Ollie - Yes. Although wind tunnel data is obtained physically, it still isn't the real (practical) world, and results from various sources would, to be meaningful, have to be obtained under conditions that are matching and proper, fat chance. Conversely, real aircraft have aspect ratio, surface defects, appendages, and aren't flown in air that's been "straightened."
BMatthews
Wildblueyawner, you're quoting profili / Xfoil. Can I take this to mean that the advanced features unlocked when I pay uses an embedded Xfoil to calculate the polars?
Yes. Profili simply melds a graphical front and back end onto XFoil to ease data entry and the output of plots - Basically turns XFoil keystrokes into mouse clicks. The analysis and computation has already been performed by XFoil when the databased airfoils were pre-processed by Stefano, or will be performed by XFoil when considering a modified or custom airfoil (see below re pre-processing).
ptxman
Is there generic 'code' available that takes airfoil shape + Re information & spits out resultant polars? If so, is this reliable? What are the limitations?
XFoil is anything but generic, based on its ability to handle low-Re conditions and the associated separation bubbles. It's capabilities are summarized here, plus links for downloading the actual XFoil files
http://raphael.mit.edu/xfoil/
If the program spits out polars based on actual tunnel tests at measured Re's, smoothed or otherwise, isnt this basically a glorified graphical database? Doesnt this then pose a limitation if you modify an airfoil or request a different Re because it is then forced into some sort of intrepolation routine?
Whoa! Careful here - That plot of wind tunnel data I posted was NOT from Profili / XFoil. That came from here:
http://www.nasg.com/afdb/index-e.phtml
XFoil doesn't simply interpolate chart data - It performs a real CFD analysis on the airfoil in question. A simple interpolation routine wouldn't handle the discontinuous function presented by phenomena such as separation.
A limitation when modifying an airfoil (or picking some weird Re other than the standard values in Profili) relates more to convenience - The time to generate a custom polar. This is addressed with a pre-processing option that can run XFoil on modified or custom airfoils unattended, then saves the polar data. Depending on the number of new airfoils, pre-processing can take considerable time (Stefano states that it took over 500 hours of pc run time to pre-process the 2200+ airfoils ==> call it 15 minutes per airfoil) but, again, it can be done unattended, i.e. click mouse, come back later. Generation of the polar charts for the included airfoils only takes a few seconds.