cA and cW calculation from airfoil data


I downloaded the profile coordinates for the s818-profile from NREL and then tried to calculate the cA and cW-values with javafoil However there are huge differences compared to the values supplied with fast. As I do not know much about airfoils I wonder how what I need to change to get the same results from javafoil or any similar code?
The reason for this calculation is, that I also need the cM-value for the calculation of the pitching moment.

Best regards,

By the way: NRELs server says “Forbidden - You don’t have permission to access /designcodes/simulators/fast/ on this server.” Is there something wrong with the server configuration?


If the airfoil-coefficient file you used was S818_2703.dat in the WP 1.5 model, the Reynolds Number used for it was 4M. Did you use 4M with javafoil?

I have an S818 file that Dan Somers (the airfoil inventor) generated using the Eppler code. It has many Re, but not 4M. The highest is 3.5M. I have attached it to this post if you are interested. The last column in the file is Cm.

Since generating the file (in 2004?), the Eppler code has been improved and Dan says the airfoil coefficients really should be regenerated with the newest version. You may also want to compare javafoil to MIT’s XFoil, which I think is freeware.

Whenever I think about predicting airfoil coefficients with software, the old saying, “Here there be dragons,” comes to mind. :stuck_out_tongue:

S818E.TXT (4.98 KB)


thank you for supplying the S818 file. The results at Reynolds Number 3.5M are much closer to the S818_2703.dat compared to what I got from javafoil for 4M. For what I am doing I also need the cm values for s825_2103.dat and s826_1603.dat. Do you also have data for these airfoils including cm values?

Is the Eppler code public, so I can do a recalculation of the airfoil coefficients?



It may be that Dayton Griffin used the Eppler predictions to create the S818_2703.dat file, so I would not be surprised they are close.

I’ve attached the Eppler predictions that Somers generated for the S825 and S826. You should also check out my Airfoils web page ( There is a lot of useful information there–even results from OSU’s wind-tunnel test of the S825.

The Eppler code is not free. You can buy it from Dick Eppler or Dan Somers ( I have no idea what it costs, but I suspect it is thousands of dollars.

S826E.TXT (4.13 KB)
S825E.TXT (4.15 KB)

For anybody interested in using XFoil, I would also recommend XFLR5 (another free code): XFLR5 allows the XFoil code to be driven by a graphic user interface, which makes it much easier to use. XLF5 also adds several other bells and whistles…it’s a nice code.

“Here there be dragons”, hahah. Agreed.

Danny Sale
University of Washington
Mechanical Engineering


All the results based on “the old” eppler code provide too good results.
That means that especially the Reynolds-sensitivity is underestimated.
This was shown especially at the model airfoils of eppler, which were all not good-
this was measured in Stuttgart obout 20 years ago.
This costed me years of my life :frowning:

A detailed explanation can be found at , why this is the case.
But it’s in german language. It’s written in a funny style.
“air particles are like car drivers- they don’t like to brake”.

Xfoil of Mark drela is closer to wind tunnel tests (I do not write “truth”!).
Because wind tunnel tests provide sometimes different results, too.
2-D is usually OK.
Especially maximum lift can become a problem because of wind tunnel blockage.
(35% thick airfoils and high reynolds numbers of about 5Mio and 15 degrees angle of attack -
is there any wind tunnel being below 10%?)
Xfoil is known to produce slightly too good results in drag and maximum lift
relative to wind tunnel tests.
Therefore I reduce the Reynolds number to bring results closer.
I digitized some results from reports with freeware “engauge digitizer” to checkout.

Kind regards


Thanks for adding to the topic. Maybe this will help others.

Yes, blockage problems do cause wind tunnels grief at high angles of attack. I guess someone needs to spend a fortune and use a really big tunnel. I see no other solution.

What makes you think tunnel data is correct? :laughing:


Hallo Marshall!

Why? Hmm- Several years ago I read an open vacancy.
One person was searched for this open position.
This person should make wind tunnel tests,
write the evaluation programs
and by the way should make CFD-calculations.

Therefore it must be very easy to find the correct data :slight_smile:


And in 3 or 4 years all engineers will be redundant.
The secretaries will do the work with all the blackboxes.
Don’t you think so?

I think your timeline is a bit off. I’ve been redundant for years…