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.
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.
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.
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.
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
@@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 :)
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"
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.
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.
@@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.
"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.
@@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.
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.
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.
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.
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.
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.
@@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.
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.
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
@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.
@@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
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!
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.
@@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.
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'
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.
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.
@@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).
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.
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.
@@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.
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
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.
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?
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.
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.
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. 👍🏼
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 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.
@@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.
@@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.
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 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.
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??
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.
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
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.
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.
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.)
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
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.
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.
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.
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
"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.
'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.
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.
@@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.
@@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!
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.
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
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.
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?
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.
@@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.
@@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
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.
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.
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…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.
….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.
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...
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.
"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
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 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.
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.
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.
@@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.
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.
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...
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.
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.
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.
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.
@@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.
@@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.
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.
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
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 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
@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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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...
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.
@@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.
@@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
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
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.
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.
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.
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
@@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 :)
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.
@@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👩🏫
@@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.
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.
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... 🙂
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.
@@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... 🙂
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.
@@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.
@@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.
@@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.
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 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.
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 ❤
I dropped a Physics class in college because I objected to this “longer path” explanation of lift. When fruitlessly trying to discuss this with a couple of professors in the department, using examples and logic to highlight how the explanation in the textbook must be wrong, I realized I was not in a science class. I prefer Galileo’s method of understanding Physics - you know, using experimental evidence to back up conclusions, not just “accept what the book says.” This was around 1990 that I dropped the class. Since then, I’ve been amazed at how many people who should know better are still duped by an incorrect theory. Anyhow, at least I did gain a deeper understanding of Bernoulli’s principle as I tried working through understanding the issue.
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.
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.
@@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.
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
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.
@@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 )
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.
@@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
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.
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.
@@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.
@@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.
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.
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.
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
(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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
I was confused by this video and not clear on its point until the end (I think). The first half is unnecessary, and thereafter very little time is spent on the actual subject. I hadn't ever heard it said that airflow over the top doesn't travel a farther distance. The theory that I had heard and has since been discarded is that when air flow splits at the stagnation point split the previously adjacent 'packets' have to rejoin each other at the trailing edge. That is the theory that has been disproven, not necessarily the further distance theory.
Yes the first half is meant to provide context, but this is still one of my shorter videos. I've heard and seen the argument that both the longer path and pressure differential are disproved by the upside down argument. Maybe I was too subtle with putting in high and low pressure areas in the animations.
@@LetsGoAviate I've also never heard the argument that lift from an upside-down airfoil disproves the longer path theory. Only the most uninformed person thinks a cambered airfoil is necessary to produce lift, but I guess there are a few around.
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.
Lift is generated by the stagnation point, which is located under the wing due to angle of attack. On-coming air is accelerated upwards to counter downward pressure, that's how pressure is decreased above.
Thank you for this. It seems bizarre that the stagnation point shifts to what seems to be below the tip of the wing, but as you say, that creates a longer path for the top airflow which then gives the wing lift even when flying upside down. Crazy but now it makes sense.
It's just as well you're right. It's scary enough as it is when you find yourself flying upside down for the first time ! Nice video, very well explained.
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.
Wind tunnel tests show that even at angle of attack close to the stall the pressure drop over the wing is much greater than the pressure increase below the wing. Now technically yes, the pressure under then pushes the wing up but really you must consider the differential (the CL x q in the formula Lift = CL x q x S), most of which is derived from the pressure drop in the increased speed of flow over the wing.
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.
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.
You want low pressure on top high pressure on bottom. And slapping the air down, as someone said, the angle of attack (AOA, the angle of direction to the direction of motion*) is best option. However i have noticed that it's a bit of both, with AOA being the main cause. *Further AOA for those that don't know. If you're in perfect orbit. So perfectly level flight direction of motion. If you nose up at 5° and still go the same direction you have 5° AOA.
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?
For anyone who knows anything about fluid flow, they know that the lift is created by the change in momentum from the leading edge of the wing to the trailing edge. This change in momentum is created by the angle of attack deflecting the airflow as it passes the wing. A flat plate can fly. The so called wing effect is to make it more efficient and to achieve what is called the Kutta condition, which is the seamless joining of the air flow on top of the wing with the flow on the bottom. If the upper flow does not rejoin the flow from the bottom, we have separation and the upper flow separates from the wing surface before reaching the rear of the wing. If this condition reaches an extreme, the wing stalls, quits flying, and the plane drops like a rock. The pilot must get the plane's nose down, establish air speed, and then pull up to the proper angle of attack to start "flying" again.
I have a simpler way of putting it. The air will always take the line of least resistance over a wing, and that line is generally towards the centre of a vortex. Upper camber induces a gentle but almost continuous vortex over the upper surface, giving the best lift/drag. In inverted flight, the only significant vortex is at the leading edge, while the flat underside is doing very little, requiring a higher angle of attack to achieve significant lift. It is quite valid to consider the centrifugal forces caused by the curved flow, so we can see that the curvature now below the wing will produce a net pressure drop, reducing efficiency.
Every time the wing send an air molecule down then the wing go up by an equivalent amount. Everything else is just different way the wing can move those air molecules down. Air vortex, deflection, low pressure ect... Name it, It is all moving air molecule down enough to move the wing up. If you look at the flow to see what happen then you can notice that the trailing air molecule are going down relative to the wing. So everything that happens in between is just the way the energy of the molecule that go down is transferred to the wing so that it can go up. I do not know if I simplified too much. But it is pretty much it. Really complicated fluid simulation just help to know exactly how the energy was transferred trough the fluid. But it is basically the good old Newtonian action reaction physics at play.
I have an interesting perspective on this issue. My father was a private pilot, he owned two different planes and was in a club which had a number of planes to fly. One of them was a Cessna 150 Aerobat with the aircraft equivalent of a million miles! A stall is a maneuver pilots train for and perform at higher altitude. Anyhow, this Cessna has aluminum skin and the skin was a bit loose. When we'd approach a stall, you'd hear an oil can noise right before we dropped. It gave me an ability to almost see lift as a low pressure system attached to the top surface of the wing, versus imagining air flow and lift occurring somehow. I'm sure the oil can noise was lift detaching from the skin because every pilot in the club talked about it, and they were a very intelligent group. Other than that, I'm not sure of the physics, but I thought this might be a point of interest, maybe even a data point. Cheers!
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.
@@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.
All airfoils are a compromise in an attempt to improve the lift and drag curves of a flat plate. Since a flat plate is very limited for structural purposes thickness must be added for strength. The classic curves applied to airfoil shapes were derived from educated guesses in an attempt to optimize lift and drag characteristics observed in experimentation. Over time improvements in the understanding of the behavior of airflow over the surface of wings has been much better understood. However, to this day, even the best fluid dynamics computer models are not 100 percent accurate. The simplest answer is that enough air must deflected downward to carry the weight not being offset by any upward thrust angle. Those fluid equations do include Bernoulli’s principles, as well as simple mass deflections caused by the chord line angle of attack. The airfoil shape in turn can mitigate turbulence in the wake thus lowering induced drag and separation bubbles on the upper surface thus increasing usable angle of attack. These are well understood principles today yet the math is still incomplete as the actual airflow is not really a fluid and is chaotic. And, most of this understanding goes out the window as the airflow reaches supersonic.
Side note: Almost all the lift of a wing is created by the shape of the section ahead of maximum thickness. The rest of the airfoil is primarily for controlling induced drag.
@@gilbertgauger3380 Don't you mean "form drag"? My understanding of "induced drag" is the drag due to the work of creating downwash. Which is an inevitable consequence of generating lift, though for a given amount of lift it can be reduced by strategies such as increasing the wingspan.
Crispin Miller, actually both. Typically, though form drag is technically seperate , in the case of a wing the drag is calculated/ measured as the combination of drag forces. For the entire aircraft the non lifting surface area contributes primarily form drag and is calculated accordingly. The absolute simplest calculation is simply frontal area of the plane as a whole but that value can obviously be reduced by strangling the shape. Most airfoil and wing studies compare coefficients of lift and drag and coefficient of drag is both induced and form drag combined. Just convention.
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.
I am a soil and water scientist so don't particularly study wings but I understand fluid dynamics, turbulence, reynolds number, stokes law etc... I had no idea there was controversy over how a wing worked- here's my take which up until now I assumed everyone else believed... get an air compressor and a wing. Blow air directly upwards at the wing. I presume everyone is OK with the idea that a mass of air pressure under the wing should push it up... And if you have just a normal mass of air being pushed under a wing, boundary layer resistance and the skin friction will result in turbulent flow under the wing, in essence bunching up the air underneath- high air pressure. In order to do this you need a concave surface under the wing or an appropriate angle of attack that forces more air under the wing than over it. Similarly the shape of the wing seems to decouple the air from above the wing, creating a vacuum. This is what I thought lift was. Loop I think it's wrong but can't be bothered explaining... High speed would be different because the air essentially becomes more viscous, and the wing would be more to cut a path through a nearly solid medium rather than make lift.
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.
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
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.
I've considered this very issue since I was a teenager making balsawood airplanes in the '70's. Seems to me that both the pressure difference above/below the wing, AND the fact that the angle of incidence of the wing produces a downward deflection of the airflow produce lift. The more interesting question to me is what percentage of the lift is produced through each means. My bet has always been that about 80% of lift is produced through deflection, and only about 20% of lift is produced through the pressure differential on the top and bottom of the wing. But I have no hard data to confirm this suspicion. I knew that symmetrical airfoils worked, that kites work, despite the fact that they have no camber, and that inverted level flight was possible. These all point to the idea that deflection of the airstream provides most of the lift force.
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.
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.
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.
Before watching: As far as I remember there are at least 3 effects that together make airfoil as efficient in producing lift as necessary for planes to fly. Pressure differential is just that hand-wavy explanation that provides couple percent of the lift at most. From memory the biggest part of lift comes from conservation of momentum. And of course the "lower pressure" bit does matter, but just a tiny bit. You can fly upside down, it's just that it will be suboptimal (and other effects will get penalty as well IIRC).
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.
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.
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.
Another measure to look at is the mass of the air deflected downward in the vertical direction in the wake of a passing aircraft. This deflected air will need to have net change in vertical momentum, to counteract the weight of the aircraft. Picture a hypothetical hovering object (vehicle) with a rocket engine mounted at the bottom, pointing downward. In order to hover, this craft will need to expel exhaust gases with volume and mass sufficient to provide an upward change in momentum equal to the craft's weight. If we disregard the horizontal component and take solely the vertical component into account, the air in the wake of a fixed-wing aircraft's passage must have a similar effect. Rather than expelling exhaust mass downward as in the hypothetical vehicle mentioned, the moving aircraft instead deflects air downward, and the reaction force generated by the downward deflected air is Lift. Likewise, for an aircraft to climb, as it passes it needs to deflect a greater amount of air downward per unit time, passing downward momentum to the air in exchange for upward momentum for the aircraft. This thought experiment, involving nothing except the application of Conservation of Momentum (which applies to everything in the universe), allows a person to intuit what a wing actually "does". It might then become intuitively apparent that maintaining laminar flow over the wing's surfaces might be important for other reasons, but the pressure differential between the top and bottom surfaces is not a critical factor in examining how a wing generally functions.
You didn't mention symmetrical wings. Which is better for inverted flight. It's lift coefficient is rather low too. But it has equal performance inverted or not. Lift forces are caused by two phenomena: impulse and pressure differential. Impulse is the AOA driving the air mass downward by deflection, whereas Bernoulli taught us about velocity and pressure differential. The air travelling faster has a lower pressure than that taking a shorter path. Very nice presentation, Sir.
It's the relative differential in air flow velocity that creates the pressure differential from top to bottom of the airfoil. The angle of attack defines the separation of the upper airflow from the lower. Given the right AOA, camber becomes less of a factor. Inverted airfoils can be much less effecient and will cause a change in stall characteristics. But given enough thrust, most airfoils are capable of inverted fight. Length of airflow path contributes to the difference of air velocity, but it is not as critical to generation of lift as does pressure differential.
I learned most of this back when I was between 12 and 14 years old and my Dad was teaching me to fly model airplanes. This was many years before RC flight took off (pun not really intended). The first planes I flew were control line and had flat wings being a simple slab of balsa. My first good (good in my opinion) plane was a Ringmaster from Sterling which had a semi-symmetrical wing and flew quite well upside down with only a slightly nose high attitude. My dad won a bet with an acquaintance who bet him $50 (a lot of money in those days) that he could not fly a 1x12 board which my dad proceeded to do. It didn't fly well but it flew. Big engine for the time, a Fox 59 up front and a large elevator out back. I seem to remember it having about a 3 foot "wing" span. I learned that with a flat bottom wing you could life heavy loads pretty easily but you had to readjust the elevator trim as you increased or decreased speed. You could counteract this with downthrust on the engine but it had to be just right. In my adult years I flew a lot of differently designed planes. Proportional radio controls had become practical by then and one of my many different aircraft was a Senior Telemaster. My Telemaster addressed that problem in a different way by having a lifting horizontal stabilizer. So while this video sounds simplistic to me, I did learn some new stuff especially about the stagnation points. I'd never heard of those before. Also, I've pretty much figured out that propellers, no matter what kind, develop their thrust because they are just very specialized wings. Same for the turbines in a Jet engine or a Turbo Supercharger.
