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JohnDeere

A better way out of line twist!

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An interesting physics lesson. However, I've never seen a line twist a jumper could get his hands ABOVE and you'd be on the wrong end of it. I guess the only way to test your hypothesis is to pack yourself a few and try it...:)

You're mostly kicking to get the 'untwist' started (or to try and stop it as it spins you the wrong way)...once it starts to untwist it builds up momentum.
Keith Abner
D-17590

"Those who do, can't explain; those who don't, can't understand"

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You seem to be pretty determined to prove that kicking does nothing when suspended under canopy. I will leave my knowledge on the subject out of this because there has been one with a great deal of physics knowledge than mine try to tell you different and you refute it. So instead, I am just going to pose a simple and small challenge to you. Next time you are under canopy try kicking yourself into a line twist. By your theories it can't be done and if you can report back and explain it.

If still are determined to say it does nothing, then I must have done something super special when I kicked myself into a line twist for a photo. :ph34r:

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But, but, but...practicality isn't as much fun as arguing theory.
Fly a wingsuit. You'll quickly learn the best ways to get out of twists (turning the twist on itself while kicking).
Theory is wonderful and the stuff bonfires n' beer are made for, yet applying an effort will often prove the theory wrong.

One guy had written quite a few words on what to do in a water-wingsuit landing. People believed it for years. Until someone actually took the time to enter the water in a wingsuit, and we discovered that following his written theory (he'd never been in the water in a suit) would have likely killed the otherwise safe wingsuiter.

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It is also interesting to see that the parachute keeps flying in the same direction, it is the jumper that spins which means that although the jumper has a higher mass the parachute has a much higher overall inertia (at this axis).



The parachute keeps flying in the same direction because you and the parachute are flying and because it's designed to be stable in yaw, not because it has a larger moment than the jumper.

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My last post was supposed to say "For the apparent translation he achieved". I assume he's swimming through the air he's breathing, since again it's impossible to translate without ejecting mass or suffering an external force.



This is correct.

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Not without an external force. It is not possible to change the angular momentum of a free body by applying only internal stresses.



You're right that you can't vector your angular momentum (by "kicking" or "squirming" or whatever you want to call the motion) without applying a torque on something... If only there was something we were attached to that we wanted to rotate relative to by applying a torque on it... hmm...

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One thing you neglect, is that a parachutist in line twists is not a free body. Because of drag you're behind the canopy some, giving you some relative potential energy. Also, there is friction in the twisted lines, which allows you to add momentum similar to spinning in an office chair.
Brian

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One thing you neglect, is that a parachutist in line twists is not a free body.



Ah, a very good observation.

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Because of drag you're behind the canopy some, giving you some relative potential energy.



wait, what?

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Also, there is friction in the twisted lines,



abort! abort!

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which allows you to add momentum similar to spinning in an office chair.



:|

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So instead, I am just going to pose a simple and small challenge to you. Next time you are under canopy try kicking yourself into a line twist.


I will do that, though kicking into a twist is not identical to kicking out of a twist, since the twist reduces your number of connections to the canopy from two (your 3-rings) to one (the twist). I'm also debating whether or not to build a hanging harness to try stuff out on. I think this is a pretty fascinating problem. :S

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Worse part is, The Op, has a good point, in many cases that is a good way out of twist...then someone came up with this "kicking does nothing" story, and blew the conversation to hell!


It was actually mentioned in the very first post, so I didn't exactly pull it out of nowhere.

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If only there was something we were attached to that we wanted to rotate relative to by applying a torque on it... hmm...


First, thanks for considering it seriously rather than just throwing me under the bus. :)

Second, very true. My reasoning for considering the jumper as a free body (at least as far as kicking into a rotation) is that the single connection to the canopy (the twist) has very little torsional rigidity, and as such would be difficult to apply a torque to without reaching up and twisting the line groups, which is the method advocated in the OP.

Now, It hadn't occurred to me until just now that you can still develop some momentum by swinging under the canopy, which you might then be able to change the direction of and use. I may try hanging myself (yikes!) and fiddling with this. Perhaps two risers to a bearing/swivel, then the bearing to some line groups to the ceiling. That would eliminate any friction at the "twist" location and let me play with it. :P

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I've never seen a line twist a jumper could get his hands ABOVE and you'd be on the wrong end of it.



Getting my hands above the twists helped on this jump. Heck, there was no room BELOW the twists.



Did you do any kicking to try to get out of this?
Be the canopy pilot you want that other guy to be.

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Hey, you're under a round! Pretty sneaky.
Different laws of physics.



I do it all the time with a square as well. Especially when training other jumpers on canopy skills, flying in formation and camera. It doesn't matter which canopy you are under, this is not a difficult thing to do.

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I will do that, though kicking into a twist is not identical to kicking out of a twist, since the twist reduces your number of connections to the canopy from two (your 3-rings) to one (the twist). I'm also debating whether or not to build a hanging harness to try stuff out on. I think this is a pretty fascinating problem.



You can keep on doing it. I have done 5 line twists just to see how many I could do. That wasn't the final amount I just got bored with it. You can do this exact same thing on a swing set. Give it a shot. You are overlooking a few things but experience with trying it will teach you better than I would plus it won't go into larger debates on this thread until you tried it for yourself.

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the twist reduces your number of connections to the canopy from two (your 3-rings) to one (the twist).



Ummm...... :S

The last time I had linetwists, both of my 3-rings remained connected. I must have been doing it wrong.

Having linetwists in no way changes the number of connection points you have with your canopy. It just changes the angle of force on your connections.
Owned by Remi #?

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The last time I had linetwists, both of my 3-rings remained connected. I must have been doing it wrong.




Well, I'm glad at least a couple people have been willing to think about what I'm saying and respond with some reasonable suggestions and comments, rather than just making willfully stupid remarks. :-/


I'll try some things out, see what I can see.

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in re to "Well, I'm glad at least a couple people have been willing to think about what I'm saying and respond with some reasonable suggestions and comments, rather than just making willfully stupid remarks. :-/ "
.............................

I'm wondering if it is possible for a seasoned skydiver NOT to kick when they've got line twists.
The only time I didn't kick I chopped .
Every other time (hundreds under f111) kicking worked just fine.

Perhaps with modern canopies its a good idea not to even try and kick out due to loss of height awareness , imminent death ,etc.

I'm sure many people would still be alive if the instant reaction to line twists under ZP was ....CHOP.

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Whatever you say, Chief.;)



John,

Sure hope he doesn't have a problem under canopy, he will go in talkiing it to death.

Sparky

And kicking does help with line twists, I have done a time or 2. :P
My idea of a fair fight is clubbing baby seals

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The last time I had linetwists, both of my 3-rings remained connected. I must have been doing it wrong.




Well, I'm glad at least a couple people have been willing to think about what I'm saying and respond with some reasonable suggestions and comments, rather than just making willfully stupid remarks. :-/


I'll try some things out, see what I can see.



I, for one, am glad to see that you are finally coming to the realization that you do not understand what you are talking about when it comes to kicking out of line twist.

(It does work, by the way)

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I'm not totally convinced, but save that argument for a later date. I have an interesting (and far more related to the OP) observation.

I just made up a dummy thing hanging from my ceiling, with four line groups going to two attachments on a weight.


Just spinning this up normally produces a nice, even line twist which comes out very easily on its own. I think that's pretty likely to be the type of line twist I see under a nice docile canopy, which flies straight even with the twist and comes out quickly without any real user input.
http://imgur.com/GwYM8.jpg


However, spinning it up with a slightly slack line, or forcing the twist to move up the lines with my finger, produces a much weirder and interesting twist. The twist runs up all four lines, then splits (usually unevenly) up the right and left groups. I bet this is what throws an otherwise good canopy into a turn. EDIT: actually, I just looked more carefully and this develops very repeatable if the jumper is not centered directly below the canopy when the twist occurs. Smaller, twitchier canopies would obviously be much more susceptible to this since they are much more sensitive to line length.
http://imgur.com/YSWNF.jpg

This configuration has almost no natural tendency to untwist. I looked for it in videos posted online and saw it pretty often (most times the video is not clear enough to tell), especially in line twists people chop.

The OP's method of twisting the problem down towards the risers (which I tried on the hanging thing) seems to change the weird twist into a nice even one, which then comes out easily. I don't know the mechanism by which this happens, but it's very repeatable. Spreading the risers apart doesn't seem to help unless you get the twist to travel a pretty long way up the lines.

It's actually pretty easy to see in the OP's video: http://www.youtube.com/watch?v=5FD2I8OqbS0#watch-vid-title



EDIT: also, does this board's markup allow embedded images? If not, can it be done using HTML instead?

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But, but, but...practicality isn't as much fun as arguing theory.
Fly a wingsuit. You'll quickly learn the best ways to get out of twists (turning the twist on itself while kicking).
Theory is wonderful and the stuff bonfires n' beer are made for, yet applying an effort will often prove the theory wrong.

.



Not all theories are created equal, Spot.
...

The only sure way to survive a canopy collision is not to have one.

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