, we can use that to our advantage without needing to explain the why. The why doesn’t always matter as much as understanding what, what happens if…I don’t remember the details from 30 plus years ago, but I remember the explanation for this effect being explained as continuity and conservation of mechanical energy, in geology (sediment transport) it is explained as a difference in the chaos/collisions occurring in the medium (increased velocity increase laminar-smoother-flow).
I am left to wonder what has happened to our education system.
U can’t trick me Ted-ed! I know that centripetal acceleration is actually directed radially inward to the center of the circle!
The centripetal acceleration part really threw me off, having shown outward acceleration vectors. Isn’t the perceived outward force in a turning car known as centrifugal force? And yes I know centrifugal force isn’t a real force, but conventionally centripetal acceleration is toward the center of rotation
Tie a rock to a string. Swing the rock in a circle above your head. Feel the tension (force) on the string. That is centrifugal force. How can anyone say it's not real?
Centrifugal force is the reaction to the centripetal force required to keep the rock traveling in a circle.
That special shape of the wing is called airfoil which alter air pressure above and below to be of different speed while also change the direction of the air flowing downward giving action and reaction forces
Please help me understand the coanda effect in aviation and which factor influences lift the most. If it is bernoulli effect then why do we even raise the elevators up? We can just make it fly easily by just making bigger flaps and increasing velocity.
**please help and clear my confusion**
"It's just turbolence, physics of fluids in motion"
"No, a wing detatched, look!"
"Hey, I'am the pilot here, just let physics do its things!"
Even as a little kid, the first time I stuck my hand out of a car window, I understood exactly how wings generate at least most lift.
When talking about fowler flaps on modern aircraft, the flap seperates from the main wing and the gap between them allows the air to "add energy to the boundry layer delaying stall" how does this come about and not add turbulence to it?
No mention of the role of the angle of attack factor here. AOT, about 3 degrees, causes the wing to push the air down which, combined with lift as detailed in this article, enables flight to occur. The wing is “skiing” on the air, in much the same way that a water ski glides over water. When the skier’s speed drops too low to sustain “flight” the skier and the ski “stall” and both fall back into the water. This is precisely how a fixed wing aircraft lands, it stalls gently (usually!) onto the ground. At least that is how it was all explained to me when I was gaining my (humble) private pilot’s license over 40 years ago.
Also, note that when you are just getting up the ski needs to be at a high angle of attack, then only a slight angle at speed. As you slow down, the angle needs to increase until it "gives up" and you are back in the water.
So how do planes fly upside down without changing their wing shape? According to this model the air meeting the now downward facing bulge in the cross-section would accelerate, causing lower pressure on the lower side of the wing, in which case the plane should be pulled downward and crash.
Angle of attack. One very important part that wasn't really talked about here is the angle of the wing relative to the airflow. If you are upside down and the wing has a relatively symmetric shape it will create lift if you point the nose of the plane above the horizon. One of my physics professors always explained lift via the angle of attack. On the upper side of the wing towards the trailing edge it will create an area of lower pressure as the air is forced to go towards the ground by the tilted wing. Just like on almost any object that moves through a fluid medium the air is creating higher pressure infront of the object and a lower pressure behind the object. This causes the air to form a vortex at the trailing edge of the wing. In the extreme case of a too high angle of attack this vortex is what detaches the airflow from the upper side of the wing creating turbulent airflow (just like directly behind a normal car). If the angle of attack is in the right range the angular momentum of the vortex is compensated by creating a big vortex around the entire wing with opposite direction. The two movements add up to the speed at the upper sider beeing higher than at the lower side of the wing. It is a way to explain it that is good for calculations but not necessarily 100% correct. You could argue that the big vortex movement around the wing is the same that is talked about with the centripetal force in this video. They are closely connected. The wing has to have a shape that alows the air to stick to it as good as possible and for planes that are not primarily designed for flying upside down that is a rather asymmetrical shape as you know it. The more the plane is supposed to go upside down the more symmetric it will be. Some cars like F1 cars use this effect on wings too to create downforce. They have a negative angle of attack in that case. The angle of attack is pretty much the most important thing on aircrafts. It decides whether the wing stalls, what direction the aerodynamic forces are pointing at and how strong the lift is.
2:10 The force you experience in a car during a turn is centrifugal (toward the outside of the turn, as the animation shows), not centripetal. In addition, if a wing curvature is not the only way to generate lift, i.e. a difference in pressure, it is indeed what cause lift if the wing incidence is null. I can't even see what you think you have demonstrated. What exactly, in your opinion, causes the force that accelerates the air flow above the extrados ? Calling it "centripetal" is not an explanation.
Finally someone else notices this mistake. It’s quite worrying for me that I had to scroll down so far to find someone else who noticed. Did no one here pay attention in high school physics class?
Also, the diagrams in the video seem to imply that the upper-front part of the wing has lower pressure than the bottom of the wing, which makes absolutely no sense. That part of the wing is almost perpendicular to the direction in which the wing moves. I don't care how much fluid dynamics and centripetal mumbo jumbo you throw at me, the part of the wing that is hitting the incoming air HEAD ON can't have lower pressure than the bottom or the back of the wing.
Very nice. And this is very basic as opposed to the computation of V-rotate in of airliners.
A second equally important aspect is the " angle of attack" of the wings and tail wings measured in degrees, say 15 degrees. This causes the air striking the undersurface of the wing to exert an upward force related to air density and plane speed.
2:11 isn’t this centrifugal force rather than centripetal?
Another interesting fact about the Navier-Stokes equation is it's one of the "millennium prize problems" in math. You will be awarded $1 million if you solve it.
Yes, that's correct. The Navier-Stokes equations are one of the seven unsolved problems in mathematics known as the "Millennium Prize Problems." The Clay Mathematics Institute announced in 2000 that a $1 million prize would be awarded to anyone who could provide a solution to any of the seven problems, including the Navier-Stokes equations. So far, the problem remains unsolved and the prize is still unclaimed.
Please explain....when the air particles experience centripetal force at the beginning of the curvature, why does it result in an increase in velocity? I think Centripetal Acceleration does not necessarily mean an increase in velocity, it's the change in direction of the velocity that results in acceleration. Bottom line, why does the speed increase when the air particles hit the wings curvature ?
No mention of starting vortices?
There is a transient period when the wing first begins to move through the air, where the air on the lower surface actually flips around the trailing edge and comes back up the wrong way some distance over the upper surface of the wing, with a stagnation point where it meets the main airflow coming the other way. That creates a vortex that peels off the back of the wing, rotating counterclockwise in the view shown in the video. Due to conservation laws, this generates a clockwise vortex superimposed over the airfoil, whose velocity adds to the free stream velocity. On the top surface of the wing, this vortex is in the same direction as the free stream, so the velocity increases. On the bottom surface, the vortex is in the opposite direction, so the air slows down. That means the lower surface air can’t flip around as far up the top surface, so the stagnation point migrates aft until it reaches the trailing edge and the condition becomes stable (the Kutta Condition).
Ironically, lift is much easier to explain at supersonic speeds, where you simply get an oblique shock off the leading edge on the lower surface that increases the pressure, and you have an expansion wave on the top surface which lowers the pressure.