Quadrotor configuration I think is the most common configuration for drones. Why has it not been made into a full-size production aircraft? I think that it could be better than helicopters as it does not need a tail.
A quad rotor will immediately lose stability if one of the rotors fails. If a helicopter engine fails, it’s actually possible for the pilot to still land it relatively safely and gently. Safety tends to be the top priority for human-piloted vehicles, especially flying ones.
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