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AS350 Astar/Squirrel

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AS350 Astar/Squirrel

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Old 8th Dec 2001, 13:07
  #61 (permalink)  
 
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Not a combat aircraft? - dont say that around the French!

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?
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Old 8th Dec 2001, 19:31
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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.
 
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Old 10th Dec 2001, 08:43
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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?
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Old 10th Dec 2001, 09:10
  #64 (permalink)  
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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.
 
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Old 11th Dec 2001, 17:31
  #65 (permalink)  
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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...
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Old 15th Dec 2001, 05:21
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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.
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Old 15th Dec 2001, 19:25
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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
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Old 16th Dec 2001, 00:49
  #68 (permalink)  
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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.

[ 15 December 2001: Message edited by: Nick Lappos ]
 
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Old 16th Dec 2001, 06:48
  #69 (permalink)  

 
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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 ]
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Old 16th Dec 2001, 20:16
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"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.
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Old 17th Dec 2001, 00:06
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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
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Old 17th Dec 2001, 00:50
  #72 (permalink)  

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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!! I think you're a crap designer!"
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Old 17th Dec 2001, 00:53
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"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?
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Old 17th Dec 2001, 03:32
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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.
 
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Old 17th Dec 2001, 03:39
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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
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Old 24th Jan 2002, 08:14
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Post 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 0kts 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">
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Old 24th Jan 2002, 14:03
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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.
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Old 24th Jan 2002, 14:35
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SlingLoad. .The 80kt is in the RAAF fligth manual that I had a look through today... .Must be a Defence Force standard.
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Old 24th Jan 2002, 14:36
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SlingLoad. .The 80kt is in the RAAF fligth manual that I had a look through today... .Must be a Defence Force standard.
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Old 12th Mar 2002, 03:00
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Post 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?
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