Weads asks:
Why does nr decay occur in zero airspeed autos. When I say zero airspeed I'm referring to a high decent rate and no forward airspeed. Thanks again
Here is the drag curve for the Robinson R22:
(from
Helicopter Aviation).
As you know, in the R22 the bucket airspeed (minimum drag) is 53 kts at 104% Nr. In both powered and unpowered flight, drag goes up as you increase or decrease airspeed.
The other thing you need to know is that rotor drag decreases as you decrease RPM. This is in fact why minimum sink in the R22 occurs at 90% RPM. That's the slowest you can safely turn the rotor system in flight according to RHC.
When you autorotate at speeds below or above 53 kts, the total drag increases. The result is that the excess drag will cause the Nr to decrease. As the Nr decreases, rotor drag also decreases as we mentioned above. At some point the decrease in rotor drag balances the increase in total drag, and the rotor will maintain that RPM in steady state autorotation.
I think there is also an effect due to the shift of relative wind to be more from below the aircraft causing an increase in angle of attack as you descend more vertically, and this has the same effect as raising the collective would. However, I'm not prepared to defend that statement in open forum!