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Foreword: I am an Aerospace Engineering student and next year is my third and final year, starting September 2019.

I was taught approximately 2 semesters ago that the correct theory of lift is that airflow circulates around the wing and so the upstream increases the velocity on the top surface while reducing the pressure, and reduces the velocity on the lower surface while increasing the pressure, which causes a difference in pressure that generates lift. Prior to that, I had the incorrect, yet simpler 'equal transit time' theory, which I'm guessing is taught at all universities at the early stage due to the simplicity of it. My problem is that I still base a lot of things on incorrect theories of the production of lift.

  1. What causes air to circulate around a wing, or even a cylinder? My answer to that would be due to viscosity and boundary layer effects, is that right or wrong? I'm honestly unsure.

  2. Since this theory was first being explored by analysing a cylinder, and it was determined that a cylinder can produce lift due to circulation of air (and the direction of upstream air), why do we have airfoils shaped in the way they are? I have many answers to this question which now I'm honestly unsure of due to the mix of theories in my head and some are based upon incorrect lift production concepts. Please give me a detailed answer for this question especially.

Ali Al Kayyali
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  • As a general comment, keep in mind that theories about lift are not mutually exclusive (except the equal transit time, that's just wrong). Splitting flow in translation and circulation is mathematically appealing but not necessarily good for an intuitive understanding. – Sanchises Jul 29 '19 at 16:25
  • I can't believe they are leaving out the whole acceleration of mass aspect which is what most "lift" is. Stand behind a propeller, or under a helicopter's rotor, which are both just wings going in a circle, in the blast of the accelerated air mass, and it's kind of hard to miss. – John K Jul 29 '19 at 16:33
  • John K, by accelerated mass are you trying to say that upper surface particles accelerates more due to the longer distance covered and it must meet with lower surface particles? If that is what you are saying, then it is wrong.. but please do elaborate on what you mean. – Ali Al Kayyali Jul 29 '19 at 16:38
  • @AliAlKayyali what I am saying is that the wing induces a large package of air, some below but mostly above, the wing to move downward as the result of the wing passing through it.The circulation and pressure distributions are all a part of that process. A flat sheet of plywood at a positive angle will produce lift just on the air deflected by the bottom of the sheet, but an airfoil influences a large part of the air above it as well to also move down, so it's way more efficient at doing it. When u stand behind a prop you are getting blown by the accelerated mass made by the "revolving" wing. – John K Jul 29 '19 at 16:57
  • @AliAlKayyali Since Newton's Law is a factor here, I suggest you google some word strings like "newton airfoil lift" or similar and you will find articles that explain the action/reaction aspect to lift generation. – John K Jul 29 '19 at 17:05
  • @JohnK okay so your point is that the downwash at the trailing edge is what causes lift using Newton's third law? So can I combine theories, such as the theory of momentum and the theory of flow circulation to get a correct explanation of lift and why airfoils are designed the way they are? Cause for me it now makes everything clearer with the theory of momentum. I mean what really matters is how to obtain lift and how to improve it (mathematically) but I still also rather have a good intuitive understanding of the correct theory. – Ali Al Kayyali Jul 29 '19 at 17:51
  • @JohnK Also, can you explain to me the reason why air circulates around a wing? Does a cylinder have to be spinning in order for the air to circulate around it, and why? – Ali Al Kayyali Jul 29 '19 at 17:52
  • Yes the cylinder has to spin so that its surface friction can induce circulation by making the boundary layer want to follow the moving surface of the cylinder, otherwise the air just goes around the top and bottom equally and rejoins aft somewhere. Try this out: http://www.onemetre.net/design/downwash/Circul/Circul.htm – John K Jul 29 '19 at 18:28
  • I'm going to suggest this is duplicate of https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift/, which has a pretty good, and correct, explanation, in the accepted answer. But since you mention circulation specifically, you may also want to refer to https://aviation.stackexchange.com/questions/47251/what-is-the-physical-meaning-of-circulation-found-in-kutta-condition. – Jan Hudec Jul 29 '19 at 20:29
  • @JanHudec thank you very much, that second link was very helpful. I can now conclude that a low RN causes circulation, and it is typically for a wing or some body with a sharp trailing edge? Though just one more comment, was the Kutta Condition determined experimentally? Because I can't find any mathematical model to represent it.. it seems as though it is only useful in solving the Kutta-Joukowski theorem. The reason I want to look more into the theory of circulation is just to widen my vision in the field of 2D and 3D flow over an airfoil. I am planning to base my M.Sc. thesis on this. – Ali Al Kayyali Jul 30 '19 at 18:37
  • @AliAlKayyali, yes, Kutta condition, and Kutta-Joukowski theorem were determined experimentally, and you need some experimentally determined values to calculate anything with them. It is possible to model the fluid flow mathematically using the Navier–Stokes equations, but those don't have analytical solution for any non-trivial case, so you need computer program to calculate them. For the simple 2-D case there is the famous XFOIL. – Jan Hudec Jul 30 '19 at 18:48

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