Part of the confusion over whether airspeed or groundspeed
matters for timing exits has come because nobody has clearly stated
what their goal is -- what we need to do to be safe.  We all say that
we want to maximize the distance between where one group and the next
opens -- but what does this really mean> Another way of making this
more explicit is that we can put a party balloon exactly in the
airspace where one person (group exiting together) opened, and we want
to know how to keep the next person (group) opening as far
(horizontally) from that balloon as possible.
	<For the anally technical, we're assuming that the balloon
stays at basically the same altitude, the wind conditions don't change
over time but may vary with altitude, and everybody opens at the same
altitude and falls straight down (tracking just adds horizontal
distance from out result and is <hopefully!> under the jumper's
control... we want to know about the things the jumper has no control
over, namely wind drift).>
	Imagine yourself sitting inside this balloon (Hey! No jumping
out yet! Pay attention here!) watching the plane above.  If the plane
doesn't appear to be moving, this can be pretty bad news -- as a
couple of people have already described, no matter how long the next
group waits, their path will look exactly the same to you: identical
to the path of the first group.  BLAM, they smack into the balloon.
We knew this already.
	If you see the plane moving at 60 mph with a 12 second wait
between exits, you see the plane move 12/60 = 1/5 mile, or about 1000
feet.  Just like Bobby V's ascii graphics showed, their path will be
"parallel" to the path of the first group, just shifted over 1000', so
they will open 1000' away from your balloon.  We knew this already.

	So if I've ended half of the paragraphs with "we knew this
already," why am I wasting net.bandwidth and your reading time?
Because of one key difference: in these explanations, you were sitting
inside a balloon drifting with the wind at opening altitude; everybody
but Joe Peterson forgot to mention that (and Joe got slammed and gave
up)!  Notice above that we always say *you see* the plane...
Physicists call this a reference frame: you measure motion, distances,
etc. as *you see* them, i.e., relative to something you specify (here,
it's just you floating in the balloon).  Bobby V and others had the
concept right, but forgot what their reference frame was.  As I
covered in the first paragraph, we want maximum separation *relative
to the jumpers who already opened* (who are drifting with the wind!).
	OK, so what's the answer?  If the ground were the reference
frame, we'd call the plane's speed *as you see it* the groundspeed,
and that's what would matter.  But instead, we really want everything
relative to you in the balloon (call it `balloonspeed?').  From where
you stand (float?), the equivalent of `winds aloft' will be the
difference between your drift rate and the wind speed at exit
altitude: (Wind@exit - Wind@opening).  (These wind speeds are what we
normally think of as wind speeds: they're relative to the ground.)  A
whuffo standing on the ground would see the plane moving at
(PlaneAirspeed - WindsAloft); similarly, for us, this is
[PlaneAirspeed - (Wind@exit - Wind@opening)].  The distance between
groups of jumpers will be exactly the distance covered at that speed
(that is [PA - (WE - WO)]) in the time jumpers wait between groups.

	In summary, the people saying ground was all that mattered
were almost right; Joe Peterson went out on a pretty sturdy limb and
had the right idea saying it was "the difference in air speed between
jump altitude and opening altitude" that mattered.  A couple of people
tried to mistakenly cut down that limb, including the unjustified,
loud, and just plain wrong response:
>From: bonitz@ebony.eecs.ucdavis.edu (Bob Bonitz)
>Makes no sense.  Your limb broke.  The only relevant factor, I repeat
>"only," is your ground speed.  Let's move on folks!
No further explanation was given.

	So, if the wind at 12K is the same as the wind at 2K <or
wherever you happen to exit and open>, exit as soon as you're ready
because you'll get maximal separation (at the jump-run speed of the
aircraft).  This is true even if the plane's groundspeed is 0!
	But, if the wind at 12K is a lot faster than the wind at
2K, take a long count before following the group in front of you,
since you're not getting quick horizontal separation between your
opening points.  Again, this is true regardless of ground speed.

Sorry about the flame-------+
                            !
                            +-------=> Greg Galperin
                                       B-15037, PP-ASEL,
                                       Private Pilot Ground Instructor
Newsgroups: rec.skydiving