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I was looking at the great site code7700.com, but I don't really understand their explanation of lift and drag:

enter image description here The so-called infinite wing displaces air for a while, but the air particles return to their original positions along the relative wind. The relative wind experience up wash as it approaches the airfoil and down wash as it leaves the airfoil. But the up wash and down wash are equal so the net effect is cancelled. Because of this, the aerodynamic force occurs perpendicular to the relative wind and their is no induced drag. In other words, all of the aerodynamic force is producing lift, none of it is pulling the wing backwards.

Source

It seems this is a big confusion. Could someone just confirm that lift is created solely by the downwash and Newton's 3rd law of motion?

(Why air is deflected is not part of the question.)

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  • $\begingroup$ In the world of physics everything "just works that way" and anything we call "laws" are just models. Like the earth goes around the sun once a year not because the law of gravity "causing" it that way, but describe it well. Even now I don't think our understanding of physics knows for sure what "causes" gravity. Same thing here. Most people find the pressure-lift model more useful than the downwash-lift model to study an airfoil. Downwahs-lift model works better for a tunneled device like duct fan or jet where you can measure flow mass and flow velocity accurately not pressure distribution. $\endgroup$
    – user3528438
    Oct 20, 2017 at 19:34
  • $\begingroup$ is the car acceleration created by pushing the road backwards? or by the engine? or maybe by the torque? it all depends on how you want to understand the question $\endgroup$
    – szulat
    Oct 20, 2017 at 20:48
  • $\begingroup$ @user3528438 (and szulat) When you feel the recoil of a gun, you can't tell for sure if the recoil is due to the reaction to the bullet acceleration, according to the 3rd law? Is that what you suggest? $\endgroup$
    – mins
    Oct 20, 2017 at 22:06
  • $\begingroup$ Of course you can model it that way, or you can model it as the chamber pressure pushes the back of the chamber backwards. The choice of the model depends on the purpose. If you study the projectile's ballistics, then pressure model (2nd law) is more convenient. If you study the gun's recoil then the 3rd law's model is more convenient. Sure you can do it the other way but why? $\endgroup$
    – user3528438
    Oct 20, 2017 at 22:40
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    $\begingroup$ Although they don't directly explain why code7700 is right or wrong here, Dan Hulme and others have good explanations on lift, Newton's law, and oversimplications like this at this question, just in case anyone hasn't read it: aviation.stackexchange.com/questions/16193/… $\endgroup$
    – Cody P
    Oct 21, 2017 at 0:11

4 Answers 4

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If no downward acceleration occurs, there can be no lift.

However, a wing of infinite span will use an infinite amount of air for lift production, so an infinitely small amount of downward acceleration already suffices to produce lift.

Clearly, this is a thought experiment that cannot realistically be reproduced in reality.

Edit five years later: Essentially, this is d'Alambert's paradox of lift. Force without counterforce. This is only possible in 2D flow, as soon as reality intrudes it will disappear.

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    $\begingroup$ you have to use lift per length or down wash per length for all calculations or everything goes to infinity $\endgroup$
    – user3528438
    Oct 20, 2017 at 22:42
  • $\begingroup$ @user3528438: Since the downwash intensity is inversely proportional to the square of wingspan, this does not really help. $\endgroup$
    – Peter Kämpf
    Oct 20, 2017 at 23:48
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    $\begingroup$ @PeterKämpf "However, a wing of infinite span will use an infinite amount of air for lift production, so an infinitely small amount of downward acceleration already suffices to produce lift." A lift of infinite span will have aerodynamic properties in each section exactly equivalent to an airfoil. So, you don't have a downwash at trailing edge because it's of infinite length? That's wrong. You have a downwash at the trailing edge but it's because of the airfoil's circulation and is not the combined downwash of 3D wing induced flow plus airfoil local flow. $\endgroup$
    – ares
    Oct 21, 2017 at 21:08
  • $\begingroup$ @AlexandrosK.: No, you misunderstood the question. If you have measurable downwash with infinite span, lift will also be infinite. Here the question is about any lift at all without any downwash. And that cannot happen - an infinitely small amount is needed even for very little lift. $\endgroup$
    – Peter Kämpf
    Oct 21, 2017 at 23:14
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    $\begingroup$ @PeterKämpf Now you're playing with the equation relating wing induced downwash with aspect ratio $AR$ and lift coefficient $C_L$ when I was referring to the cambering of the flow behind the airfoil which is due to the circulation $\Gamma$ (only 2D flow). Check the second figure in my answer below. $\endgroup$
    – ares
    Oct 22, 2017 at 2:57
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To complete some of the other explanations here, lift is produced (at least the most part, there are other mechanisms by which lift is produced in real aircraft, such as engine mount angle) by downwash. Most if not all phenomenons producing lift can by explained by or can be linked to downwash.

By deflecting air downward, even if the velocity field at infinity is strictly unaffected by the immediate downwash of the wing, it still creates a slight deviation of the airflow close to the wing. This is translated to a pressure field, which combined with Bernouilli's theorem, gives rise to lift.

Basically, pressure lift can be explained by downwash and vice-versa.

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    $\begingroup$ Well, the inclined, lift-producing propeller or jet engine also accelerates air downwards. No need to make an exception - all cases of dynamic lift require downwash to work. $\endgroup$
    – Peter Kämpf
    Oct 22, 2017 at 8:28
  • $\begingroup$ @PeterKämpf Absolutely true. Just didn't cross my mind since we were talking about airfoil downwash, but it's basically the same. Thanks. $\endgroup$
    – Cedric
    Oct 22, 2017 at 18:18
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Sorry but this source is pretty bad. The explanation is clearly wrong. You should download a basics aerodynamic book...

Here's what happen in a finite (real world) wing, the lifting of the wing generates a pressure field which induces vortices running from the lower surface to the upper, these vortices generate the downwash which is explicitly related to induced drag by a simple equation. The strength of the downwash is mainly depended on the lift coefficient and the aspect ratio of the wing, and in the limiting case of infinite wing (which is a theoretical aerodynamic's term) the wing's induced downwash doesn't exist and induced drag is zero.

So, downwash or upwash would depend on the wing geometry and lift, it's lift tha generates the upwash/downwash and also affected by it since it modifies the local angle of attack on each wing's section.

If you're considering the full airplane, the upwash/downwash of wing is also affected by the fuselage near the root.

This picture you posted is very wrong and confusing, because even in 2D flow, you have a cambering of the flow behind the airfoil (of course flow isn't just going flat after trailing edge). The flow then re-alignes with the freestream direction.

enter image description here

This picture. taken by a Stanford's Aerodynamic's lecture course, perfectly answers your questionenter image description here

The Kutta-Jukowski theorem for lift states that lift is proportional to density, freestream velocity and circulation $\Gamma$ by the equation $L=\rho V_{\infty} \Gamma$. For a 2D airfoil the circulation is as depicted in the far upper sketch and will produce an upwash in leading edge region and a downwash in the trailing edge region, then in the second sketch you see the downwash induced by the wing's free vortices. Lastly, you see the combined induced flowfield.

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  • $\begingroup$ I explicitly mentioned "why air is deflected is not part of the question" to prevent entering useless debates around the cause of the downwash, I'm interested in the consequence of it as lift. So unless you rewrite it, your post is not an answer. $\endgroup$
    – mins
    Oct 21, 2017 at 9:55
  • $\begingroup$ @mins Deflected air changes local angle of attack and hence lift, and the change of lift changes the deflected air, this is strongly coupled. $\endgroup$
    – ares
    Oct 21, 2017 at 21:26
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    $\begingroup$ There are few points pretty unclear (and that seems false in fact). First, there can be lift without downwash-producing wingtip vortices. A 3D wall-to-wall wing (i.e. 3D airfoil) still produces lift, without wingtip vortices. Kutta-Jukowski's circulation is simply be negative on the underside, and positive and bigger on the upper side, which integrated over the airfoil curve gives a non-zero lift (circulation is greater over than under). Basically, lift is created by air downwash. $\endgroup$
    – Cedric
    Oct 22, 2017 at 2:35
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    $\begingroup$ @AlexandrosK. I didn't say you said it. I said it is unclear, because given your explanations, one can assume lift is created by vortices and given the image you reference, wingtip vortices are the most apparent thing on it. Moreover, the second referenced image about circulation do tend blend in with your "upwash" explanations. Which is why I wanted to clear this up. $\endgroup$
    – Cedric
    Oct 22, 2017 at 2:53
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    $\begingroup$ @Cedric Ok, I hope the second sketch made it clear, by separating 2D and 3D upwash/downwash. Again, just to know, it's not only the wingtip vortices, as you see from figure 1 there is a whole vortex system on the wake. Which is why winglets can suppress only locally the effect of vortices. $\endgroup$
    – ares
    Oct 22, 2017 at 3:04
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Downwash is in no way a cause of lift.

Newtons third law (equal and opposite reaction) is a consequence not a cause of Lift. It is a consequence of the principle of conservation of momentum, which must be due to required spatial-translation symmetry. According to Noether's Theorem, every physical symmetry leads to a conservation law. For example, time-translation symmetry (the laws of physics don't change over time) implies conservation of energy, and space-translation symmetry (the laws of physics are the same everywhere) implies conservation of momentum. But it is Newtons second law (F=dP/dt, where P is Momentum (mass x velocity)) that is most directly associated (but not causal to) with aerodynamic lift.

Lift is a FORCE. Forces can and do exist even in the absence of motion, (see below). If lift can be said to be caused by anything, it is caused by thr exchange in momentum (dP/dt), that occurs when molecules bounce off the surface of an airframe.

A simple thought experiment demonstrates that downwash is not necessary to have Lift. Inflate an expandable compartment under a weight with gas (or air) under pressure. What is holding up the weight? Clearly there must be a LIFT force! Just as clearly, there is no downwash as there is no no fluid movement, (except Brownian motion). To see a more detailed and academic explanation of this, (and other aerodynamics misconceptions), review this you tube video

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  • $\begingroup$ +1. You're right to remind action-reaction is a case of momentum conservation, and therefore not an initial cause. Downwash is nevertheless a convenient intermediate result to compute lift without restarting to Navier Stokes equations. If I'm not wrong this is actually the idea behind Kutta Condition which by itself is an answer to my initial question: "Can lift be created without air downwash?" $\endgroup$
    – mins
    Dec 4, 2022 at 10:13
  • $\begingroup$ @mins and CharlesBretana: Newton's third law has nothing to do with energy. Newton's first, second and third laws translate in simple words the conservation of momentum. There's no causes or consequences involved, just sides of the same coin. Molecules do not bounce off of the surface of an airframe unless the airframe is flying hypersonic. $\endgroup$
    – sophit
    Dec 4, 2022 at 17:56
  • $\begingroup$ @Sophit, yes my bad, you are correct about 3rd law. But Simple thought experiment that demonstrates that downwash is not necessary to have Lift. Inflate an expandable compartment under a weight with gas (or air) under pressure. What is holding up the weight? Clearly there must be LIFT! Just as clearly, there is no downwash. $\endgroup$
    – Charles Bretana
    Dec 5, 2022 at 1:58
  • $\begingroup$ @sophit, thanks did as you suggested. $\endgroup$
    – Charles Bretana
    Dec 5, 2022 at 15:24
  • $\begingroup$ "Inflate an expandable compartment under a weight with gas (or air) under pressure [...]" if I understand correctly this looks like a balloon. There is no lift in such aircraft, only buoyancy. $\endgroup$
    – mins
    Dec 5, 2022 at 19:10

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