Blade.dat file angles

Wind & Water Rotor Aerodynamics
1

I have the radius, chord, thickness, twist, centerposition and airfoil type of blade. Is there a way to calculate the following data in the blade.dat file: BlCrvAC BlSwpAC BlCrvAng

2

Dear @Karthik.Prakash,

By “center position”, presumably you are referring to the location of the aerodynamic center, that is, the reference point about which the airfoil lift, drag, and pitching moment data are defined about. This location is BlCrvAC and BlSwpAC, which are the out-of-plane and in-plane offsets of the aerodynamic center from the pitch axis. You can derive BlCrvAng from BlCrvAC and BlSpn as follows:

BlCrvAng(n) = atan ( ( BlCrvAC(n+1) - BlCrvAC(n-1) ) / ( BlSpn(n+1) - BlSpn(n-1) )*180/pi

Best regards,

3

Dear @Jason.Jonkman

Thanks a lot for the quick response. The ‘center position’ data I have is the aerodynamic center of each airfoil on its chord. In that case, id I need to get the BlCrvAC and BlSwpAC, can it be done via calcuation or does it require testing data.

Best regards,
Karthik

4

Dear @Karthik.Prakash,

If you are defining the “aerodynamic center” the same way I did–i.e., the reference point about which the airfoil lift, drag, and pitching moment data are defined about–then this is BlCrvAC and BlSwpAC.

Best regards,

5

Dear @Jason.Jonkman

Many Thanks for the reply.
I am still trying to get a clarity on the definition of “aerodynamic center” data that I have. In the meantime, I have 2 questions, can it so happen that BlCrvAC = BlSwpAC.
Also, what can be the high level effects if it turn both of them to zero. Also, Does blades less that 95 m usually have BlSwpAC. or is it usually kept only for longer rotors?

6

Dear @Karthik.Prakash,

Here are my responses:

I am still trying to get a clarity on the definition of “aerodynamic center” data that I have. In the meantime, I have 2 questions, can it so happen that BlCrvAC = BlSwpAC.

That sounds highly unlikely to me to have the transverse offsets of the aerodynamic center from the pitch axis be the same in both transverse directions.

Also, what can be the high level effects if it turn both of them to zero. Also, Does blades less that 95 m usually have BlSwpAC. or is it usually kept only for longer rotors?

When both BlCrvAC and BlSwpAC are zero, this means that the aerodynamic loads are applied along the pitch axis. If you set them this way incorrectly, this would impact the accuracy of the pitching moment on the blade. Many modern utilility-scale blades have some built-in prebend / precurve or presweep whereby BlCrvAC and BlSwpAC would be nonzero.

Best regards,

7

Thanks a lot for the reply dear @Jason.Jonkman

From Definition of parametersI found the following equations. Should the AeroTwist be in “radians” since for my calculations for upwind turbine its coming as +ve.
BlCrvAC and BlSwpAC are related to AeroCent and AeroTwist from FAST v7.02 as follows:
BlCrvAC = -(0.25-AeroCent)Chord SIN(AeroTwst)
BlSwpAC = -(0.25-AeroCent)Chord COS(AeroTwst)

8

Dear @Karthik.Prakash,

Whether or not AeroTwst in these equations should have the units of radians or degrees depends on if the SIN() and COS() functions you are using expect radians or degrees (they should be consistent).

Best regards,