PPRuNe Forums

Go Back   PPRuNe Forums > Aircrew Forums > Rotorheads
Forgotten your Username/Password?

Rotorheads A haven for helicopter professionals to discuss the things that affect them


Reply
 
Thread Tools
Old 3rd Dec 2001, 23:46   #21 (permalink)
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

As far as I remember Servo transparency is a nice name for "jack stall". It is common in smaller helicopters with parallel jacks. AS350, SA340/341/342 Gazelles etc. Jack Stall is often included as part of the flight envelope definition. Sorry, I don't know of any repositories of knowledge but I'll ask my contact at Marignane.

PS Lu, thanks for the input, I'm sure the Eurocopter Bureau D'Etudes will jump for joy.
Grey Area is offline   Reply
Old 5th Dec 2001, 10:19   #22 (permalink)
 
Join Date: Sep 2001
Location: CA
Posts: 828
Post

An AS350 crashed in NZ in the early 90's. The pilot was in a high speed decending turn when 'Jackstall' occurred.
My understanding is the huge feedback forces generated by a high 'G' manouver cause the controls to lock.
Correct?
Steve76 is offline   Reply
Old 5th Dec 2001, 23:41   #23 (permalink)
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

1. Servo Transparency - Discussed with a Eurocopter flight tester engineer today. The phrase is a direct translation from French - To a Frenchman it makes perfect sense, interestingly another ALAT pilot I spoke to translated the French phrase immediately to "Jack Stall" (heavy French accent required). The implication in French is that the jack appears to be no longer there.

2. Steve you are just about there. The point is that the aerodynamic loads fed back by the rotor system to the flying controls exceed the maximum force that the servo-jacks can exert. In effect you now have manual flying controls which, given that the jacks are stronger that you (I assume ) is impossible to move in the desired direction. If you ease the loading the problem disappears, ie reduce the manoeuvre or lower the collective. Unfortunately as you are most likely at the edge of the flight envelope if you are close to the ground the consequences can be disasterous.

This is not so much a design flaw as a flight limitation, and common on small helicopters with parallel flying controls.
Grey Area is offline   Reply
Old 6th Dec 2001, 00:46   #24 (permalink)

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,117
Question

To: Grey Area

“This is not so much a design flaw as a flight limitation, and common on small helicopters with parallel flying controls”.

You’re statement above is basically the same as the cautions placed in the Robinson POH which restricts the pilot from flying out of trim and from sideslipping the helicopter as something bad will happen if the pilot violates the cautionary statements in the POH.

Both the restrictions on the Aerospatiale products and the Robinson are to compensate for a design flaw.

From what I understand the Aerospatiale helicopter in this discussion has a hydraulic system that could be considered low pressure by comparison to the systems used in other helicopters. I would assume that the pump used is a constant displacement pump and I would assume that because of the existence of the jack stall problem it does not have an accumulator and if it does, then it is of low displacement.

What I visualize happening is that under certain maneuvering conditions there is a high demand on the hydraulic supply system. This causes a pressure drop in the system and if the pressure drop is sufficient, the bypass valves in the servos open and the jack is now part of the control linkage and the feedback forces can pass through the jack. Upon the cessation of the demand on the hydraulic system the pressure builds up and the bypass valves close and the jack is now operational.

This is a killer just like having high flapping loads on the Robinson. The protection of the pilot rests on a few words in the respective POHs and not by proper system design.


Regarding the French logic in using words to describe something when I was working in the Service Department at Sikorsky we got a telex from the French Forces in Algeria asking something about the dirty link. This caused some confusion so we asked them to identify the part number under discussion. The number provided was for the sloppy link on the primary servos. It seems that the French did not have a word for sloppy so they figured that sloppy equated to dirty.

[ 05 December 2001: Message edited by: Lu Zuckerman ]
Lu Zuckerman is offline   Reply
Old 6th Dec 2001, 01:25   #25 (permalink)
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

Lu,

What you invisage happening is not what I described and is, frankly, rubbish. The feedback forces exert a force equal to the maximum available from the jack so it can no longer move. It has nothing to do with bypass valves.

"From what I understand the Aerospatiale helicopter in this discussion has a hydraulic system that could be considered low pressure by comparison to the systems used in other helicopters."

Really? Its the same as many production helicopters.

"Both the restrictions on the Aerospatiale products and the Robinson are to compensate for a design flaw."

Helicopters are designed to do a job. If you exceed the design spec or limits you lose your right to sue. The helicopter concerned conforms to FAR 27, I would suggest you back your comments up with hard facts before making sweeping and rather slanderous allegations about things you patently understand close to nothing.

Why not get a copy of FAR 27, or even AC27 (flight test specs), read it then sue the FAA because you don't like what it says?
Grey Area is offline   Reply
Old 6th Dec 2001, 02:49   #26 (permalink)
 
Join Date: Sep 2001
Location: CA
Posts: 828
Wink

Grey Area, as soon as I read your last paragraph I could hear Lu's fingers tapping.
It is also unfair to the Astar/squirell/AS350 to be mentioned in the same context as a Robinson. A far superior aircraft.
Steve76 is offline   Reply
Old 6th Dec 2001, 02:50   #27 (permalink)

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,117
Post

To: Grey Area

“What you invisage happening is not what I described and is, frankly, rubbish. The feedback forces exert a force equal to the maximum available from the jack so it can no longer move. It has nothing to do with bypass valves”.

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. To do this it must be properly designed and be supported by an adequate supply system. If you have what you call jack stall then the system is not designed properly. In order to do this; the hydraulic system must be able to support the maximum demands placed on the system by manipulation of the controls. That is why hydraulic systems incorporate an accumulator large enough to take care of these maximum demands while the pump catches up. Most new hydraulic systems do not incorporate an accumulator but use a constant speed variable displacement pump. In a few cases the system may incorporate both an accumulator and a constant speed variable displacement pump.

Again, I state that if you get jack stall then the system is not designed correctly and I don’t give a damn what Aerospatial thinks.

I have seen helicopter flight control systems that incorporate constant speed variable displacement pumps and in monitoring the pressure gage the system pressure can drop by one third or more when the cyclic stick is moved in a circle or if the collective is raised and lowered. When the movement stopped the pressure immediately went up to the green line. At two thirds rated pressure the system could still resist the maximum design feedback forces. If this type of pump can’t keep up with the heavy demand then think about what a constant displacement pump does relative to system pressure when a high demand is placed on it.

On a Sikorsky servo on the H-34, H-37 and H-19 the bypass valve would open at pressures lower than 750 pounds PSI. Please tell me what the hydraulic pressure is on the AS-350 and tell me at what pressure the bypass valves on the servo open up?
Lu Zuckerman is offline   Reply
Old 6th Dec 2001, 05:42   #28 (permalink)
 
Join Date: Jul 2001
Location: the cockpit
Posts: 1,070
Cool

To GA: I agree.

To Lu: your sweeping statements are why I love this forum so much. Lets look at your call that >>something bad will happen if the pilot violates the cautionary statements in the POH<<

Hello?

Lu?

Isn't that WHY there is a POH?

As for "design flaw", I assume you were joking.

Jack stall is a detail inless you execute a high G (generally high speed) manuever. Why would you do that? The condition was demonstrated on the endorsement course (and all AS350 pilots should have seen it). The AS355 even has a limit light to tell you that jack stall is approaching (though I cannot imagine having my eyes inside during high G rapid manuevering).

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.

helmet fire is offline   Reply
Old 6th Dec 2001, 08:04   #29 (permalink)

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,117
Question

To: helmet fire

“To Lu: your sweeping statements are why I love this forum so much. Lets look at your call that >>something bad will happen if the pilot violates the cautionary statements in the POH<< “.

This is probably the tenth time I have said this. The cautionary notes in the Robinson POH were incorporated as a result of an engineering study made by the Georgia Tech Aero department and the FAA commissioned the report. The FAA wanted to know the causes of over 22 mast separations and rotor incursions on Robinson helicopters since they entered into service. The report was made just prior to 1995. Although the engineering study was never completed there was sufficient evidence that prior to the mast separations or the rotor incursions the helicopter experienced violent flapping excursions. (I will explain the design problem later in this text). It was determined that these flapping excursions could be initiated in three different ways.

1) By flying out of trim

2) By sideslipping the aircraft

3) By the application of left cyclic while recovering from a zero G incident.

In 1995 Robinson incorporated an unnumbered page in section 4 of the POH outlining restrictions from performing the above (1-2-3) and instead of making the information mandatory they made it in the form of a suggestion. The CAA indicated to me that they would make it mandatory in all POHs for G registered Robinson helicopters. That was about 8 months ago and nothing has been done. During the past year or so there have been several mast separations in GB.

This is what I meant by the pilot being protected by words and not design.

Regarding the design problem on the Robinson head it involves the “droop stop” which is referred to as a tusk. The tusk establishes the low position of the rotor blade while static. Once the rotor is up to speed and collective applied the tusk moves downward from its' limiting stop. Under normal flight conditions the tusk will never contact the stop. However, when you experience extreme flapping excursions and the tusk hits the stop then it turns the rotor system into a first class lever with the fulcrum at the teeter hinge. If the kinetic energy is sufficiently strong the force exerted by the blade will cause rotorhead contact with the mast.

Regarding your comment about sweeping statements this is what I get paid for as a RMS Engineer and that is to identify and eliminate if possible design problems that will impact Reliability, Safety and even Maintainability.

No helicopter should be designed and certified if there is a possibility of experiencing “jack stall” or to have a mechanical part of the rotorhead that under certain circumstances can cause the loss of the helicopter.

Now, here is something to think about. If the Robinson was restricted from certain flight conditions in 1995 because these conditions could result in the loss of the helicopter, how could the helicopter pass certification if these same situations had to be demonstrated (sideslip and out of trim).
Lu Zuckerman is offline   Reply
Old 6th Dec 2001, 09:25   #30 (permalink)
 
Join Date: Jul 2001
Location: the cockpit
Posts: 1,070
Cool

Lu,

I see we are getting some where. I point out that jack stall is a detail, and you talk about the rotor heads flying off Robbos. I talk about the comparison of Jack Stall to other design gotchas, and you talk about the rotor heads flying off Robbos.

Shall we ignore the Robbo, and return back to the fact that Jack Stalls are NOT a design flaw?

You said : >>No helicopter should be designed and certified if there is a possibility of experiencing “jack stall” or to have a mechanical part of the rotorhead that under certain circumstances can cause the loss of the helicopter.<<


I say: Dont all helicopters have a certain circumstance that can cause loss of the helicopter? What negative G and applied roll rate in a teetering head helicopter? What about fully lowering the collective at the bottom of an auto? What about flying them into a fog bank and trying to hover? How did anyone manage to get any of them certified?


Again: The AS350 is an outstanding aircraft. Enjoy it's huge capability and dont fly it against the POH - like EVERY OTHER AIRCRAFT.


helmet fire is offline   Reply
Old 6th Dec 2001, 09:50   #31 (permalink)
 
Join Date: Jan 2001
Location: North Queensland, Australia
Posts: 2,178
Post

I must endorse helmet fire's comments here - the fact that the AS350 hydraulic jacks can reach a point where they can't overcome the flight loads being placed on them is just something that happens with this particular machine (and others, no doubt) and defines a boundary on the flight envelope.

No matter what type you fly, there will be physical limits to what you can do with it because of the design - for instance, don't try to do a level slow roll in a teetering head helicopter, and whatever other such ridiculous examples you want to name.

I have seen incipient jack stall demonstrated in an AS350, and shown it to others myself as a controlled learning exercise to promote awareness of what can happen if the aircraft is mishandled. This is not a design flaw in the aircraft, it's simply a facet of the machine that people need to be aware of, like VNE or the transmission torque limits; or vortex ring for that matter.

I have flown agressive manoeuvres in the aircraft on many occasions, and have never come anywhere near to jack stall. I would go so far as to say that if you get in a situation where jack stall occurs and you crash because of it, you have caused the crash yourself by placing the aircraft in a situation from which it couldn't recover - something which can be done in any aircraft type!
Arm out the window is offline   Reply
Old 6th Dec 2001, 11:09   #32 (permalink)
 
Join Date: Jan 2001
Location: AB, Canada
Posts: 396
Post

This does bring up an interesting question. What is the difference between a design flaw and a published limitation?

Obviously if a flaw in the design is discovered and it is found to be more economical to narrow the envelope than fix the flaw, then that flaw becomes a limitation. Doesn't mean it's not a flaw anymore.

______________

My take is this. When you design a helicopter, you have a purpose for that helicopter in mind. That purpose defines a minimum envelope. A published limitation that narrows the envelope to an extent that any part of that minimum envelope is excluded could be considered a design flaw.

Let's say that to post on PPRuNe, the programmers decided you must hit the "Add Reply" button. They write the software and find that you have to double click it. They are perplexed and can't fix it. Instead, they add some text to say to double click the button. It's still a flaw.
_______________

It sounds like the AS350 limitation is at an extreme of an envelope. From my line of thinking, that means it's not a design flaw.

What's been said about sideslipping a Robinson seems unusual for a training aircraft. That may qualify as a design flaw that became a published limitation.

________


Of course, I haven't flown or touched either aircraft so what the hell do I know?
heedm is offline   Reply
Old 6th Dec 2001, 11:22   #33 (permalink)
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

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.
Grey Area is offline   Reply
Old 6th Dec 2001, 14:38   #34 (permalink)

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering in hyperspace and the FIR
Posts: 9,357
Post

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...
ShyTorque is offline   Reply
Old 6th Dec 2001, 15:09   #35 (permalink)
 
Join Date: Aug 2000
Location: Asia
Posts: 155
Post

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.
Mark Six is offline   Reply
Old 6th Dec 2001, 20:39   #36 (permalink)

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,117
Question

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.
Lu Zuckerman is offline   Reply
Old 6th Dec 2001, 21:01   #37 (permalink)
widgeon
Guest
 
Posts: n/a
Post

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 ).
  Reply
Old 6th Dec 2001, 21:06   #38 (permalink)
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

1. What "Jack Stall Light"?
2. If you really understand the certification process why not take issue with FAR 27 instead of aircraft that conform to it?
Grey Area is offline   Reply
Old 6th Dec 2001, 23:19   #39 (permalink)

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,117
Question

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 ]
Lu Zuckerman is offline   Reply
Old 7th Dec 2001, 01:05   #40 (permalink)

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering in hyperspace and the FIR
Posts: 9,357
Exclamation

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....

ShyTorque is offline   Reply
Reply
 
 
 


Thread Tools


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT. The time now is 05:42.


vBulletin® v3.8.7, Copyright ©2000-2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
1996-2012 The Professional Pilots Rumour Network