Samoa,
As a followup, I used JavaFoil to give me polar plots for a comparison of Cl and L/D.
I choose the NACA4425 that I am using, , NACA4415 which is thinner, USPNS$ which is the Jacobs profile, and the SG6050 which the Sandia project chose.
Some are softer stall than others, some smoother transitions over AOA
three RE# were choosen : 50K, 150K and 300K
At design TSR=4.5, RE#300K is about max for 20mph WS for 4.5 inch cord at tip.
Lower RE# is for stations closer to hub in non tapered case.
Mid RE# is typical for lower WS ie 12mph.
http://www.otherpower.com/images/scimages/7526/LD_ratio.jpg
http://www.otherpower.com/images/scimages/7526/Cl_vs_AOA.jpg
I am trying to get my hands around these analyis tools to see how they give guidance.
If my presumptions are correct ...the L/D dictates the max TSR you will actually get at any WS, since drag at the tips limits how fast those tips can fly (thinner is better), but the actual lift coeff Cl coupled with the cord width determines how much torque you deliver at that TSR (wider is better).
Two diff rotors at same rpm in same WS, the greater the torque , the better the efficiency.
So, the balancing act is deciding on first a TSR to match the generator rpm, choosing the right blade pitch for AOA so that L/D is best at the 75% station, and then choose the profile and cord width so that the tip is as thin as reasonably possible, while still having enough meat at the root to keep the thing from flying apart. The ideal blade would have the same torque at all stations along the blade at some optimum WS at which point the maximum efficiency will be obtained.
In my prototype , I choose a non-twist, tapered 1:3 so that the RE# is fairly constant,
root is thick for strength, wide for low starting and high torque, but tip is thin to get to correct TSR = 3.5. By mounting on tubular spars, I can reset the pitch angle at will.
I would appreciate comments on the above conclusions.
Stew Corman from sunny Endicott