From: grg@boo-berry.ai.mit.edu (Gregory Galperin)
Newsgroups: rec.skydiving
Subject: Re: Spotting and Ground Speed
Date: 2 Oct 92 18:50:04 GMT
Organization: MIT Artificial Intelligence Lab

In article <1992Sep30.093227.11885@pta.pyramid.com.au> bob@pta.pyramid.com.au (Robert Vernon) writes:
>In article <1992Sep25.002602.10036@gateway.ssf-sys.DHL.COM> bdaniels@ssf-sys.DHL.COM (Ben Daniels) writes:
>>Maybe this seems complicated because people have been framing it in
>>terms of rates and times.
>>
>>If I look out the door when the group in front of me gets out and
>>see what their exit point was
>>And if I want to open 1000 ft upwind of them
>>Then I have to wait until my exit point is 1000 ft upwind of theirs.
>
>Now THATS a simple explanation.  Forget ground speed and air speed and
>drift and everything.
	It is simple, but I think it's too simple.  It "forgets" too
many things that really *do* affect your path through the air, and all
it really is is a good way of using the groundspeed rule-of-thumb
(it's just restating the rule using distance rather than time; but
 (time)(groundspeed)=(distance) -- it's an easy relation.)
	To get technical for a second, it still assumes that winds at
exit are the same as winds at opening, like some did early in the
discussion.  We know this just isn't true.  But I'm not going to go
off on that one here.  I think the consensus here says that
groundspeed *is* important, but that it isn't the *only* important
factor -- some common conditions can mess that rule of thumb up.

	I'd like to give one more (different!) way of thinking about
how we drift, and then ask why we don't just fly jump run slightly
crosswind to avoid this problem.  Bear with me a bit here...

	Let's start by looking at a no-wind day: your freefall path
will be almost straight down (OK, let's assume you (can :-) and do)
fall stably in your "column" of air -- a pretty boring dive, I'd say).
The top is pushed around a bit since the plane was moving when you got
out, but the same effect will happen at that airspeed for every jump,
so we let's just ignore that -- call it a straight line, completely
vertical.  If we strap smoke on you (or a bag of flour, for the old
days), it'll leave a straight trail that's completely vertical.
	Now build the idea by considering the strange day in which we
have wind, but it's the same speed and direction all the way down
through opening.  If you look up when you open, you will see your
smoke trail is a *straight line* that is *now completely vertical
above your opening point!*
	"What?!  But I'm drifting!  It can't be vertical, above me!
It doesn't even go to the plane, and I know I got out of the plane!"
some confused net.reader cries.  You are drifting... but that means
you are moving sideways _with the wind_, that is, with the air.  Your
smoke trail marks the air you went through; we have all of it drifting
at the same same rate, so your vertical line just slides along with
you, always straight up and down, always vertically above you.  And of
course it doesn't lead to the plane; even if the "plane" were a cliff,
stationary with respect to the ground, the air that was on the cliff
when you jumped has long since drifted past.  Air "drifts" too!
	So, a bunch of jumpers exiting 8 sec. apart from a plane going
75 knots would leave a bunch of vertical lines in the air (all
parallel, all vertical) each about 1000' apart.  Since they open at
the bottom of these lines (just say they turn their smoke off when
they open), their opening points are separated by 1000' of air.
(Notice, by the way, that we didn't mention winds aloft or groundspeed
at all yet!)
	Convinced of that yet?  Good; now let's confuse this some
more.  Realistically, the wind speed changes with altitude.  To
picture this effect, first consider a steady change with altitude,
say, from higher wind speed to lower.  The air at the top of our
freefall path is going faster than the air at the bottom... so instead
of being a straight line, our smoke line gets slanted.  And the longer
we look at it, the more slanted it gets.  Say the wind is blowing from
left to right (jump run is to the left) in a picture; the top of the
line would be farther to the right than the bottom.  Taking Bob V's
lead, let's try some ascii graphics: we'll draw a straight smoke trail
(left) and watch it slant as it drifts (other to the right):

      *            *            *            *
      *           *            *           *
      *           *           *           *
      *          *           *          *
      *          *          *          *
      *         *          *         *
      *         *         *         *
      *        *         *        *
      *        *        *        *
      *       *        *       *
      *       *       *       *

And your trail will look something like one of these, depending on the
difference between winds at exit and the winds at opening (no
difference = the leftmost path, big difference = rightmost).
	(Note for physics weenies: I've assumed a linear dependence of
altitude on wind speed.  You can easily make any other dependence and
draw the corresponding deformation.)
	And I'd again like to claim that "separation" means you want a
bunch of distance between the bottom of these lines, so you get a
bunch of air between you and the other canopies.  Try it with
different combinations of winds at exit, opening, and plane's
groundspeed to convince yourself of your arguments... I say no more.

>But I'd like to modify it slightly, just in case you need to do
>a down or cross-wind spot.
>
>If I want to open 1000 ft away from them
>Then I have to wait until my exit point is 1000 ft away from theirs.

	This relates to something I was thinking about before.  Why
don't we have jump run fly over the spot at an angle (like 30 degrees?)
off the windline?
	The only thing we've *all* agreed on is that this whole issue
is confusing.  We all see problems that a couple of groups of jumpers
could end up trying to use the same bit of air with some strange
combination of windspeeds and airspeeds, not all of which can be
easily known, that cancel each other out.  If we fly the plane about
30 degrees off the wind line, we *guarantee* a separation that *can't*
be cancelled by any winds along the wind line -- the distance the
plane gets across the wind line.  At about 20 or 30 degrees, you don't
lose much in the way of wind penetration, and you won't get too far
off the spot.  (Heck, too often we do this without intending to
anyway... :/)
	If you've actually bothered to read this far, what do you
think about this idea?  Any obvious safety issues I've brain-locked on
here?  Anybody?

Bueller?----+
            !
            +-----=> Greg Galperin