This start-up with a trillion-dollar idea could be a big winner.Helicopters are one of transportation's most versatile inventions. They can take off and land vertically, they can hover in place, they can operate without runways, and they can reach remote areas, all of which make them indispensable for medical, military, and rescue purposes.They're also noisy, expensive, and not very fuel-efficient. Now, if only we had an aircraft like a helicopter that ran electrically and was a skosh more practical for eve ...