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The process of perfecting the windmill sail, making incremental improvements in efficiency, took 500 years. By the time the process was completed, windmill sails had all the major features recognized by modern designers as being crucial to the performance of modern wind turbine blades, including 1) camber along the leading edge, 2) placement of the blade spar at the quarter chord position (25% of the way back from the leading edge toward the trailing edge), 3) center of gravity at the same 1/4 chord position, and 4) nonlinear twist of the blade from root to tip (Drees, 1977). Some models also featured aerodynamic brakes, spoilers, and flaps. The machine shown in Figure 4 (which was operating with two of its buddies pumping water about one meter up from one irrigation pond to another in the Netherlands in 1994) features leading edge airfoil sections.