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.
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?