In addition to Chris' answer, I have had a look at your project file and could not readily reproduce your problem. I suspect the issue was in a combination of poor discretization (I increased the resolution of the airfoil and wing somewhat) and incorrect methodology (fixed-speed instead of fixed-lift).
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Save project and switch to "Wing and Plane Design". You would want to set metrics at "Options/Units" to feets and lbs prior, as JSBSim uses these - I did not do that, since my initial model was at meters, but preferred to recalculate output later at Libre Office Calc.
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