ARGH! - please help, separation questions (yet again)

[Newbie 0]

everytime i think i have this sorted out, i read more stuff on here, that get's pretty indepth and heavy and i have to think about this again, which is hard as im pretty stupid when it comes to physics.

I know the ideal is to try and leave 1000ft of ground distance travel between exiting groups, but as this is not always easy to do, especially if a JM is stopping you from getting too close to the door because they think you will just dive straight out, i usually do a combination of a count while watching the previous jumper(s), and the ground - obviously increasing the length of count when uppers are stronger.

However, can someone please tell me how i should be adjusting my count for different disciplines/group sizes that go before i go please? I know that the flatter groups have more drift than the freeflyers due to being in the air longer, and at my home dz, flat flyers go first, then the freeflyers, big to large groups, to account for this (and extra climbout time needed) - what happens if im a solo/2 way freeflyer out after a 2 or 4 way flat group? Is there a way to say, "well uppers are fairly strong so i know there will be some significant drift, and they are a 4 way, so maybe i should add on x more seconds to my count" or something?

I KNOW you should look to see distance covered on the ground, i know this, but not always being able to see straight down out the plane makes this difficult, and also my inexperience means i'm still not 100% what 1000ft across the ground will look like at different points along different run ins on jump run. Am i making any sense?

Thanks

"Skydiving is a door"
Happythoughts

[darkwing 5]

Unfortunately you are going to get referred to several threads that contain conflicting information. Part of the problem is that every time this comes up, people want to get 3 years of physics classes compressed into a paragraph, and it just doesn't work. You will get some convincing information that is wrong, and some unconvincing information that is right.

I give up.

-- Jeff
My Skydiving History

[kallend 2,106]

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Unfortunately you are going to get referred to several threads that contain conflicting information. Part of the problem is that every time this comes up, people want to get 3 years of physics classes compressed into a paragraph, and it just doesn't work. You will get some convincing information that is wrong, and some unconvincing information that is right.

I give up.

Well, you can always try my interactive article on the topic, at
www.iit.edu/~kallend/skydive/

...

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

[councilman24 37]

The answer depends on the skill of the various groups. If freeflyers are good the exit into the relative wind and get thrown farther. If they're not so good they catch the relative wind, don't get throuwn as far, and then transition to high speed fall. If relative workers funnel the exit, they may get thown farther (slightly) and fall faster for awhile. I don't believe there is one answer for all plane loads, either on order or timing. I usually watch the freeflyers going out before me to see if they're exiting "head down" into the realtive wind or the RW guys to see if they catch air rightaway and adjust my timing accordingly. I've found that a lot of people fail to take into accout the differences caused by the skill of the groups.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[Newbie 0]

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.......air rightaway and adjust my timing accordingly

how do you adjust your timing? Add more time to a count, or watch the ground or what? And by how much?

"Skydiving is a door"
Happythoughts

[councilman24 37]

I'm usually watching both ground travel and the body position and group position relative to the airplane. I also try to spot the previous group in freefall but that can be difficult and you may not be in the appropriate position orientation. If the group get a good forward throw, head downers into the relative wind, then I'll see them keep up with the airplane and wait longer. If the ground speed is high I know there's not much freefall drift and lateral separation is happening fast so I can wait the standard amount. If ground speed is slow and I'm doing RW after a FF group that gets a good throw I'm going to wait longer because they will be closer and get less drift and I'll get more drift than they did. In spite of all the calculations that have been done, it becomes a judgement between ground speed, wind speed, types and order of groups, and skill of groups.

Disclaimer, I don't have to do this a lot because I jump at a small DZ. But having gone to boogies for 20 years I've heard all the theories, many of which are valid in certain cases, and still there always seems to be an exception. Looking out the door, judging your own exit and offering advice to any groups behind you who might take it is still my best advice.

And sometimes if I don't want to watch but I know I need some more time, I count slow. or start over. How much. There are some times I've waited 10 or 12 seconds. There are sometimes that the old standard 5 (about minimum) is plenty.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[Newbie 0]

thanks for that

"Skydiving is a door"
Happythoughts

[billvon 3,073]

To be very brief:

Leave more time if the plane is headed into strong winds. Leave even more time if the winds at opening altitude are from the opposite direction. Leave more time for larger groups. The key on larger groups is actually breakoff altitude - the higher the breakoff, the longer the separation you need.

In no winds on an Otter, with all 4-ways, 6 seconds is generally sufficient - that gives you about 1000 feet between exits. As the above factors come in to play, leave more time. Note that 6 seconds means exit to exit, so if you take a second to check the spot, start the climbout, get into position, and give the count, you have your 6 seconds.

How do you figure out how much time you really need? Well, you can do the math. I actually do it, but most people don't. Or you can use "DZ lore" and just ask people what amount of time they use on windy days.

[tombuch 0]

I'm the S&TA at The Ranch and covered this topic in a monthly post on our site. It's available at http://ranchskydive.com/safety/tb_article15.htm. You may need to adjust the specific times for your aircraft, or for the separation that is expected at your own drop zone, but the article should give you enough background to understand the concepts.

Tom Buchanan
Instructor (AFF, SL, IAD, Tandem)
Commercial Pilot (IAMSEL,G)
Author, JUMP! Skydiving Made Fun and Easy

Tom Buchanan
Instructor Emeritus
Comm Pilot MSEL,G
Author: JUMP! Skydiving Made Fun and Easy

[goose491 0]

Hey Dan,

Everything Bill said is gold.... One thing I could add is that I've just gotten used to the visual of the jumpers before me. That is to say, when they are at an approximate 45 degree (down and back relative to me in the plane), I start my count. It's real simple, once you've nailed that spot, it's like target-lock... that way, your separation time is ajusted for all factors.

Works like a charm for me because I can use all my fingers to help me climb out and hold the aircraft instead of counting .

lol.

Just my $0.02CND

My Karma ran over my Dogma!!!

[fred 0]

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That is to say, when they are at an approximate 45 degree (down and back relative to me in the plane), I start my count.

For a second, I thought we were going to get through a separation thread without seeing this falsehood.

The 45 degree rule doesn't/can't work.

[goose491 0]

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That is to say, when they are at an approximate 45 degree (down and back relative to me in the plane), I start my count.

For a second, I thought we were going to get through a separation thread without seeing this falsehood.

The 45 degree rule doesn't/can't work.

I..... see


hahahaa care to elaborate?

My Karma ran over my Dogma!!!

[elfanie 0]

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For a second, I thought we were going to get through a separation thread without seeing this falsehood.

The 45 degree rule doesn't/can't work.

this is what I was trained to do and all I really have done..

Can you share why you believe this doesn't work?

--------------------------------------------
Elfanie
My Skydiving Page
Fly Safe - Soft Landings

[billvon 3,073]

>>The 45 degree rule doesn't/can't work.

>care to elaborate?


The 45 degree thing:

a) often doesn't work at all. An exiting group generally never gets to 45 degrees from the aircraft's point of view. (Try it next time with a transit or something.) So it's what the next jumper _thinks_ is 45 degrees, and that varies from jumper to jumper.

b) tells you nothing about winds. If you jump in zero winds, the angle the previous group makes after 5 seconds is X degrees. If you jump in a 70 knot headwind, the angle the previous group makes after 5 seconds is exactly the same.

c) tells you to do the wrong thing with freeflyers. A head-down launch makes a more acute angle with the plane sooner (more throw) and thus would tempt you to exit sooner after freeflyers, which is a time that you need to be leaving a lot more time.

The 45 degree thing can help with separation because it makes people stick their head out the door, look for the previous group, and then scratch their head for a moment while they try to figure out what 45 degrees is. That usually gives them enough separation from the previous group, if conditions are good. So I don't get too annoyed when I hear about the 45 degree thing, because it gets people to look out the door before they jump (which is a good thing) and it delays them a bit (which is also, generally, a good thing for separation.) On windy days, though, it may not be enough. Perhaps adding a caveat that they have to check the main landing gear as well if winds are high might do the trick.

[fred 0]

The simplest explanation I can give is that, in order for there to be a 45 degree angle, the plane must have traveled as far from the jumper horizontally as the jumper has fallen vertically.

[winsor 236]

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That is to say, when they are at an approximate 45 degree (down and back relative to me in the plane), I start my count.

For a second, I thought we were going to get through a separation thread without seeing this falsehood.

The 45 degree rule doesn't/can't work.

I..... see


hahahaa care to elaborate?

Sure thing.

When you step out the door you are subject to two effects that come to mind - gravity and wind resistance. Gravity applies a constant force in the down direction, and wind resistance applies a force proportional to the square of airspeed in a direction opposite airspeed.

Right out the door airspeed is often close to terminal, so you can expect almost 1 g of acceleration directly aft from wind resistance (drag); add to that orthogonal (right angle) acceleration downward, and it turns out you follow close to a 45 degree path out of the door.

Thus, the angle of departure is a poor parameter upon which to base exit timing.

Bill's suggestion that 7 seconds to start with from an Otter is a minimum between groups is a good one - I'm prone to count off 5 seconds before climb-out.

BTW, if you're putting out a series of solos who can be trusted to stick to their column of air, you can shorten it up significantly.

While playing Army, we used to hook our reserve under the main of the guy closer to the door and run out. We could clear out a C-130 like it was a tube of toothpaste that got stepped on.

In retrospect that was none too fucking bright, but it sure was fun.


Blue skies,

Winsor

[kallend 2,106]

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> Perhaps adding a caveat that they have to check the main landing gear as well if winds are high might do the trick.

What about the "counting the windows" thing?

...

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

[billvon 3,073]

>What about the "counting the windows" thing?

I don't want to be stuck behind a group who tries to count the windows on a CASA.

[kallend 2,106]

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Quote

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That is to say, when they are at an approximate 45 degree (down and back relative to me in the plane), I start my count.

For a second, I thought we were going to get through a separation thread without seeing this falsehood.

The 45 degree rule doesn't/can't work.

I..... see


hahahaa care to elaborate?

Here's a nice picture of skydivers exiting airplanes on a big way. Care to indentify the point at which the stream of jumpers first makes 45 degrees with the vertical from the plane?

...

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

[goose491 0]

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Here's a nice picture of skydivers exiting airplanes on a big way. Care to indentify the point at which the stream of jumpers first makes 45 degrees with the vertical from the plane?

Are you kidding?

I dindn't think Seperation was the idea on a BIG WAY.

My Karma ran over my Dogma!!!

[kallend 2,106]

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Here's a nice picture of skydivers exiting airplanes on a big way. Care to indentify the point at which the stream of jumpers first makes 45 degrees with the vertical from the plane?

Are you kidding?

I dindn't think Seperation was the idea on a BIG WAY.

I suppose it didn't occur to you that the stream of jumpers falling away from the planes traces out very nicely the path taken, and hence the angle made with the vertical, at various times after exit?

And maybe you didn't notice that there's no clear and well defined point at which that angle becomes 45 degrees (or any other value) and which could be used as an indicator for anything at all, let alone a safety parameter.

And therefore you couldn't conclude that the "45 degree rule" is bogus.

The 45 degree rule has no basis in physics or mathematics, but many people disregard that and want evidence from their own eyes, so that's what this picture provides.

...

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

[goose491 0]

I was just teasing you.

I'll try and find where the 45 degrees is... but bear in mind that this pic is taken from
down and to the left of the flight path... If you had the same pic but right from the side (i.e. no extra dimension) it would be much easier.

Finding 45 degrees (down and back) from your own eyes is a LOT easier than finding 45 degrees (down and back) from a plane flying in the air... You've got to figure out the angle at which you're looking at the plane first.

My Karma ran over my Dogma!!!

[PhillyKev 0]

I think Tom Buchanan's article that he linked above explains why 45 degree doesn't work very well. I hope he doesn't mind me quoting a piece of it here.

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if we imagine an airplane flying directly into a wind so great that the plane isn't moving, it will still be possible to achieve the 45 degree standard as jumpers are initially blown back, but every jumper will be getting out over exactly the same place on the ground, they will be blown back along exactly the same path, and will open at the same place with no separation at all. The separation doesn't suddenly disappear when the airplane stops covering ground in the wind, but rather deteriorates as the wind speed increases, and the 45 degree rule can't be expected to compensate

http://ranchskydive.com/safety/tb_article15.htm

[AndyMan 7]

Here's what you should take from looking at that picture.

The line of jumpers leaving each plane is almost perfectly striaght. With a few exceptions, the angle doesn't change at all. It's virtually constant.

Now wether or not any jumpers in that pic happen to be at a 45 degree angle or not is meaningless, because they're all roughly at the same angle.

Those who have just exit, and those who exited 10 seconds ago are ALL at the same angle.

The angle doesn't change, so the angle tells us nothing about separation.

_Am

__

You put the fun in "funnel" - craichead.

[kallend 2,106]

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I was just teasing you.

I'll try and find where the 45 degrees is... but bear in mind that this pic is taken from
down and to the left of the flight path... If you had the same pic but right from the side (i.e. no extra dimension) it would be much easier.

Finding 45 degrees (down and back) from your own eyes is a LOT easier than finding 45 degrees (down and back) from a plane flying in the air... You've got to figure out the angle at which you're looking at the plane first.

That really doesn't matter. You can clearly see that at no point does the path make a change abrupt enough to provide a clear indication that the separation has gone from unsafe to safe condition.
I expect you don't carry a protractor on the plane with you.

I will add a little to Winsor's comments: At exit altitude if you leave in a belly flying attitude, your airpseed inherited from the plane will be less than your terminal velocity at the same altitude. hence the drag force on you will be less than the gravitational force (these are equal at terminal) and you will fall away at less than 45 degrees to the vertical. When you reach terminal, your vertical velocity is greater than the plane's airspeed at the same altitude, so you are still making an instantaneous angle to the vertical of less than 45 degrees, measured from the door of the plane. In between exit and terminal there will be some variation in angle, but not much (it can be computed with some difficulty - see my web site). Only when you have fallen many thousands of feet into denser air will your vertical velocity slow down enough that the angle you make starts creeping up towards 45 degrees. And as Billvon said, it generally won't ever get there.

...

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