Re: [motherhucker] Determining Fall Delay
In reply to:
I hope if you take 5 secs slider up from a 600' object, you are over water and wearing indestructible diapers. And if you take 5 secs slider "on", but not up, be prepared to land on 3 or 4 cells.
be careful.
Not necessarily. One thing I carry with me all the time (loaded in my Palm, and yes I'm a geek) is a fall rate chart. A five second delay until PC deployment is likely approximately six seconds to canopy pressurization.
[note that this is a big assumption - despite the critical nature of post-PC extraction length calculations to anyone doing really low pulls, I've never seen a consensus on exactly how long after PC deployment until a given canopy is pressurized and flying. . . clearly, one would expect this number to vary depending on slider up/slider down, direct/indirect stow or no stow slider up, venting or unvented canopy, etc. . . however, as a general rule I've heard others talk about a standard, 1 second post-PC deployment delay until pressurization. . . mostly, those who play the low pull game develop a very good "gut feel" of how long until PC deployment is possible by doing lots of jumps and building a virtual model in their mind of where the fringe of the chart is and how close to the fringe is possible without going to the hospital or morgue. . . an interesting topic, at least to me.]
A body falling for six seconds with no decceleration (either from aerodynamic drag or partial canopy extraction drag) would eat up 504 feet, and be going 94.09mph. One hundred feet of canopy time is not on the low end of the spectrum - I've seen (and partaken in) lower than this many times, even slider up.
Which brings me to an important issue. First, it is important to actually have a good feel for the "fall rate chart," particularly when dealing with subterminal objects (less than about 850feet to deployment). I know several jumpers who have simply memorized the zero to seven second fall rate chart. While this is (obviously) a continuous function, some data points extracted from the curve are:
1 10.91 16
2 31.36 62
3 51.81 138
4 70.91 242
5 84.54 366
6 94.09 504
7 100.91 652
8 terminal 808
9 terminal 971
Note that the distance travelled in freefall is far from linear; in fact, it's approximately geometric until about 7 seconds. This is why, in my opionion, timing delays accurately between 2 and 6 seconds is an advanced BASE skill mastered by few folks. Things happen fast in that zone- acceleration and distance fallen are increasing at an increasing rate (humans think mostly in linear terms, so most folks don't naturally cotton to this math). It takes alot of jumps to get a feel for how this curve really unfolds in freefall.
This is, of course, particularly relevant when "low pulling." Taking 4 seconds off a 440 foot object requires pretty good timing, for example. Take five (assuming the opening didn't break your back slider off), and you'll likely impact at line stretch. Take three, and well you are wasting valuable freefall time
If we are not using laser rangefinders (or if the exit is underhung and makes a laser reading impossible without climbing gear to set an anchor and partially rappel off the exit for a clean line of sight), then the way to see how high an object is (and thus what delay is possible, assuming no glide to landing area problems, etc.) is by rock drop.
WARNING Doing rock drops incorrectly can result in substantively incorrect readings and potential fatalities! A rock - in order to overcome aerodynamic inertia - must be (by conventional wisdom) at least as big as a softball and roughly spherical. A smaller rock - or one with a flatter shape - will slow down and hit "terminal" much earlier than a falling human body before transition to track (and without a wingsuit).
Thus, throwing a flat or small rock off a cliff, timing (with a watch) eight seconds, and planning a 5 second slider up delay is a likely fatality. The object may well be only 500 feet or so high - and a 5 second slider up delay off that will result in impact at line stretch most likely. Fatality.
Also remember that "lighter" rocks (like volcanic tuft) will float no matter how big they are, within reason. Their density is too low to offer accurate rock drops.
I've seen many experienced jumpers make these rock drop mistakes, thereby systematically
over-estimating how high cliff exits are. This is extremely dangerous.
Yes, low pulls are a dangerous game. It is stupid - why would anyone do it? Don't do low pulls. In other words, do as a I say and not as I do (as in so many other areas).
Peace,
D-d0g
ddog@wrinko.com
http://www.wrinko.com
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