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 Then propeller diameter is for example 0.457 m and pich is 0.254 m (18x10 inches) and propeller rotation speed is for example 4000 rpm then propeller max eficienty be at speed (propeller eficienty max be then sliding propeller is 20-30 %): 4000/60 s = 66.666666666666666666666666666667 revolution per second. 66.666666666666666666666666666667 * 0.254 = 16.933333333333333333333333333333 meters per second. 16.933333333333333333333333333333 * 3.6 = 60.96 km/h 60.96 * 0.75 = 45.72 km/h. So max propeller efficienty of propeller 18x10 inch with 4000 rpm be at speed 45.72 km/h. If propeller is 12x6 inch (305 cm diameter and pitch 152 cm) and rpm is 4000, then propeller max efficienty be at speed: 66.67 rps * 0.152 = 10.13384 m/s 10.13384 * 3.6 = 36.481824 km/h 36.481824 * 0.75 = 27.361368 km/h. If propeller is 12x6 inch (305 cm diameter and pitch 152 cm) and rpm is 6000, then propeller max efficienty be at speed: 6000/60 = 100 revolution per second 100 * 0.152 = 15.2 m/s 15.2 m/s * 3.6 = 54.72 km/h 54.72 km/h * 0.75 = 41.04 km/h If you not ride on that speed, then propeller eficienty is max ( maybe 60-80 %), then propeller efficienty can be 10 precent if you ride about, not 41.04 km/h, but 7 km/h. Propeller efficienty posible maybe known use ventilator and blow to propeller. If we know ventiliator motor output power, ventilator efficienty and know power what generates generator on that is propeller and we know generator efficienty, then we can calculate propeller efficienty. I read what some propellers max efficienty is 0.9. Thats propellers max efficienty is 0.9: http://www.aerosila.ru/index.php?act..._content&id=17 http://www.aerosila.ru/index.php?act..._content&id=18