Hello everybody,

in the header of the aerodynamic data files include following informations:

Stall angle (deg)
Zero Cn angle of attack (deg)
Cn slope for zero lift (dimensionless)
Cn extrapolated to value at positive stall angle of attack
Cn at stall value for negative angle of attack
Angle of attack for minimum CD (deg)
Minimum CD value

How important are these values for the calculation in FAST and how exact should they be. I have a problem to estimate them from the coefficients below. I know that they are 3D-correkted but the 2D values doesn’t fit, anyway.

For example:
DU21_A17.dat from the NREL 5 MW Turbine

The “Zero Cn angle of attack (deg)” is -5.0609°
If I say Zero Cn has the same angle of attack like Zero Cl (because the angle is small), the angle has to be between -4.50 ° (Cl = -0.048) and -4.00 ° (Cl = 0.016). This comes from the coefficient table below and does not fit to -5.0609°.
How can I reach the correct value?

Best Regards,
Nils

Dear Nils-Christian,

I don’t think I can comment on how exact the values must be, but our AirfoilPrep preprocessor (wind.nrel.gov/designcodes/prepro … rfoilprep/) can be used to derive these values. I’ve attached the AirfoilPrep spreadsheet that I used for the DU21 airfoil of the NREL 5-MW turbine so you can see how the values for this airfoil were derived.

Best regards,
AirfoilPrep_v2p0_DU21_A17.xls (444 KB)

Dear Jason,

thank you for the answer. It helps alot. I usesd a interpolation to find alpha_0 and not the Ca / Cn slope-function. This avowed the difference in the results. I w’ll make test calculations with both and check the differences, soon.

Best regards,
Nils