Quadricopter seem like a perfect candidate for personal air-vehicles. The maneuverability rocks and the problem of computer control seems to be pretty well understood. You could design a control system that anyone who's ever played a video-game could master, the only real danger would be people doing stupid things with them on purpose.
I haven't seen any examples larger than models though, do they not scale trivially?
Nothing scales trivially in the material world. The fundamental problem is that as you scale, the strength of the structure increases as N^2 but the mass increases as N^3.
> the only real danger would be people doing stupid things with them on purpose
Not even close. The quadricopter design is very non-robust in the presence of a failed engine. That alone is probably a show-stopper for ever having one of these as a passenger vehicle.
They used to have man-sized quadrotors in the early 20th century. My understanding is they went out of style because the material tech wasn't up to stuff yet and there were control issues.
Also, is there a reason these couldn't auto-gyro? The mechanics of how that works has always been a bit fuzzy for me.
Auto-gyroing would require pitch control of the blades. The toy quadcopters control by differential thrust and torque. For attitude, they control the thrust of the different motors to get a vectored thrust effect. For orientation, the pairs of motors are spinning in opposite directions, so if you spin a pair faster and a pair slower, the torque imbalance between the CW and CCW spinning motors will spin the quadrotor body.
Autorotation in a helicopter is done by setting the blade pitch negative so that the rotor is driven by the air wooshing up through the rotor. When you are close to the ground, you pull collective, trading the energy stored in the spinning of the rotor blades for vertical thrust to slow your descent. If you do it right, you land gently with a much slower rotor speed. If you do it wrong, you fall the last five feet. :-O
(Auto-gyro works like a helicopter auto-rotation. I suspect auto-gyros typically don't have as much blade pitch control as a helicopter. As I understand it, it is much safer to land an auto-gyro with forward velocity and a roll-out rather than vertically like a helicopter.)
You would only need to auto-gyro in the event of fuel starvation or multiple engine failure. That's not the hard part. The hard part is recovering from a single engine failure. Now you have an unbalanced torque. If you don't notice the problem and do something about it VERY fast then you will flip over. It might be possible to develop a control system that can use the engine opposite the failed one for attitude control while slowing the descent (or maybe even maintaining altitude) with the other two still-balanced engines, but that is one gnarly control problem.
Twin engine airplanes have this problem too but it's much less severe. Adverse roll is a second-order effect in an airplane, it's a first-order effect in a quad. An airplane has the tail tending to stabilize it. Despite all that, losing an engine on takeoff in a twin engine aircraft is often catastrophic. In a quad, I'd wager good money (though not my entire life savings) that it's unrecoverable. But if you're going to put human passengers in it you have to plan for that because it will happen sooner or later.
I haven't seen any examples larger than models though, do they not scale trivially?