Tom asked 5 years ago
I have opened an xflr5 model in flow5 and re-run the analysis.
The plot for Fx vs V is completely different between the two programs, while everything else appears to be the same…
The Fx vs V plot in xflr5 is as expected (looks roughly like an x^2 curve) while the Fx vs V plot in flow5 seems wrong (looks more like an x^0.5 curve)
Is Fx calculated for a different coordinate system in flow5 compared to xflr5? Am I missing something?
It seems indeed that there is an issue with the calculation of the forces. I’ll investigate and post back.
André
The problem is that the Fx_FF variable in flow5: (1) only includes the induced part of the drag, and (2) is given in geometric/body axes and not in wind axes.
This was a design choice for the alpha phase, but is has no reason to be anymore. It will be corrected in beta 08 so that the results match those of xflr5.
André
Is it possible that the calculation of Fx (FF and sum) is broken again? It looks like Tom described it. The same file and analysis without rerunning it looks diffrent in flow5.
Patrik
Other than a potential sign issue in the output, looks good as far as I can tell.
Things have changed significantly between the latest versions of flow5 and xflr5, so differences are unavoidable.
André
To me it does not look like just a sign issue. Sadly it is not possible to attach photos as a file, so i try to describe it: for a Fx_FF/Fx_sum vs alpha plot the graph looks roughly like the graph -1,5x+1,2 for a wing with 3000mm wingspan and MAC of 150mm. You can see a straight graph with a negative slope. In comparison, the Drag vs alpha plot has a square progression. If you would like further information, i can provide you with the fl5 files.
Patrik
I forgot to mention that my comment reffered to a T2 analysis. A T1 analysis shows a negativ quadratic progression.