What causes lift [Aerodynamics

[@9wyn]

Based on the concept that a longer upper surface should give more lift, a German designed a wing using airfoil with a bulge on top that came to be known as a cat’s-back wing. A test pilot reported that the airplane waddled around in the air like “a pregnant duck.” Einstein the airfoil designer later noted, with chagrin, that he had goofed.

[@prof.vanburen]

Einstein better stick to relativity!

[@nelaturinikhil9238]

best explanation I have ever heard

[@prof.vanburen]

Aw thanks!

[@manaoharsam4211]

Best explanation I have seen . Looks like this person has a good grasp on the subject. I was just curious. Tried to find an explanation on my own . I am still curious what really transpires in Coanda effect . Thanks for great lecture.

[@prof.vanburen]

Thank you so much! I appreciate the kind words. In this case, while it has the name of the Coanda effect, I find it more relatable to think of vacuum/voids. If the fluid wants to lift off from the surface, it creates a void or vacuum underneath, which pulls it back. In order to successfully separate, it needs to find enough convincing (usually from adverse pressure gradients) to overcome the force to fill that void. Thus, flow wants to stay attached unless otherwise convinced. Hope this helps!

[@XPLAlN]

@@prof.vanburen Whilst I think it is a decent video, I don't think the Coanda Effect should even be in it. The definition being “the tendency of a JET to follow a curved surface” [paraphrasing Coanda himself]. We have a plain airfoil passing through a uniform control volume: there is no jet. Coanda effect is not even mentioned in most of the standard aerodynamic texts. It appears that this phrase has been widely subverted on YT to explain flow attachment. I would argue that the common example of the stream from a faucet v spoon is not even a demonstration of the Coanda Effect. Until that stream comes into physical contact with the surface you get zero deflection, or at least none that can be seen. And yet as soon as the spoon touches the stream you immediately see the deflection begin. It stands to reason that the Coanda Effect increases with proximity of the jet to the surface, but no such behaviour is seen in this experiment. Furthermore, the point where the 'air gap' goes to zero, should be the upper limit of the effect and yet it is clearly seen to be only the start. Very evidently then, it is not a demo of Coanda Effect. I presume there is some negligible effect, indiscernible due to the massive density of the liquid stream v the ambient air and the minimal entrainment between these two fluids in their different states. I am not saying Coanda Effect is a myth, I am saying it has nothing to do with the flow attachment or lift of a plain airfoil, and that the spoon/faucet experiment is not a demo of Coanda anyway.

[@prof.vanburen]

Okay@@XPLAlN!"

[@rolandotillit2867]

If you consider that flow redirection is caused by the downwash of the induced vorticity, something interesting happens. Suddenly you have an explanation as to why air flows faster over the top of the wing, as the vortex carries momentum across the upper surface of the wing, keeping the boundary layer energized. Suddenly Coanda and Bernoulli make sense. The vortex takes away lift at the tips to enhance it at the center, an easy trade off to make since it makes lift possible to begin with.

[@prof.vanburen]

Interesting perspective!

[@leshengzeng1967]

Good vedio, Just comments, when you talking about the where the lift comming from, please emphasize the Bernouli's equation is not the only thing applied to the lift. Also you could attach some experiment did by the Babinsky's research to obviously show why some misunderstanding are wrong. Cause the lift actually made by the part of the deflecting of the air flow. And also I believe that the Cnda effect did not applied this situation becasue that only working for Jet flow.

[@prof.vanburen]

Hi and thanks for your comments! Coanda was traditionally attached to jet flows, but the physical concept is the same regardless of the name. Coanda actually applies to jets attached to the surface, at which point I would argue they are starting to become wall-bounded or film flows.

[@tom_winguill]

Hi prof. Tyvan! Can we write lift as the change in momentum of the flow in y-direction? If so, then why there isn't a formula for lift that is based on the change in momentum of the flow? Measuring the flow momentum (along with the velocity vector) before and after the flow passes along the airfoil is easier than measuring the pressure at each point, isn't it? (same approach as used in thrust equation). And one more question, if the flow is inviscid, will there be a Coanda effect.

[@prof.vanburen]

Hi and sorry for the late reply! The semester got a way from me. Great questions! 1. You could consider a flow volume approach where lift comes from overall change in flow momentum. This is actually done for drag, sometimes, where you measure the velocity deficit in the wake and then back out what the drag force must have been. However, this requires a volumetric approach, and doesn't lend itself to the clean momentum equations we are used to. In essence, you could consider the Kutta -Joukowski as a form of this, where you get lift from the total added circulation to the flow. 2. I'm not entirely sure if it is still technically the Coanda effect, which is mostly used for wall jets. However, you will get the physical features of Coanda even if inviscid, because if flow wants to pull away from a surface it needs to fill the volume it is creating or there will be a vacuum, which means there is effectively an attraction force to the wall (meaning, it's easier to stay attached than separate and fill the void, unless the resistance gets strong enough). However, if flow is inviscid you no longer have the primary reason for flow separation over a lifting surface, but that is different.

[@oscarestradalopez6191]

Just excellent, your videos and explanations are really wonderful, I really enjoyed. I'd like to know which aerodynamics books do you recommend? Which ones have you used? Thank you.

[@prof.vanburen]

Thank you Oscar! For aerodynamics, anything written by Anderson is pretty well-regarded as the best in the field. "Fundamentals of Aerodynamics" and "Introduction to Flight" are both really good, most Aero courses are using these textbooks, I've taught out of both of them. Outside of that, it really depends on whatever specialty you like: turbulence, boundary layers, compressible flow, etc.

[@Cryptic_Chai]

Amazing presentation and clutter cutting there! I have a small doubt Prof Buren, can you please explain why the fluid accelerates at section one even though it's not having a roof (to squeeze in through in order to accelerate).

[@prof.vanburen]

Hi Chaitanya! You, like me, share the same first thought/doubt when it comes to the initial acceleration region. How can it accelerate, the flow could simply expand vertically (it goes off into infinity after all) to satisfy the conservation of mass? The way I like to think about it is, although there is no roof, there is still some sort of resistance to the flow expanding vertically. When the extra air tries to go around the blockage, it tries to push the extra air upwards. However, fluid can only push itself at the speed of sound, it is inherently compressible, so it takes a finite-amount of time for things to respond. In this finite amount of time, it becomes easier for the flow to just accelerate to account for the conservation of mass instead of working harder to expand. This is at least my interpretation of it, it could certainly be wrong. Does it make sense at all?

[@Cryptic_Chai]

@@prof.vanburen It does! Huge thankyou once again!!

[@crimony3054]

@@prof.vanburen Think of what you're saying with respect to 2. Air compresses because it can't move out of its own way fast enough as it is being deflected by the leading edge of the wing. Air molecules ahead in the stream give some and push back. Air molecules at the top give some and then push back. The wing gives almost none and pushes back. But it does give some, hence drag. If air impacting the leading edge creates drag, wouldn't air impacting the underneath, even at a very slight angle, create an upward force against gravity, also called lift?

[@aerospacedoctor]

@@prof.vanburen the correct statement here is that air has inertia and hence it cannot effortlessly flow around an obstacle. The fluid structure interaction results in a streamwise deceleration, that in itself becomes the source of the pressure gradient force resulting in the acceleration sideways. This sounds unfulfilling, but it is true, the structure presents an obstacle, and the nature of a fluid is it flows around it, if it cannot push it out of the way.

[@ThomasHaberkorn]

great video, but how does the starting vortex take shape in this line of reasoning?

[@prof.vanburen]

I'm not sure what you mean. Are you referring to the vortex initiated at the start of the flow---as in when the airfoil first accelerates?

[@ThomasHaberkorn]

@@prof.vanburen yes, exactly that

[@stephaniepaul6536]

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[@prof.vanburen]

Thanks I think!

[@david_porthouse]

The atmosphere is not a superfluid. As a consequence, the trailing edge of an aerofoil will dump a starting or a stopping vortex until the Kutta condition is complied with. These starting and stopping vortices can be visualised. Vorticity bound with the aerofoil generates lift by the Magnus effect. This is as simple as I can make it.

[@prof.vanburen]

Interesting viewpoint!

[@jozsefmolnar6453]

The coanda effect explanation is wrong. It is only true for jet flows, however, an object moving in a fluid can not be considered as a jet flow. Coanda effect is just giving a name for a phenomenon, not explaining the reason why the air follows the curvature of an airfoil.

[@prof.vanburen]

Okay! While I agree that Coanda might be specifically attached to jet flows in the traditional sense, accelerated flow over an airfoil in the near region of a surface is characteristically similar to a jet flow attached to a surface---a region of fluid with higher momentum than the surrounding flow. (Since we have our pedantic pants on, I would also argue that once a jet flow attaches to a surface it would lose the definition of a jet in the first place...then Coanda doesn't actually apply to jets at all). Regardless, it is the same physical phenomena that keeps flow attached to the curvature of an airfoil and that keeps a jet attached to a neighboring curved surface. If you don't believe me, a quick search of "Airfoil Coanda Effect" yields a nice result from NASA on the topic, who probably know more about it than I do.

[@flth1040]

You explained lots of misconceptions but forgot one which is Venturi effect. This effects is also wrong according to Nasa glenn research center but you base your idea on this concept. Anyway it is a good try.

[@prof.vanburen]

Thanks, but we will have to agree to disagree on this one. While I think the folks at NASA can be brilliant, I have seen the explanation you are referring to you and I don't find their rebuttal of the idea complete. And I think that's okay, because this is a very hard problem that doesn't get a simple answer! If flow passes by curvature---like one wall with a hill or the leading edge of an airfoil---it needs to react. If things are slow, the fluid just entirely (up to effectively infinitely away from the surface) moves up and over the curve and it is happy. This does not lead to the Venturi effect which is generally used to explain contracting enclosed systems. However, if it is moving rapidly by this curvature, shifting upwards takes time, and the flow might not have that time to react due to compressibility. So, if it can't get out of the way in time (and conservation of mass needs to be obeyed otherwise the world explodes) the flow can can do things like increase in density or speed up in this 2D approximation. Why is speeding up not an option for mass conservation? Would this not be a "virtual" contraction, where the "top wall" of the Venturi is really just reaction time? I know NASA in that reference discusses the flat plate as evidence, but the flat plate also leads to flow fields with curved streamlines that behave like more curved surfaces. Also, it talks about the bottom-side of the airfoil but there curvature is not nearly as rapid. Furthermore, the speed up explanation in this video is specifically attributed to traditional airfoil shapes and is only a smaller part of the big picture. It is this effect in combination with the others that lead to the supreme lift of airfoils. I am not sure my explanation is any more convincing, but I also don't see where in the aeronautics series from NASA that they explain the acceleration of flow around the curvature at the top of the airfoil, which certainly happens. It seems that course online just leads to "Euler equations are complicated". Yes they are. I have seen other explanations that include consideration of angular velocity and centrifugal things, but they didn't resonate with me (or perhaps I just didn't understand them well enough). Anyway, thanks for getting me to think about it more deeply!

[@crimony3054]

2. is wrong. Air is very hard stuff, when you move fast. In fact, the astronauts in space shuttle Colombia were killed when their massive space craft could not withstand the heat and forces generated by tiny little air molecules bouncing off of it. The shuttle wasn't destroyed as if it had been flown into the side of a granite mountain, but it was ripped apart pretty badly. By what? By air.

[@prof.vanburen]

When you say 2, you mean the "Air Pushes" portion? And I can't tell if you mean that the explanation is wrong (it is not a proper explanation for lift) or if you mean that me saying it's wrong is wrong. Anyway, I certainly agree that when moving fast air can impart very high pressure gradients and stresses on surfaces, causing them damage.

[@crimony3054]

@@prof.vanburen Believing or saying that air does not push the wing up is mistaken.

[@franciscosanudo4344]

@@crimony3054 but he does say that. Air going over the top of the wing pushes it downward. It is due to the pressure difference, with pressure being higher below the wing which induces a net force in the upward direction.

[@rocketman99]

This is sadly wrong really

[@franciscosanudo4344]

Can you explain?