Why is the top flow faster over an Airfoil? 

These 2 videos raised even more questions than they answered for me.

I will explain what happens from a mesoscopical point of view. What happens is that fluid particles tend to travel in a roughly straight line if undisturbed. This is the principle of inertia. However, the presence of the lifting surface disturbs them and they will first deviate off course due to the presence of the leading edge (or whatever they first encounter). Now particles are moving in a roughly straight line away from the lifting surface (upwards or downwards depending on where they were coming from upstream). Because of that, the flow becomes rarefied in the direct vicinity of the lifting surface which induces a pressure decrease close to it. Due to the fact that there exists a pressure gradient force pointing towards the lifting surface, fluid particles that are moving away from it are accelerated towards it. This is why flow remains attached for small values of the angle of incidence and this also explains lift: The action of the lifting surface on the fluid particles is to accelerate them in a certain direction (if the surface is generally cambered or inclined downwards, it will force fluid particles to generally accelerate downwards), hence they give away their momentum to the lifting surface in the opposite direction (principle of reciprocity). Bernoulli's "principle" kinda forgets this dynamical point of view and rather explains things from an energetical point of view: There is a transfer between kinetic energy (velocity) and potential energy (pressure and head) along streamlines: In the absence of source or sink terms in the Bernoulli equation (production and dissipation of energy), kinetic energy is won where potential energy is lost, and vice versa. This does not explain why kinetic energy is lost (or gained) and why potential energy is gained (or lost) since you lose directional information by projecting the Navier-Stokes equations onto a streamline, but along with the "third law explanation" that I gave in the first paragraph, the Bernoulli principle will help provide you with the full picture: That's because whenever fluid particles travel very quickly towards an obstacle, they contain lots of bulk kinetic energy which is transformed into pressure when they hit the leading edge and the pressure side of the lifting surface. Because fluid particles have become compressed when moving past the leading edge along the suction side, the pressure difference between patches of flow away from the surface compared to that close to the surface will become more important, which yet again reinforces the pressure gradient force caused by this rarefaction. Curvature does not magically act as is suggested in this vid: We should rather consider that the geometry curves away from the flow as flow particles move along a straight-ish line and bump into each other, getting a net momentum that forces them to go towards the rarefaction.

I think the problem is to try and use Bernoulli's theory/equations collectively for both the upper and the lower surface. If for a minute you stop and treat the upper and the bottom part separately and apply Bernoulli's equation you will find out that it's not wrong after all. Let's start with the upper surface: the flow approaches the aerofoil and the area in which it flows converges and then diverges. Bernoulli's theorem states that when the area converges the flow speeds up and the pressure drops and when it diverges the flow slows and pressure starts to increase. This is exactly what happens on the top of an aerofoil. Now considering the bottom surface: when flow approaches the aerofoil the area slightly converges prompting a slight pressure decrease and then the area diverges increasing the pressure and slowing down the flow. Now if you consider the collective contribution of the conditions below and above the aerofoil you will get lift. But I don't think Bernoulli alone is suffic

3:33 "such a sudden drop in pressure will not considerably increase the particle speed" The drop in pressure will change the speed independent of how sudden it is, as Bernoulli Equation demonstrates. The correct explanation here is that change of speed at the end of the trajectory doesn't significantly change the total time the path takes.

More classic version, still logical: if the flow-pattern is actually the sum of a 'circulation' plus a uniform horizontal flow, then, in the region above the airfoil, the circulation always adds to the average velocity. Below the airfoil, the circulation must subtract from the average velocity. (Works fine for rotating cylinders, works for more complicated 2D airfoils.) Result: any parcels which split at the leading edge, will never meet again, as long as circulation is present. Another result: if the shape and angle of the airfoil gives zero lift, also the circulation becomes exactly zero. In that case, any split parcels at the leading edge will recombine again at the trailing edge. Rule of thumb: if split parcels recombine at the trailing edge, it means that the lifting force is *exactly zero.* The infamous "transit time fallacy" turns out to be a description of a zero-lift airfoil.

Brilliant video! It's good to see the proper conclusions being drawn from observations for a change, rather than finding a convenient but wrong explanation.

Pressure of air is the amount of air molecules at a given place multiplied with the vibration speed of the molecules (higher temperature the more they vibrate and hitting a surface with a speed which gives out a force which on a given area is equal to pressure) The temperature in this scenario can be seen as constant, and the only thing we need to focus on is the amount of air molecules at a given space which will in turn give us the pressure at that area. When the air curves for example in the beginning it is pushed up and some air molecules will detach from the wing surface as the speed gives them enough momentum to leave the wing surface as it hits the sloped surface of the wing. This means it will have a smaller amount of air molecules compared to the atmosphere. Since pressure in our scenario is directly given by the amount of air molecules at a given place it must mean that we will have a lower pressure at the top of the suface of the wing. Less air molecules at the wing surface equals less drag on the air molecules due to the skin effect. This makes this air able to travel much faster than the air beneath the wing. Same thing at the bottom end of the wing. Air molecules follow the path of the wing and is smashed towards the wing at the back because of the momentun they have. This will decrease the speed of the air molecules (skin effect). Air arriving at the surface of the wing will be arriving faster than the air leaving the surface (goes further along the surface of the wing). This makes a traffic jam of air molecules where more molecules is arriving than leaving. Which in turn gives larger amount of air molecules than the atmosphere. As said, the amount of air molecules directly gives the pressure in our scenario and we will have a higher pressure here compared to the atmospheric pressure. Hope this gave any sense! :)

I've never been more exhausted from thinking about something that should be obvious.

One would have thought that this is an obvious conclusion. We must consider that there is an intention behind any shaped structure. Let us assume the conventional shape of an airfoil section has a slight angle of attack. As the foil moves forward, the lower surface is pushing the air down but also forward and the air below the wind has a downward component in addition to a horizontal component. (In a wind tunnel the lower surface slows down the horizontal airflow because of the angle of attack while reflecting the flow downwards). As the air moves above the wing, it enters a divergent shape, the divergent shape causes a downward velocity to be created in addition to the horizontal velocity. There is a pressure zone above the leading edge to accentuate the down acceleration at a later stage. Because of the tapered nature of the upper surface of the wing, towards the trailing edge, the longer vertical distance permits the air particles to gain a higher downward vertical velocity and so this results in the flow above the wing is faster than the flow under the wing. Note that when dealing with lift and drag and control surceases and propeller thrust, one should always deal with the acceleration of the mass particles at any point around the unit in question. It is acceleration that create force and not velocity or location.

that one went clean over my head...

0:29 In the curved flow, why is pressure higher outside ?

first part of the pressure is not consistent! i was lost when he started talking about the pressure in the lower part of the airfoil!

Dear friends, Here is the 2nd part of airfoil video series. We are working hard to release the 'Transistor video' by the end of this month. Please support us at Patron.com and make our efforts sustainable. www.patreon.com/LearnEngineering

Summary: "An air foil works this way because of the way it is."

now answer why the pressure is higher at the outside of a curve

As with so much of Aero. It’s slightly more subtle. The pressure gradients are sadly much harder to explain away like you would in a river or other fluid flow. There the total pressure (the integral across the river) is the same before during and after a bend assuming no energy loss to friction. With the airfoil the integral from the ground to the airfoil is higher than before whilst the integral from the top of the airfoil to space is lower than before. This I would argue is due to the velocity increasing as a result of the Chanda effect but that is a active area of debate (what causes what)

The order is The Airfoil Shape and Coanda Effect Pressure Differences (And Corresponding Forces) Velocity Gradients *special note. If the pressure gradient on the second half of upper airfoil surface becomes big enough, velocity might decrease to the extent that the flow will start reversing. This causes loss of pressure at/near that separation point, loss of lift, increase in drag and called stalling

Hello, can you please explained why Pout has greater pressure than Pin ? This is the part I am currently stuck at, I dont get why a curved line the pressure outside of the curve is higher than pressure inside the curve? if that is the case why the curve not bending downward instead is bending upward? isnt pressure suppose to flow from high to low ? thank you

It should be mention that all these explanations are valid in sub-sonic medium

There is a good bit of tautological reasoning happening in this video. What is important is that the pressure difference exists in this situation, and that a theoretical model of the flow based on first principles agrees with the observation. Attempting, after that, to come up with a simple explanation of "why" risks getting tangled up in circular reasoning. If you want to understand the theoretical models you start with the basic principles of force, mass, and acceleration of the fluid, and the principles of conversation of mass, momentum, and energy. None of which are mentioned in this video.

Admin at min 3:08 you made a mistake. For top surface V decrease and then V increase. But you made it opposite. You asked why different speed? Because the shape of airfoil is curvy downwards. Like when you are sliding from on top a hillside. Your speed is increasing right?

You should clarify the assumptions used. For example, positive cambered airfoil with no concave areas at relatively low angles of attack and subsonic flow throughout.

Bernulli's principle assumes non-viscous, non-frixtion flow and is therefore correct for such cases (i.e 2 particles should meet together at the trailing edge etc. ) . Experiments do not agree with the theory because, well. real fluids are viscous and have friction in a non linear way. Navier-Stokes equations account for all of it though.

Why does the pressure increase if air moves towards the bottom of the airfoil?

It could be due to the lower pressure above the wing than below it (higher pressure means more atoms packed together so the harder it is for the atom/ air particles to move through the high pressure(like shoving your way through a crowd). This result’s in slower air speeds) which allows the air to move faster over the wing than below it.

since bernoulli's theorem cannot be applied on two streamlines then how bernoulli's principle is resonsible for the BLOWING OFF of the ROOF DURING STORMS ????????????? PLS ANSWER

Hey man, I'd like to say thank you. I'll use this with my students in class 😊

I wish you provided more information about why the pressure gets lower above the airfoil. I just can't get what makes the pressure lower there.

I think lift is produced due to the centrifugal effect arises at both the upper and lower curved surfaces.This result in the throwing away of air to both upper and lower part of the airfoil, at the bottom surface there is the ground to provide the reaction force but throwing away of air at the upper surface will not get any reaction force thus air pressure decreases over it . Thus air plane takes off, am I correct ? Please comment on this theory...

it is because the integral is an area of the wing under the top half, above the bisection and air being disturbed must accelerate so that angular momentum may be conserved and it is inversly proportional to bernoullis equation describing increased velocity and lower pressure within a tube's bottle neck.

The initial upper part of the air foil is synonymous with an orifice inlet where your area decreases and velocity increases for the same potential of fluid flow; in which air in higher vicinity acting as wall of the orifice. That is why higher speed is achieved at the top and there is no significant curvature at the bottom to cause the very same effect.

I really like this one. Did get into my favorites playlist to share. You could also understand why trailing edge turbulence were created.

Reaction lift is indeed occurring. If a constant stream of solid particles, (say small plastic air soft pellets, for example) were aimed at the front of the wing, of sufficiently large volume to stream past the top and bottom of the wing, it's obvious that the wing would rise. And this is without the Coanda effect taking place. So, both reaction lift resulting from impact with the bottom of the wing by the aggregate mass of the air, combined with the boundary layer mass of air flowing over the top and creating a thin, negative pressure area near the top surface of the wing to which it adheres; serve to maximize lift efficiency. What occurs on the top wing surface is analagous to the effects of orbiting bodies and the "sling shot" effect used by space probes, only substituting vacuum adhesion for gravity. This is also aptly demonstrated with the "chain fountain" phenomenon. A partial orbit of a mass causing lift in the direction of a vector originating at the arc's center of radius and extending out, bisecting the arc at it's center. This increases speed and pulls on the wing surface. This partial orbit of the air mass over the wing, tugging on the wing surface via suction, added to the upward push from the air mass deflecting off the bottom, share responsibilty for lifting the wing. Both air and sea creatures make very efficient use of alternating these effects above and below the wing or tail (and to appreciable degree with sea creatures, their bodies.) Rapidly alternating negative and positive pressures enable the Black Marlin for instance, to hit a reported 82mph. Good Journeys All P.S. See - "Huge media blackout regarding supermoons" on the net

The problem I have with this explanation is that the air is not moving, the airfoil is. So the air above the wing is sucked backwards as the airfoil passes, and the air below is pushed forward. This results in a net clockwise circulation of air around the airfoil. What people fail to explain is what happens in the wake of the airfoil with two layers of air, the upper moving to the right and the lower moving to the left, meet up again behind the airfoil.

Air has weight .The shape of the wing forces the air molecules to follow a path across the wing which is changing direction . That flow following the wing is attached by a boundary layer . This transfers to the wing surface the effects of the centrifugal force providing lift when the speed of that air across the wing is fast enough . The lift is from the weight of the air being forced to change directions across the wing . There is no lift on the last 1/3rd of the wing .The Tailwind wing has very little upper camber and has a higher stall speed as a result .

This actually really made it click in my head! Thanks!!

Thanks for this Eye-opener informative video. During our study days We have been taught wrongly. God Bless you for sharing true knowledge.

The high pressure point on the leading edge reminded me of a similar problem for shooting bullets underwater. they fixed the problem with a concave tip that super cavitates and gets 60 METERs! Could a big leading edge divot work on a wing?

Anyone else find this video better at explaining how airfoils work than the official video of how airfoils work..... just me?

wait, Bernoulli says higher velocity fluids have a lower pressure, which counteracts what you said at the end about pressure distribution effecting speed and not the other way round

'wings don't suck, how wings work and planes really fly', is the best and most accurate explanation of how lift is produced.

Can you make a video explaining in detail how calculators work...?

It is faster above because of gradient in pressure (slope) between front side(stagnation point) where pressure is maximal ( and velocity is zero) and top side where pressure is lower due to fluid motion ( Bernoulli principle)

Could anyone explain for me why pressure at the outside curve is higher?

So Bernoulli's principle has no effect on generating lift? Couldn't we say that the pressure distribution due to the Coanda effect creates different speed but then these different speeds amplify the difference of pressure because of Bernoulli's principle?

Thank you for easy logical explanation!

One of the best videos I've seen about lift.

Think about it! The air is actually stationary and does not flow over the wing. The air just gives way to the wing upwards and dawn wards. The generated pressure degreases with distance. The air turns counterclockwise around the pictured profile.. Start thinking and let go the hypnosis of the windtunnel.

Why location of the pressure changes with AOA?

Thanks for explaining this in a very easy manner

you explained why the top particle goes faster, due to lower pressure gradient, but you didn't explain why the wing curvature CREATES the lower pressure. So this explanation is incomplete.

Thanks for clearing this up! I definitely had the wrong idea about this.

This is by far the best concise video on 2D airfoil flow. There are so many misconceptions out there, it's refreshing to see a video which communicates the main ideas so clearly. Whilst it doesn't directly explain lift and drag, those two forces can be inferred from integrating the pressure field around the airfoil surface. The only part of the picture it doesn't help to show is that of momentum conservation. A net lift necessarily results from a downwards component of momentum imparted to the freestream.

First you state that in curved flow pressure is greater outside. And right very next statement is that outside a flow, curved by upper surface of airfoil pressure is lower.

I did not get, why the pressure field would change on upper side of foil from being low to high at the tail end?

Actually Jukovskii theorem explains it very well using air circulation around the wing

I've spend two hours looking at videos and searching bing/google and NO ONE wants to discuss the speed of the airflow across the top of a cross-section of an airfoil vs. the speed below the airfoil vs. the relative air speed. I know this varies based on the distance from each surface, but WHY (??!!) are there no videos on the topic? If you know of one/some, please comment and direct me to them!

Is the lower pressure on the top of the wing the result of the faster flowing airflow (Bernoulli) or is the faster flowing airflow the result of the lower pressure on the inside of the curved flow that accelerates the air ?

You should have started with a symmetrical airfoil at 0° AoA and develop your arguments from there, comparing the pressure and particule speeds over and below the wing with the pressure and particle speed of the air outside of the influence of the wing, and only once it is understood what happens in a zero lift condition, develop the explanation of lift.

Well explained. Thank you

The visual is misleading after 1:46. The airfoil is shown with a negative angle of attack, but the pressures shown can only come from a positive angle of attack.

What about for symmetrical airfoil geometry

Mind-blowing explanation...

So then based off this theory using coanda effect, bernoullis principle has nothing to do with lift?

Oi i just gotta know, why are plane wings and noses blunt at the tip rather than pointed? Wouldnt it help to separate the flow seamlessly, without causing that high pressure area at the front? Why do we not use that

It means that the decreased pressure condition is the cause of acceleration of air above the wing, has been there before movement of air !!!!!

@Learn Engineering The starting vortex theory explains how the velocity over the airfoil and below differ which in turn produces a difference in pressure. Check the starting vortex theory it makes sense mathematically as well as logically.

this is very helpful, thanks!

Coanda effect is only for fluid jets. Wings don't naturally experience coanda effect except from jet engine exhaust. Airflow follows the wing due to air viscosity. While the concept of Coanda vs. flow attachment due to viscosity is similar, Coanda effect is only considered applicable for fluid jets.

pls make fluid flow analysis on aircraft fuselage

the pitch of the wing and pitch of the slope on the wing creates a pressure differential

isn't the different speeds caused by boundary layer formation?

I really don't understand why the airfoil aren't inverted, with front part in the rear and with rear part in the front.

This is like the chicken/egg question. Does the speed difference cause the pressure difference or the other way around?

great video, thank you!

provide further information for that argument he made near the end : DIFFERENT SPEEDS ---> PRESSURE DISTRIBUTION

Amanzing explanation! : Why does the pressure drop as we get close to the curvature? Because of the curvature! Like saying: Why is the sky blue? Because it's blue! Why does fire burn things? Because fire burn things! Why does the Sun illuminate the Earth? Because the Sun illuminate the Earth! It would be better to not explain anything at all if you have to explain it like this.

great video explanation sir, really appreciate it.

how a symmetrical aerofoil wing produce a lift??

May you make a video to explain the formation of vapor cone using CFD simulation? I cannot visualize in my brain how that cones formed. Your simulation is going to be very useful. Subsonic, transsonic, sonic and supersonic phases sholuld be visualized using CFD. It will be very illuminative video. Thank you.

What is the direction of the air flow in the cfd?

Beautiful!

Great Explanation!

I can see why the increase in speed but still no idea why the drop in pressure

Now, that makes sense. Pressure distribution leads to speed difference. That's why everyone who tries to explain difference in speed that causes difference in pressure fails to give a proper explanation.

Very Nice! it's so simple. thank you!!

You just clear me thank you keep it up

Excellent video!

the answer may be given by Bernoulli principle where the pressure at the top is low and hence velocity is high

i don't understand why p_out is more important than p_in ?

Coanda effect doesnt take place in order to explain this fact, just for jet fluids. Main reason is the geometry and distribution of pressures around the wing.

Very nice

Great thank you very much sir

Great video it’s to the point

this explanation is valid for inviscid incompressible and steady flow? or is only for viscid flow? thank you

MASHAALLAH khub valo video

Which software do u use for animation?

That was really good. I loved it.

LearnEngineering Can you please do a video on A/C compressors?

My explanation would be that the airfoil acts kind of like a nozzle, accelerating air by forcing it to move through a smaller area. Imagine putting another airfoil on top of this one, but flipped upside down. The two airfoils would act as a nozzle to the air flowing between them, causing the flow to accelerate, then decelerate back to normal speed as it exits the nozzle. Having just the lower airfoil does the same effect, only less pronounced; it still accelerates the flow above it, lowering its pressure, by forcing it to move through a smaller area. The bottom surface is less curved so the acceleration and pressure reduction is less. Thus upper pressure is lower and there is a net up force.