Some problem in extrapolating cl, cd and cm

Hello everyone,

This question is related to Airfoil Cl,Cd, Cm extrapolating using Airfoilprep_v2.02.03.xsl.

I would like to know the detail of thereom about it.

  1. It doesn’t use the same formula when we calculated extrapolating of Cl.

The formula corresponds to the reference(Wind turbine post-stall airfoil performance characteristics guidelines for blade-element momentum methods)
only at degree from the angle extrapolating begin to 90, which uses viterna method.

But other regions of angle have slightly change in formula, for example, from (180-valpahi)degree to 180 degree, it is Cl=Ang * CLHi * CLAdj/ VAlphaHi.

Does anyone know why the formula change?
Or is there any reference explaining the detail about these?

  1. Cm extrapolating

In this part, what i know in it is much less than the cl extrapolating.

I will like to know the theory of it.

Can anyone give me some guide or reference about it?

I hope I was quite clear, and sorry for my poor English.

Thank you for your help.