The Dauphin & the Pole (video)
 

http://www.alexisparkinn.com/photoga...-8_65LokUp.mpg

The title says "flight control lockup" but looks like some kind of "display"... :}

Regards.
Aser

We had a thread a while back on "jack stall" and there were some magnanimous ppruners who thought that when the servos stall (because they are not powerful enough to overcome the blade forces) that this was a "good" thing because it kept the rotor controls from being over stressed.

This is a film of a dauphin (one of the type helos that are jack stall prone) experiencing jack stall, going out of control, but being recovered by the pilot when the rotor forces subside enough to allow the cyclic to become effective.

Another name for jack stall is control lockup, and another is servo stall.

I recall the thread, and readings about the AS350...
Any other type with history of jack stall?

Gazelle.

Practised recovery procedure with a QFI during my type rating training, but not experienced otherwise - yet!

Jack-stall in the Whistling Chicken-leg
 

You won't be able to miss it.

Waiting for a Harrier to pitch up for some FAC in the Falklands, my pilot decided to relieve the boredom by allowing me, then just an aircrewman, to do a run onto the target.

IPS, in a shallow dive, I pulled the nose up at the end of the run to find it wouldn't come up. By unloading the disc (probably more through luck than skill), we got the cab back, and went back to our hiding place somewhat chastened.

It was discussed on my Pilots Course, but I don't recall a demonstration sortie.

Jack stall
 

The tendency for jack stall was shown on the APC and was checked for during airtests. Not a big drama to get out of as long as you have height available. just reduce collective and relax! A Gazelle crashed at Middle Wallop in the 8o's due to this!

Gazelle jack stall
 

I seem to remember that the Gazelle had a "two-stage" problem. There was "Jack Stall" and there was "Deep Jack Stall". The Gazelle IFTU crash referred to happened to an acquaintance of mine and was apparently a case of the latter. What's the difference? I can only guess that Jack Stall can be recovered by reducing the flight loads and that maybe Deep Jack Stall occurs when you have had Jack Stall for so long that your ability to reduce pitch or adjust the cyclic is no longer possible.

When I did my Dauphin conversion I had to experience Jack Stall so the instructor (a test pilot) made me fly to VNe in a shallow dive then wrap it into a climbing right hand turn. As soon as you get it you know what it is and can roll off the turn and reduce collective pitch. Scary but worthwhile.

G

:ok:

Hmmmm.....doesn't the limit light (off the load sensor) illuminate before you hit jack stall?

Yes, Limit light should illuminate before jack stall begins.

However, when it does stall, the dauphin pitches forward and left very violently. Doubt if that was what happened on the video.

"However, when it does stall, the Dauphin pitches forward and left very violently. Doubt if that was what happened on the video."

That's exactly what it is doing, is it not? From the initial pull up, the aircraft bunts and rolls left, ending up level again and rocking laterally from the subsequent PIOs.

What surprises me is that such a benign pull could induce jack stall on a twin hydraulic system aircraft. The 155, which although with a different head design, shares many common hydraulic system componants, can be thrown around with real abandon and no ill effects. There is a servo accumulator for the RHS system, but it is only there incase the system fails and it then assists the LHS system while the pilot regains level flight. Even then, it is only there for extreme manouevering.

I have to say, GIC, I'm a bit disturbed to hear that someone has shown you this effect during a conversion. The loads that must be imposed on the airframe in carrying out this demonstration would be large. I can't see it happening with much less than 3.5 - 4 g and that hardly qualifies as FLM territory!

Obviously the Dauphin/AS350 Hyd. sys. seems to be underpowered for some maneuvers.
At least AS/EC is doing something about it with the nice looking 155...

IMHO only the French products regularly exhibit such propensity (many of them), because their certification authorities have a much more tolerant attitude for such shortfalls. The only other aircraft that I know to exhibit jack stall were early model Cobras, which were corrected. I firmly believe that neither FAA nor CAA would tolerate an aircraft that can be so easily driven out of control.

It's now more years ago than I care to recall, but we had a very close call at the Navy Test Pilot School with an HH-65. Seems the crew (who shall remain nameless, but it wasn't me) were doing pullups for testing maneuver stability. The technique was questionable, and while they were doing a pretty cautious buildup, they didn't realize that the limit light had a definite delay before illuminating.
On the pullup that bit, they pitched up to about 60 degrees nose up, and the rotor blades took a nice chunk out of the vertical stab (4 distinct cuts). They then proceeded to fall for 1500 feet with the controls all locked up (hydraulic system was unpressurized, and the flight path was effectively zero G to the airframe). Once the nose dropped to something 'normal' control was regained, hearts were restarted and they landed quite soon after that.
This of course caused a major kerfuffle, and we then proceeded to look at the HH-65 maneuvering capability. After much testing, it turned out that at cruise speed, you didn't get much over 30 degrees of bank before you would get the limit light, and God help you if you decided to do a violent pullup to avoid a flock of birds...

The link to the video doesn't work for me. :(

Is there another source?

212 Man
 

Who am I to question a test pilot's contribution to my meagre skills. I was nonetheless grateful for the opportunity to explore the envelope. You will be pleased to know that even at that time (7 years ago) I was well past that stage in my life when I would want to thrash anything around at VNe let alone wazzzzz it to the control limits. The frequency with which those that indulge in such antics end up in a wooden overcoat has convinced me not to bother. Working with an EMS outfit similarly cures you of ever wanting to go motor bike cruising. There's nothing quite like picking up bits of people to make you think hard about the wisdom of trying to revisit your youth on today's unforgiving roads with todays unsympathetic "Chelsea Tractor" drivers.

G

:ok:

Hi all having flown Gazelle and Dauphin C,N,N2 and having trained to experience incipient jack stall we were always told to expect nose up and roll to the right. Having experienced jack stall for real once in more than 14,000 hrs of helicopter flying and that was enough for me the aircraft a 365N did in fact pitch violently nose up and roll to the right. I am told that this occurs because the forward right side jack stalls first because it has significantly more force to overcome when the disc loading becomes excessive ie:- high collective pitch, high airspeed and any excessive cyclic movement. When this occurs you cannot move the cyclic or the collective utill the jack is unstalled by the change in aircraft attitude then watch out for over controlling afterwards. A thorough engineering inspection afterwards revealed no damage thankfully.

The HH-65 (which is the Dauphin in everything but name and engine) pitches up. Sometimes quite violently.
The collective can only be moved down, unless the pitch up and flight path is a zero-G flight path, in which case the unpressurized hydraulic system has no fluid going to the pumps and the controls are locked until 'normal' flight is resumed.