Airfoilprep Newbie - Couple of Questions


Sorry if the following questions seem basic but aerodynamics is not my specialty and I am analysing our turbine for the first time in this manner with a view to making some control improvements.

  1. I’m unclear what the Interpolate worksheet is for. I have worked through the rest of Airfoilprep ignoring this step, put the resulting data through WT_Prof and got results which correlate reasonably well with test data so I wonder if I really need to use this worksheet? I have X,Y data for our airfoil which provides a full sweep of the profile which does not seem to be the correct input so what would be the correct input?
  2. My understanding is that the ratio r/R on the 3Dstall worksheet is that of an arbitrary radius divided by the rotor radius, if this is not the case please correct me! Can anyone give tips or guidelines on r/R selection? FYI our blade has a root to tip chord ratio of approx. 2.7 (i.e. chord at root is 2.7 times the chord at the tip) with most of the transition occurring in the first third of the radius.

Thanks in advance,

Hi Mike,

The interpolate sheet is just a general purpose interpolation tool. If you use multiple Re numbers then programs such as FAST require that all AOAs across the range of Re numbers be the same. With experimental data you may not end up with the same AOAs all the time and I think the original intent was to help users interpolate Airfoil data to a common set of angles. Although I have heard of people using it for interpolating just about anything. So to answer your question, you do not need to use it.

As for the r/R, it is the ratio of the radial location (r) of the airfoil data that you have input into the 3DStall sheet to the rotor radius (R). If you play with the r/R value you will see that the 3D stall effect dimnishes as you get closer to the tip. For each station you could modify your airfoil data to account for this effect, at least upto around an r/R of .5 or .6 beyond which the effects are probably not significant.

I hope that helps.



Thanks Subin,

That clears things up perfectly for me. :mrgreen:


I am so happy that members such as Subin have pitched in to help answer questions. We really struggle here at NREL to answer questions in a timely fashion and this really helps.

It’s also a good way for members to make names for themselves. You can even add that you are a regular contributor to the forums in your resumes!

Thanks, Subin!

Hi Marshall,

Thank you for the kind words. I have learned a lot from this forum and from all the input that you and others at NREL have graciously provided and I am just trying to do my part by paying it forward and sharing what I have picked up while trying to understand the workings of the codes. It is great to have a resource such as this and I am sure all of us newbies are grateful to NREL for providing us with this platform to share ideas and learn from each other and from people who have had lots more experience with this stuff than we have. So once again thank you and keep up the good work.