Originally Posted by
BOAC
My memories of auto-rotating a Whirlwind on the short pre-Harrier course at Shawbury have well faded, so could someone (patiently) explain the aerodynamics whereby it has been said that a stalled rotor disc could bring itself to a standstill? Surely even stalled there is still a rotational force on the blades which would keep the disc turning if air is passing through the disc (ie heli falling)? AAIB were, it seems, presumably deliberate in stating that neither main or tail rotors were rotating at impact, which is a concept I find hard to grasp.
Cue
Bastiano with his memories of Whirlwind autos!
In the meantime .. take a look at this, from 18:50 onwards:
Hopefully someone will pipe-up with an aerodynamics explanation of the physiscs behind why, if you allow Nr to decay too far below the bottom of the green, it becomes unrecoverable.
Not that this applies to modern craft but .. in some of the early helicopter prototypes the blades actually 'snapped-off' when the Nr decay became excessive.
As far as my awareness of 'modern' craft is concerned, what I understand is that the 'driving' portion of the blade used in autorotation is simply incapable of recovering the lost inertia of a 'stalled' or sufficiently slowed rotor and, even it it were, the airflow over the blades would have to be 'just so' in order to get them rotating again and which positioning of the airframe relative to the desired airflow would be impossible to control in a 'free-falling' aircraft.