Low RPM Rotor Stall
By Johan Nurmi
It was a warm summer day in late July on the East Coast. A high-time, fixed-wing and helicopter pilot was out on a business trip. He and his friend were two heavyset guys, not really made for the Robinson they were flying. When the low rpm warning horn and light came on at 97 percent, the pilot pulled the collective, pushed the cyclic forward and instantly stalled the rotor blades. Witnesses saw the helicopter plummet to the ground.
From 1979 to 1994, 29 pilots in the R22 died because of lack of training in controlling rotor rpm in this excellent helicopter. Many deadly accidents in small helicopters have been caused by pilots not being able to keep rpm in the green arc.
Rotor rpm is directly proportional to available engine power. If the rpm decreases, say, 10 percent, the power is therefore 10 percent less. Engine and rotor rpm decrease proportionally. With 10 percent less power, the helicopter starts to descend. If the pilot lifts collective to stop the descent, the rpm decreases even more, causing the helicopter to descend further. If the pilot fails to lower collective and roll on the throttle to prevent further descent, the rotor blades stall immediately and the helicopter will fall out of the sky.
The R22 and R44 are equipped with both a governor and a correlator.
A governor is a mechanical device that senses rpm and makes the necessary adjustments to keep it constant. A pilot flying will feel the throttle make the automatic changes, increasing and decreasing the throttle and rpm.
The governor keeps the rpm constant, top of green, at 104-percent rpm, as long as the pilot does not exceed the available engine power. For example, on a hot day (over 30C) the maximum engine power is 23.5 in. Hg, in the R22BII. If the pilot exceeds this, the rpm immediately starts to creep down until the warning horn and light come on to warn of a dangerously low rpm situation.
A correlator is a mechanical connection between the collective lever and the engine throttle. When collective is lifted, power is automatically increased; when it is lowered, power is decreased. The correlator maintains the rpm to a close, but not exact, value. When the governor is set to the "off" position, the pilot is therefore required to make the small throttle movements for fine-tuning the rpm.
The R22 stalls below 80 percent plus 1 percent per 1,000 ft. of altitude. There is no recovery method if the pilot stalls the rotor. If he by mistake pulls too much collective and the warning horn and light come on at 97 percent, he still has time to do an rpm recovery. Normal reaction time is 0.2 sec. to lower the collective and roll on rpm. This movement is called "milking the collective." If the warning light and horn come on, don’t panic. Relax and get it back on top by lowering the collective and rolling the rpm back to 104 percent. Thumb up is life.
If you experience a low-rpm situation close to the ground and can’t get it back (due to rpm being too low and the descent rate too rapid), it is far better to let the helicopter descend down into the ground with some rpm remaining than to stall the blades and fall out of the sky.
If you cannot recognize and react properly to the first symptoms of a low-rpm situation, trouble is on its way. The decreased rpm originates from the blade roots stalling, which results in decreased rotor thrust, accelerated rate of descent, increased rate-of-descent flow and increased induced-flow angles. The stalled region at the blade roots spreads outward toward the tips. Slower blade speeds means less centrifugal force, leading to high blade-coning angles.
Fixed-wing pilots are trained to add power and nose the aircraft over at the sound of a low-rpm warning horn. That is the correct procedure for fixed-wing aircraft, but not for helicopters. Increasing collective, adding power and pushing the cyclic forward drives the rpm down even lower and places large portions of the blades more deeply into the stalled condition. The requirement for engine torque to oppose further rotor rpm drop exceeds the engine’s ability to do that specific work.
A complete rotor stall leads to loss of directional control, severe blade flapping, possible blade failure (from the coning angles) and a nose-down pitch as the longitudinal stability aligns the fuselage with the rate of descent flow.
A low-rpm condition can come from pulling an excessive amount of collective, climbing at high altitude in thin air or mismanaging the flare in an autorotation. Whatever the reason, lower the collective to reduce blade pitch angles while simultaneously rolling on the throttle to increase power output and rpm.
You might also use right pedal and gain translational lift with slightly forward cyclic.
The rotor rpm at which the stalled condition becomes critical is remarkably below normal and safe operating rotor rpm and should be easy for pilots to avoid.
Professional training given by high-time instructor pilots is the answer, not just for avoiding low-rpm rotor stall but for avoiding all accidents. Instructor pilots should teach students to fly without the governor and practice low-rpm recovery until the correct recovery becomes an instinct.
Johan Nurmi is an FAA Gold Seal instructor pilot and vice president and owner of USA Academy of Aviation, Inc. in Murrieta, Calif.