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I'm surprised people don't start with the basics on this confusing topic. The third law of Newton's mechanics tells us that for the plane to get an up force to

I'm surprised people don't start with the basics on this confusing topic. The third law of Newton's mechanics tells us that for the plane to get an up force to counteract the gravity, the air must receive and equal amount of down force. Therefore what planes must be doing is deflect air masses down. A plane must be applying a downward force to air masses, with total force value of "mass * g", i.e. supply "mass * g * 1second" worth of downward momentum per second. As simple as that.

jvanderbot 7y ago
Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved. This principle seems to work just fine for me w.r.t. fixed wings.
- lutorm 7y ago
  Sure, that's just momentum conservation. But how do you know how much downward momentum the column of air has?
  - H8crilA 7y ago
    Airplane/helicopter mass * g * time Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.
    - lutorm 7y ago
      Sure, that has to be true if the helicopter can hover. But how do you know that's possible? You can't postulate that something flies as part of an explanation why it can fly...
      - starpilot 7y ago
        You can solve for the lift generated by each rotating blade. Then it's roughly the same calculating the lift for a fixed wing, except for the freestream flow speed varying along the span. The lift is different at each spanwise location, so you integrate to get the total lift for one blade, then multiply by the number of blades. Numerous ways to do this, but the simplest accurate way is via lifting-line theory. The resulting system of equations is solved iteratively.
      - H8crilA 7y ago
        Why not? I can see it is flying, no point in questioning that. There are some invariants to the world, I can infer information from those invariants. Just like if I weigh a bottle in the morning, and it shows 3kg, the. I weigh it in the evening, it shows 2kg - I'll know that net 1kg has left the bottle. Doesn't mean I know how.
  - jvanderbot 7y ago
    This may help. It's a guide to the equation describing lift as a function of blade angle of attack, air density, and true airspeed. https://sciencing.com/calculate-lift-rotor-blades-7680704.html https://sciencing.com/calculate-lift-rotor-blades-7680704.ht...
o_nate 7y ago
The obvious follow-up question is : why does a wing deflect the air down?
- Sharlin 7y ago
  Why it deflects air below the wing downward is trivial: it is an inclined plane (the angle of attack is nonzero) Why it also deflects air above the wing downward – pulling the air down so to speak – is more complicated.
  - o_nate 7y ago
    I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.
    - frank2 7y ago
      A flat wing at 45 degrees produces massive turbulence above and behind the wing, which in turn greatly increases the drag-to-lift ratio compared to a standard airplane wing. Also I suspect that ailerons and other control surfaces become much less useful in turbulent flow.
      - o_nate 7y ago
        Sure, but now explain drag and turbulence using Newton's laws.
        mannykannot 7y ago
        Via the Navier-Stokes equations, which are the application of Newton's laws to viscous fluids.
    - renox 7y ago
nimish 7y ago
This doesn't explain all of the induced pressure differences. You need Bernoulli (Conservation of Energy, more or less) as well. They interact in a complicated way. You really can't "just" explain lift simply.
- mannykannot 7y ago
  It is more like that there is a hierarchy of explanation. There is nothing going on that cannot be explained by Newton's laws, conservation of energy does not need to be introduced as an extra constraint, and Bernoulli's law is itself explained by Newton's laws. The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards, you still don't know how the air moves around the wing, so you can't calculate which streamlines are being accelerated by how much and in which direction. For a general solution to that question, you need to solve the appropriate Navier-Stokes equations, which take into account both the inertia and the viscosity of the air, and are also explained by Newton (up to the point where Newton does not explain the origin of viscosity.) Once you have the velocity field, you can, if and only if the velocity is low enough that the air behaves as an incompressible fluid, calculate the pressure on the wing using Bernoulli. One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before). Modelling at this level of detail is computationally very costly, and the Kutta–Joukowski theorem can be used instead, for a wide range of practical airfoil profiles. So yes, it is complicated, once you go beyond the barest hand-waving. https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem
  - nimish 7y ago
    > The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards You need bernoulli to explain why the flow field is changed beyond just the area in contact with the flow. This induces the measured pressure differential, explaining part of lift along with the reaction effects of deflected flow for momentum conservation (NS, Newton's 2nd law). It's simply not _enough_ to say that it's purely angle-of-attack or geometry, and it's definitely not enough to say it's just pressure difference caused by Bernoulli, it's _both at once_. >One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before). Not really true for 2d inviscid flows, a very useful approximation -- you need to enforce the K-J condition IIRC (which includes viscosity in a sense) In short: fluid dynamics is complex, and you need more than just newton's laws to explain it thoroughly (newton's laws don't explain conservation of mass either...)
mike_ivanov 7y ago
I also don't get it. People talk about aerodynamic lift as of a magical force, somehow different from regular physical matter interactions. Just exclude what it obviously is not (strong force, gravitation, etc), and what is left has to be it.
tathougies 7y ago
Certainly, but law is guided by empiricism. Newton's third law is not scientific fact prima facie. It was developed via observation. In this case, you say simply that 'oh it's the equal and opposite reaction', and that seems theoretically possible. But then you do the experiment, and all of a sudden, you see not only a force due to the third law, but also a pressure differential. Based on the laws of pressure, a high pressure beneath the wing and a lower pressure on top must also exert a force. These pressure laws are as fundamental (and are actually simply a consequence of the third law, since pressure is just a statistical approximation of the third law applied to invisible microscopic particles). Thus, while almost certainly true the third law does contribute, it's also true that it is not a full explanation. The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that. The third law would additionally predict that the pressure on top should be greater than that on the bottom. This is an easy experiment to do. Get a piston in a closed cylinder (the piston does not need to partition the cylinder). Place two pressure gauges on each side. Now pull the piston up. The side being compressed has higher pressure. This suggests that, if a flat board moves towards a side of a closed system, that side of the system ought to experience higher pressure. But it doesn't in the case of wings, so that's weird
- lutorm 7y ago
  The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that. There is no "extra lift" due to the pressure differential. The pressure differential IS the net force that changes the motion of the air. You can't change the motion of the air without a pressure differential, and you can't have a pressure differential without changing the motion of the air.

jvanderbot 7y ago

Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved. This principle seems to work just fine for me w.r.t. fixed wings.

lutorm 7y ago
Sure, that's just momentum conservation. But how do you know how much downward momentum the column of air has?
- H8crilA 7y ago
  Airplane/helicopter mass * g * time Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.
  - lutorm 7y ago
    Sure, that has to be true if the helicopter can hover. But how do you know that's possible? You can't postulate that something flies as part of an explanation why it can fly...
    - starpilot 7y ago
      You can solve for the lift generated by each rotating blade. Then it's roughly the same calculating the lift for a fixed wing, except for the freestream flow speed varying along the span. The lift is different at each spanwise location, so you integrate to get the total lift for one blade, then multiply by the number of blades. Numerous ways to do this, but the simplest accurate way is via lifting-line theory. The resulting system of equations is solved iteratively.
    - H8crilA 7y ago
      Why not? I can see it is flying, no point in questioning that. There are some invariants to the world, I can infer information from those invariants. Just like if I weigh a bottle in the morning, and it shows 3kg, the. I weigh it in the evening, it shows 2kg - I'll know that net 1kg has left the bottle. Doesn't mean I know how.
- jvanderbot 7y ago
  This may help. It's a guide to the equation describing lift as a function of blade angle of attack, air density, and true airspeed. https://sciencing.com/calculate-lift-rotor-blades-7680704.html https://sciencing.com/calculate-lift-rotor-blades-7680704.ht...

o_nate 7y ago

The obvious follow-up question is : why does a wing deflect the air down?

Sharlin 7y ago
Why it deflects air below the wing downward is trivial: it is an inclined plane (the angle of attack is nonzero) Why it also deflects air above the wing downward – pulling the air down so to speak – is more complicated.
- o_nate 7y ago
  I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.
  - frank2 7y ago
    A flat wing at 45 degrees produces massive turbulence above and behind the wing, which in turn greatly increases the drag-to-lift ratio compared to a standard airplane wing. Also I suspect that ailerons and other control surfaces become much less useful in turbulent flow.
    - o_nate 7y ago
      Sure, but now explain drag and turbulence using Newton's laws.
      - mannykannot 7y ago
        Via the Navier-Stokes equations, which are the application of Newton's laws to viscous fluids.
  - renox 7y ago

nimish 7y ago

This doesn't explain all of the induced pressure differences. You need Bernoulli (Conservation of Energy, more or less) as well. They interact in a complicated way. You really can't "just" explain lift simply.

mannykannot 7y ago
It is more like that there is a hierarchy of explanation. There is nothing going on that cannot be explained by Newton's laws, conservation of energy does not need to be introduced as an extra constraint, and Bernoulli's law is itself explained by Newton's laws. The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards, you still don't know how the air moves around the wing, so you can't calculate which streamlines are being accelerated by how much and in which direction. For a general solution to that question, you need to solve the appropriate Navier-Stokes equations, which take into account both the inertia and the viscosity of the air, and are also explained by Newton (up to the point where Newton does not explain the origin of viscosity.) Once you have the velocity field, you can, if and only if the velocity is low enough that the air behaves as an incompressible fluid, calculate the pressure on the wing using Bernoulli. One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before). Modelling at this level of detail is computationally very costly, and the Kutta–Joukowski theorem can be used instead, for a wide range of practical airfoil profiles. So yes, it is complicated, once you go beyond the barest hand-waving. https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem
- nimish 7y ago
  > The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards You need bernoulli to explain why the flow field is changed beyond just the area in contact with the flow. This induces the measured pressure differential, explaining part of lift along with the reaction effects of deflected flow for momentum conservation (NS, Newton's 2nd law). It's simply not _enough_ to say that it's purely angle-of-attack or geometry, and it's definitely not enough to say it's just pressure difference caused by Bernoulli, it's _both at once_. >One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before). Not really true for 2d inviscid flows, a very useful approximation -- you need to enforce the K-J condition IIRC (which includes viscosity in a sense) In short: fluid dynamics is complex, and you need more than just newton's laws to explain it thoroughly (newton's laws don't explain conservation of mass either...)

mike_ivanov 7y ago

I also don't get it. People talk about aerodynamic lift as of a magical force, somehow different from regular physical matter interactions. Just exclude what it obviously is not (strong force, gravitation, etc), and what is left has to be it.

tathougies 7y ago

Certainly, but law is guided by empiricism. Newton's third law is not scientific fact prima facie. It was developed via observation. In this case, you say simply that 'oh it's the equal and opposite reaction', and that seems theoretically possible. But then you do the experiment, and all of a sudden, you see not only a force due to the third law, but also a pressure differential. Based on the laws of pressure, a high pressure beneath the wing and a lower pressure on top must also exert a force. These pressure laws are as fundamental (and are actually simply a consequence of the third law, since pressure is just a statistical approximation of the third law applied to invisible microscopic particles). Thus, while almost certainly true the third law does contribute, it's also true that it is not a full explanation. The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that. The third law would additionally predict that the pressure on top should be greater than that on the bottom. This is an easy experiment to do. Get a piston in a closed cylinder (the piston does not need to partition the cylinder). Place two pressure gauges on each side. Now pull the piston up. The side being compressed has higher pressure. This suggests that, if a flat board moves towards a side of a closed system, that side of the system ought to experience higher pressure. But it doesn't in the case of wings, so that's weird

lutorm 7y ago
The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that. There is no "extra lift" due to the pressure differential. The pressure differential IS the net force that changes the motion of the air. You can't change the motion of the air without a pressure differential, and you can't have a pressure differential without changing the motion of the air.