I used the Simple Propeller Theory to calculate the Froude Efficiency for my theoretical propeller-driven craft (at 65k feet). Here are the results: https://ibb.co/kPPqzU.
My results were that, for a given system (of known coefficient of drag and frontal area), the Froude efficiency for given flight conditions (req'd thrust and velocity) are the same, irrespective of propeller characteristics (this is before anything about the propeller is input).
Does this efficiency thus represent an upper bound that any propeller can have for certain flight and craft conditions? If not, what does it mean? What does 47% efficiency (e.g. line one in my results posted) mean for the propeller, and incidentally the flight?