Longer Path Over Top Irrelevant to Creating Lift? Let's Put This to Bed 

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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.

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.

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.

All moot points regarding lift, and therefore, flight creation - "Money makes a plane fly" My first flight instructor :)

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.

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.

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

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.

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

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!

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.

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.

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?

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.

'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.

"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.

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.

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.

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.

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.

Retired now, but I designed aircraft for a career. You're the first guy I've seen on RU-vid 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.

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.

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.

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.

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.

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...

A very good explanation of lift. I think the confusion is that "lift" is a sum of several intertwined forces. You have the traditional Bernoulli forces, You have impact lift, and you have the equal & oposite force of the air being deflected downward relative to the wing. All three combine to create what we call lift. Good news is that outside of a FAA test you really dont need to care. Only need to know that job # 1 is keeping the air flowing smoothly over the wing. For normal, non aerobatic, flying I teach airspeed is rule #1, #2 & #3. #4 is dont't forget #1 (keep in mind the airplanes I fly do not have an AOA indicator). Keep up the great video's!!

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.

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.

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

Always been interested in aviation since childhood, but I'm not an engineer nor expert. I can honestly tell you I have always wondered how a plane can fly upside down in level flight AND this is my FIRST RU-vid video to deal with this question. Great video.

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

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...

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.

Very well done! I LOVE the sarcastic, distorted, dramatic fun you had poking fun at the ridiculous, unexplainable descriptions of why lift doesn't work the way it does. Haha. Nice, simple use of editing to create a cheap way to poke at cheap answers that are wrong. 😂

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.

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.

I have always wondered about this. I teach young people about flying, so I will incorporate this lesson into my own. Thank you.

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. 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.

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.

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.

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?)

When I was learning to fly my instructor at some point told me that the old saying was "That you could use the barn doors for wings As long as you have the right angle of attack. @InternetStudiesGuy also has it right "A perfectly flat board would work to generate lift as long as it has the right angle". Here in this video this guy only references one wing shape with a flat bottom. But there are several types of wing shapes. Two being the Symmetrical Biconvex, were the curve shape is the same top and bottom and the Asymmetrical Biconvex that is the same but thinner. The Asymmetrical Biconvex is also a laminar flow wing. During WWII they developed the laminar flow wing on the P-51 mainly for speed and to reduce what was then known as Compression or Mach tuck. Compression or wing compression, in simple terms, is where that thick leading edge on a wing at speeds approaching the speed of sound causes the air to build up, compress in front of the wing creating a shock wave that disrupts the air over the wing and control surfaces, making it impossible to pull out of a dive. So basically they moved the leading edge of the wing farther back to reduce drag of a thick leading edge and angled the wings to help prevent the compression. In the 1950's and 60's experiments with wing shape continued and wings got thinner on high speed aircraft like fighters. I spent years in the USAF and have seen fighter aircraft with practically knife edge leading edges and the wings only thicken up slightly in the middle of the wing. The F-104 Star Fighter is a good example. The wings had probably the thinnest wings and close to a knife edge leading edge. The wing was so thin it couldn't carry fuel in the wings. The F-16 is another close example, the leading edge is not as thin as the F-104 and slightly thicker. This allows it to carry some fuel in the wings but not much. Almost every section in the fusaluge that can hold fuel is a fuel tank and needs the external tanks to fly any long distance. But that thin wing makes it fast. So almost any aircraft can fly upside down as long as the angle of attack of the wing is right and the wing is capable of the loading. But in a way this video doesn't explain Symmetrical Biconvex or Asymmetrical Biconvex creates lift. Or the many other wing shapes like are shown here: web.eng.fiu.edu/allstar/Wing31.htm IT would take up more room then is possible here.

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.

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

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

I have seen enough conflicting 'theory' around how lift is generated that i have reached my own conclusion: We don't actually know EXACTLY what is going, but we know enough to create wings the work, and work reliably enough, that we have flight, in various from from kites, to hang gliders to state of the art jets. If two people build identical wings that work, one can argue Bernoulli, the other can say nah, it is all Newton, and at the end of the day both can fly off into the great yonder.

Contrary to the video, 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.

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

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.

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.

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

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.

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

It’s simple, the heavier and slower the aircraft the more pronounced the aerofoil shape has to be.

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.

Excellent points, great video. I used to teach aerobatics in the Bellanca Citabria and the Bellanca Decathlon. The Decathalon had a symmetrical airfoil and you could sustain inverted flight with two people on board. It was nearly impossible to sustain in inverted flight in the Citabria with only one person on board. Plus the decathlon head an inverted oil system.

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.

A friend of mine built an RC plane with a flat wing and I was surprised at how well it flew. Thanks for your explanation as it helps me better understand why it works.

When the Wright brothers did their wind tunnel experiments, I doubt they were thinking about the possibility of flying upside down, rather about sustained flight with a normal attitude, so the term "lift" was entirely appropriate, especially in the context of "lift off" when a flying device leaves the ground. Given that some of today's flying devices are capably of "flying" in any attitude, including straight up and straight down while remaing under full control of the pilot, the term "lift" becomes a bit of a misnomer when applied to the force imparted on the wings by the air that they pass through. Nevertheless I am happy to use the term "lift", since it applies logically to the majority of aircraft flights. By the way, I am a follower of the "force applied to a wing is a result of air acceleration as it is displaced by the wing" concept, regardless of the physical method/s that induces the air displacement. Most of my life I have contended that there is no such thing as "suction", only pressure differentials. I have worked with equipment cabaple of "pulling" (another misnomer) very high vacuum (read that as very low absolute pressure). Nevertheless I still use the term "suction", as many people simply cannot grasp the concept that atmospheric pressure is pushing the air/dust mixture into their vacuum cleaner. Any comments?

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.

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.

The problem I had with flight school describing how wings fly is that it really was one dimensional: Pressure differential. What they failed at describing is that a pressure differential creates a downward flow of air which based on newtons law creates an equal and opposite force. It’s the same thing described as two different things for what ever reason. Also no lift would be able to be generated if there was no drag.

Looking at it in the simplest way possible, the plane lift must come from some force exerted on the plane, and this force comes from the air pressure pushing on the body of the plane. A wing with a positive angle of attack will increase the pressure on the bottom and decrease the pressure on top, leading to a higher pressure on the bottom pushing up, and a lower pressure on the top pushing down. The resulting net force will therefore be upwards lift. Similarily, a wing shaped such that the top path of the air is longer than the bottom path, will cause the air on top of the wing to speed up and decrease in pressure, also leading to a net upwards force. In most cases, the angle of attack has a much bigger influence on the lift than the shape of the wing, so flipping a wing upside down will reduce its efficiency, but it can still generate lift if it has a positive angle of attack.

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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-6UlsArvbTeo.htmlsi=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.

Glad you explained something that seems so intuitive to most. Ask the keyboard geniuses why a box fan falls backwards when it is turned up to high speed. Aren't the wings just a version of the blades on that box fan (a propeller), just moving through air much slower? I worry about our future but at least we have a chance to get some people to start thinking instead of just looking for the easy way out by by simply repeating what they've read or heard on the internet.

I learnt lower pressure on top of an object creates lift when I was doing the dishes as a kid around 10 years old. I had a stack of plates in the sink of very hot water and was washing the top of the top plate with a scrubbing brush in a circular motion causing the water to start turning in a circular motion and once it was moving fast enough the plate started floating up to the surface of the water even with me pushing lightly onto the top of the plated with the brush. The plate wasn't spinning, just the water, the moving water on top of the was creating less pressure than the water under the plate that wasn't moving.

Excellent discussion topic. I always think about this when as a child I would stick my hand out of the window in the car and play with lift and angle of attack, varying the curvature of my hand to increase or decrease lift, or keep it flat and just change the angle of attack. Flat wings can fly too, they just need an angle of attack.

Bernoulli's theorm is specific to the flow in a measurable dimension. For instance, one has a volume of water flowing through a pipe of radius x, and one can measure the pressure that is exerted by the water on the wall of the pipe. If that pipe's radius becomes constricted at some point, the water will have to flow faster to maintain the same volume per unit of time, and the pressure on the wall of the pipe can again be measured. According to Bernoulli's theorem, the water pressure will show a measurable decrease, and, indeed, that is what is observed. Attempting to apply this theorem to an airplain wing is difficult because it the variables cannot be accurately measured. Newton's laws of motion are all that is necessary. Applying Newton's laws to the wing, we can easily see that the angle of attack deflects the airsteam downwards which produces an equal and opposite force upwards. Futhermore, the airflow that impacts the lower side of the wing obviously exerts increased pressure on that suface. The direction of air-flow over the top of the wing changes its direction due to the lower pressure that it exerts and that lower pressure is caused by centrifugal force. The air traveling across the top of the curved wing wants to keep on traveling in the same direction, and that induces a lower pressure on the upper wing which causes an alteration in the path of the airflow. Low speed airfoils tend to be flat on the lower side and curved on the upper side because that is the most efficient shape for that speed and wing loading. And this kind of airfoil will produce lift when inverted, contrary to what our Benoulli author maintains. The author of this video accusses those making a correct observation of a "dumb argument," but this author has to conflate Newton's laws with Bernoulli in order to make Bernoulli work.

You're looking at a consequence thinking it's the only reason it happens in the first place. Lift is produced by displacing the mass of air downwards. Canver is used to mantain the flow attached so that it flows down too. In the bottom of the wing, you're pushing air down against a volume of air, which oposes a resistance, increasing pressure and reducing its speed. At the top you're pulling air down, creating a vacuum which draws air in and thus accelerates the flow. The stagnation point is in the point from where the flow at the top and the bottom have equal energy, it is equally costly for a particle in the stagnation point to do a longer path that is pulling air in, or going the shorter path that is pushing air away.

In the end (no matter what is the cause for this fact...) lift is created, due to the air being "deflected" downwards. So both theories are right. If the path "over" the wing is longer than the path "under", that forces the air to a "downward" motion, whitch creates the lift. You can't have one without the other. :-) For me it is still easier to think of a downwards directed airflow. (Though I know, that this forces the path "over" the wing to be longer than the path "under" the wing...)

Aircraft fly because of TWO factors. 20% is Bernoulli lift, from low pressure on top of the wing. The remaining 80% is generated from Force/Reaction of air impacting the lower surface of the wing. A stall occurs when Bernoulli lift is lost, thus decreasing support for the aircraft and resulting in a descent. Air molecules DO NOT mate for life, so the Bernoulli effect, although true to a VERY limited extent, is NOT the source of the majority of lift when flying an aircraft. Just watch an airplane roll down the runway, and NOTICE when it rotates to increase the angle of attack. This is a multiplier of the force on the bottom of the wing, NOT because you suddenly gained a large amount of Bernoulli lift. Aircraft basically SURF on the air that supports them, with a gentle addition of some Bernoulli lift when they achieve flying speed. If what you say is correct, then aircraft would NEVER need to rotate on takeoff.

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😊

Air is like an elastic band, can be compressed and stretched. Its this stretching of the air over the upper curve of a wing that equates to lower pressure compared with the subtly compressed air under a wing. Stretch air too much and it separates resulting in a stall.

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.

if they insist that the longer path over top has nothing to do with generating lift. when upside down a non symetrical wing still gains lift by sending the airflow over the side which is uppermost on a longer path due to angle of attack.

Still, after 80 years, the best explanation for lift was given given by Wolfgang Langewiesche in his book "Stick and Rudder"

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...

You have the right principle but make it more complex than is required. Simply, a wing has two major mechanisms that generate the lift: the shape of the wing and the angle attack. A good aerofoil shape generates lift with low drag, even at zero degrees of attack. The second mechanism, angle of attack generates lift, but with much higher drag. So when the wing is upside-down, the aerofoil shape generates lift in a downward direction. To create a nett total upward lift, the angle of attack must be high enough to create enough upward lift to overcome gravity and the downward lift of the aerofoil, but this is at the expense of very high drag. Most people seem to get stuck by believing that lift can only be generated by one mechanism.

I'm not a pilot but rather a science teacher but I did fly hang gliders for a long time and now sail a lot which both involve "wings" with a camber. I share your frustration as lifit is obviously created simply by both diverting a fluid flow and a pressure differential above and below the camber. A simple flat board held at any angle other than parallel to a fluid flowing will cause a force on that board i.e. angle of attack but different cambers also generate different amounts of lift and the total lift is the combination of the two. If the plane is upside down still carries it's own weight then obviously the downward camber lift is smaller than the upward deflection lift. The "lift" from deflection is simple vector motion where force in the horizontal axis can be transfered to force in the vertical axis and this is where you can use trigonometry and coefficients of friction etc to estimate that. Seems pretty straight forward at a high physical level to me? Missiles with rocket engines have no cambered wing and just have flat stabilising fins and they "fly" just fine without any so called "aerodynmic lift". Fighter jets also use "vectored thrust" to alter the force vector of the plane which is a moveable nozzle on the jet engine and they also have very flat wings which could not stay stable without modern software control and extremely powerful thrust engines. This misunderstanding is caused by over simplified explainations i.e. lift is caused by a pressure differential about the wing caused by a longer path over the wing ie. camber - but like many "lay" explainations, it is only a very small part of explaination. People have written books about science and engineering misunderstandings but that's what you get when people think they know everything when in fact they are 99% ignorant and simply don't know what they don't know while smart people always are aware of what they don't know which unfortunately leads to ignorant people being very confident and smarter people doubtful about their own kwowledge and understanding. Such is life.

A or the main reason the stagnation point adjusts seemingly counterintuitively to a low point in an inverted wing with positive angle is the airfoil's (upside-down cambered surface) curvature toward and at the leading edge creating enough resistance relative to the airstream to force airflow "back" around the leading edge to the lower-pressure (gravitationally upper) side of the upside-down wing. In other words, stagnation point does not change or move anywhere nearly as much in wings with sharp-pointed leading edges and very low curvatures -- and stagnation point changes even less in such sharpened, low-profile wings when they're symmetrical.

In my simple mind, it seems pretty simple to me. It's all about angle of attack. Air pushes on the underside of the of the wing raises pressure causing a thrust vector which has a lift component and a drag component. The air going over the top lowers pressure adding to the net positive pressure increasing lift and I think reduces drag a bit but drag is still a net positive. Just imagine a flat wing with no curve at all. It will still generate lift as long as there is a positive angle of attack. In my childhood I made balsa rubber band powered model aircraft where the wing tail and fin were all flat sheets of balsa about 1mm thick. With the right balance to get a positive angle of attack, it flew nicely. As I see it, airfoils are just ways to improve efficiency but not the primary cause of lift.

I think the problem is how cool Bernouille sounds. Foreign, with some silent letters, complicated enough to be interesting, but short enough to remember. Who wants to tell their college girlfriend "I was studying air pushed downwards today"? 0:13

Yes, a flat board at the proper angle will accellerate the air downward and provide lift. However, all the lift on the flat board will be at the leading edge, as the air under it is instantly accelerated to maximum downward speed, and the air over it is likewise sucked down immediately to the same maximum downward speed. The remaining width of the wing provides strength, but not lift. And the air over the top is likely to be non-laminar. The standard modified arc provides strength in the middle where it is thicker, and gradually accelerates the air downward over most of the width of the wing so the force is spread over the wing and laminar air flow is maintained. So the curved wing provides more lift before stall, and less drag for the same amount of lift.

__ Very helpful, thank-you. I always figured an upside-down plane was just throttling more to fly at a more aggressive angle of attack since presumably you could fly a plane very inefficiently but still airborne with just a flat plane for a wing. But now that leading stagnation point moving towards the planet opens an interesting idea of a long path. A long path that is longer for being closer to, hmm, the planet. I'll watch more videos. Another thing I'm hazy on is that it occurs to me that a stalling wing with a separating boundary layer is metaphorically (at least) similar to a functioning wing because both have fewer air molecules topside. That's when I got very hazy aeronautically but I liked the poetry.

Just gotta have more air pressure under than over, and - wow - you have a lift vector. However you achieve that, it's not just one variable (wing shape), it's many variables. To ignore all but one variable is ignorant at best and motivated thinking at worst. It's amazing how hard rational thought is for so many people.

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.

I read a few of the comments below (using current time as the datum), guys..... In the beginning people looked at birds and tried to emulate their flight processes, the Wright brothers used wing-warping instead of ailerons. Then people realised what was actually happening and along came ailerons and the rear elevator. So to lift. The "highest performance" profile is rather similar to an owl's, it generates 'enough' lift whilst disturbing the air as little as possible and gives the owl its stealth. However the reality remains that lift is simply pressure differential from one side of a wing to the other. So... what advantage do you gain by choosing the owl's wing profile over any other? The required minimum airspeed is reduced and drag is reduced, the owl does not flap its wings during an attack on prey otherwise the prey would hear it coming, excepting the last second. How does that apply to aircraft? Well you can get off the ground with much less engine power, that becomes less appealing when local disruptive wind speeds are comparable to the minimum necessary air speed or if you wish to travel a long way. Instead of thinking about air path-lengths think about the mean speed of air molecules...

There are 2 factors to lift the low pressure of the longer path and also the nose up angle of attack has the forward motion air pushing the bottom of the wing. It is like having a flat surface in your hands in a windy day, you put the edge into the wind it may be unstable as you fight to keep it edge on. You expose the bottom of the surface to the wind it will pull your arms up and might lift you off the ground if big enough, there is pressure on the wind side and probably turbulant low pressure behind it. You point the top at the wind and it will be forced to the ground. A parachute is a stable form of this flat surface, the hole in the middle is kind of a rudder, the round shape allows the air escaping around the outside edge to maintain the stable wing characteristic of the air not detaching from the upper surface like an unstable flat parachute which could produce lift but the back side turbulence would kill the passenger.

The main problem with the traditional explanation is that it's often given as the ONLY cause for the lift. I once saw a Formula 1 engineer showing a spoiler that was _heavily_ curving up giving the explanation of longer-path-shorter-path... and that's it. That was the only explanation he gave for the downforce of such a curved spoiler. As if air hitting the highly curved-up upper side of the spoiler, or the low-pressure pocket formed on the underside/backside of the spoiler, or the incoming air being strongly deflected upwards, had no effect whatsoever. I was flabbergasted why a Formula 1 engineer would give an explanation so lacking that it was just borderline erroneous.

Very interesting. 5:34 I noticed the still shot drawing shows plane's nose is tilted up a bit. This change the wing's angle of attack. Now the wing os pushed up by the fatter, followed by a laminar flow as air passes the hump and roils across the rest of the wing. The wings are also tilted up like the nose. That means air is still passing over the full wing, creating low pressure vortexes, and lift. On the downward side of the wing, the hump at the front splits the air in teo with thr top flat side creating low pressure across the width of the wing. A high air pressure point is created at the leading edge of the wing. This creates inverted lift since the air pressure is lower on the up pointing width creating low pressure initially is provided mechanical lift. The air ccreated low pressure vortices on the up side and high pressure vortices on the down side as a result of the up tilt on the wing. The result is both sides of the wing provide lift, keeping the upside down plane the same lift components as upside up wing. Slightly more advance force is needed to keep the nose tilted up. The upside down wing is a key feature in F1 cars. It creates the low pressure zone under the car, and high pressure over the top. The greater the downforce, the better tyres grip the road surface. This improves handling in the corners but drag on the straightaways, which can be reduced by opening the DRS flap on the topside wing at the test by reducing the downward force on the car. Watch any F1 race practice sessions, qualifying, and GP and it's easy to see who got the wings right, and who didn't.

Just a point of order, but when you pass the CAOA, the wing still produces lift just fine. Drag just goes up drastically.

This is Aeronautics 101. Most lift, ~95%, comes from angle of attack pushing the wing up against the air pushed down, ie Newton's 3rd law, and ~5% comes from the aerofoil shape creating differential pressure, ie Bernoulli Effect. Otherwise, as kids often ask, "Then how do they fly upside down?"

I drove my instructor to shouting, arguing with him about the standard explanation of lift ! In my theory, I call it force of air mass/pressure under the wing causing the force of upward against gravity ! Nature hates unequal air pressure and the bottom wing is in the way ! I say the mass of the air flowing under the wing is pushing the weight of the plane up and the air flow over the wing is just creating a more efficient low pressure area for that to happen. At stall angle of attack all that has happened is the force of the mass has turned into drag and has exceeded it's ability to make things to go up. Ever skip a flat rock on water ? As long as the rock contacts the water at best angle of attack it will skip until it has lost it's energy/thrust due to parasitic drag. As far as I am concerned a propeller and wing are doing the same thing, just on a constant and in a more controlled manor.

A wing works the same way that a spoon does when you hold it under a stream of water. If you hold the spoon one way, it pushes it away, if you hold it the other way, it sucks the spoon into the stream. Air, like water, is a fluid. The wing is being sucked up into the air. 2/3 of the wing creates lift through suction, the other 1/3 is plate lift from the air molecules hitting the bottom of the wing, the same way that a water skiier is able to stay above the water. Newton's 3rd law. There is no one theory. It is a combination of factors. Lift is a miracle.

Maybe I'm fooling myself, at age 65, that I can hope for anything more than to see such amazing events that show a perspective of life few will see and fewer to live and tell. I'm fascinated and amazed at what is to come as well as frightened. It will be the ultimate test of my character and all I've valued in my life. I cannot help but try to prepare as best as possible: That is my nature. How this story ends is a mystery... maybe THE mystery that will be the ultimate lesson of life. Eyes open... No Fear... maybe even a smile

The Symmetrical airfoil works because there is still a low pressure "bubble" generated by the increase in air speed due to the "ramping up" from the leading edge on both top and bottom of the airfoil. The "Clark Y" or "flat bottom" will always make more lift, though. A plane with a flat bottom air foil can fly up side down if you have a high enough angle of attack AND enough power and down elevator to over come the drag and provide the correct angle. A Flat Bottom airfoil produces lift at a much lower angle of attack, when up right.

The long air flow path argument has often been made for sails as well. I've triggered numerous arguments with sailors over the years by pointing out that unlike an airplane wing the cross section of a sail is a line with path length equal on both sides. When trimming sails I simply think in terms of redirecting airflow while creating as little turbulence as possible. Redirecting the airmass efficiently creates and equal and opposite force which results in forward motion with lift from the keel and trim from the rudder largely countering the side vector when sailing on the wind (net lift is perpendicular to apparent wind direction).

I built a 3-foot model glider out of balsa for a school project. I fitted the main wing correctly but I glued the tail on backwards, so that the aerofoil was backwards with the "sharp" edge forward. It flew beautifully. I didn't realise it was backwards until teacher commented. He acknowledged that it flew very well.