Do you wait a day or 0.14 sec for an answer??????
 

GOOGLE is your friend.....!

By controlling the size of the driving region, the pilot can adjust autorotative rpm. For example, if the collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium to move inboard along the blade’s span, thus increasing the size of the driven region. The stall region also becomes larger while the driving region becomes smaller. Reducing the size of the driving region causes the acceleration force of the driving region and rpm to decrease.

and that took 0.14 seconds :D:D:D

The driven region is the part of the blade that is producing more drag then lift. as you lower the angle of attack, you are decreasing the amount of drag that the blade is producing.

So, as you drop collective, you are producing less lift, but also less drag. the driving region increases in size and rpm's increase because there is more force driving them then there is drag slowing them down. Of course, the opposite is true when you raise collective. More lift, and more drag. The driving region shrinks as the driven region grows resulting in rpm's being lost.

If you are autorotating with a constant rpm, then the driving region is spinning the rotors up at the exact same pace as the stalled and driven region are slowing it down.

(i think;))

by the way, welcome to the forums! (although I just noticed you joined before I did...oh well)

When you raise the lever in autorotation, the inner boundary between the high AOA or stalled section and the driving section moves outboard. The outer boundary doesn't move much hence the driving section is smaller and the RRPM reduce. The next question that usually gets asked is why do we reduce the RRPM to the lower end of the band to reduce R of D and increase range. The answer is that the overall L/D ratio of the whole rotor system is usually arranged to be better at the lower end of the band, beneficial if you overpitch and regardless of weight, you can always achieve it in autorotation by raising the lever.

So..

Griff - I have two books with two contradictory answers, so I figured that there was at least some rumour involved here ;-)

Rotorfossil says that the boundary stays roughly put. Flyting and Rotorwashed (from Wikipedia) that the boundary between driven and driving regions will move outwards as the collective is lowered, thus increasing the size of the driving area. Which makes some sense, as collective lowered->more RPM.

But I don't think this second view is necessarily correct. The angle of attack reduces along the blade from root to tip, and it is this reduction that produces the stalled/driving/driven regions. The point of equilibrium occurs at a particular angle of attack so, if you lower the collective, this point will move inwards along the blade. If that's not clear, consider the original point of equilibrium - its angle of attack is reduced once the collective is lowered, and this will push it into the driven region.

Given this, the increase in RPM associated with lowering the collective must be either the result of the reduction in size of the stalled region (which I'd not considered - thanks, RF) and/or an overall reduction in drag from the reduction in pitch along the blade.

What have I missed?!

--Dave