For what it’s worth, here’s my two pennyworth.
In 28 years of instructing I have probably done only hundreds of Simulated EOL (SEOL, to use EASA-speak) in piston helicopters, but thousands in turbine helicopters.
And just to clarify my own terminology a bit more, an autorotation is where the rate of descent airflow, rather than engine power, is driving the blades, and may end in a re-engagement of power, either at height, or in a flare recovery (ie the SEOL profile, but power used to recover to low speed IGE environment, no ground contact) or in a SEOL (engine running but disengaged and ground contact) or an EOL (engine not functioning, ground contact). OK, some purists may remind me that engine to MGB drive may have failed but the engine is left running to drive the TRGB…point taken!
My take is that in a piston, with the engine throttled back to idle, there would be a negligible difference, though I have never done a shut-down, thus actual, EOL in a piston.
In the turbines, it depends….
In the Gazelle, when you throttle back, the engine is completely disengaged because it has a mechanical centrifugal clutch. (BTW, 1000+ hrs as a Gaz military QHI)
In free-turbine installations there may be residual thrust at the flight idle (FI) throttle position. This is true for a couple of other light Airbus SET types, where there is a drop in NR at min collective pitch when the throttle is closed to the ground idle position. Less observable in the 206 variants, though I am open to contradiction: and only recently have I access to a 206 instrumented for flight data, so I may well have a scientific look when I next can. This may also be a function in the 206 of the high inertia rotor, so that a throttle closure to GI at committal height doesn’t manifest itself in NR drop before the flare commences.
In certification trials on a light twin with Allison engines, we found that in 4 minute OEI climbs, in identical WAT and geographical conditions, there was no discernible difference between a climb with one engine at GI and the same engine actually shut down. So conclusion: no residual thrust at GI. So with both engines at GI in autorotation in that type one might expect that to simulate faithfully the engines-out conditions.
I also had a little experience of autorotation and EOL trials in a 7-tonne class aircraft. Here with both engines shut down there was a 2-5% drop in NR at min collective pitch vs NR at GI…so definitely some residual thrust.
It is, perhaps, safest to assume that most SET may have some residual turbine thrust during practice autos, either at FI or GI throttle setting. But it's probable that for an unexpected engine failure the descent profile will result in potentially shorter range, due to the initial loss in NR before pilot intervention.
So, putting my FI(H) hat on again, two golden ‘Lessons Learned’ rules:
• In Autorotation, NR is King
• Never expect to go as far as you first think you might in auto; losing range is easy; stretching it is not!
Last edited by idle stop; 19th Sep 2017 at 11:06.