# What is the reason for circulation of air around an airfoil? Based upon the circulation of air, why are airfoils shaped the way they are? [duplicate]

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.

• 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. Jul 29, 2019 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. Jul 29, 2019 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. Jul 29, 2019 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. Jul 29, 2019 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. Jul 29, 2019 at 17:05