Forums: Skydiving: Safety and Training:
The "45 degree rule" for exit separation DOES NOT WORK

 

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Divalent  (C 40494)

Aug 27, 2012, 7:17 PM
Post #351 of 364 (738 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Which, in a nutshell, is why it doesn't work.

0ft/sec x 7 sec = 0 ft: all the watermelons land on each other.

I must be missing something here, because he clearly was advocating using ground speed to determine exit separation (the "7 sec" was his example), which is the right way to do it, yes?

[by that I mean, of course, you have to rearrange the equation to be:
X = Z / Y
Where X is exit interval time in secs, Z is desired separation distance in ft, and Y is the ground speed of the plane in ft/sec]


Premier billvon  (D 16479)
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Aug 27, 2012, 9:44 PM
Post #352 of 364 (723 views)
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Re: [Divalent] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

>I must be missing something here, because he clearly was advocating using ground
>speed to determine exit separation (the "7 sec" was his example), which is the right
>way to do it, yes?

Yes, sorry, should have said "which in a nutshell is why the 45 degree thing doesn't work."

>Where X is exit interval time in secs, Z is desired separation distance in ft, and Y
>is the ground speed of the plane in ft/sec

I think the full form is distance = time between groups x (groundspeed of airplane + winds at opening altitude)

Speeds are in feet per second, time is in seconds, airplane is assumed to be flying into the wind, winds all the way down are assumed to be roughly the same direction.


NWPoul  (D 178119)

Aug 27, 2012, 10:02 PM
Post #353 of 364 (716 views)
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Re: [phoenixlpr] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
You don't seem to have the concept of separation.
How much separation do you get if you drop jumpers with 7 seconds of delay, the plane flies with 80kts into the wind and the wind is 10, 40, 70 kts?
Horizontal Separation at the moment of opening? It should be the same for all the cases


FastRon

Aug 27, 2012, 10:08 PM
Post #354 of 364 (714 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Bill- I'm pretty sure that's what I meant. Airspeed plus or minus the vector of winds aloft parallel to that airspeed= the ground speed along jump run? If that is zero then everything ends up on top of everything else? If it's not zero then isn't there SOME built in separation by allowing whatever time between groups is appropriate? (I only suggested 7 seconds becasue someone else cited it.)
I am sort of aware we usually don't try to open at ground level...
I personally prefered about 2500 feet or so. :-)


stayhigh  (F 111)

Aug 27, 2012, 10:19 PM
Post #355 of 364 (711 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

45 degree does not work.

but I tell my students. if there is no information available, and pilot just tells you fuck off, when you ask him for the upper wind. only then apply 45 degree rule because it is better than none.


pchapman  (D 1014)

Aug 27, 2012, 10:51 PM
Post #356 of 364 (709 views)
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Re: [stayhigh] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
only then apply 45 degree rule because it is better than none.

But I thought that video tests*, and analysis, show that jumpers may or may not pass through the 45 degree line, or hover around it, depending on aircraft speed.

In typical circumstances, a jumper will go out the door and be at a relatively steep angle (relative to a line to the horizon), get to a shallower angle as they get decelerated horizontally but haven't picked up much vertical speed, and later in the fall get to a steeper angle again when they're dropping vertically faster than the typical jump aircraft is travelling horizontally.

The angle has to do with the jumper's body position to some degree (low or high drag), and largely to do with the aircraft's speed. But unless there is wind shear in the altitudes not far below the plane where the jumpers are being watched, the angle has nothing at all to do with upper vs. lower winds. Small amounts of wind shear will have only a small effect on angle.

Aircraft speed will affect it, but then one can use easier to measure time rather than trying to discern 45 degrees vs 50 degrees or the like.

So instead of the 45 degree rule, you'd be better served by just using a 7-10 second rule -- OK in a lot of cases but not in some.

--------------
*Von Novak's classic video test results once again:
http://www.dropzone.com/...nt/Detailed/628.html


JohnMitchell  (D 6462)

Aug 28, 2012, 8:15 AM
Post #357 of 364 (656 views)
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Re: [pchapman] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Aircraft speed will affect it, but then one can use easier to measure time rather than trying to discern 45 degrees vs 50 degrees or the like.

So instead of the 45 degree rule, you'd be better served by just using a 7-10 second rule -- OK in a lot of cases but not in some.


--------------
*Von Novak's classic video test results once again:
http://www.dropzone.com/...nt/Detailed/628.html
I've got a pretty good eye for angles, but a lot of people couldn't tell 45 degrees if it bit them in the ass. Besides, it's meaningless. Use time to separate the groups.


Premier billvon  (D 16479)
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Aug 28, 2012, 8:22 AM
Post #358 of 364 (653 views)
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Re: [stayhigh] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

>only then apply 45 degree rule because it is better than none.

I agree. But that's because if you used it you would never get out. (Which is inconvenient but does ensure adequate separation.) Most groups never cross the 45 degree line.


piisfish

Aug 28, 2012, 8:45 AM
Post #359 of 364 (643 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
> But that's because if you used it you would never get out. (Which is inconvenient but does ensure adequate separation.) Most groups never cross the 45 degree line.
just ask the pilot for MORE speed Laugh


kallend  (D 23151)

Aug 28, 2012, 9:09 AM
Post #360 of 364 (633 views)
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Re: [stayhigh] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
45 degree does not work.

but I tell my students. if there is no information available, and pilot just tells you fuck off, when you ask him for the upper wind. only then apply 45 degree rule because it is better than none.

There are an infinite number of incorrect ways to ensure separation. Why choose that one?


popsjumper  (D 999999999)

Aug 28, 2012, 10:43 AM
Post #361 of 364 (621 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
> If we dropped say 4 same sized/weight watermelons 7 seconds apart, shouldn't they
>impact in a rough line parallel jump run about aircraft ground speed (in ft./sec.) times
>7 seconds apart?

They will impact about 7 seconds apart.

Now take the case where the wind is 70kts at altitude and the plane is flying straight into it. Your groundspeed is zero. Winds on the ground are light and variable. Now you do the experiment again. How far apart will the watermelons be when they hit the ground?

LaughLaughLaugh
You should have used an AN-2 in nearly any wind conditions.
LaughLaughLaugh

Our separation was....when the guy before you opens and flies out from under you, then you can go.
ShockedAngelic

Or, AS Pissfish said....tell the pilot to crank it up to 120 knots and then calculate.
LaughLaughLaugh


(This post was edited by popsjumper on Aug 28, 2012, 10:48 AM)


Southern_Man  (C License)

Aug 28, 2012, 10:45 AM
Post #362 of 364 (617 views)
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Re: [kallend] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

If the pilot, for some strange reason, won't give you ground speed or headwind, stick your head outside the airplane and jump when you've moved .2 over the ground. It is not hard.


popsjumper  (D 999999999)

Aug 28, 2012, 10:47 AM
Post #363 of 364 (617 views)
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Re: [pchapman] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
In reply to:
only then apply 45 degree rule because it is better than none.

But I thought that video tests*, and analysis, show that jumpers may or may not pass through the 45 degree line, or hover around it, depending on aircraft speed.
Which is why telling the group behind you to use the 45-degree rule gives you PLENTY of separation.
LaughLaugh



(just joking 45-degree rule proponents...just joking.)


DBCOOPER  (D 24112)

Aug 28, 2012, 2:29 PM
Post #364 of 364 (556 views)
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Re: [popsjumper] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

The solution is to change the name to the 45 degree myth...


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