why aren’t there human-piloted quad-rotors?
In: 11
There are. They have existed for years.
[https://www.engadget.com/2016-01-06-184-delivery-drone-for-people.html](https://www.engadget.com/2016-01-06-184-delivery-drone-for-people.html)
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There are plenty out there. The reason you don’t see more of them is because most, like 80%, people are not competent enough to acquire a pilot’s license.
* Safety: You have four rotors. Two rotate clockwise and two anticlockwise; this controls the way the whole vehicle rotates. If you lose just one rotor then, to avoid spinning, you have to power down the diagonally opposite rotor to balance the spin. So a quadcopter has to be powerful enough to maintain altitude on only half its rotors … which is hard.
* The square-cube law: If you just scale up a toy quad-copter by a factor of 10 then its weight will go up by a factor of 1000 (because you’re multiplying length, width and height each by 10). But the strength only goes up by a factor of 100, so it’s now very fragile. The propeller area also goes up by a factor of 100 so you’d need to scale up the rotor speed by 10. Small, fast rotors are very inefficient; conventional helicopters use the largest practical rotors to keep the down-draft slow. Basically, you can’t scale 3D objects much and expect them to work the same way.
There are quad copters big enough to carry human pilots. However there are some major disadvantages to them. A quad copter relies on very precise control of the speed of each rotor in order to control the craft and prevent it from crashing. When the motors become larger it becomes harder to control them precisely. They need time to spool up and slow down. This is why you tend to see a maximum size of the motors used in multi copters and larger crafts just have more motors rather then bigger ones. But this does make the craft much more complex and you start getting interactions between the rotors that you have to account for and such.
The second issue is that there is no fault tolerance in the quad copter design. Since all the rotors are used for control if something happens to one rotor the vehicle becomes impossible to control. This results in spins and flips before the vehicle crashes into the ground. This is generally fine for a smaller unmanned craft but when you start working with larger manned crafts the safety requirements are much greater. With multiple rotors you may have enough control authority even after losing one engine but the complexity of it all just increases when you need to account for possible faults in any of the rotors.
This is in addition to the fact that quad copters are primarily used because they have much lower mechanical complexity which is important at smaller scales. All the complexity is in the software which makes the hardware much easier to build and maintain. However when you work with large enough crafts to lift a human you do not get the same advantage of the mechanical simplicity. A traditional helicopter is much better at this scale anyway.
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.
4 is the minimum amount of rotors. if one breaks it flips around and falls out of the sky. a helicopter only has 1 so it doesnt change attitude when it fails and can glide down
the square-cube law and power to weight ratio. those little toy quads barely lift themselves, and the overwhelming majority of their weight is their own motors and a battery.
if you make it 5 times bigger, it’s gonna weigh 125 times as much *at least*, and motors don’t scale like that.
gasoline has a lot more energy for its weight than batteries, and turbine engines have much higher power at the size you need to move people around. the problem with gas power is that it’s a lot better to have 1 big engine than 4 smaller ones, and splitting mechanical power is dramatically more difficult.
you also encounter problems with control. quads steer by changing the speed of their rotors. for tiny plastic rotors with variable speed electric motors, that’s no big deal. bigger blades have more momentum. they don’t change speed as readily and experience a lot more stress when they do. gas powered engines also don’t like changing speed at all. which helicopters can change rotor speed, much of the control is actually done by changing the angle the blades hit the air.
Here is a human-piloted multirotor:
It’s supposed to move ppl at the Paris olympics in 2024.
How about using 6 gas motor driven rotors and to assist quick directional/thrust response instead of using a throttle there is a vane device which could quickly restrict air flow built into each rotor housing. Sounds inefficient, but in reality keeping the gas motor at a fixed or slow changing rpm and making minor flow adjustments at the fan shroud might be a good solution.
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