The down between the engines counter-rotation of the propellers, which turn in opposite directions, allows a structural weight reduction. The arrangement preserves the symmetry of the aircraft when the four engines are operating, and reduces the adverse yaw in case of an engine failure, allowing in turn a reduction in the size of the tail fin by 17 per cent, hence reducing weight and drag. Another consequence has been the possibility to improve by four per cent the lift at low speed and thus to simplify the slats and, as a result, reduce by eight per cent the surface of the horizontal stabilizer. Furthermore, it also reduces the level of vibrations and therefore the noise inside the aircraft.
So says Airbus.