By Capt Spry

 Identification of the problem. Ffeed back from Robinson Safety Courses over the past 18 months shows post governor trained instructors and students do not really understand rotor stall, thus it is time to find a simple explanation to this complex and once very common often fatal emergency.

It all began in 1982 when 300 Robinsons were in service, mainly in the USA. A series of unexplained crashes caused the FAA and NTSB to ask some pretty awkward questions of Frank Robinson. The danger then was the FAA could revoke the certification of the R22. (It turned out to be a training and education problem and not the R22.)

So what did they find? About one third of R22 accidents were caused by the low IAS/low rotor RPM stall. (It can occur at any speed, but often starts when the machine is slowing below 53 knots.) The other one third of accidents were dynamic roll over due to hovering at three feet and not the factory recommended five feet. Today, dynamic roll over is still the major cause of losses. Fortunately, the fitting of governors has almost eliminated the low IAS/low RPM rotor stall. Flying with the governor off, (some mustering pilots do it this way), or when in autorotation, exposes the pilot to rotor stall, should he mess up the rotor RPM.

Rotor stall is not overpitching! Over pitching is a trait of high inertia rotor systems fitted to the Bell 47, Hiller 12E, Huey and other similar types. It usually happens on takeoff, when hot high and heavy. The pilot simply raises the collective to a point where there is no more power available. If the pilot cannot achieve ETL, he may raise the collective more (in desperation); the rotor will slow, and the machine fall down into something expensive to both the land and helicopter owners. The rotor is not actually stalled, just becomes very inefficient. Power is also lost as the rotor slows the engine. That is, say, 90% RRPM equals about 90% of available horsepower. In summary, it occurs usually below ETL, high blade pitch angle and full throttle.

R22 rotor stall definition. It was defined in 1982 as a stall that was not recoverable, even when full throttle was applied. They stated it occurred when the rotor RPM is below 75% plus 1% for every 1,000 feet change in density.

Later Frank Robinson changed this to 80% plus 1% for every 1,000 feet change in density altitude.

So what happens? At this point, (with the collective fully down), the engine can only produce 80% of its power and the autorotative driving area of the rotor disc has been destroyed. Thus, even if the pilot fully opens the throttle the rotor drag caused by the stalled blades is just too much. The rotor very quickly slows and the engine stalls, the blades stop rotating and the R22 pitches nose down. Videos have confirmed the chain of events. Legend has it no one has survived a rotor stall above 100 feet.

Why is it different to over pitching? In the pre-governor days the typical ingredients for this type of accident were: a new pilot just out school, probably on his first passenger carrying task. Both were focused on some task outside the helicopter, such as taking a photo or checking on a gate during mustering. Often the R22 was well above the ground, say 1,000 feet. It all starts when the collective is lowered and the R22 is slowed from cruise. By comparison to over pitching, we now have plenty of ELT and speed, often descending and a low collective position and a reduced power setting. This is totally different to overpitching!

Rotor stall entry. In the pre governor days, it was believed the pilot may have been correcting a minor low RRPM situation by lowering the collective. As the R22 has such good throttle correlation, lowering the collective will only result in reduced throttle opening – the RPM stays where it was – low! Thus an increased rate of descent occurs. At the same time, if the IAS was going below 53 knots, the minium power speed for level flight, the rate of descent is further increased. Both these actions result is an increasing rate of descent and an increased angle of attack on the rotor system. Investigators thought that when the high descent and low RPM were finally noticed, the accidental raising of the collective would quickly bring the RRPM to around 80% (in seconds) and late application of throttle was useless.

Stalled area spread is the key! When we are hovering the R22, the central portion of the rotor disc is stalled, usually just out from the blade grips, etc. As we go in to forward flight the area remains roughly the same size, but moves to the left. We correct for this by using right cyclic. If you slow the rotor, it has to work harder and the pilot unconsciously raises the collective pitch, thus making the inner stalled area bigger. Add to this an inadvertent descent caused by slowing below 53 knots and you have an overall increasing angle of attack. The stalled area will grow outwards, and if it grows too much it can often have fatal consequences.

Autorotative driven area is overwhelmed. The pilots’ friend is the autorotative driving band which appears when the collective is fully lowered. It is designed to provide enough forward thrust on its own blade section to overcome all other rotor drag and maintain rotor RPM. It ensures you have the ability to glide out of the sky when the engine fails. It usually starts just beyond half way out on the rotor blades. Now if the stalled inner area is allowed to spread out by lowering the rotor RPM, increasing the rate of descent by slowing IAS and lowering the collective, then it will move into the autorotative driving area and finally overcome any advantage there was for you.

Thus all forward thrust on the rotor blades in autorotation is destroyed and a sudden drop in RRPM will occur.

What about the engine? This is easy to explain. At normal RPM the engine happily produces 100% power. If you wind the rotor RPM back, you also wind the engine RPM back. Therefore 80% RRPM is 80% engine RPM equals only 80% power. By coincidence, at the 80% RRPM figure, corrected for density height, is the point where the engine just cannot overcome rotor drag and really gives up as rotor drag slows the whole system. And everything stops – very quickly.

Read ATSB Report on page 44. The abbreviated report has a great deal of essential information. Main points are: if the horn is on, then the collective is not fully down. To recover a low rotor RPM situation you must open the throttle as you lower the collective. If you are already in autorotation, then you must lead with the throttle; i.e. maintain flat pitch, open the throttle get the RPM back up into the green band and then raise the collective. Do it the other way around and you will be dead in a few seconds! (Do not apply forward cyclic as was done in the report! This will reduce RRPM.)

Regaining rotor RPM by opening the throttle has absolute priority over everything else!