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 level 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. O'Neill realized that a cam engine, which trades a crankshaft for a series of lobed cams, could power both propellers at the right speed without the need for a weighty, maintenance-heavy gearbox. If he could just design a cam system that produced counter-rotational force, he'd have a new kind of helicopter that was simpler and lighter.