AS350 Astar / AS355 Twinstar [Archive Copy]
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I've experienced the famous Astar "Servo Transparency" in a 350 B model.
The flight conditions were:
AUW 4200 lbs (GW 4300)
80 Kts
4,000'
+30c
Light winds
15 degree LH bank
No aggressive flight, no sudden flight control movements, the controls just simply froze when circling a spot fire.
The flight conditions were:
AUW 4200 lbs (GW 4300)
80 Kts
4,000'
+30c
Light winds
15 degree LH bank
No aggressive flight, no sudden flight control movements, the controls just simply froze when circling a spot fire.
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Apologies, perhaps I was using the term "quick stop" too loosely , I defer to your greater knowledge.
What I actually meant was going at max chat then stopping as quickly as one could, I would describe this as extreme handling (as described by Thomas Coupling) rather than gentle handling associated with slowing an aircraft under normal circumstances.
It all depends on how quick one's quick stop is, don'cha find?
Apologies, perhaps I was using the term "quick stop" too loosely , I defer to your greater knowledge.
What I actually meant was going at max chat then stopping as quickly as one could, I would describe this as extreme handling (as described by Thomas Coupling) rather than gentle handling associated with slowing an aircraft under normal circumstances.
It all depends on how quick one's quick stop is, don'cha find?
Last edited by A10 Thundybox; 2nd Dec 2004 at 02:19.
A10, in a normal quick stop you lower the collective to stay level, that in itself is going to stop any chance of a problem, plus I think the g loading required is normally well above 2-3 g.
Lu, are you saying AS350s have belt driven hydraulic pumps? Not disputing it as have no knowledge either way, just curious that they are not MGB driven directly.
Lu, are you saying AS350s have belt driven hydraulic pumps? Not disputing it as have no knowledge either way, just curious that they are not MGB driven directly.
212 Man, yes the AS-350 has belt driven hyd pump. Looks like something that came from a Citroen or Peugeot (French cars!)
I have never really heard of problems with the belt or hyd pump in the AS-350 other than the aforementioned hydraulic jack stall. Never experinced it myself flying the series but maybe thatīs because I fly them very gently.
I always heard that the controls really "freeze" on you rather than it feeling like a hydraulic boost failure.
However I have from a Dauphin AS-365 pilot that he has also experienced momentary stuck controls in extreme handling situations. I guess that you really have stuck controls if you loose dual hyd boost in the AS-365 and just really heavy controls in the AS-350.
This is why I prefer to: "FLY SMART, FLY BELL"
I have never really heard of problems with the belt or hyd pump in the AS-350 other than the aforementioned hydraulic jack stall. Never experinced it myself flying the series but maybe thatīs because I fly them very gently.
I always heard that the controls really "freeze" on you rather than it feeling like a hydraulic boost failure.
However I have from a Dauphin AS-365 pilot that he has also experienced momentary stuck controls in extreme handling situations. I guess that you really have stuck controls if you loose dual hyd boost in the AS-365 and just really heavy controls in the AS-350.
This is why I prefer to: "FLY SMART, FLY BELL"
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Jack Stall was taught to AAC students so that they would know in which situations JS could occur. The severity and onset of JS differed with each a/c and one could experiment to discover the limiting point, at and from a safe recovery situation.
Later flying the AS350 series I discovered that if the cyclic inputs were violent enough the cyclic would baulk and go into a sort of JS situation. My thoughts are that if one moves the cyclic fast enough you can feel that the fluid is not moving as fast as your input. OR it could well be that the pitch changing just cannot keep abreast of the inputs. The hyd pump is designed to provide adequate pressure and flow for all normal requirements and the system is so designed to protect the a/c from extreems of G.
Later flying the AS350 series I discovered that if the cyclic inputs were violent enough the cyclic would baulk and go into a sort of JS situation. My thoughts are that if one moves the cyclic fast enough you can feel that the fluid is not moving as fast as your input. OR it could well be that the pitch changing just cannot keep abreast of the inputs. The hyd pump is designed to provide adequate pressure and flow for all normal requirements and the system is so designed to protect the a/c from extreems of G.
QHI course, 1976. Staff used to demo jackstall on the Gazelle in a straight descent and with 60 degrees AOB to the left. My instructor, bless him, decided to do this to the right. He pulled hard from 150 odd knots. The next thing I knew was that Newport and my chinagraphs were above my head. Aircraft flipped right under. Next day, asked for a change of instructor! Jack stall on Gazelles has certainly written off a few at low level - usually when pulling hard in a descending right hand turn (but you have to pull very hard). Never had a problem in 2000hrs with the 350.
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my ignorance
please excuse my ignorance here, but I have always wondered this:
Are the cyclic & collective in a 350/355 purely a hydraulic linkage (like the brakes in a car) rather than an assisted mechanical linkage (say like power steering) as in a B206?
In the latter case jack-stall couldn't occur could it?
thanks!
Are the cyclic & collective in a 350/355 purely a hydraulic linkage (like the brakes in a car) rather than an assisted mechanical linkage (say like power steering) as in a B206?
In the latter case jack-stall couldn't occur could it?
thanks!
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The symptoms of jack stall are caused by blade stall pitch change link loads pushing through the servo, not by fluid supply limits. The hydraulic pressure warning lights would illuminate if the pump capacity were reached, since the pump could not keep up the system pressure if the fluid demand were too high.
The blades are always fighting the servos, and the pitch link loads get progressively more severe as the rotor is progressively stalled more and more (blade stall is not a flip of a switch, stalled or not-stalled).
as A10 Thundybox said, "Any aircraft will have an envelope which it is dangerous to be outside (even if you feel you can fly well beyond it in terms of your own abilities)" The real question is not that helos have envelope limits, but rather if those limits occur in normal required maneuvers. Military regulations forbid jack stall anywhere near the operational envelope, and most companies strictly avoid allowing it to occur, even with one failed hydraulic system (down to a single servo).
The consensus here seems to be that the situation in jack stall of the 350 is manageable, the propensity is predictible, and the outcome is not catastrophic. Without doubt, the French authorities examined the jack stall thoroughly in the certification tests and deemed it acceptable. Were any accident to be blamed on jack stall, I would recommend re-thinking that attitude!
Frankly, I know that Sikorsky would deem that behavior unacceptable, and require stronger servos (and swashplates, if the parts could be bent in maneuver!) I believe it is a governing philosophy of US manufacturers (certianly Sikorsky) that the controls must not lock up and the rotating control system not be damaged by virtually any maneuver the pilot can conceive, and it is a strong requirement in US Military regulations. I am also quite certain that the FAA would not approve such behavior, absent the bi-lateral agreements that make it necessary to overlook some things from foreign certifications.
The blades are always fighting the servos, and the pitch link loads get progressively more severe as the rotor is progressively stalled more and more (blade stall is not a flip of a switch, stalled or not-stalled).
as A10 Thundybox said, "Any aircraft will have an envelope which it is dangerous to be outside (even if you feel you can fly well beyond it in terms of your own abilities)" The real question is not that helos have envelope limits, but rather if those limits occur in normal required maneuvers. Military regulations forbid jack stall anywhere near the operational envelope, and most companies strictly avoid allowing it to occur, even with one failed hydraulic system (down to a single servo).
The consensus here seems to be that the situation in jack stall of the 350 is manageable, the propensity is predictible, and the outcome is not catastrophic. Without doubt, the French authorities examined the jack stall thoroughly in the certification tests and deemed it acceptable. Were any accident to be blamed on jack stall, I would recommend re-thinking that attitude!
Frankly, I know that Sikorsky would deem that behavior unacceptable, and require stronger servos (and swashplates, if the parts could be bent in maneuver!) I believe it is a governing philosophy of US manufacturers (certianly Sikorsky) that the controls must not lock up and the rotating control system not be damaged by virtually any maneuver the pilot can conceive, and it is a strong requirement in US Military regulations. I am also quite certain that the FAA would not approve such behavior, absent the bi-lateral agreements that make it necessary to overlook some things from foreign certifications.
Having reviewed my retirement portfolio last night....and considering my advanced age...seems to me....knowing the 350/355/Gazelle/365 probably the 155 can engage in recalcitrant control behaviour (....that is locking up the cyclic at a minimum....or doing its very own thing.....) which might be counter to what I (the Pilot-in-Command) desires at that moment and phase of flight.....maybe sticking to non-French gear is the answer. There's been times that I would have been much better off leaving things alone....but I think I want that to be my choice rather than some Vin swilling Gaul version of Lu.
Nick's post certainly convinced me of the advantages of riding Igor Iron.
Nick's post certainly convinced me of the advantages of riding Igor Iron.
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If what you guys are talking about is "servo-transperency" in the 350 series, I have some info. I see little bits and pieces in some of the posts above that are involved. The easiest way that I've seen to get into this phenomenon is abrupt control inputs. The higher the gross weight/airspeed, the less aggressive the inputs have to be in order to cause it. I've never had the controls actually "lock-up" on me. You can tell when you are getting close to the onset. The controls start to feel a little heavier. If you stop/decrease the input you were doing at the time, it won't progress any further. The only time I've ever felt it was when performing a hard turn at high airspeed close to max gross.
Now as far as the system itself goes, from what Eurocopter told me, its isn't a matter of the pump lacking the capacity to overcome the high control loads. The pump has the capability. It is intentionally limited to a certain boost pressure (I can't remember the exact figure in bar) as to not damage any of the components in the head.
(212man It is belt driven off of the input shaft to the m/r transmission) The belt driven pump doesn't inspire a hell of a lot of confidence in people (especially the older style green belt) but I've never really heard about as many problems with the belts. I've heard about more failures of the splines inside the pump than actual belt failures.
At high airspeed the contol loads are very high, probobly too high to be manageble at close to Vne. For this reason the aircraft has a servo accumulators is that they allow supply sufficient hydraulic pressure for inputs to be made to bring the aircraft back to about 60 kts in the event of a hydraulic failure. Other than that, it isn't much different in operation from the Bells.
Hope this helps. BTW alll this information is based on the 350 B2/B3, I don' t have any experience with the earlier models.
Now as far as the system itself goes, from what Eurocopter told me, its isn't a matter of the pump lacking the capacity to overcome the high control loads. The pump has the capability. It is intentionally limited to a certain boost pressure (I can't remember the exact figure in bar) as to not damage any of the components in the head.
(212man It is belt driven off of the input shaft to the m/r transmission) The belt driven pump doesn't inspire a hell of a lot of confidence in people (especially the older style green belt) but I've never really heard about as many problems with the belts. I've heard about more failures of the splines inside the pump than actual belt failures.
At high airspeed the contol loads are very high, probobly too high to be manageble at close to Vne. For this reason the aircraft has a servo accumulators is that they allow supply sufficient hydraulic pressure for inputs to be made to bring the aircraft back to about 60 kts in the event of a hydraulic failure. Other than that, it isn't much different in operation from the Bells.
Hope this helps. BTW alll this information is based on the 350 B2/B3, I don' t have any experience with the earlier models.
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Great thread!
One thing about the FAA and US manufacturers...
NickLappos said
Isn't a U.S. manufacturer together with FAA responsible for introducing us to the term "LTE" to cover a design-compromise, or am I completely wrong? -That happens alot 
I know that Nick is partially referring to the military but everything flying on the civilian market is a compromise, and those are beeing made by every manufacturer, no matter where they're based.
Well, I'm learning alot anyway.
Cheers!
/2beers
One thing about the FAA and US manufacturers...
NickLappos said
I believe it is a governing philosophy of US manufacturers
I am also quite certain that the FAA would not approve such behavior, absent the bi-lateral agreements that make it necessary to overlook some things from foreign certifications.
I know that Nick is partially referring to the military but everything flying on the civilian market is a compromise, and those are beeing made by every manufacturer, no matter where they're based.
Well, I'm learning alot anyway.
Cheers!
/2beers
"Just a pilot"
If servo transparency and jack stall are being used interchangeably and incorrectly, could somebody explain the difference?
As to servo transparency in the 350/355-
In more foolish days, I'd fly these like ordinary state-side helos.
Flown vigorously- Say, at better than cruise speed- load a little "G," like a zoom and control RPM with collective, add a turn- and the normally impressively agile aircraft's cyclic will stop moving in one direction( or you'll get a "limit" light in a 355). A little less turn (left, last time), or a little less collective and you get some back. Load it again, even with less pitch- and there's the wall, once more. It ruins your plans for hotting up the dog. If there's something that you were turning or zooming to avoid, well- too bad- you're not going to push the stick in that direction, right now. I hope it's a forgiving obstruction, or you left an "out."
I've never encountered this in any other mode besides playing around. And never in any other helo.
As to servo transparency in the 350/355-
In more foolish days, I'd fly these like ordinary state-side helos.
Flown vigorously- Say, at better than cruise speed- load a little "G," like a zoom and control RPM with collective, add a turn- and the normally impressively agile aircraft's cyclic will stop moving in one direction( or you'll get a "limit" light in a 355). A little less turn (left, last time), or a little less collective and you get some back. Load it again, even with less pitch- and there's the wall, once more. It ruins your plans for hotting up the dog. If there's something that you were turning or zooming to avoid, well- too bad- you're not going to push the stick in that direction, right now. I hope it's a forgiving obstruction, or you left an "out."
I've never encountered this in any other mode besides playing around. And never in any other helo.
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The terms can confuse you. We call it servo stalling, or jack stalling but "'Transparency" is a good ephimistic way to describe the same thing - the servo reaches its maximum force and then passes the forces to the crew (becomes "transparent").
The idea that this is used to protect the rotor components from flight loads in certainly novel! We always thought it was a good idea to make them strong enough to not bend!
Frankly, the FAA "Proof and Operations" tests on the helicopter controls forces the rotors to be strong enough.
Regarding LTE as an example of the pot and kettle calling each other black, I don't think you would find me supporting that either, much to Sultan's ire!
The idea that this is used to protect the rotor components from flight loads in certainly novel! We always thought it was a good idea to make them strong enough to not bend!
Frankly, the FAA "Proof and Operations" tests on the helicopter controls forces the rotors to be strong enough.
Regarding LTE as an example of the pot and kettle calling each other black, I don't think you would find me supporting that either, much to Sultan's ire!
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I wish I had had the benefit of this thread some years ago, cos it would have saved me putting some embarasing photos in my flying album.
Having sadly been on the receipt of "jack Stall" in a French made heli, there is one aspect to this topic that has not been considered. If you are taught that if you place the aircraft in a 160kt dive and pull back harshly on the cyclic it will 'js'. Why bother with all that when it does the same thing at 30deg AOB/5deg nose up and 60kts in a RH decending turn.
If I could offer any advice it would be that the 'envelope' is something of a vague line that exists in the grey areas only probed by the BOI. Dont be under the illusion that your handling has to be harsh for the aircraft to Jack stall, and the feeling through the sticks can be minimal when you are in full blown 'js'.
All the above is naturally 'IMHO'
Having sadly been on the receipt of "jack Stall" in a French made heli, there is one aspect to this topic that has not been considered. If you are taught that if you place the aircraft in a 160kt dive and pull back harshly on the cyclic it will 'js'. Why bother with all that when it does the same thing at 30deg AOB/5deg nose up and 60kts in a RH decending turn.
If I could offer any advice it would be that the 'envelope' is something of a vague line that exists in the grey areas only probed by the BOI. Dont be under the illusion that your handling has to be harsh for the aircraft to Jack stall, and the feeling through the sticks can be minimal when you are in full blown 'js'.
All the above is naturally 'IMHO'
Since the question was if any other aircraft have this kind of behaivor, yes sort of the same, the Sikorsky S-55T (sorry Nick). It might not really be a jack stall I think, more like a very veak servo system to very heavy blades, or it might be same thing but just feel diffrent to the pilot.
The S-55T is VERY easy to get in to a bladestall situation if not adhering to the VNE table and follow the WAT reduction closely, especially in mountain flying with gusts. But before it bladestalls on you it starts letting control forces through the servos to warn you that you are getting close. And on the S-55 you can't miss if you loose the servos.
I have never had the questionable "honour" to experience a blade stall in any helicopter, and I hope I never will, but I have for sure had some frightning cases of control forces through the servos on the S-55. I know of at least 3 cases of bladestall on the S-55 at our company in the past, and all went well.
The T version is flying with roughly 20 rpm higher MR rpm than with piston engine (199 to 219), so i wonder how bad the original was. But I guess it did not have the power to reach VNE with max gross as the T version does. In the RFM on the original S-55 it is actually a great deal of text regarding how to avoid and how to cope with bladestall.
Good ol days
CB
The S-55T is VERY easy to get in to a bladestall situation if not adhering to the VNE table and follow the WAT reduction closely, especially in mountain flying with gusts. But before it bladestalls on you it starts letting control forces through the servos to warn you that you are getting close. And on the S-55 you can't miss if you loose the servos.
I have never had the questionable "honour" to experience a blade stall in any helicopter, and I hope I never will, but I have for sure had some frightning cases of control forces through the servos on the S-55. I know of at least 3 cases of bladestall on the S-55 at our company in the past, and all went well.
The T version is flying with roughly 20 rpm higher MR rpm than with piston engine (199 to 219), so i wonder how bad the original was. But I guess it did not have the power to reach VNE with max gross as the T version does. In the RFM on the original S-55 it is actually a great deal of text regarding how to avoid and how to cope with bladestall.
Good ol days
CB
quote
Nick's post certainly convinced me of the advantages of riding Igor Iron.
Did you really need that post to be convinced ?
I know of some people who would have been more than happy to ride a jackass Star when a blade of the Igor Iron they were on decided to part.
Don't want to start another Airbus vs Boeing or Bell / SK vs EC, though.
Cheers
ATN
Nick's post certainly convinced me of the advantages of riding Igor Iron.
Did you really need that post to be convinced ?
I know of some people who would have been more than happy to ride a jackass Star when a blade of the Igor Iron they were on decided to part.
Don't want to start another Airbus vs Boeing or Bell / SK vs EC, though.
Cheers
ATN
ATN,
If you are referring to the Bristow S-76 that shed the blade after it was put back into service after being struck by lightning....I would suggest one might look towards the operator that trashed almost every moving part on the Lightning struck aircraft and after having done that for safety sakes....then sent the blades off for inspection and then put it back into service. The 76 had two other blade failures in its very earliest hours....and knew one of the guys killed in the second accident (a man I respected highly in all regards I might add.)
The same company lost Bell aircraft to blade issues, Wessex aircraft to unknown causes....but these are all catastrophic failures.
My point is I have concerns about knowingly flying a machine that in "normal" flight can have control problems. That does give reason for pause or should anyway. Logically, it does not follow that one would wish to fly a helicopter whose controls "lock" up in the cruise....or while maneuvering near the ground.
If you are referring to the Bristow S-76 that shed the blade after it was put back into service after being struck by lightning....I would suggest one might look towards the operator that trashed almost every moving part on the Lightning struck aircraft and after having done that for safety sakes....then sent the blades off for inspection and then put it back into service. The 76 had two other blade failures in its very earliest hours....and knew one of the guys killed in the second accident (a man I respected highly in all regards I might add.)
The same company lost Bell aircraft to blade issues, Wessex aircraft to unknown causes....but these are all catastrophic failures.
My point is I have concerns about knowingly flying a machine that in "normal" flight can have control problems. That does give reason for pause or should anyway. Logically, it does not follow that one would wish to fly a helicopter whose controls "lock" up in the cruise....or while maneuvering near the ground.
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Having only flown in the back & front of helicopters for nearly 50 years, I seem to have missed out on all these jacking problems you guys seem to have had.
The closest to it I can recall is in a Bell 47 & 206 when flying in mountainous areas parallel to a cliff face in very windy conditions, maybe 30 + kts, a gust has hit me towards the cliff face and I could not move the cyclic in the opposite direction. The cure was to move the cyclic towards the cliff to unjam the cyclic and then put in opposite cyclic. Is this what you mean by jack stall? I just considered it as running out of hydraulic control.
I can't imagine this in normal cruise unless you are flying through a massive thunderstorm when anything can happen.
The closest to it I can recall is in a Bell 47 & 206 when flying in mountainous areas parallel to a cliff face in very windy conditions, maybe 30 + kts, a gust has hit me towards the cliff face and I could not move the cyclic in the opposite direction. The cure was to move the cyclic towards the cliff to unjam the cyclic and then put in opposite cyclic. Is this what you mean by jack stall? I just considered it as running out of hydraulic control.
I can't imagine this in normal cruise unless you are flying through a massive thunderstorm when anything can happen.
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ATN,
I certainly don't want this to become an us and them thing. I believe that the servo strength of virtually every helicopter I know, civil and military, does not show any jack stall in regions anywhere close to the operating envelope. This 350 discussion is teaching me plenty. I do believe that many/most/all of the helicopter world (British, US, Russian, etc) has grown past the need for the pilot to worry about having his helo take control away from him in most maneuvers.
Collective Bias, that S-55 condition you mention is most certainly stall related servo load, but happens at or beyond Vne, and to an aircraft that is arguably three generations old. If jack stall was only experienced at Vne in 1949 (S-55 first flight Nov, 1949!), one would have hoped we stayed the same or got better in 55 years!
I did the servo adequacy tests on the S-76, and took a loaded S-76 to Vne, turned off one servo system, then maneuvered to 1.7 g's to clear the aircraft for certification. Not a hint of jack stall, believe me.
I certainly don't want this to become an us and them thing. I believe that the servo strength of virtually every helicopter I know, civil and military, does not show any jack stall in regions anywhere close to the operating envelope. This 350 discussion is teaching me plenty. I do believe that many/most/all of the helicopter world (British, US, Russian, etc) has grown past the need for the pilot to worry about having his helo take control away from him in most maneuvers.
Collective Bias, that S-55 condition you mention is most certainly stall related servo load, but happens at or beyond Vne, and to an aircraft that is arguably three generations old. If jack stall was only experienced at Vne in 1949 (S-55 first flight Nov, 1949!), one would have hoped we stayed the same or got better in 55 years!
I did the servo adequacy tests on the S-76, and took a loaded S-76 to Vne, turned off one servo system, then maneuvered to 1.7 g's to clear the aircraft for certification. Not a hint of jack stall, believe me.
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The hydraulic belt on the AS350 has caused some problems. One was lost in the GOM a few years back when the belt broke and the pilot tried to land on an offshore platform. Sadly, there were no survivors. I have around a thousand hours in the AS350D, and always had mixed feelings - lots of fuel, plenty of speed and comfort, but if things went wrong, they went badly wrong.