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|Posté le: Jeu Jan 19, 2012 06:55 pm Sujet du message: Safety Article: G-LOC, COULD IT HAPPEN TO YOU?
|See: G-LOC, COULD IT HAPPEN TO YOU? By Dr. Dougal Watson.
We have seen instances of this in our sport. Hard to prove after the fact, but our assumptions are most probably true.
Chapters include HISTORICAL,
PREVENTION OF G-LOC
WHO GETS G-LOC?
WHAT CAN I DO?
Here's the draft of that story on spirals and blacking out, for your perusal and comments...
(The program's name was G-Lock, but that would have come from the technical name G-LOC)
The human body was not built for tolerating high G-forces. Yet we can spin up enough G-force to go beyond our limits, black out and lose control of our aircraft.
Certainly in paragliding spiral dives (perhaps in hang gliding or gliding aerobatics??) there's a risk. (HG, gliding - comments?)
I'll get straight to the point. A program titled "G-Lock" (screened on ABC or SBS in the first 4 months of the year) gave the symptoms in order as:
- tunnel vision (loss of peripheral vision)
- loss of colour vision
- loss of vision completely
- unconciousness - "G-LOC" (LOC="Loss Of Consciousness")
The progression of symptoms can be quite rapid. It varies from person to person and situation to situation - generally between 3.5 and 5 G.
At any rate, around 3-4 G moving the extremities becomes difficult, or even keeping your head upright (if it's not already upright).
Any illness (even a light cold or gastro-intestinal infection), tiredness, alcohol, caffeine etc can significantly affect your tolerance of G-forces.
In extreme build-up of G-forces it's possible to skip the symptoms above and go straight to unconsciousness, but an increase of +6 G per second would take some doing!
Dr Dougall Watson has written a great paper on the subject from the powered aircraft pilot's point of view.
(Godfrey - thanks for this - could you or Dave Russell supply reference details for this? It looks like a published paper)
More brief but useful information can also be found in good encyclopedias - try "acceleration".
The guinea pigs in the program were jet fighter pilots. They were put in special training equipment with a video watching them, and spun up to a good high G force.
The pilots were told to tense all the muscles in their lower body - calves, legs, bum, etc to keep blood up in the top half of their bodies and they seemed to be doing some sort of controlled pressure breathing.
A couple of them, who hadn't yet made the pass mark, could been seen to succumb fairly dramatically.
Their eyes suddenly wandered all over the place, followed quickly by the eyes closing and the head moving as they nodded off! On deceleration they recovered quickly but were confused for about 30 seconds.
The pilot often retains no memory of the event.
Much higher G-forces can be tolerated when the direction is across the body instead of up and down, so maybe a hang glider pilot, being in a prone position,
might not be at such risk of blacking out, though "heavy" limbs might still be an issue for controlling flight. (comments?)
So how much spiral can you do? Being no "acro demon" I'm not the best person to comment on this.
In fact I didn't even do a half reasonable spiral until I'd already held my advanced rating for several years!
But I did a "crash & splash" (SIV) course in Europe and was told that for a spiral to be useful (eg for avoiding clouds) it needed to be up to 10-12 m/s (yes, they used metric. 2000-2400 ft/min) and initiated within about one turn. (Otherwise a B-line would do the job!)
Well, I duly did my 10-12 m/s spirals (with the reassurance of two radios and a speedboat on the lake under me) and felt ok. In the second one, consecutive 15" average sink rates from my barogram were:
-1.8, -9.4, -1.8 m/s.
So the whole thing was initiated and ended in about 15 seconds, losing 141 metres (460 feet) in 15 seconds.
I found that for years I had been using bad technique to finish the spiral - so the instructor suggested that when
I eased up on the inner brake, and the wing started to respond, that I pull it back down just a bit, to control the exit from the spiral.
There must be a few other pilots out there whose spirals aren't all they could be. There were other tips and I'm not qualified to give the lesson - so talk to your instructor!
So how much spiral can you do? Can't say. I've heard some pilots in Australia say they've done 15 m/s and started losing their vision.
I've heard one in Europe say he had done 20 m/s. I reckon do an SIV course if you can. You don't have to go all the way to Europe to do this and other manouevres - there are good instructors here and relatively soft lakes.
(For those who like to fiddle with numbers, with a well-balanced wind-speed meter and someone watching with a stopwatch you should be able to calculate the G-force...)
1. Radial acceleration = (speed squared)/radius
2. speed = 2 pi * radius/time taken to do one turn
(speed is the horizontal component, not the full vector which combines your sink rate. Your meter, if set up right, will measure mainly the horizontal component)
>From these, the G-force, if I've done my algebra correctly is:
G = 0.178 * speed / time taken for one turn. (again horizontal component. Speed in km/h, time in seconds)
The constant 0.178 is 2 pi/(3.6 * 9.
(I hope you're not fiddling with your meters while doing your biggest spiral - the ground comes up fast!)
I'll leave the last word to my paraglider's manual:
"... the very high G-forces make it difficult to sustain a spiral dive for long and it can place very high loads on the pilot and the glider.
As soon as any, even slight, light headedness or impaired vision is noticed the spiral should be exited immediately."
Haven't got any spiral photos - anyone got any that they might loan to the editors?
I also think there may be some misunderstanding about the testing and "G force ratings" of modern back protection.
The rated value of 8 or 15 G's mentioned in the above posts do not indicate the G-force levels the protector will protect you from, but rather the results of the DHV or other test.
The way the DHV test is conducted is to drop the harness with a dummy 50-80 kg weight in it from a height of only 2 meters (!!) and measure the deceleration experienced by the dummy in terms of "G's".
Therefore, a DHV tested back protector with a rating of 15G means that if you fall from 2 meters straight down onto an unyielding surface, you will feel 15Gs of deceleration.
Therefore, a lower "G" value is actually better, and the best Foam and airbag protectors recently tested have ratings from about 12-15 G's from 2 m. Hope this helps.
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