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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. |