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Why Groundspeed?

 

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skr  (D 981)

Apr 6, 2012, 1:50 PM
Post #1 of 87 (2264 views)
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Why Groundspeed? Can't Post

 
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/...r_coach_weekend.html

http://indra.net/...ealing_1_uppers.html
http://indra.net/...ealing_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/...niels/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


popsjumper  (D 999999999)

Apr 6, 2012, 5:59 PM
Post #2 of 87 (2103 views)
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Re: [skr] Why Groundspeed? [In reply to] Can't Post

Apparently, somebody thinks that the two groups go through completely different winds on the way down.

With your scenario, even later groups would be piling up on top of all the previous...not gonna happen.

Draw it on paper and show us what you are talking about. You'll see the fallacy.


dpreguy  (D 835)

Apr 6, 2012, 7:00 PM
Post #3 of 87 (2064 views)
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Re: [skr] Why Groundspeed? [In reply to] Can't Post

Your theory is incorrect, as you are stating the different groups are affected differently by upper air winds. I do not believe that. ALL groups are affected in exactly the same manner by upper air winds.

If you believe the second group moves closer to the first one's opening point, just because of stronger upper air winds, then how can you explain how the first group escaped the same upper air winds?

You make some good points later, but this theory that groups move together because the winds aloft are stronger than the winds at pull altitude...well, that is wrong. All are affected the same way.

Groundspeed is, and always will be, the standard to determine separation at opening altitude. If you are moving slowly over the ground at exit, because of hellacious headwinds at altitude, then all groups will be closer together for the whole dive. The reverse is also true. When we are in the King Air, which goes over 90, if there isn't any siginifcant upper air wind, we usually wait about 6+ seconds. However, if the King Air is nearly stopped or just "crawling" over the ground, we wait even many more seconds. Our pilots report ground speed when it seems like it will be factor, and we come to a consensus of how many seconds to leave between groups. The Twin Otter is even more critical, because it flies slower. Same thinking process.

You are correct in stating that the things people do once they leave the plane affects everything, and that a good discussion between groups is the safe way to proceed.


skr  (D 981)

Apr 6, 2012, 9:15 PM
Post #4 of 87 (1995 views)
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Re: [dpreguy] Why Groundspeed? [In reply to] Can't Post

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


dpreguy  (D 835)

Apr 7, 2012, 8:17 AM
Post #5 of 87 (1915 views)
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Re: [skr] Why Groundspeed? [In reply to] Can't Post

 
Hi Scratch,

I have no idea what you are talking about with red molecules. I didn't follow all of the info with that stuff. Just was commenting on your theory. If you can convince me that the first group is not affected by the wind but the second one is, I'll agree.

On looking out and down at the last group- see Brian Germain's dicussion of the 45 degree rule recently. 45 degree "rule" doesn't work so good.

Can't stand up in the door of a king air anyway. It's the devil's airplane. Too fast, on jump run, crappy tiny door, rear stabilizer too low and close to the door. I'd rather walk to altitude.(bit 'o humor here).
These discussins can get too serious sometimes.


skr  (D 981)

Apr 7, 2012, 11:49 PM
Post #6 of 87 (1842 views)
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Re: [dpreguy] Why Groundspeed? [In reply to] Can't Post

> 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


milehigheric  (C License)

Apr 8, 2012, 12:17 AM
Post #7 of 87 (1834 views)
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Re: [skr] Why Groundspeed? [In reply to] Can't Post

Am I missing something here? I understand that it is possible for groups to 'close' (relative to each other) given strong uppers and slower or opposite direction lowers however this separation remains consistent of what was decided at exit. Both groups will experience the same conditions during their fall so despite any fluctuations in separation during fall at pull height they will have the same horizontal separation that they start with.

Factoring in differing winds at differing heights to maintain separation during fall is negligible IMO...the main reference still needs to be ground speed so that we can estimate a good separation gap with reference to ground. It would take severe wind shear to even be a factor - in this case I thing the pilot and the DZSO would be letting you know before you figured it out!

I have around 5 years experience in aviation but I am still a rookie to the skydiving game so maybe I have it all wrong.


dpreguy  (D 835)

Apr 8, 2012, 8:00 AM
Post #8 of 87 (1772 views)
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Re: [milehigheric] Why Groundspeed? [In reply to] Can't Post

 Scratch has now explained explained that he was only discussing the "45 degree angle rule", so ,as I now see it, there is actually no argument here. See the thread a week ago by Brian Germain. Simpler and easier on following the theory.


My opinion:
This thread can be discontunued now, as it has all been covered by Brian last week.

Yes, groundspeed is the determinor, as that alone can achieve separation of groups. If unaffected by maneuvers (If we threw baseballs out instad of skydivers) the separation at opening would be the same as on exit. All winds and forces affect all the same. That's what Brian explained, and what you have just said.

In fact, my advice is to disregard everything on this thread and just read Brian's article. It's in the forums somewhere under "45 degree angle rule or something like that.

I doubt anyone is interested in this thread anymore anyway


JohnMitchell  (D 6462)

Apr 8, 2012, 8:37 AM
Post #9 of 87 (1758 views)
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Re: [milehigheric] Why Groundspeed? [In reply to] Can't Post

In reply to:
the main reference still needs to be ground speed so that we can estimate a good separation gap with reference to ground.
That is it, correctly and succinctly. And we still need to remember to put the slow fallers out ahead of the fast fallers any time the jump run is into the wind, regardless of it's velocity. Smile


popsjumper  (D 999999999)

Apr 8, 2012, 9:20 AM
Post #10 of 87 (1747 views)
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Re: [skr] Why Groundspeed? [In reply to] Can't Post

In reply to:
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 theword out very well.
Well, the word IS out and has been for years. It's mainly the old timers who are still propagating the myth.

Here are the references to the 45-doefree rule discussion that applies here:

Resurrected from long ago from John Kallend
"The 45-degree Rule Doesn't Work":
http://www.dropzone.com/...;;page=unread#unread

And, believe it or not, Brian started out as being a proponent of the 45-degree rule and just recently posted as such until the error of it was pointed out to him recently in this thread:

Well, whaddaya know...the thread was deleted by the greenies. Imagine that.
To be fair, maybe I couldn't find it on a thorough search.


BUT! the thread where Brian acknowledges that it doesn't work is still here. I'm impressed that Brain would man-up and admit the error in supporting the 45-degree rule.
http://www.dropzone.com/...45%20degree;#4294112


Premier billvon  (D 16479)
Moderator
Apr 8, 2012, 10:21 AM
Post #11 of 87 (1727 views)
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Re: [milehigheric] Why Groundspeed? [In reply to] Can't Post

>Both groups will experience the same conditions during their fall so
>despite any fluctuations in separation during fall at pull height they will
>have the same horizontal separation that they start with.

Not if the winds change as you descend.

Take this case. Exit winds 80kts on the nose, so ground speed is zero. Wind 0kts at 3000 feet. Your separation will be zero no matter how long you wait.

Now take this case. Exit winds zero, but a Santa Ana is causing nasty 80kt winds at 3000 feet, and they are coming from the tail of the plane. You will still get zero separation, even though your groundspeed is 80kts.

>Factoring in differing winds at differing heights to maintain separation
>during fall is negligible IMO...

Not at all. In fact groundspeed doesn't matter one bit; all you care about in terms of separation are winds at opening and winds at exit. The reason groundspeed is often a reasonable guide is that the winds at 3000 feet are often pretty close to the speed of the ground. (In other words, winds at 12,000 feet are almost always stronger than the winds at 3000 feet, and thus the ground is a good reference for what the winds at 3000 are doing.)


kallend  (D 23151)

Apr 8, 2012, 4:27 PM
Post #12 of 87 (1689 views)
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Re: [popsjumper] Why Groundspeed? [In reply to] Can't Post

In reply to:
In reply to:
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 theword out very well.
Well, the word IS out and has been for years. It's mainly the old timers who are still propagating the myth.

Here are the references to the 45-degree rule discussion that applies here:

Resurrected from long ago from John Kallend
"The 45-degree Rule Doesn't Work":
http://www.dropzone.com/...;;page=unread#unread

And, believe it or not, Brian started out as being a proponent of the 45-degree rule and just recently posted as such until the error of it was pointed out to him recently in this thread:

Well, whaddaya know...the thread was deleted by the greenies. Imagine that.
To be fair, maybe I couldn't find it on a thorough search.


BUT! the thread where Brian acknowledges that it doesn't work is still here. I'm impressed that Brain would man-up and admit the error in supporting the 45-degree rule.

Einstein is often (incorrectly) credited with inventing the A-bomb, while those who actually did invent it are largely unknown. There's a pattern here?


JohnMitchell  (D 6462)

Apr 8, 2012, 5:04 PM
Post #13 of 87 (1675 views)
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Re: [kallend] Why Groundspeed? [In reply to] Can't Post

In reply to:

Einstein is often (incorrectly) credited with inventing the A-bomb, while those who actually did invent it are largely unknown. There's a pattern here?
But Einstein emailed Pres. Roosevelt about it, right?Wink


sacex250

Apr 8, 2012, 5:14 PM
Post #14 of 87 (1667 views)
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Re: [JohnMitchell] Why Groundspeed? [In reply to] Can't Post

Quote:
But Einstein emailed Pres. Roosevelt about it, right?

Was he using AOL, Compuserve, or Prodigy?


popsjumper  (D 999999999)

Apr 8, 2012, 6:41 PM
Post #15 of 87 (1627 views)
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Re: [billvon] Why Groundspeed? [In reply to] Can't Post

In reply to:
Now take this case. Exit winds zero, but a Santa Ana is causing nasty 80kt winds at 3000 feet, and they are coming from the tail of the plane. You will still get zero separation, even though your groundspeed is 80kts.
So, what you are saying is the winds are affecting the first group but not the second. Hmmmmmm.....The first group exits and gets pushed up under the plane (80kts forward as per your example) and the second group exits but doesn't get pushed so they wind up on top of the first group at opening.

Not gonna happen.

Well, to be fair, there may have been a situation in the history of the world where the winds changed so drastically within the 7 seconds of exit separation that the two groups opened in the same airspace. It may have happened, maybe. Well, yes, maybe.....nah.

Again, draw it out on paper and show us what you mean.


(This post was edited by popsjumper on Apr 8, 2012, 6:50 PM)


theonlyski  (D License)

Apr 8, 2012, 6:46 PM
Post #16 of 87 (1622 views)
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Re: [sacex250] Why Groundspeed? [In reply to] Can't Post

In reply to:
Quote:
But Einstein emailed Pres. Roosevelt about it, right?

Was he using AOL, Compuserve, or Prodigy?

ARPAnet.


DaVinciflies

Apr 8, 2012, 6:49 PM
Post #17 of 87 (1615 views)
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Re: [billvon] Why Groundspeed? [In reply to] Can't Post

In reply to:
Take this case. Exit winds 80kts on the nose, so ground speed is zero. Wind 0kts at 3000 feet. Your separation will be zero no matter how long you wait.

So these 80kt winds are not causing any freefall drift. Is that what you are saying?


popsjumper  (D 999999999)

Apr 8, 2012, 6:55 PM
Post #18 of 87 (1609 views)
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Re: [billvon] Why Groundspeed? [In reply to] Can't Post

In reply to:
Take this case. Exit winds 80kts on the nose, so ground speed is zero. Wind 0kts at 3000 feet. Your separation will be zero no matter how long you wait.
Those of us who have jumped the AN-2 are intimately familiar with that. Well, except for the 80 kts bit.
LaughLaughLaugh

Exit separation:
When they guy before you opens up and flies out from under the plane, count to ten and then go.
LaughLaughLaugh


wildcard451  (D License)

Apr 8, 2012, 8:42 PM
Post #19 of 87 (1593 views)
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Re: [DaVinciflies] Why Groundspeed? [In reply to] Can't Post

In reply to:
In reply to:
Take this case. Exit winds 80kts on the nose, so ground speed is zero. Wind 0kts at 3000 feet. Your separation will be zero no matter how long you wait.

So these 80kt winds are not causing any freefall drift. Is that what you are saying?

Sure they are. 80kts worth. So you drift 1.5miles in FF.
Next group only gives you 15 seconds. Since the plane hasn't moved over the ground, that groups drifts the same 1.5 miles you did, and opens right on top of you.


DaVinciflies

Apr 8, 2012, 8:52 PM
Post #20 of 87 (1589 views)
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Re: [wildcard451] Why Groundspeed? [In reply to] Can't Post

Thanks. Got it.


kallend  (D 23151)

Apr 9, 2012, 7:17 AM
Post #21 of 87 (1555 views)
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Re: [milehigheric] Why Groundspeed? [In reply to] Can't Post

In reply to:
Factoring in differing winds at differing heights to maintain separation during fall is negligible IMO...the main reference still needs to be ground speed so that we can estimate a good separation gap with reference to ground.

That would certainly be the case if you intend to deploy at ground level. Speaking for myself, I usually deploy some 2,000 - 3,000 ft above that.


(This post was edited by kallend on Apr 9, 2012, 7:18 AM)


DanG  (D 22351)

Apr 9, 2012, 11:37 AM
Post #22 of 87 (1512 views)
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Re: [kallend] Why Groundspeed? [In reply to] Can't Post

In reply to:
Speaking for myself, I usually deploy some 2,000 - 3,000 ft above that.

Pussy.


milehigheric  (C License)

Apr 9, 2012, 1:17 PM
Post #23 of 87 (1491 views)
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Re: [kallend] Why Groundspeed? [In reply to] Can't Post

And your jumping in conditions that at 3K would effect separation? If you leave a good 1,000ft of horizontal separation at exit it would take some serious winds at deployment to blow you down jumprun into someone else...even then you should not be bearing down jump run after deployment anyway. Both groups experience the same wind on the way down... unless you get massive wind shift in that 5-10 (or more depending) second exit gap separation will be maintained.

I might have a chat to my DZSO about this topic. I was always under the impression that wind was negligible during freefall and the main factors for separation is the type of jump and the exit gap (with reference to the ground).


Premier billvon  (D 16479)
Moderator
Apr 9, 2012, 2:02 PM
Post #24 of 87 (1481 views)
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Re: [popsjumper] Why Groundspeed? [In reply to] Can't Post

>So, what you are saying is the winds are affecting the first group but not the second.

Nope. I am saying that the two cases are identical with respect to how far the two groups open from each other. They don't look the same to someone on the ground, but to someone at 3000 feet (which is where it matters) the two cases look the same.

Now here's a scenario that will really bake your noodle.

80kts on the nose at exit altitude, 80kts all the way down. Plane has zero groundspeed. Everyone exits 10 seconds apart. They all follow exactly the same path, at least according to an observer on the ground. But they all get plenty of exit separation - and, from the perspective of someone actually on the jump, looks no different than a jump with zero airspeed.

Until they try to land, of course.


skr  (D 981)

Apr 9, 2012, 2:19 PM
Post #25 of 87 (1478 views)
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Re: [milehigheric] Why Groundspeed? [In reply to] Can't Post

> 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


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