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Re: skydiving



In an earlier career, I was a pilot in the Marine Corps, a rated jump
master, and had 3.6 months TAD with a SEAL team. I have a little
knowledge in the area of operational sky diving (only about 121 jumps)
both over land and water.

By the time I went on active duty with USMC, I already had a Masters in
nuclear theory and was one of the earlier computational modelers. So I
did spend some time, while operational, thinking about such problems and
trying to get some information from both inside and outside sources.

On Tue, 1 Apr 1997, Dave Hamilton wrote:

Three of my students are building a computer model of a skydiver reaching
terminal velocity in freefall, opening the parachute and establishing a
new terminal velocity, then impacting the ground. They are interested in
building into their model the differences that can be expected for
skydivers of different mass.

They have the model working fairly well for a 170 lb skydiver with a 30
lb parachute but would benefit from additional information that we
haven't been able to locate in reference books. In particular, they
would like to know the following:

1. How does the terminal velocity (before 'chute deploys) of of a diver
depend on the diver's mass (all other things being equal)? They are
presently using the information that the 170 lb diver reaches a terminal
velocity of approximately 120 mph in about 12 seconds and assuming that
the force of air resistance is proportional to v^2.

Most reasonable models of the viscous drag force experienced by a jumper
involve a transition between v^1 and v^2 at about 30mph and retaining the
v^1 term with a diminished fit coefficient right up to and beyond terminal
speed. (Note that fits for a dropped shape with about 1sq-ft {foot or
head 1st "power dive"} of profile drag differ from those for 6sq-ft
{Lift/Drag max-range body glide} of profile drag in that the power dive is
usually modeled with only the v^2 term after about 20mph.

2. How long does the opening of the parachute generally take? Or, if we
know how many g's the diver pulls upon opening the 'chute, we can deduce
the time.

I'm not sure that I know what you mean by "How long does the opening of
the parachute generally take?" Do you mean from the time we pull the
D-ring until full canopy (this differs a little depending on the shape of
the canopy: circular, para-wing, regala, etc.) or the time from when the
canopy is at full extension and begins to blossom until full canopy. The
time would be from about 4 to 6s.

You cannot deduce the time for the opening of the chute from the number of
g's experienced. This is usually stated as "max g's" and will do you
little good. You would have to know the acceleration profile or curve.

3. How many g's does the diver experience as a result of impact with the
ground ('chute open, safe landing)?

The max acceleration on a good landing (involves a significant pull on the
riser cords) is about 1.5g's. This would also involve a significant bend
in the legs so that the deceleration is over a distance of about 1ft.

If any of you are skydiving experts or if you can refer us to resources,
we would greatly appreciate your help. Thanks.



David J. Hamilton, Ed.D. "And gladly wolde he lerne,
Franklin HS, Portland, OR and gladly teche."
djhamil@teleport.com Geoffrey Chaucer



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