That same graph has been endlessly referred to, over and over and over and over, for decades. often with the comment that the American farm water-pumping windmill curve, and the Savonius curve, were each mislabeled as the other. One thing that hit me decades ago was how the entire world would just keep repeating the same old graph over and over, with nobody ever changing it or coming up with a new version. I guess it persists to this day. OK if the “Professor Crackpot” Savonius can reach a Cp of .3, please tell me how. Let’s think about it logically: half of the Savonius swept area is traveling upwind, using power. The half that produces power is traveling downwind, reducing the relative wind speed, so it is not efficient. The only way you could get a Cp of .3 for the whole machine would be if the power-producing half of the machine was operating at the Betz limit of ~.6, and if the power-losing, upwind-traveling half had no losses. In other words, the idea of a Cp of .3 for a Savonius implies some miraculous “perfect performance” which is impossible since the downwind-traveling half cannot physically operate at high efficiency due that downwind travel, and the upwind-traveling half cannot possibly offer zero drag due to the enhanced windspeed it must push upwind into. On the other hand, the American farm water-pumping windmill, which was never intended to have high efficiency, so much as high reliability, and low intermittency, is still reasonably efficient, with the entire rotor fully exposed to the wind in a productive orientation, so a Cp of .3 is more believable for the American farm windmill. Therefore, I do not believe the two figures were reversed and are now correct, but instead it looks like they are reversed to this day. The silly thing is, after decades of repeating this graph, to this day people do not even know what they are looking at, no matter how many equations they can cite and repeat… Wind turbine design need not involve a lot of math or analysis. It is like riding a bike or crossing the street. Most of the factors beginners overanalyze then fail to grasp can be ignored. All the intricate analysis of airfoils etc. can similarly be ignored. Most any reasonable airfoil works fine. You just have to get the proportions right, within reason. Look at what works, copy it. The amount of info you need will fit on an index card - no need for all the complex analysis and endless equations. Wind energy is simple - either you know how to do it or you do not. Attempting to pick apart the theory in minute detail, then coming to wrong conclusions, due to still not really understanding it, is where this stuff leads. Endless formulae won;t help you if you don’t have a feel for it, informed by what is known to work. I can build great turbines in my sleep, seldom using any analytical tools beyond a measuring tape, a protractor, some calipers, and my eye. Make an airfoil that looks like an airfoil and it will work fine. Your chord and setting angles, number of blades, twist profile, etc. etc., etc., are just as influential as your choice of airfoil. No need to try to optimize the airfoil in most cases. There are so many other factors that affect performance than the choice of airfoil. And all these factors are interrelated - changing one affects all the others. In many cases, too “good” of an airfoil will work against performance due to all the other factors. Windmills are like any other aircraft - if it looks right, it flies right.