skr

> From Bryan Burke at Skydive Arizona.

I think these analyses and formulations
of situations that he does are real milestones
in the evolution of skydiving.

Skr

I read to the bottom of Bryan Burke's article

http://www.dropzone.com/safety/General_Safety/The_Horizontal_Flight_Problem_935.html

and tried to add a comment, but it doesn't show up.

I was able to rate it by clicking on one of the little gold stars but couldn't add a comment.

Skr

> One type of dive you can do is where you do nothing in freefall ... except feel the air.

I still do that fairly often, just go out
by myself and feel the amazing feeling
of being in Freefall.

Sixty seconds is a really long time!

Skr

Besides all the other good answers above
I find it helpful to get on Google Maps and
practice seeing how the runways, surrounding
ground features and so on look.

I look at the dropzone from various altitudes
then look away and practice visualizing it.

I do that enough times that I can see the
runways and know their numbers, and can
see the river, highway, town, railroad track,
race track, or whatever, and know which
way north, south, east and west are.

That helps me assimilate the avalanche
of other new stuff I will encounter at a
new dropzone.

Skr

> Writing to us the 'old' jumpers Skratch!

Yes, some of the gear, airplanes, and common
practices have changed, but lots of his stories
revolve around human nature, motives, quirks,
bravado ..

How many young guys get in trouble doing
something weird these days because underneath
it all they were just trying to get noticed by
all the pretty girls? :-) :-)

I remember a story about them jumping a
watermelon - a Texas sized watermelon.

Right away I'm visualizing something the size
of a small tank, with handles dangling off the
ends (how'd they get that thing into and out
of a 195?).

And immediately upon exit the watermelon
goes supersonic, with two jumpers fluttering
in the breeze, leaving the camera and third
jumper stranded in freefall at exit altitude.

So they seek wisdom far into the night, and
decide to try it with a mattress the next day.

A mattress?? From a 195??

But I can see that story happening today.

The jumpsuits and gear and airplanes may
have changed but ...

The "old days" really were different in some
ways, and I'm glad I got to see it, but under
the surface a lot is still the same.

Kind of scary thinking there's lots of young
guys out there now with attitudes like ours
were back then, isn't it? :-) :-)

Skr

Aha! I thought there might be more to the story
than appeared in Parachutist :-) :-)

It was good to see the uncut version, but I agree
with Guru312 that your stories reveal even more.

So if you happen to fall into a reminiscing reverie
from time to time ...

You'll be writing to us, the jumpers, not to some
Parachutist editor, so you can just kind of let go
and ...

Skr

Clarice just forwarded this email from Brian Williams:

Date: Sun, 5 Aug 2012 19:48:02 -0700 (PDT)
From: Brian Williams
Subject: Bill Newell gone

I'm sad to say, but Bill passed on sometime after 6:00 P.M. today. He battled the Grim Reaper to the very end. I've known this Great Guy for 49 years and will miss him terribly.

Brian Williams

> They are from way back in the Casa Grande era

Way back?

Whaddya mean *way back*?!

It seems like just day before yesterday.

I ran into Bob Schaeffer in Oklahoma in 1980
when I was helping Hillsy at Skydance, but I
haven't seen Mike in a long time.

Those guys really knew how to run a drop zone.

Thanks, Gary. It's good to know Mike is still
active and flying.

Skr

Worst DZO?

I've been jumping at Snohomish for the last
four years and I have a different impression
of Tyson.

He reminds me of Bryan Burke down at Eloy
who looks at the whole system and how the
various components fit into the overall
situation and tries to let people do as
much as they can without endangering others.

And I'm sure he would like to be doing some
wingsuit jumps himself.

But I think he's up against the question of
how to coordinate really different flight
modes, like swooping with regular canopy
flight this is wingsuits with straight
downers and high pulling tandems.

And I think the real sticking point is the
effort it would take to keep new wingsuiters
out of the straight downer airspace.

Multiple landing zones and airplanes add to
the complication, but I think the training
to keep the airspaces separated is probably
the stumbling block.


Didn't Mark Twain say something about people
who do what's right even when it's inconvenient
and unpopular?

Something like aggravating some and astonishing
the rest? :-) :-)

Skr

> our mom was to say the least, one of a kind.

Most of the people she jumped with were
unique and colorful characters, but yes, she
stood out even in that crowd, and not just
because we were nearly all guys and she
was a really pretty girl :-) :-)

I wasn't there for her malfunction, but now
that you mention it I remember it happening.

I remember her being on the Fairchild jumps
at Oceanside.

I was jumping at Oceanside, Elsinore, Taft,
Arvin, Lancaster and California City, and ran
into her at several places, but I imagine that
I noticed her much more than she noticed me.

People are fading fast. It's good that you have
so many memories of her.

Skr

I don't know.

Danger wasn't why I started jumping.

I wanted to go out in space and be weightless,
but I was inconveniently born a couple centuries
before that became commonly available.

And I wanted to fly, like a bird.

And when I was a kid I played WW-II pilot getting
shot down over Germany so I would have to bail
out, and I would fall and fall and fall.

And for a couple summers I spent all day jumping
off the 24 ft diving platform, accumulating several
days worth of freefall one second at a time :-) :-)

And when I finally did start I was really, really scared
until I got stable (11th jump), and that wasn't fun.

And the times after that when I would get in some
situation and be really shit scared weren't fun either.


On the other hand ..

That danger introduced me to intense focus, and
forethought, and paying attention, and I found that
I really liked that.

And I remember, in the late 70's, when life was starting
to transition, wondering why I had to jump in order to
focus like that.

Well, of course you don't. People have known that
for thousands of years. That led to a big thrash of
reading spiritual and meditation books.

And as I became more self aware, or maybe honest,
over the years I found that I was drawn to situations
of concentration and paying attention.

I remember, working at the Academy of Science in
Beijing a few years ago, the first time I decided to
try taking a taxi and get further than walking distance
from my apartment.

It was like the pre-jump jitters of my first jump,
reviewing how to say where I wanted to go, pinchecking
everything I was taking with me, going to the bathroom
again ..

And when I got to the subway and started down the
steps into the vast unknown of god-knows-what
my heart was racing like on any jumprun.


So I guess I'd change my answer from not sure to
about the same as it is now.

I don't like being afraid but I like what I've learned
from it.

Skr

Well all the words floating around in my mind
seem kind of trite and inadequate.

He used to jump at Oceanside, near Camp Pendleton,
in the early 60's, and when I'd run into him here
and there over the years he always seemed like
the same guy - a little more weather beaten, but
the same, quiet guy.

And he was always in such good shape I guess
I thought he'd just kind of go on forever.

Damn ..

Well, thanks for posting this.

Skr

Hi Al,

> The old crowd seems to be shrinking at an alarming rate now. I don't know if you've seen Bill Newell's latest post (5-5-12) on Air Trash but it snapped me back to reality.

I didn't know about that. Thanks for telling me.


I should apologize for pulling in your face before
it's too late.

It's just that I was used to pulling at two rather
than breaking at two. I actually saw you, but by
the time it registered my right hand was going
for the ripcord and it pulled before I could shift
gears.

And then when you said I should be able to handle
myself at 1,500 ft I knew you were right but I was
too embarrassed to say so.

It's funny the stuff you remember.

Skr

I got so involved in drawing that ascii diagram
that I forgot about this part:

> Why is this not true in your example:
> When J1 passes through, it will take him horizontally farther away from J2 and when J2 pass through it will take him horizontally closer to J1, back to the original horizontal separation at exit.
>
> J1 will see J2 first moving horizontally away from him and then horizontally closer to him.

Except that I woke up a couple mornings later
thinking "No, he's right. That apparent extra
separation disappears right there in the forward
throw layer."

So then I started trying to think where I got off
the track, thinking I would come back here and say
something, but I can't seem to concentrate on this
right now.

This is especially annoying because I used to think
what you just said, and even posted about it.

rec.skydiving Sept 1999:

> One way I retrain my intuition is to practice standing in the door
> with my primary focus being my relation to the ground and my motion
> across it. With high uppers I can see that I am not moving very much
> and the previous group is being swept away by the upper wind. When
> I step out I too will be swept away by that same wind and will end
> up pretty close on top of them.
>
> With no uppers, I am covering distance across the ground and basically
> leaving the previous group where they got out (except for forward throw)
> and moving away from them.

Bryan Burke has a number of pithy sayings, one of which is

"Minds are like parachutes.
"Sometimes they just don't work.

So this effort foundered on poorly formulated physics,
but I'll be back sometime later and start a new thread.

I know that 98% of the attention these days needs to be
developing customs that allow different kinds of canopy
flying to coexist, but I'd just like to have a nice, clean
exposition that takes new jumpers from their initial intuition
of looking out the door and leaving room, to why that doesn't
work with uppers, to when groundspeed is a good technique
and when conditions are in unsolved territory.

Skr

I mostly hear "hard deck" when people are talking
about when to stop trying to fix it and start taking
steps to get a reserve out.

But there are all kinds of "mental gear shift" points.

When do I decide I can't make it over the freeway
or alligator farm and land over here?

When do I decide I'm not going to make it to shore
and start preparing for a water landing?

If I have a canopy collision when am I too low to
cutaway?

----

One thing about "trying to fix it" is that it's really
easy to get focused on trying to fix it, and forget
how fast you're coming down.

So to me practicing and ingraining that "Try once,
try twice" reaction is really important.

Skr

Well that's hard to hear.

He really was one of the good guys.

He used to come over to Hillsy's dropzone
in Oklahoma.

He was not only really good with students,
he was colorful, entertaining, a good story
teller, and deep. A very perceptive guy.

One by one, they're leaving, one by one.

Skr

> It STILL seems as though you are saying that J1 didn't pass through that same higher, faster moving layer.

Right. This region during and right after exit
is hard to get into words.

The bottom is easy. J1 arrives at O1; a little while
later J2 arrives at O2.

Once you can control opening points you can talk about
group centers and tracking and canopy motion and so on.

And the middle is easy. Each jumper is falling straight
down in his little patch of air, and the higher patch
is moving slowly toward the lower patch.

But the exit stage is hard. It's not linear, and J1 and
J2 are in corresponding parts of their trajectory at
different times, and so on.

I find myself fantasizing about taking John's program
and modifying it to have one jumper exit, and then
some time later a similar jumper exit, and plot, or
somehow display, a history of their horizontal separation.

He's already done all the heavy lifting, selecting the
drag law, the numerical integration technique, step size,
and so on.

But I know I'm not going to do that. I'm just a couple
g's short of floating off to ... otherwhere.

But maybe there's some energetic math or physics student
out there who could be enticed by the fame and glory of
finally resolving the notorious separation debate :-) :-)

And then students could all look at that program.

And we experienced jumpers, too. I'd like to run a bunch
of different scenarios and get a clearer idea of exactly
how the distances play out.

----

But! To your question:

J1 and J2 are not interchangeable. It's not symmetric.
If we start the video running when J1 exits, then J2's
path doesn't look like J1's. It has an extra, horizontal
piece on the front end.

The freefall parts of the trajectories have the same
shape, but when you're comparing J1 and J2 moment by
moment you're comparing points on two dissimilar paths.

UPPERS ------ >       <------- AIRPLANE


T=J2 exit           T=0 =J1 exit

_ _ _ _ _ _ _ _ _ _    =J2 keep going straight and level

/                  /

|                  |

\                  \

\                  \

\                  \T=J2 exit <--comparing J2's freefall

\                  \           trajectory from top to

\                  \         almost bottom with J1's

\                  \       from here on down

\                  \

\                  \

\                  \

\                  \

\T=part way down   \

\                  \

|                  |

|                  |

|                  |

|                  |T=part way down

/                  /

/                  /

/                  /

/                  /

open               open



# # # # # # # GROUND # # # # # # # # # # GROUND # # #

So, I can't answer any better than that until that mythical,
long lost, skydiving, physics grad student shows up and we
can run a bunch of examples.

It's a good question.

Skr

Well! That wasn't the discussion I was expecting
from that first post. I know .. Expectations ..

I thought we were going to talk about training students.

But I'm worn out, so I'm going to restate the central
point, and go have a beer or something.

When jumper 2 exits:

|<------>|         is the separation of exit points


|<=======.======>| is the separation jumper 2 sees



UPPERS ----------- >     < ----------- AIRPLANE


|<------>|

EP2      EP1

|<=======.======>|

.        .       J1

.        .

.        .

.        .

.        .

.        .

.        .

.        .

.        .

.        .

.        .     OP2      OP1

.        .      .<------>.

.        .      .        .

.        .      .        .

# # # # # GROUND # # # # # # # GROUND # # # # #

The relevance of this is that almost everybody's
first intuition for leaving separation is to look
out the door, see |<=======.======>| for separation,
and expect that to still be there at opening time.

That's the ground based, mockup intuition of how
the world works.

They need to understand that they're going to spend
all but the first and last bits of the freefall in
a steady state situation where jumper 2 is in a higher,
faster moving layer, and that they're going to end up
with |<------>| at the bottom.

And I think it's possible for experienced jumpers and
USPA and the Safety and Training Committee to explain
this to new jumpers.

I know ..
"Look Martha! Hand me my camera!
There's an idealistic optimist!" :-) :-)

Skr

Hey, Walt, I've read this 3 times trying to get
why you're saying this.

An aerodynamic, plutonium brick will take longer
than a feather, but they both get to 80 knots.

The sideways motion is independent of the downward
motion.

Unless .. Do you mean by this:

> As the angle from vertical increases, it tells-reflects the amount of effect the moving medium has on it.

that the motion is asymptotic, that it gets closer
and closer to 80 but never actually gets there?

Skr

> Why should the position over the ground have any significance at all? I really don't get it.

You're right. As a question on a physics test,
tossing spherical, isotropic skydivers off the
tailgate and watching their horizontal separation
evolve over time, the ground is irrelevant.

And actually, in the rec.skydiving discussions
several people kept pointing this out. When a new
example or theory was being put forth, they would
wait until the jumpers were in freefall and then
bring out this giant yellow Caterpillar tractor
and start hauling the dropzone around under the
jumpers.

At the time I found this annoying because I was still
trying to understand how both airspeed and groundspeed
arguments could seem so convincing.

You would have been one of those guys :-) :-)

I finally pulled back from trying to follow other
people's arguments and sat down to think it through
from a standing start for myself.

I found that if I imagined standing on the ground,
a couple miles off to one side, and watched a jumprun
unfold, I could see all the parts in a way that I knew
the physics was correct.

The exit points on the ground, the corresponding points
in the space above the ground, the opening points, the
patches of air in the exit layer where people exited,
the way those patches moved across the sky with that
layer of wind, the patches where they opened and the
way they moved relative to the ground, and so on.

Once I had that I could listen as others layed out
their current theory and I could follow it, and I could
see if I agreed or exactly where I might disagree.

That's when I saw that the second freefall trajectory
was exactly the same as the first, just displaced upwind,
and you could separate opening points by separating
exit points.

So my answer was, when the first person goes, look
down at the ground and see where they got out, go
a certain distance across the ground, and go.

It was neither airspeed nor groundspeed, it was all
spatial distance.

So I posted about that and advocated that for several
years. But I gradually saw that it's not practical.
I had spotted a lot in the past, but these days lots
of people never get the chance to spot any.

--------

Once I had that I could see how, when the first jumper
get's out, the plane flies upwind from that point in
space, and the first jumper blows downwind from that
point in space.

And the second jumper sees a large distance to the
first jumper at exit, while at the same time separating
exit points by a smaller distance.

That excess distance is what disappears from the moving
layers effect.

--------

(I guess I'm actually now responding to later posts in
(the thread, not just to strop45.

Some time back John asked why we're still talking about
this since Newton's laws haven't changed in several hundred
years, and I said because the current problem is psychological
and social rather than physics.

Almost everybody's initial intuition for leaving separation
is to look out the door and leave some room between jumpers.

Now you can say "Use groundspeed", and some people are happy
with that. That's all they want to know.

But! They won't know why, and they won't know the conditions
under which it's appropriate.

And there are plenty of people around who don't accept proof
by authority, they want to think for themselves and know how
it works.

And for those people we need to explain the genesis and limits
of that initial intuition, and some ways to see the more complicated
situation of winds, and the approximation and limits of groundspeed.

That's what the initial post in this thread was about.

I think John and Bill and others are doing a good job of
simplifying a complicated situation.

----

Billvon, it's a good thing your 80 knot example wasn't 120 mph
with the jumper falling away from the plane at 45 degrees for
the whole jump.

That would have really stirred things up :-) :-)

Skr

> Scratch has now explained explained that he was only discussing the "45 degree angle rule"

No, I am not.

I am talking about the initial intuition that leads people,
when they first encounter the separation question, to look
out the door and leave what looks like good opening separation
between them and the person that went in front of them.

And how that leads us astray.

And how to expand that intuition so that what actually,
physically happens seems normal.

In the hopes that when enough new people have been taught this,
it will become concensus reality and this whole separation
discussion will recede into history.


I remember when I first encountered this, Pope Valley, 1970's,
Bill Dause told us one morning that we were going to start
putting multiple big groups out of the DC3.

There was horrified silence and a vast reluctance :-) :-)

But none of us discussed how to do it, we just looked out
the door and left a whole lot of room between us and the
group in front of us.

I was elsewhere in the 80's and didn't see that part of
the evolution.

My next encounter was the legendary airspeed-groundspeed
debate on rec.skydiving around 1995. After reading a thread
of wildly varying views and really clever arguments going
in every which direction I was really confused.

The arguments were so convincing, and the conclusions were
so contradictory.

If I imagined standing on the ground watching a jumprun,
then separating opening points by separating exit points
was obviously right. And I got better and better at the
physics of it all.

But I couldn't shake the feeling that if I looked out
the door and left separation that that was also obviously
right.

It was only when I saw billvon formulating the situation
as moving layers that I realized that I was unconsciously
using a ground based intuition about how the world works
in a situation with a lot of moving parts where it didn't
apply.

And I think, by the way people talk about it, that a lot of
other people are falling into the same trap.

That's what the original post was about.

I'm not trying to solve a question of skydiving technique,
I'm trying to solve a sociology problem, which is the
never-ending separation discussion.

I haven't been over to look at Brian's stuff yet, but I'm
sure that if he ever stumbled across this thread he would
understand immediately what I'm trying to do.

Skr

> Am I missing something here?

Maybe it's that some words like "separation" are
being used with different meanings in different
parts of the conversation.

"Exit point" used to just mean the place on the ground
that you got out over. When these airspeed-groundspeed
discussions came up, I'm not sure when, my first encounter
was in the mid 90's, people started also meaning a
corresponding point in the sky above the point on the
ground. There could be 3 exit points, ground, opening
layer, exit layer.

And similarly for "opening point".

Used that way separation between exit points translates
directly to separation between opening points.

But the intuitive way that almost everybody, before they
encounter the airspeed-groundspeed discussion, first thinks
of for leaving separation is to look out the door and leave
some distance between you and the person who got out in front
of you.

In that case "separation at exit" means from you in the door
to that person.

If there were no winds that would work, BUT! :-) :-)

With uppers the plane flies slowly upwind of the exit point,
the first jumper blows downwind, and when you see what looks
like the separation that you want at opening and go, you've
really only separated the exit points by ... not enough.

It's just that this conversation has been going on for so
long among so many people that people get telegraphic and
leave out background assumptions and sometimes it's hard
to be sure you're actually both in the same conversation.

It also depends on who you're talking to. I can say "groundspeed
is a good technique" to you and we're probably then in the
same conversation.

If I'm talking to a new person it's really important that
I lay out the conditons under which that's a true statement.

The winds at opening altitude can't be very strong.

And they need to be going in the same direction as the uppers.

And the uppers can't be super strong either, especially
with people pulling at such a variety of heights. A high
puller could still be up there in the kill zone even 60
seconds later.

It's a complicated situation and everybody including me
would like a simple answer.

Skr

> commenting on your theory

The theory I thought I was writing about is why
so many people start out thinking you can leave
exit separation based on how far the person in
front of you has fallen behind the plane.

It seems intuitive because that's how the world
works down here on the ground where we use the
ground as a common, rigid frame of reference.

And then on through a number of reasons why that
doesn't work with upper winds.

The point was to help new jumpers stretch their
intuitions to feel comfortable with the more complicated
situation where the ground, the layer at opening
altitude, all the layers on up to the exit layer,
and the plane, are all moving relative to each other.

Two jumpers in different layers of air may be falling
straight down relative to their local patch of air,
but the higher layer is moving relative to the lower
one, so they are not staying the same distance apart
like they would on the ground.

They are not staying the same distance apart like
they would if you had two planes, one following the
other, and a jumper leaving each plane simultaneously.

In that case the jumpers are in the same layer at the
same time and the separation stays constant.

But in the skydiving case the jumpers are separated
in time as well as space.

So that to do the initial, intuitive thing of leaving
separation by looking at the jumper ahead of you, you
have to leave the separation at opening altitude plus
enough to account for the moving layers effect.

The jumper in the door sees a distance that is roughly
airspeed * exit interval, on the way down the moving
layers effect subtracts enough that they end up with
roughly groundspeed * exit interval. And each jumper
after that sees the same thing.

Someone standing on the ground would see distance between
exit and opening points as the same but the jumper in the
door sees something different.

This is where the intuition needs stretching.

The jumper in the door is measuring the exit separation
in one frame of reference, and then opening separation in
a different frame of reference, and the frames are moving
relative to each other.

And then the theory goes from there to why it's hard
to do things that way and that's why people often
start out with an educated guess for the first load
and then adjust.

Unless a GPS is present.

With that you have the possibility, under certain
conditions, of estimating exit interval using groundspeed.

So the theory is about how our initial, ground based
intuition leads us astray, and how to learn to think
so that our intuition gives us good answers in the
upper winds situation.

I can't connect the rest of your objection to what I
thought I was talking about.

I first heard of the 45 degree rule long ago at Quincy
from John Mathews. For about an hour I walked around
feeling great relief that we finally had a simple answer
to the endless exit separation question.

Then suddenly it was like "Hey! Wait a minute! That's
the classic airspeed approach! Shit! We have to keep
on thinking!"

I don't get around much anymore, just the local dropzone
at Snohomhish, because I'm kind of fading into other...
into otherness I guess.

I didn't see Brian's stuff, but if he's having to explain
that the 45 degree idea doesn't work with uppers that
tells me that we experienced jumpers aren't getting the
word out very well.

And you're right about the King Air :-) :-)

Skr

Oh, Hi Walt,

I see it lost some formatting, I should have enclosed
it in a couple "pre" tags.

If you're standing on the opening layer the exit and
opening points look to be the same distance apart.

If your standing in the door leaving separation by
looking at the distance from you to where the first
jumper got out, the distance to the red molecule,
then you're measuring that distance in the upper
layer but measuring distance between opening points
down in the opening layer, which is a different
coordinate system.

I'll see if I can say it in another way when I get back.

Skr

I remember how confusing the airspeed/groundspeed
debate on rec.skydiving was, and I remember the aha
moment when I saw billvon formulating it as differently
moving layers of air and realized that my confusion came
from unconsciously applying my ground based, common, solid
frame of reference intuition to the moving layers situation.

I think by the way this question keeps coming up
that we, experienced jumpers, DZO's, and USPA, are
not doing very well at explaining this to new jumpers.

A few days ago I added a bit to the second paragraph
of an email I had sent to a young friend of mine and
sent it off to the Safety and Training committee.

So if Winsor comes across this and doesn't grumble too
loudly, then I'll consider it safe for home use :-) :-)

---

Why Groundspeed? Sun 2012-4-1
---------------- ------------

With typical winds, stronger on top, pretty much the same
direction and gradually decreasing on the way down, a group
in a higher, faster moving layer of air will spend the whole
freefall moving closer to a group in a lower, slower moving
layer.

Bill Von Novak is the first person I saw explain the
situation in those words, and this is something I wrote to
a young friend of mine on stretching her intuition to feel
comfortable with this.

Mockup Intuition to Layers of Air Intuition
-------------------------------------------

The name of the game here is to learn how to think so that
what physically happens with upper winds appears natural.

Once we have that, once our intuition has expanded, the
various approaches to leaving exit separation will be easier
to understand.

Specifically I'm talking about going from the intuition for
leaving exit separation at the mockup, with the ground as a
solid and common frame of reference, to the intuition for
leaving exit separation on jumprun, where different groups
will be in different layers of air which are moving relative
to each other.

At the mockup, if the first group gets out and walks a
certain distance away, and then stops, and then we step out,
and then we all just stand there, falling straight down, we
will still be the same distance apart 60 seconds later.

That's how life on the ground works, and we have a lifetime
of experience and deeply ingrained habit thinking that way.

If there were no winds we could do the same thing on jumprun.

When the first group has fallen far enough behind the plane,
we can jump out, and that initial separation will still be
there down at opening altitude.

But with uppers we have to teach ourselves to automatically
see something new, and that is that during the freefall we
will be in a higher layer of wind than the group in front of
us, and our layer will be moving faster than their layer.

The initial separation at exit will be there, but we will
spend the whole freefall gradually moving closer to them.

In order to have the separation at the bottom for opening,
we have to start with that separation plus enough more to
account for the relative motion of our two different layers
of air.

Once that new way of thinking feels natural the various
approaches to leaving more time between exits for upper
winds will be easier to understand.

Practical Difficulties
----------------------

At this point we hit the first set of practical difficulties.

The obvious, intuitive way of leaving separation, looking
out the door at the group ahead of us, and leaving enough
room for the desired separation at opening plus enough for
the different layers effect, doesn't work because

- We don't know the speeds of the different layers

- We can't accurately judge the horizontal separation from
a group that could be thousands of feet below us at exit
if the uppers are strong

- Training - We can't do the large amount of training
required for every new jumper even if we did know the
speed of the layers and could judge the distances.

That's why we turn to the more indirect method of time
between exits.

But even here there are practical difficulties

- Trying to state a procedure that would work for
combinations of all different sizes of groups is too
complicated

- And for really strong uppers the very concept of exit
separation becomes meaningless because the first group
could be practically at opening altitude before we get
enough horizontal separation. (I'm thinking here of a
King-Air load where the GPS showed a groundspeed of 12
knots.)

So the time between exits approach only works for smaller
numbers and light to medium uppers. Everything else is case
by case.

More Intuition
--------------

The final intuitive part to become comfortable with is that
people standing

- in the door
- on the layer of air at exit altitude
- on the layer of air at opening altitude
- on the ground

are all looking at the same situation, and all seeing
something different because of their relative motion.

The discomfort comes from our lifetime habit of
automatically applying our ground based, mockup intuition
where everything happens in a common, solid frame of
reference, to this new situation with lots of moving parts.

The comfort will come from practicing, and getting used to
seeing the situation with all its moving parts, from each
point of view.

First fly over and sit down on an air molecule in the
jumprun layer and watch the plane come toward you.

When the plane is even with you and the first jumper goes,
reach down and color that molecule bright red.

Now watch, the plane flies away at the airspeed of the
plane, the first jumper starts to move away from you as she
falls through progressively slower layers of air, and then
the second jumper goes.

The second jumper also starts moving away from you, but not
as fast as the jumper in the lower and slower layer. They
are actually getting closer together, but it's hard to see
that because they are getting too far away.

--

Now fly down to the opening layer and replay that last jump.

When the first jumper goes, mark that bit of sky, and a
molecule directly under it in the opening layer, as "exit
point", and do the same when the second jumper goes.

When they open, mark the corresponding molecules in the
opening layer as "opening points".

Now notice a crucial fact:

The trajectory of the second jumper is exactly the same
as the trajectory of the first jumper, just displaced up
wind by the distance between exit points.

The distance between opening points is exactly the same
as the distance between exit points.

We can separate opening points by separating exit points.

When there are uppers, the distance the plane must fly
through the air in the exit layer is from the red molecule,
which has moved some distance down wind, to the second exit
point.

That's airspeed, and that's the distance someone looking out
the door at the first jumper would have to leave to account
for the moving layers effect.

But for separation at opening, the meaningful speed, the
speed we can use to separate exit points, is the speed we
see from the opening layer. It's airspeed minus the speed of
the exit layer.

It's slower than the airspeed, that's why it takes more time
to go from exit point to exit point on windy days.

Feeling at ease with all these view points and moving parts
takes practice because our old, ground based habits are
deeply ingrained, and they tend to take over in the stress
and excitement of jumprun and exit.

If you practice a little on the ride up, you will get used to
it all and it will start to feel normal.

More Practical Difficulties
---------------------------

If we could see the exit points in the opening layer we
could just look down and jump out as we pass over them.

But we can't, plus the opening layer keeps moving relative
to the dropzone, which is where we want to land, so we would
have to have a staff member up there dragging them back into
place before each load.

We could use the aviation winds aloft forecast to calculate
the speed of the plane relative to the opening layer, but
that forecast is only done every 6 hours and local
conditions can vary. The military does this but they have a
larger budget for special equipment than most dropzones.

So probably the most common separation technique is that on
the first load, jumpers make a guess based on what's usually
needed for the current conditions, and then make adjustments
from there.

With the advent of GPS we have another possibility because
we can know the groundspeed of the plane on jumprun.

If the uppers are in the same direction all the way down,
and not too strong at opening altitude, so that the
meaningful speed of the plane relative to the opening layer
is close to the groundspeed, then using groundspeed to
calculate time between exits is a good estimate.

And it has the advantage of being accurate at the time of
the jump.

That's how "groundspeed" gets into these exit separation
discussions.

Conclusion
----------

The best situation is where the experienced jumpers talk
about how much time to leave between exits at the loading
area, and they know how much because the dropzone posts the
current groundspeed by the chart of Time Between Exits at
the manifest or the loading area.

From there it goes downhill to denial of the very concept,
just leave 5 seconds and go, with the tandem masters and
cameras screaming "Go! Go! Go!", and we are saved from more
disasters by Bryan Burke's Big Sky Theory.

This email was about intuition and conceptual framework.
My last efforts to talk about concrete details are here:

http://indra.net/~bdaniels/ftw/sg_skr_coach_weekend.html

http://indra.net/~bdaniels/ftw/sg_skr_dealing_1_uppers.html
http://indra.net/~bdaniels/ftw/sg_skr_dealing_2_tables.html

If you're feeling hard core you can skip over the arithmetic
and scan some of the words for more ideas :-) :-)

Down at the bottom of

http://indra.net/~bdaniels/ftw/index.html

are some links to some of Bryan Burke's and Bill Von Novak's
early writing. Both guys are very good thinkers about all
kinds of stuff (Google).

And finally, the link to John Kallend's Simulation Program
http://mypages.iit.edu/~kallend/skydive/ still works and
it's helpful in visualizing all these moving parts. He's
also a good thinker.


So all this is pretty confusing when you first run into it,
but I think that if you can get your intuition used to the
moving layers you can find your way through.

Skr