Skydiver

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SumWunGye
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Skydiver

Postby SumWunGye » Thu Apr 07, 2011 6:21 am UTC

Let's say you were skydiving, and your pullstring won't budge the parachute at the all. If there was a body of water you could land in, would it be best to land flat on your back, or in another position? (Considering, of course, the parachute pack on your back and the weight of the suit.)

Also, how shallow would the water need to be before it starts becoming risky? (As far as emergency parachuting landings go :roll: ) Could you safely land in a backyard pool, for example?

Just an interesting thought I had recently. :)

(P.S. - Under 'Science' because it's a pretty Physics-oriented problem.)

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Re: Skydiver

Postby Ulc » Thu Apr 07, 2011 6:35 am UTC

SumWunGye wrote:Let's say you were skydiving, and your pullstring won't budge the parachute at the all. If there was a body of water you could land in, would it be best to land flat on your back, or in another position? (Considering, of course, the parachute pack on your back and the weight of the suit.)


If the emergency chute doesn't deploy, landing on water isn't going to help you one bit. The deacceleration is still far to rapid.

Also, how shallow would the water need to be before it starts becoming risky? (As far as emergency parachuting landings go :roll: ) Could you safely land in a backyard pool, for example?


If the emergency chute does deploy, landing in water is not necessary to survive - people get hurt when they land with a emergency chute, but they don't die. And you in general *really* don't want to land in water if you can avoid it, as you do not want to be potentially tangled in the chute in a position where you can drown.
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Re: Skydiver

Postby ++$_ » Thu Apr 07, 2011 6:45 am UTC

I think you are extremely likely to be dead regardless of how deep the water is. In any case, you should not land on your back as you will probably suffer severe spinal and head injury that way. Hit feet-first (and bend your knees as though you are landing on solid ground, because the water will be pretty close to that). This will make sure that the worst injuries are farthest from your head, and it will also keep your head at the top (enabling you to more easily find your way to the surface afterwards).

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Re: Skydiver

Postby Soralin » Thu Apr 07, 2011 8:22 am UTC

Feet first, but that likely won't help you if you're falling from skydiving heights without any chute:

http://en.wikipedia.org/wiki/Golden_Gat ... e#Suicides

The Golden Gate Bridge is the most popular place to commit suicide in the entire world.[49] The deck is approximately 245 feet (75 m) above the water.[50] After a fall of approximately four seconds, jumpers hit the water at some 76 miles per hour (122 km/h). At such a speed water has been determined to take on properties similar to concrete.[citation needed] Because of this, most jumpers die on their immediate contact with the water. The few who survive the initial impact generally drown or die of hypothermia in the cold water.

An official suicide count was kept, sorted according to which of the bridge's 128 lamp posts the jumper was nearest when he or she jumped. By 2005, this count exceeded 1,200 and new suicides were averaging one every two weeks.

The fatality rate of jumping is roughly 98%. As of 2006, only 26 people are known to have survived the jump.[51] Those who do survive strike the water feet-first and at a slight angle, although individuals may still sustain broken bones or internal injuries.

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Re: Skydiver

Postby idobox » Thu Apr 07, 2011 8:46 am UTC

only one way to be sure : experiments.

If you jump in water from a few meters high, and arrive flat, you will feel a lot of pain, because you will decelerate very quickly.
If you jump head first, and your position is not perfect, you can hurt yourself pretty badly (twisted limbs)
The safest way is to jump feet first.

People with training routinely jump from 10 or 20 meters high in water without hurting themselves. How fast is terminal velocity achieved?

I also remember reading a story about an american soldier that was parachuted in France during WWII. His parachute did not open, but he landed in a tree (a cedar tree, or something like that). He suffered heavy injuries, lots of broken bones, but survived.
I'm not sure the story is real, and I don't remember the source (I was just a kid when I read it).
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Re: Skydiver

Postby Ulc » Thu Apr 07, 2011 8:59 am UTC

idobox wrote:People with training routinely jump from 10 or 20 meters high in water without hurting themselves. How fast is terminal velocity achieved?


A quick googling says that 99% of terminal velocity of 200km/h is reached in about 500-600 m of falling

Of course, terminal velocity isn't exactly staple, so that number is by it's very nature uncertain.
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Re: Skydiver

Postby Tass » Thu Apr 07, 2011 9:02 am UTC

Whereas from 20m you only get about 20m/s or 70km/h, that is one third of the momentum and one ninth of the energy.

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Re: Skydiver

Postby p1t1o » Thu Apr 07, 2011 9:18 am UTC

Given the scenario where no chute, emergency or otherwise opens, would not water - as opposed to other surfaces - be the best ("best") choice to land on? For the moment, let us put aside the possibility of drowning after impact.

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Re: Skydiver

Postby Ulc » Thu Apr 07, 2011 9:57 am UTC

p1t1o wrote:Given the scenario where no chute, emergency or otherwise opens, would not water - as opposed to other surfaces - be the best ("best") choice to land on? For the moment, let us put aside the possibility of drowning after impact.


Not really.

If aiming for land you might survive due to sheer dumb luck, such as landing 4 feet snow pile on top of a large patch of scrub, growing on very marshy land. It's exceedingly unlikely, but it's been known to happen.

If aiming for water - you're dead. There is no structures there to give you the chance for sheer dumb luck - and when falling at terminal velocity, there really isn't much difference between hitting water (it doesn't compress, and doesn't move fast enough either) and hitting concrete. You're still getting deaccelerated from ~200km/h to ~0km/h in the space of less than a second.
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Re: Skydiver

Postby Interactive Civilian » Thu Apr 07, 2011 10:44 am UTC

If your chute doesn't open, chances are you will be dead no matter what. Landing on a slope and tumbling may allow you to survive as it will be a gentler deceleration than the sudden stop of landing a flat surface like water or land (if you think water is going to just give way and let you through, do a belly-flop off the low-dive at a local pool and tell us how that feels ;) ). However, there is a big difficulty in that, since the tumble is uncontrolled, all it takes is one bad landing on the head or neck to kill you. If you do survive, you almost certainly won't be feeling very good.

I don't know what qualities of a slope would make one more survivable than another, but I'm sure there would be a mixture of angle (and change of the angle as you go down the slow), length of the slope, and the type and "clinginess" of the material (i.e. bare rock vs snow vs sand vs. grass vs bushes vs trees etc).

But, chances are you aren't going to survive. If you do, it's luck.
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Re: Skydiver

Postby p1t1o » Thu Apr 07, 2011 11:39 am UTC

Ahh well, never mind then, I'll just cross my fingers.

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Re: Skydiver

Postby Technical Ben » Thu Apr 07, 2011 1:24 pm UTC

Landing in trees might break your fall gently enough not to die. But would cause great injury again even if the off chance of you hitting something mailable happened.
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Re: Skydiver

Postby idobox » Thu Apr 07, 2011 3:17 pm UTC

Ulc wrote:If aiming for land you might survive due to sheer dumb luck, such as landing 4 feet snow pile on top of a large patch of scrub, growing on very marshy land. It's exceedingly unlikely, but it's been known to happen.

I have trouble seeing how all that would be softer than water.

I guess the air forces of many countries must have done statistics on the chances of surviving critical parachute failure.
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Re: Skydiver

Postby Angua » Thu Apr 07, 2011 3:21 pm UTC

The surface tension of water makes it almost as bad as hitting the ground when going at that speed.

I have heard (not sure if this is true) that if you throw something underneath you to hit the water first, then it helps break it up for a soft landing. Anyone know if that would work?
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Re: Skydiver

Postby p1t1o » Thu Apr 07, 2011 3:29 pm UTC

Angua wrote:The surface tension of water makes it almost as bad as hitting the ground when going at that speed.

I have heard (not sure if this is true) that if you throw something underneath you to hit the water first, then it helps break it up for a soft landing. Anyone know if that would work?


This was busted on mythbusters, I'm sure it'll be on youtube somewhere.

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Re: Skydiver

Postby gorcee » Thu Apr 07, 2011 3:46 pm UTC

It's not the surface tension. It's the density.

Even with a "penetrator" of some sort, you're going to be going from one medium, traveling 120+ MPH, into another medium with substantially high density. The immediate density gradient is basically an impulse term, and that makes for the hurting.

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Re: Skydiver

Postby jmorgan3 » Thu Apr 07, 2011 3:55 pm UTC

idobox wrote:
Ulc wrote:If aiming for land you might survive due to sheer dumb luck, such as landing 4 feet snow pile on top of a large patch of scrub, growing on very marshy land. It's exceedingly unlikely, but it's been known to happen.

I have trouble seeing how all that would be softer than water.

I guess the air forces of many countries must have done statistics on the chances of surviving critical parachute failure.

All of those things compress. Water does not.
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Re: Skydiver

Postby dukederek » Thu Apr 07, 2011 4:55 pm UTC

Damn you, you sent me on a mission of intrigue across the internet.

I think this is the most interesting thing I found http://www.greenharbor.com/fffolder/ffresearch.html

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Re: Skydiver

Postby p1t1o » Thu Apr 07, 2011 6:26 pm UTC

Would you choose water though, over say, someone's lawn? It would seem intuitive to do so (apart from the risk of drowning in the likely event of losing consciousness that is).

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Re: Skydiver

Postby Seraph » Thu Apr 07, 2011 8:18 pm UTC

I'd personally would aim head-first for the hardest looking thing I could find, preferring the quick but messy death to the slow torture of a life I'd probably have if I survived.

If I was interested in surviving I'd pick the guys lawn if only because it'll make getting post-impact treatment quicker. However, I'd guess a car, or a house, would be better choices. The roof of the car in particular would absorb a lot of energy (as opposed to you absorbing the energy) while you were crushing it.

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Re: Skydiver

Postby cphite » Thu Apr 07, 2011 9:04 pm UTC

Water is bad because it hits you just as hard as land, and you'll probably drown.

Assuming that neither chute opens, your best hope for survival is to present as much surface area as possible on the way down, and then at the last minute try to get your legs pointed down. Keep them bent, and - this is key - completely relax your entire body. When you hit the ground you're going to shatter your legs, probably your hips; but you might manage to save your spine and head if you're very, very lucky.

But chances are you're going to either die, or wish that you died.

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Re: Skydiver

Postby Ulc » Thu Apr 07, 2011 9:11 pm UTC

idobox wrote:I have trouble seeing how all that would be softer than water.

I guess the air forces of many countries must have done statistics on the chances of surviving critical parachute failure.


I doubt people have seriously sat down and tried to work out the odds of surviving without parachute. It's a horrible mess to calculate, since basically everything you can hit will work out differently, so you'd have to figure out first what the odds of hitting a a shrubbery instead of a concrete paving is (which is *hard* in practicality) - and the numbers doesn't really make a difference. From the falling persons point of view, there is no difference between odds 0.00001 and 0.00005

And the important thing is that these things compress, meaning that the de acceleration gets spread out over a longer time span. When you hit the now it starts compressing and slow you down from 200km/h to 150 km/h over 0.1 sec., this you are through the snow and hit the shrubbery, which slows you down to 75km/h over 0.5 sec. by branches bending and later breaking, then you hit the mashy ground at 75 km/h, which is slow enough that hitting water doesn't kill you, so it slows you the rest of the way down.

When hitting water you more or less instantly go from terminal velocity to 0 km/h. Say, a de acceleration of 200km/h in the space of 0.1 sec.

Sure, in the lucky case our unfortunate pilot is still to to be hurt, broken bones, wounds and almost certainly a concussion - hurt, but alive. But you really don't survive the kind of de acceleration that water causes.


*These numbers are totally made up to illustrate the point.
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Postby SumWunGye » Thu Apr 07, 2011 9:56 pm UTC

Wow, I created quite a discussion, huh? :)

Some people mentioned hitting water on your back would be a bad idea because you would decelerate falling far too quickly, which I see now. But landing feet first -- wouldn't you fall deeper and risk extreme pressure change? And of course there's recovering fast enough to avoid drowning..

And it was also mentioned by Technical Ben:
Landing in trees might break your fall gently enough not to die.
...Which I agree with! I've heard of stories where skydivers survive by landing in trees. Like this man: http://www.youtube.com/watch?v=sgaLxD3wu6M

Dudederek's link:
Falling on water at high speed is like falling on concrete, except concrete won't swallow you after breaking your bones and rendering you unconscious.

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Re: Skydiver

Postby Technical Ben » Thu Apr 07, 2011 10:23 pm UTC

Oh, the "might not die" is very slim chance. In that it may not be impossible. But never try it. Just as you might be able to catch a bullet without dying. ;)

The water is still acting like a solid at that speed, feet first. Perhaps your own feet will cushion the fall slightly. But again, we are talking about extremes, and things do not like to move or budge from the maximum effect.
Although I have no idea about the maths involved, it's pure speculation on my part.
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Re:

Postby Ulc » Fri Apr 08, 2011 8:06 am UTC

SumWunGye wrote:Wow, I created quite a discussion, huh? :)

Some people mentioned hitting water on your back would be a bad idea because you would decelerate falling far too quickly, which I see now. But landing feet first -- wouldn't you fall deeper and risk extreme pressure change? And of course there's recovering fast enough to avoid drowning..


Remember how I mentioned branches bending, then breaking would cause you to spread the de acceleration out over a longer period of time?

Landing feet first does the same thing, except your legs are playing branches for your upper body. And there isn't as many of them, so it's fairly unlikely to make a bit of difference either way. Landing in the cedar tree is still a loooong shot, but landing on water or concrete at terminal velocity is not just breaking of bones and conciousness - it's death.
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Re: Re:

Postby p1t1o » Fri Apr 08, 2011 8:19 am UTC

The first thing I think about tree braches is not "ooh! soft and springy!" so, again, you'd have to be very lucky.

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Re: Skydiver

Postby mercutio_stencil » Fri Apr 08, 2011 8:42 am UTC

I suppose it's somewhat relevant to bring up that ones terminal velocity is not exactly a fixed figure, and it varies depending on your posture while falling.

As such, it's in ones best interest to fall in a nice belly flop position, and only change to the legs down pose at the last possible moment.

This is of course assuming you maintain enough presence of mind after you parachute fails to actually keep this in mind, and that the slight difference in terminal velocity would actually have an impact on survivability. Both of those assumptions are pretty far fetched, but worth keeping in mind.

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Re: Skydiver

Postby p1t1o » Fri Apr 08, 2011 9:20 am UTC

Did you ever read "angels and deamons" by dan brown?

I don't recommend it.

One of its most special moments:
Spoiler:
Near the end, the protagonist has to jump out of a helicopter without a parachute and whilst hes falling he remembers some anecdote about terminal velocity and uses his trousers as a parachute, which gives him enough control to steer himself towards a river, where he splashes down safely due to his "parachute".

Oh and he had to bail out because there was an antimatter bomb in the helicopter. Which exploded.

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Re: Skydiver

Postby idobox » Fri Apr 08, 2011 12:05 pm UTC

It is still counter intuitive to think snow slows you down slower than water. For tree branches, or a roof, I can see it.
But snow is not much less dense than water, and is kinda solid. I demand a cantaloupe catapult and a bank of snow.

Jumping feet first in the water means you will have less surface, so less friction, less volume/mass of water moved per second. So less deceleration, and higher chances of survival. At terminal velocity, it might not be a big difference though.

The difference is huge between 1 chance in 10 000 and 1 in 50 000. You're still pretty much dead, but it's still a good idea to try the safest one.
And rough statistics like water/tree/other land might be a good start. Even if a soldier has only 1 chance in 10 000 to survive a failure, the psychological impact of knowing something can help you is important. It will not save you, but it will be easier to jump.
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Re: Skydiver

Postby Velifer » Fri Apr 08, 2011 12:53 pm UTC

This entire discussion overlooks the very simple fact that there has never been any scientific evidence in support of parachute use to reduce trauma in the first place. Smith and Pell's work on the topic (published in the British Medical Journal) showed no conclusive evidence-based research that would promote widespread parachute use for anyone exiting a plane at altitude. The notion that anything bad would happen to people who were not compliant with the "experts" guidelines (experts who no doubt have a fiscal interest in the expensive and unproven parachute technology) is entirely unsupported by the literature.
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Re: Skydiver

Postby idobox » Fri Apr 08, 2011 2:41 pm UTC

I love the part where they suggest a double-blind study.
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Re: Skydiver

Postby Moose Hole » Fri Apr 08, 2011 3:12 pm UTC

Deep Thoughts by Jack Handey wrote:If you go parachuting, and your parachute doesn't open, and your friends are all watching you fall, I think a funny gag would be to pretend you were swimming.

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Re: Skydiver

Postby gorcee » Fri Apr 08, 2011 4:07 pm UTC

idobox wrote:It is still counter intuitive to think snow slows you down slower than water. For tree branches, or a roof, I can see it.
But snow is not much less dense than water, and is kinda solid. I demand a cantaloupe catapult and a bank of snow.

Jumping feet first in the water means you will have less surface, so less friction, less volume/mass of water moved per second. So less deceleration, and higher chances of survival. At terminal velocity, it might not be a big difference though.

The difference is huge between 1 chance in 10 000 and 1 in 50 000. You're still pretty much dead, but it's still a good idea to try the safest one.
And rough statistics like water/tree/other land might be a good start. Even if a soldier has only 1 chance in 10 000 to survive a failure, the psychological impact of knowing something can help you is important. It will not save you, but it will be easier to jump.


I think maybe you haven't experienced snow that much?

We're not talking snow banks like what you would see on the side of a road. More like wind-blown snow drifts that would gather against your house. This is substantially less dense than water.

Have you ever swan dived into 2 feet of fresh snow? It's pretty fun, and not at all solid.

Also, empirical evidence for free-fall survival: http://en.wikipedia.org/wiki/Nick_Alkemade. A tree + snowdrift = no substantial injuries. Pretty badass.

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Re: Skydiver

Postby idobox » Fri Apr 08, 2011 5:03 pm UTC

It doesn't snow much where I live.
I'm not saying water is better than snow, people who know what they're talking about seem to agree it isn't the case. It's just it's not evident to me.
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Re: Skydiver

Postby Moose Hole » Fri Apr 08, 2011 5:09 pm UTC

Think of it this way. If you want to slow down while moving in air, you should try to keep moving through air that has stuff in it, because the stuff will gradually slow you down and the mostly-air will keep you mostly in the same medium you're already in. Snow is air with ice particles in it. Trees are air with twigs in it. Mattresses are air with springs and fabric in it. Water is not air.

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Re: Skydiver

Postby gmalivuk » Sun Apr 10, 2011 11:14 pm UTC

idobox wrote:But snow is not much less dense than water
Actually, it's about 1/10 as dense as water.
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Re: Skydiver

Postby masher » Sun Apr 10, 2011 11:17 pm UTC

SumWunGye wrote:Let's say you were skydiving, and your pullstring won't budge the parachute at the all. If there was a body of water you could land in, would it be best to land flat on your back, or in another position? (Considering, of course, the parachute pack on your back and the weight of the suit.)


As I was taught, if your main doesn't work, and your reserve doesn't work, try and land on your rigger's car.

It won't help you survive, but at least you've gotten revenge on the guy that stuffed up...

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Re: Skydiver

Postby pizzazz » Mon Apr 11, 2011 2:50 am UTC

The record for free-fall survival is 33,000 feet: http://en.wikipedia.org/wiki/Free_fall#Surviving_falls. The wiki article doesn't mention how, but I believe the stewardess fell into deep snow, as did a number of other survivors of high-altitude free falls (much higher than the 600 meters that was mentioned above as necessary for reaching 99% of terminal velocity). The key is that this is very powdery snow. Snow like that on ski slopes can be packed hard into near-ice that is very hard, but in any place where snow accumulates that is cold enough will build up many (30-40 maybe?) feet of soft powder, in which case getting out of the snow seems like the hardest part.

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Re: Skydiver

Postby M1k3_Nix » Tue Apr 12, 2011 10:41 pm UTC

if neither chute opens, probability and common sense say your screwed. as for landing on water, dont. at such high speeds, when you hit it, it will feel like concrete.

landing on your back or front first, the chest cavity will shatter and major organs will destroyed.
feet first, hips will shatter and bones in your legs will shatter upwards, sending shards into your chest cavity.
probably best going head first, get it over with quicker.

as someone above said, your best going for something that will absorb the energy, similar to how the crumple zones on modern cars work.


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Re: Skydiver

Postby p1t1o » Fri Apr 15, 2011 10:01 am UTC

I'm not saying that this video gives any evidence either way to this discussion, but I thought y'all'd enjoy it. Listen to that whack!

http://youtu.be/SUwfYxdrIQ0


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