Single rotor helicopters need a tail rotor to neutralize the twisting moment produced by the single large rotor.
2.
This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade.
3.
If the hand catches or pulls at an incorrect angle, the increased resistance afforded by the hand paddle will exacerbate the result twisting moment, making the defect clearer to the swimmer.
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