If the engine conks out its possible to still land the heli and keep it in one piece.
Most model helis have a 'one way bearing' this allows the blades to freewheel.....if you are flying around and the engine dies out....very possible with a nitro, petrol or gas turbine engined heli....not so common with electric helis.....because you have a free turning rotor you can use the air rushing through the rotor disc (because your heli will be going down!) to keep the rotor turning.
With the turning rotor we can slow the helis descent....and build up rotor speed....or at least maintain it....becuase once we get near the ground you need that reserve of energy to steadily slow the helis descent.....and then gently set it down on the ground.
To do this you need 'negative pitch' so with the collective stick down there will be say minus 5 degrees of negative pitch....ie the blades trailing edge will be pointing up slightly.
Autos are hard to do....at first.....you must get a simulator and try it out....Phoenix (my current sim) has an auto practice feature....you click on it and you get your chosen heli just launched at you from a height (which you can select) this forces you into having to land it....and try to keep it in one piece
Try it out....its a deep subject as you realise the forces involved....and its something that might just save your helicopter should a real engine failure happen.
Heavier blades equals good energy reserve, but basically a good sim will allow you to alter the parameters of the model...so you can experiment...its all about conditioning your reflexes....beacuse if you dither in a real emergency you could end up with a totalled heli....I would rather a slightly messy auto than a total wreck.
Rob