Questions regarding Airfoilprep

Hello everyone,

I have a few questions about the procedure of using Airfoilprep and some of the inputs.

I will state the procedure that i use and please correct me if i am wrong:

1- Since i have no access to experimental results for AOA, Cl and Cd i use Xfoil to calculate alpha Vs Cl and alpha Vs Cd for a range of AOA sat (from -3 to 23) .

2- I am using only one Re so i will skip interpolate worksheet.

3- I am only using only one airfoil so i will skip blended airfoil worksheet.

4- 3D stall worksheet:
I have checked the Selig Du equations below:


r/R and Chord: Do i have to enter r/R at each station, for example if i divide the blade to 10 elements do i have to enter r/R 10 times and
make airfoil .dat file for each station, taking in consideration that the airfoil is the same at all sections?

Rotor RPM:The same question here: since the RPM change which RPM should i use?

Min and Max alpha : Is this values related to the range that i choose at the beginning?

Alpha end: How can i choose this value?

Now i just copied the AOA, Cl and Cd into the worksheet and calculated 3D table.

5- TableExtrap worksheet:

Aspect ratio: How can i calculate this value if the blade is twisted and which chord should i use?
I know that it said: This value (CD max) is the one we’ll use. The Aspect Ratio value is provided for reference only, but i see it in the
above equations!!

Now i copied alpha, Cl and Cd to the table and update table.

6- DynStall:

Min and Max alpha: The same question as in 3D stall worksheet.

Stall angle of attack: how can i get this value, did i get it from Cl alpha curve?

Cn at stall value for negative angle of attack: How can i get this value and which parameters control it?
Is it an important parameter because i notice that when i change it nothing happened neither to the curve nor
to the values?

Now update all.

7-Editor format:

I know it some kind of long but i hope this will become reference for others.
Thanks in advance.

I can answer some of your questions:
3D stall sheet: 3D effects are more significant near the root region (inboard part of the blade) and become less significant in the outboard part. You can check this by by using different values of r/R and you will notice as r/R becomes > 0.45 or so, the difference between 2D and 3D data become negligible. Also keep in view that 3D corrections are more important for stall regulated wind turbine and not important or less important for variable speed pitch regulated wind turbine.
Blade is usually designed for a specific wind speed and/or rpm, so the rpm in the sheet should be rpm at which rated power of turbine is achieved.
Min and max alpha is used to calculate the slope of cl-alpha curve.
Alpha end is the angle until which 3D correction is applied, I guess (I’m not sure about this). But as your data is up to 23 deg so you can enter 23 deg as alpha end.
TableExtrap:: You just need to enter max Cd value. You can check entering different values of Aspect ratio to see if it has some influence. I am not sure if the equations you mentioned are for Viterna method?
DynStall: You can check the stall angle from the airfoil data which you calculated from Xfoil.

Irfan Zafar,

Thanks for your reply,
Your answers clear some of my doubts, but there are some clouds in the sky :confused:
i don’t know is it me or this stage of design is not easy. I can’t find find proper information about extending airfoil data using Airfoilprep, that made me ask myself how others do this process, did they use the same tool or there is another tool that i don’t know.

I have read Usage advice by Marshall L. Buhl Jr, and that is why i am using Airfoilprep.

Thanks again for your time

I also use AirfoilPrep except dynstall sheet but I don’t usually have much problems with it. What exactly is not clear? Can you re-phrase your question again, maybe I can answer it.

Kind regards,


Dear Hazem,

Some of your questions have been answered in early forum threads. Please use the Search feature of the forums to find previous discussions. The following link answers questions about the aspect ratio: [url]]