I have been working on an M.Sc. thesis in Kathmandu, Nepal, concerning designing and manufacturing a 5m Diameter wind turbine rotor.
I wanted to know if any forum members had any advice for me. Here is the basics that I have so far…
There are lots of people who use NACA 4415 and NACA 23015 (including some local manufacturer’s of smaller turbines) but they are generally considered by many to be outdated. LS(1) MOD and NLF airfoils were (or still are) used by some manufacturers, but they are also considered outdated.
I plan on using the NREL S-822 and S-823. From what I gather, the coordinates are now publicly available and by using a general time line, any patent from their initial creation should have expired a few years ago.
Does anyone have experience interpolating these foils? Is it just by feel? Modeling results? Is there some kind of recommended method? I can’t find a publication that directs you at what r/R to have what % interpolation between the two profiles. Does anyone know if this exists?
Below is my first choice for 10 elements in the BEM method (there is no analysis to support this as optimum).
Root
1: Pure 823
2: Pure 823
3: Pure 823
4: 20% / 80%
5: 40% / 60%
6: 60% / 40%
7: 80% / 20%
8: Pure 822
9: Pure 822
10: Pure 822
Tip
Brian
P.S. For any of you designers who are having trouble lofting between scaling/twisting airfoil profiles in a 3D cad program, I have the following tip: Remove some of the extra points near the leading edge and trailing edge of your airfoil profiles. Sometimes, since they are so close together, when lofting across the scaling/twisting profiles the 3D cad program will cross boundaries and create zero-volume errors. Reducing the number of points on the ends fixed this for me. 
Brian,
By interpolating, are you trying to get the shapes of intermediate stations between the “pure” airfoils? Is that so you can build a blade? I don’t think I can help you with that.
If you just want lift and drag data for intermediate stations, one of our summer interns did a study where he interpolated the shapes and used Xfoil to compute the coefficients. He also interpolated the airfoil data between the "pure’ airfoils and compared the resulting coefficients to those coming from Xfoil. The minor difference were not worth the effort of interpolating shapes.
Marshall
That is what I am trying to do. I am going to use XFLR5 (the 3rd party GUI frontend for XFOIL) to create the intermediate shapes. I’m just not sure what % linear interpolation to use at which intermediate stations in building a blade. I was hoping that the original NREL design specs had interpolation vs. radius information.
In the end the only limited information I could find was that the pure S-822 is at 90% radius and the pure S-823 is at 40% radius. I think my final blade will just use pure shapes outside these positions and a linear interpolation for the stations in between (I will also neglect the hub radius, meaning that station 9 of 10 is considered 90% radius).
I am currently (at this exact second) using XFLR5 to calculate the coefficients of the interpolated shapes too. At this late point I might as well finish, even if it’s not worth the effort. So far I never thought to compare the Cl Cd Cm data between the interpolated foils to see if there was a noticeable difference or not. I just put them into data files for my software program to look up.
Is the result of your intern’s work in a publicly available place? (Only so I can look it over and reference it in my thesis, not so I can perform a detailed review of the individual data sets.)
Thanks for your help.
Brian
P.S. I’ll be enrolling in Colorado School of Mines in the fall, so I’ll be headed to NREL’s neck of the woods soon.
Brian,
The mentor for the airfoil interpolation project, Pat Moriarty, says that no document was ever produced. Sorry.
It’s quite easy to interpolate Cl, Cd data between airfoils using AirfoilPrep. You’ve already done the hard part (interpreting the shapes and computing the coefficients), so you may want to consider doing that to see how the two methods compare. It would be good to get such a study documented. If you do that, please let us know so we can tell others of your work.
Marshall
Hallo!
This is not so easy.
What’s missing:
-Tip speed ratio - Reynolds number
- Blade materal (e.g. if you have only low torsional stiffness you should use an airfoil with low
pitching moment)
- This tip airfoil will break down as a first result below ~ RE= 600000.
( I took Xfoil and half Reynolds -number)
This is e.g. 30 m/s speed and 30 cm chord.
Keeping fingers away from NACA is not a bad choice.
They are created in 1945. In these times they were a revolution most probably.
Linear interpolation of tangler-airfoils is not
“allowed”- especially on pressure side.
Hermann