Something that Nigelh and Reely340 also mentions and is very inportant:
If you do an autorotation with engines in flight you arent performing a true auto before the nf shows lower rpm/ disconnected to rotor rpm on the two-/threeneedle gauge. Most turbines continue to drive the rotor until somwhere in the 102-105% ng mark. Rotor rpm has to be higher than this, otherwise the engine(-s) still are driving the rotor, very much affecting the rate of descent and glide angle.
In an auto with engine(-s) at idle, the rotor drives itself and the freewheeling unit already put the said drag( but very,very low) on the system.
I fore sure hope tou guys train autos with engines at idle. I did training somewhere where the pilots newer did this. They actually also only did the simulator autos with engines in flight(
). If you ever should be in the need for a real auto I think tou would feel very fooled, seeing a much much higher rate of descent and a lot shorter glide, and efter this tou die :-(