18 ms·

Do we still teach kids that planes fly because of Bernoulli's principle? I remember learning about it and wondering why newton's 3rd law wouldn't suffice. It's

by dguest 2y ago

Do we still teach kids that planes fly because of Bernoulli's principle? I remember learning about it and wondering why newton's 3rd law wouldn't suffice. It's pretty obvious that the wings push air down and it's not that difficult to understand (even as a kid) that newton's 3rd law works. The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift. Ok, cool, but the "Bernoulli principle" I got as a kid was "faster air is lower pressure", which is both empirically wrong (the air in a compressor hose is obviously moving faster than the air in the workshop) and logically inconsistent (speed is relative, after all). You add in a half dozen qualifiers and it becomes true, but I wonder if this is more complicated than "the wings push air down, the air pushes the wing up".

cm2187 2y ago
You can experience faster air is lower pressure when you are trying to breath in strong wing (like sky diving or by an open window on a car on the motorway). It makes you usually gasp for air. But yeah I was taught planes fly that way in the 90s.
elygre 2y ago
My mental model is that you can push hard on a wall while standing still, but the faster you run along the wall, the less you are able to push it.
lupusreal 2y ago
The Newtonian explaination of lift is partially but not completely correct. It only explains some of the lift which is empirically observed. Particularly the "push air down" model; the tops of wings also pull air down along themselves (assuming there isn't flow separation, e.g. a stall) and direct it down. To really explain that flow you need fluid dynamics.
krisoft 2y ago
> Do we still teach kids that planes fly because of Bernoulli's principle? Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.
- JumpCrisscross 2y ago
  > distinctly remember that it was how lift was explained in my PPL book TBF, the Handbook of Aeronautical Knowledge [1] does a better job. [1] https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak https://www.faa.gov/regulations_policies/handbooks_manuals/a...
  - mncharity 2y ago
    Chapter 4, PDF: https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf
rcxdude 2y ago
A lot of the difficulty of explaining lift of airfoils is that generally explanations try to follow a neat chain of cause an effect. But with the wing there isn't really a clear one. All these statements: - There is an upwards force on the wing - The pressure above the wing is lower than the pressure below it - The air around the wing follows a curved path downwards - The air above the wing travels faster than the air below it (NB: not in equal time!) - The air behind the wing has a downward momentum are related to all the others, but not straightforwardly: they all imply each other to some extent, both caused by and causing some of the others. So basically all explanations try to follow some path through the tangled web, but by doing so they always cause some oversimplification. The only top level chain is: shape of wing and angle of attack -> ????? (tangled mess of fluid dynamics few people fully understand) -> lift!
- lutorm 2y ago
  Exactly, this is why understanding fluid dynamics is so difficult. You can't look at some physical laws and assume that the right hand side "causes" the left hand side. They all represent relations and it so happens that the fluid configuration that fulfill all the relations (and that the world adopts) is the one that causes lift. Just trying to talk about cause and effect is a misunderstanding.
  - NovemberWhiskey 2y ago
    My favorite example of this was in the air-data computer I was working on for a fighter trainer. I was just on the software side rather than the aerodynamics, but it was notable that the corrections to angle-of-attack and angle-of-sideslip measured by the multifunction probes (which are way up at the nose of the plane) included terms related to the position of the flaps (which are way back at the trailing edges of the wings).
    - bquinlan 2y ago
      That's awesome! I'm not surprised about the angle-of-attack needing correction. The angle-of-attack is defined as the angle between the average chord (an imaginary line running from the leading edge of the wing to the trailing edge of the wing) and the relative wind. Since changing the flap position changes the position of the trailing edge, the angle-of-attack will also change.
fransje26 2y ago
> It's pretty obvious that the wings push air down The air being pushed down is actually a side-effect of the lift-creation process, not the cause of it. A nice "counter example" is a wing in ground effect (flying very close to the ground), where there is less downwash, because of the ground, and yet the wing produces more lift. It's an effect that can make high aspect-ratio airplanes tricky to land.
- deleted 2y ago
  [deleted]
- JumpCrisscross 2y ago
  > air being pushed down is actually a side-effect of the lift-creation process, not the cause of it The turning of the gas is absolutely what causes lift. (Where the Newtonian explanation is misleading is in “neglect[ing] the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas” [1]. Put another way: if you know the mass and acceleration of the gas about the wing, you can calculate lift. (This is impractical for many reasons.) > a wing in ground effect VTOL aircraft also experience ground effect due to the fountain effect. [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
  - HPsquared 2y ago
    Curvature of streamlines is related to pressure gradient across said streamlines.
    - JumpCrisscross 2y ago
      > Curvature of streamlines is related to pressure gradient across said streamlines Sure. Ultimately just considering pressure or mass deflection doesn’t work without elaborate workarounds. Because neither describes the reality of an airfoil turning a moving viscous fluid.
  - fransje26 2y ago
ljf 2y ago
Don't compare the pressure of the air in the workshop to the fast moving air in the nozzle - compare the air in the system of the compressor. In an air compressor, the lowest pressure air is the air moving through the hose and out the nozzle - the highest pressure air in the system is the 'still air' in the cannister. Think of an inflated balloon that you blow up and let go of, the highest pressure air is in the balloon, the lowest pressure air is immediately next to the mouth of the balloon, despite being the fastest moving. It might feel surprising, but the air that moves faster across the top of the wing is lower pressure than the slower moving air below the wing. That both the air below and above the wing are higher pressure than 'all the rest of the air in the sky' is inconsequential to the the plane - we only need to consider the air directly interacting with the wing. (though this is not to deny the impacts of angle of attack etc etc.)
- greenbit 2y ago
  The thing I never found satisfying was this notion that the air over the top moves faster because it has further to go - in what way does the length of a path that lies in the air's future have any effect on its speed now? As if the air over the top somehow has to match up with the air it was next to before the wing split it away below? What mysterious force would account for that? The best I could arrive at was that the forward motion of the wing causes the back side of the curved wing top simply to pull away from the air in that region, reducing the pressure there, and incidentally (because Bernoulli) that air then moves faster as a result.
  - ljf 2y ago
    The speed of the wing is what causes the air to move around the two faces of the wing. The air has to move around the wing as it is being pulled through it. Imagine pulling a fixed walled tube though the air, the air will move through the tube at roughly the speed that the tube is pulled through the air. Now imagine pulling a funnel that starts off large and gets smaller. The same air will now have to move faster to get through the funnel (higher pressure at the mouth of the funnel, lower at the end).
  - zardo 2y ago
NovemberWhiskey 2y ago
If Newton’s third law sufficed, then the shape of the upper surface of the wing wouldn’t be important. In fact, it matters a lot. There is no single explanation for why airfoils generate lift that works at a grade school level.
- upofadown 2y ago
  The turbulence caused by a sharp leading edge of something like a flat board causes momentum transfer to the top of the wing. The problem isn't in explaining how a conventional airfoil works, Newton's law works well enough for that. The problem is in explaining what happens when things go wrong. Turbulence has been a problem for physicists for a long time...
  - NovemberWhiskey 2y ago
    The Newton's third-law explanation is "air bangs into a bottom of airfoil and pushes it up". Without having the concepts of boundaries layers, laminar and turbulent flow, flow separation and (more generally) the entire Navier-Stokes toolbox you don't have the tools for explaining why turbulent flow is a problem, for example.
    - upofadown 2y ago
      The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards. The Navier-Stokes equations merely model fluid flows. Understanding them provides no understanding of the behaviour of such flows. That behaviour is emergent from the interaction of a great many particles.
      - NovemberWhiskey 2y ago
        >The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards. But that doesn't have any explanatory power at all. If we assume Newton's laws hold, then obviously if there's a force upward on the airfoil then there's a reaction force downward on the air. It'd be like explaining the combustion engine by saying "the drive shaft from the engine rotates this way, and the reaction force - because the engine is more-or-less rigidly mounted to the frame - is resisted through the suspension by the wheels being in contact with the ground". OK, sure, but I still don't know how the engine actually works.
HPsquared 2y ago
The qualifier is "along a streamline".
criddell 2y ago
In school when our physics teacher explained how the shape of an airplane wing creates lift and allows the plane to fly, I asked how it is that airplanes can fly upside down? I got the classic "that would be a great thing for you to research on your own time".
- ultrarunner 2y ago
  This is actually really cool, because an upside down airfoil will still create a high pressure ridge toward its leading edge. This causes air that would ostensibly flow along the bottom (high pressure) surface to sort of reverse and end up being pushed to the upper (low pressure) surface. The separation point is further down the leading edge than would be intuitively expected. This means the top stream of air still goes further, and faster, than the bottom stream of air. So inverted wings still fly, just less efficiently.
tzs 2y ago
The Newton's 3rd law explanation and the Bernoulli explanation are both reasonable approaches and both work, very similar to the way that one can explain the path of a thrown ball both by Newton's laws and by the principle of least action. NASA has a good explanation here [1]. Here's a brief summary. The gas flow has to simultaneously conserve mass, momentum, and energy. If you analyze lift by considering conservation of momentum you get that there are velocity differences in the flow at different parts of the wing. Integrate those around the whole wing and you find a net turn of the flow downward. Conservation of momentum (Newton's 3rd) requires an opposite upward force on the wing. If you analyze lift by considering conservation of energy you also get velocity differences which lead to pressure differences. Integrate pressure over the whole wing and you get a net upward force on the wing. [1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/bernoulli-and-newton/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ber...
- bumby 2y ago
  This doesn't really explain why those velocity variations occur in the first place, or am I missing something? It sounds like "We observe velocity variations on the wing and these correspond to pressure variations that create lift due to conservation of energy." But it leaves the question on what is causing the velocity variations in the first place.
- sopooneo 2y ago
  One other weird thing needed if you want your momentum numbers to balance: every plane in the sky is pulling the earth up towards it ever so slightly.
  - quietbritishjim 2y ago
    If you just mean there's a gravitational force on the Earth from the plane then, sure, but that's true for any object (including you and me). For an object on the ground, the normal force of that object on the ground balances their gravitational pull on the Earth, so the Earth experiences a net force of zero from the object and so isn't accelerated by it. But I guess you mean that the plane has a net force pulling the earth towards it. But that would violate conservation of momentum – in fact the net force is zero, just as for an object on the ground. The plane's wings push the air down (the reaction to that is what keeps the plane up) and the resulting downdraft of that air exerts a force downwards on the earth. That's all assuming the plane is in steady flight. If it's taking off, or just ascending, then overall it is pushing the Earth away from it. Conversely, when descending the Earth is pulled, overall, slightly towards it. The same thing happens when you jump: you push the Earth away from you (a much smaller distance than you travel away from it!), then on the way back down the Earth travels back towards you.
pc86 2y ago
Not only kids but pilots. I got my license ten years ago and the answer to "How do planes?" was "Bernoulli."
- ultrarunner 2y ago
  Not only that, but depending on the particular FAA designated examiner you get, failing to tell him Bernoulli can result in a disapproval. I've heard of it happening. Fortunately, none of this has ever mattered in the least for actually flying a plane, and there are plenty of sane examiners out there.
kjkjadksj 2y ago
I don’t think I learned anything about flight as a kid. Frictionless cars, yes.
hydrogen7800 2y ago
Published in 1944, Stick and Rudder[0] by Wolfgang Langeweische has this to say: >Forget Bernoulli's Theorem >When you studied theory of flight in ground school, you were probably taught a good deal of fancy stuff concerning an airplane's wing and just how it creates lift. As a practical pilot you may forget much of it. Perhaps you remember Bernoulli's Theorem: how the air, in shooting around the long way over the top of the wing, has to speed up, and how in speeding up it drops some of its pressure, and how it hence exerts a suction on the top surface of the wing. Forget it. In the first place, Bernoulli's Theorem does not really explain- the explanation is more puzzling than the puzzle! In the second place, Bernoulli's Theorem doesn't help you in the least bit in flying. While it is no doubt true, it usually merely serves to obscure to the pilot certain simpler, much more important, much more helpful facts. >The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down. >It shoves the air down with its bottom surface, and it pulls the air down with its top surface... In exerting a downward force upon the air, the wing receives an upward counterforce- by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves a bullet out forward... >Say that the wing is basically simply a plane, set at a slight inclination so as to wash the air down... But it was early found that the drag, lifting, and stalling characteristics of such an inclined plane can be improved by surrounding it with a curving, streamlined housing [emphasis mine]; hence our present wing "sections". The actual wing of an airplane is therefore not simply an inclined plane; it is a curved body containing an inclined plane. [0]https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?edition=key%3A/books/OL3323924M#editions-list https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?ed...
phkahler 2y ago
>> The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift. That argument doesn't hold up (no pun intended). Just because the distance is longer does not mean the air will go faster, it could just take longer to get there.
quietbritishjim 2y ago
Newton's third law doesn't explain stalls – or, at least, not their suddenness. As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.
bilsbie 2y ago
The Venturi effect does show that faster air is lower pressure though.

by dguest 2y ago

cm2187 2y ago

You can experience faster air is lower pressure when you are trying to breath in strong wing (like sky diving or by an open window on a car on the motorway). It makes you usually gasp for air. But yeah I was taught planes fly that way in the 90s.

elygre 2y ago

My mental model is that you can push hard on a wall while standing still, but the faster you run along the wall, the less you are able to push it.

lupusreal 2y ago

The Newtonian explaination of lift is partially but not completely correct. It only explains some of the lift which is empirically observed. Particularly the "push air down" model; the tops of wings also pull air down along themselves (assuming there isn't flow separation, e.g. a stall) and direct it down. To really explain that flow you need fluid dynamics.

krisoft 2y ago

> Do we still teach kids that planes fly because of Bernoulli's principle? Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.

JumpCrisscross 2y ago
> distinctly remember that it was how lift was explained in my PPL book TBF, the Handbook of Aeronautical Knowledge [1] does a better job. [1] https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak https://www.faa.gov/regulations_policies/handbooks_manuals/a...
- mncharity 2y ago
  Chapter 4, PDF: https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf

rcxdude 2y ago

A lot of the difficulty of explaining lift of airfoils is that generally explanations try to follow a neat chain of cause an effect. But with the wing there isn't really a clear one. All these statements: - There is an upwards force on the wing - The pressure above the wing is lower than the pressure below it - The air around the wing follows a curved path downwards - The air above the wing travels faster than the air below it (NB: not in equal time!) - The air behind the wing has a downward momentum are related to all the others, but not straightforwardly: they all imply each other to some extent, both caused by and causing some of the others. So basically all explanations try to follow some path through the tangled web, but by doing so they always cause some oversimplification. The only top level chain is: shape of wing and angle of attack -> ????? (tangled mess of fluid dynamics few people fully understand) -> lift!

lutorm 2y ago
Exactly, this is why understanding fluid dynamics is so difficult. You can't look at some physical laws and assume that the right hand side "causes" the left hand side. They all represent relations and it so happens that the fluid configuration that fulfill all the relations (and that the world adopts) is the one that causes lift. Just trying to talk about cause and effect is a misunderstanding.
- NovemberWhiskey 2y ago
  My favorite example of this was in the air-data computer I was working on for a fighter trainer. I was just on the software side rather than the aerodynamics, but it was notable that the corrections to angle-of-attack and angle-of-sideslip measured by the multifunction probes (which are way up at the nose of the plane) included terms related to the position of the flaps (which are way back at the trailing edges of the wings).
  - bquinlan 2y ago
    That's awesome! I'm not surprised about the angle-of-attack needing correction. The angle-of-attack is defined as the angle between the average chord (an imaginary line running from the leading edge of the wing to the trailing edge of the wing) and the relative wind. Since changing the flap position changes the position of the trailing edge, the angle-of-attack will also change.

fransje26 2y ago

> It's pretty obvious that the wings push air down The air being pushed down is actually a side-effect of the lift-creation process, not the cause of it. A nice "counter example" is a wing in ground effect (flying very close to the ground), where there is less downwash, because of the ground, and yet the wing produces more lift. It's an effect that can make high aspect-ratio airplanes tricky to land.

deleted 2y ago
[deleted]
JumpCrisscross 2y ago
> air being pushed down is actually a side-effect of the lift-creation process, not the cause of it The turning of the gas is absolutely what causes lift. (Where the Newtonian explanation is misleading is in “neglect[ing] the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas” [1]. Put another way: if you know the mass and acceleration of the gas about the wing, you can calculate lift. (This is impractical for many reasons.) > a wing in ground effect VTOL aircraft also experience ground effect due to the fountain effect. [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
- HPsquared 2y ago
  Curvature of streamlines is related to pressure gradient across said streamlines.
  - JumpCrisscross 2y ago
    > Curvature of streamlines is related to pressure gradient across said streamlines Sure. Ultimately just considering pressure or mass deflection doesn’t work without elaborate workarounds. Because neither describes the reality of an airfoil turning a moving viscous fluid.
- fransje26 2y ago

ljf 2y ago

Don't compare the pressure of the air in the workshop to the fast moving air in the nozzle - compare the air in the system of the compressor. In an air compressor, the lowest pressure air is the air moving through the hose and out the nozzle - the highest pressure air in the system is the 'still air' in the cannister. Think of an inflated balloon that you blow up and let go of, the highest pressure air is in the balloon, the lowest pressure air is immediately next to the mouth of the balloon, despite being the fastest moving. It might feel surprising, but the air that moves faster across the top of the wing is lower pressure than the slower moving air below the wing. That both the air below and above the wing are higher pressure than 'all the rest of the air in the sky' is inconsequential to the the plane - we only need to consider the air directly interacting with the wing. (though this is not to deny the impacts of angle of attack etc etc.)

greenbit 2y ago
The thing I never found satisfying was this notion that the air over the top moves faster because it has further to go - in what way does the length of a path that lies in the air's future have any effect on its speed now? As if the air over the top somehow has to match up with the air it was next to before the wing split it away below? What mysterious force would account for that? The best I could arrive at was that the forward motion of the wing causes the back side of the curved wing top simply to pull away from the air in that region, reducing the pressure there, and incidentally (because Bernoulli) that air then moves faster as a result.
- ljf 2y ago
  The speed of the wing is what causes the air to move around the two faces of the wing. The air has to move around the wing as it is being pulled through it. Imagine pulling a fixed walled tube though the air, the air will move through the tube at roughly the speed that the tube is pulled through the air. Now imagine pulling a funnel that starts off large and gets smaller. The same air will now have to move faster to get through the funnel (higher pressure at the mouth of the funnel, lower at the end).
- zardo 2y ago

NovemberWhiskey 2y ago

If Newton’s third law sufficed, then the shape of the upper surface of the wing wouldn’t be important. In fact, it matters a lot. There is no single explanation for why airfoils generate lift that works at a grade school level.

upofadown 2y ago
The turbulence caused by a sharp leading edge of something like a flat board causes momentum transfer to the top of the wing. The problem isn't in explaining how a conventional airfoil works, Newton's law works well enough for that. The problem is in explaining what happens when things go wrong. Turbulence has been a problem for physicists for a long time...
- NovemberWhiskey 2y ago
  The Newton's third-law explanation is "air bangs into a bottom of airfoil and pushes it up". Without having the concepts of boundaries layers, laminar and turbulent flow, flow separation and (more generally) the entire Navier-Stokes toolbox you don't have the tools for explaining why turbulent flow is a problem, for example.
  - upofadown 2y ago
    The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards. The Navier-Stokes equations merely model fluid flows. Understanding them provides no understanding of the behaviour of such flows. That behaviour is emergent from the interaction of a great many particles.
    - NovemberWhiskey 2y ago
      >The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards. But that doesn't have any explanatory power at all. If we assume Newton's laws hold, then obviously if there's a force upward on the airfoil then there's a reaction force downward on the air. It'd be like explaining the combustion engine by saying "the drive shaft from the engine rotates this way, and the reaction force - because the engine is more-or-less rigidly mounted to the frame - is resisted through the suspension by the wheels being in contact with the ground". OK, sure, but I still don't know how the engine actually works.

HPsquared 2y ago

The qualifier is "along a streamline".

criddell 2y ago

In school when our physics teacher explained how the shape of an airplane wing creates lift and allows the plane to fly, I asked how it is that airplanes can fly upside down? I got the classic "that would be a great thing for you to research on your own time".

ultrarunner 2y ago
This is actually really cool, because an upside down airfoil will still create a high pressure ridge toward its leading edge. This causes air that would ostensibly flow along the bottom (high pressure) surface to sort of reverse and end up being pushed to the upper (low pressure) surface. The separation point is further down the leading edge than would be intuitively expected. This means the top stream of air still goes further, and faster, than the bottom stream of air. So inverted wings still fly, just less efficiently.

tzs 2y ago

The Newton's 3rd law explanation and the Bernoulli explanation are both reasonable approaches and both work, very similar to the way that one can explain the path of a thrown ball both by Newton's laws and by the principle of least action. NASA has a good explanation here [1]. Here's a brief summary. The gas flow has to simultaneously conserve mass, momentum, and energy. If you analyze lift by considering conservation of momentum you get that there are velocity differences in the flow at different parts of the wing. Integrate those around the whole wing and you find a net turn of the flow downward. Conservation of momentum (Newton's 3rd) requires an opposite upward force on the wing. If you analyze lift by considering conservation of energy you also get velocity differences which lead to pressure differences. Integrate pressure over the whole wing and you get a net upward force on the wing. [1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/bernoulli-and-newton/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ber...

bumby 2y ago
This doesn't really explain why those velocity variations occur in the first place, or am I missing something? It sounds like "We observe velocity variations on the wing and these correspond to pressure variations that create lift due to conservation of energy." But it leaves the question on what is causing the velocity variations in the first place.
sopooneo 2y ago
One other weird thing needed if you want your momentum numbers to balance: every plane in the sky is pulling the earth up towards it ever so slightly.
- quietbritishjim 2y ago
  If you just mean there's a gravitational force on the Earth from the plane then, sure, but that's true for any object (including you and me). For an object on the ground, the normal force of that object on the ground balances their gravitational pull on the Earth, so the Earth experiences a net force of zero from the object and so isn't accelerated by it. But I guess you mean that the plane has a net force pulling the earth towards it. But that would violate conservation of momentum – in fact the net force is zero, just as for an object on the ground. The plane's wings push the air down (the reaction to that is what keeps the plane up) and the resulting downdraft of that air exerts a force downwards on the earth. That's all assuming the plane is in steady flight. If it's taking off, or just ascending, then overall it is pushing the Earth away from it. Conversely, when descending the Earth is pulled, overall, slightly towards it. The same thing happens when you jump: you push the Earth away from you (a much smaller distance than you travel away from it!), then on the way back down the Earth travels back towards you.

pc86 2y ago

Not only kids but pilots. I got my license ten years ago and the answer to "How do planes?" was "Bernoulli."

ultrarunner 2y ago
Not only that, but depending on the particular FAA designated examiner you get, failing to tell him Bernoulli can result in a disapproval. I've heard of it happening. Fortunately, none of this has ever mattered in the least for actually flying a plane, and there are plenty of sane examiners out there.

kjkjadksj 2y ago

I don’t think I learned anything about flight as a kid. Frictionless cars, yes.

hydrogen7800 2y ago

Published in 1944, Stick and Rudder[0] by Wolfgang Langeweische has this to say: >Forget Bernoulli's Theorem >When you studied theory of flight in ground school, you were probably taught a good deal of fancy stuff concerning an airplane's wing and just how it creates lift. As a practical pilot you may forget much of it. Perhaps you remember Bernoulli's Theorem: how the air, in shooting around the long way over the top of the wing, has to speed up, and how in speeding up it drops some of its pressure, and how it hence exerts a suction on the top surface of the wing. Forget it. In the first place, Bernoulli's Theorem does not really explain- the explanation is more puzzling than the puzzle! In the second place, Bernoulli's Theorem doesn't help you in the least bit in flying. While it is no doubt true, it usually merely serves to obscure to the pilot certain simpler, much more important, much more helpful facts. >The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down. >It shoves the air down with its bottom surface, and it pulls the air down with its top surface... In exerting a downward force upon the air, the wing receives an upward counterforce- by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves a bullet out forward... >Say that the wing is basically simply a plane, set at a slight inclination so as to wash the air down... But it was early found that the drag, lifting, and stalling characteristics of such an inclined plane can be improved by surrounding it with a curving, streamlined housing [emphasis mine]; hence our present wing "sections". The actual wing of an airplane is therefore not simply an inclined plane; it is a curved body containing an inclined plane. [0]https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?edition=key%3A/books/OL3323924M#editions-list https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?ed...

phkahler 2y ago

>> The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift. That argument doesn't hold up (no pun intended). Just because the distance is longer does not mean the air will go faster, it could just take longer to get there.

quietbritishjim 2y ago

Newton's third law doesn't explain stalls – or, at least, not their suddenness. As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.

bilsbie 2y ago

The Venturi effect does show that faster air is lower pressure though.