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On many turboprop aircraft, such as the A400M and the ATR 72, it seems that the air intake tends to be placed below the propeller (images: Wikipedia):

A400M source

ATR 72 source

Given that mounting the intake in a higher location helps reduce the risk of foreign object damage from ingesting debris into the core, why are the intakes still mounted below the propeller? Are the wings and engines already mounted high enough that the intake location doesn't matter when it comes to FOD, or does another issue (maybe accessibility for ground crews?) outweigh the risk of FOD?

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Inertial separators are augmented by gravity when located under the engine. It is easier to discharge ingested ice and debris downward, rather than some other direction, so that is where they go if possible. Inertia keeps solid matter low at the indicated point while air makes the turn into the engine.

enter image description here

C130 Nacelle

enter image description here

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    $\begingroup$ My understanding -- could be wrong -- is that inertial separators do not rely on gravity, rather the turn heavier particles have to take. $\endgroup$
    – user14897
    May 30, 2018 at 0:34
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    $\begingroup$ @ymb1 You are correct, but in this location they are augmented by gravity as I said in my answer. $\endgroup$
    – Pilothead
    May 30, 2018 at 0:35
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enter image description here
(wikimedia.org) British Aerospace Jetstream

You're right, it's the wing placement. For low-wing aircraft you find inlets above the prop, and sometimes circular inlets around the prop. The same engine shown above has different inlets depending on the plane, where you'd find an under-prop inlet in high-wing aircraft.

enter image description here
(norebbo.com)

But having the inlet above in low-wing is not a fixed rule, because a benefit of the under placement is that more air affects the wing's upper (suction) side. Inlets above the prop hub would take from that air:

Low pressure on the upper surface of the wing is really the major source of lift.

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    $\begingroup$ I think it's also worth noting that your "not a fixed rule" is of a PT-6, which is kinda its own special class of engine. Even though it's a low-wing aircraft, the benefits of that engine design and its inherent FOD reduction, make the placement of the inlets less relevant to air flow over the wing than the weight and balance of the short engine. (By the way, that's an amazing engine on an awesome aircraft.) $\endgroup$
    – Shawn
    May 30, 2018 at 15:43
  • $\begingroup$ Hi @Shawn - It is indeed a very successful engine. But why is it 'kinda its own special class of engine'? I may be missing something obvious -- I don't think it's the only 60's turboprop to be mounted backwards. $\endgroup$
    – user14897
    May 30, 2018 at 17:26
  • $\begingroup$ Sorry, "its own special class" more referred to my personal belief that all turboprop engines should be as reliable and successful as the PT-6. And my fanboism of that engine. It's not unique, but it is definitely a trend-setter of an engine. And I guess since it is used on a high number of aircraft, it's probably difficult to find a non-PT-6, low-wing aircraft with a bottom-positioned inlet. Especially one that wasn't modeled after the PT-6. $\endgroup$
    – Shawn
    May 30, 2018 at 19:49
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I believe the main reason is simply mechanical practicalities.

The bulk of the engine core is mounted overlapping the structural members of the wing, either below or above the wing spars.

The propellor is driven through a speed-reduction gearbox, which usually also offsets the propellor shaft from the turbine centreline.

That offset is used to place the propellor nearer the centreline of the wing to optimise airflow.

That in turn means the most direct air intake path to the turbine is the opposite side of the core to the propellor & gearbox. See the photos below:

A Rolls Royce turboprop engine Engine cutaway

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