Brian Abraham,
The propellers the Wright brothers said they had built in 1903 should have had an efficiency a bit over 75% to keep "Flyer I 1903" aloft during flight No. 4 on Dec. 17, 1903. This come in contradiction with the 66% efficient propellers the brothers claimed they had used in 1903. This is one discrepancy noticed by the team "Wright Experience"!
Another problem, I already mentioned, is that the efficiency obtained by the Wright Experience, for the alleged 1903 66% efficient propellers, varied between 75% and 82%. The team could not get that 66%.
"These data (the experiments done by the team Wright Experience) show that the 1903 Wright propeller had a maximum efficiency of 82 percent.
Based on Wilbur Wright's notes on the fourth flight of Dec. 17, 1903, the Flyer had an estimated forward speed of 31 mph during the steady flight portion of its path and the propellers were turning at 379 rpm, which yields an advance ratio of 0.85. Hence, the 1903 Wright propellers were operating at a mechanical efficiency of slightly over 75 percent during steady flight.
This was a remarkable feat, considering the state of propeller knowledge prior to World War I.
Since Wilbur estimated their propeller performance to be 66 percent in March of 1903, we found the results of our experimental tests to be quite surprising."
Source: (The Wright Experience; By Robert L. Ash, Colin P. Britcher, and Kenneth W. Hyde,
Mechanical Engineering "100 Years of Flight" supplement, Dec. 2003 -- "Prop-Wrights," Feature Article )