Servo-control reversibility and G limits
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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 are aircraft should be our best friend.
Last edited by EN48; 2nd Feb 2011 at 12:57.
At high altitudes +10,000' it takes very little effort to get into a servo transparency situation, very small angle of bank if you have MC power on, so that is a normal operation. This problem to my mind is a design flaw or applying older technology to a newer machine & hence a miss match?? I don't know? And yes the dual hydraulics does fix the problem as I have never experienced it in a DH ship? VF
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Just a few thoughts...
Think of a "reversible flight control system" as one in which, if you were to move the control surface by hand, you'd see the corresponding cockpit control move in unison (i.e. a fly-by-wire control system would NOT be a reversible-type).
With most hydraulically-assisted flight controls, remember that the pilot is not making a direct input to the flying surface; he/she is only moving a lever on the servo, which in turn operates a valve and allows the hydraulic force multiplication to move the control surfaces.
In the event of a hydraulic system failure (i.e. loss of hydraulic pressure) in the AS350B2 and similarly-equipped models, a locking pin in the servo drops into place, making the servo become a solid/direct link between the flight controls and aerodynamic surfaces, i.e., a reversible control system. This allows continued manipulation of the controls during a hydraulic failure, albeit with increased control loads and aerodynamic feedback.
Now, if the conditions exist, such that the aerodynamic / gyroscopic / accelerative forces are greater than the forces that can be generated by the normal hydraulic mechanical advantage, then you have overpowered (not broken) the hydraulic system, and have reached the "servo control reversibility limit." Since the flight conditions required to generate enough force to overcome the hydraulic system are generally extreme and acute, then its a good bet that your attempts to control the aircraft at that point will seem futile as well (since hydraulic pressure has not been lost, just met with equal resistance, the locking pins have not dropped). Remove the aggravating condition causing increased loads (relax backpressure, etc.), and the hydraulic system happily goes back to doing its job.
Is this a design flaw? I'll ask another question... is the maneuvering speed in an airplane a cheap attempt at covering up a design flaw? Certainly not. It's an "aerodynamic safety valve" that exists so you don't compromise the structural integrity of your aircraft by making large control inputs that can generate forces and moments capable of breaking things. As I understand it, the same idea exists behind the "servo control reversibility limit."
If a pilot told me the design maneuvering speed was dumb because someday they may need to be able to pull more G to save their own ass, and how dare the manufacturer allow the airplane to stall instead of giving them what performance they asked for (you know, the kind of performance capable of folding the wings in half), the blank stare on my face would be priceless.
Just like an airplane wing stalls when the maximum angle of attack is exceeded, no matter what its airspeed, attitude, etc., "jack stall / servo transparency / control reversibility limit" occurs whenever the hydraulic system's maximum force is exceeded by the loads imposed on the rotor system no matter what the altitude, attitude, airspeed, etc.
I hope my long-winded post gave some insight to an interesting topic.
Regards,
Frank Lombardi
Think of a "reversible flight control system" as one in which, if you were to move the control surface by hand, you'd see the corresponding cockpit control move in unison (i.e. a fly-by-wire control system would NOT be a reversible-type).
With most hydraulically-assisted flight controls, remember that the pilot is not making a direct input to the flying surface; he/she is only moving a lever on the servo, which in turn operates a valve and allows the hydraulic force multiplication to move the control surfaces.
In the event of a hydraulic system failure (i.e. loss of hydraulic pressure) in the AS350B2 and similarly-equipped models, a locking pin in the servo drops into place, making the servo become a solid/direct link between the flight controls and aerodynamic surfaces, i.e., a reversible control system. This allows continued manipulation of the controls during a hydraulic failure, albeit with increased control loads and aerodynamic feedback.
Now, if the conditions exist, such that the aerodynamic / gyroscopic / accelerative forces are greater than the forces that can be generated by the normal hydraulic mechanical advantage, then you have overpowered (not broken) the hydraulic system, and have reached the "servo control reversibility limit." Since the flight conditions required to generate enough force to overcome the hydraulic system are generally extreme and acute, then its a good bet that your attempts to control the aircraft at that point will seem futile as well (since hydraulic pressure has not been lost, just met with equal resistance, the locking pins have not dropped). Remove the aggravating condition causing increased loads (relax backpressure, etc.), and the hydraulic system happily goes back to doing its job.
Is this a design flaw? I'll ask another question... is the maneuvering speed in an airplane a cheap attempt at covering up a design flaw? Certainly not. It's an "aerodynamic safety valve" that exists so you don't compromise the structural integrity of your aircraft by making large control inputs that can generate forces and moments capable of breaking things. As I understand it, the same idea exists behind the "servo control reversibility limit."
If a pilot told me the design maneuvering speed was dumb because someday they may need to be able to pull more G to save their own ass, and how dare the manufacturer allow the airplane to stall instead of giving them what performance they asked for (you know, the kind of performance capable of folding the wings in half), the blank stare on my face would be priceless.
Just like an airplane wing stalls when the maximum angle of attack is exceeded, no matter what its airspeed, attitude, etc., "jack stall / servo transparency / control reversibility limit" occurs whenever the hydraulic system's maximum force is exceeded by the loads imposed on the rotor system no matter what the altitude, attitude, airspeed, etc.
I hope my long-winded post gave some insight to an interesting topic.
Regards,
Frank Lombardi
Last edited by rotorwash4944; 2nd Feb 2011 at 03:47.
Hmmmm please don't mix unrelated aeroplane stuff with this topic. So then if this problem is fixed with dual hydraulics what is it if not a design flaw? Why don't other servo boosted machine get it? Its just a grossly underpowered boost pump, simple answer.
Last edited by Vertical Freedom; 2nd Feb 2011 at 04:36.
OK 
OK 
I am happy to stand corrected, after all I am always learning. 
So OK yes there is ONE other model out there that suffers the same fate, the A365. 
Any other models whilst we are on the topic? 
And yes Dual Hydraulics does fix the jackstall problem in ASS350
OK I am happy to stand corrected, after all I am always learning. So OK yes there is ONE other model out there that suffers the same fate, the A365. Any other models whilst we are on the topic? And yes Dual Hydraulics does fix the jackstall problem in ASS350
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I would call it more of a design limit rather than a flaw. If AEC engineering says that is the way it was engineered, then their logic follows suit. If important things would start to break at loads higher than the hydraulics can handle, then consider it as a way the aircraft is talking to you, trying to tell you that you've effectively found one corner of the flight envelope. If anyone is routinely experiencing it, then I'd have to say they need to re-evaluate the aircraft they require to do their mission.
There are many single-point failures in a helicopter, this "feature," intentional or not, helps keep pilots from finding any of them.
FWIW, depending on the design criteria, dual hydraulic pumps may only provide redundancy, not increased hydraulic force. The choice ultimately lies with designers.
There are many single-point failures in a helicopter, this "feature," intentional or not, helps keep pilots from finding any of them.
FWIW, depending on the design criteria, dual hydraulic pumps may only provide redundancy, not increased hydraulic force. The choice ultimately lies with designers.
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Can I rephrase rotorwash's logic:
If the manufacturer has designed rotor parts that are too weak, he can cover that up by designing servos that are also too weak. That way no rotor parts break, and he can then blame the pilot if an accident happens.
The best answer? Just don't buy a substandard helicopter - that is, one with parts or servos that are too weak. There are several reputable manufacturers who seem to have no trouble finding both strong parts and strong servos.
If the manufacturer has designed rotor parts that are too weak, he can cover that up by designing servos that are also too weak. That way no rotor parts break, and he can then blame the pilot if an accident happens.
The best answer? Just don't buy a substandard helicopter - that is, one with parts or servos that are too weak. There are several reputable manufacturers who seem to have no trouble finding both strong parts and strong servos.
Sounds like a good plan Ramen Noodles but I doubt there is a better performing & more reliable work machine for hot & high than what the B3+ can achieve. She the only one of its kind.. (i am ready to be stood corrected) 
"Just a pilot"
If the EC120 works like an AS350, you'll see the phenom as follows, when you attempt a maneuver:
Cyclic 'stops' in an axis, usually left, by feedback. I never tried to 'push thru' the feedback, I don't know if it's worse than hydraulics off.
Collective resists increase.
Releasing control pressure, roll out and less reduce collective, less G, and everything is normal.
Don't push the maneuver limit without an immediate escape route open. I was 'hotdogging' every time, save once, when I've encountered servo transparency or limit lights. The single occurrence was in a max continuous power cruise- high, hot, and heavy- after a descending, right turn away from a vulture. Attempted a normal, unaggressive, non-aerobatic recovery to cruise, and the pig wouldn't roll left until I backed out a little collective.
The 355's "Limit" light is just that. Dual-hydraulics, one system (#1?) nears/at a load limit, you get a caution panel segment, but the controls feel and react normally. Same StarFlex, blades, etc. as the 350... I don't buy the "protects the StarFlex" story. The servo transparency issue is a hangover from the smaller, cheaper, Eurocopter Ecureuil days, like the rubber-band driven hydraulic pump.
Cyclic 'stops' in an axis, usually left, by feedback. I never tried to 'push thru' the feedback, I don't know if it's worse than hydraulics off.
Collective resists increase.
Releasing control pressure, roll out and less reduce collective, less G, and everything is normal.
Don't push the maneuver limit without an immediate escape route open. I was 'hotdogging' every time, save once, when I've encountered servo transparency or limit lights. The single occurrence was in a max continuous power cruise- high, hot, and heavy- after a descending, right turn away from a vulture. Attempted a normal, unaggressive, non-aerobatic recovery to cruise, and the pig wouldn't roll left until I backed out a little collective.
The 355's "Limit" light is just that. Dual-hydraulics, one system (#1?) nears/at a load limit, you get a caution panel segment, but the controls feel and react normally. Same StarFlex, blades, etc. as the 350... I don't buy the "protects the StarFlex" story. The servo transparency issue is a hangover from the smaller, cheaper, Eurocopter Ecureuil days, like the rubber-band driven hydraulic pump.
I think part of the problem is in people accepting the design philosophy of the aircraft.
During original flight testing it was probably found that the feedback forces and loading on parts of the blade and flight controls could have become extreme or unacceptable.
Understand that the 350 Series is really no different from an Alouette 2 except in the construction technique and materials used.
The impetus being on low maintenance (in the comparative sense) and high life on components. Also in the background would be lower manufacturing cost as well of course.
The hydraulic pressure was limited on purpose from information I have read.
Understand that all these feedback forces have to be transmitted through the root end of the blade. Initially most of the rotor components - Starflex, Sleeves, Blades etc had very high or no finite fatigue life. That has changed over the years with in service experience. The blades are still (I think) "on condition" i.e. no finite life. Strangely though EC will not exchange blades with more than 13,000 hours on them!
If these forces are not allowed to feed back as a kind of built in warning or safety measure and of course dual hydraulic machines have the "LIMIT" warning, who knows what it may do to the component lives.
The alternative of making things heavier or re-designed is possibly not viable. The manufacturers and certification authorities have deemed that what they are doing is acceptable.
Yes the jacks can stall, yes it says so in the flight manual.
During original flight testing it was probably found that the feedback forces and loading on parts of the blade and flight controls could have become extreme or unacceptable.
Understand that the 350 Series is really no different from an Alouette 2 except in the construction technique and materials used.
The impetus being on low maintenance (in the comparative sense) and high life on components. Also in the background would be lower manufacturing cost as well of course.
The hydraulic pressure was limited on purpose from information I have read.
Understand that all these feedback forces have to be transmitted through the root end of the blade. Initially most of the rotor components - Starflex, Sleeves, Blades etc had very high or no finite fatigue life. That has changed over the years with in service experience. The blades are still (I think) "on condition" i.e. no finite life. Strangely though EC will not exchange blades with more than 13,000 hours on them!
If these forces are not allowed to feed back as a kind of built in warning or safety measure and of course dual hydraulic machines have the "LIMIT" warning, who knows what it may do to the component lives.
The alternative of making things heavier or re-designed is possibly not viable. The manufacturers and certification authorities have deemed that what they are doing is acceptable.
Yes the jacks can stall, yes it says so in the flight manual.
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Ok, so EC aknowledge jack stall capability. Some perceive it as a major flaw/risk and others seem to perceive it as a possibility and accept it. My questions will always be: 1. how many of these machines are flying currently? 2. how many fleet hours in total and how many incidents/occurences? Where do these machines operate mostly ie VIP transfers or field work at temp/alt/heavy? In the light of the answers given above, is it really a safety disaster or is the machine capable of performing as promised despite the purposefully designed/flawed limited servo's?
Have only experienced it once in 40kt wind and extreme turbulence ...otherwise even hotdogging never had it . MAUW in the mountains in Canada , high winds etc etc never experienced it . There are some problems with the 350 ( namely a french engine ) but this servo stuff really is just not an issue IMHO.
