We're aware of this. It's not a bug and I don't lose sleep at night over it.
We did 1-1 port of VORLAX. All equations are the same. The results are identical down to more decimals than I can count (depending on your Fortran precision). We primarily compared against AVL (since we already 1-1 match VORLAX). Although I ran some cases against OpenVSP. We used an older version of OpenVSP, and as I'm aware there have been many changes. I got very unreliable results from OpenVSP on that earlier version.
When we dug into into the differences, it came down to the methods of drag integration. VORLAX integrates the surface pressures. AVL however integrates drag in the Trefftz-Plane. Mark Drela explains this in the AVL users guide
"The alternative Trefftz-Plane drag coefficient CDi is calculated
from the wake trace in the Y-Z plane far downstream. This is
generally more reliable than the CD obtained from surface force
integration, and is the appropriate wind-axes induced drag for
So, why didn't we integrate the Trefftz-Plane? - Then we wouldn't have had a 1-1 VORLAX port.
Then the question is, do we need a 1-1 port? - We chose to use VORLAX for it's ability to handle subsonic AND supersonic configurations. AVL cannot handle supersonics and other VLM's we used couldn't or gave very poor results. With VORLAX in supersonic flow it makes sense to use surface integration.
So there is a tradeoff, we went the known reliable route that allowed us to get reliable supersonic results and decent low speed results.
So in summary, it is a different method of integrating drag that worked for our needs. If folks in industry/academia are familiar with VORLAX, they should expect from SUAVE the same results they get from the FORTRAN code.
Now, if one wanted to take the VLM results and calculate the drag in the Trefftz-Plane this is doable. All the vortex strengths are correct, it's just a post-processing step.