Rotor rpm is a critically important parameter for all helicopter operations. Just as airplanes will not fly below a certain airspeed, helicopters will not fly below a certain rotor rpm. Safe rotor rpm ranges are marked on the helicopter’s tachometer and specified in the RFM. If the pilot allows the rotor rpm to fall below the safe operating range, the helicopter is in a low rpm situation. If the rotor rpm continues to fall, the rotor will eventually stall.
Rotor stall should not be confused with retreating blade stall, which occurs at high forward speeds and over a small portion of the retreating blade tip. Retreating blade stall causes vibration and control problems, but the rotor is still very capable of providing sufficient lift to support the weight of the helicopter. Rotor stall, however, can occur at any airspeed, and the rotor quickly stops producing enough lift to support the helicopter, causing it to lose lift and descend rapidly.
Rotor stall is very similar to the stall of an airplane wing at low airspeeds. The airplane wing relies on airspeed to produce the required airflow over the wing, whereas the helicopter relies on rotor rpm. As the airspeed of the airplane decreases or the speed of the helicopter rotor slows down, the AOA of the wing/rotor blade must be increased to support the weight of the aircraft. At a critical angle (about 15°), the airflow over the wing or the rotor blade will separate and stall, causing a sudden loss of lift and increase in drag (refer to Aerodynamics of Flight section). An airplane pilot recovers from a stall by lowering the nose to reduce the AOA and adding power to restore normal airflow over the wing. However, the falling helicopter is experiencing upward airflow through the rotor disk, and the resulting AOA is so high that even full down collective will not restore normal airflow. In the helicopter when the rotor stalls, it does not do so symmetrically because any forward airspeed will produce a higher airflow on the advancing side than on the retreating side. This causes the retreating blade to stall first, and its weight makes it descend as it moves aft while the advancing blade is climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes referred to as rotor “blow back” or “flap back.”
Low rotor rpm can occur during power-off and power-on operations. During power-off flight, a low rpm situation can be caused by the failure to quickly lower the collective after an engine failure or by raising the collective at too great a height above ground at the bottom of an autorotation. However, more common are power-on rotor stall accidents. These occur when the engine is operating normally but the pilot demands more power than is available by pulling up too much on the collective. Known as “overpitching,” this can easily occur at higher density altitudes where the engine is already producing its maximum horsepower and the pilot raises the collective. The corresponding increased AOA of the blades requires more engine horsepower to maintain the speed of the blades; however, the engine cannot produce any additional horsepower, so the speed of the blades decreases. A similar situation can occur with a heavily loaded helicopter taking off from a confined area. Other causes of a power-on low rotor rpm condition include the pilot rolling the throttle the wrong way in helicopters not equipped with a governor or a governor failure in helicopters so equipped.
As the rpm decreases, the amount of horsepower the engine can produce also decreases. Engine horsepower is directly proportional to its rpm, so a 10 percent loss in rpm due to overpitching, or one of the other scenarios above, will result in a 10 percent loss in the engine’s ability to produce horsepower, making recovery even slower and more difficult than it would otherwise be. With less power from the engine and less lift from the decaying rotor rpm, the helicopter will start to settle. If the pilot raises the collective to stop the settling, the situation will feed upon itself rapidly leading to rotor stall.