Longer Path Over Top Irrelevant to Creating Lift? Let's Put This to Bed
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- Опубліковано 31 тра 2024
- There's a dumb argument I see and hear everywhere that goes "If lift is created by the air over the top of the wing following a longer path than the air below the wing, then inverted flight would have been impossible." I've never understood the logic of this argument, but I've also never tried to explain why the argument is stupid, so here goes.
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Chapters:
0:00 The Dumb Argument
0:42 Wider Context that MUST BE UNDERSTOOD
2:01 Wing Loading and Angle of Attack
4:05 Principles of an Airplane Flying Upside-down
6:06 How Lift is Created - Let's Put This to Bed
9:42 Myth busted! (and disclaimer) - Наука та технологія
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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.
The last part was incredibly eye opening, just like a cool math trick
tnx & subbed! :)
buoyancy vs density and all alleged pulling forces negated... simples...
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.
Hi and congratulations on a well viewed video. I have a question for you. My replies on a couple of threads have stopped appearing. Do you know of any reason that would happen? I have experienced the same thing on one other channel. These replies still appear in my comments history as if they exist, but are not visible when I view the thread....
I think the confusion arises from many explanations of how wings work referring to the "special shape" of wings, when that's not what's happening at all. The "special shape" improves drag and possibly stall speeds, but fundamentally a perfectly flat board would work to generate lift as long as it has the right angle. But lots of explanations I've seen imply that the shape is somehow magic, when all it does is optimize the generated drag for a certain angle of attack. It's a skewed teardrop shape.
You're right.
A flat board would generate massive drag at high attack angle because of the massive eddies behind it.
Positive camber wings produce lift at zero angle of attack; flat plates don’t.
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
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.
How about a ramping or camming effect, doesn't that add to making a kite or barn door fly?
"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.
Yep, Biconvex 3.36% airfoil and something like 4 inches thick at the root.
Yep! The F-104 is my favorite example to show it's NOT Bernouli, but NEWTON that provides lift.
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.
@@rsteeb The two are not mutually exclusive.
"Money makes a plane fly"
My first flight instructor :)
Sounds like a cool CFI
@@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 :)
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.
i only clicked on this video looking for YOUR answer, not the one he gave.
well done!
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 um
newtons laws apply always
unless you suggest airplane wings are relativistic
@@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
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'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!
Any model airplane pilot could have told you that.
Laminar flow has its benefits and negatives
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.
@@anthonyb5279 There is no "Psych" portion to a medical exam that deals with neurotic pilots. Sounds good though.
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'
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.
Excellent! And the transverse motion of air towards the wing tip creates vortices.
@@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.
@@daemn42 I wanted you to explain it to viewers, I'd mess it up.
Yeah, equal transit time being false in no way substantiates that the longer path is not directly related to the very fast transit speed that does in fact exist.
How I would attempt to put the pieces logically:
When an asymmetrical solid moves through air, the asymmetry turns the air resulting in a force we call lift. The only distinguishing feature of both asymmetrical camber and angle of attack that results in this force, is the asymmetry itself.
The asymmetry accomplishes this altering of the air's direction, primarily through bending of the flow field caused by more pressure drop on the larger side of the asymmetry.
The pressure drop is produced from a conservation of energy exchange between static and dynamic pressure as a "reaction against scarcity" occurs in response the longer path length obstacle.
That is, as mass flow rate begins to falter from the longer path length imposed, the pressure drops sufficiently to supply the lower density flow at a fast enough speed to maintain the mass flow rate continuity.
This is consistent with the first principles property of conservation of momentum (quantity of motion), as the arbiter of pressure vs speed (conservation of momentum).
The false "equal transit time" fails because it in an incorrect substitute for mass flow continuity (there's an unlimited range of possible transit times associated with an unlimited range of possible mass densities).
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??
'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.
@@dougball328 Airflow across a wing - Cambridge University - Wind tunnel visualisation.
I've read that the mass and velocity of the air shoved downwards provides much of the explanation for lift
@@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!
You make many sound points. However, the oldest, most difficult, but most accurate explation of lift in a subsonic wing is the circulation theory of lift. There are reasonably simple heuristic explanations of this theory, which only require a middle high school level of mathematics. But the full explanation requires some pretty sophistocated math relating to the Kutta-Joukowski theorem. A full explation of this requires an understanding of line integrals, as well as vector and complex analysis. This is the stuff of a second year university engineering course. I have an entire textbook on it. The full story is quite a jump up in complexity and, therefore, not usually taught to pilots and people who are casually interested in all things flying. I can supply references if you're interested. Fun fact, the full explanation of 'how a wing flies' can be formulated with a series of equations that have no explicit or closed form solution. At least, not without making some pretty limiting and factually incorrect simplifying assumptions. Therefore, the only way 'solve' these equations is through computer aided simulations. But these simulations are still only approximations. Therefore, no one can say, for sure, how a specific wing will work till you fix in onto an aeroplane, stick a test pilot in the plane, and fly it! Even windtunnel testing is not quite the same as'the real thing'. Still want to be a test pilot?
As a test pilot, with a MEng degree in Aero Engineering, this comment is the one that I agree with the most. 👌🏻
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.
As an Aeronutical Engineer this is the answer I was looking for. There is a reason it is called lifting "theorems" and not lifting "laws".
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. 👍🏼
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.
If a wing has 3 degrees of Camber (curve) and can fly level, it would be able to fly upside down with 6 degrees angle of attack when upside down.
@@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.
As I recall on a classic aerofoil 20% of the lift comes from high pressure below the wing and 80% from low pressure above the wing
@@EntityWar That's how I learned too. So in this case the airplane rides on lifting force is a misconception, actually the airplane hangs in the air.
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 Actually a rocket thrust uses thrust to overcome gravity. Getting off the ground involves no drag initially because starting at zero speed creates no drag. So, at first the rocket is far surpassing the effect of gravity, but once up to speed, is then of course working against both gravity and drag.
High pressure moving air toward lower pressure, like with a weather system, is exactly how the low pressure created immediately above a wing causes the air even further above at normal pressure to be "bent" downwards. It's like wind moved to weather area of lower pressure.
This connects "Bernoulli's" pressure drop to Newton's 3rd law action-reaction of air forced down.
Increasingly I find myself boiling lift down to asymmetry of the solid object in relation to the (relative) wind.
Your trampoline has plenty lines of asymmetry running through it. Of course a parachute traditionally had symmetry and worked fine for obvious reasons - it was not creating lift but maximizing drag.
I'd assume your trampoline was dancing between a wing's asymmetry and a parachute's symmetry in taking its flight. Between moved by lift and drag.
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.
@@Lozzie74 Bernoulli only helps direct airflow downward. Newton accounts for 100% of the lift (and the drag).
@@rsteeb Wrong about everything. Well done!
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.
Where your logic fails is here:
"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."
It doesn't follow that the shape of the wing doesn't create lift.
The condition of lift depends on asymmetry in relation to the relative wind.
If a wing's shape is asymmetrical, that shape will produce lift.
Angle of attack produces lift for the same reason: asymmetry.
The combined asymmetry contributed from both of the above determines lift.
So, even an asymmetrical wing has a capability to maintain altitude as long as the angle of attack is sufficient to more than make up for the "negative asymmetry" of the shape while flying inverted.
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.
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
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
@@bruceroland5683look at the airflow over that flat wing with a positive AOA, it’s still similar to an airfoil wing
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.
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.
A very good explanation of lift. I think the confusion is that "lift" is a sum of several intertwined forces. You have the traditional Bernoulli forces, You have impact lift, and you have the equal & oposite force of the air being deflected downward relative to the wing. All three combine to create what we call lift.
Good news is that outside of a FAA test you really dont need to care. Only need to know that job # 1 is keeping the air flowing smoothly over the wing. For normal, non aerobatic, flying I teach airspeed is rule #1, #2 & #3. #4 is dont't forget #1 (keep in mind the airplanes I fly do not have an AOA indicator).
Keep up the great video's!!
The Bernoulli pressure drop acts to move mass down from above, as far as the top of the wing. This mass accelerated from the pressure drop is the vast majority of the equal and opposite, Newton's 3rd law contribution to lift. Any "impact lift" associated with the bottom of the wing is a small contributor to overall lift.
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.
What is a qualitative short description of the essence of the "origin of lift" from that book?
@@davetime5234 There is a PDF of the book available on the web so you can take a look for yourself.
@@ColinMill1 Thanks, I was able to find that. But I was also wondering in what sense you meant it "proves that the origin of lift..", implying it provides a valid explanation of lift.
Is there a short qualitative descriptive summary of what this origin of lift is, as described by this book in greater detail, that dispels some of the inaccuracies in the back-and-forth discussions?
In other words, something digestible, but effective and accessible to be used against some of the more questionable claims in these types of discussions?
For example:
When an asymmetrical solid moves through air, the asymmetry turns the air resulting in a force we call lift. The only distinguishing feature of both asymmetrical camber and angle of attack that results in this force, is the asymmetry itself.
The asymmetry accomplishes this altering of the air's direction, primarily through bending of the flow field caused by more pressure drop on the larger side of the asymmetry.
The pressure drop is produced from a conservation of energy exchange between static and dynamic pressure as a "reaction against scarcity" occurs in response the longer path length obstacle.
That is, as mass flow rate begins to falter from the longer path length imposed, the pressure drops sufficiently to supply the lower density flow at a fast enough speed to maintain the mass flow rate continuity.
This is consistent with the first principles property of conservation of momentum (quantity of motion), as the arbiter of pressure vs speed (conservation of momentum).
The false "equal transit time" fails because it is an incorrect substitute for mass flow continuity (there's an unlimited range of transit times associated with an unlimited range of mass densities).
@@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.
It's got almost nothing to do with the "longer path". Thought that this old chestnut 🌰 had been put to bed a long time ago.
Argument from incredulity.
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 Nice effort explaining way better than my weak attempts. Thanks 👍
@@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.
It has been. The rise of disinformation is an attempt to destabilize the west, like moon landing deniers and flat earthers.
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.
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.
Right, but easier to state in different terms...@@anthonyb5279
Still no.
Wings can generate lift with no change of momentum of the air.
@@Talon19 Interesting. Please explain how that would work.
@@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 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.
You put it more eloquently than I do
"sucked" how?
@@perh8258
Fluids move from higher pressure areas to lower pressure areas.
maybe 'pushed' is more accurate?
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.
"Let's Put This to Bed". Not likely, even though your reasoning is clear, well presented and factual without loosing the dominantly non-math audience (truly an accomplishment!). I hope stirring the hornet's nest has been more productive, even though we live in a time of unusually strong affiliation bonds. It is culturally more attractive at the moment to bond with fellow idiots than to challenge one's own cherished beliefs. I'm not picking on idiots, because we are all relative idiots compared to the incomprehensible volume of understanding possible. It is like we are still biologically limited cavemen, yet we are trying to understand the whole of reality.
I have always wondered about this. I teach young people about flying, so I will incorporate this lesson into my own. Thank you.
There is also newtons law effecting it, the air defected downwards by the wing angle of attack also gives lift
But it is minuscule.
@@tedmoss So you can pull/push on the air with your wing... and it doesn't move??
@@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.
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.
An rc plane is still a full size aircraft just smaller .
@@tonywright8294 in terms of weight, size matters !!!
@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.
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.
What textbooks or other sources have perpetuated your statement?:
"the Bernoulli principle applied to lift was only a small percentage"
As far as I recall, most sources have accounted for lift by leaning most heavily on Bernoulli.
Where did it ever say only a small percentage?
@@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.
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.
@@bernardedwards8461that sounds really cool :)
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.
@@j14152 which model aircraft did you design? i'm working on my own, too
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...
Well both are causes for lift however, the pressure differential is the primary cause. At least that is what the PHAK, AIM, and AFH say.
@@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 think it has more to do with the wing pushing the air down creating lift. the round shape of the wing changes the vectors of the air passing over the wing redirecting the air downwards. changes at higher speeds though right?
Newtons laws of motion say that you do not get something for nothing. The airfoil uses an angle of attack to exert a force on the airstream which alters the direction of that airstream downwards. That redirection results in an equal and opposite force upwards which we call "lift."
Very low speed airfoils, such as a sail, have exactly the same length on each side, yet they produce the required lift. Low angle of attack hydrofoils that are commonly found on sailboats also produce the required lift, but are nothing more than 1/4 inch thick flat metal plates.
Bernoulli does not explain these situations, but Newtons laws of motion work very nicely.
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.
Wings can still produce lift even with no downward movement of air.
And if they do not believe this, pick up a piece of plywood in a forty MPH wind and see what happens !!!!
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.
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.
When I was learning to fly my instructor at some point told me that the old saying was "That you could use the barn doors for wings As long as you have the right angle of attack. @InternetStudiesGuy also has it right "A perfectly flat board would work to generate lift as long as it has the right angle". Here in this video this guy only references one wing shape with a flat bottom. But there are several types of wing shapes. Two being the Symmetrical Biconvex, were the curve shape is the same top and bottom and the Asymmetrical Biconvex that is the same but thinner. The Asymmetrical Biconvex is also a laminar flow wing. During WWII they developed the laminar flow wing on the P-51 mainly for speed and to reduce what was then known as Compression or Mach tuck. Compression or wing compression, in simple terms, is where that thick leading edge on a wing at speeds approaching the speed of sound causes the air to build up, compress in front of the wing creating a shock wave that disrupts the air over the wing and control surfaces, making it impossible to pull out of a dive. So basically they moved the leading edge of the wing farther back to reduce drag of a thick leading edge and angled the wings to help prevent the compression. In the 1950's and 60's experiments with wing shape continued and wings got thinner on high speed aircraft like fighters. I spent years in the USAF and have seen fighter aircraft with practically knife edge leading edges and the wings only thicken up slightly in the middle of the wing. The F-104 Star Fighter is a good example. The wings had probably the thinnest wings and close to a knife edge leading edge. The wing was so thin it couldn't carry fuel in the wings. The F-16 is another close example, the leading edge is not as thin as the F-104 and slightly thicker. This allows it to carry some fuel in the wings but not much. Almost every section in the fusaluge that can hold fuel is a fuel tank and needs the external tanks to fly any long distance. But that thin wing makes it fast.
So almost any aircraft can fly upside down as long as the angle of attack of the wing is right and the wing is capable of the loading. But in a way this video doesn't explain Symmetrical Biconvex or Asymmetrical Biconvex creates lift. Or the many other wing shapes like are shown here: web.eng.fiu.edu/allstar/Wing31.htm
IT would take up more room then is possible here.
In my study aeronautical engineering, we tested a cambered aerofoil in the windtunnel. It had holes in it to measure the static pressure with liquid filled tubes.
Afterwards we'd calculate the force applied by the pressure differential so weve learned it adds up and why it does.
Everyone who says it doesnt, didnt do this test, and is subject to its own limited thinking.
I wish there were more comments like this.
It's so simple. The wing must accelerate air downwards to produce lift. The airfoil shape makes the process more efficient. End of story'
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.
Absolutely right, and it is perfectly established, in aerodynamic theory by Kutta-Joukowsky theorem, since 1906
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
@@daffidavit www1.grc.nasa.gov/beginners-guide-to-aeronautics/venturi-theory/#:~:text=It%20neglects%20the%20shape%20of,lift%20generated%20by%20an%20airfoil.
Nope
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
I have seen enough conflicting 'theory' around how lift is generated that i have reached my own conclusion:
We don't actually know EXACTLY what is going, but we know enough to create wings the work, and work reliably enough, that we have flight, in various from from kites, to hang gliders to state of the art jets.
If two people build identical wings that work, one can argue Bernoulli, the other can say nah, it is all Newton, and at the end of the day both can fly off into the great yonder.
But yet both require Bernoulli and Newton (3rd law) to explain lift. If you only use Newton's 3rd law to explain, then you fail to explain what causes that downward momentum of air. If you only use Bernoulli, you fail to explain how the pressure differnce is able to hold the aircraft above the ground.
A friend of mine built an RC plane with a flat wing and I was surprised at how well it flew. Thanks for your explanation as it helps me better understand why it works.
Retired now, but I designed aircraft for a career. You're the first guy I've seen on UA-cam 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
@@AussieSteve1984 Lift is the result of asymmetry of a solid in relation to the relative airflow. Asymmetry turns the airflow, and lift is the resulting reaction to turning the airflow.
The asymmetry can be from asymmetry of shape, angle of attack or a combination of both.
Bernoulli and Newton, in the usual sense discussed, are both implicated equally.
100% of the lift is accounted for by the Bernoulli pressure drop, and 100% is also accounted for by the Newton's 3rd law vertical momentum change given to the relative wind. (the lesser pressure increase at the bottom is being ignored for the moment but can be easily folded in after this always misunderstood point is grasped)
The pressure drop causes the mass displacement downwards.
The pressure drop is a conservation of energy transaction as the airflow around the top maintains its momentum as it confronts the path obstacle.
Continuity of mass flow rate must be maintained, not transit time (that's why "equal transit time" is false - the increase in speed is related to the density of mass not the distance traveled).
The inescapable primary importance of path length is its relationship to the geometry of the asymmetry that is the foundation of lift production.
@@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 :)
Flat section models fly fine with just a few degrees of incidence. Baby rog's for example were quite popular in 20's and 30's.
And chambered wings produce some lift even at ZERO degrees angle of attack.
Thanks for your effort to explain lift. The visuals were great. As a youth I built several combat Control-line models with asymetric wing profiles and they flew great upside down.
Very well done! I LOVE the sarcastic, distorted, dramatic fun you had poking fun at the ridiculous, unexplainable descriptions of why lift doesn't work the way it does. Haha. Nice, simple use of editing to create a cheap way to poke at cheap answers that are wrong. 😂
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.
You have an aerobatic rating on your pilot's license?
@@eurekamoe3744 yes, it's printed upside down
@@skyboy1956 OK yes. So your so called "aerobatic rated pilot" is total BS.
Thank you, I learned about stagnation point. I also didn't know how planes could fly upside down, but this video really cleared things up
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.
You have made an excellent video here! PMaybe you can please make a video explaining how the use of rudder alone in flight induces roll in the same direction? I'm certain it's not because the "outer" wing is passing through more air and is therefore producing more lift and thereby creating a roll effect. I think it's caused by the dihedral creating a roll coupling by presenting the "outer wing" at a greater AoA and the "inner wing at a reduced AoA am I right? I don't know if there is any roll produced in an aircraft with no dihedral (presumably not) or if indeed adverse roll is induced in an aircraft with anhedral (inverted flying?)
A Cessna 172 with no dihedral will eventually drop the right wing with sustained right rudder in a skidding turn. However, depending on the plane, a rapid right rudder can produce some amount of left roll since the rudder is usually above the center of gravity. This dynamic effect is momentary, and eventually there is right roll as above.
I thought this would be the case. I don't like comparing RC planes to full size aircraft too often but I have had a model that would do complete rolls on rudder only - the opposite way to the input! Adverse rudder roll was a thing with that plane. It had a large rudder with huge throws and a lot of area forward of rhe hinge line at the top of the rudder. No dihedral in the wing either. Was a hoot to fly 🤣
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 UA-cam video to deal with this question. Great video.
More pressure under wing than above creates lift. That simple. Angle of attack, airspeed and wing shape all contribute.
Sorry but what your school taught you is wrong
@@ninjalectualx Well, then, what is right? (true)
Contrary to the video, Bernoulli's theorem is essentially the correct way to think about lift. This has been thrashed to death because student pilots are taught Benoulli's theorem is the primary cause of lift while ignoring kite effect, that is., a flat sheet of plywood will also create lift due to the the air pressure differential top an bottom, just not efficiently. That's the purpose of airfoil shape, to optimize airflow around the wing for the performance envelope of a particular aircraft. Like a sheet of plywood, a typical plane can fly inverted just not very efficiently. The longer path along the top of the wing effectively creates a positive angle of attack, with equal distance being above the center of the leading edge.
As a practical matter, if you are learning to fly remember the old pilot's rule: throttle controls altitude, pitch controls speed. A wing generates more lift with more speed without changing pitch.
Its schools students that were taught Bernoulli's principle as the cause for lift, not pilots.
@@kennethferland5579 It's part of every ground school curriculum and there are related questions on lift on the private pilot exam. As for the video, the guy opens with two images of airflow over a wing, normal and inverted He bases his argument on the second image with the wing inverted. Whether he did it intentionally or just does not know the subject, the 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.
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.
Your explanation makes perfect sense - thank you. Stagnation point for the win.
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.
But the rearwards force is A LOT bigger than the upwards force.
So? The area of my hand is miniscule compared to the total area of the underside of a wing.
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.
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...
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
At the stall air flow curls under the wing off the trailing edge, can be seen in wind tunnels..
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.
Glad you explained something that seems so intuitive to most. Ask the keyboard geniuses why a box fan falls backwards when it is turned up to high speed. Aren't the wings just a version of the blades on that box fan (a propeller), just moving through air much slower? I worry about our future but at least we have a chance to get some people to start thinking instead of just looking for the easy way out by by simply repeating what they've read or heard on the internet.
Aerobatic aircraft, such as the Decathlon, have a symmetrical airfoil that is designed to produce lift both upside down and right side up.
Angle of attack is the biggest differential on the theory of lift of an aircraft whether inverted or not .
Most people don’t realise that when a plane is flying “ level “ it is still at about a 3-5 degree angle of attack .
Cheers 🍻 , Woody NZ 🇳🇿 .
Yeah wings are rarely mounted 100% "level", with the OV-10 Bronco as an extreme example. Some jets and some aerobatic aircraft would have 0° AoA when level though. Cheers
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.
How do paper planes produce lift?
Ah, the pompous priesthood emerges.
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 ua-cam.com/video/6UlsArvbTeo/v-deo.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
@@Alec72HD yep!
Great explanation of how some planes can fly upside down. Thanks.
all planes can fly upside down.
Excellent points, great video. I used to teach aerobatics in the Bellanca Citabria and the Bellanca Decathlon. The Decathalon had a symmetrical airfoil and you could sustain inverted flight with two people on board. It was nearly impossible to sustain in inverted flight in the Citabria with only one person on board. Plus the decathlon head an inverted oil system.
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
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.
@@thearmouredpenguin7148 And how aero-engine designers thought about propellor thrust.
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.
@@LetsGoAviate Wing shape is what makes wings efficient and allows light aircraft to fly with small engines.
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.
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.
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.
In my simple mind, it seems pretty simple to me. It's all about angle of attack. Air pushes on the underside of the of the wing raises pressure causing a thrust vector which has a lift component and a drag component. The air going over the top lowers pressure adding to the net positive pressure increasing lift and I think reduces drag a bit but drag is still a net positive. Just imagine a flat wing with no curve at all. It will still generate lift as long as there is a positive angle of attack. In my childhood I made balsa rubber band powered model aircraft where the wing tail and fin were all flat sheets of balsa about 1mm thick. With the right balance to get a positive angle of attack, it flew nicely. As I see it, airfoils are just ways to improve efficiency but not the primary cause of lift.
Excellent discussion topic. I always think about this when as a child I would stick my hand out of the window in the car and play with lift and angle of attack, varying the curvature of my hand to increase or decrease lift, or keep it flat and just change the angle of attack. Flat wings can fly too, they just need an angle of attack.
I'm not a pilot but rather a science teacher but I did fly hang gliders for a long time and now sail a lot which both involve "wings" with a camber. I share your frustration as lifit is obviously created simply by both diverting a fluid flow and a pressure differential above and below the camber. A simple flat board held at any angle other than parallel to a fluid flowing will cause a force on that board i.e. angle of attack but different cambers also generate different amounts of lift and the total lift is the combination of the two. If the plane is upside down still carries it's own weight then obviously the downward camber lift is smaller than the upward deflection lift. The "lift" from deflection is simple vector motion where force in the horizontal axis can be transfered to force in the vertical axis and this is where you can use trigonometry and coefficients of friction etc to estimate that. Seems pretty straight forward at a high physical level to me? Missiles with rocket engines have no cambered wing and just have flat stabilising fins and they "fly" just fine without any so called "aerodynmic lift". Fighter jets also use "vectored thrust" to alter the force vector of the plane which is a moveable nozzle on the jet engine and they also have very flat wings which could not stay stable without modern software control and extremely powerful thrust engines. This misunderstanding is caused by over simplified explainations i.e. lift is caused by a pressure differential about the wing caused by a longer path over the wing ie. camber - but like many "lay" explainations, it is only a very small part of explaination. People have written books about science and engineering misunderstandings but that's what you get when people think they know everything when in fact they are 99% ignorant and simply don't know what they don't know while smart people always are aware of what they don't know which unfortunately leads to ignorant people being very confident and smarter people doubtful about their own kwowledge and understanding. Such is life.
No, you've just explained that theory to a complete noob (but I played "flight simulators" in like 90s as a kid)! Congratulations, and thanks, I've learned something new.
You're looking at a consequence thinking it's the only reason it happens in the first place.
Lift is produced by displacing the mass of air downwards. Canver is used to mantain the flow attached so that it flows down too.
In the bottom of the wing, you're pushing air down against a volume of air, which oposes a resistance, increasing pressure and reducing its speed. At the top you're pulling air down, creating a vacuum which draws air in and thus accelerates the flow. The stagnation point is in the point from where the flow at the top and the bottom have equal energy, it is equally costly for a particle in the stagnation point to do a longer path that is pulling air in, or going the shorter path that is pushing air away.
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.
I built a 3-foot model glider out of balsa for a school project. I fitted the main wing correctly but I glued the tail on backwards, so that the aerofoil was backwards with the "sharp" edge forward. It flew beautifully. I didn't realise it was backwards until teacher commented. He acknowledged that it flew very well.
In the end (no matter what is the cause for this fact...) lift is created, due to the air being "deflected" downwards. So both theories are right. If the path "over" the wing is longer than the path "under", that forces the air to a "downward" motion, whitch creates the lift. You can't have one without the other. :-)
For me it is still easier to think of a downwards directed airflow. (Though I know, that this forces the path "over" the wing to be longer than the path "under" the wing...)
Yes, air HAS to be accelerated downwards to supply an equal and opposite force to the weight of the aircraft. It seems the argument is more about HOW the wing supplies this force. Here is my take on this.
The momentum of air molecules deflected downwards off the bottom surface pushing the wing up, is easy to imagine.
The air above the wing thins out and speeds up as it covers a longer distance in the same time. This causes the higher atmospheric pressure air some distance above the wing to move downwards to fill in the thinned out "vacuum" - in effect, pulling the air downwards (downwash) and creating an opposite lifting (sucking) force on top of the wing.
Combine both the down wash from the bottom and top, and Newton third does the heavy lifting.
One of the best videos on the subject I've seen. Love your style of teaching!
__
Very helpful, thank-you. I always figured an upside-down plane was just throttling more to fly at a more aggressive angle of attack since presumably you could fly a plane very inefficiently but still airborne with just a flat plane for a wing. But now that leading stagnation point moving towards the planet opens an interesting idea of a long path. A long path that is longer for being closer to, hmm, the planet. I'll watch more videos. Another thing I'm hazy on is that it occurs to me that a stalling wing with a separating boundary layer is metaphorically (at least) similar to a functioning wing because both have fewer air molecules topside. That's when I got very hazy aeronautically but I liked the poetry.
Just gotta have more air pressure under than over, and - wow - you have a lift vector. However you achieve that, it's not just one variable (wing shape), it's many variables. To ignore all but one variable is ignorant at best and motivated thinking at worst.
It's amazing how hard rational thought is for so many people.
Air is like an elastic band, can be compressed and stretched. Its this stretching of the air over the upper curve of a wing that equates to lower pressure compared with the subtly compressed air under a wing. Stretch air too much and it separates resulting in a stall.
Thank you for a very good explanation of lift. All the math calculating the amount of lift and drag are for the aerodynamicists to design the best airfoil for a given mission. But all the math addresses design theory, not lift theory, something pilots do not need to be that concerned with. Either the wing and plane will fly, or it won't. The math only confuses most of us that are actually piloting the plane, while it is critical to the designer.
Maybe at least this math would be good for the pilot to understand, that lift is F = A x P = d(mv)/dt.
Which means, the force of lift is equal to both the force of the pressure difference (Bernoulli) and to the momentum change given in forcing mass of air down (Newton's 3rd law). One is a consequence of the other (they are not separate equivalent explanations).
This leads the way to avoiding some of the illogical thinking. It also forces us to think about what downwash really means and moves us closer to what the complex Navier-Stokes equations are actually saying about the behavior of physical reality.
You have the right principle but make it more complex than is required. Simply, a wing has two major mechanisms that generate the lift: the shape of the wing and the angle attack. A good aerofoil shape generates lift with low drag, even at zero degrees of attack. The second mechanism, angle of attack generates lift, but with much higher drag. So when the wing is upside-down, the aerofoil shape generates lift in a downward direction. To create a nett total upward lift, the angle of attack must be high enough to create enough upward lift to overcome gravity and the downward lift of the aerofoil, but this is at the expense of very high drag. Most people seem to get stuck by believing that lift can only be generated by one mechanism.
A moving stagnation point explains why the leading edge of the wing is not sharp.
Very interesting. 5:34 I noticed the still shot drawing shows plane's nose is tilted up a bit. This change the wing's angle of attack. Now the wing os pushed up by the fatter, followed by a laminar flow as air passes the hump and roils across the rest of the wing.
The wings are also tilted up like the nose. That means air is still passing over the full wing, creating low pressure vortexes, and lift. On the downward side of the wing, the hump at the front splits the air in teo with thr top flat side creating low pressure across the width of the wing. A high air pressure point is created at the leading edge of the wing. This creates inverted lift since the air pressure is lower on the up pointing width creating low pressure initially is provided mechanical lift. The air ccreated low pressure vortices on the up side and high pressure vortices on the down side as a result of the up tilt on the wing.
The result is both sides of the wing provide lift, keeping the upside down plane the same lift components as upside up wing. Slightly more advance force is needed to keep the nose tilted up.
The upside down wing is a key feature in F1 cars. It creates the low pressure zone under the car, and high pressure over the top. The greater the downforce, the better tyres grip the road surface. This improves handling in the corners but drag on the straightaways, which can be reduced by opening the DRS flap on the topside wing at the test by reducing the downward force on the car. Watch any F1 race practice sessions, qualifying, and GP and it's easy to see who got the wings right, and who didn't.
I've been planning to build myself a tiny private jet.
I follow a whole lot of aviation and aerodynamics channels to [hopefully] find rare, hidden gems of insight and knowledge.
As a former USAF A&P mechanic, an employee of a U.S. contractor for Rolls Royce, and the daughter of a USAF A&P mechanic/radar technician, I have been exposed to aviation literally all my life.
Some day, hopefully I shall have my own tiny experimental single seater jet.
I recall when my brother asked me this question, which I had not thought of before. My answer seems to be understood well by non-technical people, and it takes less than the ten minutes that you took.
If gravity is pulling on you and you are attached to nothing, and you do not want to go in the direction that gravity is trying to pull you , then you need to send something else in that direction , instead of you. This creates the "equal and opposite force" on you to keep you up. This applies to rockets and wings. In the case of a wing, it sends air down, instead of you. The airfoil shape seems to be the most efficient way (less drag per lift, therefore more speed per horsepower). No wing optimized for flying one way will be as efficient if it is turned upside down; so I postulate that no airplane can fly as fast upside down as upside up.
It can also be explained in terms of air pressure, on a molecular level. The random motion of air molecules colliding with the wing surface creates pressure. Somehow, the wing shape and motion need to combine so that the total force of all the molecules colliding with the top of the wing needs to be less than the total force on the bottom. I have imagined a circular airfoil (like a UFO) that could use a small, jet engine with the intake coming from the underside of the rim and the outlet directed peripherally from the center, topside along the upper side. The upper side could be partially ducted with a steering vane in the duct for directional control. A trim tab on the trailing edge could provide pitch control (like a helicopter) to translate the lift into forward motion. It may not end up being efficient or practical but it would be really cool to show up with at a remote-controlled flying convention!
A totally symmetrical wing and airplane could. Not practical for manned flight, but could be tested with RC aircraft or gliding bombs.
An inverted airfoil is actually recommended for stability on all-wing gliders. In this case, the glider is trimmed so the upside-down airfoil is at an angle of attack to the relative wind.
Good information and a rational explanation, but PLEASE drop the sound effects!
RATIONAL doesn't equal CORRECT.
This was actually mentioned by Wolfgang Langewiesche back in 1944 in his book _Stick & Rudder_
if they insist that the longer path over top has nothing to do with generating lift. when upside down a non symetrical wing still gains lift by sending the airflow over the side which is uppermost on a longer path due to angle of attack.
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.
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?
Maybe I'm fooling myself, at age 65, that I can hope for anything more than to see such amazing events that show a perspective of life few will see and fewer to live and tell. I'm fascinated and amazed at what is to come as well as frightened. It will be the ultimate test of my character and all I've valued in my life. I cannot help but try to prepare as best as possible: That is my nature. How this story ends is a mystery... maybe THE mystery that will be the ultimate lesson of life. Eyes open... No Fear... maybe even a smile
I remember the original Lifting Bodies, which were basically upside down wing body yet they produced lift. Also remember reading Air Force studies on Waveriders which is an upside down "V" or "W" (for stability) and used "Compression Lift", could do high angles of attack.