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Articles USA Academy of Aviation Low RPM-Rotor Stall

Low RPM-Rotor Stall

From 1979-1994 the R22 helicopter was involved in 29 accidents resulting from failure to maintain rpm. Also 100 hard landings were reported, 58 accidents from autorotations, and 42 hard landings under category "other". In some of these accidents a low rpm situation might have been a contributing factor. The rotorblades of the R22 have a maximum angle of attack of about 15 degrees. If that angle of attack is exceeded the rotor blades stall, lift is cancelled, and the aircraft falls out of the sky.
 
The R22 has a Tachometer, engine rpm on left side, and rotor rpm on the right. The Tach shows the engine rpm ( engine speed ) and the rotor rpm (Blade speed). The aircraft also has a governor that keeps the engine and rotor rpm at 104 percent, but only up to a certain maximum degree of manifold pressure. If the maximum manifold pressure has been exceeded both the engine and rotor rpm will start to decrease. The green arc starts at 101-104 percent. The low rpm warning light and horn comes on at 97%, and  90% is the red line.( never allow it to go below 90% ). If the pilot allows the rpm to drop below 80% plus one percent per thousand feet of altitude the rotor blades will stall.
 
At an angle below approximately 15 degrees the air flows smoothly over the airfoil/ rotorblades, but there is an angle of attack where the air will start to separate from the blade, and when the air fully leaves the blades at approximately 15 degrees the airfoil/rotorblades will stall.
 
Before flight the pilot needs to check  Maximum Manifold Pressure. MAP is the absolute pressure of the engine intake manifold.  There is a diagram which the Robinson engineers have made to help pilots not to exceed this limit. There is a "Max Continuous" MAP and a Max "Takeoff" MAP.
 
Don't exceed any one of them. Never ever exceed the Max "Takeoff" MAP. If the pilot does so there will be a "too high" MAP with "too high" angle of attack on the blades which will cause the engine and rotor rpm to drop below safe limits.
 
For example, if the Max Map is 23 inches of mercury make sure that, when you lift collective, you are "tuned in" to the MAP gauge and the Rotor rpm gauge ( the tach ).
 
Learn how to handle the collective with ease. Never pull hard on the collective.  If you do not exceed the MAP limits you will not have a low rpm situation.
 
Remember that if you fly from lower field elevations to higher field elevation airports make sure that you check MAX MAP and MAX OGE (out of ground effect) hover ceiling before landing or taking off. If you try to land at an airport with high field elevation you might not have enough engine power available which causes a low rpm situation. Lower collective immediately and roll on rpm. If this is the case and you still want to land you have to do a running landing with ETL airpseeds 15 kts or more.  As you lift off you have to do a running take off with ETL airspeed. If you cannot obtain lift and are descending to the runway or taxiway you might have to take away fuel, baggage, or wait until the temperature drops; or you might have to fly alone to an airport at lower field elevation where you can pick up your student. Your diagrams on the helicopter display panel will inform you if a normal takeoff and normal landing are possible. This information also is carried in the POH.  
 
I flew an airplane pilot to a high-field elevation airport. We had been conducting ground school covering the OGE and IGE hover ceiling diagrams, and we checked Max MAP on different temp airports, and different weight calculations. As we performed a running landing at this high-field elevation airport of 6700 ft and came to a full stop my student tried to pull the helicopter up into a hover with low rpm.
 
In front of us was a stressed-out fixed-wing pilot who insisted upon having rights to the taxiway. My student was not capable of lifting the helicopter up into a hover, not even one inch. We experienced a low rpm horn and warning light. The student stated: "What are we going to do about this anxious fixed-wing pilot?"  I advised: "Don't bother about him; tell him that he can go around us."  So he did; and when he passed us he looked at us as if we were two blithering idiots and should not be blocking his progress.
 
We waived to him to let him know that we were friendly helicopter pilots. Then my student asked me: "What are we going to do now?"  I said to him: "We have two choices: either stop the engine here, put on our wheels, and roll the helicopter to the restaurant; or slide the helicopter 50 yards to the transient parking".
 
He decided to lift collective just to keep it light on the skid, and move the cyclic forward to create a ground-slide to the parking area. Pilots were observing us ground-sliding to transient parking and they were shaking their heads in disgust. It was a funny moment.
 
After a delightful lunch of two big, juicy burgers with large fries and drinking copious amounts of Coca-Cola my student asked me how we were going to taxi and take off. I replied: "We can do the same thing: either do a ground slide to the runway and then do a running takeoff, or put the wheels on and push it 2500 feet to the beginning of the runway."
 
My student ventured: "Let's put the wheels on and push it to the beginning of the runway". And so we did. The same pilots were pointing at us and still were shaking their heads: "Those helicopter pilots!... Hm... I don't understand them at all."
 
We pushed the Robbie to the transient parking located at the far west portion of the airport. We started the machine. Nextly we slid the Robbie to the runway and executed a running takeoff. As we received Effective Translational Lift we became airborne and could escape from this high field elevation airport.
As we looked down we saw the same pilots staring at us beneath! Funny situation! During the return flight to French Valley Airport in Murietta, Ca, my student thanked me for the incredible adventure which we both had encountered and enjoyed. I thanked him for his kind comments and I welcomed him to helicopter aviation. Flying helicopters is the most wonderful experience in the world. Never a dull moment!  
 
There are many airplane pilots who have been involved in rotor stall accidents. An airplane pilot is trained to increase power and pitch the nose of the aircraft down in order to come out of a stall situation. As the engine and rotor rpm starts to decrease and the warning horn and light comes on the fixed-wing pilots are trained to increase power and pitch the nose down in order to counteract a fixed wing stall. This is the wrong remedy for a helicopter. As the horn and light comes on at 97% in a Robinson helicopter the airplane pilot instinctively pulls in more power with collective and pushes the cyclic forward as they were trained in a fixed wing.
 
This results in a stalled rotor in less than 1.1 sec. In 14 years the R22 helicopter was involved in 29 fatal Low Rpm Rotor Stall accidents. 
 
Frank Robinson, the owner of the Robinson Helicopter Company, said on the Factory Course: "A primary cause of fatal accidents in light helicopters is failure to maintain rpm." The FAA and NTSB said that most helicopter accidents happen because of "Lack of proper pilot training and lack of situational awareness". The pilot must learn in flight school how to control the rpm and what causes the rpm to decrease below safe limits; and how to recover from a low rpm situation.
 
Frank also mentioned that Power available from the Lycoming engine is directly proportional to rpm. If, for example, the pilot has pulled in too much collective/power and the rpm starts to decrease, let's say 7 %, there is 7% less power from the engine, and 7% decrease in rotor rpm. In this situation the pilot must lower the collective and increase the throttle immediately in order to regain the lost rpm. If the pilot is slow to react the helicopter slows down and starts to descend thus it might be impossible to retrieve the rpm. If the pilot is unable to increase the rpm it is far better to descend into the ground with low rpm above 80% than to allow the helicopter to stall below 80% because, when the blades stall, they either will "blow back" with the risk of cutting the tailboom or just falling from the sky. If you descend into rough terrain with some rpm left you might walk away from the crash.
 
 Rotor stall can occur in any flight condition. If the pilot pulls in too much collective and exceeds MAX MAP and max angle of attack the blades will stall. Or if an engine failure is experienced he must lower collective immediately otherwise disaster results. If an engine failure occurs the pilot has 1.1 second to lower the collective full down disengaging the blades from the engine.
 
Carb Ice can cause an engine failure. Frank stated: "That, as the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher inflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first, allowing it to dive as it goes aft while the advancing blade still is climbing as it moves forward.  The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes called "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, frequently will allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls."
 
Johan Nurmi & Patrick Sherry
FAA Gold Seal CFI
Owner & VP
Chief Pilot & Founder & Owner
The World Record Helicopter Team
 
By Johan Nurmi and Patrick Sherry - USA ACADEMY OF AVIATION
Helicopter Flight Academy
This article was written By Johan Nurmi and Patrick Sherry - USA ACADEMY OF AVIATION and reprinted with their permission - June 4/08

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