From: NAYLORD@QUCDN.QueensU.CA
Newsgroups: rec.juggling
Subject: Skydiving & Juggling
Date: 13 Sep 92 19:41:58 GMT
Organization: Queen's University at Kingston


I was just flipping through some old copies of _Juggler's
World_ and I ran across some comments by (our fellow netter)
Arthur Lewbel about the possibilities of juggling in freefall
(Fall 1990 "The Academic Juggler"). In this article, Arthur
suggests that it might be possible to juggle in freefall or
under parachute.

Although I'm very far from an expert skydiver, several years
ago I made about 150 jumps and logged just over an hour in
freefall. In terms of skill level, let me say that I've docked
fifth in a five-person slot-perfect star formation - in juggle
terms, this is roughly the equivalent of say, a solid four
ball juggle.

As Arthur states, you'd need quite heavy props to juggle in
freefall. The props must have a terminal velocity much greater
that a human in the standard ("frog") freefall position (about
120mph).  And of course, you'd have to wait until you almost
reached "terminal" before starting to juggle (about 12sec. and
1500ft. later).

What kind of balls would you need?  I've calculated the
terminal velocity of a 2.5" silicone ball (Cd=0.4, m=157g) to
be about 50m/s (almost 120mph). Kind of a neat silicone ball
fact: if you let go of the ball in freefall, it would hover
almost perfectly in front of you (OK, you'd have to
consciously adjust your freefall rate a bit, but still it's
pretty close).  So to juggle you're gonna need something quite
a bit heavier, such as Exerballs. (Actually this might not be
completely true, see postscript).

Obviously, for the safety of the public, you're gonna require
a fairly isolated location to try this stunt.  Forward throw
(due to the plane's velocity) and freefall drift (due to often
high upper winds) of an accidentally dropped Exerball (yikes!)
must be considered.  I can tell you from unpleasant experience
that there can be a VERY big difference between the plane exit
point and the parachute opening point after a 50 sec.
freefall; the difference can be easily half a mile (or more).

Technically, it's feasible to juggle three balls in freefall;
but I suspect you'll need to be one hell of a good skydiver.
Waving your arms around in freefall will cause the body to
turn and move horizontally and vertically with respect to the
balls. You can correct for this with some skilful leg and
torso counter-movements but it might not be dead easy.
Probably it'd be best to work out the details (ball size,
weight and freefall technique) in a vertical wind tunnel. It
might take a half hour or more in "freefall" to figure out the
correct technique. In skydiving you have to learn how to "fly
your body" - it's not the cheap thrills that many people
think. OK, some cheap thrills.

I doubt it would be possible to juggle clubs (or any highly
non-spherical prop) in freefall. Keep in mind that, in the
horizontal direction, it's like being on one of those air
hockey tables.  Any slight horizontal force sends an object
careening across the sky. A juggling club would experience
fluctuating lift and drag forces as it spun. These fluctuating
forces would most likely send the clubs flying off in all
directions the instant you let go of them. (For example, it is
a common novice error to fall with the arms too far forward.
This causes the neophyte skydiver to fall at a slight head-
high angle and backslide at ever increasing speed. I've spent
entire jumps being CHASED across the sky by experienced
jumpers, wondering why they couldn't get over to me.)

As far a juggling under canopy goes, it'd be no problem.
Pickups would be a bitch. I know, many of us have seen a
picture in JW of someone juggling under a round parachute in
tow behind a boat. IMO this doesn't count since it's
essentially a no-skill carnival ride. You don't steer and you
don't have to worry about where you are going to land. Also,
you didn't have to hold onto your props during an 8g opening. ;)
Real skydivers call it Dope-On-A-Rope.

BTW, no experienced skydiver has used a round main parachute
for well over ten years. Let's get with the times. I'm talkin'
"square" ram-air (parafoil) type parachutes. These suckers are
very light low-performance gliders made from rip-stop nylon,
not the heavy silk drag machines of the past. It'd be like
juggling in a steady 15mph wind. Not too much problem. If you
wanted, you could greatly decrease the forward speed and rate
of descent by increasing the parachute's angle of attack to
near the stall point (with modified steering toggles connected
to your feet to free your hands). Then, except for the
harness, it'd be essentially juggling as normal.

I'm pretty sure it would even be possible to pass clubs
between jumpers under canopy (with substantial effort). No,
tangling would not be a problem. Not any more than usual,
anyway. Skydivers fly ram-air parachutes in close formation
all the time; it's call Canopy Relative Work and in some
places it's a highly competitive sport with specialized gear.
Those in the sport call it CREW. (Many skydivers, myself
included, don't do CREW. Why mess with a perfectly good open
parachute? But I digress....) Steering toggles could easily be
connected to the feet to free the hands for juggling.  All we
need now is some CREW experts who can pass clubs.

So why hasn't someone done it?  Well, skydivers are a much
stranger bunch than you may imagine... and I mean that in
absolutely the least flattering sense. IMO most skygods
capable of doing this stunt have a very very narrow range of
interests. If it ain't skydiving, sex, or drugs, they ain't
interested. Period.  No really, I'm not kidding.

David Naylor,
Earthling
Bonus Day #3265

 *** WARNING - STOP NOW IF YOU ALMOST PUKE WHEN YOU READ
               TECHNICAL STUFF

p.s.  It turned out when I calculated the terminal velocity of
a silicone ball, that the flow around the ball (very near the
surface) is just on the verge of becoming turbulent (Re=2x10^5
for those that know what this means). At this high speed, just
under the laminar-turbulent transition point, even the
slightest bit of surface roughness will "trip" the flow into
full blown turbulence.  When the flow near the ball becomes
fully turbulent, the drag drops by about a factor of FOUR.
This DOUBLES the terminal velocity of the ball. (The dimples
on a golf ball have the same effect - they cause turbulence
and greatly reduce the drag).  So, if you roughened NORMAL
weight juggling balls you could juggle 'em in freefall. It'd
be something like juggling in lower gravity. Neat! Kinda makes
me want to take up skydiving again.
Well, maybe not. 3^)
Newsgroups: rec.juggling