Easiest way would be to just hang with your hands. If the fan can hold you up that way, it will with a rope. Keep in mind, if you are tying the rope to just one blade, you would have to hold on to just one blade to make sure. Also, if it does hold you up, don't use the same blade with the rope. You may have weakened it.
Going to say, I highly doubt a fan would hold up 240lbs.
Edit: Never mind the power tower. A simple ceiling mounted pull up bar like this would be better because it's higher and cheaper to make. You may need a stud finder and drill, but both can be had very cheap. I would use the bar, and not just one eye-bolt. I don't know if one would hold up 240lbs.
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