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

[goose491 0]

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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

Funny how right after this, he goes onto "counting seconds... time equals distance"

Well, not if "flying into a wind so great that the plane isn't moving,"

Not sure that you can discredit the 45 based on this.

In the above example... the jumper is "initially blown back"... Hmmm I wonder if he means that the horizontal seperation would eventually cease to increase... 'cause it wouldn't.

If your aircrafts headwind did equal its airspeed, then your aircrafts groundspeed would be zero. However the aircraft is still using POWER to fight the headwind... a jumper gets out and has no power. Thusly, he/she is "initially blown back" by the planes power but even after clearing the hill, the jumper is in a stable position in air which is moving backwards (relative to dropship) so is still creating horizontal separation.

Edited to add: Furthermore, if "flying directly into a wind so great that the plane isn't moving", each jumper can count to 30 if they want... "but every jumper will (still) be getting out over exactly the same place on the ground, they will (still) be blown back along exactly the same path, and will (still) open at the same place with no separation at all"

at 45 degrees, the jumper before you has been 'blown back' the exact same distance that he/she has traveled downwards... at this point, it is safe to get out.

My Karma ran over my Dogma!!!

[kallend 1,623]

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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

Funny how right after this, he goes onto "counting seconds... time equals distance"

Well, not if "flying into a wind so great that the plane isn't moving,"

Not sure that you can discredit the 45 based on this.

In the above example... the jumper is "initially blown back"... Hmmm I wonder if he means that the horizontal seperation would eventually cease to increase... 'cause it wouldn't.

If your aircrafts headwind did equal its airspeed, then your aircrafts groundspeed would be zero. However the aircraft is still using POWER to fight the headwind... a jumper gets out and has no power. Thusly, he/she is "initially blown back" by the planes power but even after clearing the hill, the jumper is in a stable position in air which is moving backwards (relative to dropship) so is still creating horizontal separation.

Edited to add: Furthermore, if "flying directly into a wind so great that the plane isn't moving", each jumper can count to 30 if they want... "but every jumper will (still) 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"

at 45 degrees, the jumper before you has been 'blown back' the exact same distance that he/she has traveled downwards... at this point, it is safe to get out.

Put you mind where your typing hand is and put in some numbers.

Plane is flying at 100kt TAS in a 100kt headwind at 13,000ft.

due to wind shears the wind drops to zero at 4000ft and below

Jumper A exits, falls at 120mph in a stable belly to earth, gets "blown back" to any angle of your choosing, deploys at 3000ft and is open at 2,500ft

Jumper B waits 20 seconds and exits, falls just like jumper A except he deploys at 2800ft and is fully open at 2300ft, just below jumper A. How far from jumper A is jumper B as he passes jumper A?

...

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

[billvon 2,400]

>If your aircrafts headwind did equal its airspeed, then your aircrafts
> groundspeed would be zero. However the aircraft is still using
> POWER to fight the headwind...

An aircraft in level flight uses power to overcome drag. It knows nothing about any headwind.

>at 45 degrees, the jumper before you has been 'blown back' the
> exact same distance that he/she has traveled downwards... at this
> point, it is safe to get out.

Exit separation is not intended to put you X feet from someone at exit. It is to put you X feet from someone at opening; that's one of the several reasons that what you see at exit time means nothing about what will happen at opening time.

[elfanie 0]

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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.

My understanding was that the first jumper should be at a 45* angle from straight down...which means you look straight down, then at the jumper, and s/he should be about 45* from straight down...not 45* from airplane to jumper...

So that wouldn't have anything to do with how far s/he's falling...it would have to do with how fast the aircraft is going and how fast s/he's moving horizontally from the position of the aircraft in the sky...

Now..I've read all of the mathmatical posts about how to calculate separation...and honestly, it makes no sense to me. Are you guys saying that sticking your head out, watching them until they have moved horizontally from your position about |---| yeah much....that's unsafe?

Me? I usually wait until someone I know and respect (DZO, instructor, etc) nods and says, "Go!"...but I know that won't always be the case...

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

[goose491 0]

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>If your aircrafts headwind did equal its airspeed, then your >aircrafts groundspeed would be zero. However the aircraft is still >using POWER to fight the headwind...

An aircraft in level flight uses power to overcome drag. It knows nothing about any headwind.

Picky picky Bill, your speaking to a pilot but thanks. What you say is not incorrect, but it's something that is taught so as not to confuse whilst explaining the difference between airspeed and groundspeed.

If what you are trying to do with a plane is move horizontally over the ground, and your headwind is the same as your airspeed, you are not going anywhere. Any pilot would understand that you are battling a headwind.

I doensn't change that part of the argument. The plane has no groundspeed because Airspeed = Headwind. Once a jumper leaves the aircraft, the slipstream may act on him/her for a short while, this is where he/she is being (blown back) I suppose... but once clear of the hill, he/she is still in a body of air which is traveling backwards relative to dropship.

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>at 45 degrees, the jumper before you has been 'blown back' the
> exact same distance that he/she has traveled downwards... at this
> point, it is safe to get out.

Exit separation is not intended to put you X feet from someone at exit. It is to put you X feet from someone at opening; that's one of the several reasons that what you see at exit time means nothing about what will happen at opening time.

Agreed. In an example where the plane is not traveling along the ground at all, then you are relying on vertical separation to do that. Your jumper is going to accellerate downwards at the same rate (9.81m/s/s) as if the plane did have a groundspeed, but is not going to travel backwards (relative to dropship) nearly as fast... thusly, it will take longer for him/her to ever hit that 45.... thusly, you will indeed have waited longer to exit then if the plane was cooking along the ground.

I'm not disputing that counting will work (duh). I'm just saying that the 45 rule accomodates changes in all variables.

My Karma ran over my Dogma!!!

[markbaur 0]

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I'm just saying that the 45 rule accomodates changes in all variables.

Really?

No-wind day. Jump-run speed 90 mph, jumper terminal velocity 120 mph, jump altitude 13K, opening around 3K.

At opening, the jumpers have fallen about 2 miles in 1 minute. At the same time, the aircraft has flown forward 1-1/2 miles. The angle from airplane to jumper at opening time is thus less than 45 degrees, and will not increase to 45 degrees until the jumper's vertical speed is less than the aircraft forward speed, that is, until after deployment. Thus the 45-degree angle method says that on no-wind days, you should wait for the previous group to open before you jump.

Mark

[goose491 0]

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At opening, the jumpers have fallen about 2 miles in 1 minute. At the same time, the aircraft has flown forward 1-1/2 miles. The angle from airplane to jumper at opening time is thus less than 45 degrees, and will not increase to 45 degrees until the jumper's vertical speed is less than the aircraft forward speed, that is, until after deployment. Thus the 45-degree angle method says that on no-wind days, you should wait for the previous group to open before you jump.

haha okay maybe not ALL variables... On a no wind day, I'm not looking for the 45 degrees, on a no wind day, I'm not all that confused about how to get my separation either... because the plane is cookin'.

However, if you taught ONLY the 45 degree rule, then student who waited for the jumpers before them to deploy before exiting would certainly get the separation they needed... they catch a whole lot of flack but they'd certainly get their separation LOL.

and what Billl originally said about the 45 rule would hold true

"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."

lol.

My Karma ran over my Dogma!!!

[kallend 1,623]

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>If your aircrafts headwind did equal its airspeed, then your >aircrafts groundspeed would be zero. However the aircraft is still >using POWER to fight the headwind...

An aircraft in level flight uses power to overcome drag. It knows nothing about any headwind.

Picky picky Bill, your speaking to a pilot but thanks. What you say is not incorrect, but it's something that is taught so as not to confuse whilst explaining the difference between airspeed and groundspeed.

If what you are trying to do with a plane is move horizontally over the ground, and your headwind is the same as your airspeed, you are not going anywhere. Any pilot would understand that you are battling a headwind.

I doensn't change that part of the argument. The plane has no groundspeed because Airspeed = Headwind. Once a jumper leaves the aircraft, the slipstream may act on him/her for a short while, this is where he/she is being (blown back) I suppose... but once clear of the hill, he/she is still in a body of air which is traveling backwards relative to dropship.

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>at 45 degrees, the jumper before you has been 'blown back' the
> exact same distance that he/she has traveled downwards... at this
> point, it is safe to get out.

Exit separation is not intended to put you X feet from someone at exit. It is to put you X feet from someone at opening; that's one of the several reasons that what you see at exit time means nothing about what will happen at opening time.

Agreed. In an example where the plane is not traveling along the ground at all, then you are relying on vertical separation to do that. Your jumper is going to accellerate downwards at the same rate (9.81m/s/s) as if the plane did have a groundspeed, but is not going to travel backwards (relative to dropship) nearly as fast... thusly, it will take longer for him/her to ever hit that 45.... thusly, you will indeed have waited longer to exit then if the plane was cooking along the ground.



Since you bring this up, the acceleration downwards is actually less than 9.81m.s^-2 because of the (non-linear nature of the) drag force. Curiously, if an Otter passed a balloonat 13000ft and identical twin jumpers exited each at the moment they passed, and they both fell with exactly the same attitude to the relative wind, the balloon jumper would reach 3000ft sooner than the Otter jumper.


I'm not disputing that counting will work (duh). I'm just saying that the 45 rule accomodates changes in all variables.

It's very easy to make an "arm waving" argument like that and make it sound plausible, but it falls apart when you actually put numbers in and do the calculations precisely. I have done that. Have you? The 45 degree "rule" DOES NOT WORK and is UNSAFE.

I suggest that until you are prepared to do the real physics and present your results, that you refrain from arguing with those of us who have.

...

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

[markbaur 0]

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I'm just saying that the 45 rule accomodates changes in all variables.

Really?

90 mph winds. Jump-run speed 90 mph, jumper terminal velocity 120 mph, jump altitude 13K, opening around 3K.

At opening, the jumpers have fallen about 2 miles in 1 minute. At the same time, they've drifted backward 1-1/2 miles relative to the plane. The angle from airplane to jumper at opening time is thus less than 45 degrees, and will not increase to 45 degrees until the jumper's vertical speed is less than the aircraft forward speed, that is, until after deployment. Thus the 45-degree angle method says that on 90-mph-wind days, you should wait for the previous group to open before you jump.

Mark

[elfanie 0]

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The angle from airplane to jumper at opening time is thus less than 45 degrees,

I asked this previously in this thread, but wasn't responded to..

The 45* rule..
is that from "straight down to jumper" (ie: if you draw a line from door to earth, then from door to jumper, it's a 45* angle)...or from airplane to jumper (ie: if you draw a horizontal line from door to plane tail, then from door to jumper, it's a 45* angle)

And does it matter?
What I mean is...without getting into physics and mathmatical equations for those of us who aren't fans of physics and calculus...
if you look out the door...and wait until the jumper has moved X amount horizontally from the door (45* from straight down to jumper)...are you saying that is unsafe?

and if you are saying that is unsafe...what are you saying is a safer alternative to do? (again, for those of us who aren't into physics calculations)

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

[kallend 1,623]

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I'm just saying that the 45 rule accomodates changes in all variables.

Really?

90 mph winds. Jump-run speed 90 mph, jumper terminal velocity 120 mph, jump altitude 13K, opening around 3K.

At opening, the jumpers have fallen about 2 miles in 1 minute. At the same time, they've drifted backward 1-1/2 miles relative to the plane. The angle from airplane to jumper at opening time is thus less than 45 degrees, and will not increase to 45 degrees until the jumper's vertical speed is less than the aircraft forward speed, that is, until after deployment. Thus the 45-degree angle method says that on 90-mph-wind days, you should wait for the previous group to open before you jump.

Mark

How about 90mph airspeed and no wind at any altitude - a perfect day? The jumpers still haven't reached a 45 degree angle by the time they deploy, (it's around 37 degrees for belly, around 30 degrees for head-down) so the 45 degree rule STILL says wait until the previous group has opened.

...

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

[goose491 0]

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I asked this previously in this thread, but wasn't responded to..

The 45* rule..
is that from "straight down to jumper" (ie: if you draw a line from door to earth, then from door to jumper, it's a 45* angle)...or from airplane to jumper (ie: if you draw a horizontal line from door to plane tail, then from door to jumper, it's a 45* angle)

Those two are one and the same.... if the plane is straight and level, then 45 degrees downwards of the horizontal is the same as 45 degrees upwards of the vertical... being that the difference between horizontal and vertical is 90 degrees total.

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if you look out the door...and wait until the jumper has moved X amount horizontally from the door (45* from straight down to jumper)...are you saying that is unsafe?

Yes, they are saying that this is unsafe.

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and if you are saying that is unsafe...what are you saying is a safer alternative to do? (again, for those of us who aren't into physics calculations)

The 'alternative' is to do what most people do in the first place and that's count seconds... add seconds if jumprun is into a strong headwind... remove seconds if the headwind is light/nil.

Sorry for the no-response

My Karma ran over my Dogma!!!

[elfanie 0]

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Those two are one and the same.... if the plane is straight and level, then 45 degrees downwards of the horizontal is the same as 45 degrees upwards of the vertical... being that the difference between horizontal and vertical is 90 degrees total.

I just had a huge "duh" moment.
Yeah yeah yeah..
ok...
see..at least I admitted to not being a math fan. (I'm really not as stupid as I just tried to make myself look.)

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The 'alternative' is to do what most people do in the first place and that's count seconds... add seconds if jumprun is into a strong headwind... remove seconds if the headwind is light/nil.

Sorry for the no-response

Ok..
so how many seconds?
and how much do you add/take away for headwinds..? (and is the airplane always going to set up jumprun into the wind?)

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

[goose491 0]

John,

Here is my reply:

If we are ever on the same load, and I am exiting after you, rest assured. So long as you don't track back along jumprun, you will not find me in your airspace in freefall OR under canopy.

Cheers,
Nick

My Karma ran over my Dogma!!!

[billvon 2,400]

>Your jumper is going to accellerate downwards at the same rate
> (9.81m/s/s) as if the plane did have a groundspeed, but is not
> going to travel backwards (relative to dropship) nearly as fast...

He will go backwards at exactly the same speed relative to the plane. Neither the plane nor the person knows anything about a headwind. The plane is just flying at 80kts IAS like it always does. The jumper starts with 80kts IAS and slowly loses it, just like he always does. He does NOT slow down faster relative to the plane because there's a headwind. There is no appreciable change in wind speed in the 5 seconds between exits, so you won't see any difference in angle when someone jumps in a zero-knot heawind and a 100 knot headwind. Any angle based method simply does not work.

>I'm just saying that the 45 rule accomodates changes in all variables.

The 45 degree rule tells you absolutely nothing about how far you will be from someone else on opening. It doesn't work. It's a non-starter. The only reason it works at all is that it makes people look out the door, and that often delays them long enough to give safe separation.

[kallend 1,623]

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Now..I've read all of the mathmatical posts about how to calculate separation...and honestly, it makes no sense to me. Are you guys saying that sticking your head out, watching them until they have moved horizontally from your position about |---| yeah much....that's unsafe?

Me? I usually wait until someone I know and respect (DZO, instructor, etc) nods and says, "Go!"...but I know that won't always be the case...

OK, comments to your statements:

1) "45 degrees" provides no information about separation at opening time. It is therefore "unsafe" in the sense that it provides no likelihood of being safe.

2) Sometimes you just have to do some physics to understand a problem. Just like a pilot doesn't need to understand stability coefficients and pitching moments in order to know not to load an airplane aft of the design CG limits, so you don't have to know the physics in order not to trust the "45 degree" method.

However, I have already done the physics and math for you, and written a computer program that plots out the results. You can download it, along with a complete presentation on the subject of separation, from

Here

...

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

[kallend 1,623]

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Ok..
so how many seconds?
and how much do you add/take away for headwinds..? (and is the airplane always going to set up jumprun into the wind?)

See my previous post. I have put to gether a very comprehensive presentation on exit separation, along with an interactive program that allows you to play "what if", on my web site. It also pretty much answers your exact question.

...

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

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[bluewaterstream 0]

So, I've read all of the arguments and different theories on exit separation on this site posted by people that all have a lot more experience/jumps/knowledge than I do in this sport. I value all of your opinions and each one of them (even the opinions that I disagree with) helps me to develop a theory of my own that I feel comfortable and safe with.

I'm new in this sport (12 jumps), however I sincerely feel that I am super safe and conservative when it comes to skydiving. I would like the opinions of those who have been dominating this particular thread (in particular; kallend, billvon, goose491, & winsor), I am curious to hear if those with different exit separation theories would both feel comfortable/safe exiting a plane with someone like myself.

I only jump when winds are under 10 mph, and I use both the 45 degree theory and always count at least 5 seconds before exiting after a solo jumper. If a group jumps before me I'll wait anywhere from 6 - 8 seconds. If I make a 5 - 8 second count and the skydiver(s) that jumped before me are close to 45 degrees, well then I jump! This has never happened, but if I counted to 5, 6, 7, or 8 and the jumper(s) before me were still basically directly beneath me at zero to thirty degrees or so, I wouldn't jump and I'd continue counting until I felt it was cool to jump. Even if a bunch of yahoos behind me (that have no view of the spot) were screaming for me to jump, I ain't jumping until I say so. If someone is right underneath me I'm not jumping until at least 10 - 15 seconds. Also, I always track away perpendicular from the plane (flight path) and keep a close eye on everyone around me throughout most of the skydive. Just like while I'm on my motorcycle, I drive defensively while on the street and I fly defensively in the air. So, would ya'll feel safe having a cat like me jumping out after you? If not, tell me why so that I can consider re-evaluating my theory/interpretation on exit separation.

~ John

[billvon 2,400]

> I am curious to hear if those with different exit separation theories
> would both feel comfortable/safe exiting a plane with someone like
> myself.

As long as:

a) you give the sorts of times you list (5 seconds after a solo, 6-8 seconds for a larger group) and add a few second if the uppers are higher, and

b) you're willing to listen to the pilot/loadmaster/manifester/S+TA if he tells you to wait longer

then no problem. Checking for a 45 degree angle also causes you to look out the door, and that can help you see traffic, which is even better. It doesn't help you with separation, but it doesn't hurt, either - as long as you count that 5-8 seconds (more in strong uppers) before exiting.

[kallend 1,623]

Where did you receive the advice to look for a 45 degree angle? I'm sure that with 12 jumps you didn't come up with it on your own.

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

[Kris 0]

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Where did you receive the advice to look for a 45 degree angle? I'm sure that with 12 jumps you didn't come up with it on your own.

You'd be surprised who's giving that inaccurate factoid out. One very well known freeflyer, with the initials O.K., told a room full of people during a seminar at Skydive San Marcos that it was the best way to ensure separation.

Jessica and I looked at each other in bemused shock and had to fight the urge to call you or BillVon.

Sky, Muff Bro, Rodriguez Bro, and
Bastion of Purity and Innocence!™

[kyros1 0]

When I was jumping in a big busy DZ they told me to give an 8-10 delay from previous jump

[bluewaterstream 0]

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Where did you receive the advice to look for a 45 degree angle? I'm sure that with 12 jumps you didn't come up with it on your own.

No, of course I didn't come up with that. I first read about it here on dropzone.com and then asked a few people at my DZ what they thought about the 45 degree theory. A few were for it and a few were against it. Actually, one of the JM's at my DZ even recommended it.

[elfanie 0]

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Where did you receive the advice to look for a 45 degree angle? I'm sure that with 12 jumps you didn't come up with it on your own.

the person who told me that the 45* angle was a good idea was a jumpmaster who is a master rigger, been jumping for something like 15-20 years, is a tandem master, AFF instructor, PRO rating, almost 9000 jumps....someone I respect intensely.

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

[kallend 1,623]

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Where did you receive the advice to look for a 45 degree angle? I'm sure that with 12 jumps you didn't come up with it on your own.

the person who told me that the 45* angle was a good idea was a jumpmaster who is a master rigger, been jumping for something like 15-20 years, is a tandem master, AFF instructor, PRO rating, almost 9000 jumps....someone I respect intensely.

Just goes to show, doesn't it?

Now I hope you have enough information that you can set him straight.

...

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