The good old PT6

[Tom_Kitty]

Hi guys

I have recently come across a couple of interesting questions regarding the PT6 engine, unfortunately I do not have immediate access to rated engineers, so I decided to post them here: (Oh yes and the training manual does not cover them)

1) Why does the PT6-67D have two power turbines in comparison with smaller pt6's only one turbine disk?
2) Why is there only one oil line going to the nose case, but two lines returning oil to the collector tank?
3) Why is the compressor turbine of the 'open' variety, whilst the power turbines are shrouded?

Can anyone help?

[Hilife]

1) I assume you are referring to the PT6A-67D (A Being Turboprop)? All -60 Series engines (Large) have a 2 Stage Power Turbine assembly. This is as a result of the much higher power ratings as opposed to the lower power ratings of the Single Stage -20/-30/-40/-50 & -100 Series engines.

2) A single oil pressure line is sufficient to lubricate components forward of the ‘C’ Flange, but the return lines scavenge the RGB and Propeller and the #3 & #4 bearings respectively. As the return oil is aerated and unpressurised, larger and additional lines are required in order to scavenge these two separate areas efficiently.

3) The Compressor Turbine Housing incorporates shroud segments which enable not only rework and replacement in the field, but also the ability to improve tip clearance tolerances and therefore efficiency of the Compressor Turbine. Being a Gas Generator Turbine, C.T. efficiency is much more critical than for the Power Turbines which uses notched and shrouded tips on the blades.