Wind tunnel tests have proven that rotors with constant chord blades can have a high maximum Cp if the correct airfoil is used which has a low Cd/Cl ratio for a large alpha range.
Indeed, most operation of a WT should be in the range of angle of attack below the peak Cl/Cd point.
This brings to mind the fact an airfoil that achieves very high CL is counterproductive in a WT, and it is best to choose low drag instead.
Tapered wind turbine blades are normally designed for the lift coefficient for which the Cd/Cl ratio is minimal. This so called optimum lift coefficient is lying a lot lower than the maximum lift coefficient. The optimum lift coefficient is found for the point of contact of a line through the origin and the Cl/Cd curve. If we take the Gö 711 airfoil (see my public report KD 285), it can be seen in figure 4 that the optimum lift coefficient is about 0.95 for the point of contact. The Cd/Cl ratio for this point is about 0.015 which is very low for an airfoil with a flat lower side. I think that this is because the airfoil was measured for a rather high Reynolds value of 4 * 10^5. In figure 3 it can be seen that Cl = 0.95 corresponds to alpa = 3.3°. In figure 4 it can be seen that the maximum lift coefficient is about 1.5 for alpha = 12.5°.
If we draw a line in figure 4 though the origin and the point Cd = 0.025 and Cl =1, all points on the Cl/Cd curve lying left from this line have a Cd/Cl ratio smaller than 0.025. This corresponds to values of Cl larger than 0.57 and smaller than 1.42. In figure 3 it can be seen that this Cl range corresponds to values of alpha larger -1° and smaller than 10°. So this is a very large alpha range for which the Cd/Cl ratio is small enough to expect a high maximum Cp.
Constant chord blades are designed for a low lift coefficient at the blade tip, for the optimum lift coefficient about half way the blade and for the maximum lift coefficient near the blade root. So the average Cd/Cl ratio for a constant chord blade is somewhat higher than for a tapered blade and this results in a somewhat lower maximum Cp. But manufacture of a constant chord blade is much easier especially if the blade is made from massive wood and a much smaller wooden beam is needed.
Another advantage of a low lift coefficient at the blade tip is that the pressure difference in between the front side and the back side of the airfoil is smaller and this may result is lower noice production if a constant chord blade is compared with a tapered blade with the same length and the same design tip speed ratio.