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.

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......... :rolleyes:

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....... :D :D

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 ]

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 ]

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.

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'.

"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

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


:)

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?

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..

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.

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.

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?

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.

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. ;)

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.

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!

maybe servo freeze or something like that would make more sense.

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...." :D

[ 08 December 2001: Message edited by: Nick Lappos ]

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 ? ?

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! :eek:

Not a combat aircraft? - dont say that around the French! :D :D

Nick, What you are saying makes sense, but again I harp on the severity required to enter jack stall in the AS350 (not the 365), I am guessing from experience, but I would say around 3 Gs, or slightly less. As you say - jack stall occurs in or approaching RBS, but that is the point we have been trying to make - why are you flying the aircraft into RBS? Fighter maneuvering aside of course!! Thus the majority opinion was that as it only results from gross misshandling - perhaps calling it a design flaw was a bit over the top. Does this level of severity have to occur during certification? Or do you restrict it to 2 or so Gs as you indicated for the S76?

Lastly, do you think that the RBS under these conditions is a G stall?

IHL,
The higher hydraulic pressure allows more energy to be transmitted, and it allows smaller piston areas in the servo for a given force, so it saves weight. 3,000 pounds per square inch is now considered old hat. The S-92 is 4,000 psi, and the V-22 is 5,000 psi (!!) The tradeoff is against the leak potential and the need to protect against line burst.

ShyTorque,
The real issue is a subject dear to my heart. Our machines should not execute the pilot for an error, I think, but the levels of jack stall we are discussing are really just annoying characteristics, so I really agree with the group, I think.

Helmetfire,

Generally, few helicopters can pull 3 g's, and I'll bet the 350 cannot. High g capability robs hover performance, so designers give it out gingerly. The Black Hawk can pull 3.5, and an Apache can get close to that, Comanche can pull almost 4. I have hit 2.7 in the S-76, doing stuff that would make many pilots toss their cookies (I used to demonstrate helo aerobatics to the Army when they were deciding how to specify LHX maneuvers). Estimating g's is hard, I really can't without a g meter.

Max G capability is actually easily estimated based on blade chord, tip speed and diameter. I can give you a reference, if you'd like.

The issue here is really the nature of carefree handling. As long as jack stall does not cause control loss, it is not a major issue, and we really agree. If it robs the pilot of control, it should be fixed, even if the maneuver is considered extreme. FAR states that we must test to the maximum the aircraft will experience in flight, so if someone comes back and pulled more, we didn't test enough. Flight manual cautions and the "pilot error" rubber stamp are quick outs for poor designs.

We used 2 g's with a servo failure in the S-76 to assure that nobody would ever get jack stall, reasoning that folks would not pull near stall loads after a failure. With both servos on, the controls can never get to jack stall under any case, as is true with the Black Hawk.

Something I must calrify is the notion that any helicopter can hit any g if the pilot gets wild enough. The max g's for a helo are set by the rotor design, especially the solidity (amount of blade area relative to the disk area). The rotor can only pull a few g's before it stalls, and sometimes the maneuver can be quite mild. The typical rotor stall g level drops quickly with altitude, so if the maneuver took 3 g's at sea level, it might stall at 2 g's at 10,000 feet DA, a much more achievable level, and near Vne, it might stall at only 1.5 g's.

If my helicopter could lose control in jack stall at 1.5 g's, I'd find another!

You ask about g stall as opposed to speed stall, I think. There is no difference, really. The relationship between g and speed is that the stall g drops with speed until at some speed beyond Vne, the aircraft stalls at 1 g. I will be glad to email you some charts of this relationship, or give you some references, if you'd like.


You ask, "Does this level of severity have to occur during certification? Or do you restrict it to 2 or so Gs as you indicated for the S76?" Please note that the S-76 is jack stall free with only 50% of its servo strength, and jack stall with both systems healthy is impossible. The manufacturer must test to a maneuver level that he then declares to be the maximum (the phrase is that the test maneuvers are extreme enough so that the probability of exceeding them in service is "extremely remote" which is FAA speak for 1 in 10e9 hours. In other words, if 1,000 helos are building 1,000 hours per year, in 1,000 years we will have one event! Using this definition, it is not good design to experience jack stall (to loss of control) in service, having certified that your tests were so thorough.

Thanks for your response Nick. I should have clarified the G stall - what I was asking was: do you think the RBS we are talking about here was primarily induced by the G loading being experienced (as opposed to the retreating blade speed)?

your point about having the pilot executed by the machine design is well said, and I do not dispute, but I am curious as to where the limits of manuever harshness during testing are set. For example, you stated that you tested the s76 to two Gs with half the servos because you couldn't imagine anyone pulling to the stall with a servo failure. This is the point being made above - very few (none of the above so far) AS350 pilots can imagine pulling so hard as to experience jack stall, it is as you say: a manuever in which you mightlose your cookies! So if the Bell teetering head can get away with not being termed a design flaw because it can come off during gentle manuevering, and fixed wing stalling can too - jack stall is surely not a design flaw, or at least labelling it as such is a little harsh isnt it?
:)

This is truly a study in sufficiency, because you are quite right, the issue is subjective. The jack stall we are discussing is a benign type, where control is not compromised, so it could be termed a marginal but acceptable characteristic.

Regarding the differences between retreating blade stall and g stall, there really is none. The stall is at the retreating blade, and is due to too much angle of attack. This could be achieved at 1.25 g at Vne, or 2.0 at 80 knots, the cause and results are the same. If you plot the slope between those two hypothetical points, that is the relationship between thrust and speed that defines retreating blade stall. If you toss in the effects of density altitude, you create a parallel line that is at lower g for higher altitude. Another parallel line could represent the effects of lower rpm.

In other words, more g or more speed or lower rpm or higher altitude all effect retreating blade stall.

In my paranoid imaginings (result of this thread) I'm wondering where in "Chickenman's" flying life he could expect this problem to roast him?
Lots of cyclic demand, even at normal G? I can envision being rather busy if I suffered complete antitorque failure at a high hover- what with the throttle on the floor and probably over a hover hole... :eek:

interesting discussion.

i also used to fly the as350 and i used to do aerobatics in it (different environment so settle down!) and teach others to do them. i believe that jack stall was an intentional design feature and i believe that it was a carry-over from the gazelle.

the facts are that jack stall was progressive and you could fly through it if necessary. i saw it as the rotary equivalent of the plankwing "buzz" that accompanied impending stall - in other words you knew you were at the limit and if you continued there were identifiable consequences.

after my AS 350 experience with jack stall, i had cause to rethink some previous experiences in uh-1 gunships where we thought we were regularly getting thrown out of maneuvers due to retreating blade stall. some might have been rbs, but i came to believe that many of them could have been jack stall (the benign ones) and others could have been jack stall followed by rbs when we couldn't back off in time.

anyway, despite the lu hysteria, it still seems to me that "transparency" is a good design feature because it alerts you progressively to an impending limtation that in effect is being sensed through the feedback load on the control horn.

I took this from another discussion.

"except the only time I ever got into the "Servo Transparency" in my 2,000 + Astar hours was in a B model (Approx 4000 lb AUW) in a gentle turn around a fire on a calm August day at 4,000 Ft....go figure that one out??? "

I only mention it because somebody said that it only happens under extreme conditions. I have found that things like this that could happen often do just when you don't want them, despite what designers and test pilots say. In that respect, I think it should have been fixed before now. The accumulator is there to allow reduction in speed in a timely manner without going to the expense of adding a second hydraulic system.

And talking about hangovers from gazelles.... our Twinstar has a crap electrical system as well, but I won't go there....

phil

scrubba,

Save us from such wonderful design features, please! The issue is that the hydraulics are too weak to keep the blades from arguing with you. As Paco relates, the incidence of stall can occur at relatively low maneuver states.

You have been lucky that the "transparency" occurred where the recovery is simple. The problem is that the mild stick wiggles you have felt are nothing compared to what could be dealt to you in other circumstances. Should the stick simply whip around the cockpit and let the aircraft suddenly take over, you would probably not declare that a design feature! The 365 family does that, and the maneuver ends when the aircraft finally runs out of airspeed, so control can be regained. Should the aircraft find some comfortable dirt or rock to nestle into before it lets you become PIC again, the flight might come to a rapid halt, thanks to that handy-dandy design feature you like so much.

I hate to agree with Lu (and I think hard about it, believe me!) but let's all push for power steering that works all the time, and helos that don't stall too early.

I hear the same apology for LTE in this group ("crummy pilots deserve to be punished"), and I ask us all to please seek helos that don't require budding Astronauts to keep them safe. The helicopter mission is hard enough, with the demands for extra judgement, fine motor skills and great airmanship to keep the outside world from hurting us and our machines. Let us at least ask our machines to be our partners, and not our adversaries, in the quest for better safety. If our safety record can't be raised, we might always be a mere sidelight in the world of aviation.

Paco, that incidence illustrates what I was talking about above in this thread - that stall is stall, it is not g stall or speed stall. The August day was warm, I'll bet, and the aircraft was heavy, and that turn crossed the stall line by a bit. :cool:

[ 15 December 2001: Message edited by: Nick Lappos ]

Yup, stall is stall, as many beaver pilots have found when turning low and slow...

here's another from the same thread, a sobering thought...

"110 kts, straight and level, collective bottomed, 500 lbs below max gross, descent off the top of the mountain - wind sheer, a/s up to 155kts, a/c snapped to the right 90 degrees. both arms could not pull it back, cyclic was solid. you can get ST any number of ways."

You know, I don't like aircraft that bite. I know the Astar is a good machine, and has had a lot of good stuff built in from the design stages, but that sort of thing should have been fixed. To my mind a limitation is something that results from natural causes, such as something that all engineers know about, such as, say helicopters don't fly faster than a certain maximum speed. This is a design flaw. like the electrics, but I'm not going there... :)

Phil

PS Design features are a Microsoft euphemism for bugs

[ 16 December 2001: Message edited by: paco ]

"110 kts, straight and level, collective bottomed, 500 lbs below max gross, descent off the top of the mountain - wind sheer, a/s up to 155kts, a/c snapped to the right 90 degrees."

So you exceeded the max autorotation "collective bottomed" airspeed by 40 knots at sea level and it's a design flaw? Sounds to me like you put the aircraft far out of any reasonable design reserve.

First of all, I just copied that from another bulletin board - it wasn't me (read the post).

Secondly, it is a typical situation in the mountains, POH notwithstanding, and a 90 degree snap to the right is not what you want right then. It shouldn't be what you get at *any* time.

When senior test engineers or pilots admit that it was a bigger factor than originally catered for, I would suggest it is something that ought to be fixed, n'est-ce pas?

Phil

Seems to me there are two polarised opinions here.

One from pilots who observe the published aircraft limitations and have no problems.

The other from people who think that limits are to be ignored when it suits them, suffer a fright as the consequence and then call it a design fault.

"Whaddya mean, stressed to 4G?
I've only been pulling 5G all morning and then the wing came off this afternoon!! :rolleyes: I think you're a crap designer!"

"First of all, I just copied that from another bulletin board - it wasn't me (read the post)."

Sorry about that. I thought you were re-posting something you had already posted.

"Secondly, it is a typical situation in the mountains, POH notwithstanding, and a 90 degree snap to the right is not what you want right then. It shouldn't be what you get at *any* time."

I'm not sure what mountains you fly in but where I fly the "V" limitations are exactly that. Anyone near gross weight that puts the pitch down, rolls the nose over and approaches an airspeed limitation, in anything but the smoothest air is asking for exactly what happened."

"When senior test engineers or pilots admit that it was a bigger factor than originally catered for, I would suggest it is something that ought to be fixed, n'est-ce pas?"

Who are these senior people that have all this Astar experience? Lu probably has never even riden in one. Lu knows lot about why helicopters fly but he knows little about how a helicopter flies. And Nick has already stated he does not have that much experience with the AS350/355 series. What I've been reading is that the pilots with considerable Astar experience are saying there is nothing wrong.

I haven't been fortunate enough to fly one of the new generation attack helicopters but every helicopter I have flown will exhibit "Jack Stall" if pushed past it's published limits. Which model do you consider immune?

Lama Bear said:
I haven't been fortunate enough to fly one of the new generation attack helicopters but every helicopter I have flown will exhibit "Jack Stall" if pushed past it's published limits. Which model do you consider immune?

Nick sez:
The servo strength does not have to be infinate to counter the stall forces from the rotor, because the moment the blades generate reaches a practical limit in deep stall. Therefore, there is not a relationship between the designer building ever stronger servos to counter the pilot doing ever more aggressive maneuvers. A properly designed control system always wins against the blade stall forces.

In US Military service, the maneuvers must be tested to extreme limits in a structural demonstration where we squeeze everything out of the helicopter and make sure that controls, structure and systems all support the pilot. Jack stall is not allowed, and any helicopter that demonstrates it must be redesigned (usually by making the servos stronger).

In short, no Boeing, Sikorsky or Bell product of the last two generations has or can experience jack stall.

No, it's not something i already posted, but I had already alluded to the same thread a couple of messages ago - I thought you had read it, sorry :)

It's just that I'm watching two threads at the same time on the same subject. In the other one there are a lot of experienced pilots giving examples of when they have encountered this phenomenon, which range from the relatively benign to the extreme, such as the last example I gave above. When I say "typical of the mountains", I meant the situation, where windshear will often take you a great deal above your target airspeed very quickly, obviously something to be watched for, but you can't always catch it.

Obviously, one shouldn't fly aircraft to their limits, and none is bulletproof, but my problem with this one is that it isn't repeatable under the same circumstances every time. If it were, you would just avoid it and regard it as a limitation, which it clearly isn't. I still maintain it should have been fixed as part of ongoing product development, while recognising that you can't design everything right first time.

Actually, Lu (or anyone else) wasn't on my mind when I posted - here is yet another quote from the other thread:

"Within the first 50 hrs of flight after my endorsement there I was with fire crew on board headed out for the old day base in Alberta - so very close to max. gross internal. 800' ceiling "zipping" down the siesmic line enroute to day base, spring black bear eating grass on siesmic line, fire fighter wants picture, tight right hand turn at about 110knts., rate of descent approximately 200-250fpm. you do the math on how long it is prior to controlled flight into terrain. Actually it would have been uncontrolled flight into terrain as I experience "servo transparency" - cyclic & collective FROZEN - the only controls left to help reconfigure the disc loading were the pedals - however I was unaware of what was happening and it was only #$%^house luck that I was able to regain control. If I had not the A/C plus 5 people would have been spread over the Alberta jackpine muskeg for a mile and no one would ever have come up with the reason.
It took me until 1997 for a eurocopter test pilot to admit to me in "private" that yes it was a bigger factor than had been anticipated. There are at least 3 fatal's I am aware of where it is my belief that "jack stall" was involved.
My advice - for what it is worth - enjoy the A/C but learn your product well, be careful of: subject discussed, vortex state ring, Hyd. failures and although a nimble and responsive A/c is should be flown more like a medium. If you have a penchant for being a cowboy - leave the boots and spurs at home on this one. This is not meant to frighten, however knowledge is one of the gateways to professionalism. Have a awesome season."

Phil

AS-350(Max Turbulence Speed)
 

My work mates and I have been discussing wether or not the AS350 has a "turbulance penetration speed". One of the guys we work with(ex Mil) says it does :80kts which it clearly states in his Defence Force Flight Manual. But when we look for it in our civil flight manual there is no mention of it.. .I can't remember anything said about in my type rating(years ago). .So is there such a figure?. .Do the Mil guys know something we don't?. .or has Eurcopter just forgotten to add it?(which I dought)

. ."Wise man says 'make haste slowly that way accidents don't happen" <img src="rolleyes.gif" border="0">

Ive never seen it in a Squirrel flight manual either, 80 knots sounds right, its in my RAAF AS 350 pilots handbook. Perhaps it had to have a turbulence penetration speed for military acceptance.

SlingLoad. .The 80kt is in the RAAF fligth manual that I had a look through today... .Must be a Defence Force standard.

SlingLoad. .The 80kt is in the RAAF fligth manual that I had a look through today... .Must be a Defence Force standard.

Info on AS350 B2s
 

I am looking at doing my endorsement on a AS 350B2 soon.Anybody out there have any tips or info on these machines that I won't find in the FM?