Systems Question About Turbine Helicopter Autos..."Little Book of Autorotations"

[TheJudasNut]

Greetings,

I'm having a little trouble visualizing a point made by Coyle in "The Little Book of Autorotations."  On pages 64 and 65 Coyle states that in some turbine helicopter designs, specifically those that aren't built around a fixed-shaft engine, a certain amount of residual power is still coming from the power turbine to overcome losses contributed by accessories, like a hydraulic pump, etc during a practice autorotation.  He makes it clear that a real world, engine-out autorotation in such designs is steeper and has less room for error—a smaller 'flare window' one might say—because the residual power from the idling power turbine is gone. 

Coyle states, "For those who say - but the rotor is split from the power turbine and is being driver solely by the air passing up through it - why is the rate of descent more if the engine is really failed?  The power turbine is still producing power that is going to the transmission - and the transmission takes power to turn things like the hydraulic pump.  So, even if the rotor is split from [the] power turbine, the power turbine is still overcoming some of the losses.  With the engine stopped, that power has to come from the rotor."

My problem is this:  As I visualize this in my head, I see the critical accessories being installed on the rotor side of the freewheeling unit—an admittedly big assumption on my part.  Thus, I don't see how the power turbine is actually contributing to the drive of those accessories while the rotor is "overrunning" the output of the PT.  Is he only talking about frictional losses in the freewheeling unit that are being subtly overcome by power coming from the power turbine, or am I missing something here?

I think "The Little Book of Autorotations" is a fantastic text, but this is one section in which I wish Coyle had provided a simple schematic or diagram.  The diagrams in the FAA's "Helicopter Flying Handbook" aren't particularly great for this either.

Background: I'm a full time fixed wing pilot finishing a helicopter add-on rating.

Thank you in advance for your expertise and insight.

[Disguise Delimit]

Hi Judas, 

Don't overthink it. The idling engine is still providing input to the transmission by sort of "loosening up" the N2 turbine wheels and making the freewheel unit turn a bit easier. Just a bit, and if it was not present then the descent would be a bit different. Shaun was a great source of info, had a lovely lunch with him back in 2003, and he is a great loss to the industry.

His book Cyclic and Collective is also worth having, peppered with humorous anecdotes.