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[Adolph Archer]

The hovering drones would not fly as efficiently as helicopters because of their shorter rotor blades, but their enclosed rotors have the advantage of a much smaller size and safety near humans.

[Adolph Archer]

"They are less efficient than a helicopter, which has the benefit of larger diameter rotors," De Roche explained. "They do have unique performance advantages, though, as they have demonstrated flight within trees, close to walls and under bridges."