When james O'Neill,a retired marketing executive, first learned how helicopter powertrains worked a decade ago, he immediately started redesigning them. Most helicopters have a huge transmission that reduces the engine's high speed to a levei more fit for the main propeller and turns the tail rotor to keep the aircraft from corkscrewing out of control. Engineers had found a way to get rid of the tail rotor years ago: Place a coaxial propeller on the main propeller, and spin it in the opposite direction. But doing so still required a complicated assembly to achieve the proper speed and to create spin in opposite directions.
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