This is one of the better presentations I have seen. Usually, engineers are using terms that require a degree in mathematics at a very high level. This presentation, however, is easy to understand. Thank you!
@@hernanposnansky7154 Well, yes and no, I still remember an exam where I wrote the complete (developped) Navier Stokes equations and it took me one straight A4 page (yes, i don't write small and yes, it was before simplification with a bunch of hypothesis, but still)
I wish I had seen this earlier. I've slowly found almost all pieces of the information mentioned in this video but now I can finally put them together in a self-consistent picture including the effect of viscosity. Thanks!!!
Great explanation. Really appreciate the attempt to explain lift intuitively. Even though I'm an engineer, getting an intuitive feel for the forces really helps. I'm preparing a 'theory of flight' video for a drone training video, and these principles and messages will be key to explaining this stuff well. Thank you!!
SPOT ON! The equations come from the understanding of the science. . This is perhaps the best video on lift, but you may be interested in these. . . *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
it kind of peeves me when i think about my ground school textbook teaching the wrong theory of lift. Makes me think about how many other things students are mis-taught.
Great video. When I first learnt this at Delft, I started to search more and more. Your video is amazing. Thank you so much for such presentation. What it annoys me is that in school they still teach the non sense of explanation about vacuum over the wing pushing up the airplane due to a ridiculous small difference of curvature. These teachers never flew an airplane or never saw the shape of any helicopter blade or never saw a Sukhoi airplane making maneuvers. Glad to have found one more quality video about the real reason for lift !! 🙏🏆
A sukhoi has a thrust to weight ratio between 0.9 and 1. That is nothing like a conventional passenger jet that only has a thrust of a third or a quarter of its weight. While the vacuum explanation might be wrong, your counter examples operate in entirely different realms.
Nice lecture. Identifying pressure gradient below and above the wing and relating it to centrifugal force so we see air flow along the arc of a circle is a nice illustration helping us understand lift. Pressure effects are useful because a surface integral of pressure over the surface of the wing determines the lift force. This is easier to calculate then integrating over the entire volume of space to to determine total change in momentum. I know vortex filaments lead to a more practical method to calculate lift coeficients.
22:30 The difference between the 3 flows is the circulation: Negative , positive but weak, large positive. The circulation cancels the upwards flow at the trailing edge to satisfy the Kutta condition as the angle of attack is changed. The circulation is the line integral of the scalar product of the velocity vector and the path element around any closed line outside the boundary layer. The circulation is the result of conservation of angular momentum: A vortex is produced as the flow at the trailing edge separates (counter clockwise) and procedes downstream, then a clockwise circulation with the opposite angular momentum forms around the airfoil. The lift is then density times velocity times span times this circulation. This is the Kutta-Joukowsky theorem, which is the correct explanation of lift. The circulation increases the velocity on the upper side of the wing and reduces the velocity on the lower side of the wing. integrating the pressures obtained by the Bernoulli equation (conservation of energy) just outside the boundary layer will yield the exact lift force.
This video is very informative and can help understand the how lift generates, thank you! It was really helpful for me in the fact that when I would learn lift, there would always be a lot of misconceptions and after this video it helped me understand the misconceptions and why it is important not to base understanding of lift of the the misconceptions and this video did a great job in explaining it. I learned many important concepts in generating lift such as Bernoulli's effect, inviscid flows, lift theory, streamline curves and many more. This video is great in understanding lift and I recommend this video to people that want to learn how airplanes fly to watch this video.
Concepts described very clearly. In fact, these concepts describes why and how certain very non-aerodynamic airfoils like a rectangular plank can create lift and fly.
This is perhaps the best video on lift, but you may be interested in these. . . *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
Great video! I appreciate the clear explanation of the complexities of lift generation and the different factors that contribute to it. The numerical solution of the pressure distribution over an airfoil was especially helpful in visualizing the concept. One suggestion for improvement would be to provide some more concrete examples or analogies to help non-experts better understand the concepts being discussed. Overall, great job breaking down a complex topic in an understandable way.
I agree! This professor definitely provided a detailed and clear explanation of lift and how it is produced. I also appreciate how he goes into explaining how misconceptions used to explain these concepts are not helpful, even if they seem intuitive. An example of this was his rebuttal of the equal transit time fallacy, and how "air does not like voids", while seemingly intuitive, is not an accurate explanation of aerodynamics.
Yeah, this video convinced me the quote at the beginning is correct: it is an area of debate. Lift has controversial explanations in the intuitive realm. Newtons 2nd Law is a good explanation. 3rd? Not so much. Newton's 3rd Law states that forces must be balanced. It is a misinterpretation to say one half *causes* the other. That there is downward force implies upward force, exactly the same as upward implies downward. It isn't causal or sequential. Further, it is in full effect on parked aircraft. And rarely do mechanics need to pry aircraft from the hanger ceilings. Lift is a result of motion, so Newton's 2nd Law is the explanation best suited ("in my opinion", required engineering tradition). Bernoulli is a specific case for 2nd Law. The thing that trips people up is "equal transit time" nonsense; AND not realizing each flow stream surface has it's own conditions. People try to compare the 2 surfaces relative to each other, which is a no-no. Bernoulli says pressure-velocity is reciprocal; a teeter-totter. NOT that 2 different slipstreams can be compared, in terms of magnitude, to the other. So no "pressure on top vs on bottom" with Bernoulli, only within one surface, with respect to itself. Pressure differences result in acceleration, which in turn sets up a pressure gradient across the top of the wing. That pressure arc requires the wing surface to be lower pressure than the longer arc above it... And up it goes. If the aerofoil is thin, all the better, because high pressure flowstreams result under the wing (being again a longer arc than those below them)... And up it goes even faster. (There's a centripetal force argument opportunity here also: with objects requiring such to travel in a curved path. And a resultant force inward. May get confusing, though. Could skip it.) So i explain lift through Bernoulli, as a special case of Newton's 2nd Law. And since we only get lift as a result of motion: this sets up an easily approachable velocity argument: F=ma. Liked the talk though. Always good to get a range of perspectives. (P.s.: knowing an equation, like N-S, does not follow that anybody understands it. That's maybe something we should admit to ourselves, and students. Yes i know they pay a lot of money not to hear that). Just look at G.R..-- Now it's quantum gravity, and emergent space-time. We don't understand that either.) Good talk. Not implying it's "wrong", just not my preference for an intuitive explanation. NYTimes was right 😉
So many diagrams (like the one you show here) don't show the deflected air, but rather show air traveling paralell with the flight path. Air doesn't do that. We need to fix the faulty diagrams.
There is one simple thing you have forgotten at least so far,the front of the wing kicks up the inflow air at the top of the wing just enough to solicit a reaction which causes the streamline to follow the wing down
Any theory has to fit the factual observations. The air velocity over the wing can be measured as can the drop in pressure it produces. That pressure drop multiplied by the wing area gives you the lift and is greater than the weight of the aircraft. There is some lift from the air deflected below the wing, about 20%. A flat wing will fly from the downward airflow but a very powerful engine is needed as the flat wing is very inefficient. Likewise, sustained inverted flight (not a loop!) also requires a high angle of attack and a powerful engine as it is also inefficient, not as much as the flat wing. The majority of the lift is coming from the UPPER surface and that is where the airbrakes/lift spoilers are put. The upper surface has to be kept clean and free of ice as it is critical. At the stall, the lift disappears because it is the airflow over the upper surface that becomes turbulent. The lower surface is relatively unimportant and that is where all the hardware is put, undercarriage (!) jet engines, flap hinges and screw jacks, and on fighter aircraft the ordnance, guns and rockets.
One additional thing... I love this video but I am kind of left empty about the reason the streamlines follow the contour of the body. I am coming away with the reason this occurs can be explained by the fact that an inviscid flow MUST follow the body contour. But, the air molecules probably don't know much about inviscid flow so how do they "know" to follow the contour?. What am I missing? Any help is appreciated.
@@GH-oi2jf Thank you for the reply. We did see in one of the slides a case where the streamline was separated and did not follow the contour. But I agree that a vacuum cannot form.
The atmospheric pressure does not go away when the wing moves. It is still trying to push air against all surfaces as best it can. Folks seem to forget that. . This is perhaps the best video on lift, but you may be interested in these. . *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
@@wcottee Yes, you have to listen to what he says about that one slide. .Also, if you understand and think that our one minute video works for you, I'd appreciate an upvote. My 11 YO granddaughter did the video editing. It really helped motivate me to finally make something. *WHY There is Lower Pressure Above a Wing* * ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
We should turn things around when trying to explain airplane wing interaction: The air molecule does not flow from leading to trailing edge, eventually separating or not. It waits for the wing to arrive and is then pushed up or down, and just a little longitudinally and here actually forward in the direction of the wing (and more the nearer the wing surface the particle is.) One should have this physical picture in mind when explaining airplane wing lift. In a wind tunnel though (or on a sailing yacht) the air flows along the surface of the wing profile, but on an airplane wing the molecule waits for the airplane to arrive (as we all normally do - a good clue for remembering this phenomenon. /Henrik Segercrantz - M Sc. Naval Architecture
Great lecture. Thanks! I couldnt understand the logic of vacuum not possible at 21:01? Why is that? Because even the streamline on the bottom surface would get sucked up? At 21:16, cant an air element move in the loop without rotating? An element can maintain its orientation and yet move along the circuit.
This is perhaps the best video on lift, but you may be interested in these. . *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
Just watched Doug McLean, Common Misconceptions in Aerodynamics, and lasted 10 minutes... this guy got me... all the way!! Don't really know what that means... 🤔
_Not saying professor Fidkowski is wrong, simply commenting._ 1) 6:43 When this real phenomenon is illustrated, no lecturer I've seen has pointed out that if the initial `longer path, equal transit time, faster flow, lower pressure` has an effect (however moderate,) the illustrated phenomenon means the top flow is even faster and thus has a less moderate effect. 2) My `simplified intuitive model` is the bottom side is for lift, the top side is for control, to wit: spoilers. (yes, incomplete)
I recommend using calculations and numbers proving how 10 tons of thrust can generate lift that leviates 150 tons of a plane. Otherwise all the rest are just words...for example from this presentation one should be able to calculate how much lift a directed air flow downwards can generate lift. Because for directing air flow downwards, the airfoil or the pressure field around it or both must have applied force on the air block flowing around. As a reaction to this force, the lift is generated and this can be calculated, maybe?
Let's be real. Y'all clicked because y'all thought "damn son this guy has the whole alphabet in his name". Joking aside though this video has good info.
Thanks. I recognize he addresses and dismisses the Coanda effect, but placing the back side of a spoon against a stream of water (eg. from your kitchen sink) causes that water flow to readily bend to follow the spoon curvature and also applies a marked force on the spoon towards the stream. If you haven’t tried it, please do so. It’s hard to totally ignore that effect (whatever is causing it).
@@BobbieGWhiz True, but the effect is much weaker (assuming the speed of the stream is similar in both cases). What's more there's no flow on the other side, unlike in the case of a wing. I'm not saying the phenomenon isn't real, interesting and at all relevant to the functioning of a wing, but wing doesn't need that phenomenon to create lift (as explained on the video). The upper side of a wing is curved mainly to keep the airflow nicely streamlined. Quick turns are bad for the airflow, again, as explained on the video (nowadays it also allows wings to hold fuel). A perfectly flat wing creates plenty of lift when it's faces the airflow at an angle as all wings do in normal flight.
He didnt properly explain the curving. He just said there will be a mismatch of pressure. From bernoullis equation wouldnt the stagnant air have more pressure?
Yes. At 21:05 with the upper circulating example, he mentions a pressure difference, but that doesn't make sense because the two regions he seems to be pointing to are not tin the same flow and, therefore are not related by velocity and pressure. I want to call and talk to him about that.
@@Observ45er In inviscid flows, Bernoulli's equation applies everywhere, not just along the same streamline. If you are curious about the proof, you can probably find it in one of the introductory aero textbooks. Hope that helps
@@nfarvolden No. Can you summarize the key concept? I have several of the well known texts, but don't want to spend hours re-reading for that one concept. The way you phrase that appears to indicate that a jet of fluid injected into an otherwise still ambient will then have a lower pressure than the ambient. Is that what you mean by"not just along the same streamline."?
12:34 - Hello. Do I understand correctly that the airflow on the top of the wing "follows" its shape, because when trying to "break away" from the wing, a vacuum is created between the airflow and the wing?
The geometry of the foil creates an upwash of the on-coming air. Air is forced to curve upwards before the wing arrives, by this it is easy to understand what's going on, on either side of the stagnation point. On one side, air is accelerated out of the wings' way , while on the other side the converse is happening, air is rammed into the bottom of the wing. This causes the pressure difference. On the upper surface, air is opposing gravity hence less pressure. This creates potential energy which will be converted into kinetic energy(due to gravity) in the downward direction so a downwash of air aft the wing's camber is produced. The downwash is vital as it aids in pressure recovery to resist drag. This interplay between gravity and airfoil manipulation of the fluid, through which it is moving, is the fundamental reason why planes fly. The speed differential of air flow between top and bottom can be attributed to the obstacle effect. Provided the wing is in an asymmetric orientation towards the on-coming air, the air under the wing is slowed down at the expense of the air which is sped up on top of the wing. If the the air under the wing is slowed too much(essentially just drag) , the air above is sped up too much and so cannot make the turn to stay attached to the upper surface hence a stall. The airflow will follow the wing's shape until the pressure required to force the air down is exceeded by the pressure required to hold a flow moving so fast, it's inertia cannot be overcome by gravity.
A wing generates lift by placing the stagnation point below the wing just behind the leading edge, that's how an upwash is induced, which is what is needed to reduce upper surface pressure 👍
Lift is due to centrifugal force caused by the mass of the air flow over the top of the wing.Replace air with mercury . tr3 b. Lift from sealed container.
I feel like I am missing something. So he shows two pictures of flow being deflected downwards and the pressure differential it causes 11:00 he shows low pressure on the bottom and high pressure on top 12:45 the high pressure is on the bottom and low pressure on top Why do the areas of pressure switch between the two examples?
At the 11.00 minute mark he is referring to the streamline only, not the airfoil. The streamline experiences a pressure differential which is why it curves downward, higher pressure on top. At 12.45 he is referring to the airfoil, high pressure on the bottom which creates lift.
You have to notice that in the wing diagram, pressure P∞, is the same at the top and bottom edges of the diagram. Unlike the drawing at 11:00 where one side is at higher pressure. It's key to understanding Just look at the lower streamlines only, since they curve downward, the pressure at the lower surface must be higher than P∞ (as explained by the drawing at 11:00). Now look at the upper streamlines only, again they curve down so the pressure at the upper surface must be lower than P∞. Therefore the pressure at the lower surface must be higher than at the upper surface.
Fundamental principle #1: Pressure in a curved flow *must be* higher on the putside of the curve and lower *toward the center* of curvature. The higher pressure pushes the flow around the curve. *
Rubbish - everybody knows planes fly by flapping their wings, like birds. I've been in a plane & looked out and seen them moving up & down with my own eyes. No idea why they have those whiry things at the front, though - must be to power the air conditioning.
Excellent expositions of empirical shaping laws and deductions for Intuitive Actual Intelligence. An accurate assessment and assembly of observational oscillation resonance phenomena in Mathematical Theoretical Formulae. (That's what we see) How it happens is superimposed logarithmic condensation modulation wave-packaging formation of Navier-Stokes type formatting that you get in the time-timing arrangement of anything, not only flying. So the why, of all is vibration-> Superspin, quantitative timed time mechanics-> 0-1-2-ness Entanglement be-cause-effect self-defining superposition-> sync-duration assembly, which is Fractal Quantum Operator Fields Modulation Mechanism.., at this Centre of Logarithmic Time Singularity, ..QM-TIME pure-math Reciproction-recirculation relative-timing ratio-rates Perspective Principle. (Basic QM logic "says" it is because it is, => Principle) Engineering likes to build a fence around reliability with a Safety Factor that obscures the fine structure of phenomena. "Long may they continue". But if you analyse nuances, then the inevitable result is that "All things must pass", change changes across the 1-0 probability dominance spectrum to continuously Form this Universe. The finest quality of information existence is derived from e-Pi-i sync-duration connectivity function that operates via Singularity, ie @.dt zero-infinity sync-duration eternally, in Primes and Cofactors of vector-value Timing-spacing coordination, AM-FM Quantum-fields Condensates forming Math-Phys-Chem and Geometrical Drawing Conception of the real-time wave-package Periodic Table, Standard Model and related QM Temporal Chemistries. This is the ultimate sense of experiencing existence as "floating in flat-space ground-state No-thing-defined eternally, "flying" as coherence-cohesion objectives in temporal cause-effect continuity.
The equal transit time hypothesis states that "lift causes lift". This confuses the cause with the effect. You don't create low pressure, then get lift. Because the low pressure is the lift. You can only get the effect of lift by causing the action of downwash.
No it doesn't meet up. Why should it? I really don't understand why so many physics keep saying it meets up. - And so, explain lift of wings wrong :-((
Imagine the fluid element on the upper surface is a rabbit; the fluid element under the lower surface is a tortoise. Of course rabbit can run faster than the tortoise, but the reason of rabbit running faster is not because rabbit will meet tortoise at the same time in the final (with running a comparative long distance). The rabbit is faster because of its genetic (unbalance of varying pressure distributions); it always runs faster than the tortoise.
Regarding your simple intuitive answer about how chemical rockets work: According to Doug McLean's Michigan Engineering video "Common Misconceptions in Aerodynamics" it's really more about Newton's second law, not the third. See the section "Stretching of Newton's Third Law."
You don't use the pressure difference to get lift. Rather, the pressure differential IS the lift, gotten by acceleration of air downward. Lift doesn't cause lift.
Exactly. At time 11:40 the provided diagram indicates high pressure on top of the wing, and low pressure below the wing. The 'streamline curvature' explanation is immensely "hand-wavy." A higher pressure above the wing than below, shown in that diagram, *_is NOT lift_* - just the opposite. If an effort were made to explain lift in 10 minutes (not 31 minutes), I'm pretty sure the instructor is smart enough to do that. As it is, I get a sense that he's not 100% sure himself.
At high Reynolds numbers , turbulence forms after the laminar boundary becomes unstable and transitions to turbulent boundary layer whose displacement thickness grows as the square root of the length. The Navier Stokes equations do not describe this process, a turbulence model must be added. There are several models that are used , approximately.
Rather than the arbitary term "flow turning", it would be much more helpful to introduce the concept of circulation. Once the concept of circulation is grasped, it is easy to establish the induced force from this circulation to the force generated with Kutta-Joukowski.
The geometry of the foil creates an upwash of the on-coming air. Air is forced to curve upwards before the wing arrives, by this it is easy to understand what's going on, on either side of the stagnation point. On one side, air is accelerated out of the wings' way , while on the other side the converse is happening, air is rammed into the bottom of the wing. This causes the pressure difference. On the upper surface, air is opposing gravity hence less pressure. This creates potential energy which will be converted into kinetic energy(due to gravity) in the downward direction so a downwash of air aft the wing's camber is produced. The downwash is vital as it aids in pressure recovery to resist drag. This interplay between gravity and airfoil manipulation of the fluid, through which it is moving, is the fundamental reason why planes fly. The speed differential of air flow between top and bottom can be attributed to the obstacle effect. Provided the wing is in an asymmetric orientation towards the on-coming air, the air under the wing is slowed down at the expense of the air which is sped up on top of the wing. The aerofoil does not turn the air, the atmospheric pressure does. It's a fluid so wants to take the shape of the object , as dictated by gravity.
The equal transit time hypothesis confuses the action with the reaction. It says that you create a pressure differential, then you get lift. However, a pressure differential is lift. Lift does not cause lift. Downwash causes lift. Lift is an upward force caused by a downward force. Wings that don't send air down, don't get lift. The question of how a force transmits to a wing is a different question than what the causes the force is. The how question is explained by Bernoulli, while the what question is answered by Newton.
Pressure is just force over area, ALL forces exerted on ALL macroscopic objects are ultimatly just pressure over an area of the object, if the force is from another solid object in contact with them or from fluids in contact with them.
Nothing. This is perhaps the best video on lift, but you may be interested in these. . . *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
i am no flight student but i came here because i had this question. "If airfoil workrd on the basis of bernoulis principle then how planes fly upside down ."
@@ameunier41 Wrong. ALL airplanes can fly upside down; it's just a question of how efficient they are in those two conditions. As usual, you've made a simple declaration that is worthless. If you aren't ready to discuss the nuances, STFU. All you're doing is increasing the confusion WRT the subject.
@@craigwall9536 Although I would agree that all airfoils can be flown upside down but with different efficiency, in a way the more pointed example is why until very recently it was thought that a helicopter cannot be flown upside down (each helicopter blade is also an airfoil, and it was thought that upside down could not create lift). I haven't looked closely at the recent demos that show it is possible to fly a helicopter upside down but would suspect extending the limitations of angle of attack in a negative direction (Just guessing!)
To expand on this, wings can produce lift upside down, it doesn't need to be symmetric or flat. A symmetric airfoil produces lift equally well if the angle of attack is positive or negative. It produces no lift if the angle of attack is 0. A naca0012 airfoil does not care if it is upside down or right side up. Cambered airfoils can also be flown upside down and create lift, but it might still before attaining an angle of attack that can support the weight of the plane. Cambered airfoils can create lift at 0 angle of attack. And so it would need more negative AOA to fly upside down. An example of a cambered airfoil is the Clark y.
EXACTLY! If a curved airfoil redirects the air downward, then a flipped airfoil would redirect the wind upward.. So is not the redirection of airflow only, so there could be many reasons LIFT is caused not only one...sometimes is redirecting and sometimes is angle of attack sometimes is both. what about flying sideways?
illustrate your point using sailboats. The properties of airflow may mimic traffic flow,both people and vehicles. Sailplanes have no thrust, they're always going downhill. Can you explain a curveball?
It works exactly the same. You have the sail at a positive angle of attack. This redirects the airflow, resulting in a lifting force. Finally, on a sailboat, through the keel the forward component of the lift vector accelerates the sailboat. This requires that there's also an angle between the ship and the sail/lift vector, ideally around 45°.
21:12 can someone olease explain why can't we have stagnant air there, he said its because of Bernoulli principle but Bernoulli's principle is applicable to a SINGLE streamline flow not two spearate flow like this, the air above is moving that flow is completely separate form the stagnant air, why qre we relating the two?
You can compare the static and dynamic pressure of two streamlines if they started out with the same initial conditions. It is the same principle used in the Pitot system.
And these are all lacking in one aspect or another: Newton's (secret, apparently) 4th law hodge podge- the Kutta condition, viscosity, and the Coanda effect, and various takes on centripetal/centrifugal circular logic....not
Does an air stream not posses the quality of both adhesion and COHESION. Stream deflection upsets or imparts energy to. the inherent energy natural to the cohesion of the air in the stream.
Oh. You know I mean. Air is not a localized entity. Drawing a section thru it for the convenience of a geometric solution is under rating the nature of it all. Go flying in a j3.
Uhhh... No. If it's about deflecting, turning, or otherwise reacting against the air, you need more thrust than both the weight and drag. Also, you have to account for the energy of the rising air that meets the leading edge (which I suppose you could say is part of induced drag.) Air isn't all that dense compared to the aircraft. You know that gravity is accelerating the mass of the aircraft at a constant rate. Let's just say a 500 KG aircraft at 9.8m/s^2. You can give yourself a simple graph and ask yourself what is a reasonable amount of mass to acceleration to still get 4900 Newtons for level flight; accelerate how much air, how quickly, and how much air do the wings actually touch anyways? The only forces that affect the wing are the fluids that actually interface with it. Yes, stuff is going on at significant distances from the wing... but the wing doesn't care. So as long as there is more net pressure on the bottom surface of the wing than on the top, you get lift. We know that pressure is not uniformly distributed across the wing, but it also corresponds to local air velocity. Air over the top of the wing is moving at a great initial velocity and then slows at the back. So when you look at a pressure distribution, the very end of the wing is typically positive in pressure (pushing the wing down.) So there's no argument that lift is from a pressure difference, that the air velocity corelates to the pressure that the wing actually sees, so the question is: Why is the air moving faster over the top of the wing? Specifically, why is it moving faster than the wing is? The only obvious answer is that there is lower pressure (vacuum) above the wing. But, logically, this would appear circular in reasoning. What creates the vacuum, that accelerates the air, to create the vacuum? To that end, I would pose the question: What was the initial state of the wing? Answer: Not. Moving. If you have a wing in dead-still air with smoke or fog, what will you see the air do when you start everything up from the perspective of the wing?
What? Why would you need more thrust than both the weight and drag? Lift increases with velocity, so at some point lift will equal weight. At this point thrust needs only to be equal to drag. What you suggest maybe applies to a rocket, but not a wing. What you write about pressure is all in the video as are answers for some of your questions. Did you even watch it?
Just to beat a dead horse, a 747 has 224000 lbs of thrust and a max take off weight of 987009 lbs. Tell me again how thrust must equal weight plus drag to fly?
A rocket doesn't push out gas. The gases pressure is what causes the gas to expand and thus push the rocket and the gas particles equally away from the epicentre of that pressure. Since gas flow can deform to suit a conduit, and the rocket can't, and an equally emparting momentum into the gas and rocket creates a higher flow velocity for the less massive gas and a lower velocity for the more massive rocket, the gas is manifestly jetting away from the rocket faster than the rocket is accelerating away from the starting point of the ground.
Yes. Lower pressure means higher velocity. That's Bernoulli. Lower pressure also means that the air is changing direction, curving towards the lower pressure and away from the relatively higher pressure on the outside of the curve.
@@xnoreq Can you explain why there is lower pressure there because I normally get the answer "because the air has a high velocity, bernoulli", so you are stuck in a bit of a loop if you understand me.
@@guypaterson9503 Imagine a symmetric airfoil at 0° angle of attack. In front of the leading edge a high pressure point ("stagnation point") will form, air slows down, simply because there's an obstacle. From this high pressure point, the airflow has to accelerate/pressure has to drop as it curves over the leading edge and curved airfoil - around the obstacle so to speak. Note that this even happens on the lower side on a cambered airfoil, just to a lesser extent. The reason for that is that the much tighter curve around the upper side causes higher acceleration/drop in pressure. As you noted correctly, Bernoulli doesn't explain why there's lift. It only points out the relationship between pressure and velocity and is only valid within a flow field.
I imagine you just re-do the calculations with the airfoil in its new orientation (upside down). Believe all the concepts described would still be in play.
The same reasons, but oNLY if you have the correct reasons. . This is perhaps the best video on lift, but you may be interested in these.
. *WHY There is Lower Pressure Above a Wing* *ua-cam.com/video/3MSqbnbKDmM/v-deo.html* A LIKE would be appreciated. .. And read this: *Understanding Lift Correctly* *rxesywwbdscllwpn.quora.com/*
Through angle of attack. This means placing the stagnation point just just below the leading edge, so airflow will travel back to the leading edge and be forced to make a turn to reverse its direction of flow. Air has inertia , so any directional changes require a force , which is provided by the atmosphere. However flat wings have high drag to lift ratio, so are inefficient lift devices at subsonic speeds. That is why he states that his lecture pertains to subsonic flight. At supersonic and hypersonic speeds, flat swept back wings generate lift differently. Lift from shock waves , known as compression lift.
@@singh2702 So misconception of lift is really the misconception that there is only one way to create lift... Rather lift can be created in many ways. and the most efficient depends on speed, airfoil shape, etc etc
@smooth-dancer That is correct. Wing geometry and design are optimized for a certain application. This video deals with lift at subsonic speeds and fluid behavior at these speeds. At hypersonic speed, the fluid behaves very differently, so bizarre that Newton's sine square law actually applies, for example, the X-15 rocket plane. There are misconceptions of wings generating lift at different speeds, but one thing will always be true, lower pressure above and higher pressure below, at all speeds.
@@singh2702 why am i not flying? Its not my shape... or weight.. its because gravity pushes me down. So lift is when enough energy pushes me up stonger than gravity. So a rocket pushes down enough stronger than gravity until theres no gravity. An airfoil cuts the air in a way to pushes up the airplane. A sail cuts the air in a way that pushes the boat sideways and the keel cuts the water so pushed the boat the other way and therefore the boat moves forward. Interesting topic
@smooth-dancer The atmosphere we are immersed in puts pressure on us at 14.7psi or 10 metric tons per square meter. This pressure is applied in every direction, top bottom left right diagonally, hence all pressures cancel each other out so there is no net force to counter the force of gravity, this is why you are not flying. Lift is about reducing that pressure applied in the downward direction so that the pressure applied upward can overcome weight. All the examples, besides the rocket, require this pressure differential to overcome forces.
Is he saying that Bernoulli's theory can be disregarded as an explanation for lift? I understood from him that lift is purely related to the pressure gradient around the aerofoil. I'm left a bit confused. surely the reason you have a pressure gradient is because there is greater and less pressure on different sections of the aerofoil. Difference in pressures is therefore Bernoulli's theory.
+bhfireblade, he says to avoid using the formula to explain lift, as will be of no help. But as you say, he states that pressure differences are in accordance to Bernoulli.
bhfireblade Bernoulli only applies along a streamline, but it doesn't explain anything other than the link between velocity and pressure with a streamline. It is the shape of the aerofoil that creates the lift force by curving the flow over and under the aerofoil.
So as a flight instructor, I haven't found any lecture done by Ph.D. or professors explaining why does air gets accelerated on the airfoil. because every book in the field for student pilot explains the lift with newton's 2nd la and Bernoulli's equation. Here's what does the book say: because of the curvature of the airfoil, the speed of airflow above the airfoil is accelerated, and therefore pressure drops. why does the airflow is accelerated by airfoil's curvature? well, this is how it is. I've never put my self in trouble with this explanation(because it's pretty enough for pilots) but is there anybody who can explain why does air gets speeding up?
If something accelerates and it has mass, a force is responsible. That's Newtons 2nd law: F=ma The force is pressure. Higher pressure upstream is accelerating the air as it moves downstream into the lower pressure area above the wing. The low pressure above the wing is the result of the airflow being curved as it flows over the top of the wing. The airflow tries to turn the corner and feels a "centrifugal force" pulling it to the outside of the curve as it stretches and tries to stay attached to the top of the wing. This generates a pressure gradient from low to high, from inside to outside of any curved streamline.
A real observation flying through a rain zone. The water droplets on the wing should have been whipped away past the trailing edge, but could be seen creeping towards the leading edge. Boundary layer? Does not seem likely. The boundary layer would be moving forward at a speed somewhat less than that of the wing, so it would be moving slowly backwards relative to the wing, not forwards. I asked the same question elsewhere, but got no takers. Anyone here, please?
Boundary layer is defined by a turbulent zone that has a velocity gradient from 0 airspeed at the wing surface to the freestrean velocity at the outside. Inside the BL flow is somewhat unpredictable. Turbulent areas have flow vectors that can be independent of freestrean vectors. If it was an airplane window on the side of the plane with water on it, the water was in a turbulent area of separated flow.
That's one of the more incomprehensible talks on lift I've seen on UA-cam. How about keeping things simple? First imagine two chaps engaged in arm wrestling. Let’s say instead of joining hands they both push against a plate with their hand flat. So the left guy pushes against that plate, the right guy pushes against it, and as long as they push with the same amount of force, that plate won’t move. Let’s say the left surface of that (vertical) plate which is exposed to the forces of the left guy’s hand pushing against it could be compared to the lower surface of a wing (mentally turning the plate 90° counterclockwise) and the right surface of that plate which is exposed to the forces of the right guy’s hand pushing can be compared to the upper surface of a wing. That wing won’t move and will not experience lift as long as the net sum of forces below and above de wing (or the opposing forces of the left guy and the right guy) are the same. Now imagine the right guy becomes exhausted and his force weakens. The left guy is now able to push the plate towards the right, however the forces exercised byt the left guy didn't increase, it stays the same (in this example). The plate is moving right only because there is less amount of countering pressure from the right guy’s hand. The plate will move even more forcefully to the right if when the countering forces of the right guy weakens, the left guy decides at the same time to push even more forcefully against the plate. The net sum of both forces, that is the force of left guy pushing against the plate (or, if you rotate that image counterclockwise by 90°, pushing from below upwards against the wing) and the countering force of the right guy (pushing from the right or downwards if you rotate everything by 90° counterclockwise) is lift in the case of an airfoil or wing, if the net sum pushes the airfoil or wing upwards. Now who are those two guys in the case of the airfoil or wing ? They are nothing more and nothing less than the *static air pressure* pushing at all times against the wing, if there is no airflow, then the "plate" will stay in the middle and the wing will not experience lift, but if there is airflow, aerodynamic effects generated by the AoA and the camber of the wing influence the pressure the surrounding gas molecule can exercise on the wing by aerodynamic effects which act on both the lower and the upper surface of the wing. That is, by aerodynamic effets, the force air pressure can exercise against the wing is either weakened (above the wing) or increased (below the wing). That what is known as pressure differential. Now the upper surface alone cannot create lift, it only contributes to lift. The reason for that is that there is no such thing as suction or negative pressure. Pressure is always positive and air is always pressing against the upper and lower surface of the wing, at all times. There is no force pulling the upper surface of the wing upwards, as air cannot grip the wing and pull away from it. The wing is always lifted because air pushes the lower part of the wing upwards. Back to those arm wrestlers : if there is no left guy present, it does not matter how much the force of the right guy will be weakened, there will never be any push towards the right (there will never be any lift if the image is rotated counterclockwise by 90°). So there can only be lift if gas molecules around the wing are exercising pressure against the wing (an atmosphere featuring pressurized air needs to be present) and if there is air present pushing against the lower surface of the wing. Imagine putting the wing in a mold filled with low amount of water just so that the wing will float and that the forward and aft stagnation points are just level with the water surface and the upper end of the mold. If wind blows against that wing, airpressure pushing downwards on the upper side of the wing will decrease as air flow hits the upper part of that wing , however as there is no airflow below the wing, the wing will be unable to lift from the mold. Also airflow will not be bend downwards against the water surface, it will just flow around the upper part of the wing and continue afterwards horizontally in a laminar flow. No lift is generated by the upper surface of the wing alone. Therefore, the pressure differential between the air under the wing and the air above it is decisive. It explains lift. But how is the pressure differential formed ? I’m with Bernoulli on this one. Airspeed of local air flow must be different in order to influence the effect static air pressure has on the wing. That is, the faster the airflow, the less force static air pressure can apply on the wing. So in order to get lift, local airspeeds below and above the wing must be different, locally increasing or decreasing the pressure created by the static air pressure. This works even in other situations. Behind a long 40 ton semi-trailer truck, there is a lot of slipstream. Now you might think that this slipstream is enough for a cyclist to be able to travel at high speeds, like 60 - 65 km/h on the flat, without much effort. Actually, that’s not the case. 5 meters behind that truck, the air feels like not moving at all (calm air), however a considerable amount of force is still needed to overcome rolling resistance in order to not lose the semi-trailer. However, there is a small region about 2 m behind that semi-trailer where very turbulent air dominates, and 20-30 cm farther away, it calms down to what feels like total calm. Suprisingly, when the whole cyclist is situated in that calm air, the cyclist must steadily pedal in order to stay with the truck, but if the distance is decreased so that only the back is in calm air but not the frontal side of the cyclist (the effect is increased by getting as upright as possible on the bike), there is a net force pushing the cyclist forward against the truck. This can only be explained by more air pressure hitting the back of the cyclist than his frontal aspect. The effect is maximized by staying with the face and torso in the most violent turbulent air (it feels as if one is continously slapped in the face) whilst exposing the lower back to calm air. Not only is it no longer necessary to pedal, it becomes necessary to slightly brake. My explanation is that the highly turbulent air slapping against the upper part of the body (face, torso) diminishes the effect static air pressure has on that region, whilst the calm air hitting the back is not diminishing the effect of static air pressure, creating therefore a pressure differential which pushes the cyclist against the semi-trailer. One has to very carefully keep the body in that « window of opportunity » as the region this effect happens is no wider than about 10 to 20 cm (a distance of about 1 to 1.2 meters between the end of the semi-truck and the front wheel). I was therefore surprised that the best spot behind a semi-trailer would also be most noisy place where I would constantly be hit hard against my face and torso from both sides of the slip stream. Another example : imagine a vaccum suction cup holder, once installed on a window pane, it actually does not suck on the window to stay put. The part facing the window pane can be compared to the upper wing, the part facing you can be compared to the lower wing. So the "vaccum suction cup holder" remains put because static air pushes it against the window pane (that could be compared to lift) the only difference is, in order to create it, there is no need for airflow because the surface facing the window is hermetically sealed of and the lower static pressure is permanently maintained so there is no need for dynamic airflow over a curved surface at an angle of attack in order to create a local reduction of static air pressure hitting the wing. Now, it would be foolish to say the inner part of that suction cup holder created "more lift than the outer part" - as no force is pulling the inner surface against the window pane, only the outside static air pressure is pushing the suction cup against it. Since the inner side is hermetically sealed and no airflow is occuring, there is also no need to send air masses downwards. You could install a suction cup holder on the lower side of a horizontal ceiling window and it will still hold your objects because the static air pressure on the outside and lower surface of that suction cup pushes that cup upwards against the window pane. The difference with a wing, there is no hermetical seal and therefore the local decrease of air pressure on the upper surface of the wing must be created aerodynamically and dynamically by airflow streaming against the wing. Therefore I still think that Bernoulli is a good explanation why air passing along the surface of a wing decreased the effect static air pressure exercises against the wing and therefore explains the pressure differential.
Hi guys, thanks for a video however I found it quite confusing. Especially the explanation of pressure gradient around the airfoil. Below the airfoil as well as above the airfoil, there is lower pressure compared to the atmospheric pressure (to be exact, there is a few spots along the airfoil where this does not apply and the pressure on the airfoil surface is higher than atmospheric pressure - area around the stagnation point and area close to the trailing edge), so how can you explain the pressure gradient this way? How can you even say that below the airfoil, there is high pressure. There is not higher pressure, there is a lower pressure compared to the atmospheric pressure, meaning pressure of free stream. Only reason why the airplanes can fly is the fact that above the airfoil there is even lower pressure compared to the pressure below it, yet both of them are lower than atmospheric pressure - pressure of free stream. The explanation of the pressure gradient and the fact that streamlines are curved is nonsense in my opinion however I really like the idea of using the conservation of momentum when explaining the lift force. It makes much more sense than the Bermoulli's equation. Could you please answer to my comment and make your explanation more clear? Maybe I just misunderstood your concept.
Jiri Dolinsky the airflow below the aerofoil is actually much slower than that goes over the aerofoil. If two air molecules get to the aerofoil at the same time the one that goes over the top of the aerofoil takes about 2/3 of the time taken by the molecule that goes under the aerofoil. According to Bernoulli the slower flow has to have a higher pressure than the flow prior to reaching the aerofoil and the faster flow much have a lower pressure. The pressure gradients as described here are normal for aerofoils that are not described as having laminar flow (as fitted to the P-51). Most of the lift generated by a non-laminar flow aerofoil is from the front 1/3 of the upper surface. After this point the pressure rises back to normal air pressure at the trailing edge.
I have the same doubt. He first says that pressure difference is caused by curved streamlines. Then, he says that streamlines are curved because of pressure difference ... It is not clear what generates the pressure gradient.
@@rauldeira116 I find this, and most others like it, just focus on the fallacies, and then have a simple statement that because we curve the airflow downward we must be generating lift. This video (terrible recording, watch in parallel with another show the slides) goes deeper into how this works, any why Bernoulli only relates to straight streamlines. ua-cam.com/video/QY2pS-xXC_U/v-deo.html Note that he actually appears to give some credit to the Coanda effect, so they still can't seem to agree
Why do you consider streamlines nonsense? There are many ways of looking at and estimating the effect of a wing moving through air. One has to come to grips with the fact that a wing moving through fluid is the same as a fluid moving over a wing. Once this leap is taken, it is easier to visualize streamlines as the effect the wing is having on the fluid. Once you are able to see the effect the wing has on a single particle of air, it might help you understand lift by assuming the streamlines do not collide and the effect is on a large number of particles. But understand that it is one more to look at the forces involved to intuit the goals of a successful airfoil. Saying, I want to understand all the complexities of why a shape moves a certain direction when propelled through a nearly inviscus unbounded medium without using an explanation that simplifies the visualization of particles interaction in a way that can be used to estimate lift is a little short sighted. Pressure and partical movement are all part of the same elephant.
So a wing (air foil) with a zero angle of attack has no lift??? Obviously, flat wing can generate lift with a positive angle of attack (Hand out of window or balsa wood glider). Presumably the air foil shape causes more lift than a flat wing.
Idea of lift is simple. Introducing of obstacle into flow will change velocity of flow (direction and speed). Any changes like this call fluid acceleration/deceleration. Any acceleration/deceleration will cause fictitious force ( remember what happen when we push brake pedal in car or turn car ) . Such force pushes fluid and causes pressure changes 9 as your fasten belt in car) . As long as pressure distribution around obstacle is not symmetrical ( as around not-rotating ball) obstacle will experience some net force named lift .
+Andrew Pa i agree that the explanation in the video still fails to explain exactly how a low pressure area is created above the wing-saying it must exist by examining the air flow is not an explanation. The conclusion that to explain lift one must consider viscous and inviscid flow simultaneously is, unfortunately, the conclusion-i expected it to be the starting point for explaining the colorful pressure distribution diagram that seems to be produced using Finite Element Analysis (?) Since the simulation model seems to produce flows and pressures that match practical observations, maybe examining the FEA parametric variables step by step would help explain what happens and why. Your explanation: I'm not used to dealing with fictitious forces as part of a valid solution in physics, so I did not understand most of your explanation. You seem to be getting inertia involved but it got quite confusing-likely due to the english structure. There may be a valid explanation lurking there... would you please re-write it perhaps just using Newtonian forces that I can understand and that can be validated in a lab, wind tunnel, etc.? Regarding the ending part of your comment, you may have missed that part early in the video which points out that your explanation of non-symmetrical shapes and different lengths of flow does not account for how the symmetrical camber wings of some aerobatic airplanes produce lift or how an airplane flying inverted produces lift on the side of the airfoil that is opposite to your explanation. Also, per your explanation, an airfoil with a relatively flat lower surface and a "wavy" corrugated upper surface should produce an incredible amount of lift - in reality it doesn't. You seem to have a typo error while giving an example of non-symmetrical flow: "as around not-rotating ball" should probably be "as around rotating ball." This is because a non-rotating ball would have symmetrical flow around it and net zero lift, whereas a rotating ball WOULD "rotate the airflow with it" to a new direction (the Magnus Effect), which.will result in lift in the opposite direction.
The media are idiots and cannot tell the difference from a Cessna 150 and an Airbus A380, much less understand aerodynamics, or anything to do with aviation.
At 21:06 Prof Fidkowski does seriously misspeak. I am very surprised he said that. . He glosses over just what the "pressure difference" would be when using that "Bad Bernoulli" misconception. _IF_ speed had something to do with it, the "moving air" would have lower pressure. That is the air farther above and, therefore, [if' moving' air had a lower pressure - it doesn't] the flow would curve UPWARD away from the wing. SO this is a double error on his part. OOPS!! So, yes, what he said is wrong. O U C H ! ! . . . . In addition, the air right above the wing IS MOVING! In fact, it is moving in the opposite direction of flight - WHEN viewed from the ground as a wing passes by. SO. . . NOT ONLY can't you have stagnant air there, you don't! PERIOD! We SEE it moving toward the trailing edge BOTH in the wind tunnel and in flight. . . I think I should talk to him. . . . Also around time 23:38 He glosses over the fact that that "sharp turn" around the trailing edge is a HIGH acceleration. He only mentions needing a sharp Pressure Gradient. He doesn't explain WHY viscosity and boundary layers make the physical flow off the trailing edge - that is very vague. I claim that it is the need tor a very high Pressure Gradient and the air's inertia that helps us understand why BOTH flows leave the trailing edge like that. Because it must. Don't confuse things with inviscid flows. . You must cover the important fundamentals first, then when the student understands that, THEN you can add the more complex and issues beyond the basics. . For a wing's lift, it is best to start with the fact that lift comes from ONLY the Top-Bottom pressure difference, Then, explain how/why those two are formed. . AFTER THAT, then. . . talk about the stall, upwash, downwash, tip spill, tip vortex, induced drag, form drag and intersection drag. . I also feel that he should NOT start with all this "separation" talk and the turbulence examples. That is not part of flying. That is part of NOT FLYING!. I've seen this in my initial field of Electrical Engineering. They want to explain everything up front and put too much unnecessary things in too early.
As shown in the figure(1), water bends downward and moves downward along the curved surface. Obviously, water does not produce pressure gradients by viscosity. At the same h depth, the force of water on the curved surface decreases. Why does it decrease? Because it is a curved surface, the flow of water requires centripetal force, so part of the gravity of water is used to provide centripetal force. Obviously, even if the water is not viscous, the force acting on the curved surface will decrease, that is to say, the pressure gradient will occur in the normal direction of the curved surface. So your understanding is incorrect. It is not the curve movement that supports the lower pressure, but the lower pressure support curve movement. First of all, there must be centripetal force, then there will be curve movement. You violated Newton's law. Newton's law says that force is the cause of the change of motion state of an object. Without centripetal force, how can there be curve motion? There must be centripetal force first. Precisely speaking, there must be a force perpendicular to the direction of motion. And because the curvature of the surface is not the same everywhere, the centripetal force is also changing. Therefore, centripetal force can not only be provided at the beginning, so it must be adjusted constantly. Therefore, it is wrong to provide the theory of centripetal force only at the beginning of the movement. So, how to constantly adjust the centripetal force? You said that viscosity is one of the reasons for centripetal force, so what other reasons? Obviously you admit there are other reasons, so what are the other reasons? You have to admit that when water flows along a curved surface, centripetal force can be generated without viscosity. This is because water is heavier than air, and gravity makes water on a curved surface. It is not because of the viscosity that water moves along the curved surface, but because gravity makes water move along the curved surface. Gravity provides centripetal force for water to move along the curved surface. These facts negate your theory. (1): qph.fs.quoracdn.net/main-qimg-b46fe6a57a535484aa14bce3bd684154) Come from qr.ae/TWN85L
Totally misses the point. It is not about airflow .Lift is created by the change in the properties of the disturbed airmass in that the kinetic energy of the particles is not random but parallel to the direction of movement of the airfoil. Since gasses occupy only a small fraction of the space in which they are present the amount of gas in any region of space can be increased without a change in pressure if the direction of the motion of the particles can be controlled
In the case of the Coanda effect it doesn't matter whether you're blowing a laminar jet over a wing or moving an airfoil through still air. Both framesets are correct.
No, thats not right. A Jet (or Free-Jet) pulls additional air from its surrounding which "pulls", for example, the sheet of paper up. It's actually visualised pretty well in this presentation (18:57). There is a specific velocity distribution (also visible on 18:57) needed to get this effect. This velocity distribution doesnt exist if you just flip around the coordinate system to move through stationary air, because where should this distribution now come from?
Actually yes it does. The Coanda effect only applies to jets and doesn’t apply to the airfoil, this is because jets (such as those in a hot tub) have to do with entrainment if surrounding fluids. What happens on the airfoil looks similar to the Coanda effect but it’s actually not. This was explained much better by a P.H.D fluid dynamics professor but unfortunately I can’t find the link to the page where he explained
A good explanation - but I think he should get together with Holger Babinsky at Cambridge (perhaps he has by now). Nevertheless, there are major shortcomings in the presentation. 1) It is coming from the 'conventional' viewpoint of a stationary wing with air flowing over it, as in a wind tunnel, 2) It does not describe the reason why the air moving over the upper surface of the wing is moving faster than the air moving beneath it. 3) It does not describe 'holistically' the total flow field around a wing. Thinking about item 1), in the 'real world' the air is stationary until it encounters a wing moving through it. The wing must, therefore, be displacing the air, which, at some time after the wing's passage, returns to a state of rest. It was not and never was "flowing". 2) Circulation theory is not mentioned - but circulation theory is not merely theory, it is a physical fact. A wing is effectively generating a vortex around itself, which is stripped away or shed, as the wing continues on its way. 3) The downward deflection of the air (Newton's Third Law), as a result of the passage of the wing, is a manifestation of the vortex being shed. It can be seen in the 'real world' when an aircraft flies low over the cloud tops. There are numerous photos showing the 'trench' in the cloud tops, caused by the passage of the aircraft.
How can 10 tons of thrust keep a 100 ton aircraft in the air? We all know that on an inclined plane, we can lift an object by a force much smaller than its weight. The same is true of airplanes, which are also on inclined planes and lift themselves into the sky with a small force. This inclined plane is made up of wings and air. One is the wing inclined plane, the other is the air inclined plane. The wing has an angle of attack, so the wing is an inclined plane. Air flows through the surface of the wing, so air is in fact an invisible inclined plane. The air inclined plane consists of two parts, one is the air inclined plane at the bottom of the wing and the other is the air inclined plane at the top of the wing. The bearing capacity of an inclined plane is determined by the material of the wing. The bearing capacity of air inclined plane is determined by the relative speed of air and wing. Under the condition of no stall, the greater the relative speed, the greater the bearing capacity of the air inclined plane, and the heavier the air inclined plane can bear. The angle of inclined plane is related to the angle of attack, but not necessarily equal to the angle of attack. If the angle of attack is too large, the air inclined plane on the top of the wing will be destroyed. In this way, the support of air inclined plane will be greatly reduced. qr.ae/TWIy3q
This gives us flight instructors a dilemma. The FAA uses Bernoulli and Newton to explain life. This fella says, nah, don't mention those guys. So either teach the wrong thing and the student passes the test, or teach the wrong thing and have them totally confused.
But Newton does apply here and is a very important component of the explanation. The airflow is directed downward which pushes the wing upward. Bernoulli also applies, but doesn't really provide anything useful to explain why there is lift. Bernoulli's principle only applies within a flow field and describes the relationship between pressure and speed. So it can tell you that higher speed also means lower pressure, but it doesn't tell you why this happens in the first place.
@@xnoreq If the airflow is directed downward via the combination of laminar flow and the shape of the wing, how does it actually push the wing upward if the downwash doesn't occur fully until it flows behind the wing and is separated from it?
@@careywaldie6735 By the time the airflow separates from the top of the wing at the trailing edge, the airflow's direction has already changed, so that's not a point where we'd expect any lift being created in simple Newtonian physics. In terms of pressure, it drops on the top where the airflow actually curves and bends downwards. Imagine a symmetric airfoil at 0° AoA. In front of the leading edge you'll get a stagnation point on axis with the cord with higher pressure, then as the airflow bends symmetrically around the upper and lower part of the airfoil you get lower pressure (and higher velocity) and this increases again (slows down) towards the trailing edge where you again get a point of higher pressure. The airflow did not change direction, so no lift was created. Now imagine the same airfoil at 15° AoA and assume laminar flow. The leading edge stagnation point will actually move down and back a bit. The airflow bends from this point over the leading edge and the upper curvature of the airfoil. In terms of pressure, you will see a significant drop in pressure on the upper side near the very front of the leading edge. This is where the curvature of the airflow is now highest. (At the lower side, the boost in pressure from the stagnation point will drop rapidly, as the airflow also has to curve a bit in the other direction on the lower side.) If we average the location of all pressures into a single "center of pressure" point then we see that this point has moved forward significantly vs. the 0° AoA case. The creation of lift has moved towards the leading edge, and this is in line with the simplistic explanation that lift is created due to directing the airflow downward. (As you probably know, this is why wings themselves are an unstable system.)
How does a wing whose cord is in line with the relative wind deflect air downward? I can't draw it for you here, but imagine a flat-bottomed airfoil moving through the air at the same angle as the relative wind. It's differential pressure, not wind deflection.
This is one of the better presentations I have seen. Usually, engineers are using terms that require a degree in mathematics at a very high level. This presentation, however, is easy to understand. Thank you!
Magnar
Complex numbers, some knowledge of differential equations is sufficient for a quantitative treatment of most aspects of fluid dynamics
@@hernanposnansky7154 Well, yes and no, I still remember an exam where I wrote the complete (developped) Navier Stokes equations and it took me one straight A4 page (yes, i don't write small and yes, it was before simplification with a bunch of hypothesis, but still)
I wish I had seen this earlier. I've slowly found almost all pieces of the information mentioned in this video but now I can finally put them together in a self-consistent picture including the effect of viscosity. Thanks!!!
same
Great explanation. Really appreciate the attempt to explain lift intuitively. Even though I'm an engineer, getting an intuitive feel for the forces really helps. I'm preparing a 'theory of flight' video for a drone training video, and these principles and messages will be key to explaining this stuff well. Thank you!!
Thanks for the comment Don--come back and share your video in the comments when you're done!
He hit the nail on the head. Equations are a DESCRIPTION and NOT an EXPLANATION. Excellent video.
SPOT ON! The equations come from the understanding of the science.
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This is perhaps the best video on lift, but you may be interested in these.
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. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
..
And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
Just like gravity, black holes, electrons, the function of the human body, only descriptions, NO idea how they work.
This is awesome! A lot of clarifications in one shot.
I am taking ground school tomorrow, and wanted to learn how a plane generated lift. Thank you for what you do.
it kind of peeves me when i think about my ground school textbook teaching the wrong theory of lift. Makes me think about how many other things students are mis-taught.
@@joebuckaroo82 Perhaps better yet, if you don't parrot back the misinformation, you fail the exam.
@@commentatron absolutely. That's part of the problem with public education as a whole. We become parrots rather than thinkers.
Great video. When I first learnt this at Delft, I started to search more and more. Your video is amazing. Thank you so much for such presentation. What it annoys me is that in school they still teach the non sense of explanation about vacuum over the wing pushing up the airplane due to a ridiculous small difference of curvature. These teachers never flew an airplane or never saw the shape of any helicopter blade or never saw a Sukhoi airplane making maneuvers. Glad to have found one more quality video about the real reason for lift !! 🙏🏆
A sukhoi has a thrust to weight ratio between 0.9 and 1. That is nothing like a conventional passenger jet that only has a thrust of a third or a quarter of its weight. While the vacuum explanation might be wrong, your counter examples operate in entirely different realms.
Wow, this is the best explanation I hvae ever seen !!!!
Nice lecture. Identifying pressure gradient below and above the wing and relating it to centrifugal force so we see air flow along the arc of a circle is a nice illustration helping us understand lift. Pressure effects are useful because a surface integral of pressure over the surface of the wing determines the lift force. This is easier to calculate then integrating over the entire volume of space to to determine total change in momentum. I know vortex filaments lead to a more practical method to calculate lift coeficients.
22:30
The difference between the 3 flows is the circulation:
Negative , positive but weak, large positive.
The circulation cancels the upwards flow at the trailing edge to satisfy the Kutta condition as the angle of attack is changed.
The circulation is the line integral of the scalar product of the velocity vector and the path element around any closed line outside the boundary layer.
The circulation is the result of conservation of angular momentum:
A vortex is produced as the flow at the trailing edge separates (counter clockwise) and procedes downstream, then a clockwise circulation with the opposite angular momentum forms around the airfoil.
The lift is then
density times velocity times span times this circulation.
This is the Kutta-Joukowsky theorem, which is the correct explanation of lift.
The circulation increases the velocity on the upper side of the wing and reduces the velocity on the lower side of the wing.
integrating the pressures obtained by the Bernoulli equation (conservation of energy) just outside the boundary layer will yield the exact lift force.
Great little summary.
One of the understandable presentatios for this topic. Thank you.
Man, i really enjoy watching this
Sure he can tell me why planes fly, but can he explain how he's so fly? Damn son, nice lecture.
What 😂
He's not a filthy boomer that's how
This video is very informative and can help understand the how lift generates, thank you! It was really helpful for me in the fact that when I would learn lift, there would always be a lot of misconceptions and after this video it helped me understand the misconceptions and why it is important not to base understanding of lift of the the misconceptions and this video did a great job in explaining it. I learned many important concepts in generating lift such as Bernoulli's effect, inviscid flows, lift theory, streamline curves and many more. This video is great in understanding lift and I recommend this video to people that want to learn how airplanes fly to watch this video.
Concepts described very clearly.
In fact, these concepts describes why and how certain very non-aerodynamic airfoils like a rectangular plank can create lift and fly.
This is perhaps the best video on lift, but you may be interested in these.
.
. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
..
And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
Great video! I appreciate the clear explanation of the complexities of lift generation and the different factors that contribute to it. The numerical solution of the pressure distribution over an airfoil was especially helpful in visualizing the concept. One suggestion for improvement would be to provide some more concrete examples or analogies to help non-experts better understand the concepts being discussed. Overall, great job breaking down a complex topic in an understandable way.
I agree! This professor definitely provided a detailed and clear explanation of lift and how it is produced. I also appreciate how he goes into explaining how misconceptions used to explain these concepts are not helpful, even if they seem intuitive. An example of this was his rebuttal of the equal transit time fallacy, and how "air does not like voids", while seemingly intuitive, is not an accurate explanation of aerodynamics.
Excellent presentation - thanks!
A description of the vortex system of a wing would be useful.
Yeah, this video convinced me the quote at the beginning is correct: it is an area of debate. Lift has controversial explanations in the intuitive realm.
Newtons 2nd Law is a good explanation. 3rd? Not so much.
Newton's 3rd Law states that forces must be balanced. It is a misinterpretation to say one half *causes* the other. That there is downward force implies upward force, exactly the same as upward implies downward.
It isn't causal or sequential. Further, it is in full effect on parked aircraft. And rarely do mechanics need to pry aircraft from the hanger ceilings.
Lift is a result of motion, so Newton's 2nd Law is the explanation best suited ("in my opinion", required engineering tradition).
Bernoulli is a specific case for 2nd Law. The thing that trips people up is "equal transit time" nonsense; AND not realizing each flow stream surface has it's own conditions. People try to compare the 2 surfaces relative to each other, which is a no-no. Bernoulli says pressure-velocity is reciprocal; a teeter-totter. NOT that 2 different slipstreams can be compared, in terms of magnitude, to the other.
So no "pressure on top vs on bottom" with Bernoulli, only within one surface, with respect to itself.
Pressure differences result in acceleration, which in turn sets up a pressure gradient across the top of the wing. That pressure arc requires the wing surface to be lower pressure than the longer arc above it... And up it goes. If the aerofoil is thin, all the better, because high pressure flowstreams result under the wing (being again a longer arc than those below them)... And up it goes even faster. (There's a centripetal force argument opportunity here also: with objects requiring such to travel in a curved path. And a resultant force inward. May get confusing, though. Could skip it.)
So i explain lift through Bernoulli, as a special case of Newton's 2nd Law. And since we only get lift as a result of motion: this sets up an easily approachable velocity argument: F=ma.
Liked the talk though. Always good to get a range of perspectives.
(P.s.: knowing an equation, like N-S, does not follow that anybody understands it. That's maybe something we should admit to ourselves, and students. Yes i know they pay a lot of money not to hear that). Just look at G.R..-- Now it's quantum gravity, and emergent space-time. We don't understand that either.)
Good talk. Not implying it's "wrong", just not my preference for an intuitive explanation. NYTimes was right 😉
So many diagrams (like the one you show here) don't show the deflected air, but rather show air traveling paralell with the flight path. Air doesn't do that. We need to fix the faulty diagrams.
Love this explanation video. Thank you!!
The Fundamental equation is the
Kutta - Joukowsky vector expression for Lift, involving density, velocity, span and most importantly the CIRCULATION.
There is one simple thing you have forgotten at least so far,the front of the wing kicks up the inflow air at the top of the wing just enough to solicit a reaction which causes the streamline to follow the wing down
Any theory has to fit the factual observations. The air velocity over the wing can be measured as can the drop in pressure it produces. That pressure drop multiplied by the wing area gives you the lift and is greater than the weight of the aircraft. There is some lift from the air deflected below the wing, about 20%. A flat wing will fly from the downward airflow but a very powerful engine is needed as the flat wing is very inefficient. Likewise, sustained inverted flight (not a loop!) also requires a high angle of attack and a powerful engine as it is also inefficient, not as much as the flat wing.
The majority of the lift is coming from the UPPER surface and that is where the airbrakes/lift spoilers are put. The upper surface has to be kept clean and free of ice as it is critical. At the stall, the lift disappears because it is the airflow over the upper surface that becomes turbulent.
The lower surface is relatively unimportant and that is where all the hardware is put, undercarriage (!) jet engines, flap hinges and screw jacks, and on fighter aircraft the ordnance, guns and rockets.
One additional thing... I love this video but I am kind of left empty about the reason the streamlines follow the contour of the body.
I am coming away with the reason this occurs can be explained by the fact that an inviscid flow MUST follow the body contour. But, the air molecules probably don't know much about inviscid flow so how do they "know" to follow the contour?. What am I missing? Any help is appreciated.
If the airflow didn’t follow the contour, there would be a vacuum. Air pressure forces the airflow down into that space.
@@GH-oi2jf Thank you for the reply. We did see in one of the slides a case where the streamline was separated and did not follow the contour. But I agree that a vacuum cannot form.
The atmospheric pressure does not go away when the wing moves. It is still trying to push air against all surfaces as best it can. Folks seem to forget that.
.
This is perhaps the best video on lift, but you may be interested in these.
. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
..
And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
@@Observ45er Thank you for your reply...makes sense.
@@wcottee Yes, you have to listen to what he says about that one slide.
.Also, if you understand and think that our one minute video works for you, I'd appreciate an upvote. My 11 YO granddaughter did the video editing. It really helped motivate me to finally make something.
*WHY There is Lower Pressure Above a Wing*
* ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
We should turn things around when trying to explain airplane wing interaction: The air molecule does not flow from leading to trailing edge, eventually separating or not. It waits for the wing to arrive and is then pushed up or down, and just a little longitudinally and here actually forward in the direction of the wing (and more the nearer the wing surface the particle is.)
One should have this physical picture in mind when explaining airplane wing lift.
In a wind tunnel though (or on a sailing yacht) the air flows along the surface of the wing profile, but on an airplane wing the molecule waits for the airplane to arrive (as we all normally do - a good clue for remembering this phenomenon. /Henrik Segercrantz - M Sc. Naval Architecture
correct, the air is not flowing through the airfoil, the airfoil hits the air and displaces it... correct?
if run towards a tight crowd... most likely i will bounce. an airfoil hitting air is doing that, bouncing.
Great lecture. Thanks!
I couldnt understand the logic of vacuum not possible at 21:01? Why is that? Because even the streamline on the bottom surface would get sucked up?
At 21:16, cant an air element move in the loop without rotating? An element can maintain its orientation and yet move along the circuit.
This is perhaps the best video on lift, but you may be interested in these.
. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
..
And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
Just watched Doug McLean, Common Misconceptions in Aerodynamics, and lasted 10 minutes... this guy got me... all the way!!
Don't really know what that means... 🤔
Absolutely excellent explanation! Thank you!
_Not saying professor Fidkowski is wrong, simply commenting._ 1) 6:43 When this real phenomenon is illustrated, no lecturer I've seen has pointed out that if the initial `longer path, equal transit time, faster flow, lower pressure` has an effect (however moderate,) the illustrated phenomenon means the top flow is even faster and thus has a less moderate effect.
2) My `simplified intuitive model` is the bottom side is for lift, the top side is for control, to wit: spoilers. (yes, incomplete)
I recommend using calculations and numbers proving how 10 tons of thrust can generate lift that leviates 150 tons of a plane. Otherwise all the rest are just words...for example from this presentation one should be able to calculate how much lift a directed air flow downwards can generate lift. Because for directing air flow downwards, the airfoil or the pressure field around it or both must have applied force on the air block flowing around. As a reaction to this force, the lift is generated and this can be calculated, maybe?
Fascinating
Loved this. Can you do one about wind creating rotation in wind turbine generators?
Let's be real. Y'all clicked because y'all thought "damn son this guy has the whole alphabet in his name".
Joking aside though this video has good info.
Thanks. I recognize he addresses and dismisses the Coanda effect, but placing the back side of a spoon against a stream of water (eg. from your kitchen sink) causes that water flow to readily bend to follow the spoon curvature and also applies a marked force on the spoon towards the stream. If you haven’t tried it, please do so. It’s hard to totally ignore that effect (whatever is causing it).
Water on one side and air on the other is not the situation a wing faces, though.
@@adoatero5129 If you blow a stream of air past the back of a spoon, the same thing happens.
@@BobbieGWhiz True, but the effect is much weaker (assuming the speed of the stream is similar in both cases). What's more there's no flow on the other side, unlike in the case of a wing. I'm not saying the phenomenon isn't real, interesting and at all relevant to the functioning of a wing, but wing doesn't need that phenomenon to create lift (as explained on the video). The upper side of a wing is curved mainly to keep the airflow nicely streamlined. Quick turns are bad for the airflow, again, as explained on the video (nowadays it also allows wings to hold fuel). A perfectly flat wing creates plenty of lift when it's faces the airflow at an angle as all wings do in normal flight.
That is fantastic…well done. Great to listen to his explanation.
@veritasium I would thoroughly enjoy a video from your team on the misconceptions in lift generation
pure gold, thank you
10:00 Flow turning theory of lift
How is it that air flowing off the back of the wing pushes it up but air bouncing off the bottom of the wing does not?
He didnt properly explain the curving. He just said there will be a mismatch of pressure. From bernoullis equation wouldnt the stagnant air have more pressure?
Yes. At 21:05 with the upper circulating example, he mentions a pressure difference, but that doesn't make sense because the two regions he seems to be pointing to are not tin the same flow and, therefore are not related by velocity and pressure. I want to call and talk to him about that.
@@Observ45er In inviscid flows, Bernoulli's equation applies everywhere, not just along the same streamline. If you are curious about the proof, you can probably find it in one of the introductory aero textbooks. Hope that helps
@@nfarvolden No. Can you summarize the key concept? I have several of the well known texts, but don't want to spend hours re-reading for that one concept.
The way you phrase that appears to indicate that a jet of fluid injected into an otherwise still ambient will then have a lower pressure than the ambient. Is that what you mean by"not just along the same streamline."?
12:34 - Hello. Do I understand correctly that the airflow on the top of the wing "follows" its shape, because when trying to "break away" from the wing, a vacuum is created between the airflow and the wing?
The geometry of the foil creates an upwash of the on-coming air. Air is forced to curve upwards before the wing arrives, by this it is easy to understand what's going on, on either side of the stagnation point. On one side, air is accelerated out of the wings' way , while on the other side the converse is happening, air is rammed into the bottom of the wing. This causes the pressure difference.
On the upper surface, air is opposing gravity hence less pressure. This creates potential energy which will be converted into kinetic energy(due to gravity) in the downward direction so a downwash of air aft the wing's camber is produced. The downwash is vital as it aids in pressure recovery to resist drag. This interplay between gravity and airfoil manipulation of the fluid, through which it is moving, is the fundamental reason why planes fly.
The speed differential of air flow between top and bottom can be attributed to the obstacle effect. Provided the wing is in an asymmetric orientation towards the on-coming air, the air under the wing is slowed down at the expense of the air which is sped up on top of the wing. If the the air under the wing is slowed too much(essentially just drag) , the air above is sped up too much and so cannot make the turn to stay attached to the upper surface hence a stall. The airflow will follow the wing's shape until the pressure required to force the air down is exceeded by the pressure required to hold a flow moving so fast, it's inertia cannot be overcome by gravity.
I'm not an engineer but... isn't the picture in 12:40 (high pressure above) just the exact opposite to the picture in 12:45 (high below)?
Ever have a dream where you're in a lecture, and slowly realize you've taken none of the prerequisite courses?
A wing generates lift by placing the stagnation point below the wing just behind the leading edge, that's how an upwash is induced, which is what is needed to reduce upper surface pressure 👍
Lift is due to centrifugal force caused by the mass of the air flow over the top of the wing.Replace air with mercury . tr3 b. Lift from sealed container.
08:43 Venturi theory of lift.
I feel like I am missing something.
So he shows two pictures of flow being deflected downwards and the pressure differential it causes
11:00 he shows low pressure on the bottom and high pressure on top
12:45 the high pressure is on the bottom and low pressure on top
Why do the areas of pressure switch between the two examples?
At the 11.00 minute mark he is referring to the streamline only, not the airfoil. The streamline experiences a pressure differential which is why it curves downward, higher pressure on top. At 12.45 he is referring to the airfoil, high pressure on the bottom which creates lift.
You have to notice that in the wing diagram, pressure P∞, is the same at the top and bottom edges of the diagram. Unlike the drawing at 11:00 where one side is at higher pressure. It's key to understanding
Just look at the lower streamlines only, since they curve downward, the pressure at the lower surface must be higher than P∞ (as explained by the drawing at 11:00). Now look at the upper streamlines only, again they curve down so the pressure at the upper surface must be lower than P∞. Therefore the pressure at the lower surface must be higher than at the upper surface.
Fundamental principle #1: Pressure in a curved flow *must be* higher on the putside of the curve and lower *toward the center* of curvature. The higher pressure pushes the flow around the curve. *
Rubbish - everybody knows planes fly by flapping their wings, like birds. I've been in a plane & looked out and seen them moving up & down with my own eyes. No idea why they have those whiry things at the front, though - must be to power the air conditioning.
10:55 Streamline curvature
Excellent expositions of empirical shaping laws and deductions for Intuitive Actual Intelligence. An accurate assessment and assembly of observational oscillation resonance phenomena in Mathematical Theoretical Formulae. (That's what we see)
How it happens is superimposed logarithmic condensation modulation wave-packaging formation of Navier-Stokes type formatting that you get in the time-timing arrangement of anything, not only flying.
So the why, of all is vibration-> Superspin, quantitative timed time mechanics-> 0-1-2-ness Entanglement be-cause-effect self-defining superposition-> sync-duration assembly, which is Fractal Quantum Operator Fields Modulation Mechanism.., at this Centre of Logarithmic Time Singularity, ..QM-TIME pure-math Reciproction-recirculation relative-timing ratio-rates Perspective Principle.
(Basic QM logic "says" it is because it is, => Principle)
Engineering likes to build a fence around reliability with a Safety Factor that obscures the fine structure of phenomena. "Long may they continue". But if you analyse nuances, then the inevitable result is that "All things must pass", change changes across the 1-0 probability dominance spectrum to continuously Form this Universe.
The finest quality of information existence is derived from e-Pi-i sync-duration connectivity function that operates via Singularity, ie @.dt zero-infinity sync-duration eternally, in Primes and Cofactors of vector-value Timing-spacing coordination, AM-FM Quantum-fields Condensates forming Math-Phys-Chem and Geometrical Drawing Conception of the real-time wave-package Periodic Table, Standard Model and related QM Temporal Chemistries. This is the ultimate sense of experiencing existence as "floating in flat-space ground-state No-thing-defined eternally, "flying" as coherence-cohesion objectives in temporal cause-effect continuity.
The equal transit time hypothesis states that "lift causes lift". This confuses the cause with the effect. You don't create low pressure, then get lift. Because the low pressure is the lift. You can only get the effect of lift by causing the action of downwash.
I never understood why the air on top MUST meet up with it's buddy under the wing.
It doesn't?
No it doesn't meet up. Why should it?
I really don't understand why so many physics keep saying it meets up. - And so, explain lift of wings wrong :-((
Imagine the fluid element on the upper surface is a rabbit; the fluid element under the lower surface is a tortoise. Of course rabbit can run faster than the tortoise, but the reason of rabbit running faster is not because rabbit will meet tortoise at the same time in the final (with running a comparative long distance). The rabbit is faster because of its genetic (unbalance of varying pressure distributions); it always runs faster than the tortoise.
It’s a misconception. It doesn’t have to meet its “buddy” under the wing, as you put it. The idea of equal transit times is wrong.
@@loouuiisssss296 How many rabbits does it take to keep a 152 aloft?
Regarding your simple intuitive answer about how chemical rockets work: According to Doug McLean's Michigan Engineering video "Common Misconceptions in Aerodynamics" it's really more about Newton's second law, not the third. See the section "Stretching of Newton's Third Law."
man great job!!!!
You don't use the pressure difference to get lift. Rather, the pressure differential IS the lift, gotten by acceleration of air downward. Lift doesn't cause lift.
Exactly. At time 11:40 the provided diagram indicates high pressure on top of the wing, and low pressure below the wing. The 'streamline curvature' explanation is immensely "hand-wavy." A higher pressure above the wing than below, shown in that diagram, *_is NOT lift_* - just the opposite.
If an effort were made to explain lift in 10 minutes (not 31 minutes), I'm pretty sure the instructor is smart enough to do that. As it is, I get a sense that he's not 100% sure himself.
At high Reynolds numbers , turbulence forms after the laminar boundary becomes unstable and transitions to turbulent boundary layer whose displacement thickness grows as the square root of the length.
The Navier Stokes equations do not describe this process, a turbulence model must be added. There are several models that are used , approximately.
Rather than the arbitary term "flow turning", it would be much more helpful to introduce the concept of circulation. Once the concept of circulation is grasped, it is easy to establish the induced force from this circulation to the force generated with Kutta-Joukowski.
@ 28:40 reason for lift is "Downward turning" but see here: ua-cam.com/video/QKCK4lJLQHU/v-deo.html from Common Misconceptions in Aerodynamics
Speed Bird
Exactly
your link is essential to debunk the confusion this guy is generating, some is real garbage!
14:04 How does the aerofoil turn the flow down?
The geometry of the foil creates an upwash of the on-coming air. Air is forced to curve upwards before the wing arrives, by this it is easy to understand what's going on, on either side of the stagnation point. On one side, air is accelerated out of the wings' way , while on the other side the converse is happening, air is rammed into the bottom of the wing. This causes the pressure difference.
On the upper surface, air is opposing gravity hence less pressure. This creates potential energy which will be converted into kinetic energy(due to gravity) in the downward direction so a downwash of air aft the wing's camber is produced. The downwash is vital as it aids in pressure recovery to resist drag. This interplay between gravity and airfoil manipulation of the fluid, through which it is moving, is the fundamental reason why planes fly.
The speed differential of air flow between top and bottom can be attributed to the obstacle effect. Provided the wing is in an asymmetric orientation towards the on-coming air, the air under the wing is slowed down at the expense of the air which is sped up on top of the wing.
The aerofoil does not turn the air, the atmospheric pressure does. It's a fluid so wants to take the shape of the object , as dictated by gravity.
Very helpful, thanks
20:08 Inviscid flow
The equal transit time hypothesis confuses the action with the reaction. It says that you create a pressure differential, then you get lift. However, a pressure differential is lift. Lift does not cause lift. Downwash causes lift. Lift is an upward force caused by a downward force. Wings that don't send air down, don't get lift. The question of how a force transmits to a wing is a different question than what the causes the force is. The how question is explained by Bernoulli, while the what question is answered by Newton.
Pressure is just force over area, ALL forces exerted on ALL macroscopic objects are ultimatly just pressure over an area of the object, if the force is from another solid object in contact with them or from fluids in contact with them.
Perhaps at 2:56 he could mention the underwater wings boats for similarity with airplanes.
Hay, what is the effect on a wing's total lift due to the air pressure INSIDE the aerofoil ???
Nothing.
This is perhaps the best video on lift, but you may be interested in these.
.
. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
..
And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
i am no flight student but i came here because i had this question. "If airfoil workrd on the basis of bernoulis principle then how planes fly upside down ."
Most plane that can fly upside down have flat wing, they mostly play with the angle of attack.
@@ameunier41 Wrong. ALL airplanes can fly upside down; it's just a question of how efficient they are in those two conditions. As usual, you've made a simple declaration that is worthless. If you aren't ready to discuss the nuances, STFU. All you're doing is increasing the confusion WRT the subject.
@@craigwall9536 Although I would agree that all airfoils can be flown upside down but with different efficiency, in a way the more pointed example is why until very recently it was thought that a helicopter cannot be flown upside down (each helicopter blade is also an airfoil, and it was thought that upside down could not create lift). I haven't looked closely at the recent demos that show it is possible to fly a helicopter upside down but would suspect extending the limitations of angle of attack in a negative direction (Just guessing!)
To expand on this, wings can produce lift upside down, it doesn't need to be symmetric or flat. A symmetric airfoil produces lift equally well if the angle of attack is positive or negative. It produces no lift if the angle of attack is 0. A naca0012 airfoil does not care if it is upside down or right side up. Cambered airfoils can also be flown upside down and create lift, but it might still before attaining an angle of attack that can support the weight of the plane. Cambered airfoils can create lift at 0 angle of attack. And so it would need more negative AOA to fly upside down. An example of a cambered airfoil is the Clark y.
EXACTLY! If a curved airfoil redirects the air downward, then a flipped airfoil would redirect the wind upward.. So is not the redirection of airflow only, so there could be many reasons LIFT is caused not only one...sometimes is redirecting and sometimes is angle of attack sometimes is both. what about flying sideways?
illustrate your point using sailboats. The properties of airflow may mimic traffic flow,both people and vehicles. Sailplanes have no thrust, they're always going downhill. Can you explain a curveball?
It works exactly the same. You have the sail at a positive angle of attack. This redirects the airflow, resulting in a lifting force. Finally, on a sailboat, through the keel the forward component of the lift vector accelerates the sailboat. This requires that there's also an angle between the ship and the sail/lift vector, ideally around 45°.
21:12 can someone olease explain why can't we have stagnant air there, he said its because of Bernoulli principle but Bernoulli's principle is applicable to a SINGLE streamline flow not two spearate flow like this, the air above is moving that flow is completely separate form the stagnant air, why qre we relating the two?
You can compare the static and dynamic pressure of two streamlines if they started out with the same initial conditions. It is the same principle used in the Pitot system.
Came here from Fly with Magnar's riposte of an inaccurate explanation of heavier-than-air flight by a certain famous astrophysicist!
And these are all lacking in one aspect or another:
Newton's (secret, apparently) 4th law hodge podge- the Kutta condition, viscosity, and the Coanda effect, and various takes on centripetal/centrifugal circular logic....not
Does an air stream not posses the quality of both adhesion and COHESION. Stream deflection upsets or imparts energy to. the inherent energy natural to the cohesion of the air in the stream.
Oh. You know I mean. Air is not a localized entity. Drawing a section thru it for the convenience of a geometric solution is under rating the nature of it all. Go flying in a j3.
Uhhh... No.
If it's about deflecting, turning, or otherwise reacting against the air, you need more thrust than both the weight and drag. Also, you have to account for the energy of the rising air that meets the leading edge (which I suppose you could say is part of induced drag.) Air isn't all that dense compared to the aircraft. You know that gravity is accelerating the mass of the aircraft at a constant rate. Let's just say a 500 KG aircraft at 9.8m/s^2. You can give yourself a simple graph and ask yourself what is a reasonable amount of mass to acceleration to still get 4900 Newtons for level flight; accelerate how much air, how quickly, and how much air do the wings actually touch anyways?
The only forces that affect the wing are the fluids that actually interface with it. Yes, stuff is going on at significant distances from the wing... but the wing doesn't care. So as long as there is more net pressure on the bottom surface of the wing than on the top, you get lift.
We know that pressure is not uniformly distributed across the wing, but it also corresponds to local air velocity. Air over the top of the wing is moving at a great initial velocity and then slows at the back. So when you look at a pressure distribution, the very end of the wing is typically positive in pressure (pushing the wing down.)
So there's no argument that lift is from a pressure difference, that the air velocity corelates to the pressure that the wing actually sees, so the question is: Why is the air moving faster over the top of the wing? Specifically, why is it moving faster than the wing is?
The only obvious answer is that there is lower pressure (vacuum) above the wing. But, logically, this would appear circular in reasoning. What creates the vacuum, that accelerates the air, to create the vacuum?
To that end, I would pose the question: What was the initial state of the wing?
Answer: Not. Moving.
If you have a wing in dead-still air with smoke or fog, what will you see the air do when you start everything up from the perspective of the wing?
What? Why would you need more thrust than both the weight and drag?
Lift increases with velocity, so at some point lift will equal weight. At this point thrust needs only to be equal to drag.
What you suggest maybe applies to a rocket, but not a wing.
What you write about pressure is all in the video as are answers for some of your questions. Did you even watch it?
Just to beat a dead horse, a 747 has 224000 lbs of thrust and a max take off weight of 987009 lbs. Tell me again how thrust must equal weight plus drag to fly?
A rocket doesn't push out gas. The gases pressure is what causes the gas to expand and thus push the rocket and the gas particles equally away from the epicentre of that pressure. Since gas flow can deform to suit a conduit, and the rocket can't, and an equally emparting momentum into the gas and rocket creates a higher flow velocity for the less massive gas and a lower velocity for the more massive rocket, the gas is manifestly jetting away from the rocket faster than the rocket is accelerating away from the starting point of the ground.
The question of how a force transmits, is a separate question as to what causes that force. You should have separate answers.
Did he even explain why the velocity of air increased over the top surface?
Yes. Lower pressure means higher velocity. That's Bernoulli.
Lower pressure also means that the air is changing direction, curving towards the lower pressure and away from the relatively higher pressure on the outside of the curve.
@@xnoreq Can you explain why there is lower pressure there because I normally get the answer "because the air has a high velocity, bernoulli", so you are stuck in a bit of a loop if you understand me.
@@guypaterson9503 Imagine a symmetric airfoil at 0° angle of attack. In front of the leading edge a high pressure point ("stagnation point") will form, air slows down, simply because there's an obstacle. From this high pressure point, the airflow has to accelerate/pressure has to drop as it curves over the leading edge and curved airfoil - around the obstacle so to speak.
Note that this even happens on the lower side on a cambered airfoil, just to a lesser extent. The reason for that is that the much tighter curve around the upper side causes higher acceleration/drop in pressure.
As you noted correctly, Bernoulli doesn't explain why there's lift. It only points out the relationship between pressure and velocity and is only valid within a flow field.
difference between the top and bottom surfaces of the wing produce lift. how does an airplane fly upside down?
I imagine you just re-do the calculations with the airfoil in its new orientation (upside down). Believe all the concepts described would still be in play.
The same reasons, but oNLY if you have the correct reasons.
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This is perhaps the best video on lift, but you may be interested in these.
. *WHY There is Lower Pressure Above a Wing*
*ua-cam.com/video/3MSqbnbKDmM/v-deo.html*
A LIKE would be appreciated.
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And read this:
*Understanding Lift Correctly*
*rxesywwbdscllwpn.quora.com/*
If curved or flow turning is answers, how does a flat wing turn air enough to generate lift
Through angle of attack. This means placing the stagnation point just just below the leading edge, so airflow will travel back to the leading edge and be forced to make a turn to reverse its direction of flow. Air has inertia , so any directional changes require a force , which is provided by the atmosphere. However flat wings have high drag to lift ratio, so are inefficient lift devices at subsonic speeds. That is why he states that his lecture pertains to subsonic flight.
At supersonic and hypersonic speeds, flat swept back wings generate lift differently. Lift from shock waves , known as compression lift.
@@singh2702 So misconception of lift is really the misconception that there is only one way to create lift... Rather lift can be created in many ways. and the most efficient depends on speed, airfoil shape, etc etc
@smooth-dancer That is correct. Wing geometry and design are optimized for a certain application. This video deals with lift at subsonic speeds and fluid behavior at these speeds. At hypersonic speed, the fluid behaves very differently, so bizarre that Newton's sine square law actually applies, for example, the X-15 rocket plane. There are misconceptions of wings generating lift at different speeds, but one thing will always be true, lower pressure above and higher pressure below, at all speeds.
@@singh2702 why am i not flying? Its not my shape... or weight.. its because gravity pushes me down. So lift is when enough energy pushes me up stonger than gravity. So a rocket pushes down enough stronger than gravity until theres no gravity. An airfoil cuts the air in a way to pushes up the airplane. A sail cuts the air in a way that pushes the boat sideways and the keel cuts the water so pushed the boat the other way and therefore the boat moves forward. Interesting topic
@smooth-dancer The atmosphere we are immersed in puts pressure on us at 14.7psi or 10 metric tons per square meter. This pressure is applied in every direction, top bottom left right diagonally, hence all pressures cancel each other out so there is no net force to counter the force of gravity, this is why you are not flying. Lift is about reducing that pressure applied in the downward direction so that the pressure applied upward can overcome weight. All the examples, besides the rocket, require this pressure differential to overcome forces.
gurney flaps?
Is he saying that Bernoulli's theory can be disregarded as an explanation for lift?
I understood from him that lift is purely related to the pressure gradient around the aerofoil.
I'm left a bit confused. surely the reason you have a pressure gradient is because there is greater and less pressure on different sections of the aerofoil. Difference in pressures is therefore Bernoulli's theory.
+bhfireblade, he says to avoid using the formula to explain lift, as will be of no help. But as you say, he states that pressure differences are in accordance to Bernoulli.
bhfireblade Bernoulli only applies along a streamline, but it doesn't explain anything other than the link between velocity and pressure with a streamline. It is the shape of the aerofoil that creates the lift force by curving the flow over and under the aerofoil.
So as a flight instructor, I haven't found any lecture done by Ph.D. or professors explaining why does air gets accelerated on the airfoil. because every book in the field for student pilot explains the lift with newton's 2nd la and Bernoulli's equation. Here's what does the book say: because of the curvature of the airfoil, the speed of airflow above the airfoil is accelerated, and therefore pressure drops. why does the airflow is accelerated by airfoil's curvature? well, this is how it is. I've never put my self in trouble with this explanation(because it's pretty enough for pilots) but is there anybody who can explain why does air gets speeding up?
If something accelerates and it has mass, a force is responsible. That's Newtons 2nd law: F=ma
The force is pressure. Higher pressure upstream is accelerating the air as it moves downstream into the lower pressure area above the wing.
The low pressure above the wing is the result of the airflow being curved as it flows over the top of the wing.
The airflow tries to turn the corner and feels a "centrifugal force" pulling it to the outside of the curve as it stretches and tries to stay attached to the top of the wing. This generates a pressure gradient from low to high, from inside to outside of any curved streamline.
@@GZA036Great explanation! Bravo! This falls in line with what I have read.
A real observation flying through a rain zone. The water droplets on the wing should have been whipped away past the trailing edge, but could be seen creeping towards the leading edge. Boundary layer? Does not seem likely. The boundary layer would be moving forward at a speed somewhat less than that of the wing, so it would be moving slowly backwards relative to the wing, not forwards. I asked the same question elsewhere, but got no takers. Anyone here, please?
Boundary layer is defined by a turbulent zone that has a velocity gradient from 0 airspeed at the wing surface to the freestrean velocity at the outside. Inside the BL flow is somewhat unpredictable. Turbulent areas have flow vectors that can be independent of freestrean vectors. If it was an airplane window on the side of the plane with water on it, the water was in a turbulent area of separated flow.
Just like when he showed the wing with separated flow, some of that air is moving backwards compared to the freestream.
@@anonymous12345678935 Thanks. These were droplets on the wing which was directly under my window
That's one of the more incomprehensible talks on lift I've seen on UA-cam. How about keeping things simple?
First imagine two chaps engaged in arm wrestling. Let’s say instead of joining hands they both push against a plate with their hand flat. So the left guy pushes against that plate, the right guy pushes against it, and as long as they push with the same amount of force, that plate won’t move.
Let’s say the left surface of that (vertical) plate which is exposed to the forces of the left guy’s hand pushing against it could be compared to the lower surface of a wing (mentally turning the plate 90° counterclockwise) and the right surface of that plate which is exposed to the forces of the right guy’s hand pushing can be compared to the upper surface of a wing.
That wing won’t move and will not experience lift as long as the net sum of forces below and above de wing (or the opposing forces of the left guy and the right guy) are the same.
Now imagine the right guy becomes exhausted and his force weakens. The left guy is now able to push the plate towards the right, however the forces exercised byt the left guy didn't increase, it stays the same (in this example). The plate is moving right only because there is less amount of countering pressure from the right guy’s hand.
The plate will move even more forcefully to the right if when the countering forces of the right guy weakens, the left guy decides at the same time to push even more forcefully against the plate.
The net sum of both forces, that is the force of left guy pushing against the plate (or, if you rotate that image counterclockwise by 90°, pushing from below upwards against the wing) and the countering force of the right guy (pushing from the right or downwards if you rotate everything by 90° counterclockwise) is lift in the case of an airfoil or wing, if the net sum pushes the airfoil or wing upwards.
Now who are those two guys in the case of the airfoil or wing ? They are nothing more and nothing less than the *static air pressure* pushing at all times against the wing, if there is no airflow, then the "plate" will stay in the middle and the wing will not experience lift, but if there is airflow, aerodynamic effects generated by the AoA and the camber of the wing influence the pressure the surrounding gas molecule can exercise on the wing by aerodynamic effects which act on both the lower and the upper surface of the wing. That is, by aerodynamic effets, the force air pressure can exercise against the wing is either weakened (above the wing) or increased (below the wing). That what is known as pressure differential.
Now the upper surface alone cannot create lift, it only contributes to lift.
The reason for that is that there is no such thing as suction or negative pressure. Pressure is always positive and air is always pressing against the upper and lower surface of the wing, at all times. There is no force pulling the upper surface of the wing upwards, as air cannot grip the wing and pull away from it. The wing is always lifted because air pushes the lower part of the wing upwards.
Back to those arm wrestlers : if there is no left guy present, it does not matter how much the force of the right guy will be weakened, there will never be any push towards the right (there will never be any lift if the image is rotated counterclockwise by 90°).
So there can only be lift if gas molecules around the wing are exercising pressure against the wing (an atmosphere featuring pressurized air needs to be present) and if there is air present pushing against the lower surface of the wing.
Imagine putting the wing in a mold filled with low amount of water just so that the wing will float and that the forward and aft stagnation points are just level with the water surface and the upper end of the mold. If wind blows against that wing, airpressure pushing downwards on the upper side of the wing will decrease as air flow hits the upper part of that wing , however as there is no airflow below the wing, the wing will be unable to lift from the mold. Also airflow will not be bend downwards against the water surface, it will just flow around the upper part of the wing and continue afterwards horizontally in a laminar flow. No lift is generated by the upper surface of the wing alone.
Therefore, the pressure differential between the air under the wing and the air above it is decisive. It explains lift. But how is the pressure differential formed ?
I’m with Bernoulli on this one. Airspeed of local air flow must be different in order to influence the effect static air pressure has on the wing. That is, the faster the airflow, the less force static air pressure can apply on the wing.
So in order to get lift, local airspeeds below and above the wing must be different, locally increasing or decreasing the pressure created by the static air pressure.
This works even in other situations.
Behind a long 40 ton semi-trailer truck, there is a lot of slipstream. Now you might think that this slipstream is enough for a cyclist to be able to travel at high speeds, like 60 - 65 km/h on the flat, without much effort. Actually, that’s not the case. 5 meters behind that truck, the air feels like not moving at all (calm air), however a considerable amount of force is still needed to overcome rolling resistance in order to not lose the semi-trailer. However, there is a small region about 2 m behind that semi-trailer where very turbulent air dominates, and 20-30 cm farther away, it calms down to what feels like total calm. Suprisingly, when the whole cyclist is situated in that calm air, the cyclist must steadily pedal in order to stay with the truck, but if the distance is decreased so that only the back is in calm air but not the frontal side of the cyclist (the effect is increased by getting as upright as possible on the bike), there is a net force pushing the cyclist forward against the truck. This can only be explained by more air pressure hitting the back of the cyclist than his frontal aspect. The effect is maximized by staying with the face and torso in the most violent turbulent air (it feels as if one is continously slapped in the face) whilst exposing the lower back to calm air. Not only is it no longer necessary to pedal, it becomes necessary to slightly brake. My explanation is that the highly turbulent air slapping against the upper part of the body (face, torso) diminishes the effect static air pressure has on that region, whilst the calm air hitting the back is not diminishing the effect of static air pressure, creating therefore a pressure differential which pushes the cyclist against the semi-trailer. One has to very carefully keep the body in that « window of opportunity » as the region this effect happens is no wider than about 10 to 20 cm (a distance of about 1 to 1.2 meters between the end of the semi-truck and the front wheel). I was therefore surprised that the best spot behind a semi-trailer would also be most noisy place where I would constantly be hit hard against my face and torso from both sides of the slip stream.
Another example : imagine a vaccum suction cup holder, once installed on a window pane, it actually does not suck on the window to stay put. The part facing the window pane can be compared to the upper wing, the part facing you can be compared to the lower wing. So the "vaccum suction cup holder" remains put because static air pushes it against the window pane (that could be compared to lift) the only difference is, in order to create it, there is no need for airflow because the surface facing the window is hermetically sealed of and the lower static pressure is permanently maintained so there is no need for dynamic airflow over a curved surface at an angle of attack in order to create a local reduction of static air pressure hitting the wing.
Now, it would be foolish to say the inner part of that suction cup holder created "more lift than the outer part" - as no force is pulling the inner surface against the window pane, only the outside static air pressure is pushing the suction cup against it. Since the inner side is hermetically sealed and no airflow is occuring, there is also no need to send air masses downwards. You could install a suction cup holder on the lower side of a horizontal ceiling window and it will still hold your objects because the static air pressure on the outside and lower surface of that suction cup pushes that cup upwards against the window pane.
The difference with a wing, there is no hermetical seal and therefore the local decrease of air pressure on the upper surface of the wing must be created aerodynamically and dynamically by airflow streaming against the wing.
Therefore I still think that Bernoulli is a good explanation why air passing along the surface of a wing decreased the effect static air pressure exercises against the wing and therefore explains the pressure differential.
Ah and just to be clear: Bernoulli is *not* the same as "equal transit-time theory" which, of course, is totally wrong
21:59 - But why? I am still missing something.
It's helpful to distinguish two separate questions. How a force transmits to a wing? And what causes that force?
Hi guys, thanks for a video however I found it quite confusing. Especially the explanation of pressure gradient around the airfoil. Below the airfoil as well as above the airfoil, there is lower pressure compared to the atmospheric pressure (to be exact, there is a few spots along the airfoil where this does not apply and the pressure on the airfoil surface is higher than atmospheric pressure - area around the stagnation point and area close to the trailing edge), so how can you explain the pressure gradient this way? How can you even say that below the airfoil, there is high pressure. There is not higher pressure, there is a lower pressure compared to the atmospheric pressure, meaning pressure of free stream. Only reason why the airplanes can fly is the fact that above the airfoil there is even lower pressure compared to the pressure below it, yet both of them are lower than atmospheric pressure - pressure of free stream. The explanation of the pressure gradient and the fact that streamlines are curved is nonsense in my opinion however I really like the idea of using the conservation of momentum when explaining the lift force. It makes much more sense than the Bermoulli's equation. Could you please answer to my comment and make your explanation more clear? Maybe I just misunderstood your concept.
Jiri Dolinsky the airflow below the aerofoil is actually much slower than that goes over the aerofoil. If two air molecules get to the aerofoil at the same time the one that goes over the top of the aerofoil takes about 2/3 of the time taken by the molecule that goes under the aerofoil. According to Bernoulli the slower flow has to have a higher pressure than the flow prior to reaching the aerofoil and the faster flow much have a lower pressure.
The pressure gradients as described here are normal for aerofoils that are not described as having laminar flow (as fitted to the P-51). Most of the lift generated by a non-laminar flow aerofoil is from the front 1/3 of the upper surface. After this point the pressure rises back to normal air pressure at the trailing edge.
I have the same doubt. He first says that pressure difference is caused by curved streamlines. Then, he says that streamlines are curved because of pressure difference ... It is not clear what generates the pressure gradient.
@@rauldeira116 I find this, and most others like it, just focus on the fallacies, and then have a simple statement that because we curve the airflow downward we must be generating lift. This video (terrible recording, watch in parallel with another show the slides) goes deeper into how this works, any why Bernoulli only relates to straight streamlines. ua-cam.com/video/QY2pS-xXC_U/v-deo.html Note that he actually appears to give some credit to the Coanda effect, so they still can't seem to agree
Why do you consider streamlines nonsense? There are many ways of looking at and estimating the effect of a wing moving through air. One has to come to grips with the fact that a wing moving through fluid is the same as a fluid moving over a wing. Once this leap is taken, it is easier to visualize streamlines as the effect the wing is having on the fluid. Once you are able to see the effect the wing has on a single particle of air, it might help you understand lift by assuming the streamlines do not collide and the effect is on a large number of particles. But understand that it is one more to look at the forces involved to intuit the goals of a successful airfoil. Saying, I want to understand all the complexities of why a shape moves a certain direction when propelled through a nearly inviscus unbounded medium without using an explanation that simplifies the visualization of particles interaction in a way that can be used to estimate lift is a little short sighted. Pressure and partical movement are all part of the same elephant.
So a wing (air foil) with a zero angle of attack has no lift??? Obviously, flat wing can generate lift with a positive angle of attack (Hand out of window or balsa wood glider). Presumably the air foil shape causes more lift than a flat wing.
Idea of lift is simple. Introducing of obstacle into flow will change velocity of flow (direction and speed). Any changes like this call fluid acceleration/deceleration. Any acceleration/deceleration will cause fictitious force ( remember what happen when we push brake pedal in car or turn car ) . Such force pushes fluid and causes pressure changes 9 as your fasten belt in car) . As long as pressure distribution around obstacle is not symmetrical ( as around not-rotating ball) obstacle will experience some net force named lift .
+Andrew Pa i agree that the explanation in the video still fails to explain exactly how a low pressure area is created above the wing-saying it must exist by examining the air flow is not an explanation.
The conclusion that to explain lift one must consider viscous and inviscid flow simultaneously is, unfortunately, the conclusion-i expected it to be the starting point for explaining the colorful pressure distribution diagram that seems to be produced using Finite Element Analysis (?)
Since the simulation model seems to produce flows and pressures that match practical observations, maybe examining the FEA parametric variables step by step would help explain what happens and why.
Your explanation: I'm not used to dealing with fictitious forces as part of a valid solution in physics, so I did not understand most of your explanation. You seem to be getting inertia involved but it got quite confusing-likely due to the english structure. There may be a valid explanation lurking there... would you please re-write it perhaps just using Newtonian forces that I can understand and that can be validated in a lab, wind tunnel, etc.?
Regarding the ending part of your comment, you may have missed that part early in the video which points out that your explanation of non-symmetrical shapes and different lengths of flow does not account for how the symmetrical camber wings of some aerobatic airplanes produce lift or how an airplane flying inverted produces lift on the side of the airfoil that is opposite to your explanation.
Also, per your explanation, an airfoil with a relatively flat lower surface and a "wavy" corrugated upper surface should produce an incredible amount of lift - in reality it doesn't.
You seem to have a typo error while giving an example of non-symmetrical flow: "as around not-rotating ball" should probably be "as around rotating ball." This is because a non-rotating ball would have symmetrical flow around it and net zero lift, whereas a rotating ball WOULD "rotate the airflow with it" to a new direction (the Magnus Effect), which.will result in lift in the opposite direction.
so it is magic
David Walter Hughes
The media are idiots and cannot tell the difference from a Cessna 150 and an Airbus A380, much less understand aerodynamics, or anything to do with aviation.
At 21:06 Prof Fidkowski does seriously misspeak. I am very surprised he said that.
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He glosses over just what the "pressure difference" would be when using that "Bad Bernoulli" misconception. _IF_ speed had something to do with it, the "moving air" would have lower pressure. That is the air farther above and, therefore, [if' moving' air had a lower pressure - it doesn't] the flow would curve UPWARD away from the wing. SO this is a double error on his part. OOPS!!
So, yes, what he said is wrong. O U C H ! !
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. . . In addition, the air right above the wing IS MOVING! In fact, it is moving in the opposite direction of flight - WHEN viewed from the ground as a wing passes by. SO. . . NOT ONLY can't you have stagnant air there, you don't! PERIOD! We SEE it moving toward the trailing edge BOTH in the wind tunnel and in flight. . .
I think I should talk to him. . .
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Also around time 23:38 He glosses over the fact that that "sharp turn" around the trailing edge is a HIGH acceleration. He only mentions needing a sharp Pressure Gradient. He doesn't explain WHY viscosity and boundary layers make the physical flow off the trailing edge - that is very vague.
I claim that it is the need tor a very high Pressure Gradient and the air's inertia that helps us understand why BOTH flows leave the trailing edge like that. Because it must. Don't confuse things with inviscid flows.
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You must cover the important fundamentals first, then when the student understands that, THEN you can add the more complex and issues beyond the basics.
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For a wing's lift, it is best to start with the fact that lift comes from ONLY the Top-Bottom pressure difference, Then, explain how/why those two are formed.
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AFTER THAT, then. . . talk about the stall, upwash, downwash, tip spill, tip vortex, induced drag, form drag and intersection drag.
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I also feel that he should NOT start with all this "separation" talk and the turbulence examples. That is not part of flying. That is part of NOT FLYING!.
I've seen this in my initial field of Electrical Engineering. They want to explain everything up front and put too much unnecessary things in too early.
28:33 Summary
As shown in the figure(1), water bends downward and moves downward along the curved surface.
Obviously, water does not produce pressure gradients by viscosity. At the same h depth, the force of water on the curved surface decreases. Why does it decrease? Because it is a curved surface, the flow of water requires centripetal force, so part of the gravity of water is used to provide centripetal force.
Obviously, even if the water is not viscous, the force acting on the curved surface will decrease, that is to say, the pressure gradient will occur in the normal direction of the curved surface. So your understanding is incorrect.
It is not the curve movement that supports the lower pressure, but the lower pressure support curve movement. First of all, there must be centripetal force, then there will be curve movement. You violated Newton's law. Newton's law says that force is the cause of the change of motion state of an object. Without centripetal force, how can there be curve motion? There must be centripetal force first. Precisely speaking, there must be a force perpendicular to the direction of motion.
And because the curvature of the surface is not the same everywhere, the centripetal force is also changing. Therefore, centripetal force can not only be provided at the beginning, so it must be adjusted constantly. Therefore, it is wrong to provide the theory of centripetal force only at the beginning of the movement. So, how to constantly adjust the centripetal force?
You said that viscosity is one of the reasons for centripetal force, so what other reasons? Obviously you admit there are other reasons, so what are the other reasons?
You have to admit that when water flows along a curved surface, centripetal force can be generated without viscosity. This is because water is heavier than air, and gravity makes water on a curved surface. It is not because of the viscosity that water moves along the curved surface, but because gravity makes water move along the curved surface. Gravity provides centripetal force for water to move along the curved surface. These facts negate your theory.
(1): qph.fs.quoracdn.net/main-qimg-b46fe6a57a535484aa14bce3bd684154)
Come from
qr.ae/TWN85L
Totally misses the point. It is not about airflow .Lift is created by the change in the properties of the disturbed airmass in that the kinetic energy of the particles is not random but parallel to the direction of movement of the airfoil. Since gasses occupy only a small fraction of the space in which they are present the amount of gas in any region of space can be increased without a change in pressure if the direction of the motion of the particles can be controlled
In the case of the Coanda effect it doesn't matter whether you're blowing a laminar jet over a wing or moving an airfoil through still air.
Both framesets are correct.
No, thats not right. A Jet (or Free-Jet) pulls additional air from its surrounding which "pulls", for example, the sheet of paper up. It's actually visualised pretty well in this presentation (18:57). There is a specific velocity distribution (also visible on 18:57) needed to get this effect. This velocity distribution doesnt exist if you just flip around the coordinate system to move through stationary air, because where should this distribution now come from?
Actually yes it does. The Coanda effect only applies to jets and doesn’t apply to the airfoil, this is because jets (such as those in a hot tub) have to do with entrainment if surrounding fluids. What happens on the airfoil looks similar to the Coanda effect but it’s actually not. This was explained much better by a P.H.D fluid dynamics professor but unfortunately I can’t find the link to the page where he explained
A good explanation - but I think he should get together with Holger Babinsky at Cambridge (perhaps he has by now).
Nevertheless, there are major shortcomings in the presentation. 1) It is coming from the 'conventional' viewpoint of a stationary wing with air flowing over it, as in a wind tunnel, 2) It does not describe the reason why the air moving over the upper surface of the wing is moving faster than the air moving beneath it. 3) It does not describe 'holistically' the total flow field around a wing.
Thinking about item 1), in the 'real world' the air is stationary until it encounters a wing moving through it. The wing must, therefore, be displacing the air, which, at some time after the wing's passage, returns to a state of rest. It was not and never was "flowing". 2) Circulation theory is not mentioned - but circulation theory is not merely theory, it is a physical fact. A wing is effectively generating a vortex around itself, which is stripped away or shed, as the wing continues on its way. 3) The downward deflection of the air (Newton's Third Law), as a result of the passage of the wing, is a manifestation of the vortex being shed. It can be seen in the 'real world' when an aircraft flies low over the cloud tops. There are numerous photos showing the 'trench' in the cloud tops, caused by the passage of the aircraft.
How can 10 tons of thrust keep a 100 ton aircraft in the air?
We all know that on an inclined plane, we can lift an object by a force much smaller than its weight. The same is true of airplanes, which are also on inclined planes and lift themselves into the sky with a small force.
This inclined plane is made up of wings and air. One is the wing inclined plane, the other is the air inclined plane. The wing has an angle of attack, so the wing is an inclined plane. Air flows through the surface of the wing, so air is in fact an invisible inclined plane. The air inclined plane consists of two parts, one is the air inclined plane at the bottom of the wing and the other is the air inclined plane at the top of the wing.
The bearing capacity of an inclined plane is determined by the material of the wing. The bearing capacity of air inclined plane is determined by the relative speed of air and wing. Under the condition of no stall, the greater the relative speed, the greater the bearing capacity of the air inclined plane, and the heavier the air inclined plane can bear.
The angle of inclined plane is related to the angle of attack, but not necessarily equal to the angle of attack. If the angle of attack is too large, the air inclined plane on the top of the wing will be destroyed. In this way, the support of air inclined plane will be greatly reduced.
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I prefer Dilbert's teeter-totter law of levitation.
@@johnsherman7289 why？
Bending the airflow down on top of the airfoil creates lift 13:40, but bending the airflow down on the bottom has no meaningful effect 7:35. Hmmm.
This gives us flight instructors a dilemma. The FAA uses Bernoulli and Newton to explain life. This fella says, nah, don't mention those guys. So either teach the wrong thing and the student passes the test, or teach the wrong thing and have them totally confused.
But Newton does apply here and is a very important component of the explanation. The airflow is directed downward which pushes the wing upward.
Bernoulli also applies, but doesn't really provide anything useful to explain why there is lift. Bernoulli's principle only applies within a flow field and describes the relationship between pressure and speed. So it can tell you that higher speed also means lower pressure, but it doesn't tell you why this happens in the first place.
@@xnoreq If the airflow is directed downward via the combination of laminar flow and the shape of the wing, how does it actually push the wing upward if the downwash doesn't occur fully until it flows behind the wing and is separated from it?
@@careywaldie6735 By the time the airflow separates from the top of the wing at the trailing edge, the airflow's direction has already changed, so that's not a point where we'd expect any lift being created in simple Newtonian physics. In terms of pressure, it drops on the top where the airflow actually curves and bends downwards.
Imagine a symmetric airfoil at 0° AoA. In front of the leading edge you'll get a stagnation point on axis with the cord with higher pressure, then as the airflow bends symmetrically around the upper and lower part of the airfoil you get lower pressure (and higher velocity) and this increases again (slows down) towards the trailing edge where you again get a point of higher pressure. The airflow did not change direction, so no lift was created.
Now imagine the same airfoil at 15° AoA and assume laminar flow. The leading edge stagnation point will actually move down and back a bit. The airflow bends from this point over the leading edge and the upper curvature of the airfoil. In terms of pressure, you will see a significant drop in pressure on the upper side near the very front of the leading edge. This is where the curvature of the airflow is now highest.
(At the lower side, the boost in pressure from the stagnation point will drop rapidly, as the airflow also has to curve a bit in the other direction on the lower side.)
If we average the location of all pressures into a single "center of pressure" point then we see that this point has moved forward significantly vs. the 0° AoA case. The creation of lift has moved towards the leading edge, and this is in line with the simplistic explanation that lift is created due to directing the airflow downward.
(As you probably know, this is why wings themselves are an unstable system.)
How does a wing whose cord is in line with the relative wind deflect air downward? I can't draw it for you here, but imagine a flat-bottomed airfoil moving through the air at the same angle as the relative wind. It's differential pressure, not wind deflection.
fastest way to put me to sleep is watch a guy talking in front of a screen or whiteboard