[QUOTE=askeptic]
Furthermore a skydiver under a deployed chute does not reach terminal velocty. Terminal velocity refers to an object in free fall.
[/QUOTE]
Yes, she does. Terminal velocity refers to the free-falling (that is, not powered) velocity at which the the force of gravity and the force of air resistance balance out and the object no longer accelerates.
[QUOTE=Q.E.D.]
Yes, she does. Terminal velocity refers to the free-falling (that is, not powered) velocity at which the the force of gravity and the force of air resistance balance out and the object no longer accelerates.
[/QUOTE]
True in a physics class but not at most dropzones.
You might find this helpful:
[QUOTE=askeptic]
True in a physics class but not at most dropzones.
[/QUOTE]
???
[QUOTE=yelimS]
Does acceleration have anything to do with it? Is hitting the ground at x kmph while accelerating worse than hitting it at x kmph while at terminal?
[/QUOTE]
No - it’s the amount of kinetic energy you have to get rid of that does you in, and that’s solely speed-dependent.
[QUOTE=LiveOnAPlane]
???
[/QUOTE]
I have been talking about the terminal velocity of a human. When at a dropzone we don’t look up and see a person under fully deployed chute and say or think “Wow they are at terminal velocity”. While it may be true that for the deployed chute person “system” it has acheived its functional terminal velocity but it is not the TV for the person alone. That person under the chute is falling much slower than it woulod without the chute. The chute person “system” itself would fall faster if the chute collapsed. Hence my asertion that a fully deployed functional parachute does not fall at its terminal velocity because if the parachute deforms it will fall faster. This is all semantics to determine TV of a chute should we through a fully packed undeployd chute out and calculate that or do we calculate the speed of a properly deployed chute. Keep in mind that modern ram air chutes are more like wings in that they not only function by creating drag to offset gravity but they also generate a certain amount of lift.
When I read the question posed by the OP it was clear that he was talking about the TV of a person without a chute. Now can we stop with the symanticism?
However you may choose to use the term colloquially, terminal velocity has a very specific meaning: you are no longer accelerating. The how or why is incidental. In every measured setting I am aware of, from model rocketry to the scientific literature, the orientation/configuration is noted in the results.
Given enough time, an unconscious body is likely to end up either a) straight vertical head down, b) flexed at the hips, supine; or c) a spinning tumbling. (Though “feet down, straight” may seem a plausible “minimum resistance” position, the force on the feet/legs would flex the knees, and the added resistance of flxed legs would pull them up into position B or (more likely?) C
[20,000 ft wouldn’t always be enough free-fall for to cancel out the tumble of an unconscious body, based on what I have seen, but A and B are two relatively stable positions that don’t freely interconvert, depending on one’s limb/torso proportions and weight distribution.]
Face down, limbs deliberately spread, is almost certainly not the best position at impact, but it is possibly the best position to decrease downward velocity, as evidenced by its widespread use by parachutists to maximize air time. More importantly, it offers a limited steerable glide for a larger radius of potential landing zones, and is a good position to observe the ground to select your landing site – in the second of clear vision you’ll get with ungoggled eyes facing into super-hurricane level “wind”. (I don’t know anyone who jumps without goggles)
Recreational vertical wind tunnels are springing up everywhere. We have one near me, and I’ve heard rumors of a second. Most have airspeed gauges, and could be valuable empirical resource for this class of perennial question, even allowing our ‘divers’ to assess terminal velocity and stable positions in street clothes.
Minimizing terminal velocity by body configuration seems to be the best initial step, and possibly the only one with a significant and consistent effect. Changing position before impact (e.g. to land in water “feet first” or as I was taught, legs flexed against the torso to land “soles and butt first”) seems likely to be useful, but would be hard to test definitively, and probably couldn’t have nearly the same magnitude of effect. On the plus side, you wouldn’t need superhuman reflexes – changing position a second or two before impact would be almost as good.
The OP did specifically ask how best to “hit the ground”, but that may be too late to do very much to maximize your survival, compared to the glide position.
A person falling with a deployed and perfectly operating parachute is falling at terminal velocity at the time he ceases accelerating, as is a person who is falling without a parachute at all.*
“Average human terminal velocity” does not equal “terminal velocity.” Terminal velocity is a value that varies depending on not only the object in question, but even the direction it is facing in mid-air.
I was nearby when a window-jumper failed to kill himself. He was saved by the same thing John DiFool mentioned-- his legs crunched (telescoped) and absorbed the impact. The hospital this happened at wasn’t equipped to deal with said trauma, and doofus was sent to another hospital to have his lower self rebuilt.
The thing is, that only works if your pelvis is strong enough. Otherwise you get femur popping out the top of your shoulder, and, well, game over. Sorry, Chronos.
What height is required for terminal velocity?
I’ve heard the statistical curve flattens at about three-stories; i.e. you’re (essentially) just as likely to die falling from a three-story building as you are from any higher point.
But this might be a myth.
The time to reach terminal velocity is not directly linked to the free-fall distance. There are a lot of factors that can together vary the terminal velocity by up to 50% (e.g. air density due to altitude, body mass ratio, habitus, position)
However, 3 stories is nowhere NEAR the asymptotic limit for fatality. I know many people (including myself) who have fallent that far without serious injury. Ignoring air resistance, it only takes 1.4 sec to hit the ground from ~9.8m (32ft)-- a terminal velocity of ~13.7 m/s (45 ft/s or 30 mph), which most people will survive.
You might be interested in reading one of the classic papers in the field: “Mechanical analysis of survival in falls from heights of fifty to one hundred and fifty feet” by Hugh De Haven of Cornell (a seminal injury prevention researcher inspired by his own experience surviving a plane crash). It was originally published in War Medicine (1942; 2:586–96) (an AMA publication) but was recent;y reprinted in a British Medical Journal publication (Inj. Prev. 2000;6;62-68) which you should be able to access here. If that link doesn’t work, try this one which requires a free registration to access the full article.
It’s more anecdotal than epidemiological (hey, it was an early paper in the field) but you should be able to find more recent work over the past 65 years by Googling the title, to find papers that cited it in their bibliographies.
Just want to add that if you are already falling at 100+ mph, if you don’t have any previous skydiving experience, and in a discipline called free flying in particular, it is unlikely that you can successfully manuver yourself into a feet first position just before impact. At that speed, the wind is exerting a very significant amount of force and it doesn’t take much to tumble completely out of control.
[QUOTE=flight]
When you hear someone say, “Their chute didn’t open, but they lived,” they usually mean, “Their chute was in a big streamer-like mess behind them that, while not acting like a regular parachute, created a huge amount of drag to slow them a great deal so they lived.”
[/QUOTE]
When I took a parachuting class, that type of arrangement of your chute was indeed called a “streamer.”
The other common name for it was “screamer.” 
Quoth askeptic:
What part of what I said are you disputing? That you decelerate at the bottom of a fall, or that an upright person has more height than a supine one? In any event, I didn’t say that was the best orientation to fall in; I said that it was the best orientation to land in.
Quoth Darth Nader:
No apology necessary. I admitted from the start that no matter what you do, your odds are pretty darned bad. But “you might have a very slim chance of survival, if your pelvis happens to be unusually strong” is better than “you don’t have any chance at all of survival”.
can’t believe nobody has offered this:
“miss the ground”.
Or this:
“falling at terminal velocity is not fatal by any means.”
[QUOTE=Key Lime Guy]
What height is required for terminal velocity?
I’ve heard the statistical curve flattens at about three-stories; i.e. you’re (essentially) just as likely to die falling from a three-story building as you are from any higher point.
But this might be a myth.
[/QUOTE]
Lets figure it out. We will do two cases, the first for for a flat orientations (TV of 180 fps) and vertical (TV of 300 fps). In both cases you reach TV when drag=gravity. Drag is a function of the velocity squared, so:
C*V[sub]t[/sub][sup]2[/sup]=g
Here I have already eliminated mass, g is the acceleration due to gravity, and C is a constant that depends on your orientation, air density, viscosity, etc. So, knowing the terminal velocity we can determine the coefficient:
C=g/V[sub]t[/sub][sup]2[/sup]
That is at TV, what about before? For this we have
a=g-C*v[sup]2[/sup]
We integrate that to get:
v=(1-e[sup]-2tsqrt(Cg)[/sup])/(1+e[sup]-2tsqrt(Cg)[/sup])*sqrt(g/C)
Someone might want to check that. Lets sub in our equation for C now.
v=(1-e[sup]-2tg/Vt[/sup])/(1+e[sup]-2tg/Vt[/sup])*Vt
So, we see that we will not actually reach terminal velocity, but asymptotically approach it. So, how about 90% terminal velocity?
I numerically solved that to give: For the standard (flat) parachutists position it will take you 8.2 seconds to reach 90% of your 180 fps terminal velocity. For the vertical position it will take you 13.7 seconds to reach 90% of your 300 fps terminal velocity.
I decided not to try to get a closed form solution for how far you fell, but numerically integrated it to find that it takes 831 feet to reach 90% of terminal velocity in the flat position, and 2316 feet in the vertical position.
[QUOTE=flight]
I numerically solved that to give: For the standard (flat) parachutists position it will take you 8.2 seconds to reach 90% of your 180 fps terminal velocity. For the vertical position it will take you 13.7 seconds to reach 90% of your 300 fps terminal velocity.
I decided not to try to get a closed form solution for how far you fell, but numerically integrated it to find that it takes 831 feet to reach 90% of terminal velocity in the flat position, and 2316 feet in the vertical position.
[/QUOTE]
Just to give a reality check here, the general rule of thumb for skydivers is that it takes ten seconds to reach terminal velocity from the time you go out the door, and you’ll fall 1,000’. This jibes quite nicely with your numbers. That’s based on ~120mph terminal velocity, relaxed arch and a jumpsuit that’s not skintight nor baggy.
[QUOTE=Valgard]
Just to give a reality check here, the general rule of thumb for skydivers is that it takes ten seconds to reach terminal velocity from the time you go out the door, and you’ll fall 1,000’. This jibes quite nicely with your numbers. That’s based on ~120mph terminal velocity, relaxed arch and a jumpsuit that’s not skintight nor baggy.
[/QUOTE]
Yeah, that does work well. 120 mph is about 180 fps, and you would reach 1,000’ and ten seconds somewhere around 95% or so of terminal velocity. Cool.
[QUOTE=Chronos]
But “you might have a very slim chance of survival, if your pelvis happens to be unusually strong” is better than “you don’t have any chance at all of survival”.
[/QUOTE]
I think this is probably a bit of hyperbole - nobody has a pelvis strong enough to survive a feet-first landing - your legs are going to end up thrust inside your torso, unless you have an unusually strong pelvis - that is, one perhaps made from titanium and carbon fibre.
[QUOTE=Sam Stone]
Then you might increase your chance of surviving from .000000000000000001 to .0000000001 or something.
[/QUOTE]
Really? A 100-million-fold increase? Result! :dubious: