LBarron
Well-Known Member
What does one do to overcome the problem of the variation in calculated/published airfoil data when designing a new project or analyzing an existing one? My issue right now is the pitching moment coefficient for the Pelican/Vampyr flying wing airfoil, but the question is obviously applicable to any other airfoil. I get completely different Cm results after loading the ordinates into Profili (Xfoil) and AeroFoil (which claims to be more accurate than Xfoil). While the data from AeroFoil look more reasonable, when I use them in calculations (Roncz stability spreadsheet adjusted to eliminate the horizontal tail component) the moments produced by the wing make balanced level flight impossible as currently designed. But, the plane did fly, and according to the designers pirep it was quite stable. Of course I could have screwed up the calculations, but the other moments are more accurately calculated so the biggest variable is the Cm. So now I'm questioning both results.
Just for grins I compared Cm data for several 4-series and 6-series NACA airfoils using NACA wind tunnel data, Profili, Airfoil Optimizer and AeroFoil. Needless to say, they were all over the map (some results look similar, but .01 really does make a difference). I've also looked at reflexed airfoil Cm data using those programs and got the same results. Indeed, we've all read the comments (here and in other places) and disclaimers (from the software companies) about the accuracy of computer generated data compared to wind tunnel tests, as well as the variability of wind tunnel tests themselves. So, back to the first line, how do you know which data to use? - to have the flight characteristics reasonably resemble the calculations (especially in the case of the Cm for a flying wing). I would hope there's a happy medium between the $5k aero analysis (done by computer) and the "let's just build it and see how it flies" approach.
Or is this why test pilots were created?
Just for grins I compared Cm data for several 4-series and 6-series NACA airfoils using NACA wind tunnel data, Profili, Airfoil Optimizer and AeroFoil. Needless to say, they were all over the map (some results look similar, but .01 really does make a difference). I've also looked at reflexed airfoil Cm data using those programs and got the same results. Indeed, we've all read the comments (here and in other places) and disclaimers (from the software companies) about the accuracy of computer generated data compared to wind tunnel tests, as well as the variability of wind tunnel tests themselves. So, back to the first line, how do you know which data to use? - to have the flight characteristics reasonably resemble the calculations (especially in the case of the Cm for a flying wing). I would hope there's a happy medium between the $5k aero analysis (done by computer) and the "let's just build it and see how it flies" approach.
Or is this why test pilots were created?
