why aren’t there human-piloted quad-rotors?


why aren’t there human-piloted quad-rotors?

In: 11

There are. They have existed for years.



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.