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What's the point of the centrifugal compressor stage in the P&W PT6A?

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What's the point of the centrifugal compressor stage in the P&W PT6A?

Old 6th Oct 2015, 19:31
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Join Date: Apr 2012
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What's the point of the centrifugal compressor stage in the P&W PT6A?

Hi everyone,

I've recently been reading into free-turbine turboprops and by extension, the PT6. I understand the maintenance advantages of doing away with any mechanical link between the power shaft and gas generator and in turn, why the engine must be reverse-mounted and airflow rotated through 180 degrees by the intake and again at the exhaust. What I don't understand is the advantage of reciprocating direction of airflow twice more in the gas generator; once following compression by the centrifugal, 4th stage, after it passes through the diffuser ducts and again as it exits the combustion chamber. The only potential benefit I could think of would be that it shortens the overall length of the engine, as it allows some of the combustion chamber to extend beyond the turbine. I would however have thought that the additional friction that comes about by virtue of this would have a severely negative effect upon efficiency and not be a worthwhile trade-off.

Kind regards,

Charlie Reed
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Old 11th Oct 2015, 01:54
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The turns at the burner can make for a shorter engine for sure. The turn going into the compressor seems to help separate the air from ice/rain/fod. Almost never see fod damage in a PT6 on a King Air.

One big advantage of the air coupled power coupling is starting in cold weather. A Garret King Air (B100) has a second battery and the ability to put them in series for a 48 volt start in cold weather, the PT6 powered ones don't require it.
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Old 13th Oct 2015, 18:48
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Thanks a lot, very interesting.

Kind regards,

Charlie
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Old 8th Nov 2015, 17:21
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Join Date: Jan 2001
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Charlie,

Don't mix up why the CF compressor is there with why the free turbine is not mechanically connected - two entirely separate issues.

The CF compressor is there to compress - nothing more. The fact that it's preceded by a few axial stages simply serves to give it something denser to compress in the first place.

Quoting some gash figures from my youth (I'm sure more educated folks than I will have better numbers but mine will do for this argument ...)

A single stage of axial will give maybe 1.25:1 compression, while a single stage of CF will give maybe 5 or 6:1 so if you can feed that CF stage with air at 50psi than crank it up 6 times you'll get that from a much shorter engine than if you had 10 stages of axial.

Rest assured if the trade off in efficiency was too bad the clever designer chappies would have thought again, but they aren't always focussed on efficiency above all else, and the dear old PT6 has been turning a good long while now, and paid my mortgage for donkey's years :-)

PT6 is a great engine with many good attributes, but perhaps not the last word in sophistication compared with today's designs.
Resistance to FOD is greatly improved by the shape of the intake with its 90 degree bend (although that's further helped in the KingAir by the bypass feature in the airframe part of the inlet system too), but FOD is still possible in there as anyone who's ever looked can confirm (but the pilot doing his walkround on a KA will never see it as it's too far round the bend! - that's another story)

Why the free turbine is there is a whole other story too, but enjoy your journey :-)

Last edited by Coriolis; 8th Nov 2015 at 17:26. Reason: Tweaking numbers to increase believability
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