Jackstall: What is it? Any experiences?
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Nick, nothing wrong with your design philosophy except that many can't afford it. If you were writing international certification standards I'd be very much on your side. Lets only make good product, from now on. Unfortunately, people don't want to spend $10 million on a training helicopter. Hence we have training helicopters that have noticeable accident rates in areas that designers could have prevented.
For the foreseeable future, helicopters will continue to carry a pilot. It is not a legacy item, it is a necessary piece of equipment that can override the errors of all the other pieces. Until helicopters are reliably designed without pilots, the pilot will be responsible to follow limitations. Whether a limitation is the result of an obvious efficiency in the design stage or if it is a compensation for a deficiency will be up for debate.
It's funny how helicopters seem to rely on piloting skill so much more so than fixed wing aircraft, yet the limitations to prevent Jack Stall are akin to the most accepted limitations of fixed wing aircraft: g limits. Yes, modern aircraft prevent the pilot from overstressing the machine, but almost every fixed wing aircraft I've flown had g limits that were easily exceeded by inciting the conditions that lead to Jack Stall.
For the foreseeable future, helicopters will continue to carry a pilot. It is not a legacy item, it is a necessary piece of equipment that can override the errors of all the other pieces. Until helicopters are reliably designed without pilots, the pilot will be responsible to follow limitations. Whether a limitation is the result of an obvious efficiency in the design stage or if it is a compensation for a deficiency will be up for debate.
It's funny how helicopters seem to rely on piloting skill so much more so than fixed wing aircraft, yet the limitations to prevent Jack Stall are akin to the most accepted limitations of fixed wing aircraft: g limits. Yes, modern aircraft prevent the pilot from overstressing the machine, but almost every fixed wing aircraft I've flown had g limits that were easily exceeded by inciting the conditions that lead to Jack Stall.
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Matthew, I have to disagree. You seem to think that designing a helo properl;y is more expensive or heavier. The ONLY helos that can experience jack stall are those mentioned in these threads, a small handfull (I want to avoid mentioning brands, this is not a commercial rant). It is not necessary to design substandard servos to be cheap enough or competitive enough, a thousand successful helos prove otherwise.
Don't give in so easily to the demon of crappy helicopters!
Don't give in so easily to the demon of crappy helicopters!
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Nick, I have to admit that the business side isn't my thing, but the thinking that brought me to my previous point went something like this:
Weren't the helicopters that we aren't talking about very commercially viable at one point, and remain relatively so today? Didn't they offer a significant increase in performance over the types that they were replacing, despite very similiar overall operating costs? If they were beefed up, would they have increased costs and decreased performance?
I don't think we disagree on what we want, and I know we both admit that throughout helicopter design there are compromises, I just think that jack stall was a compromise.
I'm not sure if there is any new product that suffers jack stall with systems operating properly, so the designers apparently have taken care of this.
Unfortunately I suffer the "demon of crappy helicopters" in other areas. When will we get the perfect machine?
Cheers,
Matthew.
Weren't the helicopters that we aren't talking about very commercially viable at one point, and remain relatively so today? Didn't they offer a significant increase in performance over the types that they were replacing, despite very similiar overall operating costs? If they were beefed up, would they have increased costs and decreased performance?
I don't think we disagree on what we want, and I know we both admit that throughout helicopter design there are compromises, I just think that jack stall was a compromise.
I'm not sure if there is any new product that suffers jack stall with systems operating properly, so the designers apparently have taken care of this.
Unfortunately I suffer the "demon of crappy helicopters" in other areas. When will we get the perfect machine?
Cheers,
Matthew.
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Jack Stall
Jack Stall used to be a requirement that had to be achieved as part of the Gazele MTF (5M Checks), until one day my colleague nearly chopped the tail off !
Requirment was removed rabidly from the 5M and his cheeks slowly reurnedto their normal colour !
Fling overalls subjected to a quick launder and assessed 'S'
Requirment was removed rabidly from the 5M and his cheeks slowly reurnedto their normal colour !
Fling overalls subjected to a quick launder and assessed 'S'
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Nick
As a pretty inexperienced helicopter chap I have had the pleasure of learning a lot of good stuff from your contributions to this forum. I was, therefore very surprised by your comments regarding EC products. I am British and therefore not overly disposed towards our neighbours across the channel. However, having been part of a military school which has operated the Squirrel for over 75,000hrs with no recorded cases of Jackstall (intentional or otherwise) I think I might br forgiven for suspecting a case of 'American is best' here. I really hope I am wrong. Have you personally encountered Jackstall in the Squirrel? If so then please describe how easy it is to encounter. 130 instructors wait with bated breath to hear how close to disaster they come every day flying this deathtrap.[/I]
As a pretty inexperienced helicopter chap I have had the pleasure of learning a lot of good stuff from your contributions to this forum. I was, therefore very surprised by your comments regarding EC products. I am British and therefore not overly disposed towards our neighbours across the channel. However, having been part of a military school which has operated the Squirrel for over 75,000hrs with no recorded cases of Jackstall (intentional or otherwise) I think I might br forgiven for suspecting a case of 'American is best' here. I really hope I am wrong. Have you personally encountered Jackstall in the Squirrel? If so then please describe how easy it is to encounter. 130 instructors wait with bated breath to hear how close to disaster they come every day flying this deathtrap.[/I]
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dak,
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.
My contention is that we apologize for crappy systems that fail to do their job by bragging "any pilot worth his salt should be able to handle the fact that the controls lock up or the tail rotor fails to control the machine!"
Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.
Our passengers do not want to ride in substandard machines built with flaws that were engineered out decades ago just because our pilots are so proud of their testosterone driven skills that they do not fear a helicopter that can go out of control unless babied. What do we tell your widow when you try to avoid a bird at altitude and get jack stall and hit the dirt? That you were a bad pilot and deserved it?
Have I experienced jack stall? Yes, in several helo types, as far back as the AH-1G in 1969, but I have also flown an S-76 with one servo system turned off at maximum gross weight to Vne, and pulled 2.0 G's without feeling any difference in the controls. Why did I do that? Because that is how we test to be sure you and your passengers did not have to experience jack stall.
I contend that it is time for us to seek the best for our machines, and stop impuning the skills of our pilot brothers when an accident happens.
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.
My contention is that we apologize for crappy systems that fail to do their job by bragging "any pilot worth his salt should be able to handle the fact that the controls lock up or the tail rotor fails to control the machine!"
Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.
Our passengers do not want to ride in substandard machines built with flaws that were engineered out decades ago just because our pilots are so proud of their testosterone driven skills that they do not fear a helicopter that can go out of control unless babied. What do we tell your widow when you try to avoid a bird at altitude and get jack stall and hit the dirt? That you were a bad pilot and deserved it?
Have I experienced jack stall? Yes, in several helo types, as far back as the AH-1G in 1969, but I have also flown an S-76 with one servo system turned off at maximum gross weight to Vne, and pulled 2.0 G's without feeling any difference in the controls. Why did I do that? Because that is how we test to be sure you and your passengers did not have to experience jack stall.
I contend that it is time for us to seek the best for our machines, and stop impuning the skills of our pilot brothers when an accident happens.
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I think the Jack stall demo is no longer taught officially on the UK Gazelle, but I was certainly demo'd one and tried one for myself when I went through.
Vne dive and pull up hard till it tried to flick itself on its back. Lots of fun.
Got to say though, having flown a large number of Helicopters from Seakings to Seahawks Squirrels to Hips, you can't beat the Gazelle. The French do make very fine Helicopters, (even if they always do have a flaw or two to keep you on your toes!)
A true gentlemans sportscar.
You dont fly it, you strap it on.
Vne dive and pull up hard till it tried to flick itself on its back. Lots of fun.
Got to say though, having flown a large number of Helicopters from Seakings to Seahawks Squirrels to Hips, you can't beat the Gazelle. The French do make very fine Helicopters, (even if they always do have a flaw or two to keep you on your toes!)
A true gentlemans sportscar.
You dont fly it, you strap it on.
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Wow. Light the blue touch paper and stand back.
Have I? I thought I was asking a simple question or two.
Did I say that? I have re-read my post a dozen times but can't find it.
This doesn't answer my question ref the Squirrel. From all the comments in this thread, we all know that the Gazelle could be jackstalled in extreme manouevring. Does the Squirrel, a later design, have the same characteristics? If no improvement in jackstall threshold has been achieved then I agree with you that this is unacceptable.
I do not have figures on the number of accidents caused by jackstall, but if they are occuring with the frequency that you imply, then surely the offending helicopter types should have their certification withdrawn!
I realise that this is an issue which you feel very strongly about. That is why I am looking for reasoned answers from someone I consider an expert - not abuse.
Dak
dak,
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.
Please do not generalize my words, you then wouldn't be so surprised. I have avoided putting manufacturer's labels on this discussion because of exactly what you have done, put your nationalistic/manufacturistic (is there such a word?) ethos first and your safety judgement second.
My contention is that we apologize for crappy systems that fail to do their job by bragging "any pilot worth his salt should be able to handle the fact that the controls lock up or the tail rotor fails to control the machine!"
Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.
Meanwhile, the accident reports prove that assertion is simply bull, since accidents can and do happen all too frequently BECAUSE of jack stall and LTE. If accidents do not happen because of Jack Stall, I would not take this point of view. Since they do, your contention that pilots ought to be better is simply arrogant bull, since it implies that pilots and passengers get what they deserve, since they don't fly with perfect pilots like you. Such a position has no place when we speak of how to make our industry better.
Have I experienced jack stall? Yes, in several helo types, as far back as the AH-1G in 1969, but I have also flown an S-76 with one servo system turned off at maximum gross weight to Vne, and pulled 2.0 G's without feeling any difference in the controls. Why did I do that? Because that is how we test to be sure you and your passengers did not have to experience jack stall.
I do not have figures on the number of accidents caused by jackstall, but if they are occuring with the frequency that you imply, then surely the offending helicopter types should have their certification withdrawn!
I realise that this is an issue which you feel very strongly about. That is why I am looking for reasoned answers from someone I consider an expert - not abuse.
Dak
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My only experience of jack stall was in a single engine helo at MAUW, which occured over 25 years ago. We were asked to carry out a single pass over a military parade ground having just disembarked from 'mother' a few miles away. Approached the target downwind in a slight descent for maximum speed (must have been approaching 95 kts, at least), as we on topped the target, the aircraft was pulled up and right and instantly the most violent vibration was experienced.
The aircraft then continued on the original approach hdg in a slight descent, as opposed to the control input of up and right, (probably because the controls were relaxed!) the speed washed off and the vibration stopped. Ht loss was maybe 50-100 feet, which was OK because our escape route was over ground that dropped away. We made our excuses and left.
I have experienced many flight regimes in helicopters with nothing scary happening, but the factors on this occasion for onset of 'Jack stall' were (I believe), very heavy aircraft, in a descent for max speed with collective applied and forward cyclic, and the application of more collective and cyclic to pull up and right which resulted in a lot of 'G'.
Certainly caught us out, and it never happened again!
The aircraft then continued on the original approach hdg in a slight descent, as opposed to the control input of up and right, (probably because the controls were relaxed!) the speed washed off and the vibration stopped. Ht loss was maybe 50-100 feet, which was OK because our escape route was over ground that dropped away. We made our excuses and left.
I have experienced many flight regimes in helicopters with nothing scary happening, but the factors on this occasion for onset of 'Jack stall' were (I believe), very heavy aircraft, in a descent for max speed with collective applied and forward cyclic, and the application of more collective and cyclic to pull up and right which resulted in a lot of 'G'.
Certainly caught us out, and it never happened again!
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dak,
Lets chalk this up to the difference between verbal banter and written miscommunication. I read your "america is best" "bated breath" and "deathtrap" different than you meant them, I guess.
Yes, there have been several jack stall accidents mostly involving out-of-control helos smacking the ground. My guess is that your demo does a nice job of showing a nibble, and those poor fellows swallowed the whole whale. I know of one Pax River TP whho flipped upside down while having all the control of a passenger. He actually enjoyed it!
Lets chalk this up to the difference between verbal banter and written miscommunication. I read your "america is best" "bated breath" and "deathtrap" different than you meant them, I guess.
Yes, there have been several jack stall accidents mostly involving out-of-control helos smacking the ground. My guess is that your demo does a nice job of showing a nibble, and those poor fellows swallowed the whole whale. I know of one Pax River TP whho flipped upside down while having all the control of a passenger. He actually enjoyed it!
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So...we are all agreed then? 
a. Jack Stall occurs when the limits of the designers chosen hydraulics jacks are exceeded by the aerodynamic forces of stalled rotor blades.
b. Both roll &/or pitch control (or both) are lost for the period of exceedence.
c. It is violent, potentially fatal & probably reduces the airframe & component fatigue life &....
d. Can, like LTE, be significantly reduced (even eliminated) with more robust design efforts.
**
My 2cents:
Risk management is the very essence of being a pilot (esp a helo pilot).
I agree with Nick entirely but the manufacturers also have valid points. Eg: Mr Robinson designed a the R22 to commute from A to B. If you're throwing it around mustering or deer hunting then you must bear some responsibility if things break.
RBS and Jack Stall are not always the result of deliberately being a cowboy. Scooting around a ridge into a string wind is a common cause.
(other factors are weight, blade angle, rate of control inputs, temperature/humidity/density altitude and TAS)
Perhaps the compromise is that if you're NOE at 8000ft, ISA+20 and dodging gunfire in the Afgan mountains......you surely deserve powerful hyd jacks.
If you're helo was designed for flying tourists on city tours of NYC, then I you should be flying within the most limiting factor (be that mechanical or crew experience).
So where does that leave us?
Should manufacturers be providing a comprehensive list of tasks, locations or environments that void the helos warranty?
btw: The Huey jackstalls with carefree abandon. (Euro vs USA isn't a rivalry worth anyones time/effort).
Nick...I can only agree with your points but following that same route:
a. We'd all be flying twin engined helos that can hover at MAUW on 1 at ISA+15 'just in case' the engineers failed us and,
b. We'd all have anti-icing and full IFR avionics just in case, in poor weather with poor airmanship, we failed ourselves?
a. Jack Stall occurs when the limits of the designers chosen hydraulics jacks are exceeded by the aerodynamic forces of stalled rotor blades.
b. Both roll &/or pitch control (or both) are lost for the period of exceedence.
c. It is violent, potentially fatal & probably reduces the airframe & component fatigue life &....
d. Can, like LTE, be significantly reduced (even eliminated) with more robust design efforts.
**
My 2cents:
Risk management is the very essence of being a pilot (esp a helo pilot).
I agree with Nick entirely but the manufacturers also have valid points. Eg: Mr Robinson designed a the R22 to commute from A to B. If you're throwing it around mustering or deer hunting then you must bear some responsibility if things break.
RBS and Jack Stall are not always the result of deliberately being a cowboy. Scooting around a ridge into a string wind is a common cause.
(other factors are weight, blade angle, rate of control inputs, temperature/humidity/density altitude and TAS)
Perhaps the compromise is that if you're NOE at 8000ft, ISA+20 and dodging gunfire in the Afgan mountains......you surely deserve powerful hyd jacks.
If you're helo was designed for flying tourists on city tours of NYC, then I you should be flying within the most limiting factor (be that mechanical or crew experience).
So where does that leave us?
Should manufacturers be providing a comprehensive list of tasks, locations or environments that void the helos warranty?
btw: The Huey jackstalls with carefree abandon. (Euro vs USA isn't a rivalry worth anyones time/effort).
Nick...I can only agree with your points but following that same route:
a. We'd all be flying twin engined helos that can hover at MAUW on 1 at ISA+15 'just in case' the engineers failed us and,
b. We'd all have anti-icing and full IFR avionics just in case, in poor weather with poor airmanship, we failed ourselves?
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Nick
We don't demonstrate jackstall in the 350 as was done years ago on the Gazelle. We do a lot of low flying, however, and your bird avoidance scenario is what makes me wonder where that jackstall threshold lies. I shall certainly be seeking information from the people who ought to know about the more murky corners of the envelope.
Cheers
Dak
PS American was certainly best when it came to producing a certain twin engined fixed wing beauty - you called it the Skytrain. We named it Dakota.
We don't demonstrate jackstall in the 350 as was done years ago on the Gazelle. We do a lot of low flying, however, and your bird avoidance scenario is what makes me wonder where that jackstall threshold lies. I shall certainly be seeking information from the people who ought to know about the more murky corners of the envelope.
Cheers
Dak
PS American was certainly best when it came to producing a certain twin engined fixed wing beauty - you called it the Skytrain. We named it Dakota.
Avoid imitations
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Watch this linked video, it shows what can happen.
http://www.youtube.com/watch?v=9mzT1...elated&search=
http://www.youtube.com/watch?v=9mzT1...elated&search=
having flown the squirrel for a few years, sometimes pretty heavy, sometimes a little enthusiasticly, with no sign of this jack stall, i would love to see what you need to do to induce it.
Also flown regularly in hills with 40 - 50 kt with severe turbulence again with no problem . Am i just lucky ?
Also flown regularly in hills with 40 - 50 kt with severe turbulence again with no problem . Am i just lucky ?
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dak - Re: the Squirrel, one pilot reported the problem in Canada coming off a mountain at 8000 feet, and another reported it flying at 40 kts around a fire - "in a 15 degree banked LH turn around a fire at 80 Kts, 4500' +30 C" Yet another got it about once a month in the Rockies. There have also been two reported incidents of a hardover on the ground to my knowledge, one at Remote helicopters (to a friend of mine) and one at Canadian, which wasn't originally reported but subsequently discovered.
I certainly think more pilot understanding of the hydraulic system is required before the average type rating is given.
Phil
I certainly think more pilot understanding of the hydraulic system is required before the average type rating is given.
Phil
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The fact is that the controls loads that are produced by stall are those that cause jack stall, and it does NOT take a wild maneuver to cause them, all it takes is some degree of blade stall. At high weight and high density altitude, the maneuver required can be shockingly small, and there is little or no warning.
I repeat what I have posted earlier: A control system is supposed to control, not lock up. Jack stall is a design problem that is only experienced in ONE manufacturer's production helicopters, because all the other manufacturers make their servos powerful enough.
Like LTE, jack stall makes pilots need to fear their machines, and like LTE, jack stall is not a condition that need be suffered by pilots anymore.
I repeat what I have posted earlier: A control system is supposed to control, not lock up. Jack stall is a design problem that is only experienced in ONE manufacturer's production helicopters, because all the other manufacturers make their servos powerful enough.
Like LTE, jack stall makes pilots need to fear their machines, and like LTE, jack stall is not a condition that need be suffered by pilots anymore.
Dak, in the days of the military Gazelle we used to low fly at IPS or MPS to give 120 kts or 135-140. At those high pitch settings, the risk of jackstall in a bird avoidance manoeuvre was real. We also demoed the dive to VNE and recovery (including jackstall if you were harsh in the manoeuvre).
Nowadays the DHFS Squirrel is artificially torque limited and so high pitch settings at low level are uncommon and the dive to VNE is seen as unneccessary. I suspect that if the Squirrel was actually owned by the military and used on the front line as the Gaz used to be, then you proably would demo the limits of its flight envelope but because it is owned by a civilian company and operated in a training environment to their limits, you don't.
That doesn't mean that the Squirrel won't jackstall (or suffer servo transparency if you prefer that term) - the only way to find out is to calculate the VNE (if it has a graph for it) and then dive at max pitch/Torque and pull - if it doesn't do it then great - write and tell us.
Nowadays the DHFS Squirrel is artificially torque limited and so high pitch settings at low level are uncommon and the dive to VNE is seen as unneccessary. I suspect that if the Squirrel was actually owned by the military and used on the front line as the Gaz used to be, then you proably would demo the limits of its flight envelope but because it is owned by a civilian company and operated in a training environment to their limits, you don't.
That doesn't mean that the Squirrel won't jackstall (or suffer servo transparency if you prefer that term) - the only way to find out is to calculate the VNE (if it has a graph for it) and then dive at max pitch/Torque and pull - if it doesn't do it then great - write and tell us.
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nigelh - this wasn't jackstall on the ground, but a hardover, which nobody has yet really come up with an explanation for, as far as I am aware, despite taking a machine into the deepest Arctic to see if cold was affecting the servos. Transport Canada banned all hydraulics-off training for a while, which upset all the type conversions, since it's on the syllabus!
My friend was sitting there at the run up and the cyclic just motored over to the left and he couldn't pull it back, which is why there is now an extra hydraulics off check at low RPM before you wind the machine up. I believe the Canadian Helicopters problem was similar.
I only mentioned it in my previous post because it is a problem with those machines that is hydraulics related and some people might not have heard about it.
Phil
My friend was sitting there at the run up and the cyclic just motored over to the left and he couldn't pull it back, which is why there is now an extra hydraulics off check at low RPM before you wind the machine up. I believe the Canadian Helicopters problem was similar.
I only mentioned it in my previous post because it is a problem with those machines that is hydraulics related and some people might not have heard about it.
Phil
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Talking of gazelle Jack stall In the early eighties flew into a small airshow out Bedford way as i was in a helo they parked me with a display gazelle that was RN but being flown by RAf pilot . He did his display, when he landed back I asked him did he normally leave his pull out so late, it was then I first heard the term jackstall, he said he had been lucky it bounced off the ground air cushion as it was jack stalled at the time . 
lucky
lucky