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For those that want a TL;DR explanation: Almost all object that are airborne, stay so by pushing air in the opposite direction that they want to go. This is Ne

For those that want a TL;DR explanation: Almost all object that are airborne, stay so by pushing air in the opposite direction that they want to go. This is Newton's third law. Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards. It is really that simple

closewith 5y ago
The second page of the article seems to address this and calls it an incorrect lift theory. > It is really that simple. The whole point of the article is that it's not that simple: > The real details of how an object generates lift are very complex and do not lend themselves to simplification.
- mansoor_ 5y ago
  > The second page of the article seems to address this and calls it an incorrect lift theory. If it does, it's wrong. Edit: Now that I have read it, I would have to disagree with you. The article only references the Bernoulli pressure arguments, it does not mention Newton's third law or airflow circulation. > The real details of how an object generates lift are very complex and do not lend themselves to simplification. There is no scientific contention about flight. Nobody is currently publishing papers on this topic ("journal of fluid mechanics" if you are interested), it was well understood in the previous century. Newton's third law explains at a high level how object fly. Yes you can dig into the details of fluid motion, turbulent flow, boundary layers etc, but this does not detract from the high-level picture.
hugh-avherald 5y ago
That's half of it. Lift is produced by airflow over both sides of an aeroplane wing not just the underside.
- mansoor_ 5y ago
  It's not just the underside that is responsible for the circulation of the airflow. In fact the majority of the circulation is generated by the top surface.
lordnacho 5y ago
Airplanes can fly upside down. How do you explain that?
- gear54rus 5y ago
  They don't do so at the same angle of attack I'd wager. Meaning to fly upside down they have to control pitch in a way that turns the wing to the exact needed angle. Same way they have to control pitch when flying normally to achieve lift. This pic of real jets doing that side by side confirms what I'm saying (nose is pitched up on the normal-flying plane and 'down' but still up relative to the ground on the inverted one): https://imgur.com/loVcrgO https://imgur.com/loVcrgO
- ColinWright 5y ago
  (Please note ... we are not in disagreement, but I wanted to address this) Asking how 'planes fly upside-down and putting one's theory to the test in this thought experiment is useful, but it doesn't refute the "downwash" theory. If you have a wing at an appropriate angle of attack, even if upside-down, then provided the flow remains attached, it (the flow) can still be deflected downwards. If the wing is reasonably asymmetrical then the angle of attack needs to be large, which makes it more likely that the flow will become detached, so we deduce that a 'plane designed to fly upside-down needs to have symmetrical, or near symmetrical, aerofoil profiles ... which they do. So the downwash theory is not refuted by the "upside-down" question. (Please note: All that notwithstanding, the unadorned downwash theory is not the complete explanation of how lift is generated)
  - lordnacho 5y ago
    Yes very true. It is just one among many things that needs to be explained by a theory though. Things like why do stalls happen, why are there deltas on the ends of commercial airliners, and so on. Point is it's one of the more complicated everyday phenomena to explain.
- mansoor_ 5y ago
  Great question, but the physics is still the same. The aircraft still generates lift by direction air in a downwards direction. For some aircrafts (based on the wing geometry), you can achieve this upside down, within a certain range of angle-of-attack of the wing. Obviously, the aerodynamic efficiency is not the same. Airfoil sections are anti-symmetric across this direction because these designs achieve higher L/D (lift to drag) ratios.
gear54rus 5y ago
In that case the 2nd theory must be correct but they say its incorrect for some reason. Which doesnt make sense to me at all. They say it's incorrect because it doesnt account for the airflow on the top. Well the flow on the top doesnt strike the wing (except its forward facing part, the wing is pitched 'up' slightly) so it doesnt push it down. But the bottom flow does push it up (it strikes the whole surface of the wing). Really cant see whats wrong with this simple explanation.
- ColinWright 5y ago
  The top surface does cause the air to move down. Unless stalled, the air "clings to the top surface" and is pulled down by the shape of the foil. But the simple explanation is wrong because when you actually do the sums the numbers don't add up ... there is more going on.
  - gear54rus 5y ago
    OK well we can add that to the simple explanation. If the top airflow is diverted down when it wants to go straight, it would still want to go straight therefore pulling the top surface up, right?.
    - ColinWright 5y ago
      The numbers still don't add up, you still need to account for the pressure difference (lower on top, higher underneath) due to Bernoulli. The "downwash theory" doesn't give enough lift on it's own. So it's more complicated than that.
      - mansoor_ 5y ago
        I think you have a misunderstanding here. You are right that downwash accounts for a tiny proportion of the overall lift, in fact it only become significant close to the wing tips. Downwash (in the context of lift generating wings) refers to the downwards motions of air generated by tip vortices (look-up "induced drag" for more details). This is not the same as the net downwards motion of the air in the wake, relative to the free-stream air. Take for example, an infinite wing (or a 2D wing). Here, there is no downwash but we still generate lift and have a net downwards motion of the air in the wake. You can see this with online fluid simulators for airfoil sections, many of them are 2D.
ColinWright 5y ago
That's a very bold statement, so I'm compelled to ask: In the light of evidence that conflicts with your statement, why should we believe you? In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong. [0] Even faster than the (incorrect) equal transit time idea would imply.
- simonh 5y ago
  The statement only referred to the net motion of the vehicle and the deflected air. The fact that during that process the air here of there had higher or lower pressure is incidental, and not contradictory to this explanation. The air might have other things changed about it by the process, such as turbulent motion, but again those are incidental and are not contradictory to the general statement.
- mansoor_ 5y ago
  > In the light of evidence that conflicts with your statement, why should we believe you? Can you clarify what you mean? Which evidence? >In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong. These two perspective are in fact congruent. I decided to explain lift by reasoning about the circulation generated by the wings (https://en.wikipedia.org/wiki/Circulation_(physics) https://en.wikipedia.org/wiki/Circulation_(physics)). In this picture, the motion of the air in the wake of the wings has a net rotation (clockwise when the wind is coming from left to right) that generates an upwards resultant force on the wing. This is analogous to the pressure argument. I feel this perspective provides a clearer understanding. With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is. As I stress, it really is a simple and well understood problem. Complexity with fluid dynamics arises from problems like turbulence, multi-phase flow and computational modelling.
  - 5y ago
throwawaylinux 5y ago
> Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards. Well wings typically don't provide thrust but drag, i.e., a resultant force that is backwards and upwards. So according to Newton they would be pushing air down and forwards. The engines overcome drag and give a net forward force.
- mansoor_ 5y ago
  This is true, I was referring to the net motion of the air in the wake. You are right, the wing does technically push the air forwards.

closewith 5y ago

The second page of the article seems to address this and calls it an incorrect lift theory. > It is really that simple. The whole point of the article is that it's not that simple: > The real details of how an object generates lift are very complex and do not lend themselves to simplification.

mansoor_ 5y ago
> The second page of the article seems to address this and calls it an incorrect lift theory. If it does, it's wrong. Edit: Now that I have read it, I would have to disagree with you. The article only references the Bernoulli pressure arguments, it does not mention Newton's third law or airflow circulation. > The real details of how an object generates lift are very complex and do not lend themselves to simplification. There is no scientific contention about flight. Nobody is currently publishing papers on this topic ("journal of fluid mechanics" if you are interested), it was well understood in the previous century. Newton's third law explains at a high level how object fly. Yes you can dig into the details of fluid motion, turbulent flow, boundary layers etc, but this does not detract from the high-level picture.

hugh-avherald 5y ago

That's half of it. Lift is produced by airflow over both sides of an aeroplane wing not just the underside.

mansoor_ 5y ago
It's not just the underside that is responsible for the circulation of the airflow. In fact the majority of the circulation is generated by the top surface.

lordnacho 5y ago

Airplanes can fly upside down. How do you explain that?

gear54rus 5y ago
They don't do so at the same angle of attack I'd wager. Meaning to fly upside down they have to control pitch in a way that turns the wing to the exact needed angle. Same way they have to control pitch when flying normally to achieve lift. This pic of real jets doing that side by side confirms what I'm saying (nose is pitched up on the normal-flying plane and 'down' but still up relative to the ground on the inverted one): https://imgur.com/loVcrgO https://imgur.com/loVcrgO
ColinWright 5y ago
(Please note ... we are not in disagreement, but I wanted to address this) Asking how 'planes fly upside-down and putting one's theory to the test in this thought experiment is useful, but it doesn't refute the "downwash" theory. If you have a wing at an appropriate angle of attack, even if upside-down, then provided the flow remains attached, it (the flow) can still be deflected downwards. If the wing is reasonably asymmetrical then the angle of attack needs to be large, which makes it more likely that the flow will become detached, so we deduce that a 'plane designed to fly upside-down needs to have symmetrical, or near symmetrical, aerofoil profiles ... which they do. So the downwash theory is not refuted by the "upside-down" question. (Please note: All that notwithstanding, the unadorned downwash theory is not the complete explanation of how lift is generated)
- lordnacho 5y ago
  Yes very true. It is just one among many things that needs to be explained by a theory though. Things like why do stalls happen, why are there deltas on the ends of commercial airliners, and so on. Point is it's one of the more complicated everyday phenomena to explain.
mansoor_ 5y ago
Great question, but the physics is still the same. The aircraft still generates lift by direction air in a downwards direction. For some aircrafts (based on the wing geometry), you can achieve this upside down, within a certain range of angle-of-attack of the wing. Obviously, the aerodynamic efficiency is not the same. Airfoil sections are anti-symmetric across this direction because these designs achieve higher L/D (lift to drag) ratios.

gear54rus 5y ago

In that case the 2nd theory must be correct but they say its incorrect for some reason. Which doesnt make sense to me at all. They say it's incorrect because it doesnt account for the airflow on the top. Well the flow on the top doesnt strike the wing (except its forward facing part, the wing is pitched 'up' slightly) so it doesnt push it down. But the bottom flow does push it up (it strikes the whole surface of the wing). Really cant see whats wrong with this simple explanation.

ColinWright 5y ago
The top surface does cause the air to move down. Unless stalled, the air "clings to the top surface" and is pulled down by the shape of the foil. But the simple explanation is wrong because when you actually do the sums the numbers don't add up ... there is more going on.
- gear54rus 5y ago
  OK well we can add that to the simple explanation. If the top airflow is diverted down when it wants to go straight, it would still want to go straight therefore pulling the top surface up, right?.
  - ColinWright 5y ago
    The numbers still don't add up, you still need to account for the pressure difference (lower on top, higher underneath) due to Bernoulli. The "downwash theory" doesn't give enough lift on it's own. So it's more complicated than that.
    - mansoor_ 5y ago
      I think you have a misunderstanding here. You are right that downwash accounts for a tiny proportion of the overall lift, in fact it only become significant close to the wing tips. Downwash (in the context of lift generating wings) refers to the downwards motions of air generated by tip vortices (look-up "induced drag" for more details). This is not the same as the net downwards motion of the air in the wake, relative to the free-stream air. Take for example, an infinite wing (or a 2D wing). Here, there is no downwash but we still generate lift and have a net downwards motion of the air in the wake. You can see this with online fluid simulators for airfoil sections, many of them are 2D.

ColinWright 5y ago

That's a very bold statement, so I'm compelled to ask: In the light of evidence that conflicts with your statement, why should we believe you? In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong. [0] Even faster than the (incorrect) equal transit time idea would imply.

simonh 5y ago
The statement only referred to the net motion of the vehicle and the deflected air. The fact that during that process the air here of there had higher or lower pressure is incidental, and not contradictory to this explanation. The air might have other things changed about it by the process, such as turbulent motion, but again those are incidental and are not contradictory to the general statement.
mansoor_ 5y ago
> In the light of evidence that conflicts with your statement, why should we believe you? Can you clarify what you mean? Which evidence? >In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong. These two perspective are in fact congruent. I decided to explain lift by reasoning about the circulation generated by the wings (https://en.wikipedia.org/wiki/Circulation_(physics) https://en.wikipedia.org/wiki/Circulation_(physics)). In this picture, the motion of the air in the wake of the wings has a net rotation (clockwise when the wind is coming from left to right) that generates an upwards resultant force on the wing. This is analogous to the pressure argument. I feel this perspective provides a clearer understanding. With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is. As I stress, it really is a simple and well understood problem. Complexity with fluid dynamics arises from problems like turbulence, multi-phase flow and computational modelling.
- 5y ago

throwawaylinux 5y ago

> Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards. Well wings typically don't provide thrust but drag, i.e., a resultant force that is backwards and upwards. So according to Newton they would be pushing air down and forwards. The engines overcome drag and give a net forward force.

mansoor_ 5y ago
This is true, I was referring to the net motion of the air in the wake. You are right, the wing does technically push the air forwards.