Why do line twist openings happen?

460

Any opinions?

In skydiving, the consensus was/is that the bag got into a spin during deployment. Now skydiving canopies are small and highly loaded so any asymmetry and loading issues are accentuated.

Other than being highly asymmetric or out of control during deployment of a free-packed canopy, what would cause a big-ass base canopy to open in line twists? I think other than a pilot chute that could spiral or do weird things, it is the free-packed canopy catching air during line stretch and creating an out of sequence opening.

Other opinions or evidence, please?

HOBO #7

dan_inagap

Re: [460] Why do line twist openings happen?

on sub terminal jumps I've noticed that if one leg is higher than the other at opening, it consistently ends up with a twist or the beginning of a twist.
If the right leg is up it will twist left (anti clockwise)and if the left leg is up it will twist right(clockwise).

Any thoughts on this?

It's all fun and games till someone burns a site
"I'm so fucking happy I could shit rainbows"

MBA-PATTO

Re: [dan_inagap] Why do line twist openings happen?

Incorrect pc ,not attached crectly

bad body asymetry on opening

reaching one hand up earlier then letting canopy open

bad packing

look at someone whilst pitching

slight wind on that opening or high winds..

you have a bad canopy that is out of trim

or maybe its just pure bad luck..

as long as ya dont get em and then smash into teh wall your gonna be good

http://www.blincmagazine.com/...=Fatality_Statistics
http://www.blincmagazine.com/...e=BASE_Fatality_List

TomAiello

Re: [460] Why do line twist openings happen?

99% of the time that I've seen twists, they develop after the canopy is inflated. The canopy opens off heading, and the jumpers body is "slingshotted" back around to the canopy heading, spinning up into twists as a result.

These twists do not develop from the canopy twisting as it moves to line stretch--rather they occur after the canopy is fully inflated. The process is very fast, though, and most of the jumpers under the twists have thought that the twists were "already there" when they hit line stretch until they were shown the video (often in slow motion).

Anything which increase the velocity differential between the jumper and canopy during an off heading opening, like a good track or shallow brakes, can increase the severity of these twists (because the "slingshotting" has more force).

Tom Aiello

Tom@SnakeRiverBASE.com
Snake River BASE Academy

GreenMachine

Re: [dan_inagap] Harness Input

I've noticed if one leg is higher than the other
at opening, it consistently ends up with a twis
or the beginning of a twist.

During a normal deployment your shoulders
affect the symmetry of the risers taking load
as the drag stands you up in the harness the
load is transferred to the hip rings/junction.

On small SKY canopies you can turn the wing
just by leaning in the harness. Hence having
your legs symmetrical affect the deployment.
Try closing your legs after you pull.

Day BASE 1366
Night BASE 293

-rm

Re: [TomAiello] Why do line twist openings happen?

TomAiello wrote: Anything which increase the velocity differential between the jumper and canopy during an off heading opening, like a good track or shallow brakes, can increase the severity of these twists (because the "slingshotting" has more force). I always thought shallower brakes made line twisting less likely. Wouldn't deeper brake settings increase the velocity differential?
I've tried packing sky canopies with a round of twists or two so as to practise getting out of them for base, but it never worked - canopy came out straight anyways. Spectre 135 and PD student canopy.

Perhaps people should get more used to harness turns and carving, to get more used to the "opposite slingshot" way of handling unbalance. And that people who girl-land (meet the ground with the toggle nearest to the ground in crosswind) shouldn't consider base jumping at all.

Jester: That was some of the best flying I’ve seen to date – right up to the part where you got killed.

mariobat

Re: [TomAiello] Why do line twist openings happen?

TomAiello wrote: 99% of the time that I've seen twists, they develop after the canopy is inflated. The canopy opens off heading, and the jumpers body is "slingshotted" back around to the canopy heading, spinning up into twists as a result.

These twists do not develop from the canopy twisting as it moves to line stretch--rather they occur after the canopy is fully inflated. The process is very fast, though, and most of the jumpers under the twists have thought that the twists were "already there" when they hit line stretch until they were shown the video (often in slow motion).

Anything which increase the velocity differential between the jumper and canopy during an off heading opening, like a good track or shallow brakes, can increase the severity of these twists (because the "slingshotting" has more force).
+1
Thanks Tom

#1913

Fledgling

Re: [460] Why do line twist openings happen?

460 wrote: Other than being highly asymmetric or out of control during deployment of a free-packed canopy, what would cause a big-ass base canopy to open in line twists?
Other opinions or evidence, please?
Line twists on square canopies is a problem inherent to setting brakes. Just as having brakes set is a contributing factor to tension knots in the D Line cascade. I think it's just one of those fundamental flaws that we simply have to work with.

TomAiello

Re: [-rm] Why do line twist openings happen?

-rm wrote: I always thought shallower brakes made line twisting less likely. Wouldn't deeper brake settings increase the velocity differential?
Deeper brake settings slow the canopy down, which decreases the forward surge at opening, thereby reducing the difference in velocity (speed+direction) between the canopy and jumper during an off heading opening.

Tom Aiello

Tom@SnakeRiverBASE.com
Snake River BASE Academy

Fledgling

Re: [TomAiello] Why do line twist openings happen?

TomAiello wrote: 99% of the time that I've seen twists, they develop after the canopy is inflated. The canopy opens off heading, and the jumpers body is "slingshotted" back around to the canopy heading, spinning up into twists as a result.
I also believe this to a certain point. Though I think this to be the leading cause only on slider off jumps. There are also ways to combat the slingshot effect on rougher openings.

TomAiello

Re: [Fledgling] Why do line twist openings happen?

Fledgling wrote: There are also ways to combat the slingshot effect on rougher openings.
The most effective I've found is to get your hands between the risers and push outward to stabilize your body and stop it spinning up.

Trying to sit up during deployment also seems to help because it reduces the distance of travel as the body is brought around to the canopy's heading.

Tom Aiello

Tom@SnakeRiverBASE.com
Snake River BASE Academy

Fledgling

Re: [TomAiello] Why do line twist openings happen?

TomAiello wrote: Fledgling wrote: There are also ways to combat the slingshot effect on rougher openings.
The most effective I've found is to get your hands between the risers and push outward to stabilize your body and stop it spinning up.

Trying to sit up during deployment also seems to help because it reduces the distance of travel as the body is brought around to the canopy's heading.
I've always found that a nice wide body position helps to reduce the inertia of the slingshot too. Some of the worst line twists I have seen happened to people who would continually rush into assuming the classic compact box man for a 1 or 2 second delay.

JSol

Re: [Fledgling] Why do line twist openings happen?

Don't have experience with BASE yet, but what Tom says makes a lot of sense physically.

Imagine a 180: if your canopy opens with large forward speed(shallow breaks) then it will twist you with a lot of force since there is a higher speed, but if it opens at almost zero speed the force it will exert is less. Or something like that I think

littlestranger

Re: [-rm] Why do line twist openings happen?

In reply to: And that people who girl-land (meet the ground with the toggle nearest to the ground in crosswind) shouldn't consider base jumping at all.
hello mr. [-rm],

girl-land?

that's pretty insulting, to say the least.

what generation are you from?

thank you,

teresa

TomAiello

Re: [Fledgling] Why do line twist openings happen?

Fledgling wrote: I've always found that a nice wide body position helps to reduce the inertia of the slingshot too.
Definitely. A wider moment arm increases the rotational inertia (of rest), reducing the total rotation.

In other words, yes, what he said.

Tom Aiello

Tom@SnakeRiverBASE.com
Snake River BASE Academy

-rm

Re: [littlestranger] Why do line twist openings happen?

littlestranger wrote: hello mr. [-rm],

girl-land?

that's pretty insulting, to say the least. well, we call it that up here, I guess as a joke. Lots of student girls and downsizers were getting hurt and after we started calling it girl-landings they kind of saw the flaw and worked with it. Brought attention to it. If being a joking chauvinist keeps people from getting hurt, is it still wrong?

littlestranger wrote: what generation are you from?

thank you,

teresa 1984, but I have a gf. thanks for asking though.

Jester: That was some of the best flying I’ve seen to date – right up to the part where you got killed.

littlestranger

Re: [-rm] Why do line twist openings happen?

In reply to: 1984, but I have a gf. thanks for asking though.
you're welcome : )

xnewmanx

Re: [littlestranger] Why do line twist openings happen?

After about 250 base jumps I experienced my first linetwist this weekend. After a 6.5s delay I had an off heading opening and just spun up at least twice. Fortunately I was at the NRGB on BD. I was headed for the tracks on the north side of the bridge but was able to muscle my way above the twist and get some lines in my hand to peel it into the water before I hit the trees.

I certainly experienced the phenomenon that Tom A mentioned. The canopy probably only opened 90 left but did so in such a way that I just instantly spun up. I think my body position was OK. I know my packjob was garbage... but so were the 3 that followed it and all opened straight.

Zebu

Post deleted by Zebu

Fledgling

Re: [Zebu] Why do line twist openings happen?

Zebu wrote: Maybe also even keeping knees together until opening and then (in event of off-heading) star fishing them out as you open up your elbows?
It would be far easier to simply adopt a wider body position for all openings.
On a side note. You shouldn't be "reaching" to catch your risers. It's a bad habit that is pointless and will eventually lead to you smacking the shit outta your hands.