Explain like I’m five: Why do these modern electric helicopters have many small rotors, but traditional helicopters have only one big horizontal rotor? If many small rotors is more efficient, why haven’t non-electric helicopters done it?
Almost certainly because electric motors can be directly coupled to each small rotor. It would be much harder to have individual engines directly mounted to small rotors, requiring additional weight and complexity), and there would be a mechanical complexity and efficiency loss if using a single engine to distribute torque to a number of different rotors through complex gearing and driveshaft mechanisms. Possible probably, but perhaps not lighter or more efficient.
More rotors aren’t more efficient: the more rotor area you have, the more efficient.
Whether you achieve that with 4 small rotors or 1 big rotor is immaterial.
The reason eVTOLs use many small rotors is because an eVTOLs only operate in vertical mode for a short period of takeoff and landing. They aim to be normal fixed wing aircraft for most of the flight!
A helicopter like big rotor would be inefficient- adding weight and drag during forward flight. Helicopters on the other hand, spend the entire flight in vertical mode, for which a big rotor is most efficient.
this stems from newtons second law, but is neglecting second order effects like interference
The modern electric multirotor drones (e.g., quad-, hexa-, octo-copters) are the result of computer controls allowing a configuration that is so unstable that no human could fly it to be actually implemented as aircraft that are much more agile, reliable, robust, and easier to fly (onboard or remotely) than helicopters. They also have a smaller footprint than a helicopter.
Integrating multirotor technology into a fixed-wing aircraft in a VTOL configuration allows for the small takeoff/landing footprint of a helicopter with the flight efficiency of a fixed-wing aircraft.
Not a helicopter, nor an expert in them, but modern electrical engines are significantly more efficient than traditional ICE motors. Also, the scalability of existing small-form quadcopters (etc) means it's much easier for companies to start from the ground up with that sort of tech, rather than developing a complicated rotor/contra rotor set up.
Basically, it would have been very difficult to go the contemporary route in decades gone by, whilst it doesn't quite make as much sense for the usage scenarios to go the traditional route.
My lay understanding is that more rotors are more efficient, but also more unstable due to a concept known as disc loading (the ratio of weight vs swept area of the blades). I'd guess the main reason we don't see more than two rotors on traditional helicopters is partly due to the complexity of transmitting power to multiple locations from a single engine (or having multiple engines), and partly because making such a system responsive enough for stable flight is hard. Electric motors neatly solve both problems.
? Why do these modern electric helicopters have many small rotors, but traditional helicopters have only one big horizontal rotor? If many small rotors is more efficient
so that it can easily maneuver around skyscrappers while beating city traffic and picking their customers
You can rotate them in such a way to use the lifting surfaces (wings) for efficiency during ranged flight while preserving VTOL capabilities. They're more redundant versions of the Osprey.
