While the vision of whisking passengers between rooftops in aircraft capable of vertical takeoff has largely solidified in the general shape of multiple rotors, some with airfoils and others without, the question of how to power this new kind of aircraft is less so. Some envision pure electric with lots of high-capacity batteries; others believe batteries alone will never be ready to produce the power required, and pitch hybrid designs—with a fossil-fueled engine producing electricity to assist the motors during some or all phases of flight. However, one recent entrant in the field of some 150 such aircraft being...