Within a quadcopter, I know usually it's all the propellers facing upwards or all of them facing downwards. Why hasn't anyone tried making two propellers facing upwards and the other two facing downwards? Do some forces cancel out and renders that method ineffective?
closed as unclear what you're asking by fooot♦, xxavier, kevin, DeltaLima, anshabhi Nov 15 '18 at 12:30
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The elegance and simplicity of a quad-copter design lies in how it achieves control in all axis with simple variation of engine power (and thus speed) using just simple fixed-pitch rotors:
- varying power to the forward pair of rotors compared to the aft pair controls pitch
- varying power to the left pair of rotors compared to the right pair control roll
- varying power to the clockwise spinning pair compared to the counter-clockwise spinning pair control yaw (this means the rotors placed diagonally spin in the same direction)
These commands work independently if all the rotors generate the same lift and torque for the same power. They also need to be laid out in a square or not too narrow rectangle. And the easiest way to achieve that is if both the rotors and their mounts are all identical (except for mirroring for counter-rotation).
Does not really matter if they are facing up or down, it matters how they are pitched and what direction they are spinning. All 4 with the same orientation is a good common sense approach to a quadcopter, though you can have opposite spinning pairs to cancel torque. This is common on twin engine aircraft.
How ever, notice in a helicopter the tail rotor is tilted 90 degrees out of plane with the main rotor to control yaw torque from the main rotor.
And there is nothing to say you can not have more than 4 rotors, if you find an application where more is better. Really a matter of matching (any) available technology that can be practically applied to the idea.
There might be slight differences in performance up vs down due to air circulation interferences near the prop, so the safer bet is to keep the design consistant, but 2 up and 2 down is not outside the realm of possible.