disclaimer - not a helicopter pilot - but still technically adept
now on a piston helicopter if the rpm of the engine decreases there can't be any torque transmitted from the engine to the rotor system / gear box. also it does not matter which accessories are driven before or after the sprag clutch. it will be the same no matter how far the engine is below output shaft rpm if it is minus 100 or minus 10 percent.
on a turbine helicopter with a gas generator and a power turbine on the other hand... you can reduce the rpm of the gas generator only. that running at idle still makes air go through the power turbine which i believe will still turn at sufficient rpm although with relatively little torque to transmit some power to the rotor system.
so we see we have a difference regarding autorotation:
piston helicopter there is no difference between idle and engine off
turbine helicopter there a difference between idle and engine off
now add to that: i'm not so sure a turbine helicopter needs a sprag clutch. you could well say the power turbine as a load is not significant enough to warrant one. but i guess they put one in in case turbine or compressor blades decide to rampage through the engine .... which could be a significant load.
still idle power should impart enough torque onto the power turbine wheel so that it makes a noticeable difference compared to engine out even if there is a sprag clutch.
so nigel: tc might have said no friction but might have meant no residual torque being transmitted.
but i guess that's the different viewpoint piston vs turbine.
anyway there haven't been any concise explanations on the issue so maybe my train of thought will help with that maybe not. at least you got my brain working that's always nice so thanks for the discussion
