Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

AS350 Astar/Squirrel

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

AS350 Astar/Squirrel

Reply

Old 7th Dec 2001, 00:35
  #41 (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: 2,132
Question

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.
Lu Zuckerman is offline  
Reply With Quote
Old 7th Dec 2001, 03:06
  #42 (permalink)  
 
Join Date: Jul 2001
Location: the cockpit
Posts: 1,077
Cool

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 ]
helmet fire is offline  
Reply With Quote
Old 7th Dec 2001, 06:13
  #43 (permalink)  
 
Join Date: May 2000
Location: Corvallis, Oregon
Posts: 58
Post

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 ]
Jiff is offline  
Reply With Quote
Old 7th Dec 2001, 10:20
  #44 (permalink)  
 
Join Date: Jan 2001
Location: North Queensland, Australia
Posts: 2,866
Post

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.
Arm out the window is offline  
Reply With Quote
Old 7th Dec 2001, 10:45
  #45 (permalink)  
 
Join Date: Jan 2001
Location: AB, Canada
Posts: 420
Post

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'.
heedm is offline  
Reply With Quote
Old 7th Dec 2001, 15:36
  #46 (permalink)  
 
Join Date: Feb 2000
Location: UK (Wilts)
Posts: 118
Post

"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
Grey Area is offline  
Reply With Quote
Old 7th Dec 2001, 18:30
  #47 (permalink)  
 
Join Date: May 2000
Location: Corvallis, Oregon
Posts: 58
Post

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


Jiff is offline  
Reply With Quote
Old 7th Dec 2001, 18:33
  #48 (permalink)  
 
Join Date: Nov 2000
Location: USA - Mexico
Posts: 127
Post

In 12,000 plus hours I have never flown a helicopter that would not "Jack Stall" if the aircraft was over flown. Has anyone besides Lu flown one that is immune to this?
Lama Bear is offline  
Reply With Quote
Old 7th Dec 2001, 20:19
  #49 (permalink)  
 
Join Date: May 2001
Location: perth
Posts: 231
Post

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..
sling load is offline  
Reply With Quote
Old 7th Dec 2001, 20:54
  #50 (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: 2,132
Question

To: All

Once and for all understand this: 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.
Lu Zuckerman is offline  
Reply With Quote
Old 7th Dec 2001, 21:51
  #51 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 11,625
Post

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.
ShyTorque is online now  
Reply With Quote
Old 8th Dec 2001, 02:08
  #52 (permalink)  
 
Join Date: Jul 2001
Location: the cockpit
Posts: 1,077
Cool

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?
helmet fire is offline  
Reply With Quote
Old 8th Dec 2001, 02:23
  #53 (permalink)  
Nick Lappos
Guest
 
Posts: n/a
Post

Uh, Oh, I might have to agree with Lu on this. heedm, note the date and time!

Jack stall is a sign of a servo system with too little force capability, and is not acceptable in modern helicopters, as a rule. It is caused when the stall of the retreating blade causes the blade to change its pitching moment sharply downward, which makes the blade try to dive. The strong downward force is fought by the pitch change links, swashplate and servos. At Sikorsky, we set the design requirement of the servos so that with one stage failed, the other has the full capability to fly the flight envelope. That means that we have twice as much servo force capability as needed in normal unfailed circumstances.

I personally set the S-76 flight test points to prove this, with max weight, one servo turned off, we pulled over 2 g's at Vne with no change in stick feel.

A helicopter that shows jack stall is on the edge of taking over for the pilot, and turning him into a passenger for what could be a very short ride. Treat all jack stall cases as bad ju-ju, because the difference between a stick that throbs a few cycles, and one that thrashes around and allows the aircraft to go ape is a very small difference in maneuver load factor.
 
Reply With Quote
Old 8th Dec 2001, 03:39
  #54 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 11,625
Post

Nick,

2G equates approximately to a sustained level turn at 60 degrees of bank, if I remember correctly.

The manoeuvres I have experienced as required to induce jackstall are beyond that.

If the aircraft are flown within their published limits there is no problem.

However, I may well agree that Sikorsky aircraft may be built with more margin in hand in some areas, obviously in the case of the S-76 and other aircraft with an input from your good self. It doesn't mean to say that another manufacturer's product is unsafe though.
ShyTorque is online now  
Reply With Quote
Old 8th Dec 2001, 05:26
  #55 (permalink)  
 
Join Date: May 2001
Location: perth
Posts: 231
Post

To get transparency in the AS 350, the manouevers are so severe that you could fly without ever experiencing it, i have had it demonstrated and as one of the other posters said,it is quite a deal to set it up, I don't see it as a design flaw, it was designed at the time with the technology available, its an excellent aircraft and an absolutely stable platform to work from. Ive flown up to and including the B2, i havn't seen or read the B3 data, but if it was a serious concern, the B3 may have changed.

Anyone who operates an As 350 outside the performance specs and limitations is asking for trouble, doing anything, its a light helicopter with limits, the manufacturer has identified them for the pilot, fairly straightforward.

It is an extremely violent manoeuver to get to transparency. I have only had it when its demonstrated, never encountered it in normal ops or had other pilots experience it in normal ops.
sling load is offline  
Reply With Quote
Old 8th Dec 2001, 05:35
  #56 (permalink)  
 
Join Date: Jan 2001
Location: North Queensland, Australia
Posts: 2,866
Post

Funny term, transparency.
You would think that in the normal course of events, the servos would feel 'transparent', i.e. you wouldn't know they were there, so to speak.
I'd say that jack stalling would be when they weren't transparent!
Arm out the window is offline  
Reply With Quote
Old 8th Dec 2001, 08:30
  #57 (permalink)  
 
Join Date: Apr 2001
Location: Hartford, Connecticut, USA
Posts: 64
Exclamation

maybe servo freeze or something like that would make more sense.
baranfin is offline  
Reply With Quote
Old 8th Dec 2001, 08:52
  #58 (permalink)  
Nick Lappos
Guest
 
Posts: n/a
Post

Shytorque,

I agree that the 2 g's is not terribly severe, but recall that is with half the total servo force, since we had turned one system off prior to the maneuver.

The test pilots who took the AS-365 through air combat trials frequently experienced jack stall to the point that the aircraft would wind up tightening its own turn. This occurred at 2.4 g's or less, which is not terribly far out in the spectrum of possible maneuvers. One can argue if this is acceptable or not, but the ideal helicopter should not have the ability to out-fight its pilot, in my humble opinion. None of this should be interpreted as impuning a very fine helicopter, just stating what we should be striving for.

In any case, the concept of jack stall is clearly understood by the bunch, we can argue (politely, of course!) the specific level at which it should occur (clearly far out in the envelope).

Like weak tail rotors (which create the need for careful LTE training), jack stall is a fact of life, but I hope we all get to help raise the bar and make it something we tell newbies about with the phrase, "Sonny, I remember the time when...."

[ 08 December 2001: Message edited by: Nick Lappos ]
 
Reply With Quote
Old 8th Dec 2001, 09:39
  #59 (permalink)  
IHL
 
Join Date: Feb 2001
Location: Canada
Posts: 579
Post

Hey Nick : I always wondered why the S-76
has such High Hydraulic pressure (3,000 psi)for a helicopter with a Gross Weight under 12,000 Lbs. Is it because the servo actuators are relatively small ? ?
IHL is offline  
Reply With Quote
Old 8th Dec 2001, 11:50
  #60 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 11,625
Wink

Nick,

I certainly agree that a combat aircraft should be able to endure more than its pilot can stomach.

But the AS-350 could hardly be described as a combat aircraft!
ShyTorque is online now  
Reply With Quote

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service