[dTal]

>A properly-tuned quadcopter is more stable than a helicopter and performs better in most situations

This is not correct. All quadcopters are fiendishly unstable and require computer control. Helicopters can get away without computer control, but a computer-controlled helicopter is just as stable. The performance of a helicopter is vastly better in every way - more efficient due to increased swept area, and more maneuverable due to variable pitch. You won't see a quad fly like this: https://www.youtube.com/watch?v=AXSfFLGeVZA

What quadcopters are is mechanically simpler, and therefore cheaper. This is why they are popular, and also why efforts to scale them up to human size are misguided.

[afarviral]

Why is it misguided for transport? I would have thought reducing costs would be good.

I think there is a real simplicity to the hardware with multirotors which is superb, but I wonder if its a good trade-off with complexity of the flight computer. Is the surface area for failure pretty similar?

RC helicopters are the most fragile and catastrophic beasts, whereas kwads can take a walloping and still perform incredible feats.

[dTal]

A few reasons:

* Efficiency. To get a quad to the same level of efficiency as an equivalent monorotor, the total area of the four prop discs must equal that of the large rotor. Simple geometry makes this impractical, as you end up with a machine far larger (and with four booms instead of one).

* Power distribution. You must now power four props instead of one. Your choices are batteries (very poor autonomy with the current state of the art), mechanical distribution from a combustion engine (heavy and just as complex as the helicopter), or hybrid-electric (very heavy since you need electric motors and a combustion engine and a generator).

* Control. Speeding up and slowing down rotors to modulate lift works great when they're inches across. It's not so good when they're huge - it's laggy, and you waste energy, and it's difficult to do mechanically (see previous point about power distribution). And sure, you could use variable pitch instead - and once again we're on complex helicopter turf, except multiplied by four.

* Safety. Although a helicopter only has one rotor, if it loses power, it can autorotate safely to the ground. Catastrophic mechanical failure is extremely rare, because the only absolutely critical components are the rotor and the bit that keeps it attached, and those are highly overengineered for obvious reasons. Quads on the other hand do not tolerate the loss of any part of their lift system. If the battery runs out, you die. If a prop fails, you die. If a motor burns out, you die. You just can't overengineer the entire power train to the same degree as a single "Jesus nut", and you now have four critical points of failure instead of just one.