Wanted: University to predict airfoil coefficients


A long-time goal of mine has been to have someone use a program such as XFoil to generate lift, drag, and moment data for our S-series airfoils (wind.nrel.gov/airfoils/). The shapes of the airfoils are listed on the site. Dan Somers of Airfoils, Inc. used an old version of the Eppler code to do it when he designed the airfoils for us. He has told me recently that the results may not be accurate and that he has made improvements to the Eppler code since then. We also do not have data for all our airfoils.

I believe the Eppler code costs money, but I don’t know if he is willing to let it be used for free by universities.

If a university is looking for a project for a student, this is something that would help the wind industry, as I still get requests for the data.

Another extension to the project would be to generate 360-degree data using flat-plat theory or the Viterna method. We use our AirfoilPrep spreadsheet to do this, but a real aerodynamicist should examine the results to ensure they are realistic. Many engineers who do this are loads analysts and not aerodynamicists.

If you are interested, please let me know.


Hello Marshall!

A comment:

  1. I think it’s well known that “the old” Eppler code predicts airfoil performance too optimistic.
    xfoil is here better. There are several publications on the internet which show that.
    Nevertheless, checking out measurement results with xfoil tell that it’s a little bit optimistic.

  2. Thin airfoils do IMO not necessarily mean that there’s low drag. Because you need camber to produce
    circulation- therefore the appropriate thickness depending on Reynolds number is needed.
    In fact it’s correct that an airfoil running in dirt with much turbulent flow- here thin airfoils are always better.

  3. Do you know the philosophy in which way the Tangler-airfoils are made dirt- insensitive?
    From my point of view their leading edge has a low radius. This is resulting in a high sensitivity, because
    there are high overspeeds and the lam-turb transition is triggered by const/sqrt(Re)
    if I remember correctly from my red fluid-mechanic book of my university teacher.

And for laminar flow there is a note in the old Abbott-Doenhoff (I think NACA-Report 824)
I found half a year ago that they found there’s an optimum leading-edge-radius fitting to the airfoils.

Kind regards

Dear Hermann,

Thank you so much for your insight. I’m sorry it took so long to respond, but I was reassigned to a high priority project for a half year and just did not have time to keep up with the forums.

I am a loads analyst and not an aerodynamicist, so I cannot comment on some of your points. You may be able to find information about how Somers designed the airfoils for roughness insensitivity in one of his documents here:


Despite being designed by the Eppler code (which you say overpredicts performance), the S809 did perform much better when dirty than the original airfoils used on the Micon 65/13.

Thanks again for your response.


Dear Marshall,

We have been working on the combined research (international research center and Nanjing University of Aeronautics and Astronautics) with the group of scientists from Jiangsu Key Laboratory of Hi-Tech Research for Wind Turbine Design.
Other contributions: Turbulent flow simulation of the NREL S809 airfoil
https://www.researchgate.net/publication/228358590_Turbulent_flow_simulation_of_the_NREL_S809_airfoil_by_Ouahiba Guerri

Best regards,
Renee Degutis

The last article was Combined Effect of Rotational Augmentation and Dynamic Stall on a Horizontal Axis Wind Turbine. I can give access to the full archive of materials, if you are interested, and I would highly appreciate if you could share your recent publications on the S809.