Plotting
Table of contents
Polar calculations and plotting
The polar calculation facility driven from the OPER menu deserves a detailed description. It has been considerably upgraded from previous XFOIL versions.
The simplest way to create a polar is to issue the PACC command which sets the auto polar accumulation toggle and asks for the optional save and dump filenames. If either filename is given, each computed operating point will be stored internally and also written to the specified file. If no filename is given, the automatic writing is not performed.
The polar’s operating points can be computed individually with ALFA, or more conveniently en masse with ASEQ. One can also use CL or CSEQ, although these will not work close to CLmax.
The polar can be plotted anytime with PPLO. If previous polars have been computed or read in with PGET, they can be plotted as well. If a polar is deemed incomplete, additional points can be computed as needed.
If automatic writing of a polar was not chosen (no filename was given for PACC), the polar can be written later all at once with the PWRT command. The only drawback to this approach is that if the program crashes during a polar calculation sweep for whatever reason, the computed polar and all other stored information will be lost.
If existing filenames are given to PACC, the subsequent computed points will be appended to these files, but only if the airfoil name and flow parameters in the file match the current parameters. This is to prevent clobbering of the polar file with “wrong” additional points. Messages are always produced informing the user of what’s going on.
Off-line polar plotting
Polar save file(s) can also be plotted off-line with the separate program PPLOT. This is entirely menu driven, and is simply executed:
% pplot
The file pplot.def contains plotting parameters, and is read automatically if available. If it’s not available, then internal defaults are used.
Like the RGET,FREF commands in OPER, PPLOT permits reference data to be overlaid. A reference polar data file has the following form:
CD(1) CL(1)
CD(2) CL(2)
. .
. .
999.0 999.0
alpha(1) CL(1)
alpha(2) CL(2)
. .
. .
999.0 999.0
alpha(1) Cm(1)
alpha(2) Cm(2)
. .
. .
999.0 999.0
Xtr/c(1) CL(1)
Xtr/c(2) CL(2)
. .
. .
999.0 999.0
The number of points in each set (CD-CL, alpha-CL, etc.) is arbitrary, and can be zero.
The contents of a polar dump file can be selectively plotted with the separate menu-driven program PXPLOT. It is executed with:
% pxplot <dump filename>
This allows surface plots of Cp vs x, H vs x, etc. for any or all of the saved operating points. Of course, these plots can be generated in XFOIL for any individual operating point, so PXPLOT and the dump file itself are somewhat redundant in this respect.