# AirfoilPrep Understanding

[color=blue]Hi everybody,

I am learning AirfoilPrep and have some understanding questions.

1. The tables of airfoil characteristics (AOA CL CD CM) are measured in wind tunnels. So they could be augmented by 3DStall function of AirfoilPrep. Rotor R means TipRad?How to choose Alpha End? Rpm and wind speed are average rated values?

2. The 3D table results could now be extended in TableExtrap. Is Aspect Ratio= (Span^2)/Area also right for wind turbine blade? I do not know how to estimate the AR of tapered and twisted blade? Is Max CD looked up from the red table? or Just use the calculated value from AR.

3. The same input table as TableExtrap is used to calculate dynamic stall parameters. We choose stall angle from the CL column. How to understand " Cn at stall value for negative angle of attack" ?

Thanks

Best regards

I find some descriptions for the model used in 3DStall from a Phd thesis.
R is the blade span, constants a and b and d are given as 1 by the author of the model. V is the incoming wind speed. RPM is rotaional velocity. c and r are respectively the local chord and radius.

1. Are V, RPM rated average values? For example, V has cut in and cut out speed.
2. How could we choose the local chord? If one airfoil is used in different elements, these elements have different local chord. In the 5MW example, the last airfoil NACA64_A17 is used from 12th to 17th element, those chords are different. Which chord should be used to calculate the 3DStall augmentation for NACA64_A17 airfoil? r/R should have the same problem for different element?

Hi,everyone:
I also have this question:How to choose Alpha End?
Thanks.
Best regards.