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.
In: Engineering
manned vehicles require failsafes for control in case of mechanical failure, unmanned drones do not (unless they are big and/or fly over populated areas).
electric dronecopters don’t have swashplates, which are far more complicated than adding additional independently controlled propellers. to completely replace the swash plate mechanism, you need at least three propellers because you need three points to establish a plane (tri-copter). these three points draw a triangle within which the center of thrust can be placed anywhere by the control system to achieve pitch and roll control. when the COT is over the center of gravity, the drone is stable.
in case of motor failure however your control of the COT is now limited to a line that connects the remaining two propellers. your cg is outside the line which will cause the copter to roll and crash. this is just a pick up, repair and replace scenario for an unmanned copter but impossible to sell as a manned one.
if you had four propellers, and lose one propeller, the control area turns into a triangle. if your cg was exactly in the center of the original square, it is now right on the line of the remaining triangle and you are on the edge of losing control. additionally, the motor #3 (the motor diagonally opposite the failed one) must also turn off to prevent a roll/pitch crash. so you are down to two motors reduced thrust and also either no roll or no pitch control. crash.
five is probably the minimum configuration where loss of a single will still maintain COT control vs the CG.
you can add arbitrarily many more to improve redundancy, but you also run into a limiting case eventually since the efficiency of thrust is proportional to the swept area of the propellers; adding more propellers greatly increases the footprint of the craft while losing efficiency. i.e. if you have a given allowed footprint, putting one big propeller is more energy efficient than fitting many small ones in the same footprint. today’s manned craft can (actual working ones), i have seen up to eight (octocopter).
tldr; drone copters don’t have swash plates to control pitch and roll and replace this mechanism by having at least three propellers. the loss of one is a confirmed crash with likely fatalities. in quads, the loss of one requires another to voluntarily shut down to prevent total loss of control, leaving two and still the loss of either pitch or roll control and also halved thrust leading to likely crash and fatalities.
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