I am a bit perplexed about the CM=0.0062 that is computed for ALFA=-160, and from the jump to 0.4 after only 10 degrees (ALFA=-170). Is this behavior physically correct? I would have thought that the CM curve should be about symmetric to the positive side, for so high values of ALFA…
Thanks a lot for your answers!
Your correct, these numbers are a little suspect. The explanation of where these come from can be found in Appendix D of the AeroDyn User’s manual (AirfoilPrep used to be known as FoilCheck, just to make things even more confusing) wind/designcodes/simulators/aerodyn/AeroDyn.pdf
It turns out that when you have reverse flow on an airfoil you can get some big moment coefficients. And apparently this has led to pitch system failures of turbines in the past. Therefore when AirfoilPrep was written, the writers hard coded large values at alpha = -170 Cm = 0.4 and alpha = 170 Cm = -0.5 (both of which are in your dataset). These values are based on some wind tunnel measurements of airfoils done at OSU - see report for actual papers if you’re interested.
I’m not sure how accurate these numbers are for any airfoil including the one you are examining, as these data are hard to come by. If you can find some more accurate results I would suggest using those.
I am old enough for having used Foilcheck… but my question is more about the LOW values before the “fixed ones” at big angles.
I copy here, for easy reference, the phrase from the Foilcheck manual, pagg 44:
It is clear to me why the values are big, what I would like to understand is why the CM goes to such a SMALL value immediatly before. In the positive region of ALFA, the behaviour is relatively regular, in the negative it is not.
Unfortunately I have no better data available, we are a small startup company, I have to do with this. My intention would be to correct manually the data in order to have a regular curve. What do you think about this idea?
Best regards & thanks again for your patience and help!
I’d be inclined to do a literature search for pitching-moment data for high angles of attack and use it. I’d manually fair it in with the data you started with. You can plot the data in Excel and then click on individual points and drag them up and down to get a smooth curve.
I think that the large jump you are seeing between the low values and high values at the angles is not physical. This is highly dependent on the airfoil shape, but for a smoothly varying airfoil (not a lot of camber) I would expect that Cm would also smoothly vary.
As Marshall suggests I would look in the literature and start with the OSU reports listed in the FoilCheck manual, of which most are available on the NREL publications server.
I noticed this jump in the CM for negative values of angle of attack when modeling rotor fragments. I believe this comes from an error in the function GetCM in the table extrapolation for negative angles of attack, which was not following the algorithm mentioned in the AeroDyn manual (Laino 2002). I have added a corrected version and will send to Khahn Nguyen for verification. Note that this error makes all moment coefficient extrapolation suspect for tables developed with AirfoilPrep, which might impact pitch moment calculations for extreme load calculations. AirfoilPrep_v2p2_SML.xlsm (183 KB)
Scott Larwood’s version of AirfoilPrep has been verified and this version is released as v2.02.01 (Dec 13, 2012). It includes a correction to the calculation of CM in the negative angle of attack range on the TableExtrap tab.
Regarding the jumps in CM at 170 and -170 angles of attack, these CM values are still hard coded as -0.5 and 0.4, respectively.