Is Airflow Distance Over the Top of a Wing Always Longer?

[@LetsGoAviate]

Airplane Propeller Effects - kzbin.info/www/bejne/jZ-tqaV-otmHlcU Understand Airplane Propellers - kzbin.info/www/bejne/nqfFoJxsrJaNb6M

[@tonywilson4713]

Aerospace Engineer here: I thought this was going to be another hair pulling video, but instead you've done a great job of explaining it. In future I wont bother trying to explain this anyone and just send them here.

[@Eduardo_Espinoza]

The last part was incredibly eye opening, just like a cool math trick tnx & subbed! :)

[@evolutionCEO]

buoyancy vs density and all alleged pulling forces negated... simples...

[@Munakas-wq3gp]

If lift was produced by pressure differential alone, you could create an extremely efficient lift wing by having a jet engine intake channeled underside the wing.

[@DocScience2]

Is there any particular reason you feel the need to explain basic science to people who refuse to take basic physics classes.

[@InternetStudiesGuy]

I think the confusion arises from many explanations of how wings work referring to the "special shape" of wings, when that's not what's happening at all. The "special shape" improves drag and possibly stall speeds, but fundamentally a perfectly flat board would work to generate lift as long as it has the right angle. But lots of explanations I've seen imply that the shape is somehow magic, when all it does is optimize the generated drag for a certain angle of attack. It's a skewed teardrop shape.

[@michaeldunlavey6015]

You're right.

[@chrissmith2114]

A flat board would generate massive drag at high attack angle because of the massive eddies behind it.

[@Talon19]

Positive camber wings produce lift at zero angle of attack; flat plates don’t.

[@JHe-f9t]

This right here. The special shape makes it a 'lifting body'. A lot of time when someone asks 'how do planes fly', they are actually being told what a lifting body is, and not how planes actually fly. An upside down wing has negative lift at 0 Angle of Attack. That negative lift is overcome by an increase in AoA, as this video explains. A lifting body makes a plane efficient, but it's not what makes it fly. Stick your hand out the window palm down at 70mph, if you hold it at a positive AoA, your hand will rise. Your flat hand is relatively symmetrical and as such is not a lifting body. If it were, your hand would go up slightly faster, that's about it. Planes fly because they push air downward. A shaped wing makes it push 10% more air downward than a flat one.

[@jured7383]

Yeah for flat board is only about the angle of attack ( with big drag ) while for cambered wing ( in practical limitations ) it is Bernouli with low drag but high lift

[@MarcPagan]

"Money makes a plane fly" My first flight instructor :)

[@VETTERACER96]

Sounds like a cool CFI

[@MarcPagan]

@@VETTERACER96 Even better perhaps? I cleaned this gem up a bit - "If it flies, floats, or fornicates ....it's cheaper to rent it" He said "cheaper" mind you, not better :)

[@jimviau327]

@@MarcPagan - the most accurate answer so far :)

[@mhughes1160]

If you want to go higher or faster then Bring more money 💰 . LoL 😂 👍

[@tomtufore3426]

On the topic of flight instructors. "Those that can make a living by doing, will do; those who can't will teach." And now "those that can't make a living by teaching will teach how to teach"

[@mikeb.7068]

You can generate lift with a sheet of plywood. But a sheet of plywood stalls at a very small angle of attack. The answer to what creates lift is the change in momentum of the air forced downwards by the passage of the wing. This creates an equal and opposite lift force on the wing. The low pressure above the wing also makes a contribution. The purpose of the airfoil shape is to avoid boundary layer separation above the wing as long as possible.

[@LetsGoAviate]

I don't necessarily dispute this. But I would like to say a sheet of plywood is no different than a symmetrical wing (other than being very inefficient). The stagnation point still forms under the leading edge assuming positive AoA, and that's what this video is about, showing that fundamentals doesn't change with wign shape, right side up, upside down.

[@mikeb.7068]

@@LetsGoAviate I agree. For aerobatic flight, you need a symmetrical wing. To fly slowly you need a highly cambered wing. To fly fast you need a thin airfoil. All of these wings will fly inverted.

[@oneninerniner3427]

How about a ramping or camming effect, doesn't that add to making a kite or barn door fly?

[@adb012]

"The low pressure above the wing also makes a contribution." It is not A CONTRIBUTION (I am "highlighting", not "yelling"). It is the same thing at different levels or perspectives of explanation. The only way for the wing to push the air down (and for the air to push the wing up) is through a distribution of pressures, and that distribution of pressures can be explained with high accuracy through Bernoulli. In the same way that you would not say that X+1=3 and X-1=1 both contribute to X being equal to 2.

[@LetsGoAviate]

@@adb012 Nice viewpoint. I don't like binary viewpoints, rarely are things either fully black or white, fully right or wrong. This is similar to how I see it. Ofcourse the video was only to counter an argument not to explain lift fully, but nonetheless.

[@Eyes-of-Horus]

Many a military pilot has said, "If you put a jet engine on a brick you can make it fly." If you look at the wings of the F-104 the wings are practically razor sharp and pretty much flat with no discernable lift surface. But that thing flew and quite well.

[@Ranchpig67]

Your right. Google a picture of two F16's flying one up and one upside down for airshows and you will see that they have the SAME angle of attack. This is fact, not perception. It destroys the nonsensical idea that angle of attack is causing lift. The same for "airfoil" shaped wings. Those wings are basically symmetrical.

[@Hornet135]

Yep, Biconvex 3.36% airfoil and something like 4 inches thick at the root.

[@rsteeb]

Yep! The F-104 is my favorite example to show it's NOT Bernouli, but NEWTON that provides lift.

[@flybobbie1449]

Yes but look at the speed it needed for take off and landing. What range angle of attack? I bet 5 degrees max. Reason why so many crashed, pull and snap stall. Reason we have rounded leading edges is to smooth any transition as a of a is increased.

[@Hornet135]

@@rsteeb The two are not mutually exclusive.

[@rowlybrown]

It's so simple. The wing must accelerate air downwards to produce lift. The airfoil shape makes the process more efficient. End of story'

[@daffidavit]

That's only part of it. The wing creates a "Venturi effect" with the smooth undisturbed air above it. Both ways contribute to lift, so it's not the end of the story.

[@Trompicavalas]

Absolutely right, and it is perfectly established, in aerodynamic theory by Kutta-Joukowsky theorem, since 1906

[@petervanderwaart1138]

All the diagrams illustrting the K-J theorem show the wing with a positive angle of attack, leading edge higher than trailing edge, with airflow deflected down. Gotta obey Newton's Third Law.

[@daffidavit]

All of the old NACA smoke stream videos show that the highest induced lift was produced just before the stall occurred. The air at the trailing edge of the wing burbled first as the burbled air crawled backward toward the leading edge of the wing. None of the air was flowing downward past the trailing edge of the wing. Thus, at high angles of attack there must be something more than Newton's laws that are creating the lift. It's a combination of Newton and the Venturi effect that produces lift on the wing. @@petervanderwaart1138

[@rmack9226]

@@daffidavit www1.grc.nasa.gov/beginners-guide-to-aeronautics/venturi-theory/#:~:text=It%20neglects%20the%20shape%20of,lift%20generated%20by%20an%20airfoil. Nope

[@vandal968]

I've been a licensed pilot since 1993, and this is the best explanation I've ever seen of how a non-symmetrical wing is able to generate lift despite being upside-down. Bravo!

[@flashgordon3715]

Any model airplane pilot could have told you that. Laminar flow has its benefits and negatives

[@davidsoom1551]

Since we're here to learn ,let me say this, you re a certificated pilot. There is no "license" issued in the US. know why? The Interstate Commerce Act. Go don this rabbit hole you'll be astonished.

[@davidsoom1551]

@@anthonyb5279 A bit touchy. You didn't need to take offense. At any rate you don't have a "Pilot's License', and you were speaking to a group of pilots not laypersons.

[@davidsoom1551]

@@anthonyb5279 There is no "Psych" portion to a medical exam that deals with neurotic pilots. Sounds good though.

[@BulletsRubber]

I bet there are literally dozens of pilots who don't live in the US, and they wouldn't have to take any notice of the Interstate Commerce Act. In the UK for instance pilots require a PPL, or Private Pilots Licence, and are very much 'licensed pilots'

[@mikehunt8968]

All down to angle of attack...

[@davefoord1259]

Not quite all. A section with no camber has zero lift at zero aoa. A cambered section has zero lift at a small negative aoa. But otherwise yep i agree with you. Mind you an aircraft is often designed to fly at its most efficient aoa in cruise, that is if efficient cruise is its most important design parameter, and thats usually at a fairly small aoa. A cambered section will always br more efficient at a small positive aoa than a symmetrical section

[@BlakeBigfoot]

.. no. Did you watch the video?

[@davefoord1259]

@@BlakeBigfoot yep i did, what did i get wrong?

[@ketilrkke4552]

The theory that lift on an airfoil is created by the static pressure differential between the air on the lower and upper surfaces of the air foil has been left behind years ago. It has its origin in Bernoullis principle that states: “If in the same mass of fluid og gas, part of the fluid or gas moves faster , the dynamic pressure increases and the static pressure decreases” It is possible to measure this pressure differential, and what you will find is that it creates just a fraction of the force that is needed to keep the airfoil and structures connected to it, off the ground. The lifting force is instead a matter of mass movement. Air is viscous and it has mass. That means that if you move a part of a mass of air, the air around it will immediately fill the gap created by the displaced air. The process continues a distance through the air mass, depending on the volume and acceleration of the initially displaced air. So it is the downward deflection of a variable amount of air that keeps the aircraft defying gravity. This can easily be seen in wind tunnel tests where air particles that initially were many meters above the airfoil, is way below it when the airfoil has passed through, all depending an the speed and angle of attack of the airfoil.

[@victormuckleston]

i only clicked on this video looking for YOUR answer, not the one he gave. well done!

[@JulianDanzerHAL9001]

bernoullis principle still applies and htings get ab it ocmplciated if you try to look at every atom of air but when udnerstanding lift bernoullis principle is useful the other way round to how people think the wing produces lift and because of this and because of bernoullsi principle the air speed sup above htewing NOT the other way round

[@calvinnickel9995]

@anthonyb5279 Bernoulli is pretty much the _only_ thing that makes lift in most aircraft. The only aircraft that can take advantage of Newtonian lift are modern fighter jets as doesn’t start increasing lift coefficient until well beyond the stall (this is what vortexes from delta wings and leading edge root extensions bridge) and doesn’t reach a maximum until about 45 degrees angle of attack.. still producing a lower lift coefficient than Bernoulli lift with about ten times the drag coefficient. The only practical applications of Newtonian lift are things that don’t need to support themselves or where the drag can also be useful.. things like square rigged ships, paddle wheels, and impulse turbines.

[@JulianDanzerHAL9001]

@@calvinnickel9995 um newtons laws apply always unless you suggest airplane wings are relativistic

[@JulianDanzerHAL9001]

@@calvinnickel9995 "The only practical applications of Newtonian lift are things that don’t need to support themselves or where the drag can also be useful" nope, read the wikipedia article on lifti nduced drag and have your mind utterly blown

[@warriorson7979]

The confusion arises because people think it must be an either/or... Lift is generated by 2 simultaneously occurring phenomena, one is the pressure difference between the top and bottom of the plane (causes a small amount of lift and almost no drag), and the other is the impulse due to air being accelerated downwards (causes most of the lift but also a lot of the drag, called "Drag due to lift"). The rest of the drag is due to the form factor and skin friction.

[@thrall1342]

To be fair, those are one and the same thing, if I'm not mistaken. Air acceleration downwards can only happen with that pressure differential created by something that's thusly accelerated upwards.

[@davetime5234]

@@thrall1342 They are linked by cause and effect and numerical equality, but the two must be considered separately. A suction cup stuck on a refrigerator door is held up by a pressure difference, but the weight of that suspended mass is supported through the structure of the fridge against the floor. As an aircraft has no such rigid support structure, it is required to expel mass downwards, as a consequence of that pressure difference, to supply the necessary force suspending it above the ground. So, what manifests as a result of the pressure difference, must be considered separately to get at the full description. An airplane suspended by a suction cup from a crane is also opposing weight from a pressure difference, but the crane is substituting for the need for vertical expulsion of mass (change in vertical momentum equals the difference in pressure for an aircraft in flight).

[@davetime5234]

And those result from conservation of mass flow rate (continuity equation), conservation of energy and conservation of momentum. The airfoil puts stress on the mass flow rate continuity resulting in the pressure drop, as conservation of energy requires this to increase the velocity to maintain flow continuity (deprivation of flow increases pressure differential consistent with lateral acceleration of the flow). This conservation of energy induced pressure drop alters the path of the flow field in the vertical direction (adjacent air moves towards the area of lower pressure) such that the change in momentum vertically equates to the force from the vertical difference in pressure. All phenomena intimately tied together, and each separately requiring consideration. Navier-Stokes equations.

[@burnttoast111]

All you need to understand is Bernoulli’s Principle. When you have a fluid (air follows fluid dynamics), there are 2 kinds of pressure: 1. Static Pressure. Pressure all around an object. At sea level, there is ~15 lb / in ^2 of pressure acting on you. 2. Ram Pressure. If you are in a speeding car, and you stick your arm out the window, your arm moves very quickly through the air, exerting pressure on your arm. What's the relationship? As ram pressure goes up, static pressure goes down. If you roll the windows down on your car, and drive fast, light objects in the car, such as a piece of paper, can get sucked out of the car. On a wing, where the air splits, as long as the flow is laminar (not disrupted, stalled, etc.), it will meet on the other side. The air that goes further has a higher ram pressure and lower static pressure. Lift is static pressure pushing from high to low pressure through the wing. What seems to not be considered is that control of an aircraft is done through changing the shape of the various airfoils through control surfaces, which change their lift, drag, etc.

[@thrall1342]

@@davetime5234 Of course it’s all in there, but the air that changes momentum never touches the wing. It mediates its momentum change by exerting force on the air around, which is pressure. Newton’s law: no momentum change without a force, which in this case arrises from pressure. All those conservation laws you stated necessitate that pressure and mass flow cease to be separate quantities and are linked. Its basically a reduction of dimensionality, like a singular matrix describing an equation system with less degrees of freedom than variables.

[@daemn42]

While most of what was said in this video is correct, what was left unsaid is that the idea that the air above the airfoil follows a longer path than that below is often associated with the "equal transit time" hypothesis, which is false. This was taught to many in K12 physics classes and even in some older textbooks. It goes like this.. "If a packet of air is split in half at the forward stagnation point into two, then the upper packet of air *must* arrive at the trailing edge at the same time as the lower packet, and thus follows a longer path over the curved upper surface". This is false. But the truth is stranger than that. In reality the upper air packet does follow a longer path, but it is both accelerated rearward and downward, and it arrives at the trailing edge *before* the lower packet of air and *before* the free stream air well above and below the airfoil (outside of its direct influence). The net result of the upper air arriving first is that it displaces a mass of air behind the airfoil, downward. This total mass of downward forced air integrated over time equals the total generated lift, and in steady flight that equals the weight of the aircraft. At higher angles of attack and higher wing loadings this air is accelerated over the top of the airfoil faster still relative to that below. It naturally wants to follow a curved shape (ref "bound vortex") , and the curved airfoil shape is more a reflection of the shape the air *wants* to flow for a given angle of attack and wing loading, rather than something that forces the air to flow along that path.

[@ChimeraActual]

Excellent! And the transverse motion of air towards the wing tip creates vortices.

[@daemn42]

@@ChimeraActual The wingtip vortices occur simply because the wing is displacing a volume of air downward continuously as it moves forward. The surrounding air has to fill in the space it previously occupied and there's a sharpish transition out at the end of the wings that creates a swirl of inrushing air. The greater the vertical displacement (caused by high wing loading + high angle of attack such as during takeoff/landing) the greater the vortices. As you fly faster the displacement is decreased. If you use a longer wing (distributing the displacement over a larger area) the displacement is decreased. One way to see this mechanism directly is to drag your hand, or a board or paddle through the surface of still water. It'll make a temporary "trench" in the water which will then spill in from the sides to fill it.

[@ChimeraActual]

@@daemn42 I wanted you to explain it to viewers, I'd mess it up.

[@grantfrith9589]

Are we over thinking this? I'm more of an intuitive type thinker and my sense of it has always been in terms of pressure differentials. A wing with angle of attack will create a low pressure on the appropriate side to create lift. A brick as many might mention can fly given enough thrust. The classic explanation of how an aerofoil works as criticised here makes sence to me from an economical perspective. Am I perceiving it correctly??

[@__-1234]

But why would it split in half ? Below Mach 1 i don't see why it should, the flow field is already influenced by the wing before it reaches it. I'm wondering what is the actual proportion, I guess it has been computed using CFD.

[@keithharrison1453]

As an ex-pilot (Rotary and Fixed Wing), I remain impressed to have observed our otherwise flat kids trampoline, now ex-trampoline, when a few decades ago, it elected to fly solo one fine windy day and without a pilot. It took off effectively vertically, cleared my wheel digger, then went a very long way up a steep hill, before arriving in a shitty heap, never to fly, or trampoline, ever again. However, the point I'm getting at, is that I can say for certain that it was quite happy flying both the right way up, and also inverted. Who'd have thought that was a thing.

[@eurekamoe3744]

You have the right idea. It's always been fairly simple to me, thrust overcomes drag (that is how a SpaceX rocket gets the Falcon 9 off the ground) and lift overcomes weight (that is how all wings get an airplane off the ground like an Airbus A380). High pressure air moves towards low pressure air (that is why there are wing tip vortices). That is why they have all the H's and L's on the weather maps. That's why when someone is smoking in a car, all you have to do is crack a window and the smoke gets sucked out of the inside of the car.

[@gerrys6265]

@@eurekamoe3744 Well, unfortunately, if someone is smoking in a car it is a little bit more difficult than that. I found not found it easy to get the smoker out through that window! I guess that is where drag comes in.

[@Andy-df5fj]

Lift is created by deflecting the air downwards which is a factor of the angle of attack as the wing moves through the air. The airfoil only optimizes the lift to drage ratio.

[@jpdemer5]

The airfoil also prevents stalling by greatly increasing the critical angle - a wing can't achieve optimum lift if it can't reach the optimum angle of attack. The F-104 stalled easily because the thin wings allowed separation of the flow across the top of the wing, even at small angles of attack. With a conventional airfoil, Newton rules, but Bernoulli does have something to contribute: at a 0° angle of attack, Bernoulli is the only thing keeping you airborne (and your aircraft had better be very light, or very fast.)

[@nerys71]

Lyft is created by throwing air downwards it's quite literally a mass thrower action reaction for every action there's an equal one opposite reaction throw 10 lb of air downward you get 10 lb of lift upward This is not in dispute people think it's indispute because they lack understanding The pressure differential around an air foiled wing creates this downward path of air can you simulate this with a flat plate? Yes you can what's the difference? Efficiency Will your flat plate generate lift? Yes it will it'll also do so incredibly inefficiently requiring a stupid amount of power on your part in order to effectively use that wing Air foil efficiency is all about getting the angle of attack as close to zero as possible because the closer I can get it to zero the less drag I'll produce while still producing lift and less drag I produce means I don't need as much thrust from my engine in order to maintain flight not requiring as much thrust means I can use a lighter engine using a lighter engine means I can use a lighter airframe which means I can use a lighter engine which allows me to use a lighter airframe see how that works? There's a point of diminishing returns but that's the basic concept The closer I can get to zero The less drag I produce less drag our produce less thrust I need less thrust I need less mass I need Air foils are about efficiency that's why we have so many different types of airflows for so many different types of applications because different applications have different requirements and a different shapers required to get the efficiency needed for the application Showing that a flat plate can generate lift does not make the theory wrong it just makes your understanding of it wrong because the flat plate working proves a theory it doesn't disprove it

[@Lozzie74]

Andy did you watch the video? What you have explained is the Newtonian component for lift generation, which I agree is a component of lift generation. However, he cleared up why the Bernoulli effect is also a component. You have just dismissed that.

[@rsteeb]

@@Lozzie74 Bernoulli only helps direct airflow downward. Newton accounts for 100% of the lift (and the drag).

[@jpdemer5]

@@rsteeb Wrong about everything. Well done!

[@praevasc4299]

It's easy to check: - do aircraft with symmetrical airfoil exist, and can they fly? Yes, they exist and yes they can fly. - can aircraft fly upside down? Yes, they can. If the shape of the wing was responsible for pulling the aircraft upwards, then an aircraft flying upside down would be pulled faster towards the ground than how it would be falling without wings. That's it, question settled. The typical shape of the wing is to increase performance. Aircraft are perfectly capable of flying with a completely flat wing, it would just be inefficient.

[@b.s.7693]

Shidd... So alot of text books are wrong 😮

[@__-1234]

@@b.s.7693 Actually less and less, I've seen the equal transit time story vanishing slowly from some. A few years ago it was in the IKO textbook, I remember heated debates with instructors, but then it vanished.

[@plektosgaming]

​@@b.s.7693 Almost all of them, in fact. The reason the shape is curved is to minimize turbulence and eddies/drag. Plus materials, as a flat surface has to be incredibly strong compared to something with internal bracing, as the transition from lift to being thrown backwards is rather abrupt. A teardrop shape is a good compromise. And the first wings were usually curved on the bottom as well. With stronger materials such as metal wings, the need to have the bottom half curved was reduced, saving weight and materials. Though many planes undersides are also very slightly curved as well. Again, to improve efficiency as air doesn't really like a perfectly flat surface, either.

[@plektosgaming]

@@b.s.7693 Wouldn't be the first time, won't be the last. My astronomy professor the first day of class would bring in a stack of 10 books. And then explain that THIS book was the correct one with all of the most up to date information, starting with ones from the 1600s. Then proceed until he was holding our textbook. The idea being that all of our research is a work in progress and most of it is eventually going to be proved wrong or too simple in its explanation.

[@Ettridge]

So happy to find a true human mind behind this series of informative videos. I'm so tired of coming across AI generated generalities. You teach the very things I want to know. How can I support you further. Do you have a Patreon account?

[@LetsGoAviate]

I don't have Patreon yet but thanks so much for the support!

[@SlowMonoxide]

I don't know if you actually need the first several minutes of the video to make this point, but your explanation of the way airflow divides at the stagnation point rather than necessarily the tip of the wing and how that is effected by angle of attack was perfect, that was an excellent clarification

[@PhilipFly11]

The simplest wing is just a flat plate. Some small model aircraft have a wing made of a slice of Balsa wood. The distance above and below the wing is the same. The lift is created by deflection of the air, which has mass and, therefore, the lift force is f=ma ie mass of the air x acceleration of it.

[@LetsGoAviate]

From what starting point are you measuring if you get the same distance over and below a plank wing with a positive angle of attack? Or are you saying the stagnation point isn't where airflow visualization shows it is?

[@bruceroland5683]

It is my belief that the model airplanes that can fly with a perfectly flat top and bottom (and with a blunt leading and trailing edge) i.e. no airfoil, are doing so strictly by angle of attack. The positive angle of attack is trimmed for level flight and when the plane is inverted, it simply requires a larger elevator deflection in the opposite direction to maintain level flight. I have seen this firsthand having flown models with this type of wing. I am sure that is it is extremely inefficient and that a full scale airplane would not be able to achieve this for host of reasons. My guess would be that drag would be the primary reason.

[@lenrichardson7349]

@@bruceroland5683 Part of the answer can be the turbalance over the top of the wing, a traditional wing shape can be created with a small amount of turbalance instead of the solid wing. Drag helps in this.

[@JulianDanzerHAL9001]

@@LetsGoAviate that visualisation is not of a flat plate but yes stagnation poitn can shift in a very thin cambered plate with airflow aligned with its leading edge its very close to its leading edge though despite it having almost not lenght difference nad producing plenty lift

[@sledawgpilot]

@@bruceroland5683look at the airflow over that flat wing with a positive AOA, it’s still similar to an airfoil wing

[@David-hm9ic]

Dr. Michael Selig's excellent research on "Airfoils at Low Reynolds Numbers" tested a huge variety of sophisticated and traditional airfoils in a wind tunnel experiment. The flat plate was the standard by which all other airfoils were evaluated. The flat plate was 90% as efficient as the best airfoils tested. That last 10% is where the highly refined airfoils make a difference in Lift/Drag, fuel economy, high speeds, heavy lifting capability and all of the differences we perceive as being the result of the airfoil that was chosen for an application. @9:01 - The wing is not at zero degrees AoA assuming a horizontal airflow. The wing is actually at +5° or so (calibrated eyeball ;-) because the measurement should be taken on a line projected through the red dot and through the trailing edge.

[@7up-weee]

'Path length' creating faster flow above the wing and therefore lower pressure above the wing than below, is still wrong. Just because some people have come up with the wrong arguments for why it is wrong doesn't make it right. As one of your videos correctly illustrated, air flowing over the top of the wing arrives at the trailing edge 'before' air flow under the wing. This can be seen in wind tunnel experiments with dyes and aerofoils. There is no physical requirement for the divided air molecules / parcels / whatever to rejoin at the same time at the trailing edge. Path length therefore doesn't explain the velocity difference above and below the wing. There is no physical basis for appealing to path length to explain the lift force. I think the confusion in trying to intuit lift forces arises as we try to assign cause and effect at one instance in time to a flow. In reality, the pressure field affects the velocity field (or flow field) and the flow field affects the pressure field over a continuum in time and space which is why lift force is so difficult to intuit from a static picture. Bernoulli's equation only relates pressure and velocity, it doesn't attribute cause and effect. It would be fantastic to have a really intuitive way of picturing the explanation for the lift force on an aerofoil but I've yet to see one. There's a reason why there are so many incorrect attempts to wrap it up in a simple way. You still need a lot of heavy maths to calculate the numbers required. And when people say it's 'simply' Newtons Laws, fluid dynamics/mechanics embodies newtonian mechanics, it doesn't break it. In first year University Physics we derived Bernoulli's equation from Newton's laws. We use concepts such as pressure instead of force because it makes the maths more convenient and you can easily measure it in a flow.

[@dougball328]

So then tell us where the mass goes that does not make it to the trailing edge? Place a vertical plane right at the leading edge and one at the trailing edge. The same amount of mass must pass through those planes at the same rate. Otherwise you are stacking it up somewhere (and that does NOT happen) So you see, there IS a physical basis for it. Confusion comes from all the armchair aerodynamicists who never actually studied the subject. I did, for five years and two degrees.

[@7up-weee]

@@dougball328you must have seen wind tunnel experiments with dye injected into the flow? Conservation of mass is not broken just because the flow is separated at the leading edge and air is accelerated over the top surface. It might help to search for videos of wind tunnel dye experiments over aerofoils to visualise it.

[@7up-weee]

@@dougball328 Airflow across a wing - Cambridge University - Wind tunnel visualisation.

[@jmevb60]

I've read that the mass and velocity of the air shoved downwards provides much of the explanation for lift

[@7up-weee]

@@jmevb60 So that's the conservation of momentum (mv) but you also have to conserve energy which is essentially what Bernoulli equation conserves. Along with conserving mass, all three give you the Euler equations and you have to consider all of them together. If you want to consider viscosity as well, you get the complete picture which are the Navier-Stokes equations. But the Euler equations are a pretty decent approximation for subsonic flight and idealised aerofoils. They are all linked which is why it's hard to intuit. The path length and equal transit time are not physics though - which is why this video doesn't really have a point. Just because the upside down plane example isn't a good argument against path length, doesn't make path length and transit time true. There are several arguments against flat earth theory that aren't very good and you could ague against - but the Earth still isn't flat - for some other very good reasons!

[@les8489]

Video at 7:13: this "large angle of attack" for inverted wing is only due to the fact that we measure the angle of attack between the direction of far-field flow and the GEOMETRIC chord (which is convenient). The "actual" angle of attack can be measured between a ZERO-LIFT line, which for Clark-Y is about 5 degrees up relative to the geometric chord. So - when we align the geometric chord with the flow direction - the Clark-Y profile actually works at an effective angle of attack of 5 degrees. Inverting the wing results in flipping the ZERO-LIFT line by 5 degrees DOWN - that's why the apparent angle of attack is higher. Same applies to any profile with a camber...

[@jamestucker1126]

You make many sound points. However, the oldest, most difficult, but most accurate explation of lift in a subsonic wing is the circulation theory of lift. There are reasonably simple heuristic explanations of this theory, which only require a middle high school level of mathematics. But the full explanation requires some pretty sophistocated math relating to the Kutta-Joukowski theorem. A full explation of this requires an understanding of line integrals, as well as vector and complex analysis. This is the stuff of a second year university engineering course. I have an entire textbook on it. The full story is quite a jump up in complexity and, therefore, not usually taught to pilots and people who are casually interested in all things flying. I can supply references if you're interested. Fun fact, the full explanation of 'how a wing flies' can be formulated with a series of equations that have no explicit or closed form solution. At least, not without making some pretty limiting and factually incorrect simplifying assumptions. Therefore, the only way 'solve' these equations is through computer aided simulations. But these simulations are still only approximations. Therefore, no one can say, for sure, how a specific wing will work till you fix in onto an aeroplane, stick a test pilot in the plane, and fly it! Even windtunnel testing is not quite the same as'the real thing'. Still want to be a test pilot?

[@peteohead]

As a test pilot, with a MEng degree in Aero Engineering, this comment is the one that I agree with the most. 👌🏻

[@davetime5234]

But isn't Kutta-Joukowski a subset of Navier-Stokes along with Bernoulli, the Coanda effect etc. etc.? And the best numerical simulations of the Navier-Stokes equations approach the results of real physical testing of the forces and flow across airfoils? Therefore, while avoiding detailed case solutions of the Navier-Stokes equations, can we not still look at the fundamental physical laws encoded in the Navier-Stokes relationships for a disciplining guidance on how to better basically describe the nature of lift? Navier-Stokes: conservation of mass flow rate, conservation of energy and conservation of momentum (simultaneous partial differential equations connecting the interrelationships of these fundamental laws) The path length obstacle imposed (by the combined effects of camber and angle of attack) create a lateral pressure difference consistent with conservation of energy as demanded by continuity of mass flow rate. The gradient and the accelerated speed of mass around the imposed contour go hand in hand. This lateral pressure gradient maintaining flow rate consistent with energy conservation, changes the vertical momentum of adjacent air, which creates the vertical momentum change consistent with Newton's second law, which is the force of lift? I guess I am disputing your statement: "But the full explanation requires some pretty sophisticated math relating to the Kutta-Joukowski theorem," in the sense that we can better provide a basic explanation of the drivers of lift, even though an actual wing design requires much computational fire power, and real-world testing. While Kutta-Joukowski is required for a more accurate mathematical representation of a particular case, a simpler yet more full basic explanation only requires that we describe the process logically in terms of the fundamental laws of Navier-Stokes. And for some reason we historically fail to do that. Example: equal transit time is used incorrectly as a shorthand for mass flow continuity. Bernoulli is somehow stated to be equivalent to Newton's second law applied to the vertical change in momentum, even though the implied conservation of energy and conservation of momentum consequences must both be integrated into our more reliable simpler explanation, because they are integrated in the most reliable theoretical explanation, Navier-Stokes, with that being the most comprehensive description of the physical reality.

[@ruandurand3971]

As an Aeronutical Engineer this is the answer I was looking for. There is a reason it is called lifting "theorems" and not lifting "laws".

[@CristiNeagu]

Explaining how a wing generates lift and solving the equations of lift for that wing are two very different things. The fact that you seem to confuse these two things is somewhat concerning.

[@fjohnson9749]

Thank all of you for your statements. From someone who loves aero-D but went to work on the flight deck. The fact that a flat or symmetrical surface will only create lift in the direction of the angle of attack always dispelled both the camber/deflection theories in my thoughts. 👍🏼

[@deezynar]

The stagnation point moving far below and behind the leading edge demonstrates the strong differential in pressure between the areas above and below the wing. Air moving toward the leading edge will get sucked over the top to fill the low pressure zone if it arrives at an area of the wing where the air pressure differential is strong enough to pull it upwards.

[@LetsGoAviate]

You put it more eloquently than I do

[@perh8258]

"sucked" how?

[@deezynar]

@@perh8258 Fluids move from higher pressure areas to lower pressure areas.

[@perh8258]

maybe 'pushed' is more accurate?

[@mikester1290]

That was how I was taught it, by a book mind, the book only showed the classic aerofoil and explained that the air moving the further distance was basically being "stretched" (same air, more distance) thus creating a vacuum and therefore lift. Now adding in the new knowledge of the splitting point of the air hitting the aerofoil it makes a lot more sense, although I've been told that is NOT how it works. I'm not saying I know now but it's interesting.

[@garyradtke3252]

I graduated high school in 1974 and we where taught that the air going over the top of the wing with the angle of attack caused a more negative pressure and lifted the wing and not the pressure under the wing pushing it up. That never sounded right to me so the way I started looking at it (write or wrong) is the combination of the angle of attack, the forward motion of the wing, and the speed at which it is moving all combine to create a pressure differential between the top and bottom of the wing. I am not a scientist, mathematician, a physics or aeronautical engineer but I do observe things and when I can't make sense out of what someone is telling me I have to question it. Come to find out it seems they where teaching theory as fact back in those days too. I may not be anywhere close to right but it's what I see in my minds eye. What I do know is someone knows something because the sky is full of airplanes.

[@jsbrads1]

If a wing has 3 degrees of Camber (curve) and can fly level, it would be able to fly upside down with 6 degrees angle of attack when upside down.

[@XPLAlN]

⁠​⁠@@jsbrads1…camber is measured as percentage of chord, not degrees. Perhaps you mean if a cambered wing enables level flight at a given angle of attack and airspeed, it will require approximately double the angle of attack to fly level when inverted at the same speed.

[@XPLAlN]

….airspeed and angle of attack (as a proxy for coefficient of lift) are the only two variables within the general lift equation that are directly controlled by the pilot hence the consideration of lift in those terms makes a lot of sense. Then, the understanding of why a given angle of attack results in a given coefficient of lift, whilst required knowledge for the aeronautical engineer, is of academic interest to the pilot, except perhaps when it comes to the stall.

[@EntityWar]

As I recall on a classic aerofoil 20% of the lift comes from high pressure below the wing and 80% from low pressure above the wing

[@GaborSzabo747]

@@EntityWar That's how I learned too. So in this case the airplane rides on lifting force is a misconception, actually the airplane hangs in the air.

[@MarkStafford9]

"Let's Put This to Bed". Not likely, even though your reasoning is clear, well presented and factual without loosing the dominantly non-math audience (truly an accomplishment!). I hope stirring the hornet's nest has been more productive, even though we live in a time of unusually strong affiliation bonds. It is culturally more attractive at the moment to bond with fellow idiots than to challenge one's own cherished beliefs. I'm not picking on idiots, because we are all relative idiots compared to the incomprehensible volume of understanding possible. It is like we are still biologically limited cavemen, yet we are trying to understand the whole of reality.

[@jasone3166]

Amen brother!

[@dbell1016]

Thanks!

[@LetsGoAviate]

Thank you!

[@johanhelberglongbowsknifes1185]

Hi buddy something not many can explain to me why do designers of light aircraft and some Bush planes put Ailerons lower on wing and not strait. Is there a magic there like let the plane land like floating on landing like flaps is doing.

[@cabanford]

It's got almost nothing to do with the "longer path". Thought that this old chestnut 🌰 had been put to bed a long time ago.

[@bashkillszombies]

Argument from incredulity.

[@nerys71]

If you want to see a perfect example of why the pressure differential is in fact how you get lift you can actually do this with asymmetrical shape just find yourself a Bic pen The kind where you can remove the actual pen from the inside and the tail cap and end up with a simple plastic tube that's even along the whole length now place that tube onto a desk or countertop aimed for the edge of the countertop place your fingers on top of the tube and press down hard flicking the tube out from under your fingers and once you get the hang of it you can make that pen fly across the room and it does this generating lift in fact you can generate so much lift that you can actually make the pen do a loop The loop This is called the Magnus force this is how curveballs work and how soccer players are able to make the balls fly a curve path With the backwards spin the air going under the pen is slowed down by the drag against the pen body while the air on top of the pen body is accelerated again because of the drag against the pen body effectively giving you a very inefficient wing the path over the top is longer and the path over the bottom is shorter in time length relationship because of the difference in drag from the rotating surface Butt but it's the deflection of air that creates the lift except that deflection of air happens because of the pressure differential :-) That's what gives you the deflection :-) That's why a wing that is perfectly level produces lift Do you get more lift when you angle the wing which will deflect more air? Absolutely you also reduce efficiency because as you angle that wing you are dramatically increasing drag which means you now need to exert more work from your engine in order to fly the aircraft The closer you can get that wing to no angle of attack while still producing Lyft the more efficient your flight will be this is why we design airfoils :-) to increase efficiency reducing drag and reducing how much power we need for flight this is why an acrobatic airplane that's designed to fly upside down inverted etc as a symmetrical airfoil it's not as efficient as a highly cambered airfoil but it's more efficient at more angles of attack than a highly cambered air foil which is only really efficient in one configuration so you're giving up ultimate efficiency for a broader range of so-so efficiency. It all comes down to efficiency to reducing how much power you need in order to generate the necessarily lift for sustained flight.

[@cabanford]

@@nerys71 Nice effort explaining way better than my weak attempts. Thanks 👍

[@sasjadevries]

@@nerys71 Well, that still doesn't explain why a NACA 6 series is more efficient than a NACA 4 digit equivalent 😆. I do kinda agree with your explanation though, but still scientists say that the downwash is a result of lift, and not its cause. While it's still being the pressure difference that's creating the lift, because the pressure difference is the actual force acting on the wing. And the pressures are pushing and pulling, deflecting the airstream. 😆It's quite a rabbit hole to get into.

[@hoytoy100]

It has been. The rise of disinformation is an attempt to destabilize the west, like moon landing deniers and flat earthers.

[@ericlarue8010]

It's impossible to accelerate air downward without getting an upward force. And impossible to get an upward force without accelerating air downward. A pressure differential doesn't cause lift ,because a pressure differential IS lift, caused by accelerated air.

[@Talon19]

Wings can still produce lift even with no downward movement of air.

[@NAMCBEO]

And if they do not believe this, pick up a piece of plywood in a forty MPH wind and see what happens !!!!

[@randomxnp]

You just contradicted yourself. The pressure differential causes lift because the pressure differential is lift. The pressure differential causes the airflow changes: a gas can only transfer force by pressure differential.

[@wbeaty]

Bingo, that's it! But it's really too bad that wings fly 100% by downwash. If only they would fly by pure pressure-difference alone, without having to press downwards against the Earth. Then we could make flying saucers! Just put your magic aircraft inside a disk-shaped empty box. (Perhaps punch a few holes, to let the pressure-diff escape.) When you fly the aircraft, the pressure-diff on the wing surfaces can also lift the hollow box. Next, use a very sturdy box, and seal it up. Then the pressure-force will still work, even when the hollow box flies up into outer space! Bernoulli without Newton would be pure magic, ...if it existed.

[@kennethferland5579]

People need to remember that Pressure is not force, it is force per unit area. And every point on a solid object is experiencing its own vector of force, the sum of ALL the forces then gives us the force on the object.

[@pacresfrancis1565]

Thank you, I learned about stagnation point. I also didn't know how planes could fly upside down, but this video really cleared things up

[@dougball328]

Swept wings don't have stagnation points or lines. They have attachment lines. The flow never stops (like the author discusses) but it attaches to the leading edge and flows outward. You can think of the airplane velocity as having a component perpendicular to the wing leading edge and one parallel to it. Look up simple sweep theory for more details.

[@pacresfrancis1565]

@@dougball328 thanks for more info, do you recommend a video that goes in-depth for that? I'm interested on airfoils because im designing a wind turbine blade for my school project👩‍🏫

[@dougball328]

@@pacresfrancis1565 It's not that simple that a You Tube video is going to show you how. And if you can get to You Tube, you know how to use Google, so try googling wind turbine design. You will find there is a variety of ways to design a wind turbine. You should study this presentation: www.nrel.gov/wind/assets/pdfs/systems-engineering-workshop-2019-wind-turbine-design.pdf Good luck.

[@keesvandenbroek331]

Good explanation of the stagnation point. This is also the principle of a stallwarning vane: During normal flight, the vane is pushed down, because the stagnation point of the flow lies above the vane and the flow pushing it down. When approaching critical angle of attack, the stagnation point moves below the vane, the flow pushing it up and triggering a warning. One point of critique though: even when flying with a wing past the critical angle of attack there is a pressure differential over the wing. This means the wing is still producing lift (and enough to conquer the, component of the, weight), albeit with a very high drag. Which most likely will prevent horizontal flight

[@boosterhuiz2779]

Thanks, be it engines or wings, you explain them so well. Only 2 vids in and I am hooked

[@ronboe6325]

MIT covered this a while back in their series on flight (aimed at folks wanting to get their pilot certificate). Basically path lengths over wings is not involved in lift; but momentum transfer as the wing hits all the little air molecules does, at least at speeds below transonic speeds, then shock waves start to take over. Reviewing the MIT video will do a better job of clearing things up.

[@paradoxworkshop4659]

Right, but easier to state in different terms...​@@anthonyb5279

[@Talon19]

Still no. Wings can generate lift with no change of momentum of the air.

[@ronboe6325]

@@Talon19 Interesting. Please explain how that would work.

[@Talon19]

@@ronboe6325 Positive-camber, flat-bottom, zero AoA wings produce lift even with long thin plates extending behind the trailing edge. No vertical change of airflow after the trailing edge of the camber.

[@ronboe6325]

@@Talon19 Well this was a rabbit hole. Graphs show the Clark Y having a lift coefficient of about 0.3 at zero AoA (likely OK for model airplanes but not people carrying craft - even Piper Cubs seem to carry a positive AoA). Further looking lead to Kutta -Jaukowski theorem and Wiessiing's Approximation for modeling wings and airflow - they tend to treat momentum transfer only for pressure - which is critical - but you run into fluid dynamics and other ugly factors that become important at speed and if you have to pay for the gas to fly your craft. Good ol' Bernoulli's is not mentioned.

[@paulhelman2376]

Flat section models fly fine with just a few degrees of incidence. Baby rog's for example were quite popular in 20's and 30's.

[@ColinWatters]

And chambered wings produce some lift even at ZERO degrees angle of attack.

[@tekelili1]

If RC models have a lower wing loading, it is not because they are made of light materials (although it can be a factor). But it is mainly because, for a given shape, the volume is related to the cube (³) of the length and the surface in related to the square (²) . I.e. if you divide by 2 the length of a plane keeping the same overall shape, its wing area will be divided by 4 and its weight will be divided by 8 ! So it will have a lower wing loading.

[@tonywright8294]

An rc plane is still a full size aircraft just smaller .

[@tekelili1]

@@tonywright8294 in terms of weight, size matters !!!

[@AllanTheBanjo]

@tonywright8294 but the dimensions don't all scale the same way. If you double the length of a model you square its area and cube its volume. Identical shape models of different sizes have vastly different characteristics.

[@gerardpenman6615]

Yes, the actual density is not that different. Compare hollow aluminum frames with solid balsa or other woods and it is not that much of a difference. The scale has much more of an effect. Look at the Mosquito compared to others of it's time.

[@deaftodd]

Very well said. That's why supersonic wings profile looked almost upside down because of the stagnation point to cover all ranges from sub, trans and super sonic. If it goes fast enough, it doesn't need a profiled wings anymore, it ends up like a missle fins. B-52 has a very THICK profile wings almost like a tear drop to handle extra heavy loads and they have no needs to fly upside down. Thet's why they have 8 engines to overcome that drag. Awesome power.

[@stevemarks9360]

It's simple, the aerofoil shape creates lift, the angle of attack also generates lift.

[@daszieher]

The theory of different-length paths and resulting pressure differential has been disproven a long time ago. Lift is created by accelerating air around the wing downwards (and slightly forwards with respect to the wing, i.e. drag). The different pressures observable are a side effect of said acceleration, but not the root cause of lift.

[@JulianDanzerHAL9001]

there's a pretty in depth aerodynamics explanation of this on youtube, once oyu look at it as a linear addition of voritces it gets really fascianting but yeah, equal transit is just some cleverish sounding nosnense someone came up with and set back sceince education by centuries with

[@NQR-9000]

I agree.The fact is that the "different lenght" theory has just a lesser explanatory value than the "flux deviation by the profile" one. For example, if the "different length" theory is true, how to explain how "thickless" wings fly (wings like the one of the pre WWI planes or the hang gliders), which are basically of the same profile on both side...

[@buppy453ds]

Well both are causes for lift however, the pressure differential is the primary cause. At least that is what the PHAK, AIM, and AFH say.

[@JulianDanzerHAL9001]

@@buppy453ds the pressure differential caused by what precisely? a change in speed or a changei n velocity? cause that is an important difference velocity includes direction change in velocity is absically redirecitng air change in speed would imply that bernoulli comes first but hen I'd wonder what causes that since equal transit is completely and utterly nonsensical, its about as well debunked as flat earth

[@buppy453ds]

@JulianDanzerHAL9001 Velocity. I am interested in being disproven, though. The facts I have are from the FAA. Do you have any sources that I could review?

[@michaelpettett3087]

As an aerobatic rated pilot, all statements in this video are valid/true. The level of English is better than required to be a pilot. Some so called pilots who do videos have atrocious English skills.

[@eurekamoe3744]

You have an aerobatic rating on your pilot's license?

[@skyboy1956]

@@eurekamoe3744 yes, it's printed upside down

[@eurekamoe3744]

@@skyboy1956 OK yes. So your so called "aerobatic rated pilot" is total BS.

[@Ken-fw9dh]

Ask ten people if they know how an aeroplane remains in the air and you won't get two right answers. I'm a pilot and I have tried

[@alfredwong1489]

Honest question: if you have a highly cambered wing with super sharp leading and trailing edges (so that the stagnation point will always be on the chord line), if the chord line is parallel to the relative airflow, will there be lift?

[@crispinmiller7989]

With a 2-D foil of infinite span (so there's no induced drag from air escaping upward past the wingtips) I think the answer is yes, but otherwise you'll need some positive AoA to pay for the need to fling air downward more sharply than you received it coming up toward you. To explain that "coming up toward you" (ref. provided below) -- in any subsonic flow, the pressure under the wing is "felt" by the air both ahead and behind, so that ahead of the wing the air begins to rise up to escape that pressure region carried along under the wing -- then the wing gets hold of it and flings it back down again, and it's this change in direction that creates the "bubble" of pressure below the wing. (Then after the wing has gone past, if it's 2-D so there's no escaped air, the flow aft of the wing mirrors the flow ahead of it, with the downward-flung air being deflected back to up a level direction by the pressure below it. OTOH if it's got a finite span so that it's had to fling extra air downward to make up for the escaped air at the sides, then the pressure below will be insufficient to flatten out the path of the air behind, and it will continue in a somewhat downward direction as "downwash.") For a more detailed description see Prandtl & Tietjens, /Applied Hydro- and Aeromechanics/, chapter VI: B: topic 101, The Velocity Field in the Vicinity of the Airfoil.

[@soniakolasinska3850]

This is amazing explanation for someone like me. I'm a PLL student and I'm just starting to learn all this. Your explanation is extremely good. Thank you ❤

[@procurion8934]

Just a small point...Wings are designed and attached at angles relative the the expected attitude that the aircraft will fly. That is to say the example of an aircraft flying upside needing to rotate the fuselage dramatically is because the wings are attached with a positive angle when right-side-up. When upside down, the pilot must pitch the fuselage at a steeper angle because the is "negative" attack that must be compensated for. Roughly 2/3 of the lift comes from from the lower air pressure on the top of the wing, 1/3 from the bottom.

[@michaelm7299]

"Roughly 2/3 of the lift comes from...on the top of the wing, 1/3 from the bottom.".... This is an interpretation of the definition of 'lift', and variable to the angle of attack. If an airfoil is ideally shaped for a given airspeed, and has a zero AoA, then 100% of the lift is from the topside lower pressure. In real-world configurations, and as the AoA is increased to exploit dynamic pressures of relative wind force (by airspeed), the lift - meaning the total force supporting the plane in flight - becomes much more dependent on the wings' (and fuselage) undersides

[@stuntmanmike37]

This is a futile argument over semantics. Lift is created by Newtonian mechanics. Shove the air down, it pushes the airplane up. That's it. However you do it, it's lift either way. Curved wings, flat wings, propellers or rocket engines; they lift by shoving air down.

[@WINCHANDLE]

Nope. not like a windmill. There's more to it than just a deflection of air flow.

[@ColinMill1]

Forget about the various "arm-waving" qualitative explanations of lift. Just get a copy of Glauert's " Elements of Aerofoil and Airscrew Theory" and read it. It was written in 1926 and proves that the origin of lift and a quantitative understanding for flat plates, cambered plates, aerofoils and wings of finite span were well developed nearly 100 years ago. Just follow the maths.

[@ColinMill1]

@@davetime5234 There is a PDF of the book available on the web so you can take a look for yourself.

[@ColinMill1]

@@davetime5234 Well, I think the starting point is that lift is a result of the interaction of the free-stream flow with the bound circulation associated with the object (aerofoil). Glauert develops this in a methodical fashion and uses the analytical solution for the stream function in invicid flow around a cylinder to produce an analytical result for a Joukowsky areofoil using conformal mapping. By applying the Kutta condition to eliminate the singularity that would exist in the flow at the trailing edge he produces a result for the circulation around the body that can and has been shown to be in excellent agreement with experiment. While the conformal mapping approach is not applicable to aerofoils more generally this approach provides a very solid basis for our understanding of lift more generally. It provides an excellent starting point for the understanding of lift for wings of finite span from which accurate predictions for induced drag etc can be made. Personally, I have been working with this for well over 50 years and I'm quite happy that the maths of this approach provides a sold foundation.

[@ColinMill1]

@@davetime5234 Well, I wrote a lengthy reply to this which has just disappeared so, if you didn't see it before it got deleted, I guess you are stuck with the book.

[@ColinMill1]

@@davetime5234 Many thanks for the reply. I used to always edit and save long replies in a word processor and save them because this used to happen a lot. I need to get back into the habit as it seems to be happening a lot again these days.

[@evantspurrell]

i think it has more to do with the wing pushing the air down creating lift. the round shape of the wing changes the vectors of the air passing over the wing redirecting the air downwards. changes at higher speeds though right?

[@MathIndy]

The truth is that most of the common explanations for how lift is created are correct. A good aeronautical engineer knows all of the explanations, when to use them and when not to. For some reason people get married to the idea that only one explanation is correct.

[@Talon19]

What are the “common explanations of lift”? The air turning, deflection, momentum change, and Newtonian explanation of lift are all false because wings can generate lift without any deflection of air.

[@donlyons6556]

More pressure under wing than above creates lift. That simple. Angle of attack, airspeed and wing shape all contribute.

[@ninjalectualx]

Sorry but what your school taught you is wrong

[@ronaldlindeman6136]

@@ninjalectualx Well, then, what is right? (true)

[@jodeldk]

There is also newtons law effecting it, the air defected downwards by the wing angle of attack also gives lift

[@tedmoss]

But it is minuscule.

[@godfreypoon5148]

@@tedmoss So you can pull/push on the air with your wing... and it doesn't move??

[@davetime5234]

@@tedmoss The newton's 2nd law part is not small. The change in vertical momentum of the air (the air turned downwards) must create a force equal to that of the pressure difference between top and bottom of the wing. Though we have to be careful of what we mean by the word "deflection." So the motion of air moving down is as substantial as the weight of the aircraft.

[@Alec72HD]

​@@tedmoss No, you CANNOT generate ANY lift without transferring downward momentum to air. IF you could do that, that would violate Newton's 3rd law. IF, IF that was possible, you could theoretically create a propulsion device for use in vacuum. BUT WE CANNOT. In other words, lift is 100% equal to mV/t. No more, no less.

[@keithjurena9319]

Flow on the bottom doesn't change at stall. Flow radically changes on the top of the wing during stall. This puts a big hole in the momentum hypothesis and solidifies the idea planes are sucked into the air

[@RB-bd5tz]

There is no such thing as suction. There is only differential pressure. Objects are pushed (blown) from areas of high pressure (in this case, below the wing) to areas of low pressure (above the wing). Even a vacuum cleaner operates on "push": The fan moves air, which creates low pressure in the vacuum and differential pressure between the vacuum interior and the outside atmosphere, and objects are pushed into the hose by the atmospheric pressure.

[@MegaDeano1963]

@@RB-bd5tz you are correct, I blame poor terminology , suction should never be referred to as a force ( and on a personal peeve neither should gravity be referred to as a force but after a hundred years what you gonna do )

[@demondoggy1825]

A stall doesn't actually remove all lift, it's a reduction in lift that changes as the stall progresses. The momentum transfer argument never actually says the pressure lift doesn't happen, it says it's not the majority. So boundry separation along the top can both cause a stall and not account for the majority of the lift.

[@MegaDeano1963]

@@demondoggy1825 I've seen wind tunnel wing modelling that shows that the increase in pressure over a wing during a stall, can reduce the lift of a wing to 30% of that it had when it had a heathy flow over the wing ( constant fluid velocity ) . Its wild the efficiency you get from the fluid over the wing with a good design

[@flybobbie1449]

At the stall air flow curls under the wing off the trailing edge, can be seen in wind tunnels..

[@SVAyouTube]

if possible, please answer: is there centrifugal force acting on the air when air moves along the surface of the wing, or is the centrifugal force insignificant? I want to understand why the air flow comes off the upper surface of the wing. In addition to the frictional force that slows down the flow, what causes it to break away? Thank you

[@davetime5234]

Isn't it just that the airflow has an upper limit on how "tight a turn" it can make successfully, depending on the properties of the air flow. If the turn is too tight for the conditions, turbulence and separation occurs.

[@crispinmiller7989]

The centrifugal/centripetal chestnut is one I won't try to sort out here, but what I will say is that any time you have curved streamlines, the reason they're curving is that there's less pressure on the inside of the curve than there is on the outside, and it's that difference which is causing the air to curve. This means that if you have air shooting up past the leading edge and ripping around the shoulder of the upper surface and then continuing to follow that surface to the trailing edge, then the fast-moving, sharply-curving air over the shoulder of the wing can only be moving like that if the pressure inside its path is sharply reduced. This is the reason for the upper side of the wing to have a sharper difference from atmospheric than the underside does, where the flow doesn't bend near as much. This flow pattern breaks down if the wing's flow "stalls," which means that it fails to stay attached to the upper surface and jumps off before it reaches the trailing edge. This can happen if you tilt the wing too much, asking for more lift than it can provide. In doing this -- tilting the wing too sharply -- part of what you're doing is sabotaging the streamlined shape of the wing. In the flow of a fluid around any solid object, the "boundary layer" (the nearby fluid, slowed down by viscous drag against the object) will have a tendency to slow down on its way to the back end -- and if it stops completely, then the flow that comes afterward is blocked from reaching the back, and has to jump off ("separate"). The reason for a gradual taper on the rear part of a wing, or of anything else streamlined, is to hold the boundary layer of flow around the object "out in the breeze" so it's exposed to viscous drag from the surrounding flow to help keep it moving. (Ref: a very approachable little book by MIT fluids prof. Ascher Shapiro, /Shape and Flow/.) (By contrast, flow crosswise around a cylinder routinely separates, because the back side of the cylinder is sheltered from the surrounding flow so there's no nearby flow to help drag it along.) So then if you tip the wing up too sharply, the rear portion of the upper side is not being held out in the breeze so well any more, and the flow over it is more likely to stagnate before it reaches the trailing edge, and if it does, then this forces the flow to separate and then you don't have lifting flow any more.

[@SVAyouTube]

@@crispinmiller7989 thanks a lot for such great answer🤝

[@davetime5234]

@@crispinmiller7989 In saying the following: "if you have air shooting up past the leading edge and ripping around the shoulder of the upper surface and then continuing to follow that surface to the trailing edge, then the fast-moving, sharply-curving air over the shoulder of the wing can only be moving like that if the pressure inside its path is sharply reduced. This is the reason for the upper side of the wing to have a sharper difference from atmospheric than the underside does, where the flow doesn't bend near as much." Are you saying that the pressure is reduced because the pressure is reduced?: sharply curving air--> because the pressure is sharply reduced reason for the ... sharper difference from atmospheric --> ...the underside...doesn't bend near as much In terms of flow separation (correct me if I'm wrong), you're saying mixing "untainted" higher energy flow to energize the boundary layer, is the cure (in terms of the shape design) to delay the onset? But in terms of the pressure drop associated with the curved flow lines, do you believe that prerequisite for lift generation absolutely requires consideration of: the boundary layer and/or viscosity, and/or the kuta condition? (I realize that's a bit of a broad question, caused perhaps by watching, and trying to synthesize, too many KZbin professors).

[@crispinmiller7989]

@@davetime5234 First of all, sorry if any of this repeats stuff in the previous post you were commenting on. I wasn't seeing that I could just scroll up and see how much I'd said already. “Are you saying that the pressure is reduced because the pressure is reduced?”: Well, I’m saying *what* the flow pattern is, and what the pressure distribution is in such a flow pattern, and that it has to be that way in order for the flow acceleration (the curving of the streamlines) to obey F=ma - but whether that’s saying “why,” or “because” anything, is a bit of a chestnut. We often say “because” when what we really mean is “or else it would violate principles we believe to apply,” i.e. that it needs to be that way to be *consistent with* the rest of our knowledge. But whether that’s really *why* it happens is a philosopher’s question. (You could search KZbin for Feynman talking about “why?” and you get to see him roasting an interviewer for asking that.) It's true you can get into a chicken-and-egg runaround with this, but I wasn’t meaning to give a circular argument of lowered pressure caused by the flow chasing the low pressure. In other scenarios (for instance, sticking a vacuum-cleaner wand into a crossflow) you *can* use low pressure to make streamlines curve -- but in the airplane-wing case, a better answer for me to have written would be that the pressure is reduced just because the air *is* following a curved surface, so then by F=ma the streamlines at the inside of the curve have to have lower pressure than the atmosphere outside those curved streamlines. A more interesting “why” question, if you want to ask one, is just “why *does* the air follow the curved surface like that, instead of jumping off?” - and to that, my “because” answers, i.e., my remarks on what other principles it’s consistent with, are that *when* it follows the surface (whereas, in a stall, it doesn’t any more), it’s doing it “because” - given (1) its momentum, (2) the surface beside it, and (3) the atmospheric pressure pressing it against that surface - there’s no easier place for it to go. Even though the pressure at the surface is below atmospheric, it’s still well above zero (a vacuum) - so the air is clamped against the surface and will follow it, unless something gets in its way. [this next chunk of the answer -- down to "do you believe that prerequisite for lift generation... requires consideration of: the boundary layer and/or viscosity, and/or the kutta condition?" -- is an amplified repeat of stuff I said last time] That situation applies *as long as the air is able to keep moving along the surface.* If something blocks it, then it will have to jump off, and then you have a stall instead, and you don’t have much lift any more. In a typical stall this is because the air passing over the wing comes to a stop before it reaches the trailing edge (I’ll say “why” in a minute), so it sits there like a proverbial bump on a log, blocking the passage of any air that’s following it, so the subsequent flow has to jump off. Some wings are also equipped to *force* a stall, by using little crosswise fins that can be popped up out of the wing, called “spoilers” - to descend quickly, or to help stay on the ground, in the case of lightweight and delicate sailplanes, recently landed and not tied down yet, that don’t want to get lifted off the ground if a wind comes up. So in a stall that’s not produced by spoilers, what’s happened? What’s happened is that in the “boundary layer” - the region of flow that’s close to the solid surface, so it’s affected by drag along the surface - my condition #(1), momentum, has died before the flow has reached the trailing edge. “Why”? - well, with any shape with bulging sides immersed in a surrounding flow, the pressure around the bulges of it will be lower than atmospheric but then, from there on back to wherever the flow stops and jumps off, the pressure will progressively come back up to atmospheric pressure (or else it wouldn’t be able to flow away downstream). This pressure increase along the rear portion of the surface is referred to as an “adverse” pressure gradient because it opposes the flow. In the boundary layer, this means the flow over this area has two factors opposing it - it’s always got the drag from being near the solid surface, and now it’s got the adverse pressure gradient too. In shapes with a blunt rear portion, this double threat slows the flow completely to a stop (“stagnation”) somewhere before it reaches the trailing end, and then the flow has to jump off without reaching all the way to the back. In streamlined shapes, including wings, the rear portion is tapered gradually in order to keep the boundary layer exposed not only to the solid surface but also to the influence of the surrounding air stream, tending to drag it along until it reaches the trailing end after all. BUT if you tilt the wing up too sharply, asking too much lift from it, then you can end up tucking the rear part of the upper surface down into the wind-shadow of the forward part of the wing, so it’s no longer being helped along by the surrounding air - you’ve oriented it so it’s no longer streamlined. In this state of affairs the adverse pressure gradient combined with the surface drag can bring the boundary layer to a stop somewhere before the trailing edge, so the air on the rear part becomes stagnant, the flow that comes after that has to jump off, and you no longer have the nicely-curved flow pattern that was giving you the lift. This sabotaged flow pattern is what’s called the stall. “In terms of flow separation (correct me if I'm wrong), you're saying mixing "untainted" higher energy flow to energize the boundary layer, is the cure (in terms of the shape design) to delay the onset?” - right, as I seem to have repeated or paraphrased just now. “But in terms of the pressure drop associated with the curved flow lines, do you believe that prerequisite for lift generation absolutely requires consideration of: the boundary layer and/or viscosity, and/or the kutta condition?” You do need to avoid separation before the trailing edge, and that requirement corresponds to the Kutta condition, although the Kutta condition is purely mathematical. EXCEPT for understanding the stall, you can get decent rough approximations of flow and lift without considering the viscosity. Where viscosity cannot be ignored is, first of all, in understanding the stagnation/separation/stall issue, and second of all, in understanding the physical reason that successful flow does correspond to the Kutta condition. (Since analytically that condition is just a rule inserted in the algebra to make it match what’s observed.) So for what *physical* reason does the flow separate at the trailing edge (if it hasn’t already)? It’s simply that the edge has a sharp enough curvature that if the underside air were to come up around such a sharp turn, involving a sharp pressure drop in the interior layer of air (curvature of streamlines again) - and then run head-on into air coming the other way, the pressure difference at that collision would present such a rapidly-increasing pressure gradient that it would force the rounding-the-edge boundary layer to die right there and separate after all. (UNLESS the wing has stalled, and then there’s not much upper-side velocity back there to collide with, and then the underneath air *can* burble around onto the top, and short out the lift, after all.) “(I realize that's a bit of a broad question, caused perhaps by watching, and trying to synthesize, too many KZbin professors).” No worries. It was a reasonable question and was asked nicely. Hope the kind of wordy response has made some sense. (“adb012,” please fix any misstatements.)

[@fahembree]

My question is, is the wing being pushed up from below the wing or pulled up from the top. I think it's being pushed up from below otherwise fabric covered wings would have fabric covering being torn off from the revit attachments

[@crispinmiller7989]

Nope, the fabric is just held on well. In addition to rivets it may well be glued. Also in the region where the pressure is especially low (the place where the airflow is most sharply curved), the fabric is most sharply curved too (hugging the ribs), so it's not in a shape that can be peeled easily.

[@buffdelcampo]

Retired now, but I designed aircraft for a career. You're the first guy I've seen on KZbin that has mentioned the stagnation point. When tufting some wings, I have seen the leading edge stagnation point way back near the strut fitting. Of course airfoil selection and aspect ratio will make a big difference on how far the stagnation point moves. I could talk for hours about this stuff but I'll only mention one other thing now. Just look at where the stall warning sensor is on a single engine Cessna. That airplane is still flying when the horn sounds. You got a sub from me. Thanks.

[@tonyfree2691]

No one ever mentioned or emphasised the stagnation point to me before. However I felt it's presence as a boy in the creek spinning a plate around myself under water. It's very strong reactive force.

[@AussieSteve1984]

Thank you for that info, Buff. I'm neither aviator nor engineer, so I was interested in learning from both cohorts the answer to this question I asked on a major aviation site maybe 15 years ago: "Who lifts your wings, Bernoulli or Newton ?" Which provoked quite a discussion. With many saying one or the other, and many saying both :) Best, Steve

[@AussieSteve1984]

@@davetime5234 Thank you so much for that explanation, Dave. Enlightening. And having just a little physics; enough to know how much I don't know, I understood much. I appreciate your time invested here :)

[@josephgorman1275]

The stall warning light is an early warning. If it didn't come on before the wing stalled there would be a lot more dead fliers.

[@enjaymarine]

Whilst I have only been a Licensed Aircraft Engineer for 50+ years - I have also been a sailing instructor. In that environment, the Theory of Flight (in a Vertical plane) - may be demonstrated on a strong wind day, with "Lift" being generated by apparent wind speed increasing as the boat accelerates and more airflow passes "In Front Of" (over) the Foresail & Mainsail,, than Behind (under) those Sails (aerofoil sections) to act down through the mast as motive power but which may also be (technically) described as "Lift". This effect also benefits by the accelerated airflow passing through the "slot" between Genoa/jib/foresail "trailing edge" or Leech and the Mainsail leading edge or "Luff". I was also a gliding instructor for many years - and this video provides an excellent explanation of slightly-more-than-basic Theory of Flight, with the Stagnation Point position answering any queries about Lift Generation during (prolonged) inverted flight - especially with the aircraft wings at the centre of any such discussion having conventional aerofoil sections. Finally, and for your own interest, you are absolutely correct in your contention that ANY aircraft may perform simple aerobatic manoeuvres like rolls and loops - providing (as my gliding aerobatics instructor once told me) "You maintain a positive 1g throughout the manoeuver" This theory has been Proven - and recorded on film and video - by pilots of a (prototype) Boeing 707, an Avro Vulcan and - more recently - a Lockheed C-130 Hercules. Not exactly Pitts Specials, but even more impressive for that...

[@bernardedwards8461]

When I practiced falconry, my hawk had only to spread her wings in a slight headwind for her to lift off and I would tow her like a kite on the end of her leash.

[@Cap10VDO]

Excellent point. My first experiences with sailboats progressing into an oncoming wind were highly educational. I don't understand the math behind the fluid dynamics that makes it work, but I don't have to in order to know that it does.

[@marc_frank]

​@@bernardedwards8461that sounds really cool :)

[@j14152]

Where do you get a statement like this from? You do not have to maintain a positive One G throughout any aerobatic maneuver. If your loops are really round, there's likely negative G's at the top of a loop. A Citabria or a Super Cub could do a truly round loop. When you pull on the control wheel, G's increase, and - vice versa - relax and G forces decrease. So - how can one possibly perform an aerobatic maneuver without moving the elevators, which vary the G force on any aircraft? I was educated as an aeronautical engineer. I am a RC flyer, model aircraft designer, aerobatic pilot, and - a flight instructor. I have 15,000 hours flight time in full size aircraft, both in a number of light airplanes and a few commercial passenger jet aircraft. Years ago, when I taught the "aerobatic" maneuver known as a chandelle, depending on how it was performed, G forces clearly less than (and more than) One G resulted. A 90 degree chandelle (hammerhead stall) should have zero G's at the top of that maneuver. Airplanes fly, because the lift comes from the force resulting from the change of momentum of air deflected downward, offsetting the weight of an aircraft, no matter the complexity of the physics behind what causes that. That's why helicopters fly, too. For every action there is an equal and opposite reaction, according to Sir Isaac Newton's third law. The law of conservation of momentum rules! The physics is generally beyond a forum like this one, where so many varying opinions can be confusing.

[@marc_frank]

@@j14152 which model aircraft did you design? i'm working on my own, too

[@gnosticbrian3980]

But, does the theory of different flow lengths enable calulation of the quantum of lift? I think a better explanation of the origin of lift is given by Newton's third law. The "wing", "lifting body", "flat plate", whatever angled in the airstream such as to cause that airstream to flow downwards; producing a downward momentum in the airstream results in an equal upward component of momentum being applied to the wing etc. Classical dynamics.

[@thearmouredpenguin7148]

When I started gliding, around 1970, one of my instructors was an aeronautics engineer involved in helicopter rotor testing, and the "Newtonian" approach was the way he explained lift. I got the impression that that is the the way that many rotary wing engineers think about lift.

[@gnosticbrian3980]

@@thearmouredpenguin7148 And how aero-engine designers thought about propellor thrust.

[@Eatherbreather]

You have made an excellent video here! PMaybe you can please make a video explaining how the use of rudder alone in flight induces roll in the same direction? I'm certain it's not because the "outer" wing is passing through more air and is therefore producing more lift and thereby creating a roll effect. I think it's caused by the dihedral creating a roll coupling by presenting the "outer wing" at a greater AoA and the "inner wing at a reduced AoA am I right? I don't know if there is any roll produced in an aircraft with no dihedral (presumably not) or if indeed adverse roll is induced in an aircraft with anhedral (inverted flying?)

[@jaywung7616]

A Cessna 172 with no dihedral will eventually drop the right wing with sustained right rudder in a skidding turn. However, depending on the plane, a rapid right rudder can produce some amount of left roll since the rudder is usually above the center of gravity. This dynamic effect is momentary, and eventually there is right roll as above.

[@Eatherbreather]

I thought this would be the case. I don't like comparing RC planes to full size aircraft too often but I have had a model that would do complete rolls on rudder only - the opposite way to the input! Adverse rudder roll was a thing with that plane. It had a large rudder with huge throws and a lot of area forward of rhe hinge line at the top of the rudder. No dihedral in the wing either. Was a hoot to fly 🤣

[@crispinmiller7989]

@@Eatherbreather No argument about you or jaywung's thoughts three weeks ago, just a footnote: there was a noted very successful family of hang glider designs in the seventies, known as Icarus I, II, and V (and maybe IV was also built) by one Taras Kiceniuk Jr., that steered by the yaw-and-dihedral scheme you initially mentioned. They were configured as flying wings with pronounced sweep and dihedral, with drag rudders on the wingtips, each rudder deployed independently and outward only. Hanging a rudder out yawed that wing aft and inward, so that the dihedral reduced the AoA of the aft wing and increased it for the forward wing, and after a brief skid the glider briskly rolled into a turn. Pitch control was by weight shift but there wasn't a typical control bar, because roll was more positively controlled by the tip rudders. They also had pronounced washout, especially the V, which made them almost unstallable because the tips, which were aft, would keep flying after you'd stalled the root, so the nose would refuse to stay up long enough to stall the whole wing. The only accidents I read about were clearly pilot error (one skimming the ocean too low above rocks and one without positive harness for the pilot). The V's were a monoplane (chosen to get a better view of the landscape below) -- 5' chord x 32" span, 6' sweep -- and had higher performance (L/D about 12) than the I, II, and maybe IV which were all biplanes. (I guess this was because the V's had less parasitic drag -- fewer vertical struts and diagonal cables, and a much finer trailing edge because the aft spar was tucked in a bit forward instead of fattening the trailing edge -- and because they had a more sophisticated custom airfoil.) But not many V's were built because they were mostly sold as plans-only, with a kit version available for a year or two. A modestly tweaked, and motorized, version of the II (or maybe IV) called the Easy Riser, produced by one Larry Mauro, was sold in much greater numbers, had a Yahoo group while those lasted (i. e., until quite recently), and might have several planes still flying. I bought a kit for a V and got it more than half done but then started engineering school and never had time to finish it.

[@robertallen6701]

Time for a science experiment. I'm not going to ask you to take my word for anything. I'm going to ask you to perform a simple experiment and see for yourself. You will need: 1 kitchen tablespoon. The kind you put next to the plate when setting the table. 1 dinner knife. Or steak knife. Or butter knife. Anything along those lines. The kitchen sink. Go to the kitchen sink, and turn on the water as high as it will go. The faster the stream, the better. Hold the knife loosely in your hands, blade down, so it can swing on its own if it wants to. Move the knife so that the flat of the blade comes in contact with the water stream. What happens? Not much. Now hold the spoon loosely, just like you did the knife, bowl part down. Move it into the water stream, with the convex (outward bulging side) being the part that makes contact with the water. What happens? Well, unless you are holding the spoon too tight, two interesting things happen. First, the spoon is pulled deeper into the water stream. Second, the water stream is no longer going straight down as it flows off the end of the spoon. It leaves at the same angle as the curvature of the spoon. At least for a couple of inches until gravity takes over and redirects the stream downward again. So...how is the spoon actually pulled up into the higher-pressure area of the water stream, rather than being pulled away from it. Laminar fluid flow. The fluid will actually naturally want to follow the shape of whatever object it's flowing around. In the case of a spoon, it has to change direction to follow the curved shape of the back side of the wing. And as Sir Isaac said, every action has an equal and opposite reaction. So as the back of the spoon redirects fluid flow, the flow redirects the spoon...deeper into the flow of water. This is actually how airplane wings create lift. As the wing moves through the air, that laminar fluid flow causes the airflow to "stick" to the skin of the wing, directing the air downward, and the wing upward. The bottom surface of a lot of aircraft wings are actually concave, bowing inward. So the wing cross section is more of a sideways, elongated C than it is an elongated teardrop. This actually increases the path the air at the bottom has to travel. If the air traveling farther over the top surface rather than the bottom is what created the bulk of an airplane's lift, this would make the wing less effective at creating lift, not more. Rather, the wings with concave lower surfaces are using both the upper and lower surfaces of the wing to direct the laminar airflow downward. Don't believe me? Try the experiment yourself. It's a simple experiment anyone can do in any kitchen in America.

[@swan77a]

When I put my arm out the window of a vehicle travelling 60 mph and rotate my hand to increase its angle of attack into the on coming air my arm lifts, no theory needed to figure that out.

[@warriorson7979]

But the rearwards force is A LOT bigger than the upwards force.

[@swan77a]

So? The area of my hand is miniscule compared to the total area of the underside of a wing.

[@karhukivi]

If you fit air pressure pressure sensors on the top and bottom of a normal aerofoil you will find that 80% of the lift comes from the top surface and 20% from the bottom surface, depending on the AoA. If you use a flat plane for a wing, then all the lift comes from the air deflected by the lower surface but you need a lot more power to get the same lift. So for a normal aerofoil, Bernoulli gives 80% and Newton gives 20%. Flying inverted the AoA had to be greater and the wing is inefficient, but will still work, as stunt pilots regularly demonstrate. It's not an issue of somebody or something being right or wrong, it is an issue of efficiency for the speed and power requirements of the aircraft.

[@LetsGoAviate]

I'm not sure about the 80/20 split (not saying it's incorrect, it may very well be correct) but even NASA says the airflow over the upper surface cannot be neglected in accurate explanation of lift. A flat plank for a wing, as long as it has a positive AoA, still forms a pressure differential below and above. It's more difficult to visualize, but a cambered wing's shape isn't the predominant factor creating lift.

[@karhukivi]

@@LetsGoAviate A flat plank tilted will generate lift below it and turbulence above it, very easy to demonstrate in a wind tunnel. As regards the 80% lift above a good aerofoil, try flying one with rime ice forming on the top surface - the lift is reduced dramatically. A theory has to account for the evidence.

[@karhukivi]

@@LetsGoAviate Wing shape is what makes wings efficient and allows light aircraft to fly with small engines.

[@dougj8186]

Bernoulli's theorem is essentially the correct way to think about lift. This has been thrashed to death because student pilots are taught Benoulli's theorem is the primary cause of lift while ignoring kite effect, that is., a flat sheet of plywood will also create lift due to the the air pressure differential top an bottom, just not efficiently. That's the purpose of airfoil shape, to optimize airflow around the wing for the performance envelope of a particular aircraft. Like a sheet of plywood, a typical plane can fly inverted just not very efficiently. The longer path along the top of the wing effectively creates a positive angle of attack, with equal distance being above the center of the leading edge. As a practical matter, if you are learning to fly remember the old pilot's rule: throttle controls altitude, pitch controls speed. A wing generates more lift with more speed without changing pitch.

[@kennethferland5579]

Its schools students that were taught Bernoulli's principle as the cause for lift, not pilots.

[@dougj8186]

​@@kennethferland5579 It's part of every ground school curriculum and there are related questions on lift on the private pilot exam. The first inverted image has the wing splitting the air in the wrong place. The split would be lower on the leading edge. The air above (bottom of the airfoil) is still traveling a longer distance, albeit not very efficiently as there is a separation of the airflow from the surface.

[@georgejleonard9410]

Great video Jaco...many pilots, even the advanced ones do not correctly understand the principles you have demonstrated and that is why we are stalling aircraft especially from base to final or on a go around. Looking forward to the next one

[@pmac_]

The flow lines in the diagrams are incorrectly drawn. At the stagnation piont the stagnation flow line must meet the surface at right angles.

[@TheMadManPlace]

HOORAY - At last someone is saying something that makes sense to me on this subject. Since I was a kid in the 60's I have been told that it is the low pressure created by the air flowing the longer path over the top of the wing that creates lift and the air flowing below the wing can basically be ignored for all intents and purposes. And I always thought to myself that this "theory" which was put forward as "fact" was VERY SUSPECT. Here is the thing... Low pressure (or vacuum) can only decrease until it is zero while air pressure basically has no upper boundary. The air UNDER the wing is what creates just about ALL THE LIFT on the wing - so what is with this "the wing is being SUCKED UP which creates all the lift" nonsense? Yes, the low pressure above the wing does create SOME lift but BY FAR the greatest amount of lift is created by the PRESSURE EXERTED by the air traveling BELOW the wing. Every time I tried to raise this when I was a kid, the so called "experts" simply shut me down and told me that I was "just a kid, and a dumb one at that and that I DIDN'T UNDERSTAND THE CONCEPT". Now, 50 or 60 years later, it seems that the "dumb kid" just maybe had a point that the "clever experts" had missed or ignored completely in their "know it all" arrogance. To paraphrase someone MUCH wiser than I : If you just sit on the bank of the river of time you will eventually see the bodies of all those who would do you harm and injustice come floating by. Here's to that idiot science teacher who put me off of following a career in the STEM fields. SCREW YOU...

[@clarkstonguy1065]

Just for fun, google "pressure at a molecular level" sometime. Technically it is not possible for a fluid to suck anything up. When someone refers to "negative pressure" or "lower pressure" on the top of the wing, that is just a way of saying the air molecules on the bottom of the wing are pushing it up harder than the air molecules on the top are pushing it down.

[@Pneuma40]

@@clarkstonguy1065 Yup. Imagine 'nothing' , a perfect vacuum with no air molecules, 'lifting' a 20 ton aircraft........ lift (and centrifugal force) don't exist. Planes fly by Newtonian physics.... period.

[@directorrepublik3575]

@@Pneuma40 a perfect vacuum on one side and atmospheric pressure on the other would 'lift' about 10ton/m², to be more exact, the atmospheric pressure would push to the vacuum with that amount

[@redbaron07]

So why are engines and other protrusions placed under the wing and not on top? I heard as a kid that "2/3 of the lift force comes from the upper surface, 1/3 from the bottom." Now I don't know where those fractions came from, or how one could measure the upper and lower lift forces separately, since they work together

[@alexeyaviator8600]

I have a simlar story of talking to guys that explained the lift of the wing talking mostly of the differences of the air pressures above and below the wing produced by its shape.

[@glenwoodriverresidentsgrou136]

Anyone who thinks the different travel paths create all the wing lift has never stood behind a propellor or under a helicopter. Force (thrust) = change in momentum (delta MV). Just like propellers deflect air backwards and helicopters deflect air downwards, wings deflect air downwards as well. The change in momentum produces most of the lift. Put your hand out the window on the highway and feel the forces. This video kzbin.info/www/bejne/bIbPpHSoq8eHm9Esi=CNghS9YtYC3fUIkH shows that slipstreams above and below an airfoil DO NOT arrive at the trailing edge simultaneously. I have seen videos where the upper path arrives after the lower path, and some (as here) show the upper path arriving before the lower path. NOTHING says they have to arrive simultaneously. By definition, there is a pressure differential between the upper and lower part of the wing or else the wing would see no net force and generate no lift. But the pressure increase on the bottom part of the wing is due to air deflection not the Bernoulli effect.

[@12345fowler]

This doesn't invalidate the longer path theory at all. An airplane prop or helicopter blade are just other cambered airfoils just like a wing and thus have also a longer path.

[@Alec72HD]

You CANNOT generate ANY lift without transferring downward momentum to air. IF you could do that, that would violate Newton's 3rd law. IF, IF that was possible, you could theoretically create a propulsion device for use in vacuum. BUT WE CANNOT. In other words, mV/t is the 100% of lift. No more, no less

[@glenwoodriverresidentsgrou136]

@@Alec72HD yep!

[@crispinmiller7989]

(1) The culprit isn't Bernoulli, but the *misapplication* of Bernoulli. Bernoulli applies perfectly well to the actual airflow. (It'd be a violation of Newton's laws for it not to.) The mistake is to think that the actual airflow is the cereal-box story about equal transit times. (2) "Suction" in any realistic context involving subsonic airplanes does not mean zero absolute pressure, it simply means air pressure lower than atmospheric. A wing can perfectly well suck air downward along its upper surface because there's several miles of atmospheric pressure above it to shove the low-pressure air down so that it follows the upper surface of the wing.

[@samuelpope7798]

I have flown identical aircraft inverted with both symmetrical and conventional lifting airfoils. It makes a huge difference on minimum power required. Inverted the lifting airfoil required over twice the power of the symmetrical. When flying normal the lifting airfoil could fly with almost half the power of the symmetrical.

[@jimbo2629]

The wing upside down is less efficient than the right way up at the same angle of attack. The difference is due to the effectiveness of the aerofoil in producing lift. The difference needs to be divided by two. None of this disproves lift from reduced pressure above the wing due to longer path. The angle of attack simple produces lift or sink due to Newtonian force. End of story.

[@Talon19]

Lift is not a one or the other thing. Different approaches arrive at the same conclusion because they are using the same information, just calculated in different ways. The pressure and velocity approach is simpler to derive and apply, but eventually will give the same result as the more advanced computational fluid dynamics approach.

[@detch01]

I've heard this same arguments that lift creation doesn't happen because of a pressure differential on the surfaces of the wing but because the "wind" is pushing against the wing and pushing it up. This is usually the argument of the same people who claim that there is no such thing as an accelerated stall - instead the stall is created by the airplane magically slowing down to below unaccelerated stall speed long enough for the airplane to stall.

[@AnMuiren]

You are so much more patient with that nonsense than I am able to be. As soon as people try to drag me into an argument on that belief, I walk away. You did an excellent job of presenting a simple, clear, and cogent rebuttal to this pseudo-scientific belief. Thank you.

[@Rampart.X]

How do paper planes produce lift?

[@crinolynneendymion8755]

Ah, the pompous priesthood emerges.

[@adbrouwer]

10:12 which theory is that, please? Has anyone a link to the mentioned theory?

[@adbrouwer]

@@davetime5234 thank you for summarising. I understood the explanation in the video. I was merely trying to get a reference to the theory he is talking about, because when I was taught aerodynamics 40 years ago, I understood the "longer distance over the wing... thus air moving faster" is an incorrect way to explain lift. Thanks again for your reply. Luckily I just encountered a video in which the phenomenon lift is explained correctly kzbin.info/www/bejne/q6u6cnVnhsSMnbcsi=-4X7jTOzpixXTMn0

[@PokoMoto-s8w]

What do you think about Coanda effect ?

[@8546Ken]

When I was 10 years old, riding in my parent's car, I would put my arm out the window. I could feel the tremendous lift force - either up or down, depending on the angle of attack of my hand. So the first time I saw a movie in school about how planes fly, I realized that the Bernoulli principle applied to lift was only a small percentage. I since read that this error was made in an early textbook on planes, and that most text books after that time repeated the error. Apparently the people who actually built airplanes knew what they were doing, ignoring those text books.

[@8546Ken]

@@davetime5234 I don't know specific texts. All I remember was that I heard (read) frequently that the Bernoulli effect, was the reason for lift, ignoring angle of attack. And I did read that this was due to an early textbook error. I have no idea what that book was or what level it was.

[@aeomaster32]

Yes, air HAS to be accelerated downwards to supply an equal and opposite force to the weight of the aircraft. It seems the argument is more about HOW the wing supplies this force. Here is my take on this. The momentum of air molecules deflected downwards off the bottom surface pushing the wing up, is easy to imagine. The air above the wing thins out and speeds up as it covers a longer distance in the same time. [edit: This happens even if flying upside down, when the bottom of the wing is now on the top, due as mentioned, to the moving stagnation points changing the distance travelled by the airflows.] This causes the higher atmospheric pressure air some distance above the wing to move downwards to fill in the thinned out "vacuum" - in effect, pulling the air downwards (downwash) and creating an opposite lifting (sucking) force on top of the wing. Combine both the down wash from the bottom and top, and Newton third does the heavy lifting.

[@Greebstreebling]

Nice explanations. Density of air above and below wings gives an Archimedean clue as to why lift occurs. Here's a nice related question: if an aircarft flies directly above you, do you feel the weight of it? Does the height at which it flies above you have any effect on what you may feel? Thanks for posting.

[@davetime5234]

Buoyancy of a balloon relates to Archimedes' principle. For a heavier than air aircraft, there is practically no effect on lift from Archimedes. You would only feel the downwash, which must have momentum equal to the weight of the aircraft, if you were close enough that it didn't dissipate in intensity before reaching your distance below. You can google images of aircraft carving out big rectangular strips in the clouds below due to the downwash.

[@crispinmiller7989]

If you had a really sensitive barometer you could theoretically detect it. (Or maybe not so sensitive if it were hedgehopping and went over at 100 feet.) See Prandtl & Tietjens, Applied Hydro- and Aeromechanics, chapter VI: B: topic 101, The Velocity Field in the Vicinity of the Airfoil.)

[@FarmerFpv]

So if I'm on the verge of stalling I could throw my passenger out of the airplane to reduce the wing loading and continue flying high alpha? I'm going to try that next time.

[@sailingsibongile]

THANK YOU !!! I have been saying this for a long time. The longer surface area creating faster air-flow is less relevant than the fact that the longer surface merely creates a longer surface that low-pressure can occur, versus the high pressure underneath. The thickest part of a P51's chord is at 50% (Spitfire is 60/40) which allowed it's insane performance, and that also debunks the aerofoil argument.

[@ChetJang]

The wing flaps can also cause upside-down lift by increasing the curvature of the bottom of the wing, but also increase the upside-down angle of attack. Like some jet incidents having to fly upside down because of a malfunction which puts the plane into an uncontrolled dive.

[@TheWilferch]

Not sure what the argument or controversy is. Simple Bernouli's equation. The longer path of the (split) airflow on the top surface.....noting Bernouli's equation..... means less pressure on the upper surface. ( example: flow in a pipe with a "pinched" center section.....this center section will exhibit higher flow but lower pressure). Besides wing "shape", then yes....the angle of attack also contributes to "lift". So, yes, ..... the plane can fly upside down but the angle of attack needs to be different than right side up. Should be no controversy here.

[@KathrynLiz1]

Depends a lot on where the mid-chord line is. A flat bottomed section like the Clark 'Y" will produce lift if the flat bottom is at zero Alpha, but the chord mid line is nevertheless at positive Alpha. Aerobatic aircraft use fairly thick symmetrical sections that produce no lift at all at zero Apha, the chord line being straight, and that means that when inverted very little trim change is needed to stabilise in pitch, assuming that the CG is fairly far back. Overdone, this can cause erratic instability in pitch due to centre of lift changing with speed. This is more likely on thin and undercambered wings...just ask any 'free flight' modeller who has had a model come off the top of a climb and just go in to a dive all the way down when it has been perfectly stable in pitch on a slow hand glide. Modellers often push things a bit to minimise induced drag to fly for three minutes off a 10 second engine run with no external controls at all... The thick sections of aerobatic aircraft make the stall less dramatic, which is compensated during aerobatics by severe yaw shifting the centre of lift laterally to flick roll, usually aided by huge ailerons.. Lift is created as a reaction to directing air downward, and with lower pressure on the top surface. You can see that pressure drop by skin bulges between the rivet lines as the aircraft takes off. Boundary layer effects are a lot more severe in small model aircraft due to the very low mass flow ratios (Reynolds #).... We don't have scale air molecules, so the boundary layers are the same size regardless of chord width, but of course low aspect ratios mean more trip losses, even with fancy tip configurations... Big wings make many of these effects fade almost into disappearance. Another thing often understood quite foggily is the effect of control surfaces. In essence, all they do is shift the overall centre of lift about relative to the CG, either laterally or longitudinally, thereby disturbing stable flight into an unstable one causing the aircraft to change direction. Also the relationship between CG, centre of lift and centre of drag are often misunderstood, and that can lead to some 'interesting' phenomena... 🙂

[@LetsGoAviate]

One of the most informed comments I have ever gotten on this subject, thank you. The difference between the flat bottom, or "apparent" 0° AoA and chord line which is actual AoA on a Clark Y is often misunderstood and is where some misconceptions start taking root. Chord line determines AoA, not the flat bottom. Also a very nice description of lift. Besides air over the top being obviously lower pressure than the air going below the wing, air pressure over the top is actually lower than ambient pressure. Ask the "air pushes a wing up from below" advocates to explain that one... I've never really thought of the pitch instability a thin symmetric wing can cause compared to a thick one, but makes perfect sense with quite a fine "line" between high pressure below, low pressure above, and at 0° slight AoA change to negative a quick reversal of pressure above and below happens. I also enjoyed the reference to CG's impact on centre of pressure (and vice versa), especially if they are not close enough. Not often mentioned in these duscussions.

[@KathrynLiz1]

@@LetsGoAviate Thank you..... I spent a large pat of my life designing and building aerobatic aircraft (little ones that didn't carry people, and we chuck them about very vigorously and sometimes become intimately familiar with the dreaded 'high speed stall', especially with higher wing loadings. Our control systems these days are so accurate that one can get away with a lot, but sadly that often leads to inefficiencies. I have spent many a happy our ballasting an aircraft to bring the CG back just short of the point of straight line stability in pitch. This gives instant pitch response, very necessary for figures requiring nice square corners, and for manoevers like the "figure M" which is a continuous series and needs a low wing loading and plenty of power to do well. The low wing loading and a thick wing helps a lot when pointing straight down to prevent excessive speed build up which makes the manoever look untidy. In case you are not familiar with it, the 'figure M' is entered from level flight... vertical climb, half roll, stall turn (wingover) at the top.. vertical down with half roll, pull out inverted into vertical climb, half roll, stall turn in same direction as the first one...into vertical dive with half roll and pull out upright and level on same heading as at the start. Takes a lot longer to describe than to do, but it's a figure that sorts out all of the balance and stability issues you may have. We usually knew we'd got it about right when you can roll into inverted level flight and only require one click of 'down' trim (which when inverter is up trim) to maintain hands off level flight. That also shows up whether you have compensated for lateral imbalance with aerodynamic trim changes... they work against you when upside down. Lots of fun... I don't do it now as it got too expensive in my old age... never mind... 🙂

[@mandst5466]

Makes perfect sense and is obviously and demonstrably true. Very well explained by your good self, thank you 👍🏻

[@bobreiber2066]

Good presentation. Always wondered why a flat wing ( no airfoil ) would fly. We use them in small indoor models. The low wing loading (and power to weight ratio ) is what makes it possible for models to fly at very low speeds without stalling.

[@JoelWetzel]

So, could a plane with a rectangular-sectioned wing fly? Drag would be high but if, say, a jet were used could an angle of attack be found which creates enough lift?

[@marks2254]

When I was in college,I was taught that two air particles that pass the stagnation point simultaneously, with one traveling over the top of the wing and the other traveling under the wing will reach the trailing edge of the wing at the same time. For that to happen, the particle traveling over the top must be going faster than the particle traveling under the wing. That speed differential causes lower pressure on the top, which causes lift. I never understood why the two particles had to reach the trailing edge simultaneously and nobody was able to explain it in a way that I could understand. Maybe you addressed that and I didn’t understand. If you didn’t explain that, would you? If you did, could you try it again? Thanks.

[@LetsGoAviate]

The air going over and below doesn't reach the trailing edge simultaniously. This is referring to the equal transit theory which has been disproven. This wind tunnel video shows it : kzbin.info/www/bejne/m2HPZGSma7d8l7ssi=X6WJ6nkzLHWh6X5X The air going over is indeed sped up, but not because it has to meet up with air going below. The path over the top of wing is also longer with a positive AoA, as is discussed in the video, and while many assume this is referring to the equal transit theory, it is not and one does not have to be true for the other to be true. Not sure if this helps.

[@tonydfixertonydfixer9113]

Ok, but what are air pressure measurements on the top of a wing relative to the bottom in a wind tunnel? With and without angle of attack.

[@davetime5234]

With some asymmetry of wing shape, without angle of attack: some pressure difference Symmetrical wing shape, without angle of attack: no pressure difference With positive angle of attack: also, a pressure difference If the wing shape is asymmetrical, then it adds to the angle of attack pressure difference.

[@BrianJohnson-sr6ok]

This was very informative! I particularly enjoyed the voices you gave to the you tube “scientists” making comments. It brought some humor to a very interesting video😊

[@b1lleman]

I used to be a glider pilot so I understand about all the terms used in this video, but still it explains quite well some of the uncertainties I had about flying upside down (which is not the kind of things you want to try with a glider). Thank you nice video.

[@davetime5234]

I was just watching a video of a glider flying upside down.

[@amitaimedan]

Pall, lift is produced seperatly and by differant methods on the upper wing and the lower wing sides. the path length means nothing. If you blow on the top side of a paper, with no air movement at the buttom, you still get lift. The buttom side lift caused by air molecules pushing the surface. Third newtons law and angle momentum. The top side lift is caused by the van der vaals force of the air particals that hitting the surface, bauncing and pulling it with them. Am I right?

[@LetsGoAviate]

Yeah you are mostly right, although I've not heard of "van der vaals force", will look into it. That isn't what this video is about though...as mentioned in the video I'm not trying to describe lift, I'm addressing that specific upside down argument.

[@JohnA000]

I read long ago that Bernoulli's principle is involved in creating lift. The faster a fluid moves past an object the less sidewise pressure it creates. Air flowing past an object wants to maintain equilibrium and in order to do that it must flow faster over the top than the bottom, creating less sidewise pressure on top than on the bottom of the wing, thus creating lift. Is this not correct?

[@crispinmiller7989]

Not really. Please actually watch the video -- the whole point of it was to get people off of that explanation. Bernoulli is perfectly correct, but the mistake is to apply it to the wrong idea about velocities. The air has no wish for the top to keep up with the bottom, and in fact the air on top typically outruns the air on the bottom by quite a margin -- the equal-transit-time idea would predict only a few percent of the lift actually observed.

[@ziad_jkhan]

In the end, does it not boil down t the amount of air pushed downwards too counter gravity?

[@davetime5234]

Only partly. Yes the air needs to be pushed down, but the question then remains what mechanism due we use to push that air down. That's were the other pieces of the puzzle are necessary for the meaningfully complete picture of the nature of lift.

[@ziad_jkhan]

@@davetime5234 You mean some mechanisms can push less air in the direction of gravity and somehow achieve a higher lifting effect? Or is the rate of air being pushed down the final determinant for lift instead, regardless of the mechanism involved?

[@ziad_jkhan]

@@davetime5234 In short, if it's not just the resultant rate of air being pushed down then what could it possibly be?

[@crispinmiller7989]

@@ziad_jkhan yes, it's the rate of air being pushed down, but what Davetime5234 was pointing out is that the mechanism for doing it is still important to understand. Modeling the air as billiard balls ricocheting of the underside of the wing isn't it -- if it were, then, just for one example, ice forming on top of the leading edge wouldn't be the crash threat that it is because there'd be no need to care about the airflow over the upper surface. If you equip an airfoil with a bunch of pressure taps along the bottom and the top, what you discover is a region of substantially decreased pressure above and behind the leading edge, and no big changes from atmospheric pressure anywhere else. Obviously "suction" is a shorthand term for "pressure below atmospheric" -- so long as the pressure variation is small compared to atmospheric. it's just a matter of semantics in the way you describe relative pressures -- but whichever way you express it, compared to having the wing sit in air of uniform atmospheric pressure, the air surrounding a wing exerts the lift on it much more by suction on the top than by compression underneath.

[@mikemarkowski7609]

Very well explained. To summarize, a Clark Y airfoil (or similar) will allow inverted flight given the correct wing loading, angle of attack and power considerations. But a symmetrical airfoil (or similar) when coupled with proper angle of attack will be more efficient. Hence, many or most truly aerobatic aircraft employ symmetrical, or nearly symmetrical, airfoil shapes.

[@ferocious_r]

One of the best videos on the subject I've seen. Love your style of teaching!

[@visarr]

For a given wing, speed and angle of attack, can one calculate how much lift (say, in pounds) is being created? Can I assume that a stall is when the pounds of lift is less than the pounds of weight?

[@davetime5234]

No, that's not correct. When an aircraft goes from straight and level flight to a controlled negative vertical speed for descent, it is definitely not in a stall in all normal operating circumstances. Initiating such a normal descent only requires that the lift be decreased to a level somewhat below the weight of the aircraft. Reducing angle of attack somewhat and/or reducing forward speed will accomplish this. And it is expected that orderly airflow over the wings will continue during this process.

[@crispinmiller7989]

@@davetime5234 A stall is more subtle. First I have to talk about "adverse pressure gradient," the pressure pattern on the upper rear slope of the wing. In any normal flight attitude this pressure increases toward the trailing edge, and as the flow over this surface encounters the pressure increase, it's obliged to slow down as it approaches the training edge. (Hence "adverse" -- acting against the necessary rearward flow.) So long as the flow doesn't slow down completely, it can spill off the edge, leaving room for the flow that follows it, and the wing can keep flying. If you push the wing too hard -- exceeding the sustainable angle of attack for a given airspeed -- then this pressure gradient becomes so great that the flow over the wing does come to a stop -- "stagnates" -- before it reaches the trailing edge. This means that the flow that comes after it is blocked, forced to stop and jump off -- to "separate" -- and in this case you no longer have an orderly rearward flow off the trailing edge, but instead a burbling region of separated flow on top of some amount of the wing (rear margin first, and increasing forward if it's allowed to continue). The only remedy is to get the nose down and build up enough speed that the flow washes the separated region away and the flow can reattach. This costs altitude -- you've got to dive a bit -- which raises two issues: (1) if you're too close to the ground for the dive you need, you're cooked; (2) if you're a rookie pilot who thinks the way to escape is to point the nose up, you're cooked until and unless you think better of it, or until someone else takes the controls. This issue (2) appears to have been the way that the person at the controls of Air France 447 on June 1 2009 held the plane stalled through a loss of several miles of altitude until it hit the ocean and all 228 aboard it died. It's true that there was fiendish sensory overload from alarms going off, and bewilderment from iced-over instrument sensors. But a quote from late in the cockpit voice record has him saying something to the effect of "I don't know what's wrong -- I've had the stick back [= nose up] the whole time!"

[@crispinmiller7989]

Please see my stall explanation sent mistakenly to davetime5234 somewhere nearby in this thread. Replied to the wrong sender, sorry.

[@daemn42]

If you want to understand what starts the unequal flow of air around an airfoil at any non-zero angle of attack (thus creating lift) check out the "Kutta condition". It boils down to this. The trailing edge of almost any decent airfoil is very sharp, which causes the rear stagnation point (referenced in this video) to almost always stay right at the trailing edge. This is because even if there's a strong pressure differential between top and bottom of the airfoil, moving air can only change directions so fast (it has inertia) so even if there's lower pressure above the middle of the airfoil, the fast flowing air below the airfoil cannot make a 180 degree turn at the sharp trailing edge and try to flow forward to equalize the pressure. Instead the air above must flow faster to equalize the pressure at the rear stagnation point. BUT.. There's no such restriction for the forward stagnation point. It can be right at the tip of the airfoil, or well below it (or above it, at negative angles of attack). The air just sort of piles up against the "front" of the airfoil. So if the airfoil is just a flat plate and you angle it upwards, the forward stagnation point moves down below the leading edge, but the rear stagnation point remains pinned to the sharp trailing edge. This means when a packet of air is split in half at the forward stagnation point, the upper packet must travel a further distance to reach the rear stagnation point. It's not equal transit time, but equal pressure at two points. The forward and rear stagnation points each have equal pressure immediately above/below, by definition (it defines where they are). In order for the upper packet to arrive at the rear stagnation point and equalize the pressure there, it must flow *further* and *faster* and thus creates a lower pressure region above the middle of the airfoil. The strangest thing is that it flows *faster* than not only the packet of air below the airfoil, but all the free stream air well above and below the airfoil.

[@imikewillrockyou]

I've built and flown stunt RC planes with wings that have the same curvature top and bottom. Seems the angle of attack matters a lot.

[@colindavidson7071]

It always amazes me that discussion of flight somehow forget or totally ignore Newton's laws. Most basically, if the air exerts an upward force on the wing (lift) then the wing must exert a equal downward force on the air, regardless of how this force is created (Bernoulli or simple deflection). This is the true basics of flight. In that regard, the diagram of stagnation points is at best misleading. The airstream of the rear of the wing shown should be backwards and down, not simply backwards. Also, all the discussion of longer or shorter paths is a bit of a red herring. Bernoulli's principle applies to speed of motion in a fluid, not the distance travelled. The longer path causes the air to move faster, relative to the wing, thus causing a pressure drop. If a longer path could be engineered that caused the airflow to be slower, the air on the longer path would be at a higher pressure.

[@thomasglessner6067]

Thanks for your effort to explain lift. The visuals were great. As a youth I built several combat Control-line models with asymetric wing profiles and they flew great upside down.

[@noema]

The critical angle of attack does not change with weight. Dropping the weight would not make the wing get out of the stall conditions unless the angle of attack is also reduced. The pilot may be able to maintain level flight with a smaller AoA when lighter but thats a different story.

[@LetsGoAviate]

Heavier weight requires a higher angle of attack. Critical angle of attack doesn't change.

[@garyakirsch]

Think Bernoulli's principal, my friend. Simply stated, increased velocity equals decreased pressure. The extreme velocity increase adjacent to the airfoil surface dramatically drops the air pressure and acts like an "air glue." The boundary layer "glues" itself to the air molecules just beyond the boundary layer. In effect "suction" of the boundary layer glues itself to adjacent air molecules. Literally, planes fly thanks to suction-which is SOMETIMES lift. Similarly, propellers and turbine blades both use this "suction glue effect" to glue themselves to the air molecules just ahead. Jets don't "push" the craft forward by blowing gases backwards - the turbine blade boundary layer sucks the plane forward.

[@KennethScharf]

Aerobatic aircraft, such as the Decathlon, have a symmetrical airfoil that is designed to produce lift both upside down and right side up.

[@martinsutoob]

In the final diagram at 9:30 you have the red dot (stagnation point) in a position so far aft on the upper wing surface that it looks cherry-picked to better support your argument. Is this location realistic? The thing that always bothered me about the longer path / shorter path explanations is the implicit assumption that two molecules that were neighbours when the leading edge impinged on them, will still be neighbours after the wing has gone by. I can see no reason why this is a valid assumption, and it is an essential requirement when the explanations switches from path lengths to airflow speeds. Longer path sure. But what if it takes a longer time too? Years ago I remember reading an article in New Scientist that offered what seemed like an overall better explanation. This involved balancing the Newtonian mechanical reaction forces between the wing/aircraft, and an appropriate volumetric mass of displaced air (something like that). This easily explains why, if you put an engine with sufficient power on almost any object at all, you can get it to fly.

[@LetsGoAviate]

I'd say my stagnation point is very realistic. Look at this video of actual airflow in a wind tunnel. Look closely at the location I said it is, and see how some air flows forwards to get over the leading edge. kzbin.info/www/bejne/m2HPZGSma7d8l7ssi=h40ed3TIR5902JOf

[@davetime5234]

Navier-Stokes equations (the gold standard of the basic physical principles and their interrelationships), say nothing about equal transit time conservation. The principles that are written into law are: conservation of mass flow rate, conservation of energy, and conservation of momentum. If the speed over the top has to be much faster than equal transit time to achieve conservation of mass flow rate, for example, then that's what happens. When we talk about lift, we best focus on these three principles of conservation, with every other method of mentally modelling the actual physical mechanisms subordinated to these known and tested principles.

[@martinsutoob]

@@davetime5234 I will just re-post the video that LetsGoAviate posted in response to my original post. kzbin.info/www/bejne/m2HPZGSma7d8l7ssi=h40ed3TIR5902JOf And by the way, I won't be getting into a long exchange over this because I know from experience just how sensitive aerodynamicists can be over their theories. B-bye now

[@davetime5234]

@@martinsutoob The video makes the point that equal transit time is wrong. No argument with that. The video also states that the speed increases over the top of the wing. No argument with that either. The fundamental established physical principles are that mass flow continuity, conservation of energy and conservation of mass must be maintained. Those are undoubtedly the rules that govern the higher flow speed over the top of the wing. There would be no disagreement amoung the experts when confronted with such established fundamentals of the physical principles, or am I wrong? And what is there to fear in a sincere and humble search for the truth according to first principles? What is there to fear when no disrepect is intended or evident? I'm happy to accept being wrong when compelling evidence and rational establishes that I have made an error.

[@charlescz1974]

It’s the Reynolds number equation or scale effect that is the dominant difference when comparing an RC plane to a full scale aircraft. Wing loading is comparable and does not affect glide ratio, just the time over distance. Check out L/D wind tunnel polar comparisons of airfoil cross sections for a better insight. It’s referenced as ‘Inverted’ flight. Of course, there’s far more to know. Soaring birds are an excellent example of how nature has solved the problem of flight. Simply stated, the broader the wing span and greater the aspect ratio, the more efficient the lift becomes.

[@flybobbie1449]

Watch bird feathers in flight, they are drawn upwards by the low pressure above wing.

[@davetime5234]

Wow, correct me if I'm wrong, but something perhaps crucial just occurred to me: Because the stagnation point marks the point of separation between air going over and under the wing, the condition for lift can be seen by whether the flowline leading to the stagnation point is rising or not. If the flow line to the stagnation point is rising from its original horizontal location, the cross-sectional area passage to air over the wing has been reduced relative to the bottom (measure the vertical height above and below the stagnation point, symmetrically as far as it needs to be until the flow pattern returns to ambient flow). With the window to the mass rate of flow simply established by that stagnation point flow line boundary, the speed of air passage difference between top and bottom becomes a simple flow rate continuity problem. And the speed change from that flow rate continuity leads directly to the conservation of energy pressure drop. Summary: The flow asymmetry around the wing is defined by where the combined geometry's (any asymmetrical camber plus angle of attack asymmetry) balance of forces places the stagnation point and the flow line leading to the stagnation point. And the asymmetry of the path windows defined by that flow line, determine the speed over the top and bottom to satisfy the continuity equation. And the conservation of energy change in pressure follows directly from those speed changes. This has immediate effectiveness for a perfectly thin sail, where it would naively appear the bottom and top paths are the same (for the surface of the sail it is the same, but the stagnation point makes the air's path length different).