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

 

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thepollster  (D 12122)

Dec 12, 2005, 6:10 PM
Post #26 of 364 (2275 views)
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Re: [DJL] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:


Also, how difficult is it to figure out wind resistance? Tell me how much you weigh and I'll tell you your very own force for drag.


You will need more information to calculate drag,such as surface area and surface friction. If you can tell me how to calculate my surface area, which changes based on body position, I'll buy you a jump ticket (valid only for DJL).


pilotdave  (D License)

Dec 12, 2005, 6:54 PM
Post #27 of 364 (2270 views)
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Re: [thepollster] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

When you're at terminal, your drag is equal to your weight. But that doesn't matter much when it comes to exit separation, since you're not at terminal...

Dave


Premier billvon  (D 16479)
Moderator
Dec 12, 2005, 7:01 PM
Post #28 of 364 (2268 views)
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Re: [thepollster] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

>You will need more information to calculate drag . . .

You really don't. Drag equals weight at terminal, period. Everything else proceeds from there - time to reach terminal, differing effects of wind on various groups etc. Heck, if you know your weight, your area (which is easy; just take a picture from below) and your terminal velocity, you can easily calculate your drag coefficient.


sundevil777  (D License)

Dec 12, 2005, 7:18 PM
Post #29 of 364 (2263 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Of course it is easy to know the final vertical component of drag, but what about the horizontal component, or the profile of the drag/speed in either direction before it reaches equilibrium.

The 45 deg rule is still crap.


Shark  (D 24499)

Dec 12, 2005, 7:39 PM
Post #30 of 364 (2256 views)
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Re: [sundevil777] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
The 45 deg rule is still crap.

....and a lot of people cannot count. Crazy

Which group is unsafe?

I've people give a 3 second delay and others more than 20.
Then again, maybe they both used the 45 degree rule.


DJL  (C License)

Dec 12, 2005, 8:20 PM
Post #31 of 364 (2246 views)
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Re: [thepollster] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
You will need more information to calculate drag,such as surface area and surface friction. If you can tell me how to calculate my surface area, which changes based on body position, I'll buy you a jump ticket (valid only for DJL).

Dammit, a few people harped on the drag=weight at terminal one before I got home and got on my computer.

Send me an accurate chalk outline of you on the ground and of you standing with your arms in tight. Your area will be somewhere between the two. Done, but it was a trick question anyway. You can start an account for me at Perris, I'll be out there sometime this year. I'll buy you a jump when I get there.


kallend  (D 23151)

Dec 12, 2005, 8:33 PM
Post #32 of 364 (2241 views)
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Re: [apoil] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
In reply to:
Not disagreeing with your statement, but all that are needed to model the situation are Newton's Laws.
Newton's laws have been well validated over the centuries when applied to situations like skydiving. Relativity and quantum mechanics are not required. I've found that a visual model to illustrate the point helps non physicists to get it.

I knew you'd understand what I'm saying.
In this case it's probably fine.
But just saying Newton's Laws is a bit general and doesn't mean you have a complete model in your program. Remember high school physics when we neglected friction and wind resistance? Well there's a lot of wind resistance going on in skydiving.

Also Newton's laws still govern highly complex dynamic systems (aka chaotic systems). As the number of variables increases the ability to predict becomes severely limited.
.

Newton showed in Volume II of Principia how to take air resistance into account in calculating a trajectory. You should read it someday, you will be impressed.

BTW my trajectory when skydiving is not chaotic, but your mileage may varyTongue


Reginald  (D 28162)

Dec 12, 2005, 9:02 PM
Post #33 of 364 (2237 views)
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Re: [Shark] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

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Quote:
The 45 deg rule is still crap.

....and a lot of people cannot count. Crazy

Shark…leave it to and Elsinore guy to hit the nail on the head.

That's the problem with timing separation is people can’t count reliably especially newer jumpers who are excited in the door. The problem is magnified the longer the count is. I don’t trust a guy with 50 jumps to count to 10 and give me an honest 10 seconds. He’s perception of 10 seconds is probably more like 5. It’s not his fault; hell, I was that guy at 50 jumps too. It’s human nature.

Someone commented that the 45 degree rule does nothing more than to get people to look out the door and try and figure out what 45 degrees looks like - and that alone takes 5 or 6 seconds. Bingo! If I want a 6 count from a new jumper going out after me, I stand a better chance of getting it by having him stare at my group and try and judge 45 degrees. That will take him a good 5 or 6 seconds. If I want an honest 12 seconds and have an experienced group going after me I stand a better chance of getting it by them counting.

Here’s my opinion, neither system is perfect and both have flaws, one has technical problems the other has human problems. Personally, I think timing is a technically superior method but you can’t just rule out the human factor…


Zenister  (A 42)

Dec 12, 2005, 10:55 PM
Post #34 of 364 (2221 views)
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Re: [Reginald] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
In reply to:
Quote:
The 45 deg rule is still crap.

....and a lot of people cannot count. Crazy

Shark…leave it to and Elsinore guy to hit the nail on the head.

That's the problem with timing separation is people can’t count reliably especially newer jumpers who are excited in the door. The problem is magnified the longer the count is. I don’t trust a guy with 50 jumps to count to 10 and give me an honest 10 seconds. He’s perception of 10 seconds is probably more like 5.

which is why there are a number of ways to 'personally enforce' a proper count, the most obvious being the one most people learned as kids.. 1 mississippi 2 mississippi, 1 one thousand, 2 one thousand etc...

i prefer the 'hand method' (move your hand at the elbow thru its full range of motion for each count)

neither are perfect ie exactly 1 second, however both make it very obvious to the counter that they when/if they are rushing the count..

even a new adrenaline soaked jumper will have a more accurate count if taught either from the start...


nathaniel

Dec 12, 2005, 11:09 PM
Post #35 of 364 (2217 views)
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Re: [Reginald] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
Someone commented that the 45 degree rule does nothing more than to get people to look out the door and try and figure out what 45 degrees looks like - and that alone takes 5 or 6 seconds. Bingo!

Gah! I didn't mean for that to be an endorsement...I'll remember to use the sarcasm tags next time.


kelpdiver  (B 7)

Dec 13, 2005, 5:14 AM
Post #36 of 364 (2207 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:
Drag equals weight at terminal, period.

Lose the "period."

Terminal isn't a constant, even if the body position is.

----
So any reason we can't have a simple device that counts down from X seconds (set on way up) and then buzzes? Each group hits the button when the prior group exits.

(Or even just a swimmer's second clock would do, though that would have people looking at it rather than out the door)


DJL  (C License)

Dec 13, 2005, 5:44 AM
Post #37 of 364 (2198 views)
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Re: [kelpdiver] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
Terminal isn't a constant, even if the body position is.

That's right...cuz when I turn on my fuckin' afterburners...


pilotdave  (D License)

Dec 13, 2005, 7:35 AM
Post #38 of 364 (2178 views)
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Re: [kelpdiver] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Terminal may not be constant, but when you are at terminal, your drag equals your weight. I'm not exactly sure what you mean by terminal not being constant... Do you mean terminal constantly slows down as you descend? Or do you mean one's fallrate is not constant due to fluctuations in the air?

Terminal velocity can be defined as the velocity that results in drag=weight. It's the equilibrium point. If you're accelerating (changing speed), you're not at terminal. You might temporarily be faster or slower than terminal.

Dave


Scrumpot  (D License)

Dec 13, 2005, 7:36 AM
Post #39 of 364 (2177 views)
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Re: [Scrumpot] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Oops, there I go, drunk posting again! Wink
Sorry for the "distraction" there folks. Angelic
Blues,
-Grant


Andy9o8  (D License)

Dec 13, 2005, 7:56 AM
Post #40 of 364 (2167 views)
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Re: [pilotdave] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Terminal velocity can be defined as the velocity that results in drag=weight.

Yes; although I find that when I tell whuffos only that, their eyes often glaze over uncomprehendingly (except for the engineers). Then they make the usual joke about "terminal" as in "illness": "yeah, skydiving's terminal, nyuk-nyuk.." When (instead of bitch-slapping them at that point) I go on to say, "It's called that because it's the velocity at which your acceleration terminates, the light goes on: "Ohhh, I get it..." Crazy

It's Reason Number 23 why I avoid discussing skydiving with whuffos...


Scrumpot  (D License)

Dec 13, 2005, 8:00 AM
Post #41 of 364 (2159 views)
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Re: [Zenister] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
i prefer the 'hand method' (move your hand at the elbow thru its full range of motion for each count)...
Yeah, and while your 20-jump wonder solo, is standing in the door doing that, behind the either 3rd or 4th RW group that has gone in front of him (maybe even taken their own sweet time in "set up"), and he now has a tandem master screaming at him "go-go-go-go-GO!" What do you think he is gonna do? Unsure


Andy9o8  (D License)

Dec 13, 2005, 8:11 AM
Post #42 of 364 (2153 views)
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Re: [Scrumpot] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
In reply to:
i prefer the 'hand method' (move your hand at the elbow thru its full range of motion for each count)...
Yeah, and while your 20-jump wonder solo, is standing in the door doing that, behind the either 3rd or 4th RW group that has gone in front of him (maybe even taken their own sweet time in "set up"), and he now has a tandem master screaming at him "go-go-go-go-GO!" What do you think he is gonna do? Unsure

From my experience seeing this, he's probably gonna GO!
Here's a thread where this was discussed not too long ago: http://www.dropzone.com/...post=1439849#1439849


Premier Remster  (C License)

Dec 13, 2005, 8:45 AM
Post #43 of 364 (2141 views)
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Re: [Andy9o8] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
Yeah, and while your 20-jump wonder solo, is standing in the door doing that, behind the either 3rd or 4th RW group that has gone in front of him (maybe even taken their own sweet time in "set up"), and he now has a tandem master screaming at him "go-go-go-go-GO!" What do you think he is gonna do?

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


From my experience seeing this, he's probably gonna GO!

Oh yes.. and that would be entirely different if he was trying to follow the 45 deg rule?????? Crazy

Using a flawed system to address a flawed situation doenst fix things. Proper education from the start is how you do it.


Premier billvon  (D 16479)
Moderator
Dec 13, 2005, 8:49 AM
Post #44 of 364 (2139 views)
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Re: [sundevil777] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

>Of course it is easy to know the final vertical component of drag . . .

Right. Drag is generally given by D = Cd * A * .5 * r * V^2, where A is area, Cd is coefficient of drag, r is density, and V is velocity. At terminal you know your speed, and at exit you know the air density, so working backwards you can get a term which is Cd*A. Now treat that as a constant, since most people don't dramatically change their surface area presented to the wind during freefall (since speeds tend to remain relatively constant.)

Now you can go back and plug in the original equation. You have Cd*A, you have r, and you have V (initial throw.) That gives you the drag force. Either integrate that or do an iterative approximation to get the decay of forward speed.


Premier billvon  (D 16479)
Moderator
Dec 13, 2005, 8:53 AM
Post #45 of 364 (2133 views)
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Re: [Shark] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

>>The 45 deg rule is still crap.

>....and a lot of people cannot count.

>Which group is unsafe?

Both. But if a skydiver can count (and most of us can!) they can get safe separation. If a skydiver is an expert at the 45 degree angle rule, he can't get safe separation.

Counting is a skill that's really not hard to master. We don't put up with it when people claim they can't thread their chest strap correctly; we shouldn't put up with it when people claim they can't count.


Andy9o8  (D License)

Dec 13, 2005, 10:25 AM
Post #46 of 364 (2111 views)
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Re: [Remster] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Quote:
Yeah, and while your 20-jump wonder solo, is standing in the door doing that, behind the either 3rd or 4th RW group that has gone in front of him (maybe even taken their own sweet time in "set up"), and he now has a tandem master screaming at him "go-go-go-go-GO!" What do you think he is gonna do?

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


From my experience seeing this, he's probably gonna GO!

In reply to:
Oh yes.. and that would be entirely different if he was trying to follow the 45 deg rule?????? Crazy

No, that would be every bit as bad; that was my point. I was just suggesting that that hapless novice might Go!! because he felt intimidated by the TM screaming at him. The linked thread condemns the screamers.

In reply to:
Using a flawed system to address a flawed situation doenst fix things. Proper education from the start is how you do it.

I agree completely. Part of that education is letting novices know that it's ok to ignore some up-jumper who can't see out the door screaming "GO!" at them, and that the DZO and S&TA will be supportive of them.


Shark  (D 24499)

Dec 13, 2005, 11:35 AM
Post #47 of 364 (2092 views)
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Re: [billvon] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quote:
Counting is a skill that's really not hard to master. We don't put up with it when people claim they can't thread their chest strap correctly; we shouldn't put up with it when people claim they can't count.

....and people wonder why the instructors are yelling at them to exit.Cool


kelpdiver  (B 7)

Dec 13, 2005, 3:20 PM
Post #48 of 364 (2047 views)
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Re: [pilotdave] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Terminal may not be constant, but when you are at terminal, your drag equals your weight. I'm not exactly sure what you mean by terminal not being constant... Do you mean terminal constantly slows down as you descend?

Precisely that. At no point does terminal qualify as a constant, though the difference due to changing air resistance is a lot less than I would have expected.


DJL  (C License)

Dec 13, 2005, 5:42 PM
Post #49 of 364 (2014 views)
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Re: [kelpdiver] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

Quick big picture question here...what are we trying to figure out?


mjosparky  (D 5476)

Dec 13, 2005, 8:51 PM
Post #50 of 364 (1983 views)
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Re: [DJL] The "45 degree rule" for exit separation DOES NOT WORK [In reply to] Can't Post

In reply to:
Quick big picture question here...what are we trying to figure out?

What we are trying to figure out has already been figured out. The 45 degree theory is so much crap. That about covers it. Tongue

Sparky


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