Jackstall: What is it? Any experiences?
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Servo Transparency (jack stall)
The current term used by Eurocopter is Servo Transparency (jack stall). The following is part of the research from 2001 AS350 accident. The report is a good outline of the effect.
http://www.ntsb.gov/NTSB/brief.asp?e...25X02148&key=1
According to American Eurocopter, the main function of the hydraulic actuators in the main rotor flight control system is to reduce the force required to fly the aircraft, and to isolate the pilot from these forces. The aerodynamic forces are constantly changing and result in rotor blade stress, which increase as a function of speed, gross weight, density altitude, angle of attack, and positive maneuvering (g-loading). Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded. In level flight, and at airspeeds less than Vne (never exceed velocity), hydraulic systems have the capability to isolate the pilot from the main rotor forces. But, as airspeed and/or g-loading increase, the forces can increase to the point that they exceed the opposing force generated by the flight control hydraulic actuators, and servo transparency or "jack stall" occurs. Jack stall results in uncommanded aft and right cyclic and down collective motion accompanied by pitch-up and right roll of the helicopter. The maneuver, often abrupt and a surprise to the pilot, tends to be self-correcting since the rapid loss of airspeed due to the pitch-up and down collective causes an equally quick reduction in feedback forces. Pilots rarely fly at speeds beyond Vne, but do occasionally induce jack stall as a result of excessive maneuvering. If jack stall is unexpectedly encountered during maneuvering, the pilot should decrease the severity of the maneuver and reduce collective.
Also see FAA bulletin:
http://www.faa.gov/aircraft/safety/a...a/SW-04-35.pdf
http://www.ntsb.gov/NTSB/brief.asp?e...25X02148&key=1
According to American Eurocopter, the main function of the hydraulic actuators in the main rotor flight control system is to reduce the force required to fly the aircraft, and to isolate the pilot from these forces. The aerodynamic forces are constantly changing and result in rotor blade stress, which increase as a function of speed, gross weight, density altitude, angle of attack, and positive maneuvering (g-loading). Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded. In level flight, and at airspeeds less than Vne (never exceed velocity), hydraulic systems have the capability to isolate the pilot from the main rotor forces. But, as airspeed and/or g-loading increase, the forces can increase to the point that they exceed the opposing force generated by the flight control hydraulic actuators, and servo transparency or "jack stall" occurs. Jack stall results in uncommanded aft and right cyclic and down collective motion accompanied by pitch-up and right roll of the helicopter. The maneuver, often abrupt and a surprise to the pilot, tends to be self-correcting since the rapid loss of airspeed due to the pitch-up and down collective causes an equally quick reduction in feedback forces. Pilots rarely fly at speeds beyond Vne, but do occasionally induce jack stall as a result of excessive maneuvering. If jack stall is unexpectedly encountered during maneuvering, the pilot should decrease the severity of the maneuver and reduce collective.
Also see FAA bulletin:
http://www.faa.gov/aircraft/safety/a...a/SW-04-35.pdf
Last edited by ichris; 1st Dec 2006 at 05:38. Reason: add information link
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ichris,
You republished, "Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded."
That circular logic is simply amazing. Let me restate it:
"Since the servos are too small to take maximum blade forces, there are circumstances where the controls stop being controls, and the pilot might die if he maneuvers too much. By the way, we provide no way to know that maximum, so hold on tight if you blunder across the boundary."
It is a fact that the maximum blade forces during stall are measurable and part of the designer's lexicon - in other words, you don't get more and more force from the blade as you maneuver, because there is a maximum moment the blade can produce, just as there is a maximum lift. The plain facts are that some helicopter designers simply don't bother to build the servos large enough to withstand this maximum force, in fact, they fall so short of the maximum that the controls lock up during relatively gentle maneuvers (at high altitude and at high weight).
Like LTE, where a tail rotor that is too small is allowed, and instead the manufacturer starts to blame the pilot, Jack stall is not a sign of pilot error in all cases, it is a sign of a poorly designed helo that does not have the basic control capability to protect its occupants.
I firmly believe these jack stall designs would never be certified in the US or CAA, they are only allowed because of the bilateral agreements. Bell, Boeing, Sikorsky, Westland and Agusta all have design rules that do not permit jack stall, because they all design the servos to be large enough to withstand the rotor forces, and protect the occupants.
You republished, "Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded."
That circular logic is simply amazing. Let me restate it:
"Since the servos are too small to take maximum blade forces, there are circumstances where the controls stop being controls, and the pilot might die if he maneuvers too much. By the way, we provide no way to know that maximum, so hold on tight if you blunder across the boundary."
It is a fact that the maximum blade forces during stall are measurable and part of the designer's lexicon - in other words, you don't get more and more force from the blade as you maneuver, because there is a maximum moment the blade can produce, just as there is a maximum lift. The plain facts are that some helicopter designers simply don't bother to build the servos large enough to withstand this maximum force, in fact, they fall so short of the maximum that the controls lock up during relatively gentle maneuvers (at high altitude and at high weight).
Like LTE, where a tail rotor that is too small is allowed, and instead the manufacturer starts to blame the pilot, Jack stall is not a sign of pilot error in all cases, it is a sign of a poorly designed helo that does not have the basic control capability to protect its occupants.
I firmly believe these jack stall designs would never be certified in the US or CAA, they are only allowed because of the bilateral agreements. Bell, Boeing, Sikorsky, Westland and Agusta all have design rules that do not permit jack stall, because they all design the servos to be large enough to withstand the rotor forces, and protect the occupants.
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I know this is a dumb question but here it goes. If you design a servo actuator strong enough to withstand the maximum force, would it then be logical to assume the other end of the force fails. Put another way, can the servo actuator be strong enough to withstand the maximum force from the blade but the result of such a strong actuator is that the pilot is allowed to put enough force into the controls such that the blade can fail, snap off and scatter but the servo actuator withstood the force? I understand a pilot can apply inputs into a robinson or a 500 which can result in catastrophic failure but in the larger hydraulic aircraft, where should the balance be? The control acutator, the part it is controlling, or the pilot?
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Good question! In fact, EC uses that excuse as a justification for the weak servos, instead of also strengthening the blade horn and pitch links. Most helos are designed so those components cannot be harmed by anything that is aerodynamically generated, so the general answer to your question is no, but for some EC designs, the answer is yes, those components are also weak enough to be damaged, and have been bent in flight during maneuvers.
For US military helos, there exists a qualification test that proves the case against allowing jack stall. Known as a Structural Demo test flight, the pilot goes out at al weights and CG's and maneuvers to the maximum, to fully stall the system, and shows that there is sufficient strength margin so that no components can fail or "jack stall" in the process. These flight tests are the most critical, and most skill dependant of any except perhaps the H-V curve hard landing tests. I know for a fact that the argument made for allowing jack stall would get not 1 mm past a US Army/Navy/AF test engineer.
For US military helos, there exists a qualification test that proves the case against allowing jack stall. Known as a Structural Demo test flight, the pilot goes out at al weights and CG's and maneuvers to the maximum, to fully stall the system, and shows that there is sufficient strength margin so that no components can fail or "jack stall" in the process. These flight tests are the most critical, and most skill dependant of any except perhaps the H-V curve hard landing tests. I know for a fact that the argument made for allowing jack stall would get not 1 mm past a US Army/Navy/AF test engineer.
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Probaly explains the pucker tucker associated with the feed back forces of trying to go right way up after rolling a '47 way too quickly - either way.
Either the light weight or heavy weight blades will do it especially when fitted with the Texas No-bar kit, and very especially on the heavy weight blades '47 3B1 etc. with the Texas kit. I.E. stab bar removed, makes them almost identical to the 206 in feel.
letting off pressure when it is trying to roll itself upside down at thirty feet is NOT an option.
Never tried it in a 206 and don't intend to, but probably would not be as bad as shorter blades would make for a lot less leverage, me thinks.
Either the light weight or heavy weight blades will do it especially when fitted with the Texas No-bar kit, and very especially on the heavy weight blades '47 3B1 etc. with the Texas kit. I.E. stab bar removed, makes them almost identical to the 206 in feel.
letting off pressure when it is trying to roll itself upside down at thirty feet is NOT an option.
Never tried it in a 206 and don't intend to, but probably would not be as bad as shorter blades would make for a lot less leverage, me thinks.
Put another way, can the servo actuator be strong enough to withstand the maximum force from the blade but the result of such a strong actuator is that the pilot is allowed to put enough force into the controls such that the blade can fail, snap off and scatter but the servo actuator withstood the force?
That's why they design the parts to be strong enough. New machines are subject to a proof & ops test, they jam the head and stall the servos. Since the big SA makes dual stage hydraulic machines, one stage has to fly the aircraft through the whole envelope, but the parts have strength to take full tilt boogie with both stages operating. Not only statically, but all those parts are good for fatigue as well.
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Aileron Jack Stall
Flight testing of the RAAF's F-86 Sabre included finding the corners of the manoeuvre envelope where the aileron hydraulic actuators/jacks stalled. This occurred at speeds in excess of Mach 1.0 during rapid aileron actuation. The stick would lock up at about half deflection and could have been most embarrassing in a combat situation.
The hydraulic system was designed with a pressure of 3,000 psi and there was no way to increase the pressure without major changes. Higher power jacks were considered but discarded.
The manoeuvre limitation was accepted as insignificant considering the infrequent excursions into the situations where it occurred.
Flight testing of the RAAF's F-86 Sabre included finding the corners of the manoeuvre envelope where the aileron hydraulic actuators/jacks stalled. This occurred at speeds in excess of Mach 1.0 during rapid aileron actuation. The stick would lock up at about half deflection and could have been most embarrassing in a combat situation.
The hydraulic system was designed with a pressure of 3,000 psi and there was no way to increase the pressure without major changes. Higher power jacks were considered but discarded.
The manoeuvre limitation was accepted as insignificant considering the infrequent excursions into the situations where it occurred.
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Actually that reminds me; I was asked the other day if any helicopters do have ejection seats! I couldn't think of any, apart from the one in one James Bond film. Know of any more? And if not, why not? Surely it wouldn't be too difficult to design into a cockpit, especially into high risk aircraft such as the Apache. Either charges on the rotor blades to lose them before the seat fires vertically, or maybe sensors on the aircraft to sense its level of pitch and roll before firing the seats one way or another horizontally. I guess with that solution there could be a problem with the G-forces involved, but I'm sure they could be solved?
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There has been some research into ejector seats and it all hinges on the guarantee of getting all of the blades off in sequence so that the out of balance forces don't get you before one of the blades left attached will.
Horizontal ejector seats are again not possible because the human body would not survive the acceleration.
IIRC the body can survive up to 45g in the vertical plane but only 5 to 10g in the horizontal. Most current ejector seats go out at something like 40g.
Even with the correct seating position and everything working properly you are going to lose around 2 - 5 cm in height in a successful ejection!
Horizontal ejector seats are again not possible because the human body would not survive the acceleration.
IIRC the body can survive up to 45g in the vertical plane but only 5 to 10g in the horizontal. Most current ejector seats go out at something like 40g.
Even with the correct seating position and everything working properly you are going to lose around 2 - 5 cm in height in a successful ejection!
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Yeah I thought the G forces might be an issue. On the other hand though, are such large forces really necessary? You'd want a decent amount to get you away from an aircraft that was perhaps in a spin (loss of tail rotor, etc), but I would have thought rather than having high forces for a short length of time, moderate forces for a longer time could also be effective. Especially bearing in mind that helicopters are likely to be travelling much much slower than your average fast jet. Would also solve the problem of having rotor blades flying in all directions at high speed - prob not a good idea if you were flying formation beforehand!
Then again though, I'm sure people have looked into the idea. This is just me thinking aloud!
Then again though, I'm sure people have looked into the idea. This is just me thinking aloud!
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The two helos that I know of that have (had) ejection seats are the Kamov KA-50 coaxial and the Sikorsky experimental S-72 RSRA (which was also the DARPA X-Wing).
They both have elaborate mechanisms to blow off the rotor blades and they use extractor rockets with thether ropes so the G forces on ejection are low.
Frankly, the weight, cost and maintenance burden of blade removal/ejection (as well as the depression it will cause in your passengers) makes them poor bets in any helicopter that I want to fly.
They both have elaborate mechanisms to blow off the rotor blades and they use extractor rockets with thether ropes so the G forces on ejection are low.
Frankly, the weight, cost and maintenance burden of blade removal/ejection (as well as the depression it will cause in your passengers) makes them poor bets in any helicopter that I want to fly.
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Weight is definitely a huge concern with helicopter ejection seats, as is cost, but operational use is also important. Generally the helicopters that could use ejection seats are flown aggressively and close to the ground. By the time you decide ejection is a good idea, then initiate the sequence, then wait for the blades to clear, then fire the seat out...you've already crashed.
Sure if you're at 500' and pull handles early you'll be alright, but under what emergencies would you choose to trust the seat and parachute rather than flying it yourself?
I'm not saying it would never be used, just that its value diminishes quickly when you realize the limitations.
Back to the original topic:
Interesting that Jack Stall is not allowed in US Military helicopters, but that doesn't mean a pilot cannot fly those machines to the breaking point, just that the breaking point doesn't include Jack Stall. There are limitations to follow on all machines, if high speed, high 'g' flight isn't allowed and is the only condition where Jack Stall occurs then that shouldn't be a problem.
To call it a design flaw is like saying any helicopter that can intentionally be oversped, overtemped, etc. is flawed.
Designers have to meet certification requirements and prove safety of flight, but they do so knowing there is a pilot in the seat and knowing that they can expect the pilot to follow rules. If that weren't the case then there would be no limitations section in your AFM/AOI/PH because the helicopter would handle that for you (maybe we'll build like that one day, but not yet).
I've seen Jack Stall and I don't want it to happen in the wrong conditions (low level), but I continued to fly that same design and occassionally had to fly it close to Jack Stall conditions.
It's not a flaw.
Sure if you're at 500' and pull handles early you'll be alright, but under what emergencies would you choose to trust the seat and parachute rather than flying it yourself?
I'm not saying it would never be used, just that its value diminishes quickly when you realize the limitations.
Back to the original topic:
Interesting that Jack Stall is not allowed in US Military helicopters, but that doesn't mean a pilot cannot fly those machines to the breaking point, just that the breaking point doesn't include Jack Stall. There are limitations to follow on all machines, if high speed, high 'g' flight isn't allowed and is the only condition where Jack Stall occurs then that shouldn't be a problem.
To call it a design flaw is like saying any helicopter that can intentionally be oversped, overtemped, etc. is flawed.
Designers have to meet certification requirements and prove safety of flight, but they do so knowing there is a pilot in the seat and knowing that they can expect the pilot to follow rules. If that weren't the case then there would be no limitations section in your AFM/AOI/PH because the helicopter would handle that for you (maybe we'll build like that one day, but not yet).
I've seen Jack Stall and I don't want it to happen in the wrong conditions (low level), but I continued to fly that same design and occassionally had to fly it close to Jack Stall conditions.
It's not a flaw.
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Hi all,
Jack stall is certainly as a result of an abrupt control input, intentional or otherwise, but as can be seen it can have a bad outcome if close to the deck.
Turbulence and low level manoevring bring it on, especially with rolls close to ridge lines and strong wind. The old bird avoidance has brought it on to.
Yes I have been there, and no I am not an idiot...........the reason, show the student so he can recognise, recover and avoid being there in the first place. Ignorance is no excuse for a death if you can prevent it.
IMHO opion if you know what where and when it makes you more appreciative of it's dangers. It is benign to recover as the load is what induced it, remove the load and the problem goes away.
To add fuel to fire of "doing things that are stupid" it is similar to demonstrating (incipient) vortex ring really. Another danger to be fully cognisant of..............
I'll watch the flak after that comment.....
Jack stall is certainly as a result of an abrupt control input, intentional or otherwise, but as can be seen it can have a bad outcome if close to the deck.
Turbulence and low level manoevring bring it on, especially with rolls close to ridge lines and strong wind. The old bird avoidance has brought it on to.
Yes I have been there, and no I am not an idiot...........the reason, show the student so he can recognise, recover and avoid being there in the first place. Ignorance is no excuse for a death if you can prevent it.
IMHO opion if you know what where and when it makes you more appreciative of it's dangers. It is benign to recover as the load is what induced it, remove the load and the problem goes away.
To add fuel to fire of "doing things that are stupid" it is similar to demonstrating (incipient) vortex ring really. Another danger to be fully cognisant of..............
I'll watch the flak after that comment.....
I guess we have said most of this on the 5,000 other jack stall threads 
but I think the arguement is always worth going over.
One consistent position is those who believe an aircraft that suffers jack stall should not be certified.
The second is that jack stall is a poor design that can be compensated for by handling skills, knowledge and training in the same way teetering heads, LTE, and critical wind azimuths, etc, can be avoided. The big issue with this position is training and knowledge!!
The consistency with which these relatively well known "flaws" kill is an indictment on our training as much as it is the aircraft we fly. In modern error management the first principle is to engineer the problem out of the system so we dont have to train and prepare for it - and that is the position adopted by Nick and the others who contend the aircraft should not be certified.
Those adhering to the second position hold that the design flaw is too expensive to retrofit versus the ease with which training and knowledge can be used to overcome it. Each of us will change between these two positions (design out problem V train to avoid it) based on our own subjective assesment of the risks V training V design cost ratios.
Personally, I think jack stall is easy to train and avoid, but should not be tolerated in new design aircraft. The flaw in my arguement is of course training! And given that the opening post of these threads has inevitably been by pilots asking for more information on the phenomena - perhaps my position has been undermined.
The question then is - what is so wrong with our endorsement or type rating training system that pilots have to ask about such basic type limitations on pprune rather than seek the answers from their instructor? Should these "instructor limitations" be designed out of our system first?????
but I think the arguement is always worth going over.One consistent position is those who believe an aircraft that suffers jack stall should not be certified.
The second is that jack stall is a poor design that can be compensated for by handling skills, knowledge and training in the same way teetering heads, LTE, and critical wind azimuths, etc, can be avoided. The big issue with this position is training and knowledge!!
The consistency with which these relatively well known "flaws" kill is an indictment on our training as much as it is the aircraft we fly. In modern error management the first principle is to engineer the problem out of the system so we dont have to train and prepare for it - and that is the position adopted by Nick and the others who contend the aircraft should not be certified.
Those adhering to the second position hold that the design flaw is too expensive to retrofit versus the ease with which training and knowledge can be used to overcome it. Each of us will change between these two positions (design out problem V train to avoid it) based on our own subjective assesment of the risks V training V design cost ratios.
Personally, I think jack stall is easy to train and avoid, but should not be tolerated in new design aircraft. The flaw in my arguement is of course training! And given that the opening post of these threads has inevitably been by pilots asking for more information on the phenomena - perhaps my position has been undermined.
The question then is - what is so wrong with our endorsement or type rating training system that pilots have to ask about such basic type limitations on pprune rather than seek the answers from their instructor? Should these "instructor limitations" be designed out of our system first?????
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That's pretty much the conclusion that I came to; you can design faults out of a system, and if possible you should (and in this case that seems completely possible), but you also need training for any that remain.
Maybe I should point out though that while I asked about this topic, I'm not actually rated on the Gazelle, so maybe it is covered in the type training. Was just it included the word 'stall' - something not generally considered to be a good thing in aviation - I just wondered if it would be applicable to any of the types I fly.
Maybe I should point out though that while I asked about this topic, I'm not actually rated on the Gazelle, so maybe it is covered in the type training. Was just it included the word 'stall' - something not generally considered to be a good thing in aviation - I just wondered if it would be applicable to any of the types I fly.
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The issue is actually how we learn to compensate for the limitations of our machine, and the consistency and clarity of the warning that the machine gives.
Every pilot believes that he is better than the others, and that a pilot error accident is really just how bad the other guy is....until his machine bites him. Read the "Right Stuff" to see how we are rationalizing wen we scoff at making our machines better to make our mistakes less costly.
The reason why jack stall is a stupid thing to allow is that it is very preventable, as is LTE. The fact iis that we log accidents in them so they are more dangerous and less trustworthy than they should be. Yes we can learn to compensate for these flaws, hairy chested pilots like crappy machines, they make us feel so successful when we get them down in one piece. Our passengers expect more.
Every pilot believes that he is better than the others, and that a pilot error accident is really just how bad the other guy is....until his machine bites him. Read the "Right Stuff" to see how we are rationalizing wen we scoff at making our machines better to make our mistakes less costly.
The reason why jack stall is a stupid thing to allow is that it is very preventable, as is LTE. The fact iis that we log accidents in them so they are more dangerous and less trustworthy than they should be. Yes we can learn to compensate for these flaws, hairy chested pilots like crappy machines, they make us feel so successful when we get them down in one piece. Our passengers expect more.