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Jackstall: What is it? Any experiences?

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Jackstall: What is it? Any experiences?

Old 29th Nov 2006, 00:48
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Exclamation Jackstall: What is it? Any experiences?

I had an instructor mention something calles Jackstall to me earlier today. He's an ex Gazelle pilot and apparently you could get it to jackstall with difficulty, but he couldn't explain what it was. I've had a look in the bible (Wagtendonk) but can't find any reference to it in there either.

Anyone know? Is it something that occurs just on the Gazelle, or a military term, or something else?

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Old 29th Nov 2006, 02:15
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The pros here will be able to explain this much better than me but, as I understand it .....

'Jack stall' occurs when the blades overpower the hydraulic servos - the jacks stall - and the controls lock solid.
If you're training in a Robinson that may not mean much because it doesn't have hydraulic controls.

It's most likely to be caused by extreme manoeuvring.
eg In the Gazelle, if you're in a descending right turn and pull really hard, you might experience jackstall.

It's very unlikely to happen in 'normal' flying.
However, if you were flying straight and level at high speed, and suddenly pulled up and to the right (eg to avoid a bird), it could happen.

Recovery from jackstall:
Release the pressure and the controls will unlock.

If it happened near the ground you'd have a problem because you wouldn't have much time/space to correct it - and possibly exacerbate the problem by instinctively trying to pull away from the ground, instead of releasing the pressure. However, as a PPL, you're unlikely to be doing extreme manoeuvres near the ground.

Is it something that occurs just on the Gazelle?
It can happen in the Squirrel - certainly in the older models. I don't know if the newer models have more powerful servos.
And possibly in some other types.
I associate it with French design helicopters, but that may be wrong.

Military term?
I don't think it is, but the FI who did my Gazelle conversion was a Mil pilot so possibly.

Or something else?
It's called 'servo stall' and 'servo transparency' in America.
I don't know what it's called in France - it’s like those French have a different word for everything. (Credit: Steve Martin)

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Old 29th Nov 2006, 03:08
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You did very well!

One tidbit to add - the force that locks the hydraulics comes from the blades as they are stalled, and is the same as the pitch down that the wing of an airplane performs when the wing stalls. This "moment shift" occurs on every airfoil when it stalls, because the orderly lift is no longer being produced, and instead, the wing behaves more like a barn door, with the wind pressure acting about the midpoint of the wing area, instead of at the 1/4 chord point when the airflow is sweet and smooth.

As the rotor blade stalls more and more (the stalled area moves outward toward the tips) the blade presses down more on the controls and can overpower the pilot or the hydraulics. In a non-hydraulic helo, the controls stiffen up, and can throb and vibrate as the stall gets worse and worse.

Jackstall is not experienced on most helos because the hydraulics are powerful enough to hold the controls against whatever the blades can dish out. Some french helos are actually designed with weaker hydraulic controls, and thus their pilots get the thrill of experiencing jack stall if they maneuver excessively.
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Old 29th Nov 2006, 03:11
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Have nothing to add other than yes you can get it in newer models---had it a few times in a B2 Astar.
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Old 29th Nov 2006, 04:31
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had a fatality @ needle rock in the coromandel, nz, bak n '94 i think.low time as350 pilot giving some locals a free ride high speed descending turn to the right and flew straight into the water.killed a female pax.fed's proved he was below 500' because the pax said they could see the mainland through the hole in needle rock & the only way that was possible was to be below a certain height at a specific seaward area.
apparently his type rating never covered hydraulic jack stall.
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Old 29th Nov 2006, 06:40
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Gents, I believe this topic was covered around Dec 04'.
Thread then called 'A-Star "Jack stall".
If searched for, it could help aswell.
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Old 29th Nov 2006, 07:37
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Thanks everyone, that was really useful. I did do a search for it but nothing came up. Seems strange that more people don't know about it - none of the rotary guys I talked to yesterday did - maybe it is just because we've all spent most of our time on robbies. Never heard of it in the 206 course either though.

Very helpful - thank everyone

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Old 29th Nov 2006, 07:56
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there is a video if you want to see it. its a US coast guard experiencing a jack stall while doing a low pass in front of a ground camera.

its pretty nasty and he does a great job of recovering it.

pm me if you want the address.

cheers BP
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Old 29th Nov 2006, 08:54
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1. With jackstall on the AS350, has it been definately been determined whether it is the hydraulics/servo not being powerful enough, or does the hyd pump drive belt slip under high demand?

2. How is the Gazell hyd pump driven?
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Old 29th Nov 2006, 10:41
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Sprocket. The pump produces constant pressure. Any excess not required when not moving the controls is vented through a pressure regulating valve. So, belt slip doesn't come into it. Gazelle pump gear driven, if memory serves me right. Known in France as 'Jaques S'tall'.
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Old 29th Nov 2006, 11:01
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Yes, the feedback forces overcome the pump output so there is a sudden "jolt" and temporary loss of control. We used to be required to demo it to studes going through the basic rotary course at Shawbury. Goodness knows what it did to the airframe, as it didn't feel pleasant! We used to go into a steep high speed descent and pull up quite hard, whereupon the aircraft "kicked" more nose up and rolled a little, which actually helped the recovery. It would be quite difficult to see this in normal ops, as it was really at the "extreme of manoeuvre".

Apologies for thread drift - I'm certain it was also possible to get close to main rotor jackstall in the military Puma. I displayed that aircraft for a while and sometimes the old girl registered her displeasure by suddenly vibrating hard, again in fairly extreme circumstances (high speed pull up, etc). The recovery was just to back off the collective pitch and/or reduce the extremity of manoeuvere, just as in the Gazelle. However, a pilot flying on normal squadron ops would have to be very hamfisted to reach those limits.

As for the two display pilots who achieved the impossible by hitting the tail rotor blades against the tail pylon...... "LSH", I think, might remember one of those occasions quite well
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Old 29th Nov 2006, 13:38
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Old 29th Nov 2006, 14:47
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I ended up with about 2000 on the Gazelle, and IIRC we did the 'Dive to Destruction (VNE) and Jack Stall' quite often. We also did it in a turn to the left I think because the aircraft rolled hard right (or the other way around !) In fact I do recall that this manoeuvre was banned after an inexperienced pilot went off to do part of the flight test schedule which called for jack stall and had to recover from 'an inverted position' !

It also reminds me of sending off solo students in the Firefly: 'My aerobatics were poor, Sir, but my spin recoveries were spotless' !
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Old 29th Nov 2006, 22:34
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Thanks for the link - very insightful! bet those dauphin pilots had some washing to do after that!
Apologies too by the way. I just tried the advanced search on this page for jack stall and came up with two pages or results that didn't show in the basic search. Life's a learning curve!
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Old 30th Nov 2006, 05:43
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Jack stall = Being stuipid and flying the aricraft in ways it was not ment to be flown ( i.e. cowboying the thing around)

Anyone who says they have experienced jackstall, has done so being an idot (IMO).....

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Old 30th Nov 2006, 09:09
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What Limits - remember it well. The CFS course used to teach recovery from descending 60 degree bank turn to the left. My instructor decided to try it in a descending turn to the right. He pulled hard and the next thing I knew was Newport and my chinagraphs were on the roof. I did ask for an instructor change after that.
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Old 30th Nov 2006, 10:07
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Part of the post 100 hour service test flight on the UH1 was to move the cyclic forward and right to aft and left in three seconds, and vice versa to check for any propensity for jack stall. This exercised only one jack at a time. The observer used to cover the hydraulic boost switch, to turn it off and then on if a stall occured!!
You could induce it fairly well by pulling up and rolling from a high speed pass!
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Old 30th Nov 2006, 19:41
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Two recent occurrences come to mind, AS350B2-
First, After a "dog and pony show" at an elementary school, clear ground all around- High speed (125 knots, yes I know...) low level pass- 300' AGL- and attempt climbing turn to the left- No left cyclic, it feels like I hit a control stop left of cyclic center- released pressure, and cyclic normal. (NO MORE HOTDOGGING, EVER) I'm very glad I wasn't NOE.
Second, max NG cruise at 2000 ft, DA approx 4000, vulture spirals up approx 200 feet dead on nose, attempted to roll right, not "agressive"- no right cyclic- a little aft cyclic and a lesser movement right. In 13 years on the airframe, that's the only "normal" control input I've ever done that was limited by hydraulics, and I couldn't repeat it.
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Old 30th Nov 2006, 21:52
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Rotorboy, I think that you are probably right that in most cases the aircraft must be flown outside the normal envelope for jack stall to occur. However, I have been been told of one instance where extreme mechanical turbulence (at high DA) caused jack stall.

What Limits - sorry for grabbing a user name so similar to yours. I'll go get another one.
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Old 1st Dec 2006, 00:40
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I have to say Rotorboy is right in many cases. However we have a little situation in the Southwest called the "Grand Canyon" For those who have flown there on shall we say Breezy days. It will seriously kick your a$$. Many pilots while trying to keep the world level there have had the sweet occasion of getting into jack stall. From their descriptions it happened while making many manuevers to stay upright.
I can say that I have not encountered it myself, and really it would scare the sh1t out of me anyway. So after thirty six years of stick time, heres hoping I never do find out what fun it can be.
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