| I am writing this post to discuss the benefits of using progressively thicker airfoils at the interior of the blade, with a somewhat limited chord, as opposed to using a constant thickness throughout. It will discuss how the benefits can be seen in either a twisted blade, or a strait blade. For this discussion, we will be using a static pitch, 17 degree total twist 3 blade, 10 ft. dia. turbine, with a N.A.C.A. 44xx series of airfoils.
Before we get into the aerodynamic benefits of using thicker airfoils on inboard sections of the blade, we must discuss how much power is in each segment of the blade sweep.
It should be noted that the segments are listed as such 100%= (100% - 90% span), and that those numbers are taken from total amount of power in the section, not the actual amount collected.
Also notice that adding 10% to the blade dia. increases the total swept area by 19%.
You can see how little power there is in the inboard sections of the blade. This proves that the dominate function of the inboard sections is structural, not aerodynamic. (i.e. The 20% section is only contains 3% of the wind power but is responsible for transferring approx 99% of the load)
Now we can discuss the apparent wind angle. The apparent wind angle is different on each section of the blade, which is the idea behind twisting blades. Also turbines work over a range of TSRs throughout there power curve, starting at 0, and working up to their rated TSR. Here is a list of apparent wind angles for 3 different TSRs
Note that the twist for the inboard sections. No one builds blades with segments with a 59 degree twist, or even a 30 degree, it would be very structurally inefficient. Twisted blades are twisted to take advantage of the outermost 60%-70% more effectively, than they are twisted to take advantage of ALL of the wind at all sections.
Now we can start looking at the aerodynamic data. Below is a list of the performance curves for 4 different airfoils. They were NACA 4415, 4420, 4425, and 4430. The last two digits of the airfoil number represent the thickness of the airfoil. These calculations were done with Javafoil, using the Eppler transition, and stall models. The Reynolds Number was set to 300,000
These calculations show optimum angle of attack and relative output, and does not represent the actual blade efficiency. Note how the 4430 has a much wider angle of attack which is logical for a wider profile, but it actually has a higher Lift/Drag (Cl/Cd) than the 4415 at any angle past 11 degrees.
Figuring that a twisted blade has 17 degrees of total twist in the blade, and is mounted at a starting AOA of 3, with a linear twist, this would be your twist
When you figure in the apparent wind angles vs. TSR you get your angles of attack:
You can see that at any TSR, the inboard sections would benefit from being a thicker profile due to their angles of attack, especially during their lower TSRs. This holds true even more-so with straight blades because they will have even higher relative angles of attack on their inboard sections due to the lack of twist. Not to mention the fact that the 4430 has peak performance at a higher AOA than that of thinner airfoils. (which is good because you can't realistically twist the inboard sections enough)
So in conclusion I believe the benefits of using thicker inboard airfoils are three fold. First, you can build a longer blade by using less material, which is the best way to increase power and the drop the $/KWH. Second, the thicker airfoils have a wider range of good AOAs, which better suits them for the wind characteristics at the interior of the blade. Lastly, given the relative wind angles, and realistic construction constraints (max-twist), an increase in efficiency is actually seen with thicker airfoils at the inboard
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