Low Rotor RPM/Blade Stall

Robinson Safety Notices SN-10 and SN-24

Helicopter pilots have a saying: RPM = life. It means that if your blades are spinning fast enough, you should be able to fly. But if you lose RPM, there’s a chance that you might drop out of the sky (yes, like a brick) .  This is called Blade Stall.  Blade stall is fatal.

Why do you drop out of the sky? Because the spinning of the rotor blades is what gives a helicopter lift. If they stop spinning, they’re not generating lift. If they’re not spinning fast enough, they’re not generating enough lift to keep the helicopter airborne.

Helicopters have low rotor RPM warning systems. In an R44, it consists of a light on the instrument panel and a “horn.” The sound of the horn is very annoying and impossible to miss. Because RPM is so important, the full system — light and horn — are required for flight.

The imposing sounding of the horn does not necessarily mean impending doom.  The horn is designed to come on at 97% Rotor RPM in the R44 while the helicopter (at max gross weight) won’t stall until 80% RPM plus one percent per thousand feet of density altitude.

Low RPM is recognized by:

  • A noticeable decrease in engine and rotor noise
  • A slight vibration and cyclic stick shake at higher speeds
  • The activation of the low rotor RPM warning light and horn system
  • Also a yaw to left and slight descent of helicopter
As you can see there is a safety factor built into helicopters, any time rotor rpm falls below the green arc and there is power, simultaneously add throttle and lower the collective. If in forward flight, gently applying aft cyclic causes more air flow through the rotor system and helps increase rotor rpm. If without power, immediately lower the collective and apply aft cyclic.