Back in the early 40's, the world's first production helicopter, the Flettner FL-282 came equipped with a rotor governor. It was totally mechanical and used a flywheel, but is said to have done the job quite well.
The FL-282 was also the first helicopter to transition between powered flight to autorotation and back to powered flight. However, the comments below do not mention if the pilot was able to override this minumum rpm after a loss of power, so that he could pull energy out of the rotor for a flare. It must be assumed that he could.
___________________
This following is an excerpt on the Flettner rotor governor.
"The collective pitch control was located adjacent to the throttle, so that both could be moved simultaneously if desired. The collective pitch lever did not actuate the collective pitch directly, but through the intermediary of a blade pitch governor.
The governor was coupled to a servo gear, which changed the pitch to correspond with the movement of the collective pitch lever. Movement of the lever, or of the governor, engaged one or other face of a clutch, which engaged a drive from the rotor to change pitch in the appropriate sense. A follow-up device disengaged the clutch when the pitch reached the correct new setting. The whole collective pitch range could be traversed in one second.
The governor was designed to control the rotor within 10 r.p.m. between full power and power-off conditions, and could be set to govern at any desirable minimum r.p.m. In early tests the governor was set for a minimum of 140 r.p.m., but in one case of power-off flight autorotation stopped and the aircraft hit the ground heavily. After this the governor was set to a minimum of 160 r.p.m. If the power unit failed, or the rotor r.p.m. were reduced to 160 by closing the throttle, the pitch was automatically reduced to that for autorotation.
The collective pitch lever allowed the pilot to override the governor and to adjust the pitch as he required, but he could only increase pitch when the rotor speed was higher than the governor minimum setting. He could not increase the pitch when the rotor r.p.m. were at the minimum. If the governor controlled the pitch erratically for any reason, the pilot could always override it by flying at low collective pitch. It only took charge when the rotor speed was dangerously low, and it was provided, therefore, not so much as an aid to the pilot in correlating pitch and throttle, but as a safety device."
[ 06 September 2001: Message edited by: Dave Jackson ]