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