From: bob@pta.pyramid.com.au (Robert Vernon)
Newsgroups: rec.skydiving
Subject: Re: Spotting and Ground Speed
Date: 9 Sep 92 10:27:26 GMT
Organization: Pyramid Technology Corporation, Sydney

In article <6730163@hplsla.hp.com> artb@hplsla.hp.com (Art Bori) writes:
>I publish a small newsletter for jumpers in the Northwest and a contributor
>wrote an article about spotting.  One point that was addressed was how 
>much time to allow between groups exiting on the same pass.  The contributor
>stated that you need to consider *ground* speed when determining how much time
>to allow between exiting groups.  In other words, if the ground speed is 
>is lower, you need to allow more time between groups to obtain the
>same relative horizontal separation.  Other jumpers I talked to argue
>that *ground* speed is irrelevant.

This old chestnut.  There is a question in our JM exam, of which 50%
of potential JMs get wrong.  And I remember once spending an hour with
my chief Instructor arguing with a very experienced, very logical
jumper on what the right answer is.  He eventually agreed with us, but
I suspect he would need convincing again if the topic ever comes up
again.

>What I wanted to ask was: 1)  Do you agree or disagree and why?

I agree with your contributor and disagree with those other jumpers.
GROUND speed is THE major factor to take into account to work out time
separation of groups on exits.  AIR speed is irrelevent.

This always seems wrong because for everything else we don't care what
the ground speed is.  We like a certain air speed for our exits,
typically 80 knots.  We don't care how much freefall drift we get other
than judging the spot.  Ground speed is irrelevent for just about
everything except this.

I know someone on this group will disagree so I'll have to try and
explain why.  I can see another long-winded posting coming up.

Imagine the plot your body makes as you fall.  You start with the
forward movement of the aircraft, called throw-forward.  This starts
equal to the ground speed.  As air-resistance takes effect you end up
moving horizontally with the prevailing winds (wind-drift).  The plane
usually makes final approach into wind, so throw-forward is opposite
to wind-drift.

If the wind speed is 0, then you won't drift.  If the plane is
stationary, then there is no throw forward.  You fall straight down and
your opening point is directly under the exit point.  Not fun with
multiple groups.

Luckily the plane is moving forward.  At power-off, lets say 80 knots
air speed.  With 0 wind, thats also 80 knots ground speed.  If each
group waits 10 seconds before exiting, the plane will travel across the
ground (mumble, mumble, mumble, why the hell don't we use metric
airspeeds), 411 metres or 450 yards.  Each group falls straight down
(with a bit of throw-forward), but they are separated by the distance
the plane travelled.  In this case lots of separation.  So on no wind
days climb out fast.

More usually, there is wind.  If say the wind (at height) is 50 knots
and the airspeed at power-off is 80 knots, then you will have 30 knots
of ground speed.  That means that the plane is moving with respect to
the ground at 30 knots.  With 10 second separation times, this means
that groups will be horizontally separated by 164 metres (168 yards).
Which is still fairly safe.

Graphically this looks something like this (not to scale and as best I
can do in ascii).

10000'   <--+--               <--+--                 <-- plane direction
            * exit group 2       * exit group 1
           *                    *                     --> wind direction
          *                    *
         *                    *
         *                    *
          *                    *
           *                    *
            *                    *
             *                    *
              *                    *
               *                    *
                *                    *
                 *                    *
                  *                    *
                   *                    *
                    *                    *
                     *                    *
                      *                    *
                       *                    *
                        *                    *
                         *                    *
                          *                    *
                           *                    *
                            *                    *
                             *                    *
                              *                    *
                               *                    *
2500'                           * opening 2          * opening 1 

Each group makes the same path through the sky.  But they are shifted
horizontally by the ground distance travelled.  If the wind speed is
70 knots and the air speed is 80 knots, then the ground speed is 10
knots.  Horizontal separation with 10 second exits is now only 51
metres (56 yards).  I track further than that.  So its easy to have
groups cross each other at opening time.  In this case you really need
to slow down the exit times.  You must emphasise to each and every
group to NOT NOT climb out quickly.  Sometimes I've had to tell
inexperienced jumpers to count to 20 before they even think of
climbing out.

In the worst case, the wind speed and air-speed are both 80 knots.
The ground speed is 0 knots, so the plane makes no progress
horizontally.  The graph above would be drawn with both lines on top of
one another.  You can wait as long as you like between groups but you
will still get out at the same spot, you both drift the same and both
open at the same point.

All the above calculations depend on ground speed alone.  Not one
calculation depends on the air-speed, or the wind-speed at various
heights or the amount of drift you expect.  All these things are
important for working out the exit point itself but they are irrelevent
for determining the exit separation time.

>			  2)  What (other) factors do you consider when
>			      multiple groups exit?

How many groups - If you have 8 two-ways then the first group has to
be spotted short and the last group will probably walk.  But you try
not to let them.  If there is not much upper wind, then its usually
better cross-winding the run-in, otherwise the spread will be much too
great.

Size of group - 2-ways climb out quicker than 4-ways which are quicker
than 8-ways.  But 8-ways get rid of more people, so they are usually
put out first.

Experience level - A hot-shot 4-way will climbout and exit in under 10
seconds, but a 4-way of just licenced jumpers may take 20 seconds or
more.  An AFF group takes anything from 0 time to 1 minute (and I've
had both) but usually about the 20 second mark.

Opening heights - Tandems open higher and have good glide ratios so
they can be spotted deeper than normal jumpers.  Put them out last.
But don't forget the cameraman who opens at a normal height.  On the
other hand, AFF students also open high.  But they are fairly erratic
with their canopy control (even with a TCO) so you need to get them
into a perfect spot.  Depending on the ground wind, the AFF may go
first or last or in the middle.

Solo jumpers take 0 time to exit.  If they are inexperienced, they have
to told NOT to exit immediately after the AFF group just before them.
Count to 5 first (or 10 or 20 or whatever suits the conditions).  I
often get a bad name with that level of jumper, because I always tell
them, every single jump run, no matter how many times I've told them
before.  But I don't care.  I've seen too many dead-brainers open right
where my 4 way was before we tracked.

>(Quite honestly, I usually jump in conditions in which getting out faster
>is more of a concern than getting out too soon.)

Me too most of the year.  But sometimes, especially during August and
September, we have real strong uppers.


Bob V!