R22 Helicopter Safety Notice SN-24
To: Date: 8 September 1986 All R22 Owners, Dealers and Pilot Operating Handbook Subscribers
LOW RPM ROTOR STALL CAN BE FATAL
Rotor stall due to low RPM is still involved in more helicopter accidents, both fatal and non-fatal, than any other contributing factor. Frequently misunderstood, rotor stall is not to be confused with retreating tip stall which occurs only at high forward speeds when stall occurs over a small portion ofthe retreating blade tip. Retreating tip stall causes vibration and control problems, but the rotor is still very capable of providing sufficient lift to supportthe weight of the helicopter. Retreating tip stall has not been a problem with the R22.
Rotor stall, on the other hand, can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky. Fortunately, rotor stall most often occurs close to the ground during take-off or landing and the helicopter only falls four or five feet. The helicopter is wrecked but the occupants survive. However, rotor stall also can and does occur at higher altitudes and when it happens at heights above 40 or 50 feet it is most likely to be fatal.
Rotor stall is very similar to the stall of an airplane wing at low airspeeds. As the airspeed of an airplane gets lower and lower, the nose-up angle or angle-of-attack of the wing must be higher and higher for the wing to produce the lift required to support the weight of the airplane. At a critical angle, (around 15 degrees or so) the airflow over the wing will separate and stall causing a sudden loss of lift and a very large increase in drag. The pilot recovers by adding power and diving the airplane to recover the lost airspeed
The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower and lower, the nose-up angle-of-attack of the rotor blades must be higher and higher to generate the lift required to support the weight of the helicopter. Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air through the rotor provides the necessary increase in blade angle-of-attack. Again at a critical angle, as with the airplane wing, the blade airfoil will stall, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing rotor RPM to quickly decrease even more, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades making recovery virtually impossible even with full down collective.
When the rotor stalls it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first allowing it lo dive as it goes aft while the advancing blade is still 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’. Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls. Due to the magnitude of the forces invoked and the flexibility of rotor blades, hub flapping stops will not prevent the boom chop. **The resulting boom chop, however, is somewhat academic. as the aircraft and its occupants are already doomed by the stalled rotor before the chop occurs. **
To prevent rotor stall and its catastrophic results the pilot must always do whatever is required to maintain a safe rotor RPM. It must take precedence over all other considerations, even if it means landing short in a swamp instead of trying to stretch your glide to the dry road beyond.
Remember the power output of the engine is proporticnal to RPM and when the RPM is low you have less power available from the engine with which to regain the lost RPM. The power-on low RPM recovery procedure of simultaneously rolling on throttle while lowering collective must be practiced until it becomes an automatic reaction to any indication of low RPM. Low airspeeds combined with high sink rates must always be avoided and full collective must never be pulled until the helicopter is within one foot of the ground.
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