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If multiple propellers were placed in a row on the same shaft, they will create less thrust than several, separate propellers, but they could be designed so that they would create more thrust than a single propeller. Normally, this is done with two propellers that are spinning in opposite directions, but powered by the same engine. They are called "contra-rotating propellers". This creates more thrust because there are two propellers, but also creates more drag because the air between the two propellers is turbulent. The advantage of contra-rotating propellers is that they eliminate the torque reaction that normally comes from a single propeller.