AS350 Astar/Squirrel
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407,
The elastomers are not any stiffer while in flight, because the internal heating from the flexure is quite sufficient. In any case, the stiffness of the elastomer is a very tiny force compared to the stall forces from the blades as they produce high thrust.
The stiffness of the elastomers is high during warmup after cold soak, that is why many helos have warmup procedures for the rotorhead, and restrictions on the amount of flapping until a few seconds of running has passed, allowing the internal friction to warm up the elastomers.
As a recap for this thread (and a chance to awaken the arguments!):
The "transparency" is experienced when the rotor pitching moment forces (blade stall forces) become great enough to overcome the hydraulic forces of the servo. It is not actually caused by low hydraulic pressure, it is a function of the design being so marginal that the servos are too weak to push the blades around as the helicopter gets near stall. The maneuvers that cause "transparency" are often quite normal, and sometimes the effect is alarming and dangerous.
The limit light on the 355 is second cousin to "transparency" in that the servo forces trigger the light when they get large enough to signal impending loss of control. However, the 355 seems to have less propensity to enter jack stall, mostly because it has a more capable servo design than the 155.
NO US helicopter, civil or military, can have such "transparency" issues, since these signal that the controls no longer are controlling, and the pilot is a passenger. I also wage my friends at CAA would have similar thoughts, were they able to post here. Only the weakness of the bilateral regs (Shawn's excellent post above discusses this) allow such marginal controls to be put into service in countries other than France.
Why do the French approve such marginal systems? Je ne sais quoi! Perhaps it is because the French government owns the manufacturing company, the design engineers, the test engineers and the approval agency, and they dislike arguing with themselves. It is so unseemly to actually regulate yourself, n'est-pas?
The elastomers are not any stiffer while in flight, because the internal heating from the flexure is quite sufficient. In any case, the stiffness of the elastomer is a very tiny force compared to the stall forces from the blades as they produce high thrust.
The stiffness of the elastomers is high during warmup after cold soak, that is why many helos have warmup procedures for the rotorhead, and restrictions on the amount of flapping until a few seconds of running has passed, allowing the internal friction to warm up the elastomers.
As a recap for this thread (and a chance to awaken the arguments!):
The "transparency" is experienced when the rotor pitching moment forces (blade stall forces) become great enough to overcome the hydraulic forces of the servo. It is not actually caused by low hydraulic pressure, it is a function of the design being so marginal that the servos are too weak to push the blades around as the helicopter gets near stall. The maneuvers that cause "transparency" are often quite normal, and sometimes the effect is alarming and dangerous.
The limit light on the 355 is second cousin to "transparency" in that the servo forces trigger the light when they get large enough to signal impending loss of control. However, the 355 seems to have less propensity to enter jack stall, mostly because it has a more capable servo design than the 155.
NO US helicopter, civil or military, can have such "transparency" issues, since these signal that the controls no longer are controlling, and the pilot is a passenger. I also wage my friends at CAA would have similar thoughts, were they able to post here. Only the weakness of the bilateral regs (Shawn's excellent post above discusses this) allow such marginal controls to be put into service in countries other than France.
Why do the French approve such marginal systems? Je ne sais quoi! Perhaps it is because the French government owns the manufacturing company, the design engineers, the test engineers and the approval agency, and they dislike arguing with themselves. It is so unseemly to actually regulate yourself, n'est-pas?
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There's a Limit light in the AS355F-1 I'm flying right now. Don't have the manual here, so I'm not sure if it's a mod or if it's original.
We flew an intentional jackstall on the Gazelle. It takes quite a bit of effort to put yourself into that condition. I agree that if it could be designed out of the system without consequence then it should, but it's not as if there's a very large accessible and easy to use "DO NOT TOUCH THIS" button in the cockpit.
Like many things helicopter, it does require the pilot to understand and be aware in able to prevent and if required, recover.
We flew an intentional jackstall on the Gazelle. It takes quite a bit of effort to put yourself into that condition. I agree that if it could be designed out of the system without consequence then it should, but it's not as if there's a very large accessible and easy to use "DO NOT TOUCH THIS" button in the cockpit.
Like many things helicopter, it does require the pilot to understand and be aware in able to prevent and if required, recover.
Nick,
The French may not have a fondness for arguing amonst themselves...but they sure enjoy arguing with everyone else. Eeeet issss Lodgeek!
The French may not have a fondness for arguing amonst themselves...but they sure enjoy arguing with everyone else. Eeeet issss Lodgeek!
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Let’s not turn this into a Francophobe forum.
As anyone in the business knows that the design and certification of any helicopter is a series of trade offs. In a good design everything is sized perfectly, keep the weight down is the battle cry in any helicopter design office, US, UK, France or where-ever. Each manufacturer has a different take on how to design a helicopter and how to achieve compliance, were that not the case then every helicopter would be the same. In the same vein safety cases rely on different mitigations; you can keep the pilot alive by making the crew station survivable in a 30 g impact but that is costly and heavy, alternatively you can reduce the risk of such an impact in other ways that do not necessarily make the aircraft less safe. Each design must be taken in entirety and a direct comparison between manufacturers design priorities in a single area will not result in an objective comparison.
Eurocopter will argue that the hydraulic system is designed to ensure that the rotor head cannot be overstressed. Something that in the early days of the Starflex head was paramount given the lack of operational data associated with any new technology. The fact that technology works and other manufacturer use elastomers is testament to a successful technology. In later Eurocopter models the Limit light is an indication that the head is close to its limiting stress loading, just as mast bending moment is displayed in other semi-rigid designs (BO105 for example). As an aside there are many in the Eurocopter who believe that hydraulics should run at the lowest possible pressure in order to minimise the safety implication of leaks and minimise weight, as SASLess put it.
Be in no doubt that the French take civil certification very seriously, in fact unlike the ETPS & USNTPS the EPNER Rotary Syllabus is based on US FAR certification and not European military compliance. After all Eurocopter has worked very hard to break into the North American and international markets and has had considerable success, they can’t be doing things too badly.
We are all different and from differing backgrounds, to really know your aircraft you need to understand the philosophy behind it not fight against it.
Fly safe chaps.
GA
As anyone in the business knows that the design and certification of any helicopter is a series of trade offs. In a good design everything is sized perfectly, keep the weight down is the battle cry in any helicopter design office, US, UK, France or where-ever. Each manufacturer has a different take on how to design a helicopter and how to achieve compliance, were that not the case then every helicopter would be the same. In the same vein safety cases rely on different mitigations; you can keep the pilot alive by making the crew station survivable in a 30 g impact but that is costly and heavy, alternatively you can reduce the risk of such an impact in other ways that do not necessarily make the aircraft less safe. Each design must be taken in entirety and a direct comparison between manufacturers design priorities in a single area will not result in an objective comparison.
Eurocopter will argue that the hydraulic system is designed to ensure that the rotor head cannot be overstressed. Something that in the early days of the Starflex head was paramount given the lack of operational data associated with any new technology. The fact that technology works and other manufacturer use elastomers is testament to a successful technology. In later Eurocopter models the Limit light is an indication that the head is close to its limiting stress loading, just as mast bending moment is displayed in other semi-rigid designs (BO105 for example). As an aside there are many in the Eurocopter who believe that hydraulics should run at the lowest possible pressure in order to minimise the safety implication of leaks and minimise weight, as SASLess put it.
Be in no doubt that the French take civil certification very seriously, in fact unlike the ETPS & USNTPS the EPNER Rotary Syllabus is based on US FAR certification and not European military compliance. After all Eurocopter has worked very hard to break into the North American and international markets and has had considerable success, they can’t be doing things too badly.
We are all different and from differing backgrounds, to really know your aircraft you need to understand the philosophy behind it not fight against it.
Fly safe chaps.
GA
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I don't want to turn this into a Francophobe rant either, but....
It does make one wonder when the hydrualics off control forces on the AS-350 series are huge (a technical term we use in flight test) and others (like the Australian military) found significant problems with the handling hydraulics off.
I remember instances of 'there is no problem with the helicopter, it is how you are flying it' on more than one model of helicopter from this manufacturer, which does lead one to wonder about the politics of the situation.
I think it will also be found that as a result of the cold weather re-test, several changes were made to the AS-350 series hydraulic systems.
Don't get me wrong- there are some wonderful design features in French helicopters, but like everything else in this world that is designed by man, they ain't perfect.
It does make one wonder when the hydrualics off control forces on the AS-350 series are huge (a technical term we use in flight test) and others (like the Australian military) found significant problems with the handling hydraulics off.
I remember instances of 'there is no problem with the helicopter, it is how you are flying it' on more than one model of helicopter from this manufacturer, which does lead one to wonder about the politics of the situation.
I think it will also be found that as a result of the cold weather re-test, several changes were made to the AS-350 series hydraulic systems.
Don't get me wrong- there are some wonderful design features in French helicopters, but like everything else in this world that is designed by man, they ain't perfect.
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I totally agree, and if there is a flaw in the helicopter I would certainly expect Eurocopter to sort it out. My point is that if the product flys as designed then there may be a transatlantic training/philosophy issue that needs addressing.
Lets face it no manufacturer wants the users to lose faith in their product, look what the DC10 tail engine issue did for MD sales.
GA
Lets face it no manufacturer wants the users to lose faith in their product, look what the DC10 tail engine issue did for MD sales.
GA
There are two types of servos in the 350, Dunlop and SAMM.
On the types I have flown there have only been Dunlop servos, and these are "easy" to demonstrate "jack stall" on.
Anybody out there that knows if the SAMM servos are the same, or better than the Dunlop. Dunlop seems to be more common.
CB
On the types I have flown there have only been Dunlop servos, and these are "easy" to demonstrate "jack stall" on.
Anybody out there that knows if the SAMM servos are the same, or better than the Dunlop. Dunlop seems to be more common.
CB
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Gray Area said:
"Eurocopter will argue that the hydraulic system is designed to ensure that the rotor head cannot be overstressed. Something that in the early days of the Starflex head was paramount given the lack of operational data associated with any new technology."
Balls! Stated another way, you contend that because the rotor is so very structurally inadequate (that it can be damaged by pilot maneuvers,) thus it is a good idea that they weaken the hydraulics so that the pilot instead loses control in the critical maneuvers!
How about simply building a helo strong enough to start with? And if it isn't strong enough, how about fixing it?
I think it is not a Gray Area. Your profile says that you are a RN TP. Is this something you think the UK military would buy?
"Eurocopter will argue that the hydraulic system is designed to ensure that the rotor head cannot be overstressed. Something that in the early days of the Starflex head was paramount given the lack of operational data associated with any new technology."
Balls! Stated another way, you contend that because the rotor is so very structurally inadequate (that it can be damaged by pilot maneuvers,) thus it is a good idea that they weaken the hydraulics so that the pilot instead loses control in the critical maneuvers!
How about simply building a helo strong enough to start with? And if it isn't strong enough, how about fixing it?
I think it is not a Gray Area. Your profile says that you are a RN TP. Is this something you think the UK military would buy?
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Nick
It is your contention that the rotor is "structurally inadequate" not mine.
The fact that you so strongly disagree simply supports my argument that there are cultural differences and different design philosophies.
I simply do not believe that bigger is always better. Whilst in military applications there is a greater need for redundancy, in the civil sector, particularly the light single market, weight eats payload and payload pays the bills. How many successful overweight or overly complex helos have there been in the civilian market? None, WG30 is a great example of how a complex military design does not necessarily translate into a commercial success.
FYI Single Squirrels operate at RAF Shawbury primarily in the Basic Helicopter Training role but also used for QHI training and misc tasks. Twin Squirrels are operated by 32 (The Royal) Sqn in the VIP role. So in answer to your last question yes, I think, in its time, the AS350 was a good design compromise. Although the Gazelle is much more fun!
GA
It is your contention that the rotor is "structurally inadequate" not mine.
The fact that you so strongly disagree simply supports my argument that there are cultural differences and different design philosophies.
I simply do not believe that bigger is always better. Whilst in military applications there is a greater need for redundancy, in the civil sector, particularly the light single market, weight eats payload and payload pays the bills. How many successful overweight or overly complex helos have there been in the civilian market? None, WG30 is a great example of how a complex military design does not necessarily translate into a commercial success.
FYI Single Squirrels operate at RAF Shawbury primarily in the Basic Helicopter Training role but also used for QHI training and misc tasks. Twin Squirrels are operated by 32 (The Royal) Sqn in the VIP role. So in answer to your last question yes, I think, in its time, the AS350 was a good design compromise. Although the Gazelle is much more fun!
GA
The British Army has always known about this with our Gazelles. When carrying out any hard right turn, I would always reduce collective a little bit to reduce the load on the servos, and never had a problem.
Is it a design fault? Possibly, but the aircraft concerned are fairly light aircraft with generally light control forces. Therefore the pressure within the hydraulic system only needs to be fairly low in order to control the aircraft. Hence the ease with which this pressure can be overcome.
Larger, heavier aircraft have heavier control forces therefore require higher hydaulic pressures. The Lynx hydraulic system is at around 3000psi and doesn't have a jackstall problem, but you can't really put a 300psi system on a 350/355.
Of course, all of the above could be b****ks.
Is it a design fault? Possibly, but the aircraft concerned are fairly light aircraft with generally light control forces. Therefore the pressure within the hydraulic system only needs to be fairly low in order to control the aircraft. Hence the ease with which this pressure can be overcome.
Larger, heavier aircraft have heavier control forces therefore require higher hydaulic pressures. The Lynx hydraulic system is at around 3000psi and doesn't have a jackstall problem, but you can't really put a 300psi system on a 350/355.
Of course, all of the above could be b****ks.
Nick,
Remind me of my hydraulics theory....something about pressure...ram diameter....and cams....could we not get the same snoot out of two systems with different pressures but different sized components? The compromise is not just operating pressure but the sheer size and weight of the rams we would need....low pressure on the bulldozers....but really huge heavy metal bits...and higher pressures on smaller lighter bits?
Say what you want to about yer 350's.....if the rotor forces can overpower the flight control hydraulics....it seems a bit daft to me to say it is okay so long as you are aware of it. When I holler whoa....I want some thing to move besides my butt in the seat as I slide forward while pulling back on the cyclic.
Seems to me...if I put the controls to the mechanical stop...collective full up....cyclic full over and forward...and pedals full right (in the french model)....I want the hydraulic system to cope with that....full flow, constant pressure, and no feedback or "jacque stalling".
Anything less than that is sub-standard engineering design.
Before anyone says anything about...."come on...no one does that....they have not seen me flying an underslung load!" I scare myself sometimes.
Remind me of my hydraulics theory....something about pressure...ram diameter....and cams....could we not get the same snoot out of two systems with different pressures but different sized components? The compromise is not just operating pressure but the sheer size and weight of the rams we would need....low pressure on the bulldozers....but really huge heavy metal bits...and higher pressures on smaller lighter bits?
Say what you want to about yer 350's.....if the rotor forces can overpower the flight control hydraulics....it seems a bit daft to me to say it is okay so long as you are aware of it. When I holler whoa....I want some thing to move besides my butt in the seat as I slide forward while pulling back on the cyclic.
Seems to me...if I put the controls to the mechanical stop...collective full up....cyclic full over and forward...and pedals full right (in the french model)....I want the hydraulic system to cope with that....full flow, constant pressure, and no feedback or "jacque stalling".
Anything less than that is sub-standard engineering design.
Before anyone says anything about...."come on...no one does that....they have not seen me flying an underslung load!" I scare myself sometimes.
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Gray Area,
Inadequate is the definition of a rotor that can be bent by the pilot during relatively normal maneuvers. This is not hypothetical, it is (according to you) why the hydraulics were weakened to the point where the control of the helo is sometimes removed. This is not cultural, it is engineering, and we are not dealing with beliefs, we are dealing with hardware.
Sasless,
The issue is not hydraulic, it is rotor structure. The Squirrel can bend and break its rotor if handled near stall. Weakening the hydraulics is the "fix" according to Gray Matter.
MightyGem, The thread was about another model, not the Gazelle. Read the first several posts to see. In my understanding, the Gazelle is very much less prone to jack stall than the 350 series, which has relatively normal maneuvers causing loss of control (a cultural problem, according to Gray Area.) I agree, because if control is lost close to the ground, the pilot will end up looking like cultured yogurt.
Also, the hydraulic pressure is applied to a piston area, so that almost any hydraulic pressure can be used, if the piston is properly sized. It is not a pressure issue, it is a design issue, where the designer has chosen a servo that gives up during some maneuvers, and the pilot becomes a passenger as a result.
Inadequate is the definition of a rotor that can be bent by the pilot during relatively normal maneuvers. This is not hypothetical, it is (according to you) why the hydraulics were weakened to the point where the control of the helo is sometimes removed. This is not cultural, it is engineering, and we are not dealing with beliefs, we are dealing with hardware.
Sasless,
The issue is not hydraulic, it is rotor structure. The Squirrel can bend and break its rotor if handled near stall. Weakening the hydraulics is the "fix" according to Gray Matter.
MightyGem, The thread was about another model, not the Gazelle. Read the first several posts to see. In my understanding, the Gazelle is very much less prone to jack stall than the 350 series, which has relatively normal maneuvers causing loss of control (a cultural problem, according to Gray Area.) I agree, because if control is lost close to the ground, the pilot will end up looking like cultured yogurt.
Also, the hydraulic pressure is applied to a piston area, so that almost any hydraulic pressure can be used, if the piston is properly sized. It is not a pressure issue, it is a design issue, where the designer has chosen a servo that gives up during some maneuvers, and the pilot becomes a passenger as a result.
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The difference is that, with the gazelle, it's reproducible in similar circumstances almost every time, whereas in the 350 it can catch you any time. Thus, with the gazelle you could call it a limitation, and with the 350 a design fault.
Phil
Phil
Rumour network, indeed!
Shawn, what's this about the Australian military finding significant problems with the hydraulics off handling?
AS350Bs and BAs were operated for years with few dramas and lots of practice hydraulics off landings. Very predictable and easy to handle, once the 'fault' was detected and the hydraulic isolate switch operated.
'Jack stall' was a discussed and demonstrated phenomenon in training courses, but I never experienced it or heard of it being a problem in normal aircraft use, and they got a bit of a caning manoeuvre-wise, much more so than the teetering-head types that were also used by the Aussie forces, because they could take it!
To paint this phenomenon (which certainly does exist but is only in my experience a problem if you like to throw the controls around like a gorilla, although to be fair, I don't know much about how they respond in extremely cold conditions) seems to be hysterical over-reaction based on a lot of hearsay.
Over...
Shawn, what's this about the Australian military finding significant problems with the hydraulics off handling?
AS350Bs and BAs were operated for years with few dramas and lots of practice hydraulics off landings. Very predictable and easy to handle, once the 'fault' was detected and the hydraulic isolate switch operated.
'Jack stall' was a discussed and demonstrated phenomenon in training courses, but I never experienced it or heard of it being a problem in normal aircraft use, and they got a bit of a caning manoeuvre-wise, much more so than the teetering-head types that were also used by the Aussie forces, because they could take it!
To paint this phenomenon (which certainly does exist but is only in my experience a problem if you like to throw the controls around like a gorilla, although to be fair, I don't know much about how they respond in extremely cold conditions) seems to be hysterical over-reaction based on a lot of hearsay.
Over...
"Just a pilot"
Collective Bias:
The present aircraft I'm in is the only one I've ever flown any significant time in that has SAMM servoes. There's no apparent difference as to jack stall.
The SAMMs have other bad habits unique to the type- they "kick" when the pressure comes up. It's in the RFM....
The present aircraft I'm in is the only one I've ever flown any significant time in that has SAMM servoes. There's no apparent difference as to jack stall.
The SAMMs have other bad habits unique to the type- they "kick" when the pressure comes up. It's in the RFM....
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First of all we need a EC tech rep on the line who actually knows what is going on - anyone, anyone?? Ken where are ya?
Secondly, has this phenomena ever led to loss of control causing an accident? Not that I am aware of, so is it really a design flaw or structural inadequacy? - Most incidences of jack stall that I have heard about have involved aircraft close to MAUW and close to Vne. Are those "normal" operating conditions?? Generally accepted procedure is to back off collective and cyclic, and control is regained. Why would you want to be in a full speed cyclic descent anyway????
And Nick - is it really a structural issue as opposed to hydraulic??
Because if this is the case, wouldn't one expect to see cracked blades, splintered stars, or bent pitch links?? I have never heard of this happening as a result of jack stall.
The aircraft has been designed to be flown with out hydraulics if necessary, and is controllable at speeds of up to 70 knots, without damage to the rotor system. If you are incredibly strong and willing, probably more - you just aren't getting any help from the hydraulics.
I believe it is more of a 'characteristic', that perhaps should be investigated a little more thoroughly, same as LTE for the 206 series, the T/R problem with the 407, or the phenomena of mast bending in the 204,205,and 412 that led to the RIN and limited torque settings in the RFM limitations.
Is it necessary or practical to change a major design feature such as the hydraulic system?? I think I can answer for Eurocopter.
Knowledge and training I think are the key to this "issue".
After all its not what you know that will kill ya...........
Arm out the window...I'm with you on this one
Secondly, has this phenomena ever led to loss of control causing an accident? Not that I am aware of, so is it really a design flaw or structural inadequacy? - Most incidences of jack stall that I have heard about have involved aircraft close to MAUW and close to Vne. Are those "normal" operating conditions?? Generally accepted procedure is to back off collective and cyclic, and control is regained. Why would you want to be in a full speed cyclic descent anyway????
And Nick - is it really a structural issue as opposed to hydraulic??
The issue is not hydraulic, it is rotor structure. The Squirrel can bend and break its rotor if handled near stall. Weakening the hydraulics is the "fix" according to Gray Matter.
The aircraft has been designed to be flown with out hydraulics if necessary, and is controllable at speeds of up to 70 knots, without damage to the rotor system. If you are incredibly strong and willing, probably more - you just aren't getting any help from the hydraulics.
I believe it is more of a 'characteristic', that perhaps should be investigated a little more thoroughly, same as LTE for the 206 series, the T/R problem with the 407, or the phenomena of mast bending in the 204,205,and 412 that led to the RIN and limited torque settings in the RFM limitations.
Is it necessary or practical to change a major design feature such as the hydraulic system?? I think I can answer for Eurocopter.
Knowledge and training I think are the key to this "issue".
After all its not what you know that will kill ya...........
Arm out the window...I'm with you on this one
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Inadequate is the definition of a rotor that can be bent by the pilot during relatively normal maneuvers.
All that aside this is a not a real issue. The recovery from jack-stall is simple, ease the manoeuvre. It is not a safety issue if the pilot flys within the capability of the aircraft - just like any aircraft there are manoeuvre boundarys.
The argument from SASLess that he should be able to "put the controls to the mechanical stop...collective full up....cyclic full over and forward...and pedals full right" is frankly daft, at low speed you might get away with it but in the cruise or faster,with the contol authority of a semi-rigid head you would end up in a very hazardous situation.
GA