The only way in which the wing surface can feel forces is either through tangential or normal forces, with tangential forces being viscous shear forces, and normal forces being pressure. The air cannot impart force on the wing surface accept through these means. What this means is that a pressure difference IS the reason a wing generates lift. Experimentation shows clearly that an airfoil shape creates differences in pressure all along the surface and when you integrate the pressure over the whole curve of the airfoil shape, the net force has some component in the normal direction to the freestream, and some component parallel to the freestream, with the normal component being what we call lift. The net result of causing this pressure difference is a turning of the flow, since the only way the flow can be turned is through the pressure gradients within it. So, from a mathematical standpoint, you are correct, the flow is turned by an amount exactly required to generate the amount of lift that is generated, but to say that lift is not actually caused by a pressure difference is incorrect, because again, the only way the wing surface can feel force is through the pressure and viscous forces acting on it directly (viscous forces barely contribute to lift, but rather contribute mostly to drag). The turning of the flow is just another result of the pressure differences within the flow field. The reason, then, that the Equal Transit Time theory is wrong, is not because it explains that pressure differences cause lift, but instead because of HOW it explains the pressure difference is created in the first place, which is admittedly much more complicated than can be explained without some very complicated math and physical principles. Equal Transit theory is not based on Bernoulli's Principle, it just uses Bernoulli's Principle to explain the result in change of pressure from the false premise it starts with. Don't equate the Equal Transit time fallacy with Bernoulli's Principle because there are plenty of valid uses of Bernoulli's equation.
correct! F1 cars use a diffuser for that reason, speeding up airflow beneath the vehicle and creating low pressure air underneath, effectively sucking the car to the ground.
Good comment, but I'm surprised you didn't comment on the completely false and incorrect graphic shown in the video at 2:50. That downdraft shows something that wind tunnels never show for such an angle of attack. Case in point, for planes cruising at constant altitude there is *no* net vertical momentum and therefore *no* net downdraft. Instead there is displacement up and down and then relaxation back to original level. Very disappointing that engineers keep repeating this mistake that flight is due to air mass being "pushed down". It's true only when a plane is ascending or rapidly accelerating.qph.fs.quoracdn.net/main-qimg-cdba626b7a727d8315c1514104ab99bb
Who says the separated air must meet at the trailing edge at the same time? Bernoulli didn't say that! Definitely Bernoulli's principle is true, but everyone is using this principle based on a wrong premise.
Thats to make the air wind in the same pattern, (laminar flow). If the air wind doesnt meet at the same time the air will become (turbulent flow) which will make the air behind not good as it was. Making it impossible for other planes to travel at the same direction after a while.
A basic symmetrical airfoil shape is a flat sheet wing with a shape added around it to allow thickness for spars, and to improve aerodynamics at different angles of attack. A flat bottom airfoil is really a cambered sheet wing with an airfoil added around it. So if you look at the centerline of that flat bottom airfoil, you can see that when the flat bottom is parallel with the direction of travel, the wing itself really has a slight positive angle of attack. Hence lift at what seems like a neutral angle of attack, but which is really slightly positive.
Maule aircraft have a flat bottom wing, the flaps can be set to negative angle to improve cruise. Other than that pretty horrible aircraft to fly in tricycle gear form.
3:38 unless I'm mistaken a wing in stall still produces lift, it's just that it's overcome by drag so not very practical, and uncontrollable due to no flow over control surfaces.
You missed a step by saying the turning of the flow is what causes lift, turning the flow creates regions of high and low pressure simply due to the flow being directed away from it's "intended/original" path. These differences in pressure is what creates the Lift force, or more specifically: the surface integral of all the vectors normal to the surface multiplied by their associated pressure value over the entire surface area of the body (airfoil in our case). Creates a resultant force, of which the Y component is LIFT and the x component is DRAG. What this means in practice is that there are some vectors pointing upwards(ish) above the airfoil which are "sucking" it up into the rest of the atmospheric pressure field, and some areas below where it is being "pushed" upwards away from the atmospheric pressure field. (The front of the airfoil actually tends to be pushed downwards in the majority of cambered airfoils, due to the region of high pressure at the leading edge) For anyone who's actually read this I'll assume you know at least a little about aerodynamics, so if I've made a mistake, feel free to correct me, I am only a 3rd year student lol.
His explanation was simple and made for the average RUclipsr. Who knows nothing about lift, drag, and much more. Not good to give TMI to people that just want the basics. Notice how he didn't get into ailerons and about nine other basic things I believe not to confuse people.
To delay a stall you can also use vortex generators on top surface of wing. Vortices tend to "stick" to the wing better than regular air, so the separation occurs at higher AoA.
@@Wrtvrxgvcf55 The F-18 uses a different principle. It uses vortices, which can be created at high angles of attack by wings with more than 45 degrees of sweep. Vortices are not the same as generic turbulent flow. They are their own phenomena. This is also used by most true delta winged aircraft.
@@HarwinSingh If I remember correctly, stall strips are intended for the opposite purpose - to initiate flow separation earlier. You place them on the root of the wing and you get pitch down instead of a spin when the aircraft is stalled.
Good video for the shortness of it. You are correct- exactly! Basically, the various "theories" of why wings work to create lift all say the same thing, but in different ways. The idea of alerting the airflow to create lift, is best explained with your explanation. Air has mass- quite a lot of mass, actually. Anytime you move mass, there is an equal and opposite force created. In order to get any aircraft off the ground, you must move (at velocity) a mass (or weight) of air equal to the mass (or weight) of the aircraft in the opposite direction in which you want the aircraft to move. Any given volume of air (a "slug") has specific weight. You can calculate what amount of force is being generated by centripetal force of moving that weight. So, basically, the wing lifts for the same reason a weight being slung around a center point get's "heavier"- the faster the weight is slung, the "heavier" it gets. the redirection of the air slugs by the wings angle of attack, eventually is going fast enough to literally force the wing upward against the weight holding it down. this is why even a flat, angular board held at an angle to the direction of flow will cause lift- the rectangular wing will fly (though with higher drag) without the curvature (I've proven this to the vast chagrin of "Bernoulli" adherents with RC airplane models. The advantage of a curved or "Bernoulli" wing is a reduction in drag AND and enhancement of more easily redirecting the airflow in a different and downward direction, thus causing more efficient lift. The fundamentalist Bernoulli people have a hard time explaining why properly curved wings can fly (still generate lift) when flying upside down...and don't even get them going on the symmetrical (curved the same on both sides of the wing surface) fly better, with less drag and more lift at higher speed than the traditional "Bernoulli" models - that is the "flat bottom" wings. It is merely the re-direction of mass that causes airplane wings to create lift- that's the whole bottom line. All the other aspects of a wings design are for reducing drag while enhancing that re-direction of mass. Excellent video. Excellent explanation of why wings create lift.
Centrifugal or Centripetal force has nothing to do with an air wing generating lift. Other than that you are mostly correct that Newtons 3rd law of motion is what provides the lift. You are incorrect though to say the poster of the video is correct in that regard because that is not what he said. He said something completely different and believes a downdraft creates lift which makes no sense whatsoever.
@@lxdimension What SHOULD have he said? What part did he say that was 'completely different'? 'Newton's 3rd law of motion' doesn't make planes fly.. More words, please. (Seen Kooper, above?)
@@lxdimension I agree re: centripetal force, that made no sense to me at all. However, in terms of the video, what do you think a "downdraft" is, if it isn't literally the "opposite" aspect of Newton's 3rd law? The air has shifted direction, due to a force being applied to it by the wing moving through it, there's an opposite force... hello lift!
I don’t get why most descriptions focus on the low pressure above the wing, instead of the high pressure beneath the wing. The high pressure is what creates the lift after all.
Not really, its the relative difference between the top and the bottom of the wing. There is no entity called "high pressure" under the wing. There wouldnt be any difference between lowering the pressure above the wing or increasing the pressure below it.
Thanks I needed this. I didn't even know what aerofoils were before this and now I understand Bernoulli's principle over it. That's some big brain time
Amazing how people are not satisfied with a simple answer to a complicated situation there are many factors involved in flight I think you did a great job and just pulling up a couple of basics unfortunately there are those who have more questions instead of bashing you they should do research on the internet that is in their hands if they really want to know
Thank you for the great explanation on the topic. I liked how you were able to debunk some of the common misconceptions generated by lift and gave a more correct answer. I also liked how you also mention some of the devices used to help lift such as flaps.
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.
While it is true that the flow tends to remain attached to the surface of the airfoil, it is not because of the Coanda effect. This effect specifically applies to jet flow, which an airfoil would not experience. The equal transit time theory is wrong as you stated, but the relative pressure fields between the top and bottom surfaces of the wing would play a role in generating lift. Pressure and velocity changes are coupled when looking at a flow like you displayed, meaning they change continuously with each other. This is a big misconception when it comes to explaining lift.
Coanda Effect and flow-attachment involve the same physics. (Simply make the diameter of the jet be wider, and the connection becomes obvious.) Attacking the term "Coanda Effect" is misguided, smacking of one-upmanship, rather than pointing out a significant error. Yes, the proper term here is Flow Attachment. But Coanda Effect is an example of flow-attachment. If you believe that the two of them employ two different kinds of physics, then you need to explain the details. Coanda Effect becomes Flow-Attachment in the limit of increasing the jet-diameter. In 1999 I saw the very first use of this same misguided attack, this "it's not Coanda Effect." The person using it insisted that Coanda Effect only involved liquid jets in air, such as water flowing over the back of a teaspoon. Why would he say this? It was quite obviously a well-poisoning fallacy. He had just been caught (in a very public situation) believing and supporting "transit-time fallacy," Rather than publicly admitting error, was trying to avoid embarrassment, by attacking his attackers, pointing out their "huge error," suggesting that they weren't competent, because Coanda Effect only involved water-jets ...the tea sticking to the teapot spout, etc. Henri Coanda didn't discover his named effect by launching any water-jets. The phenomenon appeared with air-jets coming too close to a parallel fuselage surface, where they'd unexpectedly exhibit flow-attachment. (But how wide must the "jet" become, before it's simply flow-attachment, and not Coanda Effect? Answer that, and you'll have a perhaps valid point.)
Whilst an irritating voice commentates, this is the clearest, simplest and easiest, (apologies to Easy Jet), to understand of the seven I've read today - Thank You
First off I enjoyed your visual representation of how flow goes around the airfoil. You were mentioning Newton's third law when you said the airfoil turns the fluid flow downward applying equal and opposite forces lifting the airfoil, which is a part of how lift works. However when flow is curved there is a pressure gradient. This gradient forms a pressure and velocity field around the airfoil that affects a wide area in the fluid. As you said the pressure above is lower and the velocity is faster, while below the pressure is high and the velocity is low. The pressure and velocity affect each other simultaneously and the pressure difference drives the lift force as well. Also the Coanda effect only applies to jet flow and not for ordinary flow airfoils are usually in, so it is not a factor in lift.
This "correct" explanation was given in 1944 (possibly earlier?) by Wolfgang Langewiesche in his book "Stick and Rudder" and rubbished (at the time). It is now known that BOTH principles are involved, it's just that the Bernoulli principle was incorrect regarding the speed at which air travels over the top of the airfoil..
Bernoulli Principle is not incorrect. Bernoulli Principle says nothing about the velocity profile of two adjacent streamlines, it strictly deals with flow along a single streamline.
Indulge an old fitter please. When I was a kid more than six five years ago, I built a flew model aircraft with completely flat wings. as long as these were angled upwards against the line of thrust, the models achieved lift. I figured it was like a boats rudder. Barry
1:56 Correct, The wing is indeed turning the airflow downward. But there is also a pressure difference between the top and bottom of the wing! The lift force is literally equal to the average pressure difference between. The bottom and top of the wing times the surface area of the wing! And it is in fact the pressure gradient formed arround the wing that turns the airflow downward! Please correct your mistake!
Well the actual force of the lift is created by the pressure difference between the upper and lower side of the wing. The air directed downwards is basically just the result of that. Most simply explained by newtons first law that an action must have an equal oppsite reaction. We push the plane up so the air must get pushed down as a result.
Seeking causality makes everything complicated. Ultimately, lift requires a pressure difference and requires a change in momentum, and it's hard to separate those.
@@AmbientMorality Pressure preempts movement. Its a state that begets a force, which in turn begets movement. Confusing the direction of this causal relationship is probably because the bernaulli principle is also sometimes involved, in which a change in fluid velocity begets a change in its pressure. That is not the mechamism, however, by which air compressed by a wing at high AoA gains its momentum.
@@AmbientMorality Correct. Horizontal movement of the aerofoil relative to the medium compresses the medium (or indeed vica versa), which in turn creates the vertical air movement. Neither movement however acts as a vertical force on the wing.
As an aerospace engineer, this is a useful explanation for the average person to understand what creates lift, but we can’t totally disregard the fact that there is a pressure difference. I disagree with you when you say that a pressure difference is not what creates lift. In fact, there are many times where we can actually compute the lift force (or lift coefficient) on an airfoil WITH the pressure distribution and using calculus to integrate the difference in pressure acting on the top and bottom. At the end end of the day, there are two natural sources that cause for all aerodynamic forces and they are none other than the pressure distribution and the viscous shear stress. I’m not saying you’re wrong for thinking this (although I am saying you’re wrong for saying the pressure theory is wrong), but I think they go hand in hand. You can’t have one without the other. To say one idea is more correct than the other would be ignorance. I appreciate the video though and it helped me deepen my understanding a bit because I had never heard it explained this way. Thanks! 👍🏼
I hate to argue with an aerospace engineer, and I'm also 4 years late to this party, but I do have some questions. First, take a situation where you are in a moving car and you put your hand out of the window. Where there is no AoA, there is no lift generated. There is some drag, but it's manageable. However, if you turn your hand into a slightly positive AoA, you simultaneously feel a substantial increase in drag and your hand flies upwards. If you're not prepared, it basically goes up and back as both drag and lift are generated. When I started considering lift, it seemed easy. I thought - "this has got to be because of the equal and opposite reaction of dumping flowing air downward, changing its direction by providing a force that changes its vector". Newton's 3rd law - the same force that your hand is applying to the airflow to change its direction (created by the car's engine and your muscles pushing your hand against the pressure of the airflow to overcome drag) is also applied via to your hand, causing it to want to go up. The effect is so strong and so instantaneous that I found it extremely difficult to believe that it can be caused by the development of a pressure differential above and below the "wing" of your hand. But that feeling is not scientific in nature. After all, I caution myself that an explosive decompression event causes substantial forces very quickly. And the moment you unscrew a bottle which contained a carbonated drink, the top wants to fly off as the pressure tries to equalise with the room you're in. It doesn't have to "build"... so pressure differentials can create forces quickly. So I guess the challenge in answering this question to everyone's satisfaction is that it seems hard to imagine circumstances where you can impart downward motion to air meeting an aerofoil where you're not also creating a pressure differential below and above the wing, so it feels like the Newtonian approach and one based on pressure differences seem to "come up with the same answer" and everyone is shouting about which one is right, based on the answer that feels better to them. I don't know how to bridge the gap and I don't have enough scientific understanding to try and do so - I fly planes rather than design them. But I do know that explanations of lift forces have been plagued by bad and poorly explained answers for a long, long time - it's remarkable to me that we can go to the moon, explore the wreck of the Titanic, do AI facial recognition, connect all the computers around the world, search deep space for answers to whether we're alone and image amazing parts of the universe the naked eye can't see.. to say nothing of achieving net positive energy fusion reaction. Yet a single unified and sensible theory of aerodynamic lift continues to be a challenge to describe in a way that people accept and understand. (and yes, everyone thinks their answer is right - I am not inviting more of that, just saying that there do still seem to be challenges in describing the lift forces). However, I found this quote useful, from good old Wikipedia... "A serious flaw common to all the Bernoulli-based explanations is that they imply that a speed difference can arise from causes other than a pressure difference, and that the speed difference then leads to a pressure difference, by Bernoulli's principle. This implied one-way causation is a misconception. The real relationship between pressure and flow speed is a mutual interaction." Yes. That feels right. So I think we are saying that when the air flows around (meets) the wing, all kinds of pressure differentials are created in the local atmosphere of the wing, the net result of which is upward lift. The laws of conservation still apply (it's not an either / or) because the net overall force lifting the wing is equal to, and opposite, the net overall force exerted downward on the rest of the localised atmosphere. It's just that the net overall forces are created by / are a symptom of the pressure differential.
@@cjad100 Woah, I barely remember even making this comment haha! It’s been a while so I had to rewatch the video. The main problem with the video is that he is marrying the “equal transit time” (an incorrect theory) with Bernoulli’s principle and dismisses the pressure difference idea because the equal transit theory is wrong. Newton’s law can be an explanation for how lift is created. Bernoulli’s principle and Newton’s laws suffice to explain what creates lift. But at the end of the day, you are absolutely correct. We actually do not fundamentally understand why there is pressure difference. We know the pressure decreases on the top of the wing and the pressure increases on the bottom, due to a lower velocity. However, we cannot answer the question of “Does the pressure decrease because the flow increases, or does the flow increase because the pressure decreases?” It’s basically the chicken or the egg debate.
The equal transit theory is false. Lift creation is very complex but it’s mainly caused by the pressure differential between the top and bottom of the wing and the downward deflection at the trailing edge. So it’s a combination of Bernoulli’s principle and the third law of motion.
This was perfect timing of a video! I start air cadets for the first time on Monday, and due to my age being a bit high for air cadets, they said I will need to learn why planes fly and don't sink before everyone else joins, so I can catch up and be ahead of everyone.
Some fighter aircraft have a symmetric wing shape, same on top and on the bottom. They can fly upright or upside-down. Their wing is an inclined plain to the airflow that produces lift. The angle of attack is very critical. For the traditional wing shape, the angle of attack is much less critical. QED
You are correct in identifying the Equal Transit Time fallacy, but there are other parts of that theory that still hold true. There is still a region of low pressure above the airfoil and high pressure below, and Bernoulli's Principle still holds true along a streamline. Lift is a combination of the pressure and velocity field coupling as well as Newton's conservation of momentum law, but I feel as if you dismissed the former a little too much in the video. I liked the additional information about flaps, slats, and spoilers as well as the importance of angle of attack.
He said that the pressure difference still exists, but that it works at least partially in tandem with Newton’s laws. He implied that it was both without explicitly saying it.
THANK YOU for confirming my belief that Bernoulli is BS in terms of its overall impact on lift. Sure there is some, but that's NOT the major source of the lift! BTW, there are a lot of folks saying the lift for DRONES comes from Bernoulli's Principle...even though the propeller blades are virtually flat. I point them to ceiling fans and ask them to explain THEM! LOL. Anyways, thanks for the clear and no BS video.
Other than pressure difference and AoA, i never realized that the airflow above the wing goes down to generate lift! great vid to understand the basics of the wings.
+Thanatos if lift was explained only by reaction forces from the deflection of incoming air, then only planes with a thrust-to-weight ratio above 1.0 could take off. Also, once the upper surface flow detaches from the wing the airfoil becomes stalled. Which is why fighter jets implement Leading Edge Root Extensions (LERX), slats, chines, etc to preserve the upper flow of air during high AoA maneuvers. History of aircraft design itself debunks the theory that airfoils only generate lift through deflection. Your hand doesn't try to fly up in a moving car because it's not a well designed airfoil. If you designed an airfoil just the right size for your hand and which could generate lift at low speeds, you'd feel it pull up when outside a car window.
I'm sure Thanatos is an aerodynamics expert, and all these other people who build and design planes are just cretins compared to his massive intellect. He called it, you guys. We're done here. From now on, all planes will be designed with giant hands on the sides.
I'm confused. This video tells us that it's not the pressure difference, but the deflected air that creates the lift (according to Newton's principle of action and reaction, I'd assume). Another video I've just watched tells me that it is mainly the pressure difference according to Bernoulli that creates lift, while the deflected air just creates a small percentage of the lift. Whitch is it? Or is that question still the object of brawls between physicists?
An oblique hydrofoil dragged slowly through liquid helium will not generate any transverse force. Fluid flows round the trailing edge of the aerofoil and doubles back towards the rear stagnation point. As the hydrofoil speeds up, we are putting in enough energy to warm up the helium and destroy superfluidity. Then all the vorticity is concentrated in boundary layers around the hydrofoil. As the fluid warms up, a thickening boundary layer is unable to negotiate the trailing edge and so vorticity is dumped as a starting vortex in the flow. This establishes a Kutta condition at the trailing edge, and there is residual circulation around the hydrofoil opposite in sense to the starting vortex, which generates a transverse force. This can be explained in terms of Bernoulli's Principle as well once things get going, but the formation of the starting vortex is the thing to look at. It won't happen without viscosity.
in 1:23 how we know that pressure above wing is less than pressure beneath the wing if Bernouli's equation is applied in the same streamline and not 2 different ?
Thx for the vid! But I still don't get it why the upper stream just speeds up by itself oO can it be that it travels with its normal speed whereas the lower stream is being slowed by the curvature of the wing?
There are no airstreams in flight. Air is mostly stationary, and the wing is moving through it, forcing the airmolecules apart in a mostly vertical movement pattern.
When teaching how to sail we often refer to this theory to show how a sail works. ie, the lower air pressure on the curved side of the sail allows the higher pressure on the concave side to drive the boat forward. This is counteracted by the keel/centreboard so that the boat does not simply sail sideways or tip flat on its side. Is it in fact the change of angle of the air coming off the back of the sail which creates " sideways lift" or is it the difference in air pressure?
Both. Differential pressure in a fluid (or gaseous) medium is not stable. The existance of a pressure difference necessitates the movement of fluid to equalise the pressure within the system in accordance with the 2nd law of thermodynamics. Similarly, due to the conservation of momentum, and newtons 3rd law it is necessear, that the force of the airpressure pishing the ail forward have an equal and opposite counterforce upon the air, moving it back, bringing the sum of forces and moments within the system back to 0.
3:45 So if the airflow on the top of the wing stops there will be no lift generated as you say. But earlier you said that top of the wing airflow was not what generated lift. So I am confused now!
Thank you for the excellent explanation on a very important topic in aerodynamics! I like how you started your explanation of how lift is generated by making a simple claim and then proceeding to debunk the most common misconception of the generation of lift that has been spreading in aerodynamic communities. I also found your explanations of how the angle of attack and number of flaps on aircrafts affect the lift on a wing to be very helpful. Overall, you did a great job of explaining the production of lift!
No, Flaps are used to increase lift at lower speeds, they also create more drag so more power is needed to maintain a set speed so fuel consumption actually increases. The flaps are used to get off the ground sooner, or to reduce the speed to be within a safe limit for landing.
Why does the stall configuration cause the aircraft to fall? You’d think having really low pressure on top and same amount of high pressure on the bottom would be better?
In a stall air stops flowing smoothly over the top surface - higher angles of attack lead to larger pressure gradients that try to force airflow to separate. The airflow after that separation point is recirculating, low velocity air so it’s no longer very low pressure
You know, upon revisiting this video now, i have a few more thoughts: There are wings that are basically flat, wings that have a symmetrical profile (meaning the upper and lower part of the profile is the same) and also airplanes can fly upside down, against the profile and just through angle of attack. Airplanes, even at their max speed, fly with some amount of aoa and that in my mind is responsible for the lift, the air that hits the wing from below, pushing it up and itself being turned downward. To me, the upper part of the profile is there for getting the most out of the wing for a specific type of flying by reducing the moment of air unstick and the onset of turbulent air that would start hitting the wing from above, pushing it down, all for the airspeed and aoa range at which the wing will spend most of its time in, meaning the cruise phase. Plus it conveniently gives space inside the wing for fuel and actuation. To me, its all about the angle of the wing, if it becomes too steep, the air starts to deflect in such a way that it mostly pushes the wing backwards instead of upwards, slowing the plane down until flight is impossible. Slats and flaps give you more air being turned and more sharply while maintaining lower aoa and further reduce the onset of unstick of air (slats) The air being turned down on the upper surface doesnt seem to do anything to push the wing up, also, that air meets the lower profile air behind the trailing edge of the wing, so there would be no influence on the wing itself, those things happen behind it. In my mind there is always too much propensity for attributing lift genereation to the upper profile, its there to lessen air unstick and turbulence at aoa's by giving the air there a more gradual gradient of travel, in my mind. Plus, one could argue that at low aoa's and with wings with sharp, bulbous profiles, the air that meets the leading edge on the upper surface would be deflecting upwards, pushing the wing slightly down, thats why those wings often fly very slow and at higher aoa's to negate that effect and just have that lesser propensity for unstick. What would be your thoughts ?
The air accelerates very rapidly over the upper surface and gets a very low pressure, so it's responsible for a ton of the lift. You're right that it has to meet the air at the trailing edge of the wing, which is at a much higher pressure. The fact that it's flowing from low to high pressure is why it's susceptible to stalling. You're correct that the upper surface near the leading edge produces slightly negative lift at low angle of attack. But higher angle of attack still makes the pressure gradient on the upper surface worse.
@@AmbientMorality Hmm...If what you say is true, then the wing could basically generate lift at 0 aoa ? Just through the profile ? And yet there are airplanes whose wing profiles are basically flat and/or symmetrical, made for high speed and low drag. They fly just fine too, as well as other examples i mentioned, all of them fly at some aoa even at max speeds. Hmm...the fact that air over the wing is of the lower pressure than even the air that sits above that air could perhaps aid the air under the wing in its pushing of the wing up, indirectly ? There is overally less mass "resting" on the wing from above, compared to a stationary wing, you could say ? Thats how i would imagine this at least ? But i'd say that its more of a fact of the wing itself just being a barrier for the ram air, creating that lesser pressure on the upper side and the profile being there to ease the flow of that lower pressure air, so that it doesn't unstick too early thanks to a more gradual gradient and start hitting the wing turbulently from above, at least thats how im imagining this X). Also, would you say that the lesser pressure air right over the wing accelerates, becouse of the mass of undisturbed air resting on top of it, pushing on it with its own mass ?
@@vedymin1 A cambered wing could generate lift at zero angle of attack, and a symmetric wing would generate lift on some parts but it'd be cancelled out exactly by the other side. The barrier explanation doesn't quite work in my mind because if the flow separates, the pressure on the top surface of the wing increases to roughly atmospheric pressure. The barrier analogy suggests that the wing is just blocking the air, which isn't possible because air will fill a space faster than a wing will move. I'd say the lower pressure is because the airfoil curves and the air follows that curvature: curvature means that the pressure near the wall must decrease (essentially, centrifugal force). Since nothing's actually adding energy to the flow, if the pressure decreases near the wall then the velocity must increase because energy is conserved. Therefore, you get acceleration.
Im gonna have to think about what you told me, specifically the ability of a cambered wing profile at zero aoa to still produce lift, trying to visualise what all the air particles do at all times in all parts of the wing is a challenge to say the least, more learnin required i guess xD.
If i place the back of a spoon in flowing tap water, the spoon is drawn into the water. What if now all that could be placed inside a vacuum jar, would the spoon still deflect?
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 Bernoulli/Newton dispute is a false dichotomy. They are simply different ways of describing the same phenomonem. Newtonian mechanics is arguably the more fundamental explanation, but the proximal cause of the lift (the only thing the wing feels) is the pressure differential that is a resultant of that Newtonian action/reaction. And that pressure drop is known as the Bernoulli effect, although Newton himself had observed the effect in tennis balls and concluded correctly (of course,) that it was due to action/reaction with the passing air. So, probably go with Newton since he was a couple of centuries earlier than Bernoulli.
As a flying aerobatics pilot I have 2 questions: 1) I fly Extra 300 and Extra 330. How the hell am I flying if these planes have... symmetrical airfoils? 2) I also fly RV-7. How am I able to fly inverted over runway and not smash into it?
Good start by debunking one of the common myths about lift generation. Unfortunately I think too much was thrown out with that theory. Pressure does indeed play a part in lift generation, but it's only part of the larger puzzle. To say that the pressure difference doesn't create lift is incorrect. The Coanda Effect also only applies to fluid jet flows, which is not descriptive of a fluid flowing over an airfoil. The turning of the airflow, or momentum, theory is definitely one of the ways that lift is generated. Circulation is a more complex concept, but including it would have resulted in a much more complete explanation of what causes lift.
Fun Video!! Thanks for posting! Question?.....I know nothing about nothing (no degree, not in the field), but in general it seems like Bernoulli is more accurate. For someone who knows maybe explain for me....if the opposite of Bernoulli is true and the air going over the wing is hitting the air going under the wind and firing downward....isn't ALL of that taking place AFTER the wing has left that pocket of air? If so how can that downward collision have effect on the wing that is front of it? Isn't it Bernoulli's pressure difference that form the "clouds" we sometime see around wings?
I fully agree with what's happening here, but I don't understand a stall. A stall happens when the air over the top of the wing separates from the wing itself, but how does this cause a stall if the the air moving over the wing has no relevance to the lift produced. Can anyone help? I'm happy to explain further if that wasn't enough, thanks.
Lift is created by pushing air down, and thus the air pushing back upwards against you. The explanation in the video is incorrect. If the explanation in the video was correct, and inverted plane could not fly and would just fall straight out of the sky. A stall is induced when the angle of attack is too great for the speed of the airplane. For example a aircraft that is powerful enough to climb at a 90 upward angle to the earth, will not stall until it reaches the point that the air has become too thin for the motor/engine to produce more thrust than the weight of the aircraft. That is basically how a helicopter works. So if you have a large commercial jet fully loaded, it is extremely heavy, even with big engines, it is not a high performance aircraft, and it can not climb at as steep of an angle of attack as a small, light, fast plane. Think of it like a car driving up a hill, a sports car can drive up a really steep hill because its engine is powerful, and the car is lightweight, so it can drive up the hill with ease. But if a heavy vehicle with a less powerful engine tries, the hill may be too steep, and the vehicle will either stop, or roll/slide back down the hill. So for every aircraft there is a speed+weight+wing area+altitude combination that will decide the limit of the angle of attack that it can continue flying level, or climbing. Essentially if you increase the angle of attack too much, the bottom of the wing begins to face forward, and air is just running into the bottom of the wing. So now your wing is just pushing air forward instead of down. This will create way too much drag, no lift, and slow the aircraft. I know this explanation is not very good or understandable but I hope it helps.
In a stall its because the push up under the wing, not the lift thats on top , becomes more and more of a push back. Lift is on the top, like a vacuum, and when air is no longer going over the top there is no vacuum..all you have is push back by the air underneath.
@@Rickie53 An aircraft stall and a wing stall are not the same. An aircraft can still fly with stalled wings, which specifically refers to the cessation of laminar flow of air around the wing.
Hi. I’m going to be doing some presentations for a paragliding club. Is it possible to ask for a copy of your animations used in your video called how wings actually fly. Any help appreciated.
The lift generated by a wing is due to the pressure difference between the upper and lower surface. The question is WHY is this pressure difference created. Physical testing shows that the speed of the air over the top surface IS faster than the flow over the bottom surface. There are mathamatical formulas for calulating lift that are accurate. Aircraft designers use them! The amount of lift depends on the density of the air, the coefficient of lift (determined by the cross section shape of the airfoil), the angle of attack of the airfoil, the area of the wing and the speed of the wing through the air squared. There are various sections that have been tested, the NACA sections and the coefficient of lift at various angles of attack and can been looked up for each section. The "Downwash" theory sound convincing but it forgets the "Upwash" that occurrs in front of the wing as the air rises ahead of the wing to enter the low pressure area above the wing. The "Creation of lift" is obviously far more complicated than a simple bernoulli or downward deflection explanation. The fact is lift IS created by a combination of the factors listed above and explanation involve deflection of air and the speeding up of air over the top and slowing of the air on the bottom. The pressure differences have been maesured and the force from the lower pressure on top is greater than the force from the increased pressure on the bottom. So the statement that lift is not reliant on the pressure differences above and below the wing is clearly NOT correxct.
I still don't understand why it creates lift. As you explained that downward airflow happening after it left wings. So it should be no affect right? Air should lift aircraft bottom of its wing right?
The wing creates a pressure difference between the upper and lower surface. That multiplied by wing area gives you the lift. But the air itself is deflected into a new direction by this interaction resulting in a downwash behind the wing. Before the passage of the wing the air has no vertical velocity. But afterwards it does. In principle, if you knew what this new air velocity was, times the mass flow rate of the air involved, you would have the rate of change of momentum of the air. Forrce = rate of change of momentum, so what you would have calculated is the equal and opposite force to that on the wing. So, the pressure of the air physically in contact with the wing, and the momentum change (or deflection) of the air, offer two alternative theoretical methods to calculate lift. But we are talking about contact forces here - no 'action at a distance' - so only the pressure of the air actually in contact with the wing is physically lifting it.
hey doofer, I'm a huge fan, yes, the equal transit theory is totally incorrect, but you've missed a few things out here 1. the camber on an airfoil produces lift mainly by the curvature of the streamline and the euler equation, When a streamline is curved, the air loses its pressure 2. Another thing to take into account is streamtubes, as the air meets the cambered portion of the airfoil the "Streamtubes" are slightly squished, following the venturi theorum Yes, Wings do indeed create lift by downwash too although Edit: Also the downwash eventually levels out due to its inertia
I think I’m right in saying drag is a good thing caused by flaps and not a bad thing, as drag is the opposing force to thrust therefore the flaps are keeping the plane slow whilst still maintaining lift.
This explanation has some issues. Bernoulli's principle and Newton's third law are both at work. Equal transit (which is separate from Bernoulli's principle) has been disproven, but airflow over the top does move faster and creates a lower pressure area above the airfoil while the air traveling beneath, moves slower creating a higher pressure area generating some lift.
7:00 you all keep saying the air is pushed down from the wing to give it lift. I assume when you invert your aircraft the shape of the wing dies not change. Does the air stop creating lift relative to the wing?
It's all Newton's Third Law - the mass of air pushed down offsets the mass of the aircraft. This is so obvious when you think about a rotary wing aircraft (ie. helicopter). It's very windy underneath there. The airfoil is only important for maximising air deflection while minimising drag. It's not the upper surface separating that CAUSES the lift to fail, but the increase in DRAG which decreases the forward movement, thus impacting the air deflected downwards (ie. lift).
As often happens, the same phisical phenomena can be explained under differect aspects, all true. For example, I could explain that a pendulum works by the force of gravity slowing down and accelerating the ball back and forth, while you could explain it saying that it trades potential energy for kinetic energy ciclically. Both explanation would be valid. While we all agree that the "same time" explanation is wrong, lift can indeed be explained (and calculated) by the pressure differential between lower and upper faces of the wing, which in turn are caused by profile shape and incidence. At the same time, you can also explain lift by saying that the wing pushes air downwards, creating a force upwards. Both are true at the same time.
That depends on many factors. The size and weight of the aeroplane whether it's a prop or jet whether it's made for one passenger or 300 this is something that is not a one word answer
Hi Doofer911, I think this video explains the topic really well. Could I copy your graphic at 0 min 55 sec for a RUclips video my son is making for a STEM science video on lift? I would be very grateful. Many thanks Eddie
Dont forget lift is also produced simply by the air pressure on the bottom of the wing at non-zero angle of attack. This is how planes actually take off. I would argue that this component of lift primarily enables a plane to ascend and the airflow turning on the top of the wing enables straight ahead flight.
Lift on the upper surface is higher than the lift on the lower surface at all angles of attack for virtually all airfoils. The exception is during stall (or maybe for some really weird and likely ineffective airfoil), but hopefully the plane isn't taking off in a stall.
Yes and no. This is an often misunderstood discussion about compressablilty of air, And it's a minefield :-) The pressure difference is created by aircirculation over the top of the wing towards the trailing edge and under the wing towards the leading edge. As the wing moves forward the airflow adds to the speed of the air over the wing and substracts from the air under the wing (which was moving forward). so the speed difference creates different pressures and therefor lift. It's manipulating the airstream and since air has mass forces causing the lift are therefore present.
Please check your references, the air DOES NOT join at the SAME TIME at the trailing edge of the wing, “same time” is simply a totally WRONG statement.
Note that the center of lift is approx @ 25% of chord, if directing air downwards at the rear of wing was main cause of lift, center of lift would be at the back of the wing.
Ah I didn't know that.... since making this video I've learned that I haven't covered this topic in as much detail as needed and didn't give a full, comprehensive explanation so I will revisit this topic and remake this video in the future.
check out center of pressure glen research center, then images, It seems weird to me that helicopter blade is angled up during autorotation, yet is gaining forward thrust as it falls thru the air, as is a glider wing.
+Observ45er I think the top skin produces double the lift of the bottom skin,, the air flow behind the most curved part stops being laminar, and kind of boils off the wing, precise point being determined by reynolds number
We can talk about Bernoulli and other stuff, which no doubt plays a part in lift, but the main principle is Newton's 3rd law of motion, and downward pushing of air at the trailing edge of the wing. How a wing generates lift is pretty much summarised in how wind gets pushed downward by the wing. The more wind gets pushed downward, whether more forcefully (greater airspeed) or at a steeper angle downward to a certain extent (greater angle of attack or use of flaps), the greater the lift. That's it, folks. Devices such as slats and flaps both aid that downwash of air by the wings, either by improving the flow of air over the wings such as use of slats (preventing detachment of air travelling over the wing), or by increasing the downward angle of air, deflecting it steeper down, to a certain extent.
@@AmbientMorality ...so you mean Bernoulli, then? If so, I'll refer back to my original post. There are simple wings that have no camber whatsoever (completely flat on the bottom AND top). And those still generate lift, albeit more inefficiently. That proves without a doubt the primary principle that generates lift is newton's 3rd law of motion, through the downwash of air...
@@tristanhnl They're both true. A flat plate at an angle must also have a pressure distribution that can fully explain the lift. Similarly, a turbojet does accelerate air backward and that's perfectly valid to calculate lift, but you could also sum pressures on the compressor and turbine and some other components and get the same result. Both valid.
@@AmbientMorality , both true, but again...newton's 3rd law is the *big picture* phenomenon responsible for lift, in the grand scheme of things. If I were to describe how it's possible for tennis to be played, I can explain how the molecules of the ball interact with the molecules of the floor and the tennis racquets....or I can simply explain the fact that the ball bounces. Again, not saying Bernoulli isn't true.....why do I have the feeling this exchange can go on forever LoL
@@tristanhnl nono that's totally fair. sometimes people read it as "part of the lift is produced with pressure differences and part of it is from moving air down", when really it's just two different complete explanations for the same thing
Try throwing a frsibee upside down. Then try throwing a flat disk of plastic or wood and see if it flies like a frisbee. A frisbee has a curved upper surface and that is what makes wings fly.
At time 1:58 you state: "it's NOT a pressure difference that creates lift". This statement is WRONG. It IS the pressure difference that creates the largest part of a wing's lift. Of course the downward accelleration of the air mass (deflection of the airstream that passes around the wing in a downward direction) is also part of the lift that is generated by the wing. But if your "theory" of the wing that is not related to a pressure differential between the upper side and the lower side of the wing would be correct, you could minimize the wing area by having only very minimal chord length - a profile that only deflects the air downward. But the force of the lift is clearly the mean pressure differential over the entire wing area multiplied by the wing area. If the lift would not be caused by the pressure differential then increasing the wing area would not increase lift as long as chamber (downward bending of airstream) would not be increased. But any wind tunnel experiment will show you that a longer wing chord with the same chamber will result in a greater lift. I agree with you that there are many wrong theories about how a wing generates lift - but it seems that your video is also falling into that category.
Pressure differential and downward displacement of air are in a causative relationship and not separable. A larger wing area will displace more airover a given time, and the effectiveness of wing length and width on lift varies with lreasure, airspeed and air viscosity. Air on earth is generally not very viscous, and usual airspeeds are below the speed of sound, so while the pressure wave created will dissipate relatively quickly, the air will displace faster than the wing moves through it. At speeds above the sound barrier you get the wing cutting through the medium faster than it can displace, and so a wider wing, allowing for a longer contact time becomes more drag efficient, whilst in a higher viscosity flud, such as say water, a much thinner foil suffices to produce similar lift.
To me, the explanation concerning Newton’s 3rd law always seemed more intuitive and easier to understand than Bernoulli’s. Especially observing how sharply the trailing edge of an airfoil tapers off like that. But in my simpleton brain, I just conclude that it’s a combination of the two and leave it there. I’m no engineer.
The way l see it is aircraft extend leading edge flaps on their wings at takeoff which have the effect of thickening them. A thick wing generates more lift. At V1 the air can't get out of the way quickly enough and gets bounced upwards but immediately comes down again with the weight of the atmosphere pushing down on it. This has the effect of creating a vacuum above the wing otherwise known as a shock wave. But vacuums have to go where vacuums belong, to the top of the atmosphere (equilibrium law). The atmosphere, all trillions of tons of it, detect a vacuum and push it upwards. But practically it's only the air around the plane which does the job. The A380 weighs 560 tons app. at take-off and flies! At higher speeds say north of 400 knots or so the shock wave has difficulty keeping up with the plane and tends to drag back on it. The engineers get around this by angling the wing back into the shock wave reducing its drag effect. This 'sweep' angle gives an idea of the operational speed of the plane. The bang one hears when the plane breaks the sound barrier is the shock wave, no longer able to keep up with the plane, collapsing explosively. I've seen these shock waves above planes' wings landing at airports as condensation formed along them. In fact, you could say aircraft fly by using the earth's gravity, reversing its effect of a downward force to one of lift. Aircraft fly by gravity.
Please explain how an airplane can fly upside down. If Bernoulli's principle and the other principles described here are the chief methods of lift, then an airplane flying inverted would not only tend to be forced earthward due to gravity, but also be driven downward by the "opposite lift". It also fails to explain how wings with little or no camber can also develop lift. I would submit that while Bernoulli's Principle aids in creating lift, it is not the primary source.
I have a question, what if, during a stall, the whole upper part of the wing skin lifted to help the air stick to its surface and avoid vortices that way ? The skin would lift similarly to the spoiler, maintaining its profile of course, but the center of rotation would be close to the leading edge. It would require a redesign of the wing, sure but it might increase safety in this dangerous flight regime. What are your thoughts ?
I think the cons of building such a system outweigh the benefits it could provide. To have an "active" wing surface would probably require mechanisms to move the surface, extra probes and computers to monitor airflow, would require people with incredibly specialized knowledge of aerodynamics to design such a system, would be expensive to equip/retrofit aircraft with such a system and there would be many variants required to fit many different aircraft types. I like the idea but I just think it'd be too complex and expensive to pursue considering you can achieve the same result with current stall warning systems and pilot training which are already established.
Doofer911 Well, it would require a new wing and probably a new aircraft (just like slats, flaps and spoilers were first incorporated), maybe it would be possible to make it deploy on its own depending on aoa and airflow, something like me109 slats :) ? Im not saying it could be applied without some amount of resources :) still if it could save someone from stalling close to the ground, where there is not much time to catch the plane or even pull the chute (design of which probably took even more costs and fitting it on the plane) then maybe it would be worth trying out :)
Doofer911 Plus pilots, especially weekend ones repeatedly fail to keep themselves out of harm's way, even despite warnings or aoa measuring. Some physical help could go a long way :)
I do like the idea and I completely agree that anything that can improve the safety of aircraft should be studied and implemented where possible. The question that springs to my mind is "How much of an improvement would an active wing surface to Lift performance?"... for example, a Cessna 172 in a clean configuration (Flaps Up) will stall at 53 knots. If a Cessna 172 fitted with Active Wings stalls at 50 knots.... yes it's an improvement but is that improvement worth the cost of development, design etc. There will be a critical point for any aircraft where the airspeed is just too low for the weight of the plane and it will stall anyway. Yes I agree that weekend pilots are more prone to danger but you have to ask if these pilots take their position as a person in control of an aircraft seriously? Stall recognition and avoidance is one of the first aspects of flying that a student pilot will learn. Heck, I've only done a couple of Trial Flying Lessons at my local flight school and I've even been taught Stalls on those! If a Private Pilot approaches flying with a cavalier attitude then they won't be on their toes, they won't be in full control of the aircraft and they will put themselves and any passengers in danger. It's the same when you have the driver of a car, the Captain of a boat, a firearms instructor at a gun range.... any profession where safety is of the upmost concern, it depends on the person, not the vehicle.
Doofer911 It would all come to the cost/benefit analysis, i agree completely, i can't forsee how much would such a system allow, it would all come down to tests, it may give just a few extra knots, or it may allow the plane to almost hang by the prop (almost becouse that would require sheer power to accomplish), or it may completely disturb airflow in some way i didn't think of and stall it earlier, i can't tell :) still, all such devices started somewhere, right ? :) Maybe it would work better on bigger planes with faster stall speeds, maybe the opposite would be true. At the end of the day it all comes down to the ability of the pilot to asses risk plus his own skill and then use the tools provided to him to lower the danger to an acceptable level. All of that dependent on training and its quality plus common sense. Im feeling though that with time flying may be perfected to a level of casuality similar to driving, is that good or bad...hmm, should we strive for that or not ? ...maybe ? :) Thank you for your detailed analysis and for your videos, i learned much from both :)
None of the explanations I Had heard made any sense to me until now. Great job done 5 years ago!!!
The only way in which the wing surface can feel forces is either through tangential or normal forces, with tangential forces being viscous shear forces, and normal forces being pressure. The air cannot impart force on the wing surface accept through these means. What this means is that a pressure difference IS the reason a wing generates lift. Experimentation shows clearly that an airfoil shape creates differences in pressure all along the surface and when you integrate the pressure over the whole curve of the airfoil shape, the net force has some component in the normal direction to the freestream, and some component parallel to the freestream, with the normal component being what we call lift. The net result of causing this pressure difference is a turning of the flow, since the only way the flow can be turned is through the pressure gradients within it. So, from a mathematical standpoint, you are correct, the flow is turned by an amount exactly required to generate the amount of lift that is generated, but to say that lift is not actually caused by a pressure difference is incorrect, because again, the only way the wing surface can feel force is through the pressure and viscous forces acting on it directly (viscous forces barely contribute to lift, but rather contribute mostly to drag). The turning of the flow is just another result of the pressure differences within the flow field. The reason, then, that the Equal Transit Time theory is wrong, is not because it explains that pressure differences cause lift, but instead because of HOW it explains the pressure difference is created in the first place, which is admittedly much more complicated than can be explained without some very complicated math and physical principles. Equal Transit theory is not based on Bernoulli's Principle, it just uses Bernoulli's Principle to explain the result in change of pressure from the false premise it starts with. Don't equate the Equal Transit time fallacy with Bernoulli's Principle because there are plenty of valid uses of Bernoulli's equation.
You are completely right. Very good point.
Very well said.
correct! F1 cars use a diffuser for that reason, speeding up airflow beneath the vehicle and creating low pressure air underneath, effectively sucking the car to the ground.
Bravo !
Good comment, but I'm surprised you didn't comment on the completely false and incorrect graphic shown in the video at 2:50. That downdraft shows something that wind tunnels never show for such an angle of attack. Case in point, for planes cruising at constant altitude there is *no* net vertical momentum and therefore *no* net downdraft. Instead there is displacement up and down and then relaxation back to original level. Very disappointing that engineers keep repeating this mistake that flight is due to air mass being "pushed down". It's true only when a plane is ascending or rapidly accelerating.qph.fs.quoracdn.net/main-qimg-cdba626b7a727d8315c1514104ab99bb
5:51 Spoiler alert!!
Brilliant!
@Harshit Singh Negi ok
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hahahahaha
🤣🤣🤣🤣😂🤣 I got lol!!
Who says the separated air must meet at the trailing edge at the same time? Bernoulli didn't say that! Definitely Bernoulli's principle is true, but everyone is using this principle based on a wrong premise.
Thats it! Yea, why the heck should separated air meet at the trailing edge at the same time??
Thats to make the air wind in the same pattern, (laminar flow). If the air wind doesnt meet at the same time the air will become (turbulent flow) which will make the air behind not good as it was. Making it impossible for other planes to travel at the same direction after a while.
@@OMGitsAFrEaKDuCk If you look at the movies about flow actually using a fluid, you will see that the fluid does not have to meet at the same time.
they will not meet at the same place.
Thanks for helping me out bro❤️
A basic symmetrical airfoil shape is a flat sheet wing with a shape added around it to allow thickness for spars, and to improve aerodynamics at different angles of attack.
A flat bottom airfoil is really a cambered sheet wing with an airfoil added around it. So if you look at the centerline of that flat bottom airfoil, you can see that when the flat bottom is parallel with the direction of travel, the wing itself really has a slight positive angle of attack.
Hence lift at what seems like a neutral angle of attack, but which is really slightly positive.
Maule aircraft have a flat bottom wing, the flaps can be set to negative angle to improve cruise.
Other than that pretty horrible aircraft to fly in tricycle gear form.
3:38 unless I'm mistaken a wing in stall still produces lift, it's just that it's overcome by drag so not very practical, and uncontrollable due to no flow over control surfaces.
Maximum lift, in fact. Too much for the other forces
well it kinda just becomes a wall so idk
Beautifully explained. I wish this video series was around when I first found this game about ten yrs ago
I just made an airplane using this and it worked! Thanks bra
You missed a step by saying the turning of the flow is what causes lift, turning the flow creates regions of high and low pressure simply due to the flow being directed away from it's "intended/original" path. These differences in pressure is what creates the Lift force, or more specifically: the surface integral of all the vectors normal to the surface multiplied by their associated pressure value over the entire surface area of the body (airfoil in our case). Creates a resultant force, of which the Y component is LIFT and the x component is DRAG.
What this means in practice is that there are some vectors pointing upwards(ish) above the airfoil which are "sucking" it up into the rest of the atmospheric pressure field, and some areas below where it is being "pushed" upwards away from the atmospheric pressure field. (The front of the airfoil actually tends to be pushed downwards in the majority of cambered airfoils, due to the region of high pressure at the leading edge)
For anyone who's actually read this I'll assume you know at least a little about aerodynamics, so if I've made a mistake, feel free to correct me, I am only a 3rd year student lol.
His explanation was simple and made for the average RUclipsr. Who knows nothing about lift, drag, and much more.
Not good to give TMI to people that just want the basics. Notice how he didn't get into ailerons and about nine other basic things I believe not to confuse people.
To delay a stall you can also use vortex generators on top surface of wing. Vortices tend to "stick" to the wing better than regular air, so the separation occurs at higher AoA.
exactly, this is the reason for the F18's LERX, the low energy / low speed turns it makes is insane for that reason
@@Wrtvrxgvcf55 The F-18 uses a different principle. It uses vortices, which can be created at high angles of attack by wings with more than 45 degrees of sweep. Vortices are not the same as generic turbulent flow. They are their own phenomena. This is also used by most true delta winged aircraft.
@@米空軍パイロット that is correct! and the F18's LERX creates vortices! it was specifically added for that reason
How about stall strips?
@@HarwinSingh If I remember correctly, stall strips are intended for the opposite purpose - to initiate flow separation earlier. You place them on the root of the wing and you get pitch down instead of a spin when the aircraft is stalled.
Good video for the shortness of it. You are correct- exactly! Basically, the various "theories" of why wings work to create lift all say the same thing, but in different ways. The idea of alerting the airflow to create lift, is best explained with your explanation. Air has mass- quite a lot of mass, actually. Anytime you move mass, there is an equal and opposite force created. In order to get any aircraft off the ground, you must move (at velocity) a mass (or weight) of air equal to the mass (or weight) of the aircraft in the opposite direction in which you want the aircraft to move. Any given volume of air (a "slug") has specific weight. You can calculate what amount of force is being generated by centripetal force of moving that weight. So, basically, the wing lifts for the same reason a weight being slung around a center point get's "heavier"- the faster the weight is slung, the "heavier" it gets. the redirection of the air slugs by the wings angle of attack, eventually is going fast enough to literally force the wing upward against the weight holding it down. this is why even a flat, angular board held at an angle to the direction of flow will cause lift- the rectangular wing will fly (though with higher drag) without the curvature (I've proven this to the vast chagrin of "Bernoulli" adherents with RC airplane models. The advantage of a curved or "Bernoulli" wing is a reduction in drag AND and enhancement of more easily redirecting the airflow in a different and downward direction, thus causing more efficient lift. The fundamentalist Bernoulli people have a hard time explaining why properly curved wings can fly (still generate lift) when flying upside down...and don't even get them going on the symmetrical (curved the same on both sides of the wing surface) fly better, with less drag and more lift at higher speed than the traditional "Bernoulli" models - that is the "flat bottom" wings.
It is merely the re-direction of mass that causes airplane wings to create lift- that's the whole bottom line. All the other aspects of a wings design are for reducing drag while enhancing that re-direction of mass.
Excellent video. Excellent explanation of why wings create lift.
Thank you.Your explanation helped a lot
How long did u tyle this...?
Centrifugal or Centripetal force has nothing to do with an air wing generating lift. Other than that you are mostly correct that Newtons 3rd law of motion is what provides the lift. You are incorrect though to say the poster of the video is correct in that regard because that is not what he said. He said something completely different and believes a downdraft creates lift which makes no sense whatsoever.
@@lxdimension What SHOULD have he said?
What part did he say that was 'completely different'?
'Newton's 3rd law of motion' doesn't make planes fly.. More words, please.
(Seen Kooper, above?)
@@lxdimension I agree re: centripetal force, that made no sense to me at all.
However, in terms of the video, what do you think a "downdraft" is, if it isn't literally the "opposite" aspect of Newton's 3rd law? The air has shifted direction, due to a force being applied to it by the wing moving through it, there's an opposite force... hello lift!
Can you please make a video on how gyros work, particularly the way an aircraft's gyroscopic instrument works. Thanks
Please tell me what you have learned in your journey
I don’t get why most descriptions focus on the low pressure above the wing, instead of the high pressure beneath the wing. The high pressure is what creates the lift after all.
Not really, its the relative difference between the top and the bottom of the wing. There is no entity called "high pressure" under the wing. There wouldnt be any difference between lowering the pressure above the wing or increasing the pressure below it.
Thanks I needed this. I didn't even know what aerofoils were before this and now I understand Bernoulli's principle over it. That's some big brain time
Amazing how people are not satisfied with a simple answer to a complicated situation there are many factors involved in flight I think you did a great job and just pulling up a couple of basics unfortunately there are those who have more questions instead of bashing you they should do research on the internet that is in their hands if they really want to know
Thank you for the great explanation on the topic. I liked how you were able to debunk some of the common misconceptions generated by lift and gave a more correct answer. I also liked how you also mention some of the devices used to help lift such as flaps.
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 only video about lift that actually makes sense
I AGREE, i finally understood lift now
While it is true that the flow tends to remain attached to the surface of the airfoil, it is not because of the Coanda effect. This effect specifically applies to jet flow, which an airfoil would not experience. The equal transit time theory is wrong as you stated, but the relative pressure fields between the top and bottom surfaces of the wing would play a role in generating lift. Pressure and velocity changes are coupled when looking at a flow like you displayed, meaning they change continuously with each other. This is a big misconception when it comes to explaining lift.
Coanda Effect and flow-attachment involve the same physics. (Simply make the diameter of the jet be wider, and the connection becomes obvious.) Attacking the term "Coanda Effect" is misguided, smacking of one-upmanship, rather than pointing out a significant error. Yes, the proper term here is Flow Attachment. But Coanda Effect is an example of flow-attachment.
If you believe that the two of them employ two different kinds of physics, then you need to explain the details.
Coanda Effect becomes Flow-Attachment in the limit of increasing the jet-diameter.
In 1999 I saw the very first use of this same misguided attack, this "it's not Coanda Effect."
The person using it insisted that Coanda Effect only involved liquid jets in air, such as water flowing over the back of a teaspoon. Why would he say this? It was quite obviously a well-poisoning fallacy. He had just been caught (in a very public situation) believing and supporting "transit-time fallacy," Rather than publicly admitting error, was trying to avoid embarrassment, by attacking his attackers, pointing out their "huge error," suggesting that they weren't competent, because Coanda Effect only involved water-jets ...the tea sticking to the teapot spout, etc.
Henri Coanda didn't discover his named effect by launching any water-jets. The phenomenon appeared with air-jets coming too close to a parallel fuselage surface, where they'd unexpectedly exhibit flow-attachment. (But how wide must the "jet" become, before it's simply flow-attachment, and not Coanda Effect? Answer that, and you'll have a perhaps valid point.)
Whilst an irritating voice commentates, this is the clearest, simplest and easiest, (apologies to Easy Jet), to understand of the seven I've read today - Thank You
First off I enjoyed your visual representation of how flow goes around the airfoil. You were mentioning Newton's third law when you said the airfoil turns the fluid flow downward applying equal and opposite forces lifting the airfoil, which is a part of how lift works. However when flow is curved there is a pressure gradient. This gradient forms a pressure and velocity field around the airfoil that affects a wide area in the fluid. As you said the pressure above is lower and the velocity is faster, while below the pressure is high and the velocity is low. The pressure and velocity affect each other simultaneously and the pressure difference drives the lift force as well. Also the Coanda effect only applies to jet flow and not for ordinary flow airfoils are usually in, so it is not a factor in lift.
I think both are correct. One is using the conservation of momentum. One is conservation of energy.
This "correct" explanation was given in 1944 (possibly earlier?) by Wolfgang Langewiesche in his book "Stick and Rudder" and rubbished (at the time). It is now known that BOTH principles are involved, it's just that the Bernoulli principle was incorrect regarding the speed at which air travels over the top of the airfoil..
The basic mistake is confusing cause with effect. as far as attributing increased airflow speed, lift and the Bernoulli effect over the upper surface.
Bernoulli Principle is not incorrect. Bernoulli Principle says nothing about the velocity profile of two adjacent streamlines, it strictly deals with flow along a single streamline.
Indulge an old fitter please. When I was a kid more than six five years ago, I built a flew model aircraft with completely flat wings. as long as these were angled upwards against the line of thrust, the models achieved lift. I figured it was like a boats rudder. Barry
Yup, that angle upward does the same thing as an aero foil shape to create lift!
1:56 Correct, The wing is indeed turning the airflow downward.
But there is also a pressure difference between the top and bottom of the wing!
The lift force is literally equal to the average pressure difference between. The bottom and top of the wing times the surface area of the wing!
And it is in fact the pressure gradient formed arround the wing that turns the airflow downward!
Please correct your mistake!
Well the actual force of the lift is created by the pressure difference between the upper and lower side of the wing. The air directed downwards is basically just the result of that.
Most simply explained by newtons first law that an action must have an equal oppsite reaction.
We push the plane up so the air must get pushed down as a result.
Seeking causality makes everything complicated. Ultimately, lift requires a pressure difference and requires a change in momentum, and it's hard to separate those.
@@AmbientMorality Pressure preempts movement. Its a state that begets a force, which in turn begets movement.
Confusing the direction of this causal relationship is probably because the bernaulli principle is also sometimes involved, in which a change in fluid velocity begets a change in its pressure.
That is not the mechamism, however, by which air compressed by a wing at high AoA gains its momentum.
@@ineednochannelyoutube5384 The pressure difference created around an airfoil is entirely due to movement though?
@@AmbientMorality Correct. Horizontal movement of the aerofoil relative to the medium compresses the medium (or indeed vica versa), which in turn creates the vertical air movement.
Neither movement however acts as a vertical force on the wing.
1:50 I see downwards. What if your a p51 inverted over Germany will you suddenly drop to the ground?
As an aerospace engineer, this is a useful explanation for the average person to understand what creates lift, but we can’t totally disregard the fact that there is a pressure difference. I disagree with you when you say that a pressure difference is not what creates lift. In fact, there are many times where we can actually compute the lift force (or lift coefficient) on an airfoil WITH the pressure distribution and using calculus to integrate the difference in pressure acting on the top and bottom. At the end end of the day, there are two natural sources that cause for all aerodynamic forces and they are none other than the pressure distribution and the viscous shear stress. I’m not saying you’re wrong for thinking this (although I am saying you’re wrong for saying the pressure theory is wrong), but I think they go hand in hand. You can’t have one without the other. To say one idea is more correct than the other would be ignorance. I appreciate the video though and it helped me deepen my understanding a bit because I had never heard it explained this way. Thanks! 👍🏼
I hate to argue with an aerospace engineer, and I'm also 4 years late to this party, but I do have some questions.
First, take a situation where you are in a moving car and you put your hand out of the window. Where there is no AoA, there is no lift generated. There is some drag, but it's manageable. However, if you turn your hand into a slightly positive AoA, you simultaneously feel a substantial increase in drag and your hand flies upwards. If you're not prepared, it basically goes up and back as both drag and lift are generated.
When I started considering lift, it seemed easy. I thought - "this has got to be because of the equal and opposite reaction of dumping flowing air downward, changing its direction by providing a force that changes its vector". Newton's 3rd law - the same force that your hand is applying to the airflow to change its direction (created by the car's engine and your muscles pushing your hand against the pressure of the airflow to overcome drag) is also applied via to your hand, causing it to want to go up. The effect is so strong and so instantaneous that I found it extremely difficult to believe that it can be caused by the development of a pressure differential above and below the "wing" of your hand. But that feeling is not scientific in nature. After all, I caution myself that an explosive decompression event causes substantial forces very quickly. And the moment you unscrew a bottle which contained a carbonated drink, the top wants to fly off as the pressure tries to equalise with the room you're in. It doesn't have to "build"... so pressure differentials can create forces quickly.
So I guess the challenge in answering this question to everyone's satisfaction is that it seems hard to imagine circumstances where you can impart downward motion to air meeting an aerofoil where you're not also creating a pressure differential below and above the wing, so it feels like the Newtonian approach and one based on pressure differences seem to "come up with the same answer" and everyone is shouting about which one is right, based on the answer that feels better to them. I don't know how to bridge the gap and I don't have enough scientific understanding to try and do so - I fly planes rather than design them. But I do know that explanations of lift forces have been plagued by bad and poorly explained answers for a long, long time - it's remarkable to me that we can go to the moon, explore the wreck of the Titanic, do AI facial recognition, connect all the computers around the world, search deep space for answers to whether we're alone and image amazing parts of the universe the naked eye can't see.. to say nothing of achieving net positive energy fusion reaction. Yet a single unified and sensible theory of aerodynamic lift continues to be a challenge to describe in a way that people accept and understand.
(and yes, everyone thinks their answer is right - I am not inviting more of that, just saying that there do still seem to be challenges in describing the lift forces).
However, I found this quote useful, from good old Wikipedia...
"A serious flaw common to all the Bernoulli-based explanations is that they imply that a speed difference can arise from causes other than a pressure difference, and that the speed difference then leads to a pressure difference, by Bernoulli's principle. This implied one-way causation is a misconception. The real relationship between pressure and flow speed is a mutual interaction."
Yes. That feels right. So I think we are saying that when the air flows around (meets) the wing, all kinds of pressure differentials are created in the local atmosphere of the wing, the net result of which is upward lift. The laws of conservation still apply (it's not an either / or) because the net overall force lifting the wing is equal to, and opposite, the net overall force exerted downward on the rest of the localised atmosphere. It's just that the net overall forces are created by / are a symptom of the pressure differential.
@@cjad100 Woah, I barely remember even making this comment haha! It’s been a while so I had to rewatch the video. The main problem with the video is that he is marrying the “equal transit time” (an incorrect theory) with Bernoulli’s principle and dismisses the pressure difference idea because the equal transit theory is wrong. Newton’s law can be an explanation for how lift is created. Bernoulli’s principle and Newton’s laws suffice to explain what creates lift. But at the end of the day, you are absolutely correct. We actually do not fundamentally understand why there is pressure difference. We know the pressure decreases on the top of the wing and the pressure increases on the bottom, due to a lower velocity. However, we cannot answer the question of “Does the pressure decrease because the flow increases, or does the flow increase because the pressure decreases?” It’s basically the chicken or the egg debate.
The equal transit theory is false. Lift creation is very complex but it’s mainly caused by the pressure differential between the top and bottom of the wing and the downward deflection at the trailing edge. So it’s a combination of Bernoulli’s principle and the third law of motion.
Wish I found this earlier, much more clear than all the other explanations I heard so far.
This was perfect timing of a video! I start air cadets for the first time on Monday, and due to my age being a bit high for air cadets, they said I will need to learn why planes fly and don't sink before everyone else joins, so I can catch up and be ahead of everyone.
Ah nice one, all the best for the Cadets!
So clear and understandable
I did not understand why the airflow on the upper surface speeds up?
Very good presentation. Only one thing I didn't get. Why would the air passing under the wing slow down? There is surely nothing impeding its flow?
Some fighter aircraft have a symmetric wing shape, same on top and
on the bottom. They can fly upright or upside-down. Their wing
is an inclined plain to the airflow that produces lift. The angle of attack
is very critical. For the traditional wing shape, the angle of attack
is much less critical. QED
You are correct in identifying the Equal Transit Time fallacy, but there are other parts of that theory that still hold true. There is still a region of low pressure above the airfoil and high pressure below, and Bernoulli's Principle still holds true along a streamline. Lift is a combination of the pressure and velocity field coupling as well as Newton's conservation of momentum law, but I feel as if you dismissed the former a little too much in the video. I liked the additional information about flaps, slats, and spoilers as well as the importance of angle of attack.
He said that the pressure difference still exists, but that it works at least partially in tandem with Newton’s laws. He implied that it was both without explicitly saying it.
THANK YOU for confirming my belief that Bernoulli is BS in terms of its overall impact on lift. Sure there is some, but that's NOT the major source of the lift! BTW, there are a lot of folks saying the lift for DRONES comes from Bernoulli's Principle...even though the propeller blades are virtually flat. I point them to ceiling fans and ask them to explain THEM! LOL. Anyways, thanks for the clear and no BS video.
If it's not the pressure difference that causes the lift how can we numerically calculate it? Are there any methods to calculate lift then
Other than pressure difference and AoA, i never realized that the airflow above the wing goes down to generate lift! great vid to understand the basics of the wings.
+Thanatos if lift was explained only by reaction forces from the deflection of incoming air, then only planes with a thrust-to-weight ratio above 1.0 could take off.
Also, once the upper surface flow detaches from the wing the airfoil becomes stalled. Which is why fighter jets implement Leading Edge Root Extensions (LERX), slats, chines, etc to preserve the upper flow of air during high AoA maneuvers.
History of aircraft design itself debunks the theory that airfoils only generate lift through deflection. Your hand doesn't try to fly up in a moving car because it's not a well designed airfoil. If you designed an airfoil just the right size for your hand and which could generate lift at low speeds, you'd feel it pull up when outside a car window.
I'm sure Thanatos is an aerodynamics expert, and all these other people who build and design planes are just cretins compared to his massive intellect. He called it, you guys. We're done here. From now on, all planes will be designed with giant hands on the sides.
I'm confused. This video tells us that it's not the pressure difference, but the deflected air that creates the lift (according to Newton's principle of action and reaction, I'd assume). Another video I've just watched tells me that it is mainly the pressure difference according to Bernoulli that creates lift, while the deflected air just creates a small percentage of the lift. Whitch is it? Or is that question still the object of brawls between physicists?
An oblique hydrofoil dragged slowly through liquid helium will not generate any transverse force. Fluid flows round the trailing edge of the aerofoil and doubles back towards the rear stagnation point. As the hydrofoil speeds up, we are putting in enough energy to warm up the helium and destroy superfluidity. Then all the vorticity is concentrated in boundary layers around the hydrofoil. As the fluid warms up, a thickening boundary layer is unable to negotiate the trailing edge and so vorticity is dumped as a starting vortex in the flow. This establishes a Kutta condition at the trailing edge, and there is residual circulation around the hydrofoil opposite in sense to the starting vortex, which generates a transverse force. This can be explained in terms of Bernoulli's Principle as well once things get going, but the formation of the starting vortex is the thing to look at. It won't happen without viscosity.
Air traveling below the wing gets slow down??? Why? Is there anything below the wing slowing down the air?
Thanks for debunking the common misconception about wings
in 1:23 how we know that pressure above wing is less than pressure beneath the wing if Bernouli's equation is applied in the same streamline and not 2 different ?
Thx for the vid! But I still don't get it why the upper stream just speeds up by itself oO can it be that it travels with its normal speed whereas the lower stream is being slowed by the curvature of the wing?
Valid question! If the upper stream moves faster due to the wing curvature, how then does an inverted plane still fly?
@@yezam8608 Angle of attack, which in any evwnt provides the greater part of lift.
There are no airstreams in flight. Air is mostly stationary, and the wing is moving through it, forcing the airmolecules apart in a mostly vertical movement pattern.
When teaching how to sail we often refer to this theory to show how a sail works. ie, the lower air pressure on the curved side of the sail allows the higher pressure on the concave side to drive the boat forward. This is counteracted by the keel/centreboard so that the boat does not simply sail sideways or tip flat on its side. Is it in fact the change of angle of the air coming off the back of the sail which creates " sideways lift" or is it the difference in air pressure?
Both. Differential pressure in a fluid (or gaseous) medium is not stable. The existance of a pressure difference necessitates the movement of fluid to equalise the pressure within the system in accordance with the 2nd law of thermodynamics.
Similarly, due to the conservation of momentum, and newtons 3rd law it is necessear, that the force of the airpressure pishing the ail forward have an equal and opposite counterforce upon the air, moving it back, bringing the sum of forces and moments within the system back to 0.
Which is the major factor of lift? Pressure difference or Reaction force by air on the wing.. pls reply...
3:45 So if the airflow on the top of the wing stops there will be no lift generated as you say.
But earlier you said that top of the wing airflow was not what generated lift. So I am confused now!
Thank you for the excellent explanation on a very important topic in aerodynamics! I like how you started your explanation of how lift is generated by making a simple claim and then proceeding to debunk the most common misconception of the generation of lift that has been spreading in aerodynamic communities. I also found your explanations of how the angle of attack and number of flaps on aircrafts affect the lift on a wing to be very helpful. Overall, you did a great job of explaining the production of lift!
So the Falps in 3:21 is used to take off at slower speed to reduce the amount of consumed fuel??
No, Flaps are used to increase lift at lower speeds, they also create more drag so more power is needed to maintain a set speed so fuel consumption actually increases. The flaps are used to get off the ground sooner, or to reduce the speed to be within a safe limit for landing.
Most incredible invention
How do you know that air is splitting at joining eachother at the same time? Any law or theory?
Why does the stall configuration cause the aircraft to fall? You’d think having really low pressure on top and same amount of high pressure on the bottom would be better?
In a stall air stops flowing smoothly over the top surface - higher angles of attack lead to larger pressure gradients that try to force airflow to separate. The airflow after that separation point is recirculating, low velocity air so it’s no longer very low pressure
You know, upon revisiting this video now, i have a few more thoughts:
There are wings that are basically flat, wings that have a symmetrical profile (meaning the upper and lower part of the profile is the same) and also airplanes can fly upside down, against the profile and just through angle of attack. Airplanes, even at their max speed, fly with some amount of aoa and that in my mind is responsible for the lift, the air that hits the wing from below, pushing it up and itself being turned downward.
To me, the upper part of the profile is there for getting the most out of the wing for a specific type of flying by reducing the moment of air unstick and the onset of turbulent air that would start hitting the wing from above, pushing it down, all for the airspeed and aoa range at which the wing will spend most of its time in, meaning the cruise phase. Plus it conveniently gives space inside the wing for fuel and actuation. To me, its all about the angle of the wing, if it becomes too steep, the air starts to deflect in such a way that it mostly pushes the wing backwards instead of upwards, slowing the plane down until flight is impossible. Slats and flaps give you more air being turned and more sharply while maintaining lower aoa and further reduce the onset of unstick of air (slats)
The air being turned down on the upper surface doesnt seem to do anything to push the wing up, also, that air meets the lower profile air behind the trailing edge of the wing, so there would be no influence on the wing itself, those things happen behind it. In my mind there is always too much propensity for attributing lift genereation to the upper profile, its there to lessen air unstick and turbulence at aoa's by giving the air there a more gradual gradient of travel, in my mind.
Plus, one could argue that at low aoa's and with wings with sharp, bulbous profiles, the air that meets the leading edge on the upper surface would be deflecting upwards, pushing the wing slightly down, thats why those wings often fly very slow and at higher aoa's to negate that effect and just have that lesser propensity for unstick.
What would be your thoughts ?
The air accelerates very rapidly over the upper surface and gets a very low pressure, so it's responsible for a ton of the lift. You're right that it has to meet the air at the trailing edge of the wing, which is at a much higher pressure. The fact that it's flowing from low to high pressure is why it's susceptible to stalling.
You're correct that the upper surface near the leading edge produces slightly negative lift at low angle of attack. But higher angle of attack still makes the pressure gradient on the upper surface worse.
@@AmbientMorality Hmm...If what you say is true, then the wing could basically generate lift at 0 aoa ? Just through the profile ? And yet there are airplanes whose wing profiles are basically flat and/or symmetrical, made for high speed and low drag. They fly just fine too, as well as other examples i mentioned, all of them fly at some aoa even at max speeds. Hmm...the fact that air over the wing is of the lower pressure than even the air that sits above that air could perhaps aid the air under the wing in its pushing of the wing up, indirectly ? There is overally less mass "resting" on the wing from above, compared to a stationary wing, you could say ? Thats how i would imagine this at least ? But i'd say that its more of a fact of the wing itself just being a barrier for the ram air, creating that lesser pressure on the upper side and the profile being there to ease the flow of that lower pressure air, so that it doesn't unstick too early thanks to a more gradual gradient and start hitting the wing turbulently from above, at least thats how im imagining this X).
Also, would you say that the lesser pressure air right over the wing accelerates, becouse of the mass of undisturbed air resting on top of it, pushing on it with its own mass ?
@@vedymin1 A cambered wing could generate lift at zero angle of attack, and a symmetric wing would generate lift on some parts but it'd be cancelled out exactly by the other side.
The barrier explanation doesn't quite work in my mind because if the flow separates, the pressure on the top surface of the wing increases to roughly atmospheric pressure. The barrier analogy suggests that the wing is just blocking the air, which isn't possible because air will fill a space faster than a wing will move.
I'd say the lower pressure is because the airfoil curves and the air follows that curvature: curvature means that the pressure near the wall must decrease (essentially, centrifugal force). Since nothing's actually adding energy to the flow, if the pressure decreases near the wall then the velocity must increase because energy is conserved. Therefore, you get acceleration.
Im gonna have to think about what you told me, specifically the ability of a cambered wing profile at zero aoa to still produce lift, trying to visualise what all the air particles do at all times in all parts of the wing is a challenge to say the least, more learnin required i guess xD.
If i place the back of a spoon in flowing tap water, the spoon is drawn into the water.
What if now all that could be placed inside a vacuum jar, would the spoon still deflect?
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 Bernoulli/Newton dispute is a false dichotomy. They are simply different ways of describing the same phenomonem. Newtonian mechanics is arguably the more fundamental explanation, but the proximal cause of the lift (the only thing the wing feels) is the pressure differential that is a resultant of that Newtonian action/reaction. And that pressure drop is known as the Bernoulli effect, although Newton himself had observed the effect in tennis balls and concluded correctly (of course,) that it was due to action/reaction with the passing air.
So, probably go with Newton since he was a couple of centuries earlier than Bernoulli.
As a flying aerobatics pilot I have 2 questions:
1) I fly Extra 300 and Extra 330. How the hell am I flying if these planes have... symmetrical airfoils?
2) I also fly RV-7. How am I able to fly inverted over runway and not smash into it?
Those airfoils are at an angle of attack, thus generate lift. Symmetric airfoil just means it doesn't generate lift at zero angle of attack
Good start by debunking one of the common myths about lift generation. Unfortunately I think too much was thrown out with that theory. Pressure does indeed play a part in lift generation, but it's only part of the larger puzzle. To say that the pressure difference doesn't create lift is incorrect. The Coanda Effect also only applies to fluid jet flows, which is not descriptive of a fluid flowing over an airfoil. The turning of the airflow, or momentum, theory is definitely one of the ways that lift is generated. Circulation is a more complex concept, but including it would have resulted in a much more complete explanation of what causes lift.
Fun Video!! Thanks for posting! Question?.....I know nothing about nothing (no degree, not in the field), but in general it seems like Bernoulli is more accurate. For someone who knows maybe explain for me....if the opposite of Bernoulli is true and the air going over the wing is hitting the air going under the wind and firing downward....isn't ALL of that taking place AFTER the wing has left that pocket of air? If so how can that downward collision have effect on the wing that is front of it? Isn't it Bernoulli's pressure difference that form the "clouds" we sometime see around wings?
I fully agree with what's happening here, but I don't understand a stall. A stall happens when the air over the top of the wing separates from the wing itself, but how does this cause a stall if the the air moving over the wing has no relevance to the lift produced. Can anyone help?
I'm happy to explain further if that wasn't enough, thanks.
Lift is created by pushing air down, and thus the air pushing back upwards against you. The explanation in the video is incorrect. If the explanation in the video was correct, and inverted plane could not fly and would just fall straight out of the sky.
A stall is induced when the angle of attack is too great for the speed of the airplane. For example a aircraft that is powerful enough to climb at a 90 upward angle to the earth, will not stall until it reaches the point that the air has become too thin for the motor/engine to produce more thrust than the weight of the aircraft. That is basically how a helicopter works.
So if you have a large commercial jet fully loaded, it is extremely heavy, even with big engines, it is not a high performance aircraft, and it can not climb at as steep of an angle of attack as a small, light, fast plane. Think of it like a car driving up a hill, a sports car can drive up a really steep hill because its engine is powerful, and the car is lightweight, so it can drive up the hill with ease. But if a heavy vehicle with a less powerful engine tries, the hill may be too steep, and the vehicle will either stop, or roll/slide back down the hill.
So for every aircraft there is a speed+weight+wing area+altitude combination that will decide the limit of the angle of attack that it can continue flying level, or climbing.
Essentially if you increase the angle of attack too much, the bottom of the wing begins to face forward, and air is just running into the bottom of the wing. So now your wing is just pushing air forward instead of down. This will create way too much drag, no lift, and slow the aircraft.
I know this explanation is not very good or understandable but I hope it helps.
In a stall its because the push up under the wing, not the lift thats on top , becomes more and more of a push back. Lift is on the top, like a vacuum, and when air is no longer going over the top there is no vacuum..all you have is push back by the air underneath.
@@Rickie53 An aircraft stall and a wing stall are not the same. An aircraft can still fly with stalled wings, which specifically refers to the cessation of laminar flow of air around the wing.
Hi. I’m going to be doing some presentations for a paragliding club.
Is it possible to ask for a copy of your animations used in your video called how wings actually fly. Any help appreciated.
The lift generated by a wing is due to the pressure difference between the upper and lower surface.
The question is WHY is this pressure difference created.
Physical testing shows that the speed of the air over the top surface IS faster than the flow over the bottom surface.
There are mathamatical formulas for calulating lift that are accurate. Aircraft designers use them!
The amount of lift depends on the density of the air, the coefficient of lift (determined by the cross section shape of the airfoil), the angle of attack of the airfoil, the area of the wing and the speed of the wing through the air squared.
There are various sections that have been tested, the NACA sections and the coefficient of lift at various angles of attack and can been looked up for each section.
The "Downwash" theory sound convincing but it forgets the "Upwash" that occurrs in front of the wing as the air rises ahead of the wing to enter the low pressure area above the wing.
The "Creation of lift" is obviously far more complicated than a simple bernoulli or downward deflection explanation.
The fact is lift IS created by a combination of the factors listed above and explanation involve deflection of air and the speeding up of air over the top and slowing of the air on the bottom.
The pressure differences have been maesured and the force from the lower pressure on top is greater than the force from the increased pressure on the bottom.
So the statement that lift is not reliant on the pressure differences above and below the wing is clearly NOT correxct.
I still don't understand why it creates lift. As you explained that downward airflow happening after it left wings. So it should be no affect right? Air should lift aircraft bottom of its wing right?
The wing creates a pressure difference between the upper and lower surface. That multiplied by wing area gives you the lift. But the air itself is deflected into a new direction by this interaction resulting in a downwash behind the wing. Before the passage of the wing the air has no vertical velocity. But afterwards it does. In principle, if you knew what this new air velocity was, times the mass flow rate of the air involved, you would have the rate of change of momentum of the air. Forrce = rate of change of momentum, so what you would have calculated is the equal and opposite force to that on the wing. So, the pressure of the air physically in contact with the wing, and the momentum change (or deflection) of the air, offer two alternative theoretical methods to calculate lift. But we are talking about contact forces here - no 'action at a distance' - so only the pressure of the air actually in contact with the wing is physically lifting it.
hey doofer, I'm a huge fan, yes, the equal transit theory is totally incorrect, but you've missed a few things out here
1. the camber on an airfoil produces lift mainly by the curvature of the streamline and the euler equation, When a streamline is curved, the air loses its pressure
2. Another thing to take into account is streamtubes, as the air meets the cambered portion of the airfoil the "Streamtubes" are slightly squished, following the venturi theorum
Yes, Wings do indeed create lift by downwash too although
Edit: Also the downwash eventually levels out due to its inertia
I think I’m right in saying drag is a good thing caused by flaps and not a bad thing, as drag is the opposing force to thrust therefore the flaps are keeping the plane slow whilst still maintaining lift.
Excellent explanation. You gave a very simple and straightforward explanation.
If i have understood it is a mix between bernouli and newton?
Yes, both Principles partly explain how lift is generated but there's even more to it than is covered in this video.
This explanation has some issues. Bernoulli's principle and Newton's third law are both at work. Equal transit (which is separate from Bernoulli's principle) has been disproven, but airflow over the top does move faster and creates a lower pressure area above the airfoil while the air traveling beneath, moves slower creating a higher pressure area generating some lift.
7:00 you all keep saying the air is pushed down from the wing to give it lift. I assume when you invert your aircraft the shape of the wing dies not change. Does the air stop creating lift relative to the wing?
i dont think lift will stop, there will just be less of it due to the lift being generated only on the bottom
It's all Newton's Third Law - the mass of air pushed down offsets the mass of the aircraft. This is so obvious when you think about a rotary wing aircraft (ie. helicopter). It's very windy underneath there.
The airfoil is only important for maximising air deflection while minimising drag. It's not the upper surface separating that CAUSES the lift to fail, but the increase in DRAG which decreases the forward movement, thus impacting the air deflected downwards (ie. lift).
thank you keep it up it really helped me understand better than my lecture
hello mr. can i use your video for a presentation?, the thing is that i would make it in spanish, but with the original video. would you mind?
Yeah that's fine, as long as the video is only used on the presentation and not re-uploaded online on another RUclips channel or another website.
As often happens, the same phisical phenomena can be explained under differect aspects, all true. For example, I could explain that a pendulum works by the force of gravity slowing down and accelerating the ball back and forth, while you could explain it saying that it trades potential energy for kinetic energy ciclically. Both explanation would be valid.
While we all agree that the "same time" explanation is wrong, lift can indeed be explained (and calculated) by the pressure differential between lower and upper faces of the wing, which in turn are caused by profile shape and incidence. At the same time, you can also explain lift by saying that the wing pushes air downwards, creating a force upwards. Both are true at the same time.
What is the weight of airplane wings?
That depends on many factors.
The size and weight of the aeroplane whether it's a prop or jet whether it's made for one passenger or 300 this is something that is not a one word answer
Hi Doofer911, I think this video explains the topic really well. Could I copy your graphic at 0 min 55 sec for a RUclips video my son is making for a STEM science video on lift? I would be very grateful. Many thanks Eddie
Hi Eddie, yeah that's no problem at all :)
Such a great channel I found here. Subbed instantly.
Thank you for your insightful explanations :)
Welcome to the channel! :)
Better than any explanation i have ever seen. Thanks !
Dont forget lift is also produced simply by the air pressure on the bottom of the wing at non-zero angle of attack. This is how planes actually take off. I would argue that this component of lift primarily enables a plane to ascend and the airflow turning on the top of the wing enables straight ahead flight.
Lift on the upper surface is higher than the lift on the lower surface at all angles of attack for virtually all airfoils. The exception is during stall (or maybe for some really weird and likely ineffective airfoil), but hopefully the plane isn't taking off in a stall.
So, how do symmetrical wings work, and why can planes fly upside down?
Is the low air pressure on the upper surface of the airfoil due to the air density being lower on the top? Or or those really unrelated?
Yes and no. This is an often misunderstood discussion about compressablilty of air, And it's a minefield :-) The pressure difference is created by aircirculation over the top of the wing towards the trailing edge and under the wing towards the leading edge. As the wing moves forward the airflow adds to the speed of the air over the wing and substracts from the air under the wing (which was moving forward). so the speed difference creates different pressures and therefor lift. It's manipulating the airstream and since air has mass forces causing the lift are therefore present.
Please check your references, the air DOES NOT join at the SAME TIME at the trailing edge of the wing, “same time” is simply a totally WRONG statement.
He literally says that is the incorrect theory lol
@@stevenpagillo6933In the animations he used to demonstrate the correct theory, the air meets at the trailing edge of the wing
Great video! I could understand bernoullis principle but I would have trouble explaining to a student. This explanation sums up lift very very well.
Note that the center of lift is approx @ 25% of chord, if directing air downwards at the rear of wing was main cause of lift, center of lift would be at the back of the wing.
Ah I didn't know that.... since making this video I've learned that I haven't covered this topic in as much detail as needed and didn't give a full, comprehensive explanation so I will revisit this topic and remake this video in the future.
check out center of pressure glen research center, then images,
It seems weird to me that helicopter blade is angled up during autorotation, yet is gaining forward thrust as it falls thru the air, as is a glider wing.
+Observ45er I think the top skin produces double the lift of the bottom skin,, the air flow behind the most curved part stops being laminar, and kind of boils off the wing, precise point being determined by reynolds number
This video really helped me with understanding planes, thanks!
Aeroplanes, air planes, things that plane through the air. There is always an optimum planing angle in the cruise. Boats and surf boards do the same.
That was a great explanation, thank you!
I find it astounding that something so straightforward can create such controversy among the scientifically illiterate.
Very nice explanation. Thank you.
Great explanation. Bernouli's idea does not explain why symmetrical airfoils produce lift.
Thank you. Best explanation I have seen..!
We can talk about Bernoulli and other stuff, which no doubt plays a part in lift, but the main principle is Newton's 3rd law of motion, and downward pushing of air at the trailing edge of the wing.
How a wing generates lift is pretty much summarised in how wind gets pushed downward by the wing. The more wind gets pushed downward, whether more forcefully (greater airspeed) or at a steeper angle downward to a certain extent (greater angle of attack or use of flaps), the greater the lift. That's it, folks. Devices such as slats and flaps both aid that downwash of air by the wings, either by improving the flow of air over the wings such as use of slats (preventing detachment of air travelling over the wing), or by increasing the downward angle of air, deflecting it steeper down, to a certain extent.
sure, but it's also summarized in the pressure distribution directly on the wing itself!
@@AmbientMorality ...so you mean Bernoulli, then? If so, I'll refer back to my original post. There are simple wings that have no camber whatsoever (completely flat on the bottom AND top). And those still generate lift, albeit more inefficiently.
That proves without a doubt the primary principle that generates lift is newton's 3rd law of motion, through the downwash of air...
@@tristanhnl They're both true. A flat plate at an angle must also have a pressure distribution that can fully explain the lift. Similarly, a turbojet does accelerate air backward and that's perfectly valid to calculate lift, but you could also sum pressures on the compressor and turbine and some other components and get the same result. Both valid.
@@AmbientMorality , both true, but again...newton's 3rd law is the *big picture* phenomenon responsible for lift, in the grand scheme of things. If I were to describe how it's possible for tennis to be played, I can explain how the molecules of the ball interact with the molecules of the floor and the tennis racquets....or I can simply explain the fact that the ball bounces. Again, not saying Bernoulli isn't true.....why do I have the feeling this exchange can go on forever LoL
@@tristanhnl nono that's totally fair. sometimes people read it as "part of the lift is produced with pressure differences and part of it is from moving air down", when really it's just two different complete explanations for the same thing
Try throwing a frsibee upside down. Then try throwing a flat disk of plastic or wood and see if it flies like a frisbee. A frisbee has a curved upper surface and that is what makes wings fly.
Simple and better explanation😊🤘
Great man.... total confusion is cleared now ☺️
Thank you buddy 👍
Congratulations a precise and complete explanation.
At time 1:58 you state: "it's NOT a pressure difference that creates lift". This statement is WRONG. It IS the pressure difference that creates the largest part of a wing's lift. Of course the downward accelleration of the air mass (deflection of the airstream that passes around the wing in a downward direction) is also part of the lift that is generated by the wing. But if your "theory" of the wing that is not related to a pressure differential between the upper side and the lower side of the wing would be correct, you could minimize the wing area by having only very minimal chord length - a profile that only deflects the air downward. But the force of the lift is clearly the mean pressure differential over the entire wing area multiplied by the wing area. If the lift would not be caused by the pressure differential then increasing the wing area would not increase lift as long as chamber (downward bending of airstream) would not be increased. But any wind tunnel experiment will show you that a longer wing chord with the same chamber will result in a greater lift. I agree with you that there are many wrong theories about how a wing generates lift - but it seems that your video is also falling into that category.
Pressure differential and downward displacement of air are in a causative relationship and not separable.
A larger wing area will displace more airover a given time, and the effectiveness of wing length and width on lift varies with lreasure, airspeed and air viscosity.
Air on earth is generally not very viscous, and usual airspeeds are below the speed of sound, so while the pressure wave created will dissipate relatively quickly, the air will displace faster than the wing moves through it.
At speeds above the sound barrier you get the wing cutting through the medium faster than it can displace, and so a wider wing, allowing for a longer contact time becomes more drag efficient, whilst in a higher viscosity flud, such as say water, a much thinner foil suffices to produce similar lift.
To me, the explanation concerning Newton’s 3rd law always seemed more intuitive and easier to understand than Bernoulli’s. Especially observing how sharply the trailing edge of an airfoil tapers off like that. But in my simpleton brain, I just conclude that it’s a combination of the two and leave it there. I’m no engineer.
The way l see it is aircraft extend leading edge flaps on their wings at takeoff which have the effect of thickening them. A thick wing generates more lift. At V1 the air can't get out of the way quickly enough and gets bounced upwards but immediately comes down again with the weight of the atmosphere pushing down on it. This has the effect of creating a vacuum above the wing otherwise known as a shock wave. But vacuums have to go where vacuums belong, to the top of the atmosphere (equilibrium law). The atmosphere, all trillions of tons of it, detect a vacuum and push it upwards. But practically it's only the air around the plane which does the job. The A380 weighs 560 tons app. at take-off and flies! At higher speeds say north of 400 knots or so the shock wave has difficulty keeping up with the plane and tends to drag back on it. The engineers get around this by angling the wing back into the shock wave reducing its drag effect. This 'sweep' angle gives an idea of the operational speed of the plane. The bang one hears when the plane breaks the sound barrier is the shock wave, no longer able to keep up with the plane, collapsing explosively. I've seen these shock waves above planes' wings landing at airports as condensation formed along them. In fact, you could say aircraft fly by using the earth's gravity, reversing its effect of a downward force to one of lift. Aircraft fly by gravity.
Thanks. Great explanation!
Please explain how an airplane can fly upside down. If Bernoulli's principle and the other principles described here are the chief methods of lift, then an airplane flying inverted would not only tend to be forced earthward due to gravity, but also be driven downward by the "opposite lift". It also fails to explain how wings with little or no camber can also develop lift. I would submit that while Bernoulli's Principle aids in creating lift, it is not the primary source.
I have a question, what if, during a stall, the whole upper part of the wing skin lifted to help the air stick to its surface and avoid vortices that way ? The skin would lift similarly to the spoiler, maintaining its profile of course, but the center of rotation would be close to the leading edge. It would require a redesign of the wing, sure but it might increase safety in this dangerous flight regime. What are your thoughts ?
I think the cons of building such a system outweigh the benefits it could provide. To have an "active" wing surface would probably require mechanisms to move the surface, extra probes and computers to monitor airflow, would require people with incredibly specialized knowledge of aerodynamics to design such a system, would be expensive to equip/retrofit aircraft with such a system and there would be many variants required to fit many different aircraft types. I like the idea but I just think it'd be too complex and expensive to pursue considering you can achieve the same result with current stall warning systems and pilot training which are already established.
Doofer911 Well, it would require a new wing and probably a new aircraft (just like slats, flaps and spoilers were first incorporated), maybe it would be possible to make it deploy on its own depending on aoa and airflow, something like me109 slats :) ? Im not saying it could be applied without some amount of resources :) still if it could save someone from stalling close to the ground, where there is not much time to catch the plane or even pull the chute (design of which probably took even more costs and fitting it on the plane) then maybe it would be worth trying out :)
Doofer911 Plus pilots, especially weekend ones repeatedly fail to keep themselves out of harm's way, even despite warnings or aoa measuring. Some physical help could go a long way :)
I do like the idea and I completely agree that anything that can improve the safety of aircraft should be studied and implemented where possible. The question that springs to my mind is "How much of an improvement would an active wing surface to Lift performance?"... for example, a Cessna 172 in a clean configuration (Flaps Up) will stall at 53 knots. If a Cessna 172 fitted with Active Wings stalls at 50 knots.... yes it's an improvement but is that improvement worth the cost of development, design etc. There will be a critical point for any aircraft where the airspeed is just too low for the weight of the plane and it will stall anyway.
Yes I agree that weekend pilots are more prone to danger but you have to ask if these pilots take their position as a person in control of an aircraft seriously? Stall recognition and avoidance is one of the first aspects of flying that a student pilot will learn. Heck, I've only done a couple of Trial Flying Lessons at my local flight school and I've even been taught Stalls on those! If a Private Pilot approaches flying with a cavalier attitude then they won't be on their toes, they won't be in full control of the aircraft and they will put themselves and any passengers in danger. It's the same when you have the driver of a car, the Captain of a boat, a firearms instructor at a gun range.... any profession where safety is of the upmost concern, it depends on the person, not the vehicle.
Doofer911 It would all come to the cost/benefit analysis, i agree completely, i can't forsee how much would such a system allow, it would all come down to tests, it may give just a few extra knots, or it may allow the plane to almost hang by the prop (almost becouse that would require sheer power to accomplish), or it may completely disturb airflow in some way i didn't think of and stall it earlier, i can't tell :) still, all such devices started somewhere, right ? :) Maybe it would work better on bigger planes with faster stall speeds, maybe the opposite would be true. At the end of the day it all comes down to the ability of the pilot to asses risk plus his own skill and then use the tools provided to him to lower the danger to an acceptable level. All of that dependent on training and its quality plus common sense. Im feeling though that with time flying may be perfected to a level of casuality similar to driving, is that good or bad...hmm, should we strive for that or not ? ...maybe ? :) Thank you for your detailed analysis and for your videos, i learned much from both :)