Servo-control reversibility and G limits
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Servo-control reversibility and G limits
Found the following in the EC-120B RFM:
Maximum load factor is determined by the servo-control reversibility limit. This phenomenon is smooth and presents no danger. Maximum load factor is a combination of TAS / Hσ / Weight. This servocontrol reversibility limit may be reached in a turn or in a pull-up or when maneuvering near VNE. In this case reduce collective pitch and airspeed.
Anyone care to explain this phenomenon? Also, I haven't found any reference to G limits in any other RFM... Is it that helicopters are "unbreakable" by abrupt control inputs?
Maximum load factor is determined by the servo-control reversibility limit. This phenomenon is smooth and presents no danger. Maximum load factor is a combination of TAS / Hσ / Weight. This servocontrol reversibility limit may be reached in a turn or in a pull-up or when maneuvering near VNE. In this case reduce collective pitch and airspeed.
Anyone care to explain this phenomenon? Also, I haven't found any reference to G limits in any other RFM... Is it that helicopters are "unbreakable" by abrupt control inputs?
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In a nutshell, the blades are wings, and when they are set to critical angle of attack, they stall. Just like a wing, the blades pitch downward at stall, due to the lift shifting aft on the airfoil as the smooth flow breaks up. This stall can be caused by strong maneuvering, high gross weight, high altitude, low rotor rpm, in fact, all the factors they taught you in school.
On a helicopter, the servos are all that stands between that blade downward pitching and your cyclic stick. Follow the forces on the diagram in the UK publication that puntosaurus pointed to, you can see how the blade tucks in a downward feathering motion, the pitch link resists, and passes the load to the swashplate, which then passes the load to the servo. On some model helicopters - very few- the servos are not designed to take all the force the blade can give out. This means that even at maximum servo force, the blade can push the servo backwards and shove the cyclic in your hands on these few helicopters. Sometimes, the servo has a switch in it that compresses when the servo is about to be driven backwards. This light is a warning to cool down your jets and reduce the maneuvering.
This servo back drive is called many things - Jack Stall, servo transparency, servo stall, control feedback. Few helos have the servo stall propensity, but those that do can surprise the pilot, so read and heed.
On a helicopter, the servos are all that stands between that blade downward pitching and your cyclic stick. Follow the forces on the diagram in the UK publication that puntosaurus pointed to, you can see how the blade tucks in a downward feathering motion, the pitch link resists, and passes the load to the swashplate, which then passes the load to the servo. On some model helicopters - very few- the servos are not designed to take all the force the blade can give out. This means that even at maximum servo force, the blade can push the servo backwards and shove the cyclic in your hands on these few helicopters. Sometimes, the servo has a switch in it that compresses when the servo is about to be driven backwards. This light is a warning to cool down your jets and reduce the maneuvering.
This servo back drive is called many things - Jack Stall, servo transparency, servo stall, control feedback. Few helos have the servo stall propensity, but those that do can surprise the pilot, so read and heed.
Stuck - it manifests itself like retreating blade stall - ie a pitch nose up and a roll towards the retreating side of the disc. The recovery is the same - lower the lever and reduce the disc loading/G/aft cyclic.
It is most dangerous when in a hard turn to the right in an EC helo as it can roll you upside down. You have to be heavy/fast/aggressive with the controls to get into this situation so fly sensibly and sympathetically and you won't ever see it.
EC have been touting this 'phenomenon' as a safety device since they built the Gazelle - the servos are just not man enough for the job.
It is most dangerous when in a hard turn to the right in an EC helo as it can roll you upside down. You have to be heavy/fast/aggressive with the controls to get into this situation so fly sensibly and sympathetically and you won't ever see it.
EC have been touting this 'phenomenon' as a safety device since they built the Gazelle - the servos are just not man enough for the job.
Safety feature???? - BULL****E its an accident waiting to happen if your not ready for it & your low. Once you know the machine you learn to manage the limitations she has by doing those steeper turns with the lever down some & then it don't come biting at ya. 
The dual hydraulics option eliminates this problem plus bumps up your MTOW internal to 2370kg 
The dual hydraulics option eliminates this problem plus bumps up your MTOW internal to 2370kg
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Servo Transparency also happens in straight and level flight. Had it happen to me although not dangerous gets the heart pumping for a moment when you least expect it. High altitude, heavy, windy or turbulent conditions, and close to VNE.
Was on a QHI course in '76. My instructor decided to show me 'jack-stall' in a descending turn at high speed to the right - the syllabus said left. The next thing I knew was my maps, chinagraphs and everything else were on the top of the cockpit. We eventually recovered to controlled flight having flicked right through the inverted position. Didn't fly with him again!
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Few helos have the servo stall propensity, but those that do can surprise the pilot, so read and heed.
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I spoke to a few ec engineers, the dual hydraulics will not eliminate jack stall, it has 2 pumps not 2 sets of servos. they said the servos were designed that weak on purpose, seemingly a stronger servo would shatter the star flex in such a high g manoeuvre.
Why anyone would want to demonstrate the thing is beyond me, isn't that like demonstrating dynamic roll over ? You don't have to experience something to know to avoid it!
Why anyone would want to demonstrate the thing is beyond me, isn't that like demonstrating dynamic roll over ? You don't have to experience something to know to avoid it!
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Anyone new to the AS350 and others with the same tendencies should experience servo transparency or jack stall. It can happen under different conditions and preformed correctly poses no threat except some feed back in the controls and a right roll. Reading about it is fine however experiencing it first hand may just give you that extra split second to save your a$$ in a tight situation. But hey I am definitely no expert.
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EN48 asks the right question:
Let's cut through the cheese. Jack stall or servo stall is a design shortfall, nobody wants it to happen. Accidents have been caused by jack stall, and it is yet another way for the pilot to be blamed for a marginal aircraft.
To my knowledge, only Eurocopter aircraft get jack stall, probably because their designs have been grown so much, and their designers probably decided to accept jack stall by staying with more marginal servos. There is no doubt that the aircraft can be safely flown, but the pilot must be more aware of maneuvers, weights and altitudes.
Jack stall is due to having servos that are too weak, and no modern helicopter would be passed by the US military with this "feature". In fact, most dual servo helis are designed so that ONE servo can resist the maximum blade forces, so even with a hydraulic system out, you can't get jack stall. The prime cause of jack stall is the growth of the mass of the machine without re-fitting the servos, so the original servos are asked to work harder as the all up mass is increased and the rotor comes closer to stall in normal maneuvers. It is cheaper to leave the same servos, and also lighter, so there is a strong rationale to let the aircraft come closer to jack stall with each MGW increase.
Don't be fooled by apologists, at high mass and high altitude, it takes very little to cause jack stall. The real name for jack stall could be "loss of control" as discussed by a few tales on this thread. When the servos are back-driven, the pilot is a passenger, and a crash is more likely to occur.
Let's cut through the cheese. Jack stall or servo stall is a design shortfall, nobody wants it to happen. Accidents have been caused by jack stall, and it is yet another way for the pilot to be blamed for a marginal aircraft.
To my knowledge, only Eurocopter aircraft get jack stall, probably because their designs have been grown so much, and their designers probably decided to accept jack stall by staying with more marginal servos. There is no doubt that the aircraft can be safely flown, but the pilot must be more aware of maneuvers, weights and altitudes.
Jack stall is due to having servos that are too weak, and no modern helicopter would be passed by the US military with this "feature". In fact, most dual servo helis are designed so that ONE servo can resist the maximum blade forces, so even with a hydraulic system out, you can't get jack stall. The prime cause of jack stall is the growth of the mass of the machine without re-fitting the servos, so the original servos are asked to work harder as the all up mass is increased and the rotor comes closer to stall in normal maneuvers. It is cheaper to leave the same servos, and also lighter, so there is a strong rationale to let the aircraft come closer to jack stall with each MGW increase.
Don't be fooled by apologists, at high mass and high altitude, it takes very little to cause jack stall. The real name for jack stall could be "loss of control" as discussed by a few tales on this thread. When the servos are back-driven, the pilot is a passenger, and a crash is more likely to occur.
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Steep turns in the normal course of civil operations would be an emergency manoeuver. A turn to avoid hitting a bird/a plane/Superman.Can't see any other reason to do it,except to show off to the crowd/girlfriend/peers. Further, under the right conditions over the desert mountains of the US,on a hot summer day, everything can come together like a perfect storm where a gust of wind in cruise can set off a jack stall.The onset is unmistakable and needs a contolled reaction.Not a big deal once you have a few hours in the EC models.A good training module should cover this and take the mystery away.For many years, pilots were told/trained to believe that the AS350 hydraulics were an accident waiting to happen,that a hydraulic failure could kill you etc.Nothing can be further from the truth. Like everything else (LTE anyone?) good training,increased awareness and a good knowledge of the systems involved would take away the aura of death from these aircraft.
And ,no, I am not being an apologist,merely a realist.
And ,no, I am not being an apologist,merely a realist.
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Very interesting subject whith the following comments and questions: So If I understand, with a `Jack stall’ condition, you cannot move the controls, meaning during this time the aircraft is not controllable. If the aircraft is designed up to a 2g or 3 gs depending on loading, It should also be maneuvrable within the designed envelope. Are helicopters required to have a G meter to prevent this occurence? Does this design characteristic meets the intent of Part 27 or 29-141 and 143? Have the manufacturers and the certifying agencies missed something in the certification process?
There is also the issue of the "Butee Sphérique" Spherical Bearing being wound up to its highest deformation at the same point as all the other forces are trying to push back at the servo. The bearing is at the inner end of the sleeves inside the chunky part of the starflex.
(This guy has too much time on his hands.....................but it's well done)
Enjoy
(This guy has too much time on his hands.....................but it's well done)
Enjoy
Many years ago in a galaxy far,far away:
Yes you can control the AC , but there will be feedback. The controls do not lock. Reduce power, roll to level or otherwise unload the AC.
The aircraft is completely controllable unless you are pulling so much "G" that you are way out of the flight envelope.
I am referring to an AS350D or AS350B, the AS355F1 has a limit light to advise you when you has arrived at the "G" limit due to the dual hyd system.
Long time ago but I recall the limit was 2 G.
We used to demonstrate this, very carefully, in training.
As I say that was a long time ago later models of the AC may well be more difficult to control - I don't know.
"Very interesting subject with the following comments and questions: So If I understand, with a `Jack stall’ condition you cannot move the controls, meaning during this time the aircraft is not controllable. If the aircraft is designed up to a 2g or 3 gs depending on loading, It should also be maneuverable within the designed envelope. Are helicopters required to have a G meter to prevent this occurrence? Does this design characteristic meet the intent of Part 27 or 29-141 and 143? Have the manufacturers and the certifying agencies missed something in the certification process"
Yes you can control the AC , but there will be feedback. The controls do not lock. Reduce power, roll to level or otherwise unload the AC.
The aircraft is completely controllable unless you are pulling so much "G" that you are way out of the flight envelope.
I am referring to an AS350D or AS350B, the AS355F1 has a limit light to advise you when you has arrived at the "G" limit due to the dual hyd system.
Long time ago but I recall the limit was 2 G.
We used to demonstrate this, very carefully, in training.
As I say that was a long time ago later models of the AC may well be more difficult to control - I don't know.
"Very interesting subject with the following comments and questions: So If I understand, with a `Jack stall’ condition you cannot move the controls, meaning during this time the aircraft is not controllable. If the aircraft is designed up to a 2g or 3 gs depending on loading, It should also be maneuverable within the designed envelope. Are helicopters required to have a G meter to prevent this occurrence? Does this design characteristic meet the intent of Part 27 or 29-141 and 143? Have the manufacturers and the certifying agencies missed something in the certification process"
Avoid imitations
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I don't see why some helicopter pilots get so worked up about this. The limits of controllability of these aircraft is published. Fly within the limits and there is no problem. I think of this in the same light as spinning a fixed wing - know about it, know how to recover from it, and after that concentrate on how to avoid it.
The RAF used to include a jackstall demo on the Gazelle basic rotary course to instill the idea in the students' minds. Demonstrating this to my students never frightened me as an individual (it did some others) but it did make me wonder what it did to the fatigue life of the aircraft over a period of time.
I think it safer to limit the likelihood of airframe damage by preventing the pilot from reaching a flight regime where it may occur. Having said that, obviously the pilot must be properly trained to understand what he is doing and what may happen if he pushes any aircraft too far.
Perhaps the late Colin McRae and his passengers might still be alive had he been shown this phenomenon during his training.
The RAF used to include a jackstall demo on the Gazelle basic rotary course to instill the idea in the students' minds. Demonstrating this to my students never frightened me as an individual (it did some others) but it did make me wonder what it did to the fatigue life of the aircraft over a period of time.
I think it safer to limit the likelihood of airframe damage by preventing the pilot from reaching a flight regime where it may occur. Having said that, obviously the pilot must be properly trained to understand what he is doing and what may happen if he pushes any aircraft too far.
Perhaps the late Colin McRae and his passengers might still be alive had he been shown this phenomenon during his training.
And of course one thing we don't know is exactly what the total inherent feed back forces are.
Testing during design may have shown up that the forces could exceed the design of the rotating control elements. As I said above the force of the spherical bearing has to be overcome as well.
Easy fix - don't make the control hydraulics tough enough to break things. Let the single units feed back and the dual units with a "LIMIT" indication.
This has been flogged to death here.
Testing during design may have shown up that the forces could exceed the design of the rotating control elements. As I said above the force of the spherical bearing has to be overcome as well.
Easy fix - don't make the control hydraulics tough enough to break things. Let the single units feed back and the dual units with a "LIMIT" indication.
This has been flogged to death here.
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RVDT,
a couple of thoughts:
The aerodynamic forces due to stall and blade pitching moment are considerably larger than the forces due to the rotor head bearings. I would estimate that typical elastomeric bearing forces are less than 5% of the aerodynamic forces developed by the blade. Try this experiment: remove the droop and flap stops and move the blade with your hand from the tip throughout its limits.Note the forces that you have to use to do this, and remember that in a three bladed helicopter that blade routinely carries one third of the weight of the aircraft during its normal travels around the head. Also, the rotating components of the helicopter are designed to take forces up to the stall of the servo, in fact tests are conducted to demonstrate that the components do not deform and do not spring far enough out of position to touch other components while under such load. These tests are called proof and operations tests and demonstrate that the servos should not be able to break the other components. Perish the thought that your rotor head was weak enough so that aerodynamic forces of the blades or the servos should bend the rotorhead components.
Regarding the servo's strength, the servo's purpose is to facilitate control, and the general rule should be that the servo can overcome the aerodynamic forces from the blades. On other hand if you have servos that are just not quite strong enough, and you adequately warn the crew where those limits can be reached, the aircraft can certainly be a safe machine, just not an optimal one.
In other posts at other times I have talked about the fact that we as pilots should have as few shortfalls to work around from our machines as possible. It is the measure of the success of the aircraft if these few shortfalls make the aircraft fundamentally foolproof, our piloting skills are not usually the fallback position for weaknesses of aircraft.
There are plenty of environmental and physical problems that the sky, weather and earth deliver us to adequately absorb our skills. In the battle to maintain flight safety our aircraft should be our best friend.
a couple of thoughts:
The aerodynamic forces due to stall and blade pitching moment are considerably larger than the forces due to the rotor head bearings. I would estimate that typical elastomeric bearing forces are less than 5% of the aerodynamic forces developed by the blade. Try this experiment: remove the droop and flap stops and move the blade with your hand from the tip throughout its limits.Note the forces that you have to use to do this, and remember that in a three bladed helicopter that blade routinely carries one third of the weight of the aircraft during its normal travels around the head. Also, the rotating components of the helicopter are designed to take forces up to the stall of the servo, in fact tests are conducted to demonstrate that the components do not deform and do not spring far enough out of position to touch other components while under such load. These tests are called proof and operations tests and demonstrate that the servos should not be able to break the other components. Perish the thought that your rotor head was weak enough so that aerodynamic forces of the blades or the servos should bend the rotorhead components.
Regarding the servo's strength, the servo's purpose is to facilitate control, and the general rule should be that the servo can overcome the aerodynamic forces from the blades. On other hand if you have servos that are just not quite strong enough, and you adequately warn the crew where those limits can be reached, the aircraft can certainly be a safe machine, just not an optimal one.
In other posts at other times I have talked about the fact that we as pilots should have as few shortfalls to work around from our machines as possible. It is the measure of the success of the aircraft if these few shortfalls make the aircraft fundamentally foolproof, our piloting skills are not usually the fallback position for weaknesses of aircraft.
There are plenty of environmental and physical problems that the sky, weather and earth deliver us to adequately absorb our skills. In the battle to maintain flight safety our aircraft should be our best friend.
Last edited by NickLappos; 31st Jan 2011 at 01:03.