Very low Re foam board foil polar problems

Questions, suggestions, feature requests, bug reports, feedbackVery low Re foam board foil polar problems
Tom Falls asked 4 years ago

I am trying to design very low Reynolds number airfoils using foam board for indoor RC airplane indoor flyers. I think XFoil seems to struggle with the curved flat plate foils combined with the very low Re’s. I am using Re range between 20,000 and 200,000. Questions:

  1. I get very nice looking polars using ncrit = 5 and forced top transition at 15% however,
  2. Plots at Re = 100,000 and 140,000 are off the chart, i.e. Cd of .01 and .001 respectively. I am assuming that these are not critical Re numbers, but, are issues with XFoil performing the analysis so I’ve excluded these two plots from my analysis.
  3. Are my use of ncrit = 5 and top transition of 15% and excluding wonky polars reasonable? I am getting good model plane performance and stability in XFLR5 based on these assumptions but fear these results will not accurately represent the performance of the final constructed model.

Thanks, Tom  

André Deperroistechwinder Staff replied 4 years ago

Well, my guess would be that the flow on a foam wing goes fully turbulent shortly after the leading edge, so the NCrit factor is not very important. Also, if you set the transition at 15%, the flow will go turbulent at that point for the majority of operating points, before any free transition can occur, so NCrit would not be used anyway.

Other wise, the XFoil documentation states:

situation Ncrit
—————– —–
sailplane 12-14
motorglider 11-13
clean wind tunnel 10-12
average wind tunnel 9 <= standard “e^9 method”
dirty wind tunnel 4-8

So I guess NCrit values in the ballpark of 5 can be used, but I don’t have any experience with that