# Using Viterna Method in AirfoilPrep

I have 2D lift and drag coefficient data of NACA 63415 from AOA -10 to 30 degrees at various Reynolds’ numbers. I am trying to extrapolate it from AOA 0f -180 to 180 degrees using AirfoilPrep. My question is if i should apply 3D corrections to my data before using it in Extrapolation sheet in Airfoilprep as i have noticed Viterna method in Airfoilprep requires aspect ratio (which indicates a 3D blade and not a 2D airfoil).
Also how do i calculate/guess MAX CD and aspect ratio for inputting in Airfoilprep for my tidal turbine’s blade.

Dear Zeeshan,

Yes, we would normally apply the 3D (rotational augmentation) corrections before the Viterna extrapolation.

Regarding the aspect ratio (AR), please see the following forum topic: http://forums.nrel.gov/t/extrpolation-s809-viterna-model-nrel-phase-vi-turbine/1237/1.

Regarding the max CD, AirfoilPrep recommends the equation max CD = 1.11 + 0.018*AR, which came from the following report (although I’m not familiar with the details): Viterna, L. A. and Janetzke, D. C., Theoretical and Experimental Power From Large Horizontal-Axis Wind Turbines. NASA TM-82944, September, 1982.

Best regards,

Thank you very much sir.

Dear Jonkman,

Do you know at which angle of attack the stall delay effect disappers? According to an example in Airfoilprep program, Cl and Cd data of a profile between -9 and 50 degree are given and the stall delay is applied in this range. So this means that stall delay effect continues to until 50 degree AOA or may be applied to higher degree of AOA?

Yours sincerely.

Mustafa SAHIN.
Metu Aerospace Engineering, Turkey.

Dear Mustafa,

Yes, that is possible. Of course, at such deep stall there is very limited data available (especially for a rotating rotor) and so the airfoil data may be hard to estimate. One thing to keep in mind is that for a thin airfoil operating in very deep stall, the airfoil will likely behave much like a flat plate, so, one would expect that the drag and lift curves intersect at around 45 degrees angle of attack.

Best regards,

Dear Jonkman,