DOWN BETWEEN THE ENGINES
For the A400M Airbus has developed a unique down-between-the-engines (DBE) scheme for propeller rotation direction. Engines 1 and 3 rotate clockwise, while 2 and 4 have an idler gear installed that gives them an anti-clockwise rotation. This is a marked departure from the typical counter-rotation scheme that has all engines on a wing turning in the same direction.
The DBE configuration produces a more symmetrical flow over the wing, improving lift and handling qualities while allowing for smaller (less drag-inducing) tail surfaces. The symmetric propeller slipstreams also allow for mirror image wing structure and reduced structural weight due to lower wing bending moment loads. Finally, the DBE configuration has a benefit for passengers in the cargo compartment, too. Airbus had planned to install active acoustic dampeners to muffle the sound of the propellers. The DBE configuration has the blade closest to the fuselage travelling upwards, which reduces cargo compartment noise by approximately 1dB.
1.Is the additional lift just caused by the adjacent propellers' tips moving in the same direction, or does the benefit occur specifically because the flow is downward below the wing? If the between-engine tips were simultaneously moving the air upward above the wing instead, wouldn't there be a similar benefit? Intuitively, the downward motion seems to make sense as designed, since the increased pressure under the wing would push the wing upward. But air flow above the wing is supposed to be beneficial (though I have a harder time envisioning why).
2. Why would interior noise be reduced by the nearest blade tips moving upward instead of downward?
3. Airbus is rightly proud of the smaller/lighter/less-draggy horizontal and vertical stabilizers resulting from the DBE configuration, but they went with a T-tail configuration for the A400M, which seems to be the most common setup for high-wing aircraft with engines mounted on the wings. However, the Antonov An-70 has high-mounted wings with engines on them, but it has a conventional, lighter tail. Wouldn't a similar DBE-effected size reduction still be available on a conventional empennage? Why didn't Airbus go with that configuration instead? Or is there something wrong or unsafe with the An-70?
4. Airbus specifically had the aircraft and engines go through civil certification, but has anyone ever expressed commercial interest in the A400M or TP400? How much of a hassle would it be to certify different propellers for the TP400 if a customer wanted the engine to go faster than the Mach 0.72 maximum speed of the Ratier-Figeac FH385/FH386 propellers currently certified for it?