Erroneous xfoil results?

[raptor22]

I've been trying to design a plank airfoil in xfoil, and trying to make it nice and thin for very high speed flight. There aren't many good, thin plank airfoils out there. I have finally produced something that performs very well at "normal" rn for a sloper (100,000-200,00), but when I tested it at 300K the performance did not increase at all, and at 400K I get a zigzag shaped polar.

Whats up with this? I am not surprised by the 300K results, but the zigzag shape @400K is very odd. I wouldn't expect it to be due to boundary layer irregularities, since I am going UP in RN.

--Alex

[davidfee]

This will not be a definative answer but, it is not at all unusual for low Rn-optimized airfoils to be terrible at high Rn, and vice-versa. That being said, it could also be an artifact of the computational method. Dr. Drela or others may be able to help you. Also, have you tried the x-foil list on yahoo groups?

[raptor22]

Like I said, the somewhat inferior 300K result wasn't really surprising, but I though zigzags were wierd.

I am not a member of the xfoil list, so I was hoping Dr Drela or anyone else knowsledeagle in xfoil would explain this. Especially since it seems to occur most with relatively thin leading edges.

--Alex

[davidfee]

I have seen zig-zags in my xfoil results and I usually wrote it off to user error. :) However, hopefully someone can help out with your situation.

[HELModels]

I've pretty well established my non-expert status, but did it "converge"? How about increasing your paneling? Another thing to try is increase your number of iterations. That is about what I know of Xfoil.

[biber]

Perhaps check the first 5% of the LE by zooming into it in the gdes menue if there is something happening with the spline. Sometimes deleting one node or another helps to avoid a swinging spline (wich can lead to strange results in xfoil). But sometimes I also had such zig-zag occuring in polars when I set the alpha-increase-stepwidth in the aseq command to narrow, as it seemed.

biber

[biber]

To get a comment on that by Mark Drela this maybe not the ideal place, he is checking the hand launch, thermal and sailplane talk sections far more frequently I guess. But a PM should also do it.

biber

[markdrela]

There aren't many good, thin plank airfoils out there. I have finally produced something that performs very well at "normal" rn for a sloper (100,000-200,00), but when I tested it at 300K the performance did not increase at all, and at 400K I get a zigzag shaped polar.

Whats up with this? I am not surprised by the 300K results, but the zigzag shape @400K is very odd. I wouldn't expect it to be due to boundary layer irregularities, since I am going UP in RN.

I don't know what's going on. You have to provide more information, either plots or the airfoil coordinates.