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Re: [Martini] Opening High for Bad Spots

 

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VectorBoy  (F 321)

Feb 20, 2004, 10:40 AM
Post #176 of 246 (707 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

  I finally figured out what was going on after I stopped thinking about the ground. The ground is irrelevant!
If the ground is irrelevant you could never have a bad spot = you can land anywhere with no problems. Some DZs have acres of landing outs. Others have swamps, windmill farms, angry livestock, angry livestock owners, shooting ranges, mental health and penal correctional facilities to contend with, oh and obstacles.
The fact of the matter is that the upper winds, the winds along your descent and the winds under canopy all matter, as does the ground speed of the aircraft ( GPS data for calculating jump run ) and whether you are on a floaty 20-way RW formation or a weight loaded solo speed dive.
On a particularly insanely windy day years ago those crazy enough to jump we were sternly briefed to allow a full 20 seconds between groups regardless of group size. NO exceptions! we had a JM for this reason alone ( which we don't typically do ).On jump run the aircraft was flying much faster than a standard lighter wind day those that chose to float were told. The spot looked to be about 4 or more miles out on a jump run that resembled flying a radial up wind and away from the DZ in a completely foreign direction.
Dam if all this didn't provide for a perfect spot right over the landing area. Those that opened a little higher than planned, for lack of complete faith in the plan figured out for us, actually had a little more horizontal separation than the rest. The ones that opened lower as planned had a good spot but it was tighter.
Twenty seconds is a long time when the aircraft is miles from and getting farther from the DZ and you are watching scattered clouds ZIP by you.
I know better today and would never jump in conditions like that but it was hugely educational.

The fact is the optimal group sizing and sequencing has been figured out by the DZs that have vastly different disciplines and group sizes regularly. The bigger ones. Its not a small core group that hold to these beliefs. Its what happens to work the best.
Its what we teach here if you are hosed on the spot, pull higher. If we are running multiple aircraft its probably on an offset jump run of up to a quarter mile. Even if its not you have a few minutes of separation between jumpcraft under a canopy that will descend you a few thousand feet in those minutes.
Pulling higher will not work as well if other jumpers are following the 45 degree concept or the come out right on top of you concept, to be seen in your teams video after the camera man deploys, or a mixture of concepts that don't work together like a the WFFC which is really conceptless and dangerous.


eames  (D 23844)

Feb 20, 2004, 10:42 AM
Post #177 of 246 (704 views)
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Re: [VectorBoy] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
If the ground is irrelevant you could never have a bad spot

Nobody is talking about spotting here... we're talking about separation.

Of course the ground is relevant for spotting: You're trying to land on a specific "spot" on the ground.

-Jason

Edit: Spelling


(This post was edited by eames on Feb 20, 2004, 10:44 AM)


bmcd308  (D 27472)

Feb 20, 2004, 11:03 AM
Post #178 of 246 (696 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>But my origin is moving with respect to yours. And my origin is where I am and close to where the potential collision hazard is. Your origin is 3000ft below and moving. I don't care a hoot what is going on down there. <<

It does not matter if the collision happens at my coordinates 10,-10,3000 or yours 0,0,0.

>>What if a layer of light industrial haze obscured the ground? Whast relevance does your origin have now?<<

That someone above the haze cannot see the origin does not mean that the person's position in space cannot be described relative to that origin.

A collision occurs when two people (Skydiver1 and Skydiver2) occupy the same space at the same time. Whether that space is defined relative to the Eiffel Tower, the center of the peas, or the distance from Skydiver1 does not matter.

As a matter of convenience, it might be simpler to define the origin with respect to skydiver1 or skydiver2. But it's really just arithmetic to translate between the systems.

Brent


eames  (D 23844)

Feb 20, 2004, 11:08 AM
Post #179 of 246 (692 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

New wording:

Yes, there's a relationship that depends on the characteristics of the airmass (velocity of the airmass relative to the ground and the velocity of the airmass relative to itself, and the change in velocity of the airmass between altitude and opening). But to describe those characteristics in terms of the airmass itself requires simple subtraction. To describe those characteristics in terms of the ground requires the addition of a variable, that variable being the velocity and direction of the airmass. So separation must be described in terms of the airmass. Is that correct?

Edit: Of course separation must be described in terms of the airmass; that's where the jumpers are! The apparent separation on the ground is irrelevant.

-Jason


(This post was edited by eames on Feb 20, 2004, 11:21 AM)


eames  (D 23844)

Feb 20, 2004, 11:14 AM
Post #180 of 246 (690 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
That someone above the haze cannot see the origin does not mean that the person's position in space cannot be described relative to that origin.

It can be described, but that doesn't matter. It still has to be described in terms of the airmass. The separation of the jumpers in the air, relative to the air (which is all that matters), will not change with the groundspeed.

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 11:23 AM
Post #181 of 246 (687 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>However, I dispute his thesis that you can't rely on "canopy drift" for separation, but have to ensure separation relative to some fixed point on the ground.<<

This is the definitional problem in the argument. There are those who are willing to rely on it, and those who are not.

It is not disputable that a canopy will move with the airmass it is in. What is disputable is whether we want to rely on that for separation.

I am a Mullins brainwashee, so I dismiss the notion that it is "safe" to open my container five seconds after you did above the same point on the ground as you did with a roll of my eyes.

I have never disagreed with the idea that IF we assume that there is some wind at 3,000 feet that wind will have a tendency to force the previous group's canopies downwind. However, if that wind at 3000 feet stops blowing on the way to altitude, our reliance on the wind at 3000 feet to provide separation will have been ill-advised.

Since everyone is creating unrealistic examples to prove [substitute "demonstrate"] their point, here is mine. Wind at 14K 80 knots. Jump run speed 80 KTAS into the wind.

ADD this: On the way up, there was no wind from the ground to 2000 feet. The winds gradually increased as we climbed. However, the lower winds suddenly die on jump run.

Wind from 13,999 feet to the ground 0 kts. Five seconds between groups. You are skydiver1. I am skydiver2. I plan on pulling at 800 feet, but you hear that on the way out the door and don't really process it until you are at 13,998 feet. Where do you pull?



>> If I place the origin at the point most important to ME, then I am not drifting. The ground is drifting, and I'm not worried (within a few seconds of opening) about colliding with anything on the ground. <<

ATC is not generally worried about aircraft at 30,000 feet colliding with objects on the ground. However, the location of the aircraft on the radar screen is depicted as a point over the ground whose location is defined relative to the radar antenna. If two aircraft simultaneously occupy the same point in space relative to the radar antenna, they have collided.



>>The only thing I can possibly collide with is something at the same altitude I am. <<

This is even true of objects on the ground (z=0).


I am not sure where you are going with some of this.


(This post was edited by bmcd308 on Feb 20, 2004, 11:55 AM)


eames  (D 23844)

Feb 20, 2004, 11:32 AM
Post #182 of 246 (682 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
What is disputable is whether we want to rely on [canopy drift] for separation.

Canopy drift is only one part of separation. We also rely on the plane to carry the next group to a different point relative to the airmass. This has nothing to do with a fixed point on the ground no matter how much you want it to.

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 11:48 AM
Post #183 of 246 (673 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>Can you see that we don't open our parachutes at ground level? And can you see that when you open your parachute your vertical descent rate will decrease significantly? And can you see that you will then travel horizontally away from your opening point? <<

I interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.

There is a mathematical relationship between "the airmass", which I will assume you define as a specific molecule of air, and a fixed point on the ground. That molecule of air's position relative to a fixed point on the ground can be described at any time. A collision occurs when two people attempt to occupy the same point in space at the same time.

Whether that point is defined relative to a point on the ground (they collided 3,000 feet above the peas), a point in the air (they were about a half mile behind the plane and about 11,000 feet below it when they collided) or a point in the airmass (the cloud of blood in the sky shows the spot in the moving airmass where they collided) is irrelevant to the outcome.

Brent


velo90

Feb 20, 2004, 11:51 AM
Post #184 of 246 (670 views)
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Re: [VectorBoy] Opening High for Bad Spots [In reply to] Can't Post

I think eames has pointed it out already. You are correct. The ground is very relevant when you consider where you want to land.


Quote:
On a particularly insanely windy day years ago those crazy enough to jump we were sternly briefed to allow a full 20 seconds between groups regardless of group size. NO exceptions!

Excellent! It confirms what I am saying. I am only discussing separation between opening canopies.
Again, we are not talking about spotting Smile


eames  (D 23844)

Feb 20, 2004, 12:00 PM
Post #185 of 246 (664 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Whether that point is defined relative to a point on the ground (they collided 3,000 feet above the peas), a point in the air (they were about a half mile behind the plane and about 11,000 feet below it when they collided) or a point in the airmass (the cloud of blood in the sky shows the spot in the moving airmass where they collided) is irrelevant to the outcome.

Look at a point in the sky relative to the ground. Now look at that same point in the sky five seconds later. Assuming there's wind, the same air does not occupy that space.

You seem to understand this concept... now what is it about a plane traveling through an airmass that you don't understand?

-Jason


(This post was edited by eames on Feb 20, 2004, 12:04 PM)


velo90

Feb 20, 2004, 12:02 PM
Post #186 of 246 (663 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Quote:
I interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.
Correct, but generally this is not included in the calculations for separtion. The result is we get even more separation than we bargained for in most cases.
But unless you understand why that is, you will not understand why in some cases you end up with less separation. Such conditions occur where I jump.

It's not about using the drift to determine separation, it's about knowing it can reduce separation in certain circumstances.


eames  (D 23844)

Feb 20, 2004, 12:04 PM
Post #187 of 246 (661 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
A collision occurs when two people attempt to occupy the same point in space at the same time.

Thanks, that's very profound.

In reply to:
I interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.

It does, and so does the plane carrying the next ground to a different point in the airmass.

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 12:06 PM
Post #188 of 246 (658 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>Look at a point in the sky relative to the ground. Now look at that same point in the sky five seconds later. Assuming there's wind, the same air does not occupy that space<<

Correct.

And how can I look at the same point in the sky without defining where that point is? I can't. So I define it relative to a point on the ground.


>>You seem to understand this concept<<

I do.


>> now what is it about a plane traveling through an airmass that don't you understand? <<


There are a number of things about aircraft that I do not understand, but none of those are relevant to this discussion.

Brent


eames  (D 23844)

Feb 20, 2004, 12:15 PM
Post #189 of 246 (652 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>>You seem to understand this concept<<

I do.

Apparently you don't.

Let's forget about drift under canopy for a second. As long as the plane has a positive airspeed, the groups leave at least more than zero seconds of separation in the door, and the uppers and lowers aren't going in opposite directions, there will be no collision. Because the groups will never occupy the same space at the same time RELATIVE TO THE AIRMASS!

-Jason


(This post was edited by eames on Feb 20, 2004, 12:16 PM)


bmcd308  (D 27472)

Feb 20, 2004, 12:22 PM
Post #190 of 246 (650 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

>>Correct, but generally this is not included in the calculations for separtion. The result is we get even more separation than we bargained for in most cases.
But unless you understand why that is, you will not understand why in some cases you end up with less separation. Such conditions occur where I jump. <<

This is exactly my point.

Eames is saying that it is OK for two people to open at the exact same point over the ground at staggered times because by the time the second group gets to that point, the airmass in which the first group is flying will have moved (i.e. the first group will be over a different point on the ground).

My point is that I do not want to have to rely on the airmass in which the first group deployed to have moved away from the point over the ground where the second group is deploying to achieve separation. Because if it does not, the two airmasses will be one and the same.

Canopy drift is fine for additional separation that is not counted in the time between groups. And as you point out, when jump run is opposite the opening altitude winds, the opportunity for skydivers from different groups to occupy the same space (no matter how it is defined) is increased.

Brent

Brent


bmcd308  (D 27472)

Feb 20, 2004, 12:31 PM
Post #191 of 246 (646 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>As long as the plane has a positive airspeed, the groups leave at least more than zero seconds of separation in the door, and the uppers and lowers aren't going in opposite directions, there will be no collision. Because the groups will never occupy the same space at the same time RELATIVE TO THE AIRMASS! <<

That is a bold statement.

I have never been on a plane with a negative airspeed on jump run, and I have often been told to leave five, ten, or even twenty seconds between groups, even when the winds were always out of the same direction all the way up.

Are you absolutely sure that if my 4-way team and yours left 0.5 seconds (that is greater than zero, right?) apart, there would be no chance of a collision at opening time? That seems a difficult assertion for me to swallow.

As a coach, I get out one second ahead of Cat G students, and they typically catch me no problem. Some of their docks even resemble freefall collisions.

Brent

EDITED to ADD:

So let's go through this again:

A collision cannot occur as long as:

1. the plane has positive airspeed - this one is pretty much always satisfied if you are jumping from a plane

2. >0 separation at the door - even a speed star exit leaves >0 separation at the door, so this one is pretty much always satisfied

3. uppers and lowers not in opposite directions - happens sometimes, but not that often

It sounds to me like you believe a collision is pretty much impossible. I disagree.


(This post was edited by bmcd308 on Feb 20, 2004, 12:41 PM)


eames  (D 23844)

Feb 20, 2004, 12:32 PM
Post #192 of 246 (645 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Eames is saying that it is OK for two people to open at the exact same point over the ground at staggered times because by the time the second group gets to that point, the airmass in which the first group is flying will have moved (i.e. the first group will be over a different point on the ground).

My point is that I do not want to have to rely on the airmass in which the first group deployed to have moved away from the point over the ground where the second group is deploying to achieve separation. Because if it does not, the two airmasses will be one and the same.

It's the difference in speed between the airmass and the plane (and the difference between the uppers and opening altitude). I never said we rely just on the drift under canopy, in fact I specifically said we rely on the drift under canopy and the airspeed of the plane.

In reply to:
Canopy drift is fine for additional separation....

There is a hole in your argument larger than the scope of your view. The scenario never changes with groundspeed. Actual separation only changes with the disparity between the velocity of the upper and lower winds and time left between groups.

-Jason


(This post was edited by eames on Feb 20, 2004, 12:37 PM)


Premier billvon  (D 16479)
Moderator
Feb 20, 2004, 12:34 PM
Post #193 of 246 (642 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

>Your model assumes falling straight down the column of air.

Sort of. It uses that as a baseline, then adds additional space in all directions for sliding around, tracking, and flying your canopy after opening.


>Mine (the only way someone could get above you given any real
>separation between groups) is considering sliding of either group.

Either group can slide in any direction with equal probability. The worst case reduces your separation, so that's the case I always consider.


eames  (D 23844)

Feb 20, 2004, 12:36 PM
Post #194 of 246 (641 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Are you absolutely sure that if my 4-way team and yours left 0.5 seconds (that is greater than zero, right?) apart, there would be no chance of a collision at opening time? That seems a difficult assertion for me to swallow.

As long as you waited long enough that the airspeed of the plane carried you the width of both groups through the airmass (and the groups didn't move relative to the airmass except vertically, and you accounted for the disparity in the velocity of the upper and lower winds), then YES!

If you thought about it long enough you'd understand why.

-Jason

By the way, I don't recommend trying it... I hope you found that obvious.


Premier billvon  (D 16479)
Moderator
Feb 20, 2004, 12:37 PM
Post #195 of 246 (640 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>Are you absolutely sure that if my 4-way team and yours left 0.5
> seconds (that is greater than zero, right?) apart, there would be no
> chance of a collision at opening time? That seems a difficult
> assertion for me to swallow.

If you both fell straight down the pipe, didn't slide around, didn't track, and opened unmodified rounds at the end of the jump - that would be correct. To allow for things like tracking, speedy canopies, backslides etc you have to leave more time.


bmcd308  (D 27472)

Feb 20, 2004, 12:45 PM
Post #196 of 246 (632 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

We are not worried about whether identical spaceballs pitched out of the plane will collide before they get to the ground. Everyone agrees that even if they land in the same hole, they will arrive there at different times.

That does not mean that there is adequate separation between groups.


eames  (D 23844)

Feb 20, 2004, 12:48 PM
Post #197 of 246 (629 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
That does not mean that there is adequate separation between groups.

My argument is that separation between groups does not depend on groundspeed.

Actual separation (in terms of the airmass, which is all that matters) depends on three things: Airspeed of the plane on jumprun, the difference between the velocity of the winds at jumprun altitude and opening altitude, and the time left between groups.

Nowhere does groundspeed factor into separation.

-Jason


(This post was edited by eames on Feb 20, 2004, 12:49 PM)


Premier billvon  (D 16479)
Moderator
Feb 20, 2004, 12:52 PM
Post #198 of 246 (627 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>That does not mean that there is adequate separation between groups.

That's because real skydivers do things like slide around, track, and fly their parachutes away from the center after opening. If none of those things happened you would need far less separation; just about any timing would work.

Since those things _do_ happen you have to allow for them. The figure of merit is distance at opening altitude. For me, the minimum for your typical 4-way is about 1000 feet. That means working backwards and figuring out how much time to leave between groups.


eames  (D 23844)

Feb 20, 2004, 12:52 PM
Post #199 of 246 (626 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Can you get away with using groundspeed as a reference? Yes, as was stated earlier. If that's all you're arguing then, fine, you're right.

But if you want to understand the physics of separation, you'll have to let go of the idea that the ground plays into the equation.

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 1:13 PM
Post #200 of 246 (616 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

I do understand the physics of separation, and I understand that until my feet touch it, the ground has nothing to do with anything.

However, if I were to follow your separation advice and leave half a second behind you, I might go whistling by as you were saddling out and then deploy right under you. I bet the discourse on the ground would not be as civil as the one we are having here in that case, and you'd be telling me that next time I damn well better wait until we were farther apart.

If all you're arguing is that identical spaceballs dropped from the plane under almost any circumstances will not collide on the way down, then fine, you're right, too.


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