If I decide to utilize a PT6 (ignoring for now the cornucopia of variants) as the turbine engine I will be using on my newly designed turbo-prop aircraft, do I then decide how I will orient the engine or is the this something that is engineered into the engine and thus a consideration when deciding which we choose?

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Source (adapted)

What compels an aircraft design team to utilize a reverse-flow (turbine ahead) turbine engine orientation? Is the reverse-flow a design of the engine, or maybe an option?

What are the benefits/ drawbacks of reverse-flow designs to the design engineers, mechanics, flight crew, and/or airlines?

  • $\begingroup$ So, I knew I should have refined my example, my bad. So the reference to the PT-6 was just a random choice. The question that I should have just asked is "In some turbo-prop aircraft, what is the benefit/ purpose of turning a turbine around so the fresh air intake is facing the tail and the exhaust is facing forward?". I agree with Jonathan, and realize my confusion. While mutually exclusive, are there many with free-wheeling turbo-prop acft with fwd facing turbine engines? Why would they choose that instead of the aft facing engine? Sorry for the poorly stated original question. $\endgroup$
    – BigNutz
    Mar 25, 2017 at 14:30
  • $\begingroup$ Yeah, exactly! Was that you who put that image in? I'm new to this so I don't know how everything works. That was a good image, I liked it, but when I put in the KingAir engine image to show what I was talking about, it didn't keep the other one for some reason. $\endgroup$
    – BigNutz
    Mar 25, 2017 at 21:15
  • $\begingroup$ Yeah, so what is the purpose of turning it that way? I guess it's facing aft because of the free-wheeling design. With how the exhaust energy is transferred to the propeller, if the engine were oriented intake forward, you'd need to run an additional shaft forward to the propeller gearbox like a turbofan engine. Did I Just answer my own question? $\endgroup$
    – BigNutz
    Mar 26, 2017 at 2:27

1 Answer 1


Are you talking about the reverse flow of the combustor or the reverse flow of a PT-6 installed in a tractor configuration (puller)?

The reverse flow design of a PT-6 using a power turbine separate from the gas core offers several advantages:

  • simplified gas core and power turbine without the additional complexities of nested spools.

    • modular gas core, power turbine, propeller gearbox and accessory gearbox for ease of maintenance.

    • design facilitates ease of installation in a pusher configuration e.g. Beech Starship, Piaggio P180 Avanti without complicated and heavy propeller drive trains.

    • Engine exhaust from a pusher configuration additionally provides propeller deicing without additional electrical or chemical deicing boots.

    • the reverse flow of the tractor layout allows for additional FOD protection vis a vis an intertial separator prior to air entering the first stage of the compressor. This makes the engine more adept to operating from unpaved airstrips or ones covered with rocks, ice or other debris.

    • separate gas core and power sections reduce spool inertia in the gas core, making the core - and the engine - more responsive to throttle inputs from the flight crew or autothrottles.

  • 2
    $\begingroup$ You are confusing reverse flow with free turbine. They​ are separate and independent concepts. $\endgroup$
    – J W
    Mar 25, 2017 at 13:30
  • $\begingroup$ @JonathanWalters, they are separate concepts, but definitely not independent—reverse flow only makes sense when free power turbine is used. For single-spool engines, reverse flow does not simplify anything. $\endgroup$
    – Jan Hudec
    Mar 26, 2017 at 17:16
  • $\begingroup$ @JanHudec The free turbine concept is certainly independent of the reverse flow orientation of certain engine designs, such as the PT6 or Walter. See the PW100 series: direct gas flow with a free turbine. Hence my comment: the free turbine is not a unique advantage of the reverse gas flow orientation. $\endgroup$
    – J W
    Mar 27, 2017 at 2:00
  • $\begingroup$ @JonathanWalters, yes, but that does not make them independent concepts, because reverse flow still exists only to make free power turbine simpler and wouldn't be used without it. $\endgroup$
    – Jan Hudec
    Mar 27, 2017 at 4:54

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