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The gear boxes on the big turbines are like nothing else. I used to work for one of the labs that did fatigue testing on them and they were having a lot of trouble finding test equipment that was stronger than the gear boxes.
Regarding the blades striking the tower, part of the problem is that most of the turbines currently being manufactured are upwind turbines. When the wind blows it flexes the blades toward the tower. Downwind turbines don't have that problem, but they do have what's called "tower shadow." Every time the blades pass behind the tower they enter an area where the wind is blocked by the tower. This causes a cyclic loading on the blades, which contributes to fatigue failure and an increase in noise.
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