When a helicopter is in autorotation (that is, gliding without the benefit of engine power) rotor rpm must be maintained. This is done when entering the autorotation by lowering the collective control. If the rotor rpm approaches an upper limit, the collective is raised to add pitch. This increases drag and slows the rotor rpm. A low rotor rpm situation is just the opposite, lower the collective pitch to reduce the drag and allow the rotor rpm to speed up.
Rotor rpm in autorotation is a function of several factors like density altitude, gross weight and airspeed. An over speeding rotor is easy to manage since the collective control was lowered on entry there is plenty of movement upward to add drag. However, if a pilot enters autorotation and lowers the collective control all the way down and the rotor rpm is still too low this could be a problem. Typically, in this case, the main rotor pitch is set incorrectly. The helicopter’s maintenance manual has a procedure to adjust this by either lowering the collective control’s down stop or adjusting the main rotor blades’ pitch links. To accomplish this, a mechanic will note the helicopter’s weight and the density altitude and then reference a chart to get the correct rotor rpm. A flight test will then be performed at those conditions and the actual rotor rpm will be noted with the collective all the way down. If it is not at the correct rotor rpm stated in the chart, the mechanic will make an adjustment.
This is done to insure that in the worst case scenario (light helicopter, high density altitude) the pilot will be able to lower the collective control far enough to guarantee an acceptable rotor rpm in autorotation.