Multirotors vs single rotors

i have a doubt which popped up when i was looking up rc copters.
which is better multiple rotors or a single rotor?
which provides better thrust to power ratio?
which is more efficient?
assuming ideal conditions?
 

CWatters

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In general one big rotor/propellor is more efficient than several smaller rotors. Some racing/speed model aircraft have used single blade propellers for the same reason but they can be a problem to balance.

Some cheap RC helicopters use contra rotating rotors rather than a tail rotor as it's easier and cheaper to make.
Some RC drones have multiple rotors for control reasons.

PS: The tip of a wing/rotor/propeller is a source of inefficiency. So high aspect ratio (long thin) wings are more efficient than low aspect ratio (short fat) wings. This is one reason that big rotors are more efficient. The tip area forms a smaller percentage of the overall area.
 

cjl

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One caveat to that though is that more rotor surface area is more efficient, for a given weight, so if the multirotor craft has more swept area than the single rotor craft (of similar weight), it could end up more efficient.
 

rcgldr

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One issue that could effect efficiency - Most single rotor models use a non-twisted rotor blade, and most of the generated thrust is generated from less than half of the length of each rotor blade, a bit inwards of the outer tips. Most multi-rotor models use propellers, which have a twist and more of each propeller blade contributes to producing thrust. As already mentioned, this would be offset by the fact that longer rotors are more efficient than shorter ones.
 

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