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As far as I know, all turboprop engines have mechanical continuity between the engine and the propeller except for the Pratt & Whitney Canada PT6 family of engines. What is the correct term for the setup the PT6 uses to power the propellor and how does the PT6's design differ from other turboprop engines?

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  • $\begingroup$ I can't actually find the term “direct drive” used with turboprop engine anywhere, only with turboshaft. Also as far as I can tell, the layout of PT6 is pretty typical (separate gas generator turbine and free power turbine, reverse flow so the shaft from the free power turbine does not need to go through or around the engine). $\endgroup$ – Jan Hudec Oct 19 '15 at 12:59
  • $\begingroup$ Good call. I'll edit "direct drive" out. $\endgroup$ – ryan1618 Oct 19 '15 at 14:49
  • $\begingroup$ That does not necessarily mean the term would be incorrect. Just that it is not common. $\endgroup$ – Jan Hudec Oct 19 '15 at 15:13
  • $\begingroup$ After thinking about it, it might be considered incorrect. When the term direct drive is used with piston engines the propeller is attached directly to the flywheel. No turboprop engine can work without a reduction gear. Even if there's mechanical continuity it's not really direct drive. $\endgroup$ – ryan1618 Oct 19 '15 at 15:15
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    $\begingroup$ @JanHudec A quick google seach located several references to "direct drive" turboprops such as this one. They all use the term to refer to a single shaft engine as opposed to a "free turbine." This pdf actually uses "direct drive" to refer to the fuel pump system. $\endgroup$ – TomMcW Oct 19 '15 at 19:38
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Split Shaft

Several turboprop engines, including the PT6 turboprop engine, use an epicyclic gearbox only, many other turboprop engines use a multishaft gearbox. The latter usually results in the propeller axis being offset from the turbine axis.

In common with some other engines, the PT6 turbine is also reversed. This facilitates the split-shaft arrangement where the engine and propeller are driven by separate (but in-line) shafts, each connected to their own turbine wheel.


PT6

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Image: P&W

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Image:12Charlie

A contrasting and common design

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Image:12Charlie

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  • $\begingroup$ I'm trying to understand how any turboprop/shaft with a gearbox can be described as "direct drive". Any help? $\endgroup$ – egid Oct 19 '15 at 14:16
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    $\begingroup$ @egid: I imagine it means that there is a continuous metallic contact through the power train from compressor to propellor. This is not the case in the PT6. $\endgroup$ – RedGrittyBrick Oct 19 '15 at 14:18
  • $\begingroup$ Ah, I see. That would make some sense. $\endgroup$ – egid Oct 19 '15 at 14:21
  • $\begingroup$ I think "direct drive" was the wrong term to use here. I edited it out of my original question. $\endgroup$ – ryan1618 Oct 19 '15 at 15:05
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    $\begingroup$ I don't think the layout is truly specific to PT6. Reverse flow is used in the other small turboprop engines too, at least in Allison 250/RR M250 and Walter M601. The layout of Allison 250 is rather complicated with two-shaft gearbox and compressor in the front, but at least the Walter M601 seems to have the same layout as PT6 with split shaft and in-line planetary gearbox. The layout does not appear in large engines, probably because turning larger air mass flow is no longer practical. $\endgroup$ – Jan Hudec Oct 19 '15 at 15:48
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The difference is quite simple , with the Pratt and Whitney design there is no direct mechanical link between the turbine compressor and hot section to the propeller gearbox reduction drive . With the Allison design were you to turn the propeller by hand all the turbine parts will move all the way back to the inlet compressor . There is a direct mechanical link all the way through

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As described above, it's a split shaft turboprop engine. The design isn't unique to the PT-6 (T-74) but the PT-6 variants are by far the most ubiquitous split shaft turboprops on the market today.

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