Rotation direction for adjacent propellers, how do different arrangements compare?
Initially during A400M development (when the project was still called FLA), all propellers were supposed to spin in the same direction. The reason for this was the objective to use a single engine in order to minimise logistics costs.

Windtunnel tests soon revealed that the prop wash would create asymmetric flow on the wing, causing uncontrollable rolling moments in low speed flight. Only two options allowed to remedy this:


Use of counter-rotating propellers, like on the Progress D-27 powering the An-70.
Use of two variations of the same engine with opposite direction of rotation.


Counter-rotation resulted in heavier propellers and gearboxes, so it was decided to go with the second option. The exact direction of rotation for each engine was then determined in further wind tunnel tests, with low-speed handling and maximum lift coefficient the determining objectives.