I have a pilots licence, and a science degree and when I was told about the air meeting up I said wait a second that doesn't made sense. I was told to shut up, do you want to pass your licence, so I did what I was told. Flying is alot about doing what your told by ignorant arrogant assholes.
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).
Exactly, without the Kutta condition there can be no lift in an inviscid, incompressible fluid. But most people have never heard of it. The explanation in the video seems to switch between viscid and inviscid explanations without acknowledging the differences, so you can't get the simpler explanation used in inviscid flow.
I think your answer is what I was looking for. I need to read further about this. Because the explanation about compressed air above the wing needs to travel faster, lowers the pressure, seems somewhat false: If air is compressed, I would expect the pressure to increase...
I would love to see an animation of the Kutta condition. I have been reading about it for years but never wrapped my head around it. I can see how the motion of particles of air around the airfoil will be the vector sum of the freestream air motion and some hypothetical circular motion but... so what???
@bwwwam "Compressed" is not really the correct term to explain what is happening to the flow over the top of the airfoil. The gas itself is not being compressed like you would find in a pressurized container. Instead, the streamlines are forced closer together. Since the air is not contained in a vessel, it is free to accelerate to avoid being compressed. This is the "conservation of mass" that he was talking about.
+BlazeRazgriz1 You hit on something I want to dig into. I've taken some data, but don't want to tip my hand just yet. I dislike the 'pinching' explanation as well (the Smithsonian's explanation, among othets, uses it). ... Now, we see the upper air 'accelerate' going over the top. Since air has mass & Newton reminds us it takes a force to accelerate a mass; WHAT is the force accelerating this mass of air ?? There must be a force. The Prof glosses over this and I want to ping him on this also. Comment ?
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.
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!
this is the best video about lift on TH-cam because it explains all the partially true theories separately and then combines it to show the more complete truth!
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.
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
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.
This is truly the most complete and well done video on the science of flight I have ever watched. Thank you for this + the animations. My brain feels quite happy.
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.
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.
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.
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.
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
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.
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.
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).
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.
Yes. So many think the downwash is the cause of lift. It's one of the three most common myths. 1- Equal transit & Bad Bernoulli. 2-Downwash & Newton's 3rd. 3-Half venturi above. .
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.
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!!!
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.
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.
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
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.
I have another thing I would very much like to hear discussed, the difference between force and acceleration. I hear them used interchangeably on TH-cam, but they are not the same, but related by F=Ma. This involves relativity, and Prof Merrifield should be ideal to talk about this.
Force can mean natural/applied influence to an object, which in many contexts causes especially acceleration. Acceleration generally means a change in velocity, which is a reaction to an influence.
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!
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".
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.
Those two are one and the same. Momentum change in air is caused by pressure difference, which is caused by Bernoulli effect. Prof makes it sound like those are separate for some reason. They aren't.
Yeah, as soon as you change the angle of attack Bernoulli is now involved due to the "stagnation area" moving off center as the Prof points out. You've made it an airfoil shape by not directing the air along the longitudinal axis (wing chord) of the symmetrical airfoil -- making it unsymmetrical, at least as far as the airflow is concerned. Also, take a look at the cross section of the wingtip of an F-15 or F-35. Not only is the airfoil not symmetrical but, at least at the point of the wingtip, there's an under-camber like an old fabric covered plane (it doesn't carry through the whole wing, it's more like the leading edge twists down as you move toward the wingtip -- this gives you a wing that's not doing the same thing through its whole length, exactly because different areas are specialized for different speeds and angles of attack).
That's assuming that the stagnation point remains at the same location on the symmetric airfoil as the angle of attack changes. At a = 0, the front stagnation point will obviously be at the nose, but this will change as the angle of attack increases.
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.
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.
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!
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.
I am a fluid dynamics PhD student doing 3D aerodynamic simulations, and I approve this message. Fluid dynamics on TH-cam can be really cringe-worthy, but leave it to sixty symbols to do it exactly right... Well done, more please
TheNightWatch001 I’ve always been a bit suspicious about fluid dynamics ( which includes liquids as well as gases) being applied to aerodynamics, because liquids can sustain an internal tension and gases can’t . Molecules in liquids are in direct contact with each other and gas molecules are about ten diameters apart at STP , and the mean free path of gas molecules under these conditions is closer to a hundred diameters. To take just one example--the viscosity of liquids generally decreases with temperature, but that of gases increases. Ouch!
@@davidwhite8633 fluid dynamics and aerodynamics is literally the same thing?! aerodynamic is, if you really wanted to called it that, an under-category of fluid dynamics. The same laws apply. Yes in your everyday experience water and air behave very differently, but from a fundamental stand point they can be described with the same PDEs, the navier-stokes equations. You know, an airplane would also work under water (tbh even better due to the much higher density).
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.
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 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.
There is definitely enough here to justify a part 2 video of this topic. You could expound on the Kutta condition, and you could better discuss the direction of rotation about the wing tip. I would be interested in a discussion of how the keel of a sailboat also helps to generate what we sailors describe as 'lift', acting as an underwater foil to allow us to sail closer to the wind. Traditional thoughts that the keel only reduces leeway don't seem to offer the whole explanation.
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? :)
@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.
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.
David Messer Well , that’s the problem--you see , there’s no other explanation for the pressure under the wing being LOWER than ambient static pressure at low AoA’s than Bernoulli !
Designers incorporate such things as leading edge flaps, triple slotted flaps, vortex generators, thrust vectoring to make a complex variable machine fit for purpose. The thing I find interesting is that military combat aircraft use symmetrical airfoils at very high wing loading which make them a very different beast to a commercial passenger aircraft.
Lift is action and reaction. The action is the deflection of air molecules downwards, the reaction is the wing being pushed upwards. Pressure differences are secondary. If there is no downflow of air from the wing, there is no lift. R
The graphic at 9:20 shows the vortices going the opposite direction than the reality. If you watch closely at the planes flying at about 9:30 for the next 20 seconds you can see the vortices going the correct direction. The pressure is lower on the top so the air slips around the wing tip approaching the top. The physics of from high to low pressure can be understood. Great video BTW.
Good video. Better than most I've seen that deal with lift. Personally I prefer to simplify things: The wing interacts with the airflow going around it, applying total downwards force on it. By Newton's second law, this downward action on airflow has an upward reaction on the wing, that being lift. The details on how the wing applies a force to the airflow gets more complicated, and a lot of explanations focus on pressure differentials and how they are created, but many of those explanations tend to get mired into particular details and miss the big picture. There's talk about how the reactive principle can't be correct because air isn't being deflected downwards - well, actually, it is deflecting air, but it's almost immediately stopped by the ambient air that the wing is traveling through. Basically, the downward-pushing force created by the wing is dissipated over a very large amount of air, which means the air as a whole doesn't really move a whole lot (wing tip vortices notwithstanding). If the wing traveled through the same region of air over and over again, it would eventually start to create a downwards airflow. This is actually what happens with a table fan, or an aircraft's propeller, or when a helicopter is hovering static in one position. With helicopters, it turns out that the downwards airflow through the rotor disc actually decreases lift... but that's kind of a different topic. By the way, I find it interesting that people keep saying that the air going over the top of the wing "provides more lift" than the air on the bottom of the wing. The lift is by definition caused by a pressure *differential* between the upside and downside of the wing, and you can't have a pressure differential without including both the pressure above AND below the wing in the comparison. It's true that compared to the ambient air pressure, the pressure drop above the wing is greater than the pressure increase below the wing, but the total lift force is still all about the total pressure differential and both sides are part of the lift force being generated. I also find that the low and high pressure zones are slightly over-inflated in terms of importance. The fact is that the only way gas can ever apply a force on anything is through pressure differentials, so basically the pressure differential over the top and the bottom of the wing is exactly how the wing is applying a force on the airflow (thus "deflecting" it), and the airflow is applying equal and opposite force on the wing. So I find that saying the wing "creates" areas of high and low pressure is just a roundabout way of saying that the wing is applying a force to the airflow (and vice versa, hence the lift). Just by moving your hand through air perpendicular, there's a high pressure zone in front of your hand and low pressure zone behind your hand, creating drag. If you flatten your hand relative to airflow, you can then deflect airflow up or down by tilting your hand or making a curved cup-shape: This does create high and low pressure zones, but you can intuitively feel it as your hand pushing on the airflow, as well. By the way you can create lift with a completely flat aerofoil, relying only on angle of attack. It isn't particularly efficient, but I think it could be a good place to start explaining why the simplified Bernoulli effect explanation cannot be correct, since even symmetrical aerofoils, very thin aerofoils (like paper) or completely flat aerofoils can produce lift. A completely flat aerofoil can produce lift if it's tilted (ie. has some angle of attack). A flat but curved aerofoil can produce significantly more lift; this curve is also called "camber". Many early airplanes had wings that had very narrow chord (thickness). More modern wings usually have thickness because that way it can apply a stronger force on the airflow - a thicker wing produces more lift. Wings also have asymmetric profile (camber) because this means they can produce lift even if they are at zero angle of attack or even slightly negative angle of attack; this reduces parasitic drag because the aerofoil can move through the air with the smallest possible cross-section presented to the airflow, and thus the wing becomes more efficient (at certain airspeeds, anyway). But all of this becomes very complicated very quickly, and the fundamental laws of physics, action and reaction, always apply, so the simplest way to explain lift really is to just say that the wing applies a force on the airflow going around it, and the airflow applies an equal force to the wing in the opposite direction.
@@GZA036 The numeration of the laws doesn't really matter - besides, Newton's second law is really the important bit. Newton's third law is just conservation of momentum applied to the second law.
A pilot friend once asked me "what makes an airplane fly"? I began to answer with the Bernoulli principle and he stopped me saying, no, no, that's not it. Reaching into his back pocket, he produced his wallet, waved it in the air and told me, "Money...that's what makes an airplane fly".
Nope, it is incorrect, pls watch the youtube video "Common misconception of lift "by Cambridge's proff Doug Mclean. "It is not needed for viscosity or Coanda effect for the flow to follow convex surface."
4:00 Right here is where you know this professor is legit. If you look at pressure coefficient distributions around airfoils generating lift, its very clear that the upper surface of the airfoil contributes SUBSTANTIALLY MORE to the overall pressure difference (deviation from ambient pressure) than the lower surface does. In the incompressible limit, the maximum pressure rise (anywhere) on the lower surface is +q, whereas the upper surface can get anywhere between -3q to -5q (or even more) below ambient depending on the exact shape, Reynolds number, and angle of attack. The vortices are spinning in the wrong direction for upwards-pointing lift at the end, but otherwise, cheers!
DaylightDigital And , come to think of it , the pressure on the lower surface is usually less than ambient static on the lower surface , as well , of a light aircraft in cruise flight , i.e. at low AoAs . As long as the difference gives lift you’re home and dry . It’s easy to show , as well . If you look at the bottom of the top wing and the top of the bottom wing of a doped canvas covered bi-plane in cruise flight both will be slightly bulged out. This indicates that both surfaces , top and bottom , are below the air pressure inside the wing(which is at ambient static) . There’s no other way to explain that , that I can see , without invoking Bernoulli .
There are so many topics I've learned from this series where I realize the lessons thought in school were either partly true or wholly wrong for reasons I assume are that they don't want to confuse young students. I wish they had just told the whole and correct lesson from the start.
Eddies building under the airfoil are what causes lift under turbulent flow. Using the Navier-Stokes equations, you can derive solutions to lift. Ahh...this brings back memories from my advance aerodynamics class deriving exact solutions to these equations and comparing them to experimental results...
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...
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.
I was thinking the same myself. It is somewhat analogous. In fact I personally think it is even more interesting, and a little less clearly understood.
Yes, do bicycles next! It is another one of those often debated explanations. Tho, tbh, I have no idea why it is supposed to be complicated. Seems simple enough to me. So would be happy to hear what I'm overlooking here.
They way that I like to think of it is that the rise at the front of the wing has a high resistance so it generates a large pressure differential, leaving low pressure air across the rest of the upper surface of the wing.
I would like to hear a little bit about the incredible abilities of indoor flight r/c airplanes with completely flat wings in relation to this. Airfoils are such a major part of aircraft design but indoor modelling aerobatics succeeds wonderfully and the favourite airfoils profile is flat.
I didn't like the explanation of how the air "just ends up being squeezed" up on top of the wings and it's just pawned off as being part of the equations. You're recent video on the Coanda effect really clears things up, thank you.
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!
Its because of the curvature of the streamline over and below airfoil, the greater the curvature greater the lift. From Euler equations we get two differential equations of pressure distribution one in streamline direction and other normal(radial) to it, the normal one is responsible for lift. In Stream line direction >>>>>> partial(dp/ds)=(rho*Velocity*partial(dV/ds)) In Radial(normal) direction>>>>partial(dp/dr)=((rho*square of(Velocity))/r) from radial direction equation, as the curvature of streamline is more the change in pressure is more so the change in force in that direction.
Because it is the wing that does the greatest. The fuselage may or may not produce a *tiny* amount of lift. The pressure changes caused by the motion through air are *VERY* small! That is why wings are so large. A Cessna 172 has a pressure *top-bottom* difference of only 0.1 psi!! With 25,000 square inches (147 sq feet) to lift a 2,500 pound plane. . The pressure above *decreases* about 0.08 psi and below it decreases about 0.02 psi - for a difference of that 0.1 psi.. .. The largest change of -0.08 psi is only a change of 0.57%. The greatest pressure change around the ting is only HALF a PERCENT!. ..
I am so glad that the reason the air travels faster over the wing is *not* because "it has farther to go," as I've often heard. Physics is not my area of study, but even to me that clearly makes no sense, and it always bugged me. Why does the air *have* to go that farther distance in the same time? Turns out it doesn't.
The force (full aerodynamic force and useful part of it - lift) - is a (change of) momentum per second. So the momentum is directly connected with lift. The mass and energy equations are important, but hiding behind. If you can calculate the exchange of momentum between air and wing - you can find the lift.
I have a theory called the kite effect, the heavier leading edge via gravity pushes the leading edge forwards and downwards but the air pressure underneath pushes everything up and back so most of the lift is coming from underneath pressure as it fights with angle of attack due to the front end wanting to tip forward and down. With higher angles of attack, like in climbing turbulence eddies are formed behind the top NOT some smooth coanda thing.
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.
Nothing better than having something you firmly believed you understood shown to be significantly mistaken. Excellent, thank you.
I have a pilots licence, and a science degree and when I was told about the air meeting up I said wait a second that doesn't made sense. I was told to shut up, do you want to pass your licence, so I did what I was told. Flying is alot about doing what your told by ignorant arrogant assholes.
Yeah, it's not like the air molecules are quantum entangled!
Nothing better than having something you firmly believed you understood shown to be slightly underexplained
I just wish all those who believe in pseudoscience would have the same attitude..
More people need to have this attitude towards life.
What an uplifting video.
I thought it was quite a drag.
I think he was just winging it.
The prof took it to a totally higher plane of understanding!
Spoiler alert: Everyone is Wright.
This chain of puns is really taking off....
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).
Exactly, without the Kutta condition there can be no lift in an inviscid, incompressible fluid. But most people have never heard of it. The explanation in the video seems to switch between viscid and inviscid explanations without acknowledging the differences, so you can't get the simpler explanation used in inviscid flow.
I think your answer is what I was looking for. I need to read further about this. Because the explanation about compressed air above the wing needs to travel faster, lowers the pressure, seems somewhat false: If air is compressed, I would expect the pressure to increase...
I would love to see an animation of the Kutta condition. I have been reading about it for years but never wrapped my head around it. I can see how the motion of particles of air around the airfoil will be the vector sum of the freestream air motion and some hypothetical circular motion but... so what???
@bwwwam "Compressed" is not really the correct term to explain what is happening to the flow over the top of the airfoil. The gas itself is not being compressed like you would find in a pressurized container. Instead, the streamlines are forced closer together. Since the air is not contained in a vessel, it is free to accelerate to avoid being compressed. This is the "conservation of mass" that he was talking about.
+BlazeRazgriz1
You hit on something I want to dig into. I've taken some data, but don't want to tip my hand just yet. I dislike the 'pinching' explanation as well (the Smithsonian's explanation, among othets, uses it).
...
Now, we see the upper air 'accelerate' going over the top. Since air has mass & Newton reminds us it takes a force to accelerate a mass; WHAT is the force accelerating this mass of air ?? There must be a force. The Prof glosses over this and I want to ping him on this also.
Comment ?
What didn't Euler do?
Euler is everywhere! :D
All I know is Euler fixes all my gimbal lock.
I thought Euler's method creates the gimball lock problem, which is solved with quaternions?
Euler looks a lot like that professor that is always here and at numberphile
Phonetic spelling?
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 was probably the best video on TH-cam I've ever seen for this material.
Euler looks super smug about the underwear he's wearing on his head
It's actually a crop top sleeveless hoodie with a long hood tail
I really like this video. It is quite frightening that so many textbooks and TH-cam videos give the wrong explanation.
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!
this is the best video about lift on TH-cam because it explains all the partially true theories separately and then combines it to show the more complete truth!
Veritasium made a similar video some years ago, but by Sixty Symbols making another one the chance of people understanding the physics increases.
Ahhh Euler, the source of countless nightmares of engineering students
Possibly the smartest human in history … no wonder engineers don't like him. ;-)
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.
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
That reminds me of a show I saw in Bangkok.
Nilguiri I bet that was some show.
That is what he is saying: you also have to consider the interaction between the particles
+Francois,
I also thought that was poor to say. I suspect he was trying to discount Newton's "hail of bullets" concept and went a bit off track.
I was thinking the same thing.
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.
J7 That’s a very understandable explanation . Better than most I’ve seen--and I’ve seen a lot .
This is truly the most complete and well done video on the science of flight I have ever watched. Thank you for this + the animations. My brain feels quite happy.
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.
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.
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.
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.
This channel is a gift
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
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.
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.
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).
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.
Yes. So many think the downwash is the cause of lift. It's one of the three most common myths.
1- Equal transit & Bad Bernoulli.
2-Downwash & Newton's 3rd.
3-Half venturi above.
.
Finally a TH-cam video really and fully (relatively) explains how airfoils generate lift.
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.
Best summary I've seen. Goes with the principal of superposition.
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!!!
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.
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.
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
Finally after watching a couple of videos about how lift works an - although unsatisfyingly undetailed - satisfiyngly acceptable explanation.
Life is so much simpler without Sixty Symbols.
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.
Loved the graphics for this one. Great as usual!
thats the best and most comprehensive explanation i ve seen so far - lets see what the next 10 videos of that kind trying to explain what lift is :)
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
You're right.
Yep, I spotted that too. "Parker Vortex" maybe?
Graphics guy error !
I have another thing I would very much like to hear discussed, the difference between force and acceleration. I hear them used interchangeably on TH-cam, but they are not the same, but related by F=Ma. This involves relativity, and Prof Merrifield should be ideal to talk about this.
Force can mean natural/applied influence to an object, which in many contexts causes especially acceleration. Acceleration generally means a change in velocity, which is a reaction to an influence.
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!
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!
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".
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.
Those two are one and the same. Momentum change in air is caused by pressure difference, which is caused by Bernoulli effect. Prof makes it sound like those are separate for some reason. They aren't.
Yeah, as soon as you change the angle of attack Bernoulli is now involved due to the "stagnation area" moving off center as the Prof points out. You've made it an airfoil shape by not directing the air along the longitudinal axis (wing chord) of the symmetrical airfoil -- making it unsymmetrical, at least as far as the airflow is concerned.
Also, take a look at the cross section of the wingtip of an F-15 or F-35. Not only is the airfoil not symmetrical but, at least at the point of the wingtip, there's an under-camber like an old fabric covered plane (it doesn't carry through the whole wing, it's more like the leading edge twists down as you move toward the wingtip -- this gives you a wing that's not doing the same thing through its whole length, exactly because different areas are specialized for different speeds and angles of attack).
That's assuming that the stagnation point remains at the same location on the symmetric airfoil as the angle of attack changes. At a = 0, the front stagnation point will obviously be at the nose, but this will change as the angle of attack increases.
Thank you for this thorough explanation. Or at least, much more satisfyingly thorough explanation than is commonly given.
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.
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.
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!
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.
I am a fluid dynamics PhD student doing 3D aerodynamic simulations, and I approve this message. Fluid dynamics on TH-cam can be really cringe-worthy, but leave it to sixty symbols to do it exactly right... Well done, more please
TheNightWatch001 Where are you doing your Phd from?
TheNightWatch001 I’ve always been a bit suspicious about fluid dynamics ( which includes liquids as well as gases) being applied to aerodynamics, because liquids can sustain an internal tension and gases can’t . Molecules in liquids are in direct contact with each other and gas molecules are about ten diameters apart at STP , and the mean free path of gas molecules under these conditions is closer to a hundred diameters.
To take just one example--the viscosity of liquids generally decreases with temperature, but that of gases increases. Ouch!
@@davidwhite8633 fluid dynamics and aerodynamics is literally the same thing?! aerodynamic is, if you really wanted to called it that, an under-category of fluid dynamics. The same laws apply. Yes in your everyday experience water and air behave very differently, but from a fundamental stand point they can be described with the same PDEs, the navier-stokes equations.
You know, an airplane would also work under water (tbh even better due to the much higher density).
Best explanation video for misconception of lift that I've ever seen.
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)
You British people have a talent in documentary production.
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.
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 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.
Hmm... But even the flat wing needs an angle of attack, and with increased angle the Bernoulli effect increases.
There is definitely enough here to justify a part 2 video of this topic. You could expound on the Kutta condition, and you could better discuss the direction of rotation about the wing tip. I would be interested in a discussion of how the keel of a sailboat also helps to generate what we sailors describe as 'lift', acting as an underwater foil to allow us to sail closer to the wind. Traditional thoughts that the keel only reduces leeway don't seem to offer the whole explanation.
Thank you, finally a clear and comprehensive explanation!
I'd be curious to see this same discussion extended to talk about what happens when a wing stalls.
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? :)
@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.
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.
David Messer Well , that’s the problem--you see , there’s no other explanation for the pressure under the wing being LOWER than ambient static pressure at low AoA’s than Bernoulli !
Designers incorporate such things as leading edge flaps, triple slotted flaps, vortex generators, thrust vectoring to make a complex variable machine fit for purpose. The thing I find interesting is that military combat aircraft use symmetrical airfoils at very high wing loading which make them a very different beast to a commercial passenger aircraft.
What a fantastic professor! And what a great explanation!
Lift is action and reaction.
The action is the deflection of air molecules downwards, the reaction is the wing being pushed upwards. Pressure differences are secondary. If there is no downflow of air from the wing, there is no lift.
R
The graphic at 9:20 shows the vortices going the opposite direction than the reality. If you watch closely at the planes flying at about 9:30 for the next 20 seconds you can see the vortices going the correct direction. The pressure is lower on the top so the air slips around the wing tip approaching the top. The physics of from high to low pressure can be understood. Great video BTW.
AWESOME! aeroengr here never thought I'd see a talk about aerodynamics on this channel :D
Good video. Better than most I've seen that deal with lift. Personally I prefer to simplify things: The wing interacts with the airflow going around it, applying total downwards force on it. By Newton's second law, this downward action on airflow has an upward reaction on the wing, that being lift.
The details on how the wing applies a force to the airflow gets more complicated, and a lot of explanations focus on pressure differentials and how they are created, but many of those explanations tend to get mired into particular details and miss the big picture. There's talk about how the reactive principle can't be correct because air isn't being deflected downwards - well, actually, it is deflecting air, but it's almost immediately stopped by the ambient air that the wing is traveling through. Basically, the downward-pushing force created by the wing is dissipated over a very large amount of air, which means the air as a whole doesn't really move a whole lot (wing tip vortices notwithstanding).
If the wing traveled through the same region of air over and over again, it would eventually start to create a downwards airflow. This is actually what happens with a table fan, or an aircraft's propeller, or when a helicopter is hovering static in one position. With helicopters, it turns out that the downwards airflow through the rotor disc actually decreases lift... but that's kind of a different topic.
By the way, I find it interesting that people keep saying that the air going over the top of the wing "provides more lift" than the air on the bottom of the wing. The lift is by definition caused by a pressure *differential* between the upside and downside of the wing, and you can't have a pressure differential without including both the pressure above AND below the wing in the comparison. It's true that compared to the ambient air pressure, the pressure drop above the wing is greater than the pressure increase below the wing, but the total lift force is still all about the total pressure differential and both sides are part of the lift force being generated.
I also find that the low and high pressure zones are slightly over-inflated in terms of importance. The fact is that the only way gas can ever apply a force on anything is through pressure differentials, so basically the pressure differential over the top and the bottom of the wing is exactly how the wing is applying a force on the airflow (thus "deflecting" it), and the airflow is applying equal and opposite force on the wing.
So I find that saying the wing "creates" areas of high and low pressure is just a roundabout way of saying that the wing is applying a force to the airflow (and vice versa, hence the lift). Just by moving your hand through air perpendicular, there's a high pressure zone in front of your hand and low pressure zone behind your hand, creating drag. If you flatten your hand relative to airflow, you can then deflect airflow up or down by tilting your hand or making a curved cup-shape: This does create high and low pressure zones, but you can intuitively feel it as your hand pushing on the airflow, as well.
By the way you can create lift with a completely flat aerofoil, relying only on angle of attack. It isn't particularly efficient, but I think it could be a good place to start explaining why the simplified Bernoulli effect explanation cannot be correct, since even symmetrical aerofoils, very thin aerofoils (like paper) or completely flat aerofoils can produce lift.
A completely flat aerofoil can produce lift if it's tilted (ie. has some angle of attack).
A flat but curved aerofoil can produce significantly more lift; this curve is also called "camber". Many early airplanes had wings that had very narrow chord (thickness).
More modern wings usually have thickness because that way it can apply a stronger force on the airflow - a thicker wing produces more lift.
Wings also have asymmetric profile (camber) because this means they can produce lift even if they are at zero angle of attack or even slightly negative angle of attack; this reduces parasitic drag because the aerofoil can move through the air with the smallest possible cross-section presented to the airflow, and thus the wing becomes more efficient (at certain airspeeds, anyway).
But all of this becomes very complicated very quickly, and the fundamental laws of physics, action and reaction, always apply, so the simplest way to explain lift really is to just say that the wing applies a force on the airflow going around it, and the airflow applies an equal force to the wing in the opposite direction.
You don't even know which of Newton's laws are which... you are really in over your head here buddy
Thanks for taking the trouble to write this, it's clear that you have thought a lot about it
@@GZA036 The numeration of the laws doesn't really matter - besides, Newton's second law is really the important bit.
Newton's third law is just conservation of momentum applied to the second law.
Loved this. Flying finally makes some sort sense to me.
Is he doing ok? this dude is one of my favorite in your videos
A pilot friend once asked me "what makes an airplane fly"? I began to answer with the Bernoulli principle and he stopped me saying, no, no, that's not it. Reaching into his back pocket, he produced his wallet, waved it in the air and told me, "Money...that's what makes an airplane fly".
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.
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.
Nope, it is incorrect, pls watch the youtube video "Common misconception of lift "by Cambridge's proff Doug Mclean.
"It is not needed for viscosity or Coanda effect for the flow to follow convex surface."
@@loouuiisssss296 And indeed viscosity is what allows flow to detach. Inviscid flow (in thin airfoil theory) doesn't ever separate.
This video has raised my interest in aerodynamics again.
4:00 Right here is where you know this professor is legit. If you look at pressure coefficient distributions around airfoils generating lift, its very clear that the upper surface of the airfoil contributes SUBSTANTIALLY MORE to the overall pressure difference (deviation from ambient pressure) than the lower surface does. In the incompressible limit, the maximum pressure rise (anywhere) on the lower surface is +q, whereas the upper surface can get anywhere between -3q to -5q (or even more) below ambient depending on the exact shape, Reynolds number, and angle of attack. The vortices are spinning in the wrong direction for upwards-pointing lift at the end, but otherwise, cheers!
DaylightDigital And , come to think of it , the pressure on the lower surface is usually less than ambient static on the lower surface , as well , of a light aircraft in cruise flight , i.e. at low AoAs . As long as the difference gives lift you’re home and dry .
It’s easy to show , as well . If you look at the bottom of the top wing and the top of the bottom wing of a doped canvas covered bi-plane in cruise flight both will be slightly bulged out. This indicates that both surfaces , top and bottom , are below the air pressure inside the wing(which is at ambient static) . There’s no other way to explain that , that I can see , without invoking Bernoulli .
There are so many topics I've learned from this series where I realize the lessons thought in school were either partly true or wholly wrong for reasons I assume are that they don't want to confuse young students.
I wish they had just told the whole and correct lesson from the start.
Thumbs up right even before the video started - with Prof. Merrifield it's gotta be a great video!!
"Goes back to equations by Leonard Euler" might be one of the most common phrases in modern mathematics
The ultimate video of explaining the lifting effect
Eddies building under the airfoil are what causes lift under turbulent flow. Using the Navier-Stokes equations, you can derive solutions to lift. Ahh...this brings back memories from my advance aerodynamics class deriving exact solutions to these equations and comparing them to experimental results...
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...
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.
How about analyzing how bicycles work? There are lots of false ideas there.
I was thinking the same myself. It is somewhat analogous. In fact I personally think it is even more interesting, and a little less clearly understood.
Yes, do bicycles next!
It is another one of those often debated explanations.
Tho, tbh, I have no idea why it is supposed to be complicated. Seems simple enough to me. So would be happy to hear what I'm overlooking here.
Maxwell's Demon is about thermodynamics. A bicycle is mostly about mechanics, just very hard mechanics.
Minute Physics did a video on it.
Would take more than a minute, no disrespect to the fine videos from Minute Physics.
They way that I like to think of it is that the rise at the front of the wing has a high resistance so it generates a large pressure differential, leaving low pressure air across the rest of the upper surface of the wing.
I would like to hear a little bit about the incredible abilities of indoor flight r/c airplanes with completely flat wings in relation to this. Airfoils are such a major part of aircraft design but indoor modelling aerobatics succeeds wonderfully and the favourite airfoils profile is flat.
I didn't like the explanation of how the air "just ends up being squeezed" up on top of the wings and it's just pawned off as being part of the equations. You're recent video on the Coanda effect really clears things up, thank you.
As someone who's due to fly an aeroplane over Nottingham tomorrow afternoon I'm glad all is well in the physics of lift.
Thanks so much for finally explaining this to me in a way which is logical and makes sense.
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!
Its because of the curvature of the streamline over and below airfoil, the greater the curvature greater the lift. From Euler equations we get two differential equations of pressure distribution one in streamline direction and other normal(radial) to it, the normal one is responsible for lift.
In Stream line direction >>>>>> partial(dp/ds)=(rho*Velocity*partial(dV/ds))
In Radial(normal) direction>>>>partial(dp/dr)=((rho*square of(Velocity))/r)
from radial direction equation, as the curvature of streamline is more the change in pressure is more so the change in force in that direction.
It's always interesting in lift explanations, to see the angle of attack of the fuselage ignored.
Because it is the wing that does the greatest. The fuselage may or may not produce a *tiny* amount of lift.
The pressure changes caused by the motion through air are *VERY* small! That is why wings are so large.
A Cessna 172 has a pressure *top-bottom* difference of only 0.1 psi!! With 25,000 square inches (147 sq feet) to lift a 2,500 pound plane.
.
The pressure above *decreases* about 0.08 psi and below it decreases about 0.02 psi - for a difference of that 0.1 psi..
..
The largest change of -0.08 psi is only a change of 0.57%. The greatest pressure change around the ting is only HALF a PERCENT!.
..
I am so glad that the reason the air travels faster over the wing is *not* because "it has farther to go," as I've often heard. Physics is not my area of study, but even to me that clearly makes no sense, and it always bugged me. Why does the air *have* to go that farther distance in the same time? Turns out it doesn't.
Lift - "Everyone is wrong, everyone is right: It's complicated"
Came for Euler, stayed for Prof. Merrifield.
The Nobel Prize of physics was announced and we have no news from you on that
The force (full aerodynamic force and useful part of it - lift) - is a (change of) momentum per second. So the momentum is directly connected with lift. The mass and energy equations are important, but hiding behind. If you can calculate the exchange of momentum between air and wing - you can find the lift.
The fact the plans can fly up side down proves that the Bernoulli effect can't possibly be dominant in any situation.
Well done. I love the guy who asks thr questions.. those are the questions I would ask!
I have a theory called the kite effect, the heavier leading edge via gravity pushes the leading edge forwards and downwards but the air pressure underneath pushes everything up and back so most of the lift is coming from underneath pressure as it fights with angle of attack due to the front end wanting to tip forward and down. With higher angles of attack, like in climbing turbulence eddies are formed behind the top NOT some smooth coanda thing.
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.