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The air speed vs ground speed debates in the early mid 90s on rec.skydiving were very confusing. There were so many clever and convincing arguments leading to contradictory conclusions.
This was my attempt to sort out why the different descriptions of the same underlying physical situation seemed to give different answers, and to start a FAQ that would eventually become part of standard skydiving lore.
The confusion stemmed from our ground based intuitions and there has been some progress, but we are still a long way from exit separation becoming a standard part of skydiving lore.
Winds, Exit Intervals, Separation of Groups
----------------------------------------------------------------------------
Subject: Winds/Spotting - beginnings of faq
Date: 14 May 1995 23:45:37 GMT
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.
Other frames of reference are possible - the airplane, the air itself.
Imagine we are standing on the tailgate. The outside air is moving past at
airspeed AS, the ground is moving past at ground speed GS. Jumper 1 exits
and EI seconds later jumper 2 exits. What do we see?
airplane air --> AS = air speed
*
* ground --> GS = ground speed
*
*
*
*
*
*
O2 <- S = GS * EI -> O1
. .
. .
-------------------x--------------------x------------- ground
O1 ----------------> O1
Jumper 1 falls away on some trajectory, jumper 2 falls away on the same
trajectory EI seconds later. Jumper 1 opens. While jumper 2 falls the
remaining EI seconds, jumper 1's opening point moves a distance S = GS * EI
and their opening points are separated by S = GS * EI.
Now imagine we are a wind molecule hanging out at the drop zone on a typical
day. An airplane flies by, drops jumper 1 right next to us, and EI seconds
later drops jumper 2. What do we see?
<--- airplane AS = airspeed
<--- ground WS = windspeed (not ground speed)
Jumper 2 Jumper 1 wind molecule
<----------- AS * EI ---------------> @#@
X2 <- GS * EI -> X1 <- WS * EI --> X1
* . *
* . *
* . *
* . *
* . *
* . *
* . *
* . *
* . *
O2 <- GS * EI -> O1 <- WS * EI -> O1
. . .
. . .
-x----------------x----------------x------------------ ground
O2 O1 <------------ O1
What we don't see is two airplanes in formation, one dropping jumper 1 right
next to us and the other simultaneously dropping jumper 2 at a distance AS *
EI from us. We all know that hanging out with the wind molecules changes our
view of things, but this is not that change.
What we do see is that while jumper 2 is flying the distance AS * EI through
the air to his exit point, jumper 1 starts down his trajectory and his exit
point moves a distance WS * EI. When jumper 2 exits the exit points are
separated by S = GS * EI.
And on the bottom end, while jumper 2 falls his final EI seconds, jumper 1's
opening point moves a distance WS * EI. When jumper 2 opens the opening
points are separated by S = GS * EI.
So 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.
Now as a math major I would be happy to stop right here, but since we have a
***practical application*** in mind we should do some numbers and find out
what we are actually talking about.
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
|
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
|
S = 2,000 ft | 11.8 16.9 21.5 29.6
With the free plummet case in hand we can handle tracking and canopy motion
by setting our free plummet exit and opening points far enough apart.
--------------------- <-- There .. I just drew the line :-) :-)
This is as far as I can go with this right now.
How much do people track? 300 ft each? Would free plummet opening points 300
+ 300 + 600 = 1200 ft be enough?
What if the lowers are opposite the uppers and jumper 1 flies his canopy 500
ft toward jumper 2's opening point - and they each track 300 ft toward each
other? Is 300 + 300 + 500 + 600 = 1700 ft enough?
And on another subject how can we get all this happening in practice?
Being an idealist my favorite way would be for everybody to read the FAQ
when it finally gets written, think deeply about it, and then do the right
thing on jump run.
Right.
More feasible would be some kind of load master on each plane load who knows
(estimates/guesses) what the separation should be and helps / ensures that
people do it.
Other ways would be that oreo cookie stuff - seeing the breakup as a full
fledged maneuver in its own right. Tracking perpendicular to the jump run,
using vertical separation , .. stuff like that.
So let's thrash out some of these remaining quesions here in rec.skydiving
and then someone can write a FAQ and Bob Bonitz can make a formal proposal
to USPA :-) :-)
Skratch
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