Separation Between Groups - 2nd try

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Note added in 2004: There are three things wrong with this effort:

It's here like a diary entry in my own long journey with exit separation. Dealing with Uppers and Separation Explained for Students are better.


Date:Posted to rec.skydiving July 30 1998

Separation Between Groups
-------------------------

0 - Introduction
1 - Order of Groups
2 - Separation of Groups
3 - Separation by Spotting (Spatial)
4 - Separation by Counting (Airplane Speed wrt Air at Opening Altitude)
5 - Summary
A - Number of Groups per Pass

========================================================================

0 - Introduction

At first (for me that means 1962) we often just jumped out, unrelated,
all on the same pass. Later when going out in groups and doing things
together became popular and the planes got a little bigger (mid 60's
early 70's) we would usually make one pass per group.

In the early 70's OPEC changed the oil prices and within a few years
multiple groups per pass was common. Since it has been going on for so
long you would think there would be a clear procedure by now, but as is
often the case circumstances got out ahead of generally accepted
understanding and practice.

    (Just like now with high performance canopies being way out ahead
    (of the training people get for being a canopy pilot.
    (
    (Or like the huge transition we are in because skydiving has become
    (main stream.
    (
    (I believe this is a real watershed period in skydiving.

This subject has come up several times here in rec.skydiving, the last
great wave being the air speed vs ground speed debate 3 or 4 years ago.
And while we made a lot of progress, it never really got resolved.
We just kind of collapsed from exhaustion :-) :-)

So here is my best effort to articulate the subject of separation
between groups.

But I need some help. Would some of you good engineering minds out there
hammer on this for a bit and tell me what you think?

    1 - Is all this conceptually correct? Strategically correct?

    2 - Are the numbers plausible?
        Are the simplifications and approximations valid?
        (Is the arithmetic correct?)

    3 - Is there a better summary/formulation/rule of thumb
        that's easier to use but doesn't over simplify?

At some point I'm going to send a verson of this off to USPA so they
can mull it over and perhaps make a guide line or something. Also for
all you people outside of USPA (the rest of the world) please feel free
to use this any way you want.

========================================================================

1 - Order of Groups

As Bryan Burke posted some time back Eloy uses the exit order:

   1 - Slow falling groups, largest to smallest
   2 - Fast falling groups, largest to smallest
   3 - AFF's, tandems and other high pullers

The slow falling groups are in the air longer than the fast falling
groups and are therefore blown further by the uppers.

Within the slow falling groups the groups could actually leave in any
order as long as they are leaving proper horizontal separation.

If I am doing a two way, and there are a couple students fresh off of
AFF doing solos, I will help them with their spacing and then follow
them out.

Also, perhaps the group with the longest climbout should go first in
order to get the most groups out per pass.

But largest to smallest is a good general guide line.


   Uppers ------ >                  < ------ Airplane

           \  \  \               |  |  |         \  \  \
             \  \  \              |  |  |          \  \  \
               \  \  \             |  |  |           \  \  \
                 \  \  \            |  |  |            \  \  \
    Openings -- >  O  O  O           |  |  |             \  \  \
                                      |  |  |              \  \  \
                                       O  O  O               O  O  O

            AFF's, Tandems          Fast Falling           Slow Falling
            High Pullers            Groups                 Groups


This makes sense when the winds are in the same direction all the way
down. In Colorado we often have uppers out of the west with a wind shear
at 5,000 or 6,000 ft and lowers out of the east.

This would blow the slow fallers back toward the fast fallers. Perhaps
the freefall part of this is short enough that we can ignore it, but
it is certainly a consideration in the canopy motion in the next section.

The approach of putting the fast fallers out first with the idea that
when the slow fallers start opening 30 seconds later the fast fallers
will be down out of the way sounds good but 20 ft/sec * 30 sec is only
600 ft.

Also since they are out first they will turn toward target and have
30 seconds to fly up under the following groups. The Eloy approach of
going for horizontal separaton makes more sense to me.

========================================================================

2 - Separation of Groups

How much horizontal separation do we want between groups?

Let's stand on the ground and watch two groups. They fall, they track,
they get canopy surge on opening, and, while group 2 is finishing
their freefall, group 1 has a certain amount of canopy motion.

The points O1 and O2 would be the opening points if the groups were
really just two individuals who fell to opening altitude with no track
and then pulled.

The points B1 and B2 are the outer boundaries of the two groups at
the moment group 2 gets open.

(Assume for the moment that the wind at opening altitude is zero
(with respect to the ground.

   Uppers ------ >                  < ------ Airplane

           Group 2                              Group 1
              |                                    |
              |                                    |
              |        < --  freefall -- >         |        The situation
              |                                    |        at the moment
              |                                    |        Group 2 opens
              |                                    |
             / \                                  / \
           /     \      < --  track -- >        /     \
         /         \                          /         \
    surge           surge                surge           surge
    |                   |         < ----                       ---- >
    |                   |         |  canopy                 canopy  |
    |                   |         |  motion                 motion  |
    |         |         |         |                |                |
    |         |         |         |                |                |
   B2< -------O2------ >B2       B1< --------------O1------------- >B1
          opening pt                           opening pt


Now this is the most unclear part of this whole question.

  1 - How far do people need to track to be at least say 300 ft
      from people in their own group? Is 300 ft enough? Are you
      willing to open face to face with someone who is 300 ft away?
      (Line Twists?)

               A                                     A
               |                           open      |       B
               |                               \     |     /
               |                                 \   |   /
               |                                   \ | /
  open---------X--------- B             open---------X---------open
               |                                   / | \
               |                                 /   |   \
               |                               /     |     \
               |                           open      |      open
             open                                  open

             4 way                                 8 way

             If AB = 300 ft                        If AB = 300 ft
             Then XA = 212 ft                      Then XA = 392 ft

                                                   150ft/sin22.5
                                                   x - x**3/3! +x**5/5!
                                                   sin22.5/57.3 = .38265

      So to open 300 horizontal feet from people in my own group
      I need to track 200-400 ft. (Thank you Mad John.)

  2 - How much do canopies surge on opening? 100 ft? 200 ft?

  3 - How far can the group 1 canopies go while group 2 is finishing
      their freefall? (assuming winds at opening altitude are zero)

      If typical canopy forward speed is 15-25 mph = 22-36.7 ft/sec,
      then 10 seconds between the groups would give 220-367 ft.

Let's assemble these estimates for zero winds at opening altitude
into a table:

Groups                 | 4 & 4      | 4 & 8     | 8 & 8
-----------------------|------------|-----------|----------
Tracking               | 200+200    | 200+400   | 400+400
Opening surge          | 100+100    | 100+100   | 100+100
Group 1 canopy motion  |   300      |   300     |   300
Buffer between groups  |   300      |   300     |   300
-----------------------|------------|-----------|----------
Separation of Groups     1200         1400        1600

This looks pretty plausible. The numbers are about right from experience
with a little more separation between larger groups.

There is one unfinished part - the Group 1 canopy motion.

Today's canopies travel several hundred feet in 10-15 seconds, and
if the lower winds are 20-30 mph and opposite the uppers they travel
even further (thinking now about the first groups getting out short
and turning toward the target).

    (This is getting too complicated. I therefore recommend that we
    (immediately return to 28 ft rounds and pulling below 2,000 ft.
    (Life was simpler in those days.
    (
    (Hello? Hello?
    (Anybody there?
    (
    (Ratz. They didn't go for it.   OK - Onwards ...

The problem I'm going to ignore right now because there is still a long
way to go here is that the stronger the winds at opening altitude, the
further the Group 1 canopies will go, and therefore the longer Group 2
should wait, which in turn lets the Group 1 canopies go even further.

I'm sure this iteration converges to a solution in each case but if I
get off on that this will never get done.

So let's take the 220-367 ft of typical canopy motion in 10 seconds
and add 20 mph * 10 sec = 293 ft or 30 mph * 10 sec = 440 ft and get

    220-367 plus 293 = 513-660 ft of canopy motion for 20 mph of wind
    220-367 plus 440 = 660-807          "              30 mph of wind

That is 600-800 ft instead of 300 ft for Group 1 canopy motion.

If we put this in the above table we get
for 20-30 mph lowers opposite the uppers:

Groups                 | 4 & 4      | 4 & 8     | 8 & 8
-----------------------|------------|-----------|----------
Tracking               | 200+200    | 200+400   | 400+400
Opening surge          | 100+100    | 100+100   | 100+100
Group 1 canopy motion  |   700      |   700     |   700
Buffer between groups  |   300      |   300     |   300
-----------------------|------------|-----------|----------
Separation of Groups     1600         1800        2000

Since people mostly turn toward target after opening this concern
for lowers opposite the uppers mainly applies to the first few groups
which get out short of the target.

    The groups getting out first (short of the target) should
    leave more space between groups and fly off of the jump run
    line until they can see the following group. It is kind of
    fun watching the next group break up and open anyway.

-----------

Individuals could be closer together perhaps, but if someone is sliding
3 ft/sec over the course of a jump, that's a couple hundred feet, so I
wouldn't go any closer than 800 ft which is

    100+100 for surge
    300     for Jumper 1 canopy motion
    300     for buffer

Also larger groups like 15 or 20 would need more room, but people
tracking different amounts and pulling at different altitudes would
help somewhat inside the group.

Beyond 20 they probably have the plane to themselves anyway so let's
not worry about that.

So out of all that hand waving I come up with

     800 ft -- 1 & 1 (individuals)

    1200 ft -- 4 & 4 with zero lowers
    1400 ft -- 4 & 8       "
    1600 ft -- 8 & 8       "

    1600 ft -- 4 & 4 with lowers opposite uppers
    1800 ft -- 4 & 8           "
    2000 ft -- 8 & 8           "

as typical separations of opening points.

========================================================================

3 - Separation by Spotting (Spatial)

The reason for all the effort at separating opening points is that

    To separate opening points by 800, 1200 or 1600 ft,
    just separate exit points by 800, 1200 or 1600 ft.

Simple.

That's all there is to it.

Also, assuming both groups are falling straight down (relative to the air),
separation of exit points is the only thing we have control of. That and
the groups getting out short not flying up the jump run line until they
can see the following group.

    At first long ago in the 60's I drove between landmarks at
    Elsinore to measure distance because I was trying to figure
    out how far we were tracking. (Seems like this could be done
    now by tracking with a GPS.)

    These days at Mile Hi in Colorado the runway is 4,800 ft so
    I use a quarter of it as my 1,200 ft measuring stick.

========================================================================

4 - Separation by Counting (Airplane Speed wrt Air at Opening Altitude)

The reason for all the effort at separating opening points is that

    To separate opening points by 1200, 1600 or 2000 ft,
    just separate exit points by 1200, 1600 or 2000 ft.

Simple.

That's all there is to it.

Except that ...

Many people want to count seconds between groups instead of looking
out the door at the previous group and the ground.
(insert soapbox here :-) :-)


Borrowing an excerpt from a previous post (Bradley Spatz put a
copy of the whole thing over in the Skydive Archive
http://www.afn.org/skydive/sta/skratch-spot.html):

>Winds, Exit Intervals, Separation of Groups
>----------------------------------------------------------------------------
>
>From: Ben Daniels
>Newsgroups: rec.skydiving
>Subject: Winds/Spotting - beginnings of faq
>Date: 14 May 1995 23:45:37 GMT
>Organization: National Center for Atmospheric Research, Boulder, CO
>Distribution: world
>
>Theme: Safe separation between groups at opening comes directly from
>adequate separation of groups at exit.
>
>Sub theme: When flying into high uppers leave more time between exits.
>
>We will first look at two jumpers and their exit and opening points. Then we
>will look at the effect of
>
>  1. Tracking
>  2. Canopy motion of group 1 while group 2 is still in freefall
>  3. Uppers in one direction and lowers in the opposite direction
>
>Now, using the ground as the frame of reference, imagine many layers of wind
>blowing every which way and a jump run in some arbitrary direction.
>
>Jumper 1 exits at X1 and opens at O1. Jumper 2 exits at X2 and opens at O2.
>The separation between exit points is ground speed times exit interval.
>
>     Jumper 2                    Jumper 1
>         X2 < --- S = GS * EI ---- > X1            < -- airplane
>          \                           \
>         . \                         . \
>         .  \                        .  \
>         .   \                       .   \
>         .   /                       .   /
>         .  /                        .  /
>         .  \                        .  \
>         .   \                       .   \
>         .    \                      .    \
>         .     \                     .     \
>         .      \                    .      \
>         .       \                   .       \
>         .      O2 < --- S = GS * EI ---- > O1
>         .       .                   .       .
>         .       .                   .       .
>   ------x-------x-------------------x-------x----------- ground
>         X2     O2                   X1     O1
>
>The two paths are exactly the same shape separated by S = GS * EI at all
>points top to bottom.
>
>We may achieve this separation S = GS * EI of opening points by either
>
>   * Dividing ground speed into separation distance to get exit interval EI
>     = S/GS.
>   * Looking out the door and gauging the separation of exit points by eye
>     (spotting).
>
>This is our basic picture. Separation of opening points comes directly from
>separation of exit points. Flying into high uppers causes slower ground
>speed and requires greater exit interval.

    (The rest of the post dealt with the air speed vs ground speed debate.
    (
    (Back then I still hadn't quite gotten clear that while it certainly
    (wasn't air speed, it wasn't ground speed either, but rather the
    (speed of the plane relative to the layer of air at opening altitude.
    (
    (This means that you really can haul the drop zone around with a
    (giant tractor :-) :-)
    (
    (I belong to the "Look at the Ground" school so I have cast this
    (whole discussion in terms of separation of exit points plus an
    (additional correction when the lowers are opposite the uppers.

>Let's take an airspeed of 80 kts, fly into various uppers and see what exit
>interval is required for various separations.
>
>   AS = 80 kts  6076 ft = nautical mile   EI = S/GS
>
>     uppers kts |     0      30      45      60
>                |
>     GS     kts |    80      50      35      20
>                |
>     GS  ft/sec |   135    84.4    59.1    33.8
>   --------------------------------------------
>   S = 1,000 ft |   7.4    11.9    16.9      30  < -- EI = exit interval
>                |
>   S = 1,500 ft |  11.2    17.8    25.4    44.4      (seconds between
>                |                                    (groups
>   S = 2,000 ft |  14.8    23.7    33.9    59.3
>
>Now let's do the same thing for an air speed of 100 kts.
>
>   AS = 100 kts  6076 ft = nautical mile   EI = S/GS
>
>     uppers kts |     0      30      45      60
>                |
>     GS     kts |   100      70      55      40
>                |
>     GS  ft/sec | 168.8   118.1    92.8    67.5
>   --------------------------------------------
>   S = 1,000 ft |   5.9     8.5    10.8    14.8  < -- EI = exit interval
>                |
>   S = 1,500 ft |   8.9    12.7    16.2    22.2      (seconds between
>                |                                    (groups
>   S = 2,000 ft |  11.8    16.9    21.5    29.6

========================================================================

5 - Summary

I'm still looking for a simple summary of all this that I can remember
as we're getting on the plane, or maybe as we're near jump run and the
pilot reports that the uppers and lowers have changed. But ...

800 ft       for individuals
1200-1600 ft for small groups (5 or 10)
1600-2000 ft for small groups early in the load with lowers opposite uppers

Groups getting out short should fly their canopies off the jump run line
until they spot the following group.

8-12 seconds (10-15??) is a reasonable time between exits unless there are
medium to strong uppers or a strong wind shear with lowers opposite the
uppers in which case you should get someone who knows how to look out the
door to help you make damn sure you're leaving enough horizontal separation.

-----------

How to make all this common knowledge?

Maybe it already is. I don't get around much any more but it's not
consensus reality in Colorado, nor did it seem all that well understood
in the hundreds of postings here over the years.

I remember a couple years ago a loader for one of the Casas at Quincy
was emphasizing to people on every load that 3-5 seconds was all you
need between groups.

Sport Death is still around. The Reaper Lurks.

I wonder if I will ever be able to stop saying that.

At some point I'm going to send a version of this to USPA. Perhaps they
will create a guide line or something.

-----------

As we have spent all these years figuring this out it has seemed very
complicated but I believe we are close to a real understanding. A few
years from now, in hindsight, it will probably seem obvious.

I'm off to Quincy in a few days, but after I get back I will go see if
I can figure out how Deja News works.

========================================================================

Appendix - Number of Groups per Pass

    (I was hoping to shed some light on the economic incentive to get
    (groups closer together in order to reduce go arounds but there is
    (so much variation in canopy speed that I couldn't see any real
    (conclusions.  I'll leave this here for now in case someone else
    (has some insight.

Now that we have typical separations of opening points we can see
how many groups might reasonably get out on a pass.

If people are in the saddle at 2,000 ft,
and if typical descent rate is 10-15 ft/sec,
then they are in the air for 133-200 seconds.

If forward speed is 15-25 mph = 22-36.7 ft/sec,
then they can cover 2926-7340 ft from either
side of the target and the wind cone would look like:

   |< ---------- 5852-14680 ft -------------- >|

   \       OP      OP      OP      OP      OP /    < -- Openings @ 2,000 ft
      \                                    /
         \             Zero             /
            \          Wind          /             < -- Wind Cone
               \                  /
                  \            /
                     \      /
                        \/
                      Target

   |< ---------- 5852-14680 ft -------------- >|

   \       OP      OP      OP      OP      OP |    < -- Openings @ 2,000 ft
         \                                    |
               \      Strong Wind ---- >      |
                     \                        |    < -- Wind Cone
                           \                  |
                                 \            |
                                       \      |
                                             \|
                                           Target

    5852 to 14680
    ------------- = 7 to 18 individuals per pass
        800

    5852 to 14680
    ------------- = 4 or 5 to 12 smallish groups per pass
       1200

    5852 to 14680
    ------------- = 3 or 4 to 9 largish groups per pass
       1600

??????????????? so therefore what ????????????

========================================================================

Skratch 98-7-30

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