Why are so many pilots wrong about Bernoulli’s Principle?

For decades new pilots been taught that lift is created because the air flowing over the wing travels a longer distance than the air flowing under the wing, and therefore, for the two airflows to meet at the end of the wing, the air flowing over the wing must move faster. This hypothesis is not correct and this video explains why.
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Links:
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[@jacobstump4414]

I’m an aerospace engineer who graduated from Embry Riddle, the top rated aviation school in the country. And even there, in our early aerodynamics lessons, the equal transit time fallacy was taught. I remember because I asked - Why does the air on the top HAVE TO reach the trailing edge at the same time as the air on the top? And my professor didn’t know… but to his credit, he came back the next week and taught everyone the fallacy of the equal transit time.

[@FlywithMagnar]

That's a great quality of an instructor.

[@jadneves]

No aniversário de 100 anos da Aviação eu entrei com essa questão "- Por quê um avião voa?" justamente para abolirem essa falácia que todas escolas ensinam sobre a sustentação e com o mesmo argumento do vôo invertido não aceitei tal resposta;

[@markmcgoveran6811]

Calm down it's just a math model. Ask any farmer and he will tell you it doesn't have to get there at the same time. It's like permanent magnetism and residual magnetism it's all a personal opinion which one is there. If I like it it's permanent magnetism and I sell it for being a permanent magnet. If I don't like it it's residual magnetism and I don't tell anybody

[@SwanOnChips]

@jacobstump4414 I have a technical article copied that credits Newton for wing lift, not Bernoulli. Did anyone teach that?

[@jean-pierrevandormael5315]

Only the second law of Newton related to the variation of momentum : F=d(mV)/dt, describes correctly the wing lift (vector F for the force on an air particle, scalar m for mass of that air particle, vector dV for the variation of velocity and scalar t for time). This law applies to all air particles moved by the plane flying through the air. The wing lift is equal to the vector sum of the forces exerted by all air particles on the wings. The Bernoulli's equation describes only the aerodynamic behaviour of the air due to the movement of the plane. It has noting to do with the lift.

[@petermortensen8022]

Being a carpenter I remember being taught about Bernoulli's principle and the reason why roof tiles come off a roof during high winds. I have never forgotten about this phenomenon. Love it.

[@nathanwoodruff9422]

It is the same phenomenon on why people are unable to stand up in a hurricane.

[@Chris-fn4df]

Did you use your knowledge of this principle to design roof tiles that don't come off in high winds? No? Then why do you love it? So you can tell your clients that it was the wind that knocked off their tiles? Did you _amaze_ them with this revelation?! lol

[@goodo5691]

also not enough nails

[@donfernandocolina]

@@Chris-fn4df Have you ever experienced the pure joy of discovery?

[@robbedontuesday]

Same principle applies to sail boats.

[@DD-gi6kx]

its was good to see that smoke demonstration clearly showing the air over and under do not get to back of the wing at same time...I've always wondered how people just seemed to conclude it does

[@User-jr7vf]

I have a private pilot license, but have not flown aircrafts or even being in contact with the world of aviation for about 7 years. Now KZbin recommends me this video and it reminded me of when I was taking classes, that one of our textbooks warned against these misconceptions regarding aircraft lift. I also remember the book saying that it is still not completely understood, what makes an aircraft fly. I still have the books with me as well as my (expired) private pilot license.

[@FlywithMagnar]

Lift is fully understood by aerodynamic specialists. The problem is to explain it to people without an engineering degree without oversimplifying it.

[@heathwasson7811]

@@FlywithMagnar As an aeronautical engineer (and pilot) I completely understand why the typical example is taught... It's a "good enough" explanation for almost every human on the planet, even for pilots. As you say it's very difficult to explain the totality of what's taking place, without giving a multi-day class on aerodynamics. The funny/sobering thing is... even relatively high levels academics (university science classes not focused on aero/fluid dynamic) are still teaching the incorrect science, because it is good enough most of the time. I only know better because I majored in this specific field. That leaves me to question what I think I "know" about other areas of study.

[@johnpipping3848]

However hard you try you can’t “fly aircrafts”. The plural of aircraft is……. aircraft.

[@motionsic]

In the USA, PPL is for life. Just need bi-annual flight review to be current, if I remember correctly.

[@NicholasMati]

I know what sentence you're referring to. I remember reading it and laughing / cringing. Most of that chapter's explanation for how an airplane flies is either outright wrong or misleading.

[@KenIsFlying]

I have been dying to find a good video that actually explains lift for pilots in a correct fashion. As an aerospace engineer it's really hard for me when my student pilots tell me about the Equal Transit Time theory for lift. Thank you for this video!

[@dougaltolan3017]

Show them the wind tunnel video... Not only is equal transit time shown to be wrong, the air over the top of the wing moves even faster than equal transit time would suggest. Note that that video is only valid for that profile, angle of attack and wind speed.

[@villiamo3861]

Quite right that the equal transit hypothesis is demonstrated as being false. But then it's dismaying to hear him talk about lift almost as though it were purely a function of surface curvature, when even a flat plate tipped at an angle and forced forward at speed will, also very demonstrably, generate lift.

[@boeing757pilot]

All pilots should look at the book "The Illustrated Guide to Aerodynamics" by Hubert Skip Smith. Excellent conceptual explanations without the math.. Highly recommended..

[@boeing757pilot]

@@villiamo3861 Good points!

[@romanbart5823]

The navy had some great demonstrations on lift. This wing that he is showing has such a high angle of attack that he is getting turbulence above the wing that it symbolizes a stall.

[@christophertelford]

The fact that you can make a flat square fly if you put the centre of gravity in the right place and give it control surfaces and enough thrust and a positive angle of attack tells me that everything else including the aerofoil cross section and other twiddly bits like wingtip vortex generators etc are all about improving efficiency. All you need is enough surface area to direct some air downwards, get the basics right, and it'll fly.

[@rdspam]

Any supersonic aircraft will demonstrate this.

[@FrostCraftedMC]

modern wings are mostly working on air pressure differential on top and bottom. the wing moves forward, making high pressure at the bottom, low pressure at the top, sucking the wing upwards while also sucking air over the top

[@DietmarSchlager]

@@FrostCraftedMC , you are funny.: „modern wings“? And don‘t forget, sucking is only an imagination of the real physics. You only can fix that the side which is turned to the incoming airflow (the bottom) produces more pressure and on the side which is a little bit turned away from incoming flow (upside) decreases the pressure. So it´s clear that there is a difference of pressure that can be seen as generating lift.

[@nitramluap]

@@FrostCraftedMC Funny... it's pretty windy below a rotary wing aircraft (ie. helicopter). Pretty sure it's not being 'sucked up'.

[@JohnDoe-vx3z]

@@nitramluap Yep, lift is the opposite reaction to air being pushed down. Helicopter pilots understand that better than their fixed wing counterparts.

[@austinblake4079]

As an instructor, I've been teaching Bernoulli's for straight and level briefs. I tell them it's not a direct translation from venturi tube to an aero foil, but just explain that there is a similar effect of decreasing static pressure above the wing. Didn't realize people were trying to explain the "reason" for it as an equal transit time...

[@davetime5234]

"I tell them it's not a direct translation from venturi tube" But isn't it in fact a direct translation to all situations when considered at the parcel level because each partitioned parcel also has to behave according to Bernoulli?

[@austinblake4079]

@@davetime5234 Not really because in a venturi tube we're talking about the total pressure of a closed system. An aerofoil is not in a closed system with a certain total pressure.

[@davetime5234]

Thank you for your reply. I guess what I was getting at: an individual parcel in an open system obeys Bernoulli no less than an individual parcel in a closed system? Therefore, Bernoulli relationships for conservation of energy, momentum and mass (continuity), should be just as applicable to the open system (which presumably needs to be partitioned in terms of such parcels for analysis)? I'm wondering about the above implication for those who say Bernoulli isn't applicable to a thin boat sail? One could say you make a conceptual transition from a closed venturi to an open system, with a wing having a cross section of some thickness? But, perhaps it is another conceptual jump to a perfectly thin boat sail producing lift? (here there is the challenge of convincing someone it still works without, upon first inspection, any apparent path difference between opposing sides) But in all 3 of the above cases, Bernoulli still applies, even if the analysis requires more work in the latter two cases? I'm trying to understand if, in the above, I've outlined the problem correctly?

[@austinblake4079]

@@davetime5234 I see what you're saying, but it's a little over my head really lol. I just gotta teach people that wings make lift, then tell them how to take off and land lol. I figure if the theory of lift was unified, we wouldn't really need to have these discussions. Sadly as far as I know it's not (like the theory of gravity)

[@robertweekley5926]

How about "Every Action produces an Equal and Opposite Reaction?" Nobody gives Neuton Credit! A 2,300 Lb Cessna 172 Skyhawk, or a 400,000 Lb Military Cargo Plane, MUST force a Mass of Air "Down" equal in weight and mass, to that particular Aircraft, just to maintain Level flight! Even MORE, if it wants to climb! Bernoulli never learned to Fly, himself, it seems!

[@boeing757pilot]

Thank you. This was a good explanation and properly addressed the "equal transit time" assertion, which is STILL incorrectly taught in many flight manuals...

[@kevinbarry71]

You are 100% correct. When I hear people explain this wrong idea I ask them how is it that aerobatic planes can fly and their wings are symmetrical. And how can they fly upside down?

[@andyowens5494]

Aerobatic aircraft use engine power. The angle of attach of a symmetric wing profile deflects air down, but that flight mechanism creates a lot of drag, which needs more engine power to overcome. Many aircraft can fly upside down, using the control surfaces to deflect the airflow - exactly the same forces as used to change direction, but if the control surface forces exceed the weight of the aircraft, it doesn't fall out of the sky whilst inverted. So, angle of attack, and control surface inputs, which are completely different flight mechanisms from aerofoils.

[@kevinbarry71]

@@andyowens5494 yes. Angle of attack is critical. Obviously aerobatic wings are not used on more conventional aircraft for that reason. They are too inefficient. But if the Bernoulli principle was the only thing working, this wouldn't work.

[@pk7549]

Symmetrical airfoils must always be at a positive angle of attack to produce lift, roughly +4 degrees for unaccelarated flight. Asymmetrical airfoil will still produce significant lift at even zero angle of attack and no lift at roughly -4 degrees under the same condition.

[@olddirtbiker5088]

@@kevinbarry71 Thank you for pointing out the obvious issues of angle of attack and symmetrical profile wings. If you have ever "flown" your hand out a car window, angle of attack is readily apparent.

[@rivernet62]

The angle of attack in the smoke demonstration appears to be much more than 4 degrees

[@skooter2767k]

When I used to fly RC models, we took a wing on a 3 channel trainer and put it on backwards. It flew just fine

[@einherz]

because wing was in correct aoa. backwards wing didn't make it broken, it's just make it with worse aerodynamic quality

[@josephn944]

Also worth noting an RC plane is not very representative of large aircraft as the very small/simple ones have symmetrical airfoils and don’t operate in remotely the same Reynolds number regime

[@skooter2767k]

Then why do they use models in a wind tunnel? Why do RC models have similar characteristics as the full scale airplanes they are modeled after? 🤔

[@christopherknee5756]

@@einherz This is good to know. At some stage, the clowns at Boeing are going to put something on backwards! Kidding! Kidding!

[@einherz]

@@christopherknee5756 in this world only one person doesn't make any mistakes, it's me, because i don't make shit

[@Avianthro]

Two other phenomena need mention: Vortex around the wing caused by viscosity, and simple flat plate lift. The wing's lift is actually a sum of those two, and the latter is still active even when the wing is stalled as long as its AOA remains positive.

[@LeoH3L1]

The vortex isn't caused by viscosity, it is caused by the pressure differential, and around the wing tip is the only available route the air can move to try to equalise pressure, it can't move against its own flow upstream to come back around the leading edge, or around the trailing edge to do it. You're confusing cause and mechanism. It would be like saying a ball rolls down a hill because it is round, no it rolls down the hill because of gravity, the reason it CAN roll is because it is round. The cause of the vortex is the pressure differential, the mechanism that allows it is the viscosity because without the viscosity it would immediately collapse, but again, that's not the cause.

[@Avianthro]

@@LeoH3L1 Well, if you want to get really precise, then what's the cause of the pressure differential? The ultimate cause of the wing's lift is the force (thrust) pushing on the wing to accelerate it and then maintain its relative motion with respect to the air. Then there are other co-causes and proximate-intermediary causes-mechanisms. Without the pressure differential along with the air's viscosity (See Prandtl) we would have no vorticity around the wing and it's that vortex's interaction with the air flowing past and through it that's producing the lift, along with a portion (relatively small at low aoa) of lift produced by flat plate drag if angle of attack is positive. We also should mention the shape of the wing, specifically its rounded leading edge and sharp trailing edge. That shape, especially the sharp trailing edge that starts the vortex, also is a cause...can't make lift with a cylinder, unless the cylinder is spinning...Magnus effect used on some "sail" boats. Then there's angle-of-attack...zero angle of attack...zero lift...Want to cause lift, then make the aoa positive but less than 90 degrees. So, we should really say that lift is not caused by any single thing but by a number of factors working in concert, but still the ultimate cause is thrust (from a propulsion unit or from gravity)acting to move the wing relative to the air. So, using your ball rolling downhill analogy: Thrust is gravity. The shape of the airfoil, the vorticity of the air, the aoa of the airfoil...those are the ball's roundness.

[@richh1576]

@@LeoH3L1 see above response. Recirculation effect on lift was discovered at Boeing Aircraft Research under one Arvel Gentry. The reason for the 'recirculation flow' around a wing/foil/sail is the fundamental viscostiy of the moving fluid. See previous postinjg.

[@patrickstrasser-mikhail6873]

The vortex at a wingtip is not the same as the vorticity of the flow behind the trailing edge. The second one only shows up when you subtract the free flow homogenous flow. So in sum there is no circulation arround the wing and no backward vortex behind the wing.What you have in fact is a shape that when moved forward in a fluid acclerates fluid downward, and doing so even more when angled slightly upward, while generating remarkably low drag. The downwash at the back of the wing is the reactive force to the lift. Al the same like a kite, but more efficient. The vortex at the wing tips just generates drag and reduces lift as an effect of leakage over the wing tips. By theory, simulation and experiment we know that the flow above the airfoil contributes significantly more to the lift than the flow below the airfoil.

[@Avianthro]

@@patrickstrasser-mikhail6873 Are you arguing that Prandtl was wrong? What is the reason why a wing with a rounded trailing edge produces so little lift at the same a.o.a. as a wing with sharp trailing edge?

[@californiadreamin8423]

It’s more important for pilots to understand what causes a wing to stop producing lift, so the passengers don’t get upset.

[@FlywithMagnar]

It will also be nice if the pilots understand how a wing produces lift.

[@rael5469]

"It’s more important for pilots to understand what causes a wing to stop producing lift, so the passengers don’t get upset." You mean like when the big fan up front stops cooling the pilots?

[@californiadreamin8423]

@@rael5469 You got it…..it’s doesn’t do to overheat when the passengers start screaming 😱

[@cosmicraysshotsintothelight]

As John Wayne would say... "Stop stalling and spit it out..."

[@RalphEllis]

It has nothing to do with Bernouli. It is action and reaction - it is the deflected downflow of air from under snd over the wing, that provides lift. To make the wing go up, you must deflect molecules of air downwards. No deflection, no lift. The pressure differentials are a product of molecule deflection, not the cause of lift. (ie: more molecules hitting the bottom of the wing than the top.) R.

[@BuzzMoves365]

I’ve had this argument with CFIs and FAA examiners more than once. Myths are hard to overcome.

[@boeing757pilot]

Yes.. And "equal transit time" is still the explanation written into many flight manuals..

[@erickborling1302]

CFI's should not be deficient in this fact! Really.

[@HH-mw4sq]

As a CFII and someone with an undergraduate degree in Aerospace engineering, the reason it is taught the way it is, is because the students seeking their pilots license would neither understand nor are they interested in fully learning how a wing generates lift. I know, I have tried. What an airfoil really does is to rotate the air in a clockwise manner using the diagram of the airfoil used in the video. This rotation accelerates the air above the airfoil, and retards the air below the airfoil. As mentioned, the total pressure around the airfoil is constant and the same. But with the higher airspeed above the wing, it has a higher dynamic pressure than below the wing, and therefore has a lower static pressure. Lift is generated due to the differences between these static pressures, multiplied by the surface area of the wing. Anything that rotates air, will generate lift.

[@royshashibrock3990]

Interesting, but incorrect. I am sitting in front of a fan to cool me, which is rotating air...and I assure you it is not producing "lift."

[@HH-mw4sq]

@@royshashibrock3990 - not that form of rotation. But nice try though. FYI, it is the type of air rotation which causes a golf ball to fly, and the Magnus effect.

[@dennispickard7743]

@@HH-mw4sq Ahahahahahahaha 😂😂😂

[@deang5622]

Interesting, looks as if you were taught wrongly in your aerospace engineering degree.

[@HH-mw4sq]

@@deang5622 - how so? Please elaborate?

[@aerospacedoctor]

Hi Magnar. Many have an issue with Bernoulli, and they are mistaken. As long as you are outside the boundary layer Bernoulli's principle applies. In fact, when most engineers use the pressure coefficient it is directly related to Bernoulli, as the ratio of the change in static pressure to the dynamic pressure. The ETT was initially a hypothesis of D'Alembert, and is a result of potential flow, the first real attempt to apply Newton's laws of motion to a fluid. In this situation the curvature of the streamlines at the training edge and leading edge are symmetric, and you get no resultant lift force. As such, saying that the curvature is responsible for the acceleration (while true), neglects the resolution to D'Alembert's paradox which resulted because viscosity was not understood until Navier and Stokes 100 years later. So, the asymmetric acceleration around an aerofoil is due to viscosity. There are two specific effects, the Kutta condition, which moves the rear stagnation point to the TE. and the induction of more flow upwards ahead of the wing. The end result is an asymmetric velocity of the flow (circulation) which at the surface of the wing is given as a pressure force, which will also be asymmetric, with lower pressure above and relatively speaking higher pressure below.

[@FlywithMagnar]

Thank you for your contribution!

[@tonysales3687]

the air did move faster over the top of the wing and had lower pressure, so the same principle as a sail boat.

[@jeffreyerwin3665]

nonsense. A sail has no thickness and has exactly equal lengths on each side.

[@lordnilsson]

@@jeffreyerwin3665 but the wind moves a longer and faster way over the bulb sail, because on the other side there will be an "air-bubble" along the sail, over which the wind takes a shorter (and slower) way

[@adb012]

Hi Magnar, I am a PPL but more important for this I am an Aeronautical Engineer and also was an Aerodynamics teacher at college. While it is true that Bernoulli's principle applies only to one flow line, it can be also applied to two (or more) flow lines if there is a point where the energy state (speed and pressure) was the same in both flow lines. Sufficiently ahead of the wing, the parcels of air that are going to flow just above the wing and the ones that are going to go just below have the same pressure and speed. So you CAN apply Bernoulli's principle between a point above of the airfoil and another below. Still, transit time is wrong so you can't deduct the speed just by the differences in length. How lift is generated is at the same time more simple, more complicated, and more disappointing (or unsatisfactory explanation) than most people think. Let me know if you want me to expand.

[@FlywithMagnar]

Thank you for your feedback. Lift can be both easy and complicated to explain. I understand and agree with what your wrote. In this video, I just wanted to address a common misconception.

[@davetime5234]

Finally I stumble across some making some sense! A question or two if you don't mind: How would you explain to someone who insists that Bernoulli only works for a flow contained by an enclosre such as a pipe, that in fact any parcel within a flow also behaves according to Bernoulli and therefore it also applies to a non-enclosed object such as a wing? I assume, conceptually it is the interface between parcels that causes the misunderstanding. And for sails and thin wings which can be considered infinitely thin, how do you deal with the issue of path length of the sold being identical on each side: I assume the effective actual path length is different because the total flow on each side has an average center of flow extended out some distance perpendicular to the surface?

[@adb012]

@@davetime5234 ... Let's go point by point: 1) "How would you explain [this] to someone who insists that Bernoulli only works for a flow contained by an enclosure such as a pipe". That's a strange question. Bernoulli is applicable only when several ideal conditions are met, one of which is NOT that the fluid must be contained in an enclosure. The total set of conditions can be encapsulated in 2: "Potential flow" (that is steady flow of an non-viscous fluid, without any sources, sinks or rotors) and "Between any 2 points of the same streamline" (the airflow around a wing, outside of the boundary layer, is a very good approximation to that). So how do you explain Gravity to someone who insists that it only works in Venus? Well, I suppose you tell them they are wrong, that that's not a condition of the theory. That said.... The streamlines of a steady flow, being fixed in space and tangent to the velocity vector on each point, are never "crossed" by parcels of fluid and hence they can be considered as a pipe. But that's more the reason why Bernoulli works in a pipe, rather than why it works around a non-enclosed object. 2) "And for sails and thin wings, how do you deal with the issue of path length being identical on each side". Simple: You don't deal with the issue because there is no issue to be dealt with. To generate lift, there is no requirement that the path on one side is longer than on the other side. What you said sounds like "equal transit time theory", which states that the path along the upper side of the airfoil is longer than over the bottom surface because the path along the upper side is longer. The problem is that theory is totally wrong. Why? 2 reasons: Frist, there is no reason why one would expect it to be right. Imagine the following situation. To cars going on the same highway pass at the same time in front of a Shell gas station. Some points later both cars take different roads, the red car taking a much longer route than the blue car. The roads eventually rejoin, and each car eventually passes in front of a certain Exxon gas station. Would it make any sense to say that the red car had to go faster because it took the longer road? NO! There is no reason to suppose that. UNLESS both cars pass in front of the Exxon gas station AT THE SAME TIME. But why would that be the case? There is an implicit assumption in the equal-transit-time theory, which is that 2 parcels of air that are adjacent ahead of the wing, with one passing above and one below the wing, will rejoin at the trailing edge. But there is no reason why that should be the case. Imagine that I have a hose that forks and after the fork you have one 1 ft hose and one 10 ft hose, and you put the open end of these 2 branches side to side (after the long hose doing a couple of loops). Would you expect that 2 molecules of water separated at the fork will reach their openings together? And this takes us to the second reason why it is false: Because it is demonstrably false! If you calculate the lift based on equal transit time you get a value much lower than the actual lift. And experiments show that the air flowing along the upper side reaches the trailing edge FIRST (i.e. it wins the race against the air flowing along the lower side), DESPITE taking a longer path. It goes much faster than which equal transit would require. Parcels of air separated at the trailing edge never meet again. So why does the air along the top goes faster? Circulation. But I digress.

[@davetime5234]

@@adb012 Thank you so much for your reply! On the two points: 1)I think the solid enclosure comes up as an issue with Bernoulli, because it is shown and explained in the context of such a solid macroscopic enclosure in nearly every introductory explanation. So, people are presumably left with the notion that you need some solid constriction to contain the conservation of energy swap between dynamic and static pressure as the flow progresses. In terms of generalizing the concept beyond the introduction, for a parcel along a streamline, one perhaps worries about the continual transit of mass across the parcel boundaries (random diffusion for example) to adjacent parcels - no rigid pipe walls anymore now that that conceptual device is removed. I assume the boundary interface states (between parcels) are what carry the analogy forward: defining static, and dynamic pressure differences etc.? In other words, the concept leap is that all you need is a conceptual enclosure, specifying the appropriate interface variables? (not sure if I explained well what I was thinking..) 2)Equal transit time being wrong seems so obvious, no argument on that at all. However, isn't the faster transit time due to conservation of momentum and continuity of mass flow rate? M x V to be conserved, requires V to be as fast as necessary to keep M x V constant (ignoring "momentum leaks"). So, a path length disruption imposed, such as camber, increases V as dictated by the imposed geometry (as a key dependent variable), as momentum seeks be preserved? And that's why it's not equal transit time, it's momentum conservation and continuity of mass flow rate, that are driving the higher transit speed? Is the above false? Because if it's not false, then it seems a sail may need different path flow geometries on opposite sides of the thin sail, in order to generate the pressure differential (static pressure drop, due to conservation of energy, in order to service the increased V required to maintain the lateral momentum)? I mean, after all, a parcel approaching the sail, that gets split into the two different paths around the sail, starts off at a homogenous condition: the split parcels have equal dynamic and static pressure, and temper etc. So, the sail is, in effect operating on these identical twin parcels, differently? With the differing geometries of the two different paths affecting the identical twin momentums differently, causing one to have a different static/dynamic pressure swapping experience than the other?

[@adb012]

@@davetime5234 1) A parcel of air doesn't mix with other parcels of airs by definition. Remember a parcel of air is an infinitesimally small volume of air (that measures let's say dx by dy by dz). It may interact with other particles via action/reaction forces of 2 types: pressure and viscous. But Bernoulli assumes no viscosity (otherwise mechanical energy would not be conserved). 2) You lost me there. Momentum is not conserved. I mean, it is only conserved when there are no external forces applied. But a parcel of air is exposed to external net forces everywhere because it is in a non-homogeneous pressure field. Take a Venturi tube. A parcel of air will have the same mass (and volume if we assume incompressibility) in the wide part and in the narrow part. In the narrow part the parcel will be narrower and longer, but will have the same mass. And it will be going faster. So M x V was not conserved. Which makes sense because it was moving from a zone of high pressure (wide section) to a zone of lower pressure (narrow section) so it was moving along a pressure gradient, which makes work (conservative work, but conservative forces may not change the mechanical energy of a system but absolutely change its momentum). What I am going to say next doesn't satisfy anybody, but it is what things is: Ina wing, the air flowing above the wing speeds up and reduces its pressure by Bernoulli, and the air flowing under the wing slows down and increases its pressure by Bernoulli too. Note that I said AND and not BECAUSE. It is tempting to say that the reduction in pressure in the top is due to the increase in speed, but when you ask why it increases its speed it's because it moves from a zone of normal pressure (way ahead of the wing) to a zone of low pressure (above the wing). But it cannot be the case that the reduction in pressure is due to the increase in speed which is due to the reduction in pressure. That's circular reasoning. The reduction in pressure and increase ins peed (or the opposite on the lower side) just coexist. Why? Because it is the only thing that they can do to meet the boundary conditions (the flow shall not penetrate the wing) and the Kutta condition (the air shall separate at the trailing edge). Actually it is the circulation, described via the Kutta condition, which is the real "cause" of the lift (if you are desperate to find a "cause"). And don't let the thin sail fool you. The symmetry you try to present between the parcels flowing above and under doesn't exist. The sail will be curved in ONE direction relative to the free airstream, and the angle of attack will be angled in ONE direction relative to the free airstream. Imagine that you and your identical twin are standing in the subway and grabbing one of the vertical poles. Say that the vertical pole is in the middle of the cart, and you are facing forward, just to the left of the pole, grabbing the pole with your right hand. And your twin is just to the right of the pole grabbing it with his left hand. The subway takes right curve. You will apply a pull force on the pole, and your twin will apply a push force on the road. BOTH TO THE LEFT!!! There is no symmetry. A true symmetric condition would be an airfoil at zero AoA, with no camber and with an even distribution of thickness above and below (or a flat straight sail at no AoA). And guess what? That doesn't produce lift.

[@davidmowbray6352]

Tell that Bernoulli bloke I'm fed up seeing his crappy fan heater ads.

[@dougaltolan3017]

In that particular demonstration, not only was equal transit time wrong, the air over the wing went even faster than equal transit time would suggest. Meaning that lift due to Bernouli's Principal is even greater than equal transit time would predict.

[@bernhardecklin7005]

I like Magnar's no bullshit approach to everything and his calm and easy-to-understand explanations.

[@tomg6284]

The angle of attack has an impact on the lift as well. It correlates to the under the wing lift.

[@shi01]

There's no such thing as "under the wing lift" The lift a wing generates is the result of the pressure difference between the air over the wing and under the wing. By increasing the angle of attack you increase the pressure difference, which in turn results in more lift. That by the way also explains the wingtip vortices. Because all that is, is air trying to flow from the higher pressure area under the wing to the lower pressure area above the wing. At the inside of the wing generally the fuselage of the aircraft prevents this movement, but on the outside there's nothing that prevents this from happening if you don't add things like winglets.

[@rykehuss3435]

@@shi01 Have you ever held a piece of flat cardboard (or something similar) against the wind? Tell us you feel nothing pushing against you. Its not "air trying to flow", its literally physical mass of air pushing you. So you can call it "under the wing lift", which it is. Same exact principle if I shot you with a water cannon, you'd go flying yourself momentarily, and not because the water is trying to go around you to reach lower pressure area lol. Air is a fluid too.

[@shi01]

@@rykehuss3435 If it would be only reaction force, explain the stall effect. If you increase Aoa, yes the pressure under the wing will increase slightly, but the pressure over the wing drops even further. The the flow over the wing "stalls" the higher pressure under the wing isn't nearly big enough to provide any meaningful lift. The aircraft will drop like a stone regardless of it's speed.

[@aarondg19]

@@shi01how do u explain then there’s more pressure under the wing? Exactly Bernoulli

[@shi01]

@@aarondg19 Bernoulli explains the pressure drop above the wing, sure. But why does the airflow over the wing even stay attached to it until a quite high angel of attack? Bernoully doesn't explain that. Why does the airflow accelerate over the wing to start with? Bernoulli can't explain that. Bernoulli only explains the correlation between speed of flow and static pressure. It doesn't explain how a wing works, it was never meant to. Also, be careful when using Bernoulli in combination with high speed flows. The Bernoulli theory was meant for incompressible fluids. It does work with gases reasnable well, but only a low flow speeds, when no significant compression effects occure.

[@NicholasMati]

Why can't people get this right? 1. Bernoulli is only valid along a stream tube in an inviscid flow. The inviscid flow part is usually pretty valid for high Reynolds number flows outside of the boundary layer and shear layer trailing the wing. The stream tube is much more restrictive except when you assume that the far field has a uniform velocity and static pressure. Under this assumption (which is usually pretty good), all fluid starts with the same total pressure and thus has the same total pressure around the wing. Your statement about how Bernoulli isn't valid because the wing divides the airflow into two parts is wrong. 2. Curvature in the wing is important for keeping the adverse pressure gradient under control and keeping flow attached at high angles of attack, but it still misses the point. Just as the symmetric airfoil befuddles the equal transit time nonsense, the flat plate airfoil (commonly found on small RC foam models) and the supersonic diamond airfoil (with a sharp leading edge) befuddle curvature as the source of lift. IMHO, the best conceptual explanations don't even mention Bernoulli, but instead focus on what the forces are doing to individual masses of fluid. Air is under pressure and will expand when given the chance. As it passes over the top of the wing and the upper surface deflects down, air accelerates into what would otherwise be a void behind the wing. This is accomplished through a vertical pressure gradient resulting in reduced pressure on the upper surface. However, pressure is a scalar and the reduced pressure also corresponds to a horizontal pressure gradient which first acts to speed up and then slow down the flow. At the same time, the bottom of the airfoil has to deflect flow away from it which has the opposite effect and produces a high pressure region which slows flow down and then speeds it up. This is why the transit time over the suction side is faster than the transit time over the pressure side. The exact geometry determines where the peak pressures occur. On most subsonic airfoils, fluid begins to be deflected from the pressure side to the suction side a short distance ahead of the airfoil. Because it already has an "up" component (or down component for negative lift), it has to expand around part of the leading edge resulting in much higher normal accelerations (from the high curvature) and lower pressures which move the location of the minimum pressure forward. In a completely inviscid flow, this perfectly balances out the normal force experienced over most of the wing (which is tilted back towards the trailing edge) and results in no drag. In a real flow, there is some pressure drag, but most drag comes from viscous forces in the boundary layer. I suspect someone will mention circulation. There are enough people who view circulation as fundamental that I won't completely dismiss it, but I personally view it as a convenient mathematical relationship derived from complex analysis (one derivation literally just uses a conformal map of a complex valued function) that is largely divorced from what is actually happening.

[@johnpipping3848]

Now I’ve retired, it’s fantastic to know after 35 years and 20,000 hours of flying jets (mostly in command) without accident or incident. Is this an example of what they call a ”firm grasp of the non essentials”?

[@mmichaeldonavon]

I love that statement. :-) As a 40 year flyer - of meager hours - I honestly have always been in awe of you, the airline pilots, with 20 plus thousands of hours. As a student pilot in 1980, we students thought that a guy with 250hrs was a GOD! True. :-) I know you are still flying on the weekends. God bless you - you ARE "da man." Thanks. N6395T (but the Piper Arrow was my favorite - until the wings started falling off. . :-)

[@markmcgoveran6811]

Well I'm glad your comment was 99% about how wonderful you are a wonderful your experiences are and kind of 1% of a dig at the contents of the video. Understanding anything is not important for a pilot it's all monkey see monkey do train responses regulated you don't have to understand anything you just kind of point it where the instructor told you and do what the instructor told you to do and that's how it works for you. Then arrogant pilot that you are you call this a non-essential. This is essential and it's a fundamentally simple essential thing for a person who understands airplanes enough to design one for you to fly. I had a pilot friend like you one time I found a book about engineering and airplane design in the thrift store. I told my pilot friend that if an f-15 tomcat had a thousand less horsepower The minimum turn radius at 300 miles an hour went up by 50%. He was sure his pilot experience made him absolutely correct about airplanes and me not owning an airplane made me incorrect. Next time I saw him I got out the textbook from the college and I showed him the f-15 problem and I walked him through those calculations and I said imagine that. I didn't write this book but I understand it do you understand it now?

[@mmichaeldonavon]

@@markmcgoveran6811 Was that "put down" comment for me? I was just commenting on the Airline Pilot's exploits. Quite a career, IMHO. My meager manipulation of the controls was given as a "contrast" to his exploits. How do you fit in? Thanks for commenting. p.s. I thought we had just about beat Mr. Bernoulli to death.

[@markmcgoveran6811]

@@mmichaeldonavon not really a put down. Everybody needs a different version of an airplane. The pilot needs one thing the engineer needs another. It's a very handy thing to grab as big a piece of knowledge in your version of an airplane or anything else. Bernoulli may have been beat to death for you because you lack mathematical sophistication. A big airplane manufacturer will cut out an airfoil. They fly it in a wind tunnel and they take a lot of measurements wetted area velocity direction I mean they do a lot of measuring. Then these measurements are sealed in a vault and our top secret no one can see them. Then some extremely powerful mathematicians compete at this it's called a benchmark. They use Bernoulli's principle and partial differentiation differential equations and a bunch of other miserable math stuff and they predict the behavior of the air flowing around the airfoil and the forces generated by the airfoil. Of course you just look on a chart and it tells you everything you need to know about launching it at altitude landing in an altitude loads under certain altitudes. That was written by somebody who's very well versed in Bernoulli's principle. Did you use a checklist when you were a pilot? When the first multi engine bombers came out pilots flew up in the air and crashed on takeoff the airplane was worthless, the engineers are idiots they build something that can't fly. The engineer said you guys can't remember everything you need to do to launch this multi-engine aircraft. Here's a checklist. The pilots didn't think they needed a checklist. The general thought they needed a checklist and ordered the pilots to roll down the checklist every time they launched a multi-engine airplane and they quit crashing on takeoff.

[@mmichaeldonavon]

@@markmcgoveran6811 Thank you for your in-depth comments. I really liked your comment that: "... because you lack mathematical sophistication." I'll bet you are fun at parties. Thanks. E=Mc2

[@steffanjansenvanvuuren3257]

Nothing can be accelerated instantly. Because it has inertia. That is why air molecules are literally forced apart at the top faster than it can accelerate toward the wing, becoming less dense (lower pressure), by a wing at speed. The air at the bottom of a wing is forced by the wing so fast it becomes compressed faster than it can accelerate away from the wing. (Higher pressure)

[@TesserId]

Having never heard that hypothesis that the top air would catch up to the bottom air, it was really easy for me to be skeptical about it. In my mind, the speeding up has always been about squeezing the flow to make a fluid go faster, and that applies to a single flow as well. I have no reason to believe there is much of any interaction between the streams once they split between above and below the wing. Anyway, great exploration of the topic.

[@wesleyhopmans]

When I was doing my physics teachers degree I heard my teacher say that the air particles reached the end at the same time. I immediately knew this was wrong and exclamated: "Why, they don't have a date, do they?" I still have much respect for that teacher but that day learned me never trust anybodies word for it.

[@batmandeltaforce]

Thank you for finally stating this. The pressure under the wing is FAR greater than the negative pressure about the wing. If the wing was nothing but a flat surface, it would still work just fine.

[@Quraishy]

Indeed this has been my intuitive thought for 2 decades, but scientists always talking to the wing lift due to lower pressure at the top always bothered me. If there is more pressure at the bottom, its enough to cause lift. when you hold you hand out of the car slightly tiling up wards, you feel the lift, and the wind pressure on the underside or inside of your hand, a lot more then the any pull force you feel at the top of your hand.

[@batmandeltaforce]

@@Quraishy The shape is more to avoid stall:)

[@conned]

True, in theory, but you won't have lift at Zero AoA. Hence the high lift device, n slats n flaps , THS

[@TheBlowbob]

This is simply false. The under-pressure on the upper side of the wing can far exceed the over-pressure on the lower side of the wing, especially at high angles of attack. This is obvious from the fact that the over-pressure cannot be greater than the free stream dynamic pressure, because the air cannot be decelerated to less than zero velocity, converting all available dynamic pressure to static pressure (in engineering terms, the local coeffcient of pressure cannot be greater than +1). On the other side, nothing stops the air from being accelerated to more than twice the local dynamic pressure as in the free stream, so the pressure coefficient can be well below -1. The reality is: both the over-pressure on below the wing and the under-pressure above the wing create the lift in conjunction, and you cannot have one without the other. Even a flat surface at an angle of attack will have an under-pressure on the upper side. The reason for that is that the stagnation point - that is, the point where the stagnation streamline, which separates the flow above and below the lifting surface, meets the wing or plate - is not at the leading edge but slightly behind the leading edge on the bottom side (if the plate or wing are an an angle of attack or, more specifically, when they are creating lift). So even on a flat plate, the air has to go upwards and around the leading edge, creating an under-pressure there.

[@alexandermenck6609]

Sounds nice but is wrong as the equal transit time theory. Arvel Gentry explained lift 50 years ago. No viscosity, no lift. And it is Bernoulli at the end - albeit for different reasons.

[@throughthoroughthought8064]

I still don't get it.

[@christopherknee5756]

Agreed. He complains that what was wrongly believed was a hypothesis (academic yap-yap for a guess) AND THEN promptly does the same with his guess about the curve causing the air to suddenly speed up!

[@robbedontuesday]

The video is avoiding (intentionally or not) two factors: angle of attack and speed. Bernoulli's principle does not work at slow speeds and high AOA. This video is bs.

[@singh2702]

It's amazing how everyone always focuses on the longer length of the upper surface instead of focusing on how the foil is disturbing the air as it moves through it. The stagnation point is located under the wing just behind the leading edge. This stagnation point is the source of the upwash of on-coming air and is the result of the flow pattern around a foil. The air that flows up towards the bottom of the foil hits the surface and is forced to change direction and follow the contour of the underside. First, the air flows up and then changes direction, giving air flow at the bottom a longer path(due to upwash not length of the bottom of the foil), hence a delay compared to the flow travel above. The further an airstream is away from the foil's surfaces, the faster it's moving.

[@mikefochtman7164]

That is a great demonstration. The 'meets at the tail of the wing at the same time' explanation has always troubled me. Another thing that nobody seems to consider is the change in direction. Clearly the air leaving the tail side is moving downward to some degree and that means a force was applied by the wing.

[@blusheep2]

Why would that mean a force was applied by the wing? Wouldn't that have something to do with the fact that the bottom air is lagging behind the upper air? The downward movement of air is more about induced drag I believe.

[@mikefochtman7164]

@@blusheep2 Well from a simplistic analysis, let's assume at first the air was not moving (assuming no wind at all), and then the air is accelerated downward as the wing slices through it. The underside of the wing is at a slight angle and deflects some air downward (angle of attack). The air over the wing flows over the curved surface and exits the back side in a smooth flow line along that surface (assuming the angle of attack is not so steep that the wing 'stalls'), which is slanting downward even further. Sure, AFTER the wing passes there are all sorts of eddys / whorls and turbulence in the air. But before that the air is being accelerated downward as the wing slices through. Hence, I believe, the wing is acting to push the air downward and an opposite reaction force pushing the wing up.

[@blusheep2]

@@mikefochtman7164 OK, I see. The deflection of the bottom air down demonstrates an earlier force on the bottom of the wing. Its a Newton's 3rd law thing as opposed to a Bernoullie principle thing.

[@dougaltolan3017]

@@mikefochtman7164 Absolutely this! Treat the wing as a black box. Before the wing, the air is static, after the wing the air is moving downwards. Whatever accelerates that ir must exert a force on it and experience an equal and opposite force. Only if you need to design a wing and present its characteristics do you need to know HOW it works. Which makes me wonder, why teach pilots this anyway? There are shed loads of systems on aircraft that pilots, aren't taught.

[@mikekelly5869]

@@blusheep2 Exactly. Coanda effect due to surface resistance slows the layer in immediate contact with the wing and causes the air avove to curve downwards due to velocity transfer to the slowed air below

[@Wolfie6020]

I used to be a flying instructor in the late 80's. We would explain this to our students and the easy way to confirm it was the propellor wash you could feel behind an aircraft. If Bernoullie was the reason for lift a propellor would produce no wash and a helicopter would not create any downwash. (That would be nice - I flew rescue helicopters for years and the downwash was always an issue during a winch rescue)

[@markclark4154]

At 2.39 notice how the air is being forced downwards. The lift can can also be determined using this. A wing works in the same as a propeller. Stand behind a propeller at full thrust and you will get the idea.

[@jayreiter268]

Static thrust is a special case. I had not seen that slow motion before. I only saw the old spark stop motion. That explained turbulent flow.

[@deang5622]

I think the explanation you are looking for is Newton's Third Law of motion.

[@dwmac2010]

Mark, I was going to make the same point you make, except with a helicopter "Rotary Wing". A helicopter rotor is also shaped the same way as an airplane wing. Its motion with rotor angle of attack create a tremendous movement of air downward. It is F=MA. The Mass of the air times the Acceleration of that air creates the Force upward, which lifts the copter. It is "For every Force, there is an equal and opposite Reaction." Air downward/Helicopter upward. Same with an airplane wing. Thank you for mentioning your point about the propeller. Propellers also have the same airfoil shape. I agree with you 100%.

[@mikekelly5869]

A wing doesn't work the same way as a propellor but most rotors work the same way as a wing, at least to some extent.

[@jayreiter268]

@@mikekelly5869 They all work the same way.. We just view the effect differently. With a fan or propeller stationary we feel the blast of air because the blade is drawing in air one blade at a time in the same place. When the airplane is in motion the blade describes a spiral. The British word for propeller is Aero Screw. Many propellers have Clark Y airfoil. I never worked on helicopters so do not have a full understanding. The larger the rotor "disk" the more it can lift. The same as wing span. It all has to do with Mass Airflow.

[@timrogers2638]

I have an engineering degree (not aerospace), and in all of my physics classes going back to high school, the Equal Transit Time hypothesis never made sense to me. Why does a given molecule in the air that goes over the air foil have to remain in the same place in space relative to the x axis as its "companion" molecule that moves under the air foil? Thank you for showing the slow-motion video of the pulsed smoke in the wind tunnel that belies the Equal Transit Time hypothesis.

[@davetime5234]

Mass flow continuity though, has to be maintained: rate of mass flow out has to equal rate of mass flow in. And that has to be true for not only the total mass flow around a wing, but also for the bottom and top surface of the wing considered separately. I've wondered sometimes if Equal Transit Time is a failed interpretation of the above(?). There are, however, more sophisticated studies attempting to explain the historical legacy and longevity of ETT.

[@timrogers2638]

@@davetime5234 - I would concur that that total mass in has to equal the total mass out, but the pulsed smoke in the wind tunnel seems to contradict that the mass flow rate in equals the mass flow rate out, with "Mass flow rate" as mass per unit time. The mass flow rate along the bottom of the airfoil has a slower/lower flow rate than that over the top.

[@davetime5234]

@@timrogers2638 Yeah, so I'm forced to conclude (I assume), that the total slowing of mass per unit time on the bottom must exactly equal the total increase in mass per unit time on the top: one balances out the other. And then, of course, however much mass flow rate gets "allocated" to the top flow, maintains its own in/out balance, as does the flow on the bottom. (and the top paragraph seems consistent with the wing's asymmetry inducing "circulation"?)

[@royshashibrock3990]

Great video, but I would like to say that while he does mention the lift being generated by the airflow under the wing, he skips over the reason why. It is because any time a fluid is forced to change direction (the wing's angle of attack causes a downward deflection of the air striking the bottom surface), energy in the fluid is given up, and this energy manifests itself as an increase in pressure on the deflecting surface. Something interesting to note is that while the low pressure area on top of the airfoil is more or less dependent on the airfoil, and therefore stays in the area with the most curve, this is not true of the pressure being applied on the bottom of the wing, which is quite erratic. This is why an airfoil that stalls due to excessive angle of attack becomes unstable, and why "flat" wings will never work, but symmetrical wings will.

[@FlywithMagnar]

I totally agree!

[@umi3017]

The most pressure change is on the top of the wing, not under, on a high speed low AOA condition, the button of the wing could be lower pressure than the surrounding as well, just not as low as the top. Flat wing totally works, I have made dozen of them out of KT board, and you probably have done so with paper.

[@FlywithMagnar]

I'm sorry, but I disagree with you. The most pressure change happens at high AOA where the stagnation point is below the leading edge of the wing. This forces the air to follow a larger curvature, which causes a larger acceleration and hence, a larger pressure drop. Maximum lift coefficient is achieved at critical AOA. For a reference, please read "Aerodynamics" by L. J. Clancy, pages 62-63. It can be downloaded here: www.scribd.com/document/321464060/Aerodynamics-Clancy-pdf

[@nitramluap]

Yes, Newton's Third Law. Air mass displaced downwards is counteracting the weight of the aircraft.

[@umi3017]

@@FlywithMagnar Maybe I didn't make it clear that when I say the change I mean the absolute value of the difference between ambient pressure and local pressure, and the picture on p63 shows exactly that, on normal AOA, the upper "suction" is more than the under "pushing". ofc no wing IRL have only upper or under part, but when it comes to what surface is more impotent to keep clean if you have to place things like engines or flaps guide rail or even ice, most of the time is the upper surface to keep clean

[@chomanthapa]

[Another perspective] Here is how I see it, the upper part of the aerofoil has a longer distance as it is curved, the lower part has a shorter distance. Now let's stream an imaginary 100 molecules of air to the tip of the aerofoil, 50 molecules go up and 50 go down. The shorter part (lower) has more molecules per distance than the upper part where the molecules are less dense. Now we know denser particle have more pressure compared to lower, this is why the molecules below try to push upward, Hence, lift. I completely agree with everything else.

[@bkailua1224]

I flew jet airliners for 30 years. What maters is dont crash. You dont have to understand any of this to fly safely.

[@kevinbarry71]

Then, why do you have to learn it? And wouldn't you like to know correct information?

[@hugobloemers4425]

I met an airline pilot who told me that flying a jet is like "monkey sees, monkey does". Now I understand what he means.

[@ariffpro]

Correct, pilot do their jobs, engineer do their jobs. There will be no pilot if no one built airplane.

[@royshashibrock3990]

To hear this is most troubling...since you say you "flew" I assume you no longer do so, which is good. When things go wrong, your knowledge of what you have to work with (the mechanical device you are encased in) may save the day (and the lives of many innocent people). I shudder to think that I am flying on a plane with a pilot that has an attitude such as yours.

[@FlyNAA]

@@royshashibrock3990 This particular piece of knowledge has absolutely zero value in problem-solving any normal or emergency situation. A good understanding of the AOA vs CL curve, OTOH...

[@mikekelly5869]

I'm not sure what's being said here. It seems to be a criticism of pilot education, but what is the specific criticism? Bernoulli only holds true when there's non-turbulent flow and when laminar flow starts to break down the forces on a wing (or anything else for that matter) become a far more complicated proposition. It's fine to tell pilots about lift generated by laminar flow around an aerofoil but I'd be surprised if they don't cotton on very quickly, maybe around the time of their first stall, that there's a lot more to the fluid dynamics of lift than just what Bernoulli said.

[@redseven485]

Tell me you're Scandinavian without telling me you're Scandinavian. Good video!

[@FlywithMagnar]

Magnar

[@garyradtke3252]

Glad to hear you explain the real science behind lift. In the early70's I was taught in junior high science that lift was not created by increase pressure under the wing but decreased pressure above it. I was always interested in flying but this explanation always left a question in my mind because it didn't make sense that enough negative pressure alone could create enough lift. With my education with internal combustion engines, hydraulics and other machines operating on the laws of physics I began to realize that it is the pressure differential that causes lift. Lower pressure above and higher pressure below do to Bernoulli's law creating differential pressure is the cause of lift.

[@shi01]

What is also importent to know though, Bernoulli alone does only explain why the pressure drops over the wing. But another interesting question is why does the air follow the upper wing profile. Why doesn't it simply get pushed aside by the leading edge and create a turbulent void? And that's where the coanda effect comes in.

[@jsquared1013]

The decreased pressure on the top side is still of a greater magnitude than the increased pressure on the bottom. I'm no aerodynamicist but I have seen quite a few diagrams of wing profiles showing the pressure gradients along the surfaces (granted it is for racecar wings, so the airfoils are inverted compared to an airplane, but the idea is the same).

[@TheBlowbob]

@@jsquared1013 I am an aerodynamicist and I can confirm what you say.

[@TheBlowbob]

It's not that the negative pressure on the suction side of the wing can't produce enough lift. It's that you cannot have the negative pressure on the suction side without the positive pressure on the bottom side and vice versa. Actually, with a thick enough airfoil, the pressure on the bottom side may still be lower than ambient pressure over most of the surface, due to the displacement effect of the thick airfoil, but you'll still have the stagnation point, where the pressure is increased to the total pressure, on the bottom side of the airfoil.

[@jamesspash5561]

ERAU grad here also. I note in the smoke test demonstrating the angle of attack is at what would be a stall angle. Not a good representation.

[@howardsimpson489]

The turbulence immediately behind the wing shows early stall, agreed, not a good demo.

[@ehudgavron9086]

There are 12 comments below mine, and they all mirror each other and what I would say. Danke! I'm not sure there's anything else left to say. Science is a process, and you've done it well. As a person studying to be a CFI I think your material would be helpful to future students who care about HOW AND WHY things work. Again, thank you. Ehud Gavron FAA Commercial Helicopter Pilot, Tucson Arizona US. Future CFI because I love to teach. You have helped me today!

[@riedjacobsen8620]

Last century, this was the way FAA taught it. The "FAA correct" answer on the written exam was Bernouli's theory. Even though it was wrong, that was the answer to pass the exam.

[@TheBlowbob]

Bernoulli's theory isn't wrong. The equal transit time hypothesis is. They're not the same. This confusion comes from the fact that the equal transit time (ETT) explanation then also calls on Bernoulli's theorem to explain why the pressure in the faster flow above the wing is lower than the pressure in the slower flow below. This inverse correlation between flow velocity and pressure according to Bernoulli is correct, and it is also correct that the flow above the wing is faster than the flow below the wing. Just the ETT gives a false explanation as to why the flow above the wing is actually faster. The real reason for this is the Kutta condition at the trailing edge, which is due to viscosity, in conjunction with the mass flow continuity equation.

[@georgekappland2935]

As an Instructor Pilot for 20 yrs I couldn't agree more. Very misunderstood concept. Bravo! Lift is Bernoulli living on top of the wing while Newton lives on the bottom. Two separate operations but both must work together or.... Bernulli, on it's own, would never make enough lift for flight.

[@HankHarjes]

Bernoulli's Priceable only works within a closed volume. Newton's 3rd law, "For every action there is an opposite and equal action". is what creates all the lift by simply deflecting the air flow downward. A wing with "0"deg. incidence creates NO lift.

[@davetime5234]

"Only works in a closed volume" is false. As mass density varies little in subsonic flight, air diverted adjacently means the mass density can only by maintained by the speed of flow increasing. As a result of the speed increase (higher kinetic energy), conservation of energy requires that static pressure drops. This resulting pressure gradient deflects the momentum of flow downwards. So with Newton's "reaction law", you're only getting at part of the problem.

[@bowlampar]

When i was told the faster air stream at the top meet the slower air at the bottom on trailing edge, i was confused, thinking the faster air must be waiting for the slower one at the end....until you come along saying it is just a hypothesis , not a proven theory. I 'm feeling relief. 🤗

[@rampy4963]

This explanation is incorrect as well because it is incomplete. If curvature was the sole explanation, then flate plate lift wouldn't exist. All lift is created through circulation, whether it be from wing shape (camber) or angle of attack. The effect is equivalent to the magnus effect... rotating golf ball. The viscosity of air defines the circulation flow field shape. Without viscosity, there is no lift, regardless of camber and angle of attack. Essentially, no lift means zero circulation ensuring the molecules meet each other at the trailing edge of a 2D wing. Consider a cambered aerofoil at zero angle of attack... it also generates lift through circulation. Lift generated by camber creates lift at the aerodynamic center, typically the mid-chord position in subsonic flight. Lift generated by angle of attack usually occurs at the quarter chord position. The combination of the two causes lift to advance towards the 1/4 chord position with increasing AoA. Too many experts on the internet these days. Former test pilot here.

[@FFE-js2zp]

As a trained Aeronautical Engineer, I have always had a problem with this way of describing lift. For one, the air is still until a plane moves through and smacks it. What’s generating lift is a moving baseball bat (wing) hitting ground balls (air molecules), forcing the bat to rise. The angle of attack is what ensures when the air is hit (high pressure on the bottom) it travels down instead of upward. And negative AOA does the opposite. It is that simple. A perfect example is a waterskier. He hits the water forcing it downward and him upward = lift. A fan blade/wing is another obvious example of whacking the air producing a base hit. Once that air molecule is walloped, a lower pressure flow whooshes in to take its place.

[@Not-Only-Reaper-Tutorials]

I'm telling this since decades. Thank you not only to have told, but also to have shown it in practice ❤

[@cosmicraysshotsintothelight]

The upper surface produces most of it, but "downwash" glancing off the underside also seems to aid/add. Otherwise, helicopter landing zones would not be so breezy. But wait... attach a 25 foot diameter (~8 meter) pan under the helicopter. Is it still able to fly with all the downwash hitting on itself?

[@ChrisTietjen_00]

@@cosmicraysshotsintothelight If the reaction force off of the blades is greater than the reaction force off of the pan the brick will fly.

[@wayneyadams]

I remember going to a Physics teachers workshop and Bernoulli's Principle came up. So, I asked a few Physics Teachers how it worked. I got the standard answer you heard here; pressure is reduced. When I probed further and asked about the actual physical phenomenon causing this drop in pressure, I got one of two answers, "I don't know," or "because Bernoulli's Principle says so." Teaching Physics is about imparting an understanding about how the physical world operates, not teaching students to memorize laws and equations with no understanding about the underlying phenomena. So, how does Bernoulli's Principle work, in other words, why is the pressure reduced on a surface when air flows over it? Air pressure is the result of molecules of air molecules impacting the surface. Air pressure decreases for two reasons, fewer molecules strike the surface, and the speed with which they strike the surface is reduced. If air is flowing over the surface, fewer molecules will strike the surface because they are being dragged along the surface by the air stream flowing over the surface. Greater speed of the air stream, results in fewer molecules striking the surface resulting in lower pressure. That's it, it is no more complicated than that.

[@FlywithMagnar]

You are absolutely right!

[@LTV_inc]

Uhh no.

[@romeocharlie]

I remember my aerodynamics professor at the ATPL training saying: “Do you remember Bernoulli?, well forget that stuff”, as he endeavored in a full year of complex aerodynamics about the phenomenon of circulation and the importance of viscosity of the fluids in regards to lift in airfoils.

[@davetime5234]

While viscosity is important, conservation of energy cannot be separated from the fundamentals. And Bernoulli is shorthand for conservatiion of energy. Circulation is fundamentally tied to conservation of energy because the differential speed between air on top vs bottom is a conservation of energy transaction. So, circulation needs Bernoulli.

[@TheBlowbob]

This aerodynamics professor should consider a career change...

[@rcgldr]

Note that the M2-F2 (glider) and M3-F3 (rocket powered) lifting body re-entry prototypes that predate the shuttle have the longer surface on the bottom (along with a relatively high angle of attack while gliding), somewhat similar to a wing that is upside down and backwards (leading edge is thinner and narrower than trailing edge). You can find images and youtube videos of both. The main reason for this design was the relatively high drag reduced the time from re-entry to landing. As for "why air flow over a wing remains attached", a simple explanation is "void theory": as a wing at some angle of attack sweeps through a volume of air, the air has to fill in what would otherwise be a void swept out by the wing. Since the air has viscosity, a frictionless mass of air can't simply follow along with the upper surface of a wing. Below stall angle, what would be a void is filled in by air that mostly follows the upper surface of a wing. If the wing is stalled, then vortices will fill in what would otherwise be a void, greatly reducing lift. Another side effect is that there is a normally a net increase in pressure at the trailing edge of a wing. The induced downwash continues to increase speed until it's pressure is reduced back to ambient, and the speed when pressure returns to ambient is called the exit velocity. This is more obvious in the case of a propeller or helicopter rotor.

[@oldandintheway9805]

One of the greatest books ever written about flight and how an airplanes creates lift was written by Wolfgang Langewieshe. Called Stick and Rudder. He says that an airplane cannot maintain altitude with a zero degree angle of attack. If the reduced pressure over the wing made enough lift to fly the airplane, you wouldn't need to increase the angle of attack to maintain a given altitude. The pressure increase under the wing does a lot more than the reduction of pressure over the wing. And according to Wolfgang, it makes a lot more sense to teach this as it is easier to understand.

[@FlywithMagnar]

Langewieshe also said rudder pedals were useless... Langewieshe tried to simplifly aerodynamics in a way a layman could understand. I agree with him that we can forget Bernoulli's Theory. (The correct expression is Bernoulli's Equation, which is a mathematical formula used by engineers.) The problem is that Langewieshe got several key points wrong: - A wing with a cositive camber produces lift at zero angle of attack. - Most of the lift is produced over the wing. - The only time the underside of a wing produces more lift than the top of the wing, is when the wing is stalled.

[@davetime5234]

"If the reduced pressure over the wing made enough lift to fly the airplane, you wouldn't need to increase the angle of attack" Also, you're missing the fact that the angle of attack itself generates its lift mostly from pressure drop above the wing. In no way does AoA correlate with a dominant effect happening under the wing. Like asymmetrical camber, AoA's effect is mostly above the wing.

[@0bm31770]

Good explanation. I knew Bernoulli's didn't explain it all. RC pilots have made flat boards fly (not efficient, but they fly)

[@teamheat7564]

@3:38 Its the TOTAL pressure (Ptot=Pstatic+Pdynamic) over the wing being lower than the one just below the wing (NOT the static) . The static pressure is the pressure due to your altitude and its practically the same above and below the wing - what changes is the Dynamic Pressure component of the total pressure , due to the acceleration of the flow above the wing . There is also another mechanism that produces lift by just deflecting the air downwards (there have been aircraft with upside down airfoils creating net positive lift by just setting them to aggresive angles of incidence) .Its Bernoulli (suction) + Net Deflection of airflow (Newton's 3rd law) that in combination produce lift .You can produce lift with either of the 2 mechanisms but you do it more efficiently (ie from a drag and predictable stall characteristics perspective) by combining both

[@woodpile66]

Doesn't the vacuum resultant/exist because the big metal thing moved all the air out of the way (down)?

[@teamheat7564]

@@woodpile66 NO-low pressure is created because of the acceleration of the flow and there are several ways to accelerate the flow , one of them is through the curvature in the favorable pressure gradient region of the airfoil (typically from LE to maximum thickness area of the airfoil)

[@davetime5234]

Most of the lift results from the action above the wing. And this is the drop in static pressure. The static pressure drops to maintain conservation of energy in response to the speed increase necessary to maintain mass flow rate. The static pressure drop turns the passing air downward, in the same way that wind turns toward a low pressure weather system.

[@woodpile66]

@@davetime5234 this confuses me more, you say the air above the wing is acting on more air - it seems like you have nearly left the wing out of the equation

[@woodpile66]

@@teamheat7564 I don't see how you can say "curvature" as if it wasn't the the wing. (in most cases) engine power is pulling the "curve" through the air. This function accelerates air and points it down - both as an action on air demanding a reaction. Given your viewpoint, how does an aileron manage to defeat all that suction and create a negative force with just 3 inches of flat metal?

[@rivi7197]

As a physicist, I have the impression Bernoulli's principle does not even apply to the problem, it is merely a side effect: That is because it relates a pressure gradient along the flow line with the acceleration along said flow line. However, the problem of lift is about a pressure gradient, and hence acceleration, perpendicular to the flow line. This has to do with he curvature of the flow, not it's speed.

[@thepilotdrummer]

To be honest everybody believes the equal transit theory is wrong but as per the smoke tunnel theory the smoke a.k.a relatuve airflow is not in contact on the trailing edge but if u notice more closely they will meet each other after a certain distance on the trailing edge, and more over the two main key theories are the equatioon of continuity and the Bernoullis theorem

[@EngineeringFun]

The main reason of lift generation is down deflection of flow due to the angle of attack. There are airfoils with more curvature on the bottom than the top (for instance ) and they provide ample lift provided there is enough AA. Besides, pilots generally don't understand aerodynamics past very basic level. They understand and follow rules and procedures.

[@beagle7622]

As an engineering student we stalled a wing shape in a Smoke Tunnel at around the right angle . I know roughly at the right angle. I roughly remember what the figure was but it was approximately what we hoped for.

[@josephinebennington7247]

Having watched and read much on the phenomenon of why a wing flies….While I agree that the reduced pressure above the wing ASSISTS lift, I still maintain that if you are pushing a plate into a body of air and give it an angle of attack then your plate, of itself, is definitely creating a helpful cushion of air that it sits on and will be LIFTED by that cushion just as much as it will be SUCKED up by the lower pressure above it. In years past the explanations always seem to want to state that a wing is SUCKED up, with out ever including that it is also being PUSHED up from below.

[@TheBlowbob]

The reality is: both the over-pressure on below the wing and the under-pressure above the wing create the lift in conjunction (namely, by the pressure difference), and you cannot have one without the other. Even a flat surface at an angle of attack will have an under-pressure on the upper side. The reason for that is that the stagnation point - that is, the point where the stagnation streamline, which separates the flow above and below the lifting surface, meets the wing or plate - is not at the leading edge but slightly behind the leading edge on the bottom side (if the plate or wing are an an angle of attack or, more specifically, when they are creating lift). So even on a flat plate, the air has to go upwards and around the leading edge, creating an under-pressure there. Since the under-pressure on the suction side (upper side) of the wing is typically greater than the over-pressure on the pressure side (bottom side), some lazy teachers just say that the wing is being sucked upwards, but it's always both.

[@koenth2359]

A great demo. It has always bugged me how people parrot this lift principle, as if it's very simple, without really thinking about it, and without understanding Bernouilli's principle.

[@judahrichardson3426]

Very simple and concise explanation, however I would add why the velocity increases over the top surface of the airfoil. This is actually due to curvature of the airfoil, which causes a curvature in the streamline due to the coanda effect. An increase in curvature of a streamline causes an increase in velocity. Hence why the top surface of the airfoil has a larger velocity than the bottom due to it being curved more.

[@mytech6779]

The air at the leading edge is not accelerated directly by the curve, the bottom is also curved and there the air decelerates. It is accelerated by a difference in pressure(force), the pressure gradient is created by the overall wing profile. Even adding a tiny 90 degree flap to the trailing edge will change the gradient and flow near the leading edge.

[@rolandotillit2867]

The same thing that causes the drag causes the lift, the wing vortex. It produces upwash at the tip, lowering lift, because it blocks the suction surface, but when it reverses direction it produces downwash which then blocks and slows flow on the pressure surface side, and draws air across the suction surface accelerating it. Winglets just move the tip vortex exposing more surface area to air flow, allowing slightly lower AOA, thus improving fuel efficiency. The vortex strength remains the same, ie proportional to lift. Extending flaps just increases and strengthens the vortex sheet. There's a reason Prandtl equated downwash with lift.

[@FlywithMagnar]

The wing tip vortex is an unwanted side-effect of lift. The swirl does not contribute to lift, but is known as induced drag. To reduce the vortex, the designers can increase the aspect ratio of the wing (gliders are good examples.) Another technique is to taper the wing towards the tip. Winglets reduce the vortex, and hence drag, especially at high angles of attack. When Prandtl equated downwash with lift, he ment the downwash inboard of the vortex. He also concluded that the most effective lift distribution is bell-shaped. You can learn more about Prandtl and how NASA developed the Prandtl wing here: Al Bowers - Prandtl wing update: kzbin.info/www/bejne/rV7HnGSEpbuBhKs

[@smoutezot]

3:09 I think it's beautiful to see that the first flowline under the wing gets partially sucked up even ABOVE the wing because of the lower pressure there.

[@davidzachmeyer1957]

The smoke "bursts" show that the airflow is not speeding up over the top of the wing - it is continuing at the same speed it was before encountering the wing. On the other hand, the airflow under the wing slows down.

[@WillN2Go1]

Good explanation. Especially at 3:04 where the velocity of air over the wing is shown. Someone should make that venturi apparatus with only one side pinched to model air flow over an airfoil. * Of course now you have the problem of showing the tube of liquid which shows the pressure differential. But this isn't difficult to over come. *The demonstrator as it now is shows two wings mirrored with the liquid filled tubes modeling the pressure above the air foil against the pressure in front of the 'wing'. You actually want three measuring tubes. One on the 'airfoil' (at the venturi), one before the venturi and the third under the 'wing'. These could work if you just have a reservoir of liquid feeding all three tubes, then each tube would show the air pressure before the wing, on top of the wing and under the wing (which might be made to show how angle of attack increases air pressure.)

[@tombrazier6172]

Thank you. Like so many others I had to "learn" the wrong explanation for lift when I learned to fly. It's good to hear someone debunking it.

[@einherz]

there nothing wrong with this explanation, it's just not complete

[@nikthefix8918]

@@einherz In electronics (my field) plumbing analogies are often used as teaching aids. They work fine until they don't. The model of the atom taught in school physics is utterly wrong but conceptually useful - until it isn't. I suspect that the newton / bernoulli lift contribution ratio and their respective real world inticasies are revealed on a need-to-know basis (so to speak). I have commercial pilot friends who claim that it's 2/3 bernoulli and 1/3 newton, but a quantum physicist would say that it's ultimately neither of these things!:)

[@einherz]

@@nikthefix8918 sure it's all about wing form direction aoa. some forms will use more time bernoulli, some forms - newton. but both will used all time, even flat wing with 90* aoa will forced by bernoulli too, same as laminar symmetric wing at 0* aoa will use newton force too. flight is dynamic aircraft is dynamic, air is dynamic environment. engine above wing more bernoulli, engine under wing - less bernoulli, but if imagine all airflows around there newton and bernoulli everywhere:)

[@Anthony_Harper]

I have been an instructor for about 30 years. I have argued your point with aeronautical engineers for years. If each particle at the front had to get back with its respective particle at the end then how would we be able to stir our soup? Each pea that has a neighbor pea would have to get back with its neighbor pea after traveling around the spoon?

[@FlywithMagnar]

Good point!

[@chipgarner8555]

The reason this explanation persists is the reason science is hard. Humans like explanations for things but don't really care if they are correct. Also, pilots don't need to now how lift actually works to fly the airplane.

[@BState]

As you mentioned, the wing's curvature causes the acceleration of the air over it. I'd like to add that this acceleration is further enhanced by the suction effect, drawing air towards the area of lower pressure. This pressure difference is influenced by the angle of attack, not just the air's acceleration due to the wing's curvature.

[@KajolKhan-qj5ne]

Yes but before this suction could be possible, the speed had to be increased, hence the curvature, which provoked the acceleration, then the low pressure and from that comes what you just explained.

[@BState]

@@KajolKhan-qj5ne I agree that the wing's curvature is the initial factor that accelerates the air, leading to the subsequent low-pressure area. My point was to highlight the combined effects of this acceleration and the suction effect it creates, along with the role of the angle of attack.

[@Matty88K]

I liked this demonstration. What about the Coanda Effect? The fluid flowing over the top of the airfoil clings to the wing surface, and as it follows the trailing edge, it is directed (accelerated) downwards. Following Newton's Third Law, the mass of the air flowing downwards exerts an equal force in the opposite direction: upwards. This is how most of the lift is generated. If the angle of attack becomes too great, the fluid cannot adhere to the trailing edge and the lift ceases. The flow along the bottom of the wing, can also be diverted by flaps or ailerons, and this directs even more air, or water, downwards, generating even more lift. Also, because the airflow (mass) is taking place along the trailing edge, it exerts a rotational force the wants to pitch the airframe downwards. The elevators counteract this by directing air leaving the trailing edge of the tail upwards to maintain level flight, or whatever pitch the pilot wants. Without that counteracting force, the craft would do a somersault and tumble over itself and out of the sky, like a leaf blowing in the wind. Please tell me if I'm completely wrong here.

[@cowboybob7093]

KZbin title: _Doug McLean | Common Misconceptions in Aerodynamics_ puts much less emphasis on the Coanda Effect. The clip has some chapters. Lift has not been defined. So far I'm in the "it's mainly like a rock skipping on water" - And if one can't generalize that to a steady flow model then one can't criticize it.

[@FlywithMagnar]

The Coanda effect is used to blow air over the wing's upper surface and/or flaps to control the boundary layer. Lift is not Bernoulli or Newton. They both describe the same thing. Please watch this video: kzbin.info/www/bejne/ppmUeaSontR_htU The bottom of the wing produces only a small portion of the lift. That's why a wing stalls when the angle of attack is too large and the airflow over the wing is disturbed. The downwash behind the wing is balanced by an upwash ahead of the wing, which is also part of the lift. www.av8n.com/how/htm/airfoils.html#sec-upwash-downwash The rotational force is created by the fact that the lift center is behind the center of gravity. It is balanced withe the horizontal stabilizer (and elevator). If you move the center of gravity the the location of the center of lift, the aircraft will be unstable like the F-16, which can only maitain flight with the aid of computers.

[@cowboybob7093]

@@FlywithMagnar With all due respect to you, an industry professional who is captivated by the subject, your demonstration of the Coanda effect at three minutes into the clip you linked: _"Forget Bernoulli and Newton | The easy way to explain lift"_ is misleading. The leading edge of the paper device is different in the two orientations you demonstrate. While it's tempting to address the airstream and leading edge curl, the major force difference in that specific demonstration is the natural springiness of the paper pulp matrix. That springiness assists the success of horizontal demonstration by amplifying the effect. In the vertical demonstration it contributes to stiffness which resists the effect but contributes to flutter. Held horizontally as shown, the leading edge curl imparts forces, pressure and tension on the whole matrix, including the trailing edge. Those forming forces are from within the paper matrix and would be present to form the paper to the same shape even in a vacuum. Vertically held the paper's shape is maintained by the matrix' springiness and produces flutter where the trailing edge is not assisted. Again, in a vacuum any vertical paper would conform the same vertical shape. I am not convinced you watched the clip I posted the title of. Perhaps you did. It is of a Boeing Technical Fellow's lecture at U. Michigan, it lasts about 45 minutes and presents the non-matrix factors you and I cite, and more. As I wrote before, it recognizes the Coanda effect as integral to the phenomenon of lift but does not predominate the phenomenon. However I find Doug McLean's multi-force harmony approach to be more comprehensive than yours. Ultimately though I believe _LIFT_ has not been fully defined due to the inability of real-world observations to be made. My statement is not meant to discount the importance of wind tunnels, it is simply to state that measuring aerodynamic changes at some fixed points in space relative to the moving aircraft is extremely difficult. That last point in mind, from the reference of an aircraft moving at 200 knots at an altitude 1000m above the ground plane, to discover where air initially is disturbed by the oncoming aircraft, for instance, is a challenge. Perhaps flying through a hectare large array of smoke trailing rockets would help, but I haven't seen such footage.

[@pmac_]

It would help if you included stagnation lines in your diagram at 3:21. This seperates the flow into two regimes, one above and one below the wing.Draw these lines first before you draw the other flow.lines.Except close to to wing these lines should aporoach being symmetrical about the vertical axis through the centre of lift.

[@woodpile66]

Most folks here seem to say that low pressure exists because the flow speeds up, but that group does not account for why the flow sped up in the first place, or the mandatory physics. I offer that the wing has acted on a fluid and accelerated it. The bottom by blunt force change in direction, the top by leveraging coanda effect. Velocity being preferable, as it is squared in the lift equation (the bottom does not share this advantage). This is a force which has to be accounted for with an equal and opposite force. Some say this only happens on the bottom, but the physics don't change. Once the air has been accelerated, a vacuum has formed (larger and smoother replacement flows result from more efficient acceleration). Bernoulli's EQUATION (not a force) is a great way to measure how efficiently the foil accelerates (speed and direction) air by measuring the decrease in static pressure. The group that feels the wing sucks itself up in the air is probably standing in a bucket and lifting themselves by the handle. I would hope someone can explain what happens to the wing when it causes air to accelerate. Furthermore, while they are thinking about it, what if you replaced a single engine prop with a rocket motor? Everyone would measure the force by the mass and speed of the ejected matter - but when it is a propellor, most people claim it is an airfoil and suddenly we measure the vacuum out front. No one ever puts Volkswagens in front of a 747 to watch them flip over. Sure, the engines suck in air, but what matters is when huge energy gets added to the flow - not the vacuum. Does water form a vacuum over a diving plane?

[@RichardKCollins]

You might want to look up "pressure gradient forces" and "impulse" of fuels. The pressure increases with fluid velocity at constant density. At higher pressures the density can increase giving more pressure. Rules of thumb memorized to model invisible flows are less useful than modeling every voxel individually with all that is known about the flows, fluid properties, heat, temperature, energy, three axis velocities and acceleration. Ultimately those models all have to be simple accounting, because the digital models are calculating with add subtract multiply divide mostly. Look at adjacent cubic voxels in a simulation and then check the equations for momentum, energy, pressure, velocity, and all the other things. It is an accounting model. Ways of estimating might change, but it is not complicated taken a tiny piece at a time. Magnetic fields, lasers, microwaves, mm waves, THz, UV, soft x-ray, acoustics, plasma jets, ionization, electric fields, ion and electron and nano-particle beams - there are many ways to image and modify turbulence, temperature, velocity profiles. And there are many ways to look ahead and plan for the air that will be encounters. The speed of light is thousands of times faster then the aircraft and air. And the air has a finite (albeit large) range of states and changes. The computers now can image, process, predict, verify and change in real time, and with electric and electromagnetic field, some things not possible with mechanical, hydraulic, motor and solid parts, similar to what the Wright brothers used in form and presumptions. You might want to look up "plasmonics" for electromagnetic and field methods near surfaces. Many tools are available off the shelf now (COTS - "commercial off the shelf"). James Melcher wrote 'Field Coupled Surface Waves (MIT 1963). The full title is Field Coupled Surface Waves - A Comparative Study of Surface Coupled ElectroHydroDynamics and MagnetoHydroDynamic Systems. His methods are out of date now, but the ideas are that old. Only in the last few years are sensors, computers, memories and algorithms fast enough to apply those and subsequent methods in real time. Flying fighter jets with rigid surfaces pales in comparison with flying with boundary surfaces that can have energy densities hundreds of thousands of times the ambient air or more. You can reach out ahead of the vehicle and change the air and its fields remotely. If you ever wondered how to stop a vehicle on a dime or make it hover without jets or in near vacuum, there are tools for that kind of design and have been for decades, if you could afford the parts back then, or now. Filed as (Account for all the small changes carefully. Then the system, however large, is not so difficult to predict) Richard Collins, The Internet Foundation

[@chrislee176]

Ever since I was a kid and put my hand flat out the window of a moving car, like a planar wing, it was clear to me that lift did not solely come from negative pressure on top. The wing explanations that were given to me as I grew older were an affront to the simple common-sense observation that an airplane could fly with a flat wing -though since it’d be terribly pitch-sensitive aerodynamically, people rounded it into the elongated teardrop cross-section shape.

[@a.n.7863]

If lift was totally dependent on the shape of the airfoil a plane couldn’t fly upside down.

[@mariohernandez1111]

The lift does depend on the pressure, it's the only way the fluid can apply a force on the surface. The fluid changing direction is in itself a proof of there being some pressure change. If you angle a flat plate, the exposed side will have a larger pressure than the backfacing side. I feel that most people don't really make this connection. Yes, you can treat a wing like a black box and say that the lift comes from the force required to redirect the direction of the air. But the way that happens is literally pressure. The fluid turning and the pressure differences are linked.

[@maxheadrom3088]

Excellent video, Mrl Magnar! I learned a lot including how many misconceptions I had! Thanks!

[@nashcar20]

Civil Engineer here. Lift has always conceptually made sense from observation. But, being familiar with hydraulic behaviors, the explanations by PPL instructors and videos have always left me incredibly dissatisfied. Thank you for correctly explaining this concept. It's unnerving to see the perpetuation of such a large fallacy!

[@axelBr1]

The clinching arguments for me were; a) practically any flat-ish surface can be a wing, (no fancy curves required); b) aeroplanes can fly upside down, (the lift generated on the top of the wing would now "lift" the aeroplane into the ground). But another way to think about it is, if we take a wing and whirl it around, if the lift was generated purely by Bernoulli's Principle then there shouldn't be a disturbance in the air some distance below the rotating wing. Of course anyone who has been underneath a helicopter will know that a wing accelerates air downwards.

[@programmer78]

The simulation is illustrating not only Bernouli principle: on the simulation the wing has attack angle. To simulate Bernouli principle the wing's chord should be parallel to the horizontal axis. The flow should be laminar.

[@esslar1]

Delighted that Magnar clearly lays out that "theory" and "hypothesis" are not the same thing.

[@unclesam326]

I believe this misconception comes from the effects of a venturi. In the Pilots Handbook of Aeronautical Knowledge, concerning explaining Bernoulli's principle, it says that for a venturi tube "The mass of the air entering the tube must exactly equal the mass exiting the tube. At the constriction, the speed must also increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube." In a constricted environment like the tube, this would apply, however in the open space an airfoil operates in, this is not the case and thus two particles flowing over and under an airfoil do not meet at the same time.

[@nyankorunaway2446]

Sir, I'm Japanese so I don't understand what you say unfortunately, sorry. Just I want to show my own ideas. 1) Wing has AoA upward, and wind comes. Thw wind separates up and down at the front edge of wing, and the up wind is barierd by upward AoA wing. The wind comes behind wing as ⤵voltex. Wind extends the voltex along the top of wing, and the voltex blows away at the end of wing. Upper air speed = air speed + extended voltex speed. It will be faster. Lower air will make ⤴ voltex at the end of wing, and blows away. It may disturb upper air, so wing end should be sharpen to prevent interference. 2) Wing shaped upper streamline, lower flator dimpled. Upper wind flows smoothly on wing fast. Lower wind flows as making voltexs along lower wing, so the lower air speed is slow.

[@davetime5234]

I assume the asymmetry of AoA plus any asymmetry of wing shape, cause wind to take asymmetrical paths. Circulation is then caused by 1)total mass flow rate = mass flow rate below + mass flow rate above. 2)total mass flow rate into the wing = total mass flow rate out from the wing 3)because of the asymmetry and "1)" and "2)", the increase in speed of mass flow above = decrease in speed of mass flow below. And this is circulation. This circulation is angular momentum added to the relative wind. The turning of the air caused by the total asymmetry (AoA plus asymmetrical camber) causes a downward change in air momentum that results in the "action-reaction" force of lift.

[@nyankorunaway2446]

@@davetime5234 mmmn (- -) Now I'm confusing. The front edge of wing separates air to above and below. AoA disturbs flow of above so mass flow and pressure will be low,. and below is opposit. Newton law makes lift force. However, does this theory make circulation? I think above air flow is distubed, and aborve pressure is low. then air comes there faster, and berunui law generates vaccum.

[@brandonjohnson8880]

Instead of equal transit time, think of conservation of mass. You can draw a control volume around the wing and show that mass entering and exiting are equal. From there you can derive L/D from Bernoulli

[@jeffmotsinger8203]

Yes, you can fly with air-dams or sails but fuel costs are much higher. Minimize drag by using less angle of attack and more Bernoulli lift.

[@Greggg57]

Atmopsphere has weight. That weight bears down on the wing. When the wing is moving through the air, the bernouli funnel is created by the top of the wing and the 'bottom' of the air pressure. inlet: high pressure, low velocity; middle ; low pressure, high velocity, back of wing; high pressure, low velocity. Low velocity is like a vacuum.

[@davetime5234]

There is one crucial error here though: "Low velocity is like a vacuum." That's not at all true because we need to distinguish between pressure and mass density. A vacuum is zero mass (no mass density). As you approach a vacuum, your mass density decreases but the molecules' random velocity (potential energy) may still be just as high. The problem with the mass of air around an airfoil (assuming flight is sufficiently below the speed of sound) is that the mass density is essentially constant. So, when you get the lower pressure above the wing, the mass density hasn't dropped. Therefore, lowered static pressure is the same density of molecules impacting less energetically with the upper wing's surface. What you called "bernoulli funnel" is not like a "shadowed" area swept of air by the wing to make a region of depleted mass. Bernoulli is a depletion of potential energy rather than depleted of mass. You just made me realize, here may be a crucial point of argument with the people who like to say Bernoulli doesn't apply: being shorthand for conservation of energy, the underlying principles of Bernoulli are essential to account for changes in energy associated with pressure drop (in supplying energy for the higher speed), because mass density doesn't change. (Some extra point needs to be added here I think??)

[@geoffreywilliams9324]

I like your video. However another way to demonstrate the increased force on the underside of the wing is to simply rotate the section further clockwise. As we do so becomes clear that the wing will be forced to the right . .

[@octavianr526]

The wings are pulled up mainly because the wing is tilted up in front. The air hitting under the tilted wing is lifting up the wing. Play with your palm pulled out of a running car. Tilt the palm. The lift of the palm is because the air hiting under. The Bernoully applies too, but the main force is from under the wing. Increasing the tilt will increase the loft force until at near 90 degrees there will be no lifting force.

[@jeffreyerwin3665]

Bernoulli's principle is often (and mistakenly) used by sailing instructors to try to explain how a sail propels a a boat despite the fact that a sail is a sheet of zero thickness and therefore has exactly equal distance on each of its sides. A sail works because it deflects the airstream. Newton's law: an action results in a reaction. Deflecting the airstream results in an equal and opposite reaction.

[@davidg4288]

The rubber band powered toy balsa wood planes we had as kids did actually fly a short distance from a flat surface. The wings were flat. Of course more sophisticated models did have airfoils and flew better.

[@jeffreyerwin3665]

@@davidg4288 In the Fall on 1962 I was attending some kind of introductory forum at MIT. The new students there knew how to construct super-light balsa planes that seemed to stay up forever on rubber band power.

[@rb-ex]

it's a good clarification, but it's not complete. in this and most demonstrations/illustrations the wing is always always tilted upwards from the flight path. the problem is, even a wing that is flat on both sides will generate lift that way, and that lift will depend on 1) velocity, 2) area of the wing, and 3) air density. again, this lift will occur even in the absence of any bernoulli effect. but no one seems to acknowledge this much less quantify it even roughly. how much lift is generated by the upward tilt of the wing vs bernoulli effect?

[@davetime5234]

The "Bernoulli effect" is always there because it is shorthand for conservation of energy. The 3 pillars of the more precise characterizations of lift offered by the Euler equations and the Navier-Stokes equations, are: conservation of mass flow rate, conservation of energy and conservation of momentum. Taking away the "Bernoulli" pillar of conservation of energy is like cutting off one of the legs of a 3-legged stool: it no longer can stand upright (lift can no longer be fully justified). "upward tilt of the wing vs Bernoulli effect?" It's not one or the other: the effect of the "upward tilt" is justified in terms of conservation of energy.

[@rb-ex]

​@@davetime5234 for the pressure on a 'wing' that is flat and does not involve an airfoil, the solution is given by newton's sine squared law. if you can solve this with the bernoulli equation please show us how the bernoulli principle is derived from the first law of thermodynamics, but is not interchangeable with a general principle of conservation of energy within fluid dynamics, nor does it account for a higher velocity of air over an airfoil if you carry a large piece of plywood on a windy day and are blown over, this does demonstrate conservation of energy within fluid dynamics, but would you would you think of this as 'lift' and explain it with the bernoulli equation? in fact, the fluid dynamic principles that underlie airplane flight have been intensively modeled and re-modeled but are not well-understood, just as similarly worked-over concepts in physics, such as refraction of light, remain poorly explained and poorly understood

[@rb-ex]

@@davetime5234 for the pressure on a 'wing' that is flat and does not involve an airfoil, the solution is given by newton's sine squared law. if you can solve this with the bernoulli equation please show us how the bernoulli principle is derived from the first law of thermodynamics, but is not interchangeable with a general principle of conservation of energy within fluid dynamics, nor does it account for a higher velocity of air over an airfoil if you carry a large piece of plywood on a windy day and are blown over, this does illustrate conservation of energy, but would you think of this as 'lift' and explain it with the bernoulli equation? in fact, the fluid dynamic principles that underlie airplane flight have been intensively modeled and re-modeled but are not well-understood, just as similarly worked-over concepts in physics, such as refraction of light, remain poorly understood

[@realvanman1]

Equal and opposite reaction, right? Still air encountered by the moving wing is moved from a higher place to a lower place. That requires force. The force is applied by the wing. The wing pushes the air downwards, the air pushes the wing upwards.

[@FlywithMagnar]

Yes, that's the 5 seconds explanation of lift.

[@TheAirplaneDriver]

The Bernoulli effect arguments the lift created by an airfoil. A flat wing, or fully symmetrical airfoil, or semi symmetrical airfoil will all generate lift due to dynamic forces striking the bottom of the wing. The higher the angle of incidence and/or angle of attack (to the point of stall), the higher that dynamic force and subsequent lift. At an equivalent angle of attack, a semi symmetrical airfoil will be more efficient as it will generate lift both from dynamic forces and Bernoulli effect. Hold a piece of paper by the edges and blow over the top of it. The paper will lift up because of Bernoulli forces….no dynamic forces present. The wind tunnel test in this video is interesting. Thanks for posting it.

[@wrap239]

The Equal Transit Time Fallacy was also taught to me in USAF pilot training in 1983. It was years later while flying for a major airline that I actually learned the truth.

[@rogerhardy6306]

Using Bernoulli to explain wing lift is only partially correct but is easy for students to understand. A flat plate will develop lift as its incidence is increased, it's just that it's not very efficient and stalls at low incidence. A sailboat sail develops lift even though it has no thickness (between upper and lower surfaces), only camber. It's very easy to do a calculation of the pressure difference created by a notional aerofoil in accordance with Bernoulli but you will soon find out that the lift generated is nowhere near that actually generated by a real wing. The full description of the generation of lift is best explained by newton's Third Law. Lift is the reaction to the motion of the wing deflecting the air downwards as it passes through it. Lift is equal and opposite to the vertical component of the algebraic sum of the rate of change of momentum of all the air as the aircraft passes through it. In the horizontal direction it is induced drag.

[@Thyme2sea]

As a pilot you only need to maintain speed enough to get lift at all times. The reason or explanation for the lift is irrelevant during flying. You have enough other concerns simultaneously. Just keep the speed high enough, that’s all. In a similar note: you don’t need to know biology to reproduce, or be able to build a clock to tell the time.

[@TBolt1]

But, but, but … how can you make money on a dozen monetized videos about something if you don’t make a big deal about every detail? 😅 As if anyone would fail a check ride over this shit.

[@Brad-gk9jd]

I always thought that the wind over the top part of the wing created a vacuum. That caused the bottom part to push up to fill that vacuum. That is what created lift.

[@lawrencequave7361]

I believe you are substantially correct: that the wind travels faster over the top of the wing to cause a lower pressure area there than below the wing where the wind is traveling slower. This very statement negates the 'equal transit time' addendum (I'll call it) which I don't think Bernoulli ever intended to be a part of his theory.

[@consentofthegoverned5145]

Most lift is produced by angle of attack: The airfoil shape allows for the same lift at a lower angle of attack, presenting less frontal area or lower coefficient of drag and less turbulence, thus increasing efficiency.

[@birdman4274]

3:18 This is the main point. The curvature leads to a change in direction of the air molecules creating a centripetal acceleration speeding up the airflow on the top of the wing. (velocity being a vector having both magnitude and direction)

[@Jmnp08]

He just explained Bernoullis while simultaneously trying to disprove it .... Wtaf?

[@dennisk5818]

Very interesting. It still makes sense, though. The air flow over the wing IS still traveling faster, and therefore has a reduced pressure. But, in the high speed chamber images, it's clear that the flow beneath the wing is traveling slower, and the pressure is greater. But does it need to catch up with the flow over the wing? It shows that it doesn't. I've been enlightened as a pilot. Thanks

[@davetime5234]

Pressure is less on the top and higher on the bottom because the wing's asymmetry assigns more of the mass flow continuity contribution to the top at the expense of the bottom's contribution. That speed difference directly correlates with the pressure difference. From the above is implied: conservation of mass (flow rate), conservation of energy and conservation of momentum. But also, circulation, as that is what faster over the top and slower on the bottom works out to.

[@yaglehoole5662]

In 5th grade, I wouldn't let my science teacher off without explaining the Equal Transit Time "theory" -- fallacy in fact -- to my satisfaction. I did not understand where the air molecules on the upper surface of the airfoil got the memo that "You must meet your lower counterparts at the same time at the end of the airfoil. You have a longer distance (being on a curve), so you must run faster."