# Airfoilprep questions

Sorry to disturb everyone, but now I have several questions when using Airfoilprep to get the airfoil data.
First, in the 3D stall column, how I can choose the box 10F(Alpha End, End of full correction at this angle of attack) if I have the 2D Cl and Cd data from -2 degrees to 80 degrees at a fixed Renolds number? I found that if I chose too high value of Alpha End, the Cl of high angles will be very large, so I have this trouble.
Second, when using TableExtrap column, is aspect ratio means mean chord length of the blade divided by the radius of wind tubine blade? Also which of the two Cd extention methods and are there any limitations?
Third, if I have obtained 2D experimental data of Cl and Cd from -10 degrees to 90 degrees, should I use them all to do the 3D stall corrections and the TableExtrap? Or should I take part of them(for example data from -10 degrees to 30 degrees) to do the corrections?
Hope anyone could help me.
Best regards.
Streamkite.Zhang

Hi Streamkite,

There is no physical basis for picking the value of Alpha End and for most airfoils we have found a value of 30 works best. If this leads to unrealistic results for your particular airfoil, I’d recommend using the tool to find a better value via trial and error.

In the TableExtrap column, aspect ratio is span divided by mean chord length or span squared divided blade planform area. Not sure about the limitations of the Cd extension methods - I’d recommend reading the paper by Viterna that describes his method.

And finally, I wouldn’t start by extrapolating your full range from -10 to 90, but rather use -10 to 30 (or even 20) - then compare this to the actual data you have at high angles of attack to tune the parameters in the spreadsheet. Once there’s agreement between the fitted curve and actual data, compare the extrapolated full range using the -10-20 data and the -10-90 data and see what looks to be more physically realistic. I’m sure many people on this forum would be interested in your results if you did such a comparison and are willing to share the outcome.

cheers,
Pat

That was a good explanation Pat.Moriarty thank you.

Hi Pat.Moriarty, I hope you still active in the forum so you read my question,

should the Alpha End be grater than the stall alpha where the lift starts to decrease?
is there a maximum ratio or difference that the 3D cl should not exceed the 2D cl?

Thank you,
Aziz