Nothing better than having something you firmly believed you understood shown to be significantly mistaken. Excellent, thank you.

What an uplifting video.

Euler looks super smug about the underwear he's wearing on his head

Ahhh Euler, the source of countless nightmares of engineering students

A stream of ping pong balls aimed at a wall would also splay sideways because the balls heading toward the wall would collide with the ones bouncing back

What didn't Euler do?

This was probably the best video on KZfaq I've ever seen for this material.

As an aerospace engineer I'd say that at low velocities the energy equation stops being useful, since density tends to be constant and the mass and momentum equations form a determinate system by themselves, so I'd say the momentum equation is more useful than the energy.

Aerospace engineer here. I really like the explanations and the video, but I would like to mention that the explanation of the air being 'squeezed in' near the leading edge as if there was a wall is also kind of 'fallacious' (at least from the point of view of my teachers). The real reason why the air goes faster in the upper side is because of the Kutta condition (en.wikipedia.org/wiki/Kutta_condition) which I would have liked to be mentioned here. This condition states that a body such as an airfoil, with a thin trailing edge, will create circulation around it (air rotating basically) so that an stagnation point (this means, a point where the air speed is 0) can exist at the trailing edge. The explanation for this condition is the fact that the air coming from the upper and the lower sides, when they meet, has to have the same speed vector, otherwise the conservation of mass equation does not hold, and it has to be at the trailing edge because this is where the geometry suddenly changes, and the air can be properly separated from the surface. A deeper explanation for this point involves the existance of a viscous bondary layer near the surface, and how it follows the shape of the airfoil. Actually, the reason why an airfoil 'stalls' is because the air suddenly does not follow the shape of the airfoil when the angle of attack is too steep, and decides to separate from it, causing the destruction of the circulation, the bernoulli effect and the momentum change in the air. And as for what is the real contribution to the lift, either momentum equation or bernoulli, as Prof Merrifield says, it's actually a combination of everything :), but I just like to explain to people the more simpler conservation of momentum (that is, the fact that the air gets turned down by the shape of the airfoil an the angle of attack). Also, as someone pointed out somewhere in the comments, the animation of the wingtip vortices at around 9:20 is the other way round :), air has more pressure on the lower surface so it will go upwards at the wingtips, causing a vortex going clockwise in the left wing when viewing the aircraft from behind (and also causing a type aerodynamic drag called induced drag).

Loved the graphics for this one. Great as usual!

Wow!!! This is the most accurate and thorough explanation of how an aircraft wing (or rather wing profile) actually works I've ever seen! This is a really great video!!!

I love this. This is such an honest way to talk about a simple process and the way it really is, which is extremely complicated when you want to understand every part of how it works. I understand it can be a bit maddening. But the solace found in knowing that you understand how a simple process truly functions is worth the work needed to try and understand what's going on. It also almost seems like everything is this way. A very complicated event maybe is always really a combination of very simple events that individually have a complex process behind them.

this is the best video about lift on KZfaq because it explains all the partially true theories separately and then combines it to show the more complete truth!

Now we just have to wait for professor Philips Moriarty telling us that professors Merrifield's explanation is wrong on so many levels. ;)

your wingtip vorticies in the animation at 9:21 appear to be moving in the wrong direction

Another very nice, clear explanation for the interested non-specialist. Professor Merrifield is probably my favorite presenter on this channel. He's the Dr. James Grime of Sixty Symbols.

Thin section wings! Everybody neglects the thin section examples like hang gliders and the old Wright brothers style in explaining conservation of momentum.(turning the flow, not particle bombardment) There are two primary reasons for thick wing sections; the rounder leading edge is less sensitive to angle of attack related flow separation(stall), and wing strength both in torsion and vertical bending. The majority of the net working lift has been shown to come from conservation of momentum. The Bernoulli effects are important to the general flow pattern and boundary layers and these effects indirectly add to lift generated via conservation of momentum, the actual portion of lift directly caused by Bernoulli effect is actually a rather small fraction.

Brady's videos are amazing in the way they illustrate the process of discovery through the scientific method and make it an accessible notion. You have to give him props for his talent and hard work. It was especially touching when the Professor said "I'll probably get it wrong if you want to get into it". I'm paraphrasing, but that illustrates the humility you need if you wish to get closer to a truthful fact, and it's something I've observed in the best scientists I've met.

I would personally start with conservation of momentum (a sort of distant view of the system) and work backwards into smaller details like bernoulli which is more about minimizing drag for the amount of lift. After all even a flat, infinitely thin wing can still fly. Another thing that often trips people up is the angle of attack of a wing is usually just seen as the chord line - a straight line between the bottom two points on a wing. However, since most wings are designed to minimize drag with the same side facing the ground they get quite a substantial tilt which gets hidden by their curve. Stunt planes are designed to flip over though and as such often have completely symmetric wings - without altering their angle of attack they'd have zero lift.

This channel is a gift

There is one fundamental force that keeps planes flying: money.

thanks for making this- clears things up for me a lot💛

Air being pushed by a moving wing, IS squeezed, and the air pressure, from both gravity above and the fact that the air not in motion above the "squeezed" molecules, makes the squeezed air rush out into directions "AWAY" from the direction of the hard immobile surface of the wing, thereby lowering the pressure and speeding up the air at the same time. With a sloping curve on the back side of the wing, the air has an obvious direction to "Unsqueeze" and thereby speed up. The underside of the wing has only a slope into the wind, and therefore gets a "bump" from the molecules hitting force imparted and slowing it at the same time. Thus, lift.

What a fantastic professor! And what a great explanation!

Thank you for a clear explanation. Fascinating!

A rare, comprehensive and insightful explanation that doesn't lead me to the conclusion that it would be impossible for an airplane to fly upside down when obviously it can!

I really like this video. It is quite frightening that so many textbooks and KZfaq videos give the wrong explanation.

This is the best explanation l have ever heard and l’ve been at it a long time. Also, great interviewer. Thank you both very much!

Best summary I've seen. Goes with the principal of superposition.

Really great videos of Sixty Symbols !😸

Thank you, finally a clear and comprehensive explanation!

I would just say it's simple if you take that fluids like air naturally follow the surface of the wing due to their tendency to fill any vacuums or low pressure regions. The trailing edge of the wing slopes downwards, so if the air is following that slope, then it is also being imparted with a net downwards momentum. TS;RM After the air is separated from the wing surface, it still has that downward momentum. If you think about it, air particles at a given temperature and pressure have a certain average velocity. If the trailing edge of the wing is sloping downwards, those air particles are going to collide with the wing at a lower impact velocity, meaning that the momentum of the total fluid is given a downward momentum, but also that the particle collisions on the underside of the wing will push the wing upwards against the lower collision velocity on the upper surface. So, the air pressure being lower on the top allows the bottom flow to push the wing up, while the top flow gains downward momentum due to suddenly having fewer collisions in one direction than the other.

Loved this. Flying finally makes some sort sense to me.

Thank you for this thorough explanation. Or at least, much more satisfyingly thorough explanation than is commonly given.

Great video. Thanks!

Another awesome video, thank you! :)

Thumbs up right even before the video started - with Prof. Merrifield it's gotta be a great video!!

The left and Wright brothers would be proud of this video.... I say this, and I am not joking, the Encyclopedia Britannica has a picture of the two brothers and the caption underneath reads: " Orville, left and Wilbur Wright".

Hey Ethan! Keep up the great work, proud of you!

Well done. I love the guy who asks thr questions.. those are the questions I would ask!

Thank you sir. you're my lifesaver

I really enjoy Professor Merrifield's videos. Hope he will continue for many years and not retire as the great Professor Bowley did. Oh dear I miss him and his explanations.

Thank you for clarifying this !

Great video! Glad to know my IB Physics teacher got his explanation correct!

Best explanation video for misconception of lift that I've ever seen.

It’s amazing to see all the different professors from Brady’s videos age. Then I go look in the mirror and I can’t remember how I looked all those years ago.

I really respect and enjoyed the host challenging the professor to clarify his position regarding the "fallacies". Too often we take scientific authorities for granted and accept what they say without question and don't take them to task enough when they're being obscure or obtuse. Not only was this great information (as always) but it's a great piece of journalism as well. Cheers.

Great explanation other than one thing which I have to disagree with, you can absolutely separate the bernoulli and momentum effects. Simply by creating a symmetrical wing, symmetrical wings work and are used vastly for military fighter jets, this is because at extreme speeds, a small angle of attack is more than enough to create the desired lift and aerofoil lift becomes miniscule in comparison, so to amend the final part, lift has a larger effect at slower speeds, and AOA is more important at higher speeds, aswell as allowing no change in control during inverted flight.

Yes! I've been saying this for years! To Brady's question, you can think of each portion of the total affect as having different weights depending upon the parameters and environment (e.g. foil shape, angle of attack, mach, temperature, humidity).

Thanks so much for finally explaining this to me in a way which is logical and makes sense.

This video has a lift-wing bias.

Finally a KZfaq video really and fully (relatively) explains how airfoils generate lift.

Very well presented. I've watched a ton of these videos in preparation for doing my OWN one for some drone training material I'm doing, but this is the ONE that actually admits that Bernoulli and momentum are BOTH right ... and BOTH wrong!

I am an engineer and I never excepted the “air meets up a the end of the wing at the same time because” bit. I never took the time to look up the complete physics because I don’t need it in my field. Fun to have my gut feeling being not unproven! Now I can explain this at parties ;) not sure what I want to say but I do love this kind of videos

Fantastic, I need to get my colleagues in flight training to watch this, for my own vindication at last :P That viscosity comment alludes to the coanda effect and it describes how the 'obstructed' upper surface behaves the way it does really nicely rather than just 'the equations dictate'. So glad this video was made. Coanda effect next!

I finally feel like I understand this. Finally! Thanks!

Even though im studying business, this legend is the reason im going to Nottingham! lol. Will definitely need to sneak into the physics department and say hey!

AWESOME! aeroengr here never thought I'd see a talk about aerodynamics on this channel :D

I'm so happy with this video

Why is this still a debate? Whether you compute Bernoulli's principle, finite element analysis (aka "blade theory"), or impulse (downward acceleration of the fluid air) ALL produce precisely the same result, lift+drag, and can be equated through their respective equations. A flat blade deflects airflow quite well. The reason we use airfoils is because they're far more efficient at deflecting the flow of air than a flat blade, which induces a lot of energy-robbing flow separation and turbulence. By the way, induced drag is nothing more than the x-component of the resultant force, while lift is the y-component. Add them up and reverse the sign to see what happened to the undisturbed air mass after the passing airfoil disturbs it. Essentially, it's accelerated mostly downward (opposite of lift), but also a bit forward (opposite of induced drag). Funny how that works, isn't it? :)

Brilliant explanation

From what I understand, the Coanda effect, the ability of a fluid to attach to a surface, also plays a large part in providing lift for the wing.

I am a fluid dynamics PhD student doing 3D aerodynamic simulations, and I approve this message. Fluid dynamics on KZfaq can be really cringe-worthy, but leave it to sixty symbols to do it exactly right... Well done, more please

BEST video about lift on KZfaq!!! Love you guys!

Can we all just agree that the airfoil shape of a wing is not needed for a plane to fly though? I mean if you stick a propeller on a paper airplane it would work the same, even with flat wings...

Yup, glad someone explained it in complete ways.... Some vamous youtube only mentioned the low pressure at top that caused the lift, which makes me scratch my head alot.... The 3 things plus the fluid fiscosity(as well as flow regime) are that kept changing which one is dominant in keeping the flying fly.... But under steady state, i believed its the upward momentum from the bottom of the wings that makes the lift..... the top wing low pressure would be second thing.... And if you hit dense clouds, its the fluid that contribute more (again in the form of adding momentum to the bottom wing .... upward) 3 thumbs up 4 you prof Mike

Life is so much simpler without Sixty Symbols.

Wonderful video.

I enjoyed this video and I liked how it went through multiple misconceptions and explained how each piece can not explain lift on its own. Not only did he provide examples that prove that the misconceptions are truly false, but it explains how pieces of each are used to explain lift.

Thank you! Always heard that argument that air travelling over the wing had to meet up with air travelling under the wing and thought why, (or BS)

Thank you for this awsome video! Could you make more videos about aerodynamics? It would be interesting to see, how the relevance of these theories change with airspeed. I would also like to see a video about what happens over the speed of sound.

Very nicely explained. But the Navier-Stokes only conserve mass and momentum. Where does the more general energy conservation fit in the last interpretation?

this explains its perfectly!

I miss these.

Finally after watching a couple of videos about how lift works an - although unsatisfyingly undetailed - satisfiyngly acceptable explanation.

@6.50 he is saying that the explanation of a physical phenomena COMES OUT of the equations when the physics is always there, the equations are only a MODEL that humans use to approximate nature bahavior. Here is an alternative to the TOP-DOWN explanation given: The air around the rounded part of the wing is squeezed together because the air flow is curved (why the air goes around the wing? Viscosity, momentum, all that stuff.) the only way for the air flow to perform such a curve is to have a resultant centripetal field which is the pressure gradient.

Veritasium made a similar video some years ago, but by Sixty Symbols making another one the chance of people understanding the physics increases.

This video has raised my interest in aerodynamics again.

Flying will get easier since we are running out of gravity.

Not only do you have to think about all of these things, but you have to think about how they’re effecting the air mass before, during, after, above, below, left, and right of the wing (and not just immediately close to the wing)! Even the ground disturbed the integral of lift in NS.

THANK YOU!!!!! I always kinda knew this simple explanations cant be the whole story. Now i finally know 😎👍🏻

I used to work in the aircraft engineering industry. In certain occasions including interviews I was being asked to explain how an aircraft flies in layman terms. The mechanism is rather complicated as the professor explains in the video so the real challenge was giving a short answer. I found that most people would expect 'with a sufficiently high speed and a correct range of angle of attack, an airfoil can generate lift due to pressure difference between the two surfaces'. In my job when we had to adjust the flight control systems such as ailerons or elevators, we tended to use 'bouncing particles explanation' to effectively figure out which way was the right direction. I also found that in many science museums in different cities, Bernoulli's principle is used to offer simple explanation for educational purpose.

That vortex turbulence at 9:48 is really impressive. Now I really understand why smaller aircrafts need to wait some time before departing behind a big jet.

when you analyze it, the first argument is actually the same as the first

Thank you so much for debunking the Bernoulli effect argument! I realized that that couldn't be correct a long time ago but I've never heard a scientist talk about it.

Great video! It is always important to clear up any misinformation before moving onto the real discussion. I did want to mention that the description of the streamlines being pinched results in an increase in velocity is also a misconception known as streamline pinching. The correlation between restricting the flow and increasing the velocity is true for an internal flow, bounded by a surface AKA a venturi. For this example, a flow that experiences pinching does not see a definite increase in velocity.

best video even watching in 2021

I'm still trying to get past the damage my grade school teacher did by telling me the seasons were due to the distance between the earth and sun.

I love this video. It's how I've always felt about it, but I've always been taught one or the other separately when learning helicopter aerodynamics. Conservation of momentum is used because it's "easier" to explain. "Air is pushed down so wing is pushed up". Then they brush by Bernoulli's principle because it's just "another explanation" but often "too confusing" and isn't talked about, but then nobody was able to explain how autorotation works by using conservation of momentum. They draw graphs and lines but don't actually get HOW it works and seem to not care either (fair enough, it works so we use it). However, I wasn't satisfied with not knowing how. Bernoulli's principle explains it well for me. My dad studied boat design and sailing and Bernoulli's principle explains how you can have a sail boat sailing against the wind. It's the same principle. It's the only way I was able to understand how you can have airflow coming from the front of the aerofoil yet still be pushing it forwards. It's much more exciting to not stop at the easiest, simplest explanation you can find.

the Bernoulli effect: If you isolate the volume of air affected into many tiny cells or volumes, you can analyze the effect of each individual cell. (This is how fluid flow CAD models operate). To accelerate one of these cells of air, you need less pressure on one side than the other; so the cell will accelerate towards the lower pressure side. The resultant velocity is simply due to F=mA. Less pressure actually means that the molecules are indeed further apart. The accelerated cell doesn't have less pressure "because it has higher velocity", It has less pressure because the cell actually is at less pressure. That is why it has accelerated to the speed we see near the top of the wing. For the air flowing over the top of a wing, the integral of F/m over time will be equal to the change of velocity. Air obeys F=mA like every other mass does.

You are right, you can’t really separate the aspects you described. However, we Aerospace engineers only really consider the pressure differentials when thinking about the lift. We spend a lot of time optimizing airfoil profiles (and other features) to influence the velocities over the surface and thus the pressure. The effect of the momentum change is only a byproduct for us. We call it downwash and it makes our lifes hard because it tends to influence other parts of the plane, like the horizontal stabilizers.

Sysmetric wings are really quite common, it's all AoA. Also do a follow up video on Delta wings at High AoA, which is how the concorde could fly at all at low(ish) speeds

I'd be curious to see this same discussion extended to talk about what happens when a wing stalls.

You can make a brick fly if you can keep its velocity up and find a way to control its attitude to maintain the needed angle of attack.

"Goes back to equations by Leonard Euler" might be one of the most common phrases in modern mathematics

I have another thing I would very much like to hear discussed, the difference between force and acceleration. I hear them used interchangeably on KZfaq, but they are not the same, but related by F=Ma. This involves relativity, and Prof Merrifield should be ideal to talk about this.

Lift is a wonderful thing to talk through. So many have these 'incomplete' descriptions described by Prof Merrifield, because they are quick to accept a sciencey sounding story. It is really interesting to see the different ways that trained engineers and physicists are confused when you ask a few probing questions. Few notes on the explanation given in the video: --> "the wing acquires upwards momentum" - No it doesn't, the vertical velocity of the wing in cruise is zero (sorry, no funny reference frames fix this) --> "the air goes faster on top because it is squeezed" - This doesn't help anyone understand why lift works. --> "you have to consider viscosity effects" - Yes and no. You can have lift in an inviscid fluid, but only with a little magic (circulation) that is caused by viscous forces at the trailing edge of the wing (see Kutta condition). IMO, if you say Bernoulli, Navier-stokes, or Coanda, you are just hiding the fact that there is something you haven't figured out how to explain in this context (or don't fully understand yourself). Physics don't care what your name is :) Even bringing up energy and momentum is usually a bad sign. Since these are derived quantities in classical mechanics, this is just bringing in one more conceptual step between not getting lift, and getting it... and this is just 2D.

How about analyzing how bicycles work? There are lots of false ideas there.

As someone who's due to fly an aeroplane over Nottingham tomorrow afternoon I'm glad all is well in the physics of lift.

I think they call these simple ideas "lies to children". Once you take an engineering physics course you learn just how much learning you have to do before you can even understand what you are learning....it's crazy.