AS350 Astar / AS355 Twinstar [Archive Copy]
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Lu,
You said "The very purpose of a hydraulic servo is to not only resist feedback forces from the rotor but to be able to overcome any and all feedback forces."
1. In fact in many helicopters the hydraulic systems exist only because the AFCS needs it. In the case of the AS350 you can fly an AS350 hydraulics off but because of the loads from the starflex head you wouldn't want to for too long.
2. If you took a Model T Ford to the Indy 500 would is survive? Is that a design fault? Remember that whilst reasonable margins must exist (and are defined in FAR / JAR 27 and 29) they are there for a reason.
You said "The very purpose of a hydraulic servo is to not only resist feedback forces from the rotor but to be able to overcome any and all feedback forces."
1. In fact in many helicopters the hydraulic systems exist only because the AFCS needs it. In the case of the AS350 you can fly an AS350 hydraulics off but because of the loads from the starflex head you wouldn't want to for too long.
2. If you took a Model T Ford to the Indy 500 would is survive? Is that a design fault? Remember that whilst reasonable margins must exist (and are defined in FAR / JAR 27 and 29) they are there for a reason.
Avoid imitations
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I have flown both Aerospatiale (3500 hrs) and Sikorsky products (1500 hrs), both military and civilian types. Each of them have good points and some not so good points..
(I also flew a Robinson helicopter. Only once and I have since had no desire to do so again).
If mud is to be slung about it could be done at any manufacturer, for a variety of reasons. However, as a professional pilot and Military taught QHI I do not see jack stall as a design flaw but merely as a limitation. It has already been pointed out that jack stall only occurs outside the normal flight envelope under high power and "G" (I have seen a roll off the top in an AS341 without experiencing jack stall - surely that is enough for most pilots and engineers)? The phenomena is preceded by warning signs including a big increase in vibration. I would far rather the aircraft calls "enough" by stalling a jack than something else letting go instead, either on that occasion or later for someone else to deal with.
Jack stall is just like a heart attack - it's nature's way of telling you to slow down a little...
(I also flew a Robinson helicopter. Only once and I have since had no desire to do so again).
If mud is to be slung about it could be done at any manufacturer, for a variety of reasons. However, as a professional pilot and Military taught QHI I do not see jack stall as a design flaw but merely as a limitation. It has already been pointed out that jack stall only occurs outside the normal flight envelope under high power and "G" (I have seen a roll off the top in an AS341 without experiencing jack stall - surely that is enough for most pilots and engineers)? The phenomena is preceded by warning signs including a big increase in vibration. I would far rather the aircraft calls "enough" by stalling a jack than something else letting go instead, either on that occasion or later for someone else to deal with.
Jack stall is just like a heart attack - it's nature's way of telling you to slow down a little...
I have to agree with Arm Out The Window (and others) on this one. With more than 3000 hours in the AS 350 series I have never experienced jack stall except when demonstrating it to students. Even then it usually took a couple of attempts at extreme manoeuvring to achieve incipient jack stall. As I recall the aircraft basically recovers immediately by itself as the pilot "automatically" (you have to be there) relaxes pressure on the cyclic. Any aircraft will experience problems if pushed far enough, as others have pointed out. This is not a design flaw. By the way Lu, the 350 hydraulic system does have accumulators.
Iconoclast
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The reason I mentioned the Robinson head (tusk) in the same post with the AS-350 Jack stall problem was that if you do not comply with the POH limitations you can experience a problem. How severe that problem is on the AS-350 with jack stall is totally dependent upon the conditions under which it is experienced. We already know the seriousness of the problem on the R-22 and the end result of exceeding the POH instructions.
As an RMS engineer I would think that if this condition was known at the onset of the certification process then from a safety point of view they should have eliminated the problem through redesign of the hydraulic system (if that is the root of the problem).
Here are several questions, which relate to the R-22 but are directed towards the AS-350 jack stall problem.
1) At what point in the certification process was the problem identified? Was the limitation placed in the POH and the jack stall warning light installed on the instrument panel at service entry?
2) If it was not identified in the certification process then at what point after entry into service was the problem identified? When was the warning light installed
3) At what time was the POH modified to reflect the problem and establish the flight limitations?
4) How is the jack stall light turned on? Is it by a G sensing switch or a pressure switch in the hydraulic system? Or, is it by some other means?
5) If the problem was identified after service entry was any attempt made to modify that part of the system that allowed jack stall to manifest itself?
Regarding the statement about the servos being necessary to support the AFCS or any other electronic system that requires hydraulic boost most of those helicopters had these systems installed as after market items. On those helicopters (large) that have this type of system the electronically controlled servo valves are installed directly on the servo or on systems like those on some Sikorsky helicopters there is a primary and an aux. servo system and the electronic servo valves are on the aux. servo. On the EH-101 they have dual servos at the swashplate and the electronic control is incorporated as a part of the flight control linkage. This is similar to smaller helicopters that had the system installed during production and as after market items.
However the servos are not installed to support and AFCS their primary purpose is to resist and overcome feedback forces at any level.
Even though the AS-350 can be controlled without hydraulic boost I would question what the POH indicates if you encounter this condition.
Anybody that has the knowledge please respond to the questions above.
As an RMS engineer I would think that if this condition was known at the onset of the certification process then from a safety point of view they should have eliminated the problem through redesign of the hydraulic system (if that is the root of the problem).
Here are several questions, which relate to the R-22 but are directed towards the AS-350 jack stall problem.
1) At what point in the certification process was the problem identified? Was the limitation placed in the POH and the jack stall warning light installed on the instrument panel at service entry?
2) If it was not identified in the certification process then at what point after entry into service was the problem identified? When was the warning light installed
3) At what time was the POH modified to reflect the problem and establish the flight limitations?
4) How is the jack stall light turned on? Is it by a G sensing switch or a pressure switch in the hydraulic system? Or, is it by some other means?
5) If the problem was identified after service entry was any attempt made to modify that part of the system that allowed jack stall to manifest itself?
Regarding the statement about the servos being necessary to support the AFCS or any other electronic system that requires hydraulic boost most of those helicopters had these systems installed as after market items. On those helicopters (large) that have this type of system the electronically controlled servo valves are installed directly on the servo or on systems like those on some Sikorsky helicopters there is a primary and an aux. servo system and the electronic servo valves are on the aux. servo. On the EH-101 they have dual servos at the swashplate and the electronic control is incorporated as a part of the flight control linkage. This is similar to smaller helicopters that had the system installed during production and as after market items.
However the servos are not installed to support and AFCS their primary purpose is to resist and overcome feedback forces at any level.
Even though the AS-350 can be controlled without hydraulic boost I would question what the POH indicates if you encounter this condition.
Anybody that has the knowledge please respond to the questions above.
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By POH I assume you are talking of the flight manual . Folowing Lu's logic operating the aircraft over gross weight is OK because the baggage compartment is large enough to take 10 cu ft of depleted uranium .
To suggest that you cannot change the flight manual based on service experience is patently absurd . In a perfect world the flight test program and design review would find every possible fault that might occur. I think that flight manuals carved in stone may be a little overkill not to mention overweight. Can you give a single example of an in service helicopter where the Flight Manual is still at Revision A ?. ( unless you forgot to sign up for revision service ).
To suggest that you cannot change the flight manual based on service experience is patently absurd . In a perfect world the flight test program and design review would find every possible fault that might occur. I think that flight manuals carved in stone may be a little overkill not to mention overweight. Can you give a single example of an in service helicopter where the Flight Manual is still at Revision A ?. ( unless you forgot to sign up for revision service ).
Iconoclast
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To: Grey Area
To: Grey Area
Someone in a previous post alluded to the fact that although the warning light was illuminated he did not have time to look at it as his eyes were outside the helicopter during the maneuver. I was speaking in generalities about the jack stall condition relative to the hydraulics system and the control boost servos I assumed that since a pilot mentioned the light it must be there.
What kind of warning do you get with imminent jack stall? If it is a light or a bell or a voice alarm it doesn't make any difference as I asked what caused the alarm to go off.
To: Widgeon
I did not say that the flight manual should or should not be changed due to things that occur in operation. This happens on both the POH and the maintenance manual. What I was asking, was the entry in the POH for the AS-350 jack stall problem entered at rev A when the helicopter went into service or, did the problem occur at a later date after entry into service with the entry telling the pilots to avoid this flight regime and, there was no attendant design change to prevent this from happening in the future. This is exactly what happened to the R-22 relative to sideslip and out of trim flight. It warns the pilot but no design change was made. I also asked the question, why didn’t this show up during certification testing? That goes for both helicopters.
This leads us back to the basic premise that the pilot is protected by words and not by design. Please everybody; do not attack this last statement as we have already gone through the various arguments.
I would still like to have all of the questions answered.
I should have asked a sixth question and that is why do you as pilots accept this form of protection? One day you might find yourself in an emergency situation where the last thing you need at that time is Jack Stall.
Here is a test you can perform. It works on a Sikorsky helicopter and I assume it will work on the AS-350. Go into a hover and take the cyclic stick and move it in a circle in the same direction as the rotor is spinning. Move the cyclic stick through this circular path four or five times. Do the controls get stiff. It does not happen on the Sikorsky and it may not happen on the AS-350. However if it does, it indicates that the hydraulic system can’t keep up with the demand.
[ 06 December 2001: Message edited by: Lu Zuckerman ]
[ 06 December 2001: Message edited by: Lu Zuckerman ]
To: Grey Area
Someone in a previous post alluded to the fact that although the warning light was illuminated he did not have time to look at it as his eyes were outside the helicopter during the maneuver. I was speaking in generalities about the jack stall condition relative to the hydraulics system and the control boost servos I assumed that since a pilot mentioned the light it must be there.
What kind of warning do you get with imminent jack stall? If it is a light or a bell or a voice alarm it doesn't make any difference as I asked what caused the alarm to go off.
To: Widgeon
I did not say that the flight manual should or should not be changed due to things that occur in operation. This happens on both the POH and the maintenance manual. What I was asking, was the entry in the POH for the AS-350 jack stall problem entered at rev A when the helicopter went into service or, did the problem occur at a later date after entry into service with the entry telling the pilots to avoid this flight regime and, there was no attendant design change to prevent this from happening in the future. This is exactly what happened to the R-22 relative to sideslip and out of trim flight. It warns the pilot but no design change was made. I also asked the question, why didn’t this show up during certification testing? That goes for both helicopters.
This leads us back to the basic premise that the pilot is protected by words and not by design. Please everybody; do not attack this last statement as we have already gone through the various arguments.
I would still like to have all of the questions answered.
I should have asked a sixth question and that is why do you as pilots accept this form of protection? One day you might find yourself in an emergency situation where the last thing you need at that time is Jack Stall.
Here is a test you can perform. It works on a Sikorsky helicopter and I assume it will work on the AS-350. Go into a hover and take the cyclic stick and move it in a circle in the same direction as the rotor is spinning. Move the cyclic stick through this circular path four or five times. Do the controls get stiff. It does not happen on the Sikorsky and it may not happen on the AS-350. However if it does, it indicates that the hydraulic system can’t keep up with the demand.
[ 06 December 2001: Message edited by: Lu Zuckerman ]
[ 06 December 2001: Message edited by: Lu Zuckerman ]
Avoid imitations
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My personal SWP just gave me a caution light because I hope this thread isn't going to degenerate into another where manufacturers (except Lu's apparent favourite, the big "S") get a slagging for alleged poor design.
I haven't flown the single Squirrel but Lu's cyclic stirring test doesn't let the manufacturer down on the twin with basically the same hydraulic system. On a different AS type we used to use that same test to check for a suspected faulty drag damper rather than try to test fly for a suspected design limitation of the hydraulic pump output.
Lu, a pilot is taught to understand and respect the idiosynchrasies and limitations of each machine that he flies. Most aircraft have them, because there is no such thing as care-free handling just yet although it is obviously a desirable design aim.
For example, I would not try some of the manoeuvres I have flown in the AS341 or AS355 or even AS332 in an S-76 because the rotor would overspeed as soon as you tried them. Similarly, one or two manoeuvres I used to do in my display sequence in the AS332 would possibly kill you in the S-70 due to aircraft design peculiarities (and in fact one of them was specifically prohibited). These things are not generally design faults, but aircraft limitations. I don't have any problem whatsoever with this, it's what I get paid for as a professional pilot.
From your many previous posts I can fully understand that you would like to redesign many items on many aircraft and I can sense your frustration that you haven't been able to do so. Unfortunately we don't live in an ideal world where all aircraft are 100% compliant with & safe iaw your own personal design parameters!
I used to safely enjoy 360 and 540 degree handbrake turns in the snow in my small hatchback. I wouldn't try handbrake turns in my people carrier or an articulated lorry or on a ride-on lawnmower. Does that make people carriers, artics and lawnmowers inherently dangerous or poorly dsigned? I don't think so, it would make me stupid for trying.
Watch out Frank, I think you might need your tin hat on again any minute now....
I haven't flown the single Squirrel but Lu's cyclic stirring test doesn't let the manufacturer down on the twin with basically the same hydraulic system. On a different AS type we used to use that same test to check for a suspected faulty drag damper rather than try to test fly for a suspected design limitation of the hydraulic pump output.
Lu, a pilot is taught to understand and respect the idiosynchrasies and limitations of each machine that he flies. Most aircraft have them, because there is no such thing as care-free handling just yet although it is obviously a desirable design aim.
For example, I would not try some of the manoeuvres I have flown in the AS341 or AS355 or even AS332 in an S-76 because the rotor would overspeed as soon as you tried them. Similarly, one or two manoeuvres I used to do in my display sequence in the AS332 would possibly kill you in the S-70 due to aircraft design peculiarities (and in fact one of them was specifically prohibited). These things are not generally design faults, but aircraft limitations. I don't have any problem whatsoever with this, it's what I get paid for as a professional pilot.
From your many previous posts I can fully understand that you would like to redesign many items on many aircraft and I can sense your frustration that you haven't been able to do so. Unfortunately we don't live in an ideal world where all aircraft are 100% compliant with & safe iaw your own personal design parameters!
I used to safely enjoy 360 and 540 degree handbrake turns in the snow in my small hatchback. I wouldn't try handbrake turns in my people carrier or an articulated lorry or on a ride-on lawnmower. Does that make people carriers, artics and lawnmowers inherently dangerous or poorly dsigned? I don't think so, it would make me stupid for trying.
Watch out Frank, I think you might need your tin hat on again any minute now....
Iconoclast
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To: Shy Torque
I can totally accept the fact that limitations are placed on helicopters and aircraft. In most cases it has been determined that to perform certain maneuvers you can overstress the structure. That is why they have a SCAS system in some helicopters to keep from overstressing the rotor system or the structure. In one case you described that if you performed a certain maneuver in a specific helicopter that if this maneuver was performed in another helicopter you could overspeed the rotor system. So, this maneuver is restricted on that helicopter and that is OK with me. The problem I see on the AS-350 if I understand it correctly is if you perform a specific maneuver you get jack stall, which means that there is a failure to deliver adequate fluid under pressure to combat the feedback forces generated in the maneuver. If the pump had a greater delivery capacity or they used a constant speed variable delivery pump or if they had an adequately sized accumulator the pilot would never experience jack stall. To allow jack stall to happen in a helicopter design is unconscionable in respect to systems safety and from a reliability point of view if the lower controls are exposed to flight loads.
I can totally accept the fact that limitations are placed on helicopters and aircraft. In most cases it has been determined that to perform certain maneuvers you can overstress the structure. That is why they have a SCAS system in some helicopters to keep from overstressing the rotor system or the structure. In one case you described that if you performed a certain maneuver in a specific helicopter that if this maneuver was performed in another helicopter you could overspeed the rotor system. So, this maneuver is restricted on that helicopter and that is OK with me. The problem I see on the AS-350 if I understand it correctly is if you perform a specific maneuver you get jack stall, which means that there is a failure to deliver adequate fluid under pressure to combat the feedback forces generated in the maneuver. If the pump had a greater delivery capacity or they used a constant speed variable delivery pump or if they had an adequately sized accumulator the pilot would never experience jack stall. To allow jack stall to happen in a helicopter design is unconscionable in respect to systems safety and from a reliability point of view if the lower controls are exposed to flight loads.
round & round & round &..............
Lu some answers for you:
1. The LIMIT switch is fitted on the twin (I have never seen it on the single) on a servo. It illuminates a cockpit warning light only, no bells, whistles or voices. The action required is to "reduce pitch or open out the manuever (i.e. relax the G)"
2. An actuator supplied by the LH HYD system is fitted to the end of the servo control. During flight, when the load is below an preset value, the servo pressure keeps the piston against the upper stop and the light is off. If the flight load on the servo control exceeds the limit threshold, the microswitch is actuated and the light comes on.
3. NONE of this is relevant to your statement that it jack stall is "unacceptable".
4. I have never heard of anyone getting jack stall other than during aerobatic manuevering (prohibited) or demonstrating it during endorsements. You need to be taken up and shown this how violent the manuever has to be before you reach incipient jack stall. It is unsettling. The again Lu, you COULD just believe the experienced pilots who are all telling you it is not an issue.........
5. You ask why pilots accept these seemingly dangerous limitations? This is easy Lu, they ARE NOT dangerous. More fixed wing guys die through stalling close to the ground than ever even see jack stall demonstrated (let alone experienced). Should they immediately ground all fixed wing?
Actually, thats not a bad idea.......
6. The "someone" who said they dont have their eyes in side during harsh, high G manuevering was me. Can you not look back through what has been said? If you were going around a 60mph signposted corner in your car at 100mph, would you have your eyes inside reading the speedometer? Perhaps not. Understand?
I'll say it once again (in case you cannot read previous posts):
Is this a design flaw compared to the Bell teetering head which will come off under negative G if not handled IAW the POH? Or compared to fixed wing aircraft which will stall and spin if not handled IAW the POH? Or compared to Loss of Tail Rotor Effectiveness in the B206 if not handled IAW the POH? You getting the picture?
The AS350 is an outstanding aircraft. Enjoy it's huge capability and dont fly it against the POH - like EVERY OTHER AIRCRAFT.
Edited for spelling
[ 06 December 2001: Message edited by: helmet fire ]
Lu some answers for you:
1. The LIMIT switch is fitted on the twin (I have never seen it on the single) on a servo. It illuminates a cockpit warning light only, no bells, whistles or voices. The action required is to "reduce pitch or open out the manuever (i.e. relax the G)"
2. An actuator supplied by the LH HYD system is fitted to the end of the servo control. During flight, when the load is below an preset value, the servo pressure keeps the piston against the upper stop and the light is off. If the flight load on the servo control exceeds the limit threshold, the microswitch is actuated and the light comes on.
3. NONE of this is relevant to your statement that it jack stall is "unacceptable".
4. I have never heard of anyone getting jack stall other than during aerobatic manuevering (prohibited) or demonstrating it during endorsements. You need to be taken up and shown this how violent the manuever has to be before you reach incipient jack stall. It is unsettling. The again Lu, you COULD just believe the experienced pilots who are all telling you it is not an issue.........
5. You ask why pilots accept these seemingly dangerous limitations? This is easy Lu, they ARE NOT dangerous. More fixed wing guys die through stalling close to the ground than ever even see jack stall demonstrated (let alone experienced). Should they immediately ground all fixed wing?
Actually, thats not a bad idea.......
6. The "someone" who said they dont have their eyes in side during harsh, high G manuevering was me. Can you not look back through what has been said? If you were going around a 60mph signposted corner in your car at 100mph, would you have your eyes inside reading the speedometer? Perhaps not. Understand?
I'll say it once again (in case you cannot read previous posts):
Is this a design flaw compared to the Bell teetering head which will come off under negative G if not handled IAW the POH? Or compared to fixed wing aircraft which will stall and spin if not handled IAW the POH? Or compared to Loss of Tail Rotor Effectiveness in the B206 if not handled IAW the POH? You getting the picture?
The AS350 is an outstanding aircraft. Enjoy it's huge capability and dont fly it against the POH - like EVERY OTHER AIRCRAFT.
Edited for spelling
[ 06 December 2001: Message edited by: helmet fire ]
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Oh Please,
Are you lot just arguing with Lu because you can?
Lets draw this comparison, you buy a particular hand gun knowing full well that if you wave the gun around a lot, the safety catch can move to the off position.
You are aware of this limitation therefor you don't wave it around a lot, but if you do, you always check the position of the safety catch. Anyway this hand gun in most other aspects is superb and as every one is aware of this problem it is generally accepted and called a good gun.
Now here's the revelation
Guess what
The design of the safety catch on this superb gun that every one loves is still crap.
Jiff
[ 07 December 2001: Message edited by: Jiff ]
[ 07 December 2001: Message edited by: Jiff ]
Are you lot just arguing with Lu because you can?
Lets draw this comparison, you buy a particular hand gun knowing full well that if you wave the gun around a lot, the safety catch can move to the off position.
You are aware of this limitation therefor you don't wave it around a lot, but if you do, you always check the position of the safety catch. Anyway this hand gun in most other aspects is superb and as every one is aware of this problem it is generally accepted and called a good gun.
Now here's the revelation
Guess what
The design of the safety catch on this superb gun that every one loves is still crap.
Jiff
[ 07 December 2001: Message edited by: Jiff ]
[ 07 December 2001: Message edited by: Jiff ]
Your analogy doesn't really hold water, Jiff.
The hydraulic system in question performs perfectly well under any reasonable inflight circumstances, rather unlike a safety catch that might fail at any moment.
If anyone knows anyone at Aerospatiale they should ask them, but I'd imagine that beefing up the hydraulic system and servos to fix this so-called problem would incur a weight penalty and be not worth doing considering the performance reduction.
The hydraulic system in question performs perfectly well under any reasonable inflight circumstances, rather unlike a safety catch that might fail at any moment.
If anyone knows anyone at Aerospatiale they should ask them, but I'd imagine that beefing up the hydraulic system and servos to fix this so-called problem would incur a weight penalty and be not worth doing considering the performance reduction.
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With Jiff's handgun analogy, we come back to my question. If a problem is solved by publishing a limitation, is it still a design flaw?
I posted my thoughts on this. Appreciate others'.
I posted my thoughts on this. Appreciate others'.
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"If a problem is solved by publishing a limitation, is it still a design flaw?"
The point in THIS case is that the aircraft operates quite safely within the designed limits; if you exceed this limit you will reach a possible loss of control that is refered to in the POH.
Note/ AS350 - No limit caption - CAT B
AS355 / AS365 Limit caption - CAT a
The point in THIS case is that the aircraft operates quite safely within the designed limits; if you exceed this limit you will reach a possible loss of control that is refered to in the POH.
Note/ AS350 - No limit caption - CAT B
AS355 / AS365 Limit caption - CAT a
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To Arm out the window,
I think that my analogy does hold water because I did say "wave the gun around a lot", that certainly doesn't imply that it will fail at any moment.
The beefing up of the hydraulic system in this case is probably just a very small pump capacity increase.
Heedm,
"If a problem is solved by publishing a limitation, is it still a design flaw?" Yes and no, If a helicopter can carry an underslung load of 2000 lbs. I think it is reasonable to say that's its not a design flaw because it cant carry 5000 lbs, but in the case of the hydraulic system we are talking about, it clearly is.
Grey Area,
I have designed the control systems for several safety critical systems (non aviation) which involve servo jacks and hydraulics, and I know for a fact that if the mechanical guys did not design in an additional 10% capacity above and beyond what the worst the control system could ask for, the design would have been questioned at the review stage.
Jiff
I think that my analogy does hold water because I did say "wave the gun around a lot", that certainly doesn't imply that it will fail at any moment.
The beefing up of the hydraulic system in this case is probably just a very small pump capacity increase.
Heedm,
"If a problem is solved by publishing a limitation, is it still a design flaw?" Yes and no, If a helicopter can carry an underslung load of 2000 lbs. I think it is reasonable to say that's its not a design flaw because it cant carry 5000 lbs, but in the case of the hydraulic system we are talking about, it clearly is.
Grey Area,
I have designed the control systems for several safety critical systems (non aviation) which involve servo jacks and hydraulics, and I know for a fact that if the mechanical guys did not design in an additional 10% capacity above and beyond what the worst the control system could ask for, the design would have been questioned at the review stage.
Jiff
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I suppose VNE is a limitation to cover up for a design flaw, I suppose VNE limits with doors off are cover ups of a design flaw, I suppose the Torque limit is a cover up for a design flaw, I suppose the Flight Manual should say "don't fly helicopter into powerlines" and I suppose the Flight Manual should have a special supplement that says, all flight test data conducted by Test Pilots and Flt Test Engineers are superseded subject to the opinion of Lu Zuckerman.
This forum is basically Lu taking the position of argument for the sake of argument, not for the sake of learning or sharing. I have never seen a person ignore the opinions of highly qualified people quite like this bloke.
Test Pilots found the feedback limit during flight test Lu, they call it transparency Lu, Its in my Flight Manual under NORMAL PROCEDURES, Sect 2 Paragraph 7, Crusing Flight Manoeuvres, Para B "-Maximum load factor in turns is felt in the form of servo-control "transparency"; this phenomenon is smooth, and presents NO DANGER."
Limitations Section, Para F 'Prohibited Manoeuvres'- Aerobatics.
Pretty well much says it all, once again Lu, you stray way off the subject and turn it into a design flaw of a helicopter which you have absolutely no knowledge of, aren't you glad pprune has no forums for astronauts, imagine the crap he go on with there..
This forum is basically Lu taking the position of argument for the sake of argument, not for the sake of learning or sharing. I have never seen a person ignore the opinions of highly qualified people quite like this bloke.
Test Pilots found the feedback limit during flight test Lu, they call it transparency Lu, Its in my Flight Manual under NORMAL PROCEDURES, Sect 2 Paragraph 7, Crusing Flight Manoeuvres, Para B "-Maximum load factor in turns is felt in the form of servo-control "transparency"; this phenomenon is smooth, and presents NO DANGER."
Limitations Section, Para F 'Prohibited Manoeuvres'- Aerobatics.
Pretty well much says it all, once again Lu, you stray way off the subject and turn it into a design flaw of a helicopter which you have absolutely no knowledge of, aren't you glad pprune has no forums for astronauts, imagine the crap he go on with there..
Iconoclast
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Once and for all understand this: This subject was addressed on Just Helicopters and it was alluded to as a problem. Since in my work it is imperative to try to eliminate any problem that effects reliability and safety I made a few assumptions. These were eventually proven wrong and I learned something in the process. Jack stall is an acceptable condition on the AS-350 and it does not cause a danger. I saw it as a problem because I felt that the hydraulic system was incapable of overcoming the forces and on any other helicopter I have been associated with, this problem was never addressed. I saw jack stall as a problem related to the reliability because in my misunderstanding of the conditions I felt the lower control system could react the flight loads. I saw it as a safety problem because in an emergency situation you would not want the jacks to stall. I also saw another potential problem and that was how the warning light was triggered and I was told that the warning light is on the Twin Star and not on the A Star. I thought the trigger was the system pressure and I was told that it is an external device that is mechanical with a micro switch or possibly a strain gage. The danger that I saw was that once the jack stall was taking place the pilot could not move his cyclic. To do so, would displace the servo (pilot) valve(s) opening the lower part of the cylinder to the return system and then the feedback forces could enter the control system and displace the cyclic stick quite hard and seriously hurt the pilot depending on which way the cyclic was displaced. But then again I am just a worrywart.
Once and for all understand this: This subject was addressed on Just Helicopters and it was alluded to as a problem. Since in my work it is imperative to try to eliminate any problem that effects reliability and safety I made a few assumptions. These were eventually proven wrong and I learned something in the process. Jack stall is an acceptable condition on the AS-350 and it does not cause a danger. I saw it as a problem because I felt that the hydraulic system was incapable of overcoming the forces and on any other helicopter I have been associated with, this problem was never addressed. I saw jack stall as a problem related to the reliability because in my misunderstanding of the conditions I felt the lower control system could react the flight loads. I saw it as a safety problem because in an emergency situation you would not want the jacks to stall. I also saw another potential problem and that was how the warning light was triggered and I was told that the warning light is on the Twin Star and not on the A Star. I thought the trigger was the system pressure and I was told that it is an external device that is mechanical with a micro switch or possibly a strain gage. The danger that I saw was that once the jack stall was taking place the pilot could not move his cyclic. To do so, would displace the servo (pilot) valve(s) opening the lower part of the cylinder to the return system and then the feedback forces could enter the control system and displace the cyclic stick quite hard and seriously hurt the pilot depending on which way the cyclic was displaced. But then again I am just a worrywart.
Avoid imitations
Join Date: Nov 2000
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At risk of doing this to death...
Why not fix the problem of jackstall by inceasing the pump output (and possibly having to upgrade the whole system and redesign the airframe to cope with the consequential increased feedback forces, adding weight and reducing the useful load of the aircraft)?
Firstly, because jackstall occurs outside the normal flight regime and it requires high power, high speed and harsh manoeuvre to induce it. For example, we used to demo it to basic students in the military Gazelle AS341 by diving the aircraft at high power then pulling hard through the heavy vibration until a jack stalled. It was very unnatural to do it, we didn't like doing it because it felt awful to the airframe and I believe it was later removed from the syllabus. (It also scared the $hit out of most students).
The parameters required to induce jackstall will sound familiar to any helicopter pilot because they are also exactly those required to induce retreating blade stall.
There is little point modifying an aircraft already in safe and satisfactory service to prevent jackstall if retreating blade stall will subsequently occur at only a slightly higher speed, power, or manoeuvre limit.
Secondly, market forces rule. It's a fact. Any aircraft can be redesigned (at a cost) to perform better but we aren't talking about an agile fighter here, it's a small helicopter for goodness sake! Who would pay? The manufacturer, or the customer, especially if there is nil perceived benefit?
Finally, jackstall tends to result in the aircraft throwing off pitch so that it is a short-lived phenomena, unlike retreating blade stall. As I said earlier, I would rather suffer jackstall than get RBS a few knots later. The aircraft limitations are there for a reason. Observing them is what we get paid for.
Why not fix the problem of jackstall by inceasing the pump output (and possibly having to upgrade the whole system and redesign the airframe to cope with the consequential increased feedback forces, adding weight and reducing the useful load of the aircraft)?
Firstly, because jackstall occurs outside the normal flight regime and it requires high power, high speed and harsh manoeuvre to induce it. For example, we used to demo it to basic students in the military Gazelle AS341 by diving the aircraft at high power then pulling hard through the heavy vibration until a jack stalled. It was very unnatural to do it, we didn't like doing it because it felt awful to the airframe and I believe it was later removed from the syllabus. (It also scared the $hit out of most students).
The parameters required to induce jackstall will sound familiar to any helicopter pilot because they are also exactly those required to induce retreating blade stall.
There is little point modifying an aircraft already in safe and satisfactory service to prevent jackstall if retreating blade stall will subsequently occur at only a slightly higher speed, power, or manoeuvre limit.
Secondly, market forces rule. It's a fact. Any aircraft can be redesigned (at a cost) to perform better but we aren't talking about an agile fighter here, it's a small helicopter for goodness sake! Who would pay? The manufacturer, or the customer, especially if there is nil perceived benefit?
Finally, jackstall tends to result in the aircraft throwing off pitch so that it is a short-lived phenomena, unlike retreating blade stall. As I said earlier, I would rather suffer jackstall than get RBS a few knots later. The aircraft limitations are there for a reason. Observing them is what we get paid for.
ShyT: well said. Especially that the aircraft limitations are there for a reason and that pilots get paid to observe them.
Lu: I appreciate where you were comming from, and I am glad you were able to take the points on board.
Jiff: Re your handgun example. I agree with your proposition, however, it is in no way comparable to the AS350 situation. In particular, you might note that several pilots have described how jack stall only appears during extreme maneuvering, not during normal ops. To draw the analogy with your handgun more accurately, it would be like saying that the safety catch had a design flaw if it came lose after repeatedly (and harshly) bashing the gun on a hard surface when you are reloading. In other words, the "flaw" only appears after gross mishandling. To continue the analogy, there would even be a note attached to the gun warning you that: "if you repeatedly bash the gun on the ground to reload it, the safety catch MAY come loose. To correct this, check safety catch before lifting the gun." Does this put it a little closer to context?
To heedm: The point you raise is an excellent one. Basically you are asking: where do you draw the line. Who defines flaw Vs limitation, gross mishandling Vs routine operations? I guess that is the manufacturers and certification authorities, perhaps Nick might know?
Lu: I appreciate where you were comming from, and I am glad you were able to take the points on board.
Jiff: Re your handgun example. I agree with your proposition, however, it is in no way comparable to the AS350 situation. In particular, you might note that several pilots have described how jack stall only appears during extreme maneuvering, not during normal ops. To draw the analogy with your handgun more accurately, it would be like saying that the safety catch had a design flaw if it came lose after repeatedly (and harshly) bashing the gun on a hard surface when you are reloading. In other words, the "flaw" only appears after gross mishandling. To continue the analogy, there would even be a note attached to the gun warning you that: "if you repeatedly bash the gun on the ground to reload it, the safety catch MAY come loose. To correct this, check safety catch before lifting the gun." Does this put it a little closer to context?
To heedm: The point you raise is an excellent one. Basically you are asking: where do you draw the line. Who defines flaw Vs limitation, gross mishandling Vs routine operations? I guess that is the manufacturers and certification authorities, perhaps Nick might know?