Just an idle thought on this subject I have had in the past:. .. .Why not include a bottom section of collective travel perhaps protected by a spring like the detent on the throttle. This portion of the collective travel could allow the pitch on the main rotor blades to go to a negative setting. Whilst I appreciate that this is an extremely undesirable situation in powered flight, which would probably result in a boom strike, it would at least allow rotor rpm recovery from very low values provided that enough height is available. . .. .I have just wondered why this isn't the case in the past maybe someone could enlighten me.