Mornington Sanford Aviation

Low RPM Rotor Stall is a primary cause of fatal accidents.

As air speed is the life blood to the aeroplane pilot so Rotor RPM is to the helicopter pilot! Should the aeroplane pilot stall their aircraft during flight by the loss of airspeed, they are able to recover (subject to height above the ground) by increasing the air speed and reducing the angle of attack on the wing.

However, if the helicopter pilot stalls his rotor due to a loss of rotor RPM, it is NOT recoverable because the induced flow up through the disc will never allow the angle of attack to be less than the stall angle (even with the collective lever fully down). You become a passenger until you hit the ground!

Engine power is directly proportional to the engine RPM, a 10% loss of RPM will incur a 10% loss in engine power available.

One of the reasons you are given a de-rated engine operating limit is that you have power available should you need it; a get you out of jail card to be used to prevent an accident but not to fly outside the aircraft limits.
However, if the RPM is not in the green this power reserve will be reduced directly proportional to where your RPM is at the time.

KEEP YOUR RPM IN THE GREEN