why aren’t there human-piloted quad-rotors?
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Quad rotors are efficient if you power rotors with an electric motor that can quickly change rotational speed. That makes it easy to control it without the need of changing the angle of attack of the rotor.
A problem with quad rotors is that each rotor has a separate eclectic moror and if it fails it will be quite problematic to maintain controlled flight and the likely result is a crash, it is ok for a drone but not acceptable if you lift humans.
The “quad rotor” linked by another post https://www.engadget.com/2016-01-06-184-delivery-drone-for-people.html is not a quadrotor even if the post say so it is a octarotore, there is two rotors on each and that I assume had one motor per rotor so there is redundancy. they have abandoned that design with a 8 rotors for 1 passenger, 120kg max payload and now developed a 16 rotor 2 passenger 220kg max payload variant. The max payload will include the weight of the passengers.
If you look at multirotor helicopters and tilt-rotor aircraft the rotor are mechanically linked and away spin at the same rate. They have multiple engines and if one fails the remaining can still power both rotors. You might need to defend and do a controlled landing if the lift is not enough but it is still controlled.
Even if all engines fail you autorotate and still land, You can still control the descent because you can change the angle of attack of the rotor.
If you mechanically link rotors they need to rotate at a constant speed. That makes in impossible to control the aicraft-like quadcopter with variations in rotor speed. You need to be able to control the angle of attack of the bladed and a complex swash plate and actuator needs to be added. For rotors, you can control like that require more pats and will cost more than a single large rotor on top.
The reason helicopter have multiple rotors in multiple locations is if an even large single rotor would be impractical. You have a single rotor as long a possible. You need a small rotor or fan outlet in the back to stop the helicopter from spinning around, it is a lot cheaper then root that provides lift.
Even if you have enough independent rotors so you do not need to link them for saifty reasons you still need to control the speed of the quickly. That means you need eclectic motors and not internal combustion engines. Batteries are not very energy dense and do not get light when used so drones have quite short flight time.
The 1 person human octa rotor have a range of 10miles/23 minutes lifting just 120kg payload capacity.
Compare that to a https://en.wikipedia.org/wiki/Hughes_OH-6_Cayuse that is a small helicopter with a range of 610km that is a 2.5 hour flight at max speed. It will be able to fly slower for a longer time. The payload depends on the fuel load, it weighs 557 kg empty, and max takeoff weight is 1225kg which is 668 kg for fule passenger and cargo. IT can carry a crew of 2 and 4 passengers.
This shows the enormous advantage of fuel you burn compared to batteries. You could let the internal combustion engine drive a generator but then you need even more parts and it gets heavier.
You also need to maintain aircraft humans fly a lot more than an unmaned drone to keep them safe so there is a lot of costs that is not there for drones.
Ground vehicles are in most chase the better option for humans and if you what to go where you can get by road longer range and higher capacity helicopters are often needed.
So quad rotors are from a safety perspective to bad for humans you need more rotors in design like that. The range and lift capacity are also very limited. The result is that the cost advantage that they have as drones are not relaying there for transporting humans.
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