Rotation direction for adjacent propellers, how do different arrangements compare?

The Airbus A400M uses counter-rotating pairs where the blades meet moving downward (if there is a better description, tell me please).

I played the video frame-by-frame to make sure it wasn't a tricky stroboscopic effect; use <> if you're using the HTML5 player.

How do the different arrangements compare aerodynamically? I think for the three arrangements below, the propeller blade interference will have different characteristics.

• Counter-rotating on each wing makes sense for limiting torque effects in any dual engine failure, but does the outboard rotation matching the wingtip vortice direction make sense, and does the inboard rotation throw runway debris towards the fuselage? – amI Nov 26 '16 at 18:20
• You can easily confirm the direction of rotation because of the similar blades. Your diagram is correct – TomMcW Nov 26 '16 at 23:54

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.

• Interesting to note that the AN-70 #2 prototype crashed after an overspeed shutdown only one of the counter-rotating propellers and caused massive reverse thrust. The aircraft actually broke in two when it landed short of the runway but was surprisingly repaired and returned to service, at the unbelievable repair cost of \$2m (unbelievable being that it was so cheap to repair... makes you wonder.). – Ron Beyer Nov 28 '16 at 3:24
• Do you know, do they have turbines with opposite rotation, or an extra stage in the gearbox? – Zeus Nov 28 '16 at 5:44
• @Zeus: The turbines are identical; the gearbox does the direction reversal. This is the same as for piston engines, some of which also had left- and right-turning versions. – Peter Kämpf Nov 28 '16 at 9:14
• Hmm, then this is different to piston engines, which reverse the crankshaft direction, i.e. the whole engine - it's relatively easy for them. – Zeus Nov 29 '16 at 0:30
• @Zeus, both options were used with piston engines. If the engine can be otherwise ungeared, reversing the whole engine avoids adding gearbox, but it means you now have two types of the engine, complicating maintenance. So when the gearbox is there anyway, reversing is done there and the engines are identical. And turbines have much higher RPM, so they are always geared. – Jan Hudec Nov 29 '16 at 21:59