I just remembered something I heard during the 100 birthday of the Wright Brothers. I think they said the best prop ever designed was about 88% effecient. I would imagine a typical production prop is in the low to mid 80's. So 80 % gives you about 54.4 mph and 85 % would give you 58 mph. Lets call it 56 mph. That is my final answer. No lifeline.

And that is just an ideal. As someone stated earlier, the airframe and wing have thier say so.

Well, heck, Junior Flyer has the test data !! What the heck is 68 km/h ? Hmmm, hang on......fooie, thats 42mph !!!

WOW, that is only 62% effeciency !!!