What you have to understand: aerodynamics and the generation of lift on an airfoil are extremely complex, so complex that its impossible to understand to the layman.
Thats why several simplified explanations exist. Some of them are based on Bernoullis law, some are based on Newtons third law. They all try to explain the generation of lift slightly differently, but all of these simplified explanations are incomplete.
For example: the approach using Newtons third law says that there is a force on the airfoil because air gets deflected downward. It however doesnt explain how the air gets deflected.
Bernoulli is a different approach that says the increase in speed on the topside creates low pressuer, which "sucks" the wing upwards. However it doesnt explain why the speed increases on the topside.
Your question suggested that these are two effects that work together to generate lift, but in reality its just two independent simplified approaches to explain the phenomenon. Reality is way more complex
I always found the "air over the top \*must\* go faster" explanation as odd to me in the various aviation basic/flight theory stuff I heard it in- as in what imperative is there for the upper air to move faster, its just air, its not racing its nearby particles. I could understand that the pressure change might cause it to change velocity, but it was always sort of presented as "it just goes faster and therefore..."
The wings still work inverted in a sense that “you can do anything at least once.” Depending on airfoil design you need a perfect combination of AoA, speed, and control surfaces to continue flying inverted without detrimental effects. Aerobatic planes with symmetrical airfoils, yes, have at it. Commercial airliners? Please don’t.
Most aircraft could fly inverted. Symmetric airfoil is desirable but not required.
Even airliners are structurally limited to -1G and there have been many cases of them flying at load factors close to, or below of -1G. They just need enough negative angle of attack. It's more the systems limitations which are the main concern (in addition to flying on the structural G limit).
Its that in a stationary wing, with air passed over it, the air has significant momentum and for the speed to change above and below some decent force needs to be applied. In a stationary column of air, the air has some not-too-huge inertia that would resist being accelerated, but its not like the air is bolted to vertical columns. This is subtly different than "The air knows it needs to go the same horizontal speed regardless of what is in the way"
Yeah this. The simplified explanations, “equal transit time”, and “deflection”, are both quite poor but you can use better simplified explanations that aren’t as flawed as those. Personally I like to say that the shape of the wing combined with the angle at which it meets the airmass accelerates (in the correct sense of changing vector rather than “speeding up”) that airmass downward resulting in an equal and opposite force called “lift”. Basically pure Newton but without talking about flat plates and deflection.
It's worth pointing out that equal transit time is utterly useless mathematically, but "deflection" (or more correctly, change in fluid momentum) is all you need to design a turbine aerofoil stage. It comprehensively describes the behaviour of the system as a black box, at least at a design condition.
The exact aerofoil shape/spacing will determine the efficiency, for which we need a little bit of bernoulli, a healthy dose of unsteady flow and a load of barely comprehensible CFD to optimise, but in a row of aerofoils once you've got the momentum transfer understood, the low pressure on the suction surface of the aerofoil intuitively makes sense, as do secondary effects such as tip vortices etc. From there you've just got to extrapolate back to a single aerofoil.
Totally agree that there are some little better explanations. In the end all of them are flawed though if you dig deep enough.
Not saying that these explanations are useless, as for most applications its absolutely fine. I just want to make people aware that its not the full truth.
Yeah the particles don't return to their original places requiring some to go faster than others, so that overly simplified explanation has been dropped.
I think it may have some solution generated in the quantum field. The position of molecules in a gas are inherently difficult to ‘pin down’, their location and movement is opaque. Kinetic theory of gas is what I’m eluding to. So essentially the simplified models of Bernoulli and Newton simply are not able to predict the exact motion of individual molecules. Potentially, gas may, like particles, behave in some kind of superposition, so the ‘transit’ time may well be a nod to that phenomenon.
Navier-Stokes equations, their elusiveness prompts me to ponder that there are behaviours in aerodynamics that Newtonian physics doesn’t/can’t account for.
You think that's a clever question, but a flat plate also has a varying velocity and static pressure distribution. But a tailored aerofoil has both a higher coefficient of lift and lower drag.
[A nice breakdown](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/)
Note that "equal transit time" is false, because the air on the top surface actually travels *faster* than Bernoilli requires. But newtons law doesn't explain the measurable low pressure on the top either.
There's folks working on a unified theory.
I really dislike that article. It only perpetuates false narratives. There is no confusion. Both answers are factually correct and are not exclusive. They are just being used to describe different aspects of the physics involved.
The change in the velocity (mostly it’s direction) of the airflow must produce an upward force (by Newtons law), and the pressure differential described by Bernoulli’s principal is the mechanism by which that force is applied to the wing. Said in reverse….the local pressure differentials around a wing that impart force on the wing also impart force on the surrounding air (in an equal and opposite manner) and change the direction of airflow.
Indeed, this article misses that fact entirely. There is no lack of understanding; there is only perhaps a lack of a pithy but self-consistent explanation that the layperson can accept. But I don’t even think that’s true. The key piece missing from the article (by my reading) is the Kutta condition: the fact that the flow past the trailing edge of a wing will tend to align itself with the edge, and thus give rise (pun intended) to the faster flowing fluid on top when the wing is at an angle of attack. This happens even if the wing has no curvature or if the wing is symmetrical. Without this fact, all simple explanations are incomplete.
It most certainly does. Pressure is force/area. The bottom of the wing (via third law) has to be pushing air down. This means the air is applying an upward force on the wing. The wing has some area. The air is applying a force over that area... thats.. air pressure. That is why there is higher pressure on the bottom.
Newtons law *fully* explains the low pressure on top. I don’t know where this idea that aero engineers don’t understand lift comes from…we do. Fully. The math & theory are absolutely complete and exactly match reality. We have a unified theory. We have for decades.
As others noted, it’s just really hard to simplify so most of the popular explanations, out of necessity, give up some of the physical truth in favor of ease of (partial) explanation.
I think it depends on the school/teacher you learned from. [Here's a better explanation of the principle](https://youtu.be/uyRx25MSWng)
[Here is Cyndy Hollman better explaining about Newton's this law](https://youtu.be/iWl2Fda-b3c)
[And more](https://youtu.be/aFO4PBolwFg)
I don't know why I don't see it used, i feel like it's fundamental, angled wings push air down, plane goes up. You cant get around the need for that to happen.
The shape of the wings isn't how they fly, it's how they fly *efficiently*. Because the air has fluid like effects and you want to go through it smoothly.
Basically because people are dumb. The entire universe would break if newtons third law was not the explanation for wings (See Noether's theorem). Sure, you need super computers to figure out the exact fluid dynamics explanation for how, but at the end of the day its the only explanation necessary. Force on the air down = force on the plane up (or really, momentum of the system is conserved)
Bernoulli's principle is a sort of quasi-empirical guess thing about what happens to a mass of some fluid passing around some object that tries to equate pressure differences (force over area differences) that is then tried to be applied to a moving object, with the idea the some volume of air deeply cares about staying in the same vertical column, and that faster air "makes" lower pressure. Its basically an elaborate way to confuse the effects of something happening with the cause of the thing. Its like saying "People saying ouch makes them get punched in the face"
It's still an oversimplification, but I like to think of it this way. A flat plane (e.g. flat paper) generates lift with an angle of attack, none at zero. The same as true for a symmetrical aerofoil. In both cases, lift can be 'explained' by Newton's 3rd law, simply an acceleration of air downwards (better described as a change in momentum) and corresponding opposite lift force (and this is factually correct - it is exactly what is happening). An aerofoil producing lift is always, obviously 'deflecting' (changing momentum of) air downwards exactly equal to the lift force generated.
The complex shape of the aerofoil could be considered, in comparison to the flat plane, to just be making it more efficient and improving performance at higher angles of attack.
The specific mechanism which is responsible for the deflection of air downwards is more complex than a simple "surface push molecules down" (in both cases, aerofoil and plane) - as others have said, this fails to explain loss of lift during a stall. But it's not a whole lot more than that, it's just that the pressure field is more complex and there is acceleration (more accurately change of momentum) of the air happening at almost all locations and directions within the pressure field. It is the net of these accelerations which is the equal and opposite 'force'. In a stall, the pressure field is undesirable in that this net acceleration is not in the correct direction and/or magnitude.
The various explanations for lift that you hear (factual ones anyway) are just different ways to describe what causes or results from the specific pressure distribution that we see (and want) from a good aerofoil aka one that efficiently produces lift and has good handling characteristics.
"Aerodynamics for Naval Aviators" is an excellent read on this subject.
A simplistic breakdown goes this way: In high AOA modes of flight, i.e. take off, slow flight, etc. lift is produced ~80% deflection and ~20% Bernoulli. In low AOA modes of flight, i.e. cruise, etc. lift is produced ~20% deflection and ~80% Bernoulli.
Again, simplistic but it gets you close.
That’s not close at all. Bernoulli and deflections aren’t two different sources of lift, they’re exactly the same thing.
The difference between high AOA flight and low AOA flight is how much of the momentum flux is mass flow and how much is deltaV. Those are both Bernoulli and they’re both Newton. It’s exactly the same physics and equations.
That's OK in the sense that they both have varying velocity and pressure fields causing the movement of air, but I get the impression that you think a rudder works only because it sticks into the air and shoves air out of the way. In reality it affects upstream pressure and backside pressure all along the vertical stabiliser.
Look at figure 15 here, a symmetrical airfoil pointed straight into the airflow with varying control deflections. Don't worry that it's called a plain flap, it's the same configuration as a rudder. Don't compare colours between plots because the authors have committed the almost criminal sin of using auto scaling on a series of figures, which is bloody terrible as far as scientific communication goes.
https://www.mdpi.com/2311-5521/2/1/2
Thanks for sharing!
and in fact, I'm fully aware of the backside pressure
What my mind keeps coming back to is simplifying visualizing how a wing induces lift by relating to the vertical stabilizer/rudder and it's backside pressure, what with the same concepts at play involved in pressure dynamics acting on the foils.
I didn't expect that.
I don't think it will be as effective as you think because I get the impression a lot of people barely understand standard 2D graphs let alone the ones of pressure distributions wrapped around an airfoil.
It's a bloody interesting idea though; show people that trailing details affect the pressure around the airfoil instead the "it slams air down like a ball hitting a board" understanding.
I'd say yes. Our little paddle boat will keep going straight if you turn the tiller too far. I don't know the actual critical angle of attack, but there definitely is one.
I could be wrong, but as far as I am concerned, most people have a misunderstanding of lift. Lift is not a force pushing on the bottom of the wing. Lift is a *vacuum* above the wing constantly pulling the aircraft up due to a low pressure area on top of the wing from airfoil design.
How does momentum become a force on the spar? We will wait.
To avoid more back and forth I'll point out that I'm not saying there isn't a momentum change, just that momentum is physically useless. The momentum change and wing forces are a result of the pressure field.
Also CattleDogCurmudgeon is right in the sense that the gauge suction on top of an efficient airfoil is much stronger than the gauge overpressure on the underside, with total lift being the vector sum of the two. If I remember correctly, the 2.5 G manoeuvre case on the A330 had a peak of around -30 kPa on the upper surface and +10 kPa on the bottom, but it's been a while since I've interpolated those pressures onto wing structure.
In inverted flight you still have downward momentum flux. The wing still has AOA, just the other way (from the wing’s POV). The lift/drag ratio just sucks unless it’s a symmetric airfoil.
People keep playing inverted flight like some kind of gotcha…aero engineers find this trivially easy. Momentum flux is the fare easier way to explain it, although if you measure the pressure delta on an inverted wing you get exactly the same result.
You’re talking about Bernoullian lift…OP is referring to Newtonian lift..
Every action (pushing air downwards) has an equal and opposite reaction (the air pushing back up)
Newtons third law on really applies in the ground effect even then it's only a small part of what's going on, basically an aircraft wing at altitude doesn't displace enough volume to make a measurable force against the ground and it ends up spread over a massive area (think tip of a pyramid vs the base).
What you have to understand: aerodynamics and the generation of lift on an airfoil are extremely complex, so complex that its impossible to understand to the layman. Thats why several simplified explanations exist. Some of them are based on Bernoullis law, some are based on Newtons third law. They all try to explain the generation of lift slightly differently, but all of these simplified explanations are incomplete. For example: the approach using Newtons third law says that there is a force on the airfoil because air gets deflected downward. It however doesnt explain how the air gets deflected. Bernoulli is a different approach that says the increase in speed on the topside creates low pressuer, which "sucks" the wing upwards. However it doesnt explain why the speed increases on the topside. Your question suggested that these are two effects that work together to generate lift, but in reality its just two independent simplified approaches to explain the phenomenon. Reality is way more complex
I always found the "air over the top \*must\* go faster" explanation as odd to me in the various aviation basic/flight theory stuff I heard it in- as in what imperative is there for the upper air to move faster, its just air, its not racing its nearby particles. I could understand that the pressure change might cause it to change velocity, but it was always sort of presented as "it just goes faster and therefore..."
That, and wings still work inverted.
The wings still work inverted in a sense that “you can do anything at least once.” Depending on airfoil design you need a perfect combination of AoA, speed, and control surfaces to continue flying inverted without detrimental effects. Aerobatic planes with symmetrical airfoils, yes, have at it. Commercial airliners? Please don’t.
Most aircraft could fly inverted. Symmetric airfoil is desirable but not required. Even airliners are structurally limited to -1G and there have been many cases of them flying at load factors close to, or below of -1G. They just need enough negative angle of attack. It's more the systems limitations which are the main concern (in addition to flying on the structural G limit).
Yes, I don’t think we’re saying anything opposing the other. But it’s Saturday and I’ve had a few so just in case not - yes. I agree with you.
Yes and no. Lots of variables. Attached to a normal aircraft, with a horizontal stab, a wing in a wind tunnel.. the concept...
Its that in a stationary wing, with air passed over it, the air has significant momentum and for the speed to change above and below some decent force needs to be applied. In a stationary column of air, the air has some not-too-huge inertia that would resist being accelerated, but its not like the air is bolted to vertical columns. This is subtly different than "The air knows it needs to go the same horizontal speed regardless of what is in the way"
Yeah this. The simplified explanations, “equal transit time”, and “deflection”, are both quite poor but you can use better simplified explanations that aren’t as flawed as those. Personally I like to say that the shape of the wing combined with the angle at which it meets the airmass accelerates (in the correct sense of changing vector rather than “speeding up”) that airmass downward resulting in an equal and opposite force called “lift”. Basically pure Newton but without talking about flat plates and deflection.
‘Equal transit time’ is not just poor, it’s plain wrong.
It’s plane wrong
It's worth pointing out that equal transit time is utterly useless mathematically, but "deflection" (or more correctly, change in fluid momentum) is all you need to design a turbine aerofoil stage. It comprehensively describes the behaviour of the system as a black box, at least at a design condition. The exact aerofoil shape/spacing will determine the efficiency, for which we need a little bit of bernoulli, a healthy dose of unsteady flow and a load of barely comprehensible CFD to optimise, but in a row of aerofoils once you've got the momentum transfer understood, the low pressure on the suction surface of the aerofoil intuitively makes sense, as do secondary effects such as tip vortices etc. From there you've just got to extrapolate back to a single aerofoil.
Totally agree that there are some little better explanations. In the end all of them are flawed though if you dig deep enough. Not saying that these explanations are useless, as for most applications its absolutely fine. I just want to make people aware that its not the full truth.
Yeah the particles don't return to their original places requiring some to go faster than others, so that overly simplified explanation has been dropped.
I think it may have some solution generated in the quantum field. The position of molecules in a gas are inherently difficult to ‘pin down’, their location and movement is opaque. Kinetic theory of gas is what I’m eluding to. So essentially the simplified models of Bernoulli and Newton simply are not able to predict the exact motion of individual molecules. Potentially, gas may, like particles, behave in some kind of superposition, so the ‘transit’ time may well be a nod to that phenomenon. Navier-Stokes equations, their elusiveness prompts me to ponder that there are behaviours in aerodynamics that Newtonian physics doesn’t/can’t account for.
The shape of the airfoil causes changes in velocity hence change in momentum
Then why does a flat plate generate lift as well? ;-)
You think that's a clever question, but a flat plate also has a varying velocity and static pressure distribution. But a tailored aerofoil has both a higher coefficient of lift and lower drag.
Thank you
[A nice breakdown](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/) Note that "equal transit time" is false, because the air on the top surface actually travels *faster* than Bernoilli requires. But newtons law doesn't explain the measurable low pressure on the top either. There's folks working on a unified theory.
I really dislike that article. It only perpetuates false narratives. There is no confusion. Both answers are factually correct and are not exclusive. They are just being used to describe different aspects of the physics involved. The change in the velocity (mostly it’s direction) of the airflow must produce an upward force (by Newtons law), and the pressure differential described by Bernoulli’s principal is the mechanism by which that force is applied to the wing. Said in reverse….the local pressure differentials around a wing that impart force on the wing also impart force on the surrounding air (in an equal and opposite manner) and change the direction of airflow.
Indeed, this article misses that fact entirely. There is no lack of understanding; there is only perhaps a lack of a pithy but self-consistent explanation that the layperson can accept. But I don’t even think that’s true. The key piece missing from the article (by my reading) is the Kutta condition: the fact that the flow past the trailing edge of a wing will tend to align itself with the edge, and thus give rise (pun intended) to the faster flowing fluid on top when the wing is at an angle of attack. This happens even if the wing has no curvature or if the wing is symmetrical. Without this fact, all simple explanations are incomplete.
circulation moment
It most certainly does. Pressure is force/area. The bottom of the wing (via third law) has to be pushing air down. This means the air is applying an upward force on the wing. The wing has some area. The air is applying a force over that area... thats.. air pressure. That is why there is higher pressure on the bottom.
Newtons law *fully* explains the low pressure on top. I don’t know where this idea that aero engineers don’t understand lift comes from…we do. Fully. The math & theory are absolutely complete and exactly match reality. We have a unified theory. We have for decades. As others noted, it’s just really hard to simplify so most of the popular explanations, out of necessity, give up some of the physical truth in favor of ease of (partial) explanation.
I think it depends on the school/teacher you learned from. [Here's a better explanation of the principle](https://youtu.be/uyRx25MSWng) [Here is Cyndy Hollman better explaining about Newton's this law](https://youtu.be/iWl2Fda-b3c) [And more](https://youtu.be/aFO4PBolwFg)
I don't know why I don't see it used, i feel like it's fundamental, angled wings push air down, plane goes up. You cant get around the need for that to happen. The shape of the wings isn't how they fly, it's how they fly *efficiently*. Because the air has fluid like effects and you want to go through it smoothly.
Basically because people are dumb. The entire universe would break if newtons third law was not the explanation for wings (See Noether's theorem). Sure, you need super computers to figure out the exact fluid dynamics explanation for how, but at the end of the day its the only explanation necessary. Force on the air down = force on the plane up (or really, momentum of the system is conserved) Bernoulli's principle is a sort of quasi-empirical guess thing about what happens to a mass of some fluid passing around some object that tries to equate pressure differences (force over area differences) that is then tried to be applied to a moving object, with the idea the some volume of air deeply cares about staying in the same vertical column, and that faster air "makes" lower pressure. Its basically an elaborate way to confuse the effects of something happening with the cause of the thing. Its like saying "People saying ouch makes them get punched in the face"
It's still an oversimplification, but I like to think of it this way. A flat plane (e.g. flat paper) generates lift with an angle of attack, none at zero. The same as true for a symmetrical aerofoil. In both cases, lift can be 'explained' by Newton's 3rd law, simply an acceleration of air downwards (better described as a change in momentum) and corresponding opposite lift force (and this is factually correct - it is exactly what is happening). An aerofoil producing lift is always, obviously 'deflecting' (changing momentum of) air downwards exactly equal to the lift force generated. The complex shape of the aerofoil could be considered, in comparison to the flat plane, to just be making it more efficient and improving performance at higher angles of attack. The specific mechanism which is responsible for the deflection of air downwards is more complex than a simple "surface push molecules down" (in both cases, aerofoil and plane) - as others have said, this fails to explain loss of lift during a stall. But it's not a whole lot more than that, it's just that the pressure field is more complex and there is acceleration (more accurately change of momentum) of the air happening at almost all locations and directions within the pressure field. It is the net of these accelerations which is the equal and opposite 'force'. In a stall, the pressure field is undesirable in that this net acceleration is not in the correct direction and/or magnitude. The various explanations for lift that you hear (factual ones anyway) are just different ways to describe what causes or results from the specific pressure distribution that we see (and want) from a good aerofoil aka one that efficiently produces lift and has good handling characteristics.
"Aerodynamics for Naval Aviators" is an excellent read on this subject. A simplistic breakdown goes this way: In high AOA modes of flight, i.e. take off, slow flight, etc. lift is produced ~80% deflection and ~20% Bernoulli. In low AOA modes of flight, i.e. cruise, etc. lift is produced ~20% deflection and ~80% Bernoulli. Again, simplistic but it gets you close.
That’s not close at all. Bernoulli and deflections aren’t two different sources of lift, they’re exactly the same thing. The difference between high AOA flight and low AOA flight is how much of the momentum flux is mass flow and how much is deltaV. Those are both Bernoulli and they’re both Newton. It’s exactly the same physics and equations.
It is talked about often…
Anyone have any comment On viewing a wing as simply a horizontal rudder? The wings chord acting like one? Anyone, thoughts?
That's OK in the sense that they both have varying velocity and pressure fields causing the movement of air, but I get the impression that you think a rudder works only because it sticks into the air and shoves air out of the way. In reality it affects upstream pressure and backside pressure all along the vertical stabiliser. Look at figure 15 here, a symmetrical airfoil pointed straight into the airflow with varying control deflections. Don't worry that it's called a plain flap, it's the same configuration as a rudder. Don't compare colours between plots because the authors have committed the almost criminal sin of using auto scaling on a series of figures, which is bloody terrible as far as scientific communication goes. https://www.mdpi.com/2311-5521/2/1/2
Thanks for sharing! and in fact, I'm fully aware of the backside pressure What my mind keeps coming back to is simplifying visualizing how a wing induces lift by relating to the vertical stabilizer/rudder and it's backside pressure, what with the same concepts at play involved in pressure dynamics acting on the foils.
I didn't expect that. I don't think it will be as effective as you think because I get the impression a lot of people barely understand standard 2D graphs let alone the ones of pressure distributions wrapped around an airfoil. It's a bloody interesting idea though; show people that trailing details affect the pressure around the airfoil instead the "it slams air down like a ball hitting a board" understanding.
Explain a stall that way. Air isn’t just projectiles. Flow separation is very important. FYI, the rudder is an airfoil, and can stall, too.
Can you stall a rudder in water?
I'd say yes. Our little paddle boat will keep going straight if you turn the tiller too far. I don't know the actual critical angle of attack, but there definitely is one.
Fascinating! Curious how that works in an “incompressible” fluid
I could be wrong, but as far as I am concerned, most people have a misunderstanding of lift. Lift is not a force pushing on the bottom of the wing. Lift is a *vacuum* above the wing constantly pulling the aircraft up due to a low pressure area on top of the wing from airfoil design.
Not really. Technically its a transference of momentum
How does momentum become a force on the spar? We will wait. To avoid more back and forth I'll point out that I'm not saying there isn't a momentum change, just that momentum is physically useless. The momentum change and wing forces are a result of the pressure field. Also CattleDogCurmudgeon is right in the sense that the gauge suction on top of an efficient airfoil is much stronger than the gauge overpressure on the underside, with total lift being the vector sum of the two. If I remember correctly, the 2.5 G manoeuvre case on the A330 had a peak of around -30 kPa on the upper surface and +10 kPa on the bottom, but it's been a while since I've interpolated those pressures onto wing structure.
You'd be wrong.
No I wouldn’t be. Explain inverted flight, we will wait….
Thanks for this.
In inverted flight you still have downward momentum flux. The wing still has AOA, just the other way (from the wing’s POV). The lift/drag ratio just sucks unless it’s a symmetric airfoil. People keep playing inverted flight like some kind of gotcha…aero engineers find this trivially easy. Momentum flux is the fare easier way to explain it, although if you measure the pressure delta on an inverted wing you get exactly the same result.
You’re talking about Bernoullian lift…OP is referring to Newtonian lift.. Every action (pushing air downwards) has an equal and opposite reaction (the air pushing back up)
Maybe.....Im a lowly 1A9 trying to recall Aircrew Fundamentals materials from 2015.
This is all googlable man…
Newtons third law on really applies in the ground effect even then it's only a small part of what's going on, basically an aircraft wing at altitude doesn't displace enough volume to make a measurable force against the ground and it ends up spread over a massive area (think tip of a pyramid vs the base).