Even if it's true that the Wright brothers estimated that their propellers are 66% efficient, when in fact they were 75% efficient, why doesn't this simply suggest that they were a little off on their 66% estimate or in any one of their underlying estimates re speed or RPMs.
Please read again my post from here:
http://www.pprune.org/aviation-histo...ml#post8498210
Do I have to repeat 100 times that the Wright brothers claimed they had obtained 66% efficient propellers, they built them at this efficiency. They did not just estimate a 66% efficiency for their propellers!