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Horizontal Separation

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Have any of you actually done the calcs?

The desired goal is to have 1000' (300m) of horizontal seperation at opening altitude (~3k).

The calculations presented by LouDiamond are from the military, and I'm guessing that the SF don't do head-down HALO insertions, so this is all for flatfly. If everyone is flatflying, they will all drift the same. Therefore, the only thing that will give seperation is the plane moving forward.

The 1000' seperation is designed to incorporate buffers for tracking and canopies opening, so I am neglecting that in my analysis. The 1000' is, I look up at one jumper, run 1000', and look up at the next jumper, so it's seperation with respect to the ground, so you're interested in groundspeed.

So, from LD's excel sheet:



Ground time between
speed exits
kn s
125 5
115 5
...
35 18
25 25


Tada! Those numbers correspond to a horizontal seperation of 1000'!!!



If you want to LAND 1000 feet apart, this is fine.

It has nothing to do with how far apart you are IN THE AIR, which is the frame of reference of interest when we discuss "horizontal separation."

Once you select the wrong frame of reference for your analysis, any further effort is wasted.

As Bill pointed out, the critical frame of reference is the speed of the aircraft with regard to the airmass at opening altitude. The next most relevant frame of reference is true airspeed. The least useful datum is groundspeed, so long as we're talking about horizontal separation IN THE AIR.

What the ground is doing is, in and of itself, immaterial to the behavior of objects in the air above it.

Q: If you have a layer of fog up to 2,500 feet and there is a 20 kt breeze on the ground, can you tell which way the wind is blowing by the behavior of groups in freefall above?

A: No, the ground winds and ground speed are entirely immaterial, and have no effect whatsoever on the path of bodies THROUGH THE AIR.


Blue skies,

Winsor

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>You simply don't know enough to have any idea how little you know.

Winsor, Scott, cut it out. You both have really valuable inputs here (Winsor from the theoretical side, Scott from the practical side) and they can be presented without going after each other.



Ok, trying to step in the middle here since I do talk about groundspeed too when refrencing exit seperation. Windsor and Kallend are absolutely correct when they say that the only true seperation consideration is what the winds are doing between exit altitude and deployment altitude. It's fluid dynamics and they don't change. It's that simple. However, simple for a physics professor, scholar, PhD is not so simple for the average lay person in the real world. So there are some assumptions made that generally work and we don't even realise them usually until arguements like this come up.

Now, in the real world, how often do the winds in good jumping weather really turn 180 degrees from each other between exit and deployment? Not all that often. I know in the Chicago area we have had more days with east winds than most summers I can remember but the upper winds at exit have also been more out of the north than normal so we still haven't seen that many days with true 180 degree diferentials. Sooooo......

When I'm flying the otter and we are dumping group after group out I know that I can slow the jumprun up as the plane gets lighter. This helps the tandems since they can take longer to waddle back to the door. No sense burning up airspace just because we can.

For frame of reference and in most normal jumping conditions for sport jumpers the winds on the ground are relatively close in speed to the winds at opening altitude. I know, I know (Windsor and Kallend) that it is not a perfect world. This is an assumption that I think we have to practically accept usually. So, with frame of reference being the winds on the groung being more towards the zero mark than say the 60 knots you have at altitude you can still use the aircrafts grounspeed as a tool (but not an exact measure) for determining exit seperation. And yes, the numbers I use have a little more than you really need so that when those other conditions arise you are still likely to have the adequate seperation at deployment. See what I'm saying now?

There's the theoretical world and then there is application. Those of us on the application end need to recognize that some of what we perceive to be "truths" do in fact have some fudge factored in for those oddball days. Those on the theoretical end need to be warry of using so much technical info that they only confuse the situation and not help it.

.....afraid to hit the send button...

Chris Schindler
Chris Schindler
www.diverdriver.com
ATP/D-19012
FB #4125

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Have any of you actually done the calcs?

The desired goal is to have 1000' (300m) of horizontal seperation at opening altitude (~3k).

The calculations presented by LouDiamond are from the military, and I'm guessing that the SF don't do head-down HALO insertions, so this is all for flatfly. If everyone is flatflying, they will all drift the same. Therefore, the only thing that will give seperation is the plane moving forward.

The 1000' seperation is designed to incorporate buffers for tracking and canopies opening, so I am neglecting that in my analysis. The 1000' is, I look up at one jumper, run 1000', and look up at the next jumper, so it's seperation with respect to the ground, so you're interested in groundspeed.

So, from LD's excel sheet:



Ground time between
speed exits
kn s
125 5
115 5
...
35 18
25 25


Tada! Those numbers correspond to a horizontal seperation of 1000'!!!



I have done the calculations taking into account wind speed changes with altitude, change in air density with altitude, the equations of motion of the jumpers including drag, forward throw from the plane, updating the jumpers' position and speed every 0.005 seconds from exit.

Winsor is right, and the ground is irrelevant until you get there. It does serve as an approximate guide to what is happening at opening altitude, which is why groundspeed based methods usually work OK.
...

The only sure way to survive a canopy collision is not to have one.

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Winsor is right, and the ground is irrelevant until you get there. It does serve as an approximate guide to what is happening at opening altitude, which is why groundspeed based methods usually work OK.



I rest my case. It's adequate for what we do. Never said it was perfect. Heck there was even one day at Monterey Bay that we put 2 tandems out first, then some 8-10 way, then some solos and then freefliers ending in 2 or 3 more tandems. The opening looked like a big bow. Sometimes you have to think outside the box. We made it work for the conditions we had.
Chris Schindler
www.diverdriver.com
ATP/D-19012
FB #4125

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Ok, trying to step in the middle here since I do talk about groundspeed too when refrencing exit seperation. Windsor and Kallend are absolutely correct when they say that the only true seperation consideration is what the winds are doing between exit altitude and deployment altitude. It's fluid dynamics and they don't change. It's that simple. However, simple for a physics professor, scholar, PhD is not so simple for the average lay person in the real world. So there are some assumptions made that generally work and we don't even realise them usually until arguements like this come up.

Now, in the real world, ...

...
There's the theoretical world and then there is application. Those of us on the application end need to recognize that some of what we perceive to be "truths" do in fact have some fudge factored in for those oddball days. Those on the theoretical end need to be warry of using so much technical info that they only confuse the situation and not help it.

.....afraid to hit the send button...

Chris Schindler



Chris, this is covered in detail in my presentation on exit separation at www.iit.edu/~kallend/skydive/, including real world situations and fudge factors to use when the winds at 2500ft are funny, and lots of graphics to aid understanding.
...

The only sure way to survive a canopy collision is not to have one.

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Your understanding, such as it is, is the classic "good enough for Government work." You simply don't know enough to have any idea how little you know.



Dude, I many not be a physics teacher but I have a 4 year degree so don't try to insult my intelligence with your passive aggresive replies. All your accomplishing is making yourself look like an ass. If you want to talk, then talk like a man, don't act better than thou.



It's not an act. As Dizzy Dean said, "it ain't bragging if you can do it." If you want either my credentials or peer review of my contentions, you are welcome to discover how far out of you league you are.

A four year degree is all well and good, but you could have a Ph.D. from one school and not qualify to enter another as a Freshman. All schools are not created equal.

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The term "good enought for Gov't work" doesn't hack it in my world. In SF, if it's not done right people die so it's never "good enough" it HAS to be done right, there is no margin for error. You are confusing that term with civillian gov't workers, who use that as their motto.



Actually, I'm not. Most of the policies and procedures used by SF and other elite groups are, indeed, spot on. Others are not so rigorous, and usually these are in matters about which you can't do much.

For example, I heard one of the most ill-informed treatises on interior, exterior and terminal ballistics that I've ever endured from an SF "expert," but the bottom line is that operationally he has to bet the ranch on the performance of some variant of the 5.56x45 cartridge, so it's better to believe than to doubt.

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The Excel spreadsheet you tout illustrates at a glance the level of your ignorance.



Did you even bother to look at the power point presentations? The excel spread sheet I posted is part of a bigger equation. However,civillian skydivers don't calc winds so the only thing one can do to try and be as safe as possible is to use a method safer than the 45 degree method or the 4 sec method. If you have a better way of ensuring seperation then put your money where your mouth is and show the rest of us "ignorant" people. Up until this point all you've done is waste bandwidth.



If you read the notes to my spotting and separation seminar closely, you'll see why I shy away from any formula that says "do this and you're fine."

If you want an instant improvement in your methodology, simply replace groundspeed with airspeed in all of your calculations of horizontal separation. Groundspeed is related only by coincidence.

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Your procedure is not inherently dangerous, and is better than nothing, but the physical model you use displays a fundamental lack of comprehension - as does your defense of your methodology.



You see, thats where your wrong. Becasue I don't have to defend the methodology, because what you think is not relevant to what I do. The physical model I use works in real life as I stated. We don't have this seperation issue in military skydiving. Can you explain that?



Of course I can explain it. There is enough room for error (800 vs. 900 feet, both are "plenty" or "not enough" as the case may be) that using your rule of thumb is better than nothing.

If you were working with a system where you had to be on to even two digits of accuracy, you would be screwed.

Edited for personal attacks. Discussing exit separation is fine; discussing how pompous/stupid/inferior someone else is is not, whether or not you believe it to be true.


Blue skies,

Winsor

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This is a really long thread, and nobody mentioned the practical side of avoiding close calls.

With reasonable time between groups (the number should increase with an increase in the winds at altitude), not tracking up or down jump run,and not flying your canopy up or down jump run will go a long way toward avoiding close calls.

If the time between groups on a given day is 5 sec., and you exit, freefall, and track (perdendicular to jumprun), after opening you should be able to look down the jumprun, and if the group after you was falling at the same speed, in about 5 sec., you will see them open, and it is now safe to turn up the jumprun and head toward the DZ.

For the first group of freefliers (assuming freefliers are out after RW), you will most likely open before the group in front of you, and most likely be up wind of the DZ, and needing to fly down the jumprun to get back to the DZ, you need to hold and wait for the group in front of you to catch up to you and open, and then you can trun back down the jumprun and head home.

The prevailing idea is that all movements need to be made perpendicular to jumprun untill you have cleared that airspace with a visual check. Many people seem to strive to track toward the DZ after a jump, or turn toward the DZ with a back riser the second the slider is down. That sucks and its not safe.

I have watched many people from other groups fall past me and open, but from a safe distance, because I am holding my postion on the jumprun and I am LOOKING FOR THEM. You know they are out there, you will respect their space once under canopy, just besure to respect their space through the entire skydive.

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This is a really long thread, and nobody mentioned the practical side of avoiding close calls.



I think several people did.


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With reasonable time between groups (the number should increase with an increase in the winds at altitude), not tracking up or down jump run,and not flying your canopy up or down jump run will go a long way toward avoiding close calls.

If the time between groups on a given day is 5 sec., and you exit, freefall, and track (perdendicular to jumprun), after opening you should be able to look down the jumprun...



Not a good idea on anything bigger than a 2 way, as a matter of practicality, or you'll have lots of people tracking the same way.
...

The only sure way to survive a canopy collision is not to have one.

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Like I said, it's a long thread and I may have missed some points.

Generally the bigger formations will get some addtional time from the group behind them.

There is plenty of space for several people to track on either side of the jumprun, fanning themselves out from the center of the formation. Yes this may create some possible overlap with the outer most trackers from different groups, but it's better than the guaranteed overlap you will have with people tracking up or down the jumprun.

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There are too many pissing matches on S&T right now.



You see pissing matches, I see people passionate about their safety and the safety of others. Let the pissing continue, I say.

-
Jim
"Like" - The modern day comma
Good bye, my friends. You are missed.

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Like I said, it's a long thread and I may have missed some points.

Generally the bigger formations will get some addtional time from the group behind them.

There is plenty of space for several people to track on either side of the jumprun, fanning themselves out from the center of the formation. Yes this may create some possible overlap with the outer most trackers from different groups, but it's better than the guaranteed overlap you will have with people tracking up or down the jumprun.



I doubt this will work with, say, a 16 way that has just done 8 points and breaks off from a BFR.
...

The only sure way to survive a canopy collision is not to have one.

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>There is plenty of space for several people to track on either side of
>the jumprun, fanning themselves out from the center of the
>formation.

I tell people in my formations to turn 180 from the center and track no matter what else is going on in the dive. Telling people they have to a) identify line of flight and b) _not_ go 180 from the center, but rather 90 from the line of flight will result in more people opening closer to each other.

> but it's better than the guaranteed overlap you will have with
>people tracking up or down the jumprun.

I think you got this backwards. The most likely people you will collide with are people from your own group; if you track next to them you are guaranteed to have a close opening. The only time you will have any problems with people from the next group is if they do not give you sufficient exit separation. Solution - allow more exit separation, and do not sacrifice breakoff safety.

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...I tell people in my formations to turn 180 from the center and track no matter what else is going on in the dive. ...

The only time you will have any problems with people from the next group is if they do not give you sufficient exit separation. Solution - allow more exit separation, and do not sacrifice breakoff safety.



So its settled then? Allow enough exit separation and at the end of the dive turn 180 & track away?

The rule of thumb for "enough separation" that I was given this summer was to allow 5-6 seconds per 10mph of uppers.
AMDG

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Edited for personal attacks. Discussing exit separation is fine; discussing how pompous/stupid/inferior someone else is is not, whether or not you believe it to be true.



BB wipes her head and says.."man is it Piling high and Deep in here..."



And you're adding what, exactly, to this conversation? Or was your point only to belittle those with a PhD?

-
Jim
"Like" - The modern day comma
Good bye, my friends. You are missed.

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>So its settled then? Allow enough exit separation and at the end of
>the dive turn 180 & track away?

Well, it's never been unsettled for me. That's been the rule on every RW jump I've ever been on or organized.

>The rule of thumb for "enough separation" that I was given this
>summer was to allow 5-6 seconds per 10mph of uppers.

Would probably work for an Otter, but I find the following works a little better in such an airplane:

ALWAYS wait at least 5 seconds, then add 2 seconds for every 10kts of upper wind. (assuming headed into the wind, normal exit order, winds at opening are less but same direction etc.)

This works for most cases. It doesn't work when the uppers are 70kts but that's pretty rare.

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Or was your point only to belittle those with a PhD?



Bwahhhhhhh well since I also hold this distinction...I guess I am laughing at myself!!



My original question still stands, what exactly is your contribution to this conversation? I think you may have missed Talk Back, it's a few doors down.

-
Jim
"Like" - The modern day comma
Good bye, my friends. You are missed.

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My original question still stands, what exactly is your contribution to this conversation? I think you may have missed Talk Back, it's a few doors down.



Do a search on the same topic and you will find my contributions. If you can not locate them I can point you in the right direction.
Furthermore, your last two posts did nothing to contribute to the conversation either. So now we are even.
Cheers!
BB








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OK everybody stand down, you too Jimbo. As bill said, let's keep this on topic and civilized. My intent is to enlighten as many people as possible not get into a dick contest. I think the last several posts have pretty much summed it up IMO. While not theoretically "perfect" according to physics, the method I have described(ground speed excel chart and wind calc presentations) were admitedly usefull in a "imperfect" world. Until someone comes out with a "perfect" way and a "perfect" world we will have to use what works for us in this world.

End of story , case closed. I have said all that I care to say about this , in this thread.
"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

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Winsor is right, and the ground is irrelevant until you get there. It does serve as an approximate guide to what is happening at opening altitude, which is why groundspeed based methods usually work OK.



Exactly.

From you powerpoint presentation you say that groundspeed methods work for groups of the same fallrate and if the lowers are in the same direction as the uppers.

You are going to have to make assumptions in all of these calcs. What we want is a number that will work 90% of the time. If my DZ always has lowers that are 180 out from the uppers, then my "normal" is different to another DZ whose lowers are always zero. Those are two extreme cases. "Normal" DZs normally have the lowers in the same-ish direction as the uppers (ours were ~30º last Sunday). For the "adverse" conditions in the other 10% of the time, we have to make adjustments to what we do.

We just need a number, lest we don't have a physics PhD nearby (which my DZ will have in ~year). We also need to understand the limitations behind that number, and don't treat it as gospel. Its an IF/THEN/ELSE statement. IF the lowers are "normal" THEN use the number ELSE figure it out before you go up. IF fall rates are different THEN figure it out before you go out.
--
Arching is overrated - Marlies

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