Plot: Can new emission-free electric planes replace our polluting airliners and revolutionize personal transportation in our cities? NOVA takes you for a ride in some impressive prototypes that are already in the air, from speedy single-seat planes that can take off like a helicopter but are half as noisy to “self-flying” air taxis that are already taking passengers on test flights in Chinese cities. But if electric airplanes are ever to advance beyond small, short-haul craft, significant hurdles of battery weight, energy storage, and cooling remain to be overcome