The "45 degree rule" for exit separation DOES NOT WORK

[NWPoul 1]

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>So there should be reducing of time bw groups when Groundspeed is high
Agreed there. Higher groundspeed = less time between groups, lower groundspeed = more time between groups.

UPS, my bad - I wanted to type Should not

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>(minimum interval should be taken according to AIRspeed)

I'd disagree there. If the airplane is flying into the wind, and the lower winds are less strong than the uppers (which is usually the case) then it is groundspeed that will give you separation, not airspeed.

I sad "minimum interval" I meant that at any upper wind you should only add some interval to the minimum, which taken from the airspeed (=initial interval on exit)
Yes the wind shear can cause the redusing of resulting distance (as well as proper exit order and postdeployment behavour can increase distance in case of wind shear) and simple calculation based on ground speed can be more useful that other complicated (based on true physics of affetct of wind on separation) method:)

Why drink and drive, if you can smoke and fly?

[Remster 24]

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>So there should be reducing of time bw groups when Groundspeed is high
Agreed there. Higher groundspeed = less time between groups, lower groundspeed = more time between groups.
UPS, my bad - I wanted to type Should not

Really. You need to work out the physics of the issue before you make yourself look even more foolish. What you are advocating is the exact opposite of reality.

Remster

[NWPoul 1]

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Really. You need to work out the physics of the issue before you make yourself look even more foolish. What you are advocating is the exact opposite of reality.

Really:)? Could you be so kind to explain the reality?

Why drink and drive, if you can smoke and fly?

[billvon 2,403]

>I sad "minimum interval" I meant that at any upper wind you should only
>add some interval to the minimum . . .

Agreed. You should start with some minimum (7 seconds is a good exit to exit time) and increase it from there.

.which taken from the airspeed (=initial interval on exit)

Airspeed doesn't matter. On an Otter, airpseed is always the same - but required exit separation changes dramatically based up upper and lower winds.

>and simple calculation based on ground speed can be more useful that
>other complicated . . .

Agreed. Windspeed is an excellent way to figure it out 99.9% of the time.

[popsjumper 2]

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Really. You need to work out the physics of the issue before you make yourself look even more foolish. What you are advocating is the exact opposite of reality.

Really:)? Could you be so kind to explain the reality?

Yes. Really. Please re-read many of the replies already written to you. It's been explained in a few different ways and approaches. Funny how so many are saying the same thing to you.

Really, though. All those comments are only intended to help, not hinder you. Please don't take them as bashing.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[d123 1]

Can somebody make this thread a sticky please?

Lock, Dock and Two Smoking Barrelrolls!

[NWPoul 1]

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Yes. Really. Please re-read many of the replies already written to you. It's been explained in a few different ways and approaches.

Well..."What you are advocating is the exact opposite of reality"3
what exactly of my points are exact opposite of reality?

Am I desagree, that it wise to uncrease the inteval when wind is strong (ground speed is low)? No
Am I desagree that taking ground speed as more simple value for estimating exit interval not (despite of my first post in this thread when I say that actually nor ground speed gives you a separation nor it's directly affected it, and this is true)? No

I am advocating exactly for better anderstanding of "the physics of the issue" even if it in most cases does'nt lead to any changes in actaual action:)

Once Again my points:

1. You get initial separation only from airspeed and interval, no mater what ground speed is (how strong the wind is)
2. Wind as itself does not affect the separation on deployment altitude either - only wind shear does (don't count the intentional flying to DZ into jumprun right after deployment here)
3. Ground speed don't shown you waht wind shear is (so it dont show you reduction of distance you probably get on Deployment) it's only indicate that there probably is wind shear and yes usually the stronger upper wind (low Ground speed)- the more wind shear and it's effect you get
4 So the base minimum interval should be taken based of an airspeed of plane at jump run (disagree here with Billvon) and this base minimum value can be written on the dor cos it's not depend on wind AND THAN if you have some upper wind you can ADD some sec for it
5 Yes? I Agree that another - more simple method - yes just take the ground speed as value to calculate the interval cose in 0 wind condition ground speed will be = airspeed so the
minimum interval will be the same)

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Really, though. All those comments are only intended to help, not hinder you. Please don't take them as bashing.

Always ready to learn something:)

Why drink and drive, if you can smoke and fly?

[kallend 1,633]

The physics of this were explained years ago. There really isn't anything to discuss.

...

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

[pchapman 261]

This may help:

You say, "it's all about airspeed", strongly questioning the the accepted method (as billvon wrote in this thread) of "groundspeed plus wind at opening".

That gets some people all aflutter, "OMG where's the airspeed? Surely it matters whether you are jumping a balloon or 727?"


Follow this through to see that billvon's statement still takes into account airspeed.

Let's call the billvon's calculated value the B value:

B = Groundspeed + WindAtOpening

We know
Groundspeed = Airspeed - UpperWinds

Substituting the two equations,
B = Airspeed - UpperWinds + WindAtOpening

which is the same as
B = Airspeed - (UpperWinds - WindsAtOpening)

When the uppers and lowers are the same, it is just Airspeed!
Jumpers are all in the same airmass, so it is just Airspeed that gets them separated horizontally.
Only with all winds all being the same, is Airspeed the sole answer.

When the uppers are stronger, one decreases the value of the equation by the difference between upper and lower winds. So the speed value B is lower, and one needs more seconds between jumps to give whatever one's desired separation distance is.

Voila!


To see that this works, think of the classic example of the jump aircraft in a gale up high, doing 80 kts but zero groundspeed, yet calm air down low. B= 80 - (80 -0) = 0

This gives the correct answer that one would need to wait infinitely long for horizontal separation because once under canopy, everyone stays in the same place. (In reality of course parachutes descend so eventually one will get vertical separation.)


Here's another way to wrap one's head around using "Groundspeed + WindAtOpening":*

Ground speed is what moves the airplane across the landscape away from where each jumped exited. Then the wind at opening is what moves the jumper under canopy across the landscape away from where he just opened. The combination of the two gives separation.**

Anyone unclear on the math, think through that one to understand it all!



Footnotes:
* This is for upwind jump runs. Use vector addition as necessary if this is not the case.
** All this is for "all jumpers equal", before taking into account belly vs freefly etc, where the distance blown by the wind in freefall will differ due to time of exposure etc.

[popsjumper 2]

So...where do I carry the 1?
And can I use short division instead of long division?
If I add tequila to the mix what happens then?
Inquiring minds want to know.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[rehmwa 2]

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The physics of this were explained years ago. There really isn't anything to discuss.

Guys -

I think NWPoul understands it, but he's using terms in a confusing fashion so it's coming out weird.

"separation" in his case he's only talking about "initial freefall separation" that's affected by airspeed of course, but we don't care about that. we care about "opening time separation" - which we also tend to just call "separation"

we care about the difference in uppers vs lowers (and their relative directions) - he's calling that 'shear' (another abuse of a very specific term) and seems to be prioritizing lower than freefall separation. When, it's really the biggest effect of them all.

in the end - our thumbrules are pretty decent for uppers and lowers in the same direction with uppers higher than lowers and an assumption that lowers are typically landable type speeds (less than 30 mph).

the real rule is that drift is super important and needs to be understood by using the full wind profile (rate AND directions) from top to bottom and setting TIME in separation of group exits accordingly so our openings are spaced away from each other (horizontally). I'm grateful our thumbrules are good and I can just look at 'ground speed' at jump run to figure out exit delays. I hate integration.

Our pilot always calls out ground speed when we enter jumprun - good man. Though I still look and gauge it myself.

as taught by our dear professor and mocked up in various simulations for our enjoyment

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[billvon 2,403]

>You say, "it's all about airspeed", strongly questioning the the accepted method . . .

I think he understands it now. His posts have changed subtly since the first one he made in this thread, and in his last one the math is correct.

[NWPoul 1]

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[rehmwa]
Guys -
I think NWPoul understands it, but he's using terms in a confusing fashion so it's coming out weird.

Yep, my english is far from perfect, sorry:)

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[rehmwa]"separation" in his case he's only talking about "initial freefall separation" that's affected by airspeed of course, but we don't care about that. we care about "opening time separation" - which we also tend to just call "separation"

I meant both of them, of course we are interesting of separation at deployment time

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we care about the difference in uppers vs lowers (and their relative directions) - he's calling that 'shear' (another abuse of a very specific term)

Yep

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[rehmwa]and seems to be prioritizing lower than freefall separation. When, it's really the biggest effect of them all

Sorry, don't understood what here saying

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[billvon]
>You say, "it's all about airspeed", strongly questioning the the accepted method . . .

I think he understands it now. His posts have changed subtly since the first one he made in this thread, and in his last one the math is correct.

Actually I did always understand it:) I meant the effect of windshear (sorry if this term was confusing, my english is not so good) in my first post. I wanted to distinguish the groung speed/upper wind from wind shear as factor reducing the separation (at dep time) which was initially got from airspeed.
But I do agree, that I should have been more carefull with my statement:
"Why it is wise to increase the interval between exits in case of High upper wind (low ground speed)?
Because you can!"
As there almost always is some windshear when upper wind is strong and therefore some reducing of separation

Why drink and drive, if you can smoke and fly?

[rehmwa 2]

Hey - I'm sure you've had it from the beginning.

Hell, the title of the thread is "exit separation", rather than "exit delays to enable good separation between groups at opening altitude" which is more accurate to the main concern.

But -

The "45 degree rule" for exit delays to enable good separation between groups at opening altitude DOES NOT WORK

is a bit of a long title....

My english isn't that great either.

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[fly67 0]

Great explanation. I like equations (was going to post something similar). Simple vector algebra.

If we ignore the canopy drift (assume WindAtOpening=0), the separation at opening is soley determined by ground speed.

B = Groundspeed = Airspeed - UpperWinds

So the actual separation distance D at time of opening is
D = B * DTime (between exists)

Thus:

 

Groundspeed        DTime (sec) => D (ft)

-------------	----------------------------				

knots  ft/sec	4	5	6	7

80	135	540	675	810	945

90	152	608	760	911	1063

100	169	675	844	1013	1181

Now if we add WindAtOpening:
D = (Airspeed + WindAtOpening) * DTime

I think this assumes that each group will be under canopy for about DTime and will be blown with the wind towards the other group, so this adds an extra safety margin of WindAtOpening*DTime distance, e.g 30 ft/s * 5 = 150 ft.

Am I getting this right?

This doesn't seem to take into account variations in opening altitude (say 3000 vs 2000). If so, wouldn't a higher opening group be under canopy longer, perhaps 5-6 secs and be closer to the next lower opening group?

Just trying to understand some of the other variables.

[pchapman 261]

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This doesn't seem to take into account variations in opening altitude (say 3000 vs 2000). If so, wouldn't a higher opening group be under canopy longer, perhaps 5-6 secs and be closer to the next lower opening group?

Just trying to understand some of the other variables.

Openings at different altitudes or premature openings shouldn't actually complicate the spotting issue much, even if it complicates my afternoon in spending time thinking through the scenarios.

I haven't thought it all through but:

Usually it won't be a problem. Someone opening their canopy will be driven away from the drop path of the next jumper, whether opening high or low.

See the first drawing (with 3 example).

When viewing the drop from the side, there will be a sharper angle of drift when under canopy than in freefall. So someone in the first drop who opens early, will always drift away further from the path of the second drop.

(Eg, a freefaller at 120 mph in massive 80 mph upper winds will be about 33 degrees from the vertical. If he opens, he'll be drifting downwind at an angle close to the horizontal in those winds.)

The wind effects will already have been taken care of, as long as the winds are close to the uppers vs. lowers as entered into the calculations.

Even if the lower winds extend higher than expected, both the first jumper with a premature opening, and the second to jump, still in freefall, will still be in the same lessened winds together so there won't be any loss of separation.

About the only problem I see is if the actual winds don't correspond to what was plugged into the equation. For example, if ground speed is near zero, then one gets opening separation only from the lower winds. If they suddenly die, you'd drop people ontop of each other. When in the aircraft, one can tell what one's ground speed is (to some degree), but one can't tell what the winds are actually doing down at 2000'-3000'.

For the formula to work you still have to get the winds right - garbage in, garbage out. If you are counting on the winds at opening altitude to provide a lot of the separation, be sure that those winds really are there!


===========================

As for the issue of different freefall time of exposure, i.e. belly vs freefly, that's largely a separate issue.

But can that lead to loss of separation despite "doing things right" in terms of accepted exit order?

Let's look at one possibly scary scenario, with freeflyers following flatflyers, beating the flatflyers to the time of opening, and then drifting downwind towards the flatflyers. It turns out not bad at all, despite initial fears.

The scenario: Flat flyers exit (time = 0), then freeflyers, as normal, say at t=7 seconds. The two drop paths diverge, increasing separation, as normal for a slower falling group followed by a faster falling group. I don't think it matters much what the winds are in this scenario. (Obviously the exit timing of 7 s was picked for some particular typical winds scenario.)

Although the freeflyers exit 7 seconds after the flatflyers, they race head down to opening altitude in 40 vs 60 seconds, "getting ahead of" the flat flyers by getting lower then them at the same time. The freeflyers open, at second #47.

They have 13 seconds to drift down wind until the flat flyers open, at t=60. Isn't that cutting down on horizontal separation?

In normal circumstances it isn't actually that bad:
Even at t=60, the freeflyers will only have been exposed to the wind for 53 seconds, so they won't have drifted back the same amount as the flat flyers have in their 60 seconds. So they still have more horizontal separation than between exit points.

Also, as soon as the flat flyers open, they also drift downwind quickly.

There is one way in which this scenario can get dangerous: if those under open canopy don't behave themselves and fly down jump run immediately, putting themselves closer to the flatflyers about to open.

But that's part of a common hazard in many jump run situations: If someone opens before someone else (for whatever reason), he'll create an increased hazard if flying in the direction of where someone is still in freefall. So the hazard isn't unique to flatflyers followed by fast freeflyers.


So I've shown some ways in which the "groundspeed plus wind at opening" method still works despite trying to foul it up in various ways, plus mentioned a couple caveats. Anyone got a better insight into what sort of situations might make it fail?

[billvon 2,403]

>D = (Airspeed + WindAtOpening) * DTime

Very close. D = (UpperSpeed+LowerSpeed)*DTime

UpperSpeed = speed at exit with respect to the ground (i.e. aircraft groundspeed)
LowerSpeed = speed at opening with respect to the ground of the 'average' skydiver (i.e. winds at opening altitude)

[NWPoul 1]

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D = (UpperSpeed+LowerSpeed)*DTime

I would propose:

D ~ (Airspeed-WindDelta)*DTime

where WindDelta = upper wind speed - lower wind speed (wind shear:)

Why drink and drive, if you can smoke and fly?

[billvon 2,403]

>>D = (AircraftSpeed+LowerSpeed)*DTime

>D= (Airspeed - (upper wind speed - lower wind speed)) * time

Same thing. However, I find that aircraft groundspeed is exceptionally easy to figure out (it's right there on the GPS) as opposed to airspeed, which you need to correct for altitude. And then you can neglect lower windspeed 99% of the time. So the equation devolves to distance = GPS speed * time between groups.

[NWPoul 1]

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>>D = (AircraftSpeed+LowerSpeed)*DTime
>D= (Airspeed - (upper wind speed - lower wind speed)) * time
Same thing

Of course but I like to use an airspeed and delta n wind rather than ground speed as we are calculating separation between jumpers in air not on earth so there is no earth antil we land:)

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, I find that aircraft groundspeed is exceptionally easy to figure out (it's right there on the GPS) as opposed to airspeed, which you need to correct for altitude. And then you can neglect lower windspeed 99% of the time. So the equation devolves to distance = GPS speed * time between groups.

Yep, Agree! But it's when we are start estimate the DTime (not calculate the distanse) it's wise to use the simlyfied method that work - just GSpeed on GPS if we have one))

Why drink and drive, if you can smoke and fly?

[fly67 0]

So if D = Groundspeed * DTime as a good "1st-order equation", then we have a reasonable estimate of D from group centers.

If a group tracks away from center and we know the average (or better max) tracking distance for 4-ways, 8-ways etc., then we can reverse engineer the added time.

Thus if 4-ways track off 300 feet at most and the plane has ground speed 90 knots = 150 ft/s, then:
- We should add 2 secs for one 4-way followed by a solo or 2-way (assuming 2-way tracks perpendicular to jump run)
- We should add 4 secs for one 4-way following another.

Is my logic correct here assuming we need separation of the "edges" of the group at pull time?

Also, what is a good/safe number to assume for tracking distance from group centers (ignoring big ways)?

[kallend 1,633]

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So if D = Groundspeed * DTime as a good "1st-order equation", then we have a reasonable estimate of D from group centers.

If a group tracks away from center and we know the average (or better max) tracking distance for 4-ways, 8-ways etc., then we can reverse engineer the added time.

Thus if 4-ways track off 300 feet at most and the plane has ground speed 90 knots = 150 ft/s, then:
- We should add 2 secs for one 4-way followed by a solo or 2-way (assuming 2-way tracks perpendicular to jump run)
- We should add 4 secs for one 4-way following another.

Is my logic correct here assuming we need separation of the "edges" of the group at pull time?

Also, what is a good/safe number to assume for tracking distance from group centers (ignoring big ways)?

At the risk of repeating myself:
www.iit.edu/~ugcol/separation.zip

...

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

[popsjumper 2]

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At the risk of repeating myself:
www.iit.edu/~ugcol/separation.zip

I think they should put that into the SIM.
But then, being the cynical fuck that I am, how many actually read the SIM these days.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[billvon 2,403]

>Thus if 4-ways track off 300 feet at most and the plane has ground speed
> 90 knots = 150 ft/s,

Here are my assumptions for that:

Otter jump run 80kts
Average uppers 15kts

Resultant speed 65kts
Adjust for altitude 75kts
Convert 130fps

That means 7 seconds between groups gives you 900 feet, which I think is a reasonable distance.

>Also, what is a good/safe number to assume for tracking distance from
>group centers (ignoring big ways)?

A good 4 or 8 way tracker can cover 200-300 feet on breakoff. So 900 feet would put the two closest trackers 300 feet from each other when they open, which is a reasonable distance to see and avoid someone.

[fly67 0]

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At the risk of repeating myself:
www.iit.edu/~ugcol/separation.zip

Thanks for repeating yourself. I hadn't seen this.