eames 0
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bmcd308 0
>>But my origin is moving with respect to yours. And my origin is where I am and close to where the potential collision hazard is. Your origin is 3000ft below and moving. I don't care a hoot what is going on down there. <<
It does not matter if the collision happens at my coordinates 10,-10,3000 or yours 0,0,0.
>>What if a layer of light industrial haze obscured the ground? Whast relevance does your origin have now?<<
That someone above the haze cannot see the origin does not mean that the person's position in space cannot be described relative to that origin.
A collision occurs when two people (Skydiver1 and Skydiver2) occupy the same space at the same time. Whether that space is defined relative to the Eiffel Tower, the center of the peas, or the distance from Skydiver1 does not matter.
As a matter of convenience, it might be simpler to define the origin with respect to skydiver1 or skydiver2. But it's really just arithmetic to translate between the systems.
Brent
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It does not matter if the collision happens at my coordinates 10,-10,3000 or yours 0,0,0.
>>What if a layer of light industrial haze obscured the ground? Whast relevance does your origin have now?<<
That someone above the haze cannot see the origin does not mean that the person's position in space cannot be described relative to that origin.
A collision occurs when two people (Skydiver1 and Skydiver2) occupy the same space at the same time. Whether that space is defined relative to the Eiffel Tower, the center of the peas, or the distance from Skydiver1 does not matter.
As a matter of convenience, it might be simpler to define the origin with respect to skydiver1 or skydiver2. But it's really just arithmetic to translate between the systems.
Brent
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eames 0
New wording:
Yes, there's a relationship that depends on the characteristics of the airmass (velocity of the airmass relative to the ground and the velocity of the airmass relative to itself, and the change in velocity of the airmass between altitude and opening). But to describe those characteristics in terms of the airmass itself requires simple subtraction. To describe those characteristics in terms of the ground requires the addition of a variable, that variable being the velocity and direction of the airmass. So separation must be described in terms of the airmass. Is that correct?
Edit: Of course separation must be described in terms of the airmass; that's where the jumpers are! The apparent separation on the ground is irrelevant.
-Jason
Yes, there's a relationship that depends on the characteristics of the airmass (velocity of the airmass relative to the ground and the velocity of the airmass relative to itself, and the change in velocity of the airmass between altitude and opening). But to describe those characteristics in terms of the airmass itself requires simple subtraction. To describe those characteristics in terms of the ground requires the addition of a variable, that variable being the velocity and direction of the airmass. So separation must be described in terms of the airmass. Is that correct?
Edit: Of course separation must be described in terms of the airmass; that's where the jumpers are! The apparent separation on the ground is irrelevant.
-Jason
eames 0
QuoteThat someone above the haze cannot see the origin does not mean that the person's position in space cannot be described relative to that origin.
It can be described, but that doesn't matter. It still has to be described in terms of the airmass. The separation of the jumpers in the air, relative to the air (which is all that matters), will not change with the groundspeed.
-Jason
bmcd308 0
>>However, I dispute his thesis that you can't rely on "canopy drift" for separation, but have to ensure separation relative to some fixed point on the ground.<<
This is the definitional problem in the argument. There are those who are willing to rely on it, and those who are not.
It is not disputable that a canopy will move with the airmass it is in. What is disputable is whether we want to rely on that for separation.
I am a Mullins brainwashee, so I dismiss the notion that it is "safe" to open my container five seconds after you did above the same point on the ground as you did with a roll of my eyes.
I have never disagreed with the idea that IF we assume that there is some wind at 3,000 feet that wind will have a tendency to force the previous group's canopies downwind. However, if that wind at 3000 feet stops blowing on the way to altitude, our reliance on the wind at 3000 feet to provide separation will have been ill-advised.
Since everyone is creating unrealistic examples to prove [substitute "demonstrate"] their point, here is mine. Wind at 14K 80 knots. Jump run speed 80 KTAS into the wind.
ADD this: On the way up, there was no wind from the ground to 2000 feet. The winds gradually increased as we climbed. However, the lower winds suddenly die on jump run.
Wind from 13,999 feet to the ground 0 kts. Five seconds between groups. You are skydiver1. I am skydiver2. I plan on pulling at 800 feet, but you hear that on the way out the door and don't really process it until you are at 13,998 feet. Where do you pull?
>> If I place the origin at the point most important to ME, then I am not drifting. The ground is drifting, and I'm not worried (within a few seconds of opening) about colliding with anything on the ground. <<
ATC is not generally worried about aircraft at 30,000 feet colliding with objects on the ground. However, the location of the aircraft on the radar screen is depicted as a point over the ground whose location is defined relative to the radar antenna. If two aircraft simultaneously occupy the same point in space relative to the radar antenna, they have collided.
>>The only thing I can possibly collide with is something at the same altitude I am. <<
This is even true of objects on the ground (z=0).
I am not sure where you are going with some of this.
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This is the definitional problem in the argument. There are those who are willing to rely on it, and those who are not.
It is not disputable that a canopy will move with the airmass it is in. What is disputable is whether we want to rely on that for separation.
I am a Mullins brainwashee, so I dismiss the notion that it is "safe" to open my container five seconds after you did above the same point on the ground as you did with a roll of my eyes.
I have never disagreed with the idea that IF we assume that there is some wind at 3,000 feet that wind will have a tendency to force the previous group's canopies downwind. However, if that wind at 3000 feet stops blowing on the way to altitude, our reliance on the wind at 3000 feet to provide separation will have been ill-advised.
Since everyone is creating unrealistic examples to prove [substitute "demonstrate"] their point, here is mine. Wind at 14K 80 knots. Jump run speed 80 KTAS into the wind.
ADD this: On the way up, there was no wind from the ground to 2000 feet. The winds gradually increased as we climbed. However, the lower winds suddenly die on jump run.
Wind from 13,999 feet to the ground 0 kts. Five seconds between groups. You are skydiver1. I am skydiver2. I plan on pulling at 800 feet, but you hear that on the way out the door and don't really process it until you are at 13,998 feet. Where do you pull?
>> If I place the origin at the point most important to ME, then I am not drifting. The ground is drifting, and I'm not worried (within a few seconds of opening) about colliding with anything on the ground. <<
ATC is not generally worried about aircraft at 30,000 feet colliding with objects on the ground. However, the location of the aircraft on the radar screen is depicted as a point over the ground whose location is defined relative to the radar antenna. If two aircraft simultaneously occupy the same point in space relative to the radar antenna, they have collided.
>>The only thing I can possibly collide with is something at the same altitude I am. <<
This is even true of objects on the ground (z=0).
I am not sure where you are going with some of this.
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eames 0
QuoteWhat is disputable is whether we want to rely on [canopy drift] for separation.
Canopy drift is only one part of separation. We also rely on the plane to carry the next group to a different point relative to the airmass. This has nothing to do with a fixed point on the ground no matter how much you want it to.
-Jason
bmcd308 0
>>Can you see that we don't open our parachutes at ground level? And can you see that when you open your parachute your vertical descent rate will decrease significantly? And can you see that you will then travel horizontally away from your opening point? <<
I interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.
There is a mathematical relationship between "the airmass", which I will assume you define as a specific molecule of air, and a fixed point on the ground. That molecule of air's position relative to a fixed point on the ground can be described at any time. A collision occurs when two people attempt to occupy the same point in space at the same time.
Whether that point is defined relative to a point on the ground (they collided 3,000 feet above the peas), a point in the air (they were about a half mile behind the plane and about 11,000 feet below it when they collided) or a point in the airmass (the cloud of blood in the sky shows the spot in the moving airmass where they collided) is irrelevant to the outcome.
Brent
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I interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.
There is a mathematical relationship between "the airmass", which I will assume you define as a specific molecule of air, and a fixed point on the ground. That molecule of air's position relative to a fixed point on the ground can be described at any time. A collision occurs when two people attempt to occupy the same point in space at the same time.
Whether that point is defined relative to a point on the ground (they collided 3,000 feet above the peas), a point in the air (they were about a half mile behind the plane and about 11,000 feet below it when they collided) or a point in the airmass (the cloud of blood in the sky shows the spot in the moving airmass where they collided) is irrelevant to the outcome.
Brent
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velo90 0
I think eames has pointed it out already. You are correct. The ground is very relevant when you consider where you want to land.
Excellent! It confirms what I am saying. I am only discussing separation between opening canopies.
Again, we are not talking about spotting
QuoteOn a particularly insanely windy day years ago those crazy enough to jump we were sternly briefed to allow a full 20 seconds between groups regardless of group size. NO exceptions!
Excellent! It confirms what I am saying. I am only discussing separation between opening canopies.
Again, we are not talking about spotting
eames 0
QuoteWhether that point is defined relative to a point on the ground (they collided 3,000 feet above the peas), a point in the air (they were about a half mile behind the plane and about 11,000 feet below it when they collided) or a point in the airmass (the cloud of blood in the sky shows the spot in the moving airmass where they collided) is irrelevant to the outcome.
Look at a point in the sky relative to the ground. Now look at that same point in the sky five seconds later. Assuming there's wind, the same air does not occupy that space.
You seem to understand this concept... now what is it about a plane traveling through an airmass that you don't understand?
-Jason
velo90 0
QuoteI interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.
Correct, but generally this is not included in the calculations for separtion. The result is we get even more separation than we bargained for in most cases.
But unless you understand why that is, you will not understand why in some cases you end up with less separation. Such conditions occur where I jump.
It's not about using the drift to determine separation, it's about knowing it can reduce separation in certain circumstances.
eames 0
QuoteA collision occurs when two people attempt to occupy the same point in space at the same time.
Thanks, that's very profound.
QuoteI interpreted the last sentence of this to mean that you believed that horizontal travel away from the opening point under canopy created separation.
It does, and so does the plane carrying the next ground to a different point in the airmass.
-Jason
bmcd308 0
>>Look at a point in the sky relative to the ground. Now look at that same point in the sky five seconds later. Assuming there's wind, the same air does not occupy that space<<
Correct.
And how can I look at the same point in the sky without defining where that point is? I can't. So I define it relative to a point on the ground.
>>You seem to understand this concept<<
I do.
>> now what is it about a plane traveling through an airmass that don't you understand? <<
There are a number of things about aircraft that I do not understand, but none of those are relevant to this discussion.
Brent
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Correct.
And how can I look at the same point in the sky without defining where that point is? I can't. So I define it relative to a point on the ground.
>>You seem to understand this concept<<
I do.
>> now what is it about a plane traveling through an airmass that don't you understand? <<
There are a number of things about aircraft that I do not understand, but none of those are relevant to this discussion.
Brent
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eames 0
Quote>>You seem to understand this concept<<
I do.
Apparently you don't.
Let's forget about drift under canopy for a second. As long as the plane has a positive airspeed, the groups leave at least more than zero seconds of separation in the door, and the uppers and lowers aren't going in opposite directions, there will be no collision. Because the groups will never occupy the same space at the same time RELATIVE TO THE AIRMASS!
-Jason
bmcd308 0
>>Correct, but generally this is not included in the calculations for separtion. The result is we get even more separation than we bargained for in most cases.
But unless you understand why that is, you will not understand why in some cases you end up with less separation. Such conditions occur where I jump. <<
This is exactly my point.
Eames is saying that it is OK for two people to open at the exact same point over the ground at staggered times because by the time the second group gets to that point, the airmass in which the first group is flying will have moved (i.e. the first group will be over a different point on the ground).
My point is that I do not want to have to rely on the airmass in which the first group deployed to have moved away from the point over the ground where the second group is deploying to achieve separation. Because if it does not, the two airmasses will be one and the same.
Canopy drift is fine for additional separation that is not counted in the time between groups. And as you point out, when jump run is opposite the opening altitude winds, the opportunity for skydivers from different groups to occupy the same space (no matter how it is defined) is increased.
Brent
Brent
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But unless you understand why that is, you will not understand why in some cases you end up with less separation. Such conditions occur where I jump. <<
This is exactly my point.
Eames is saying that it is OK for two people to open at the exact same point over the ground at staggered times because by the time the second group gets to that point, the airmass in which the first group is flying will have moved (i.e. the first group will be over a different point on the ground).
My point is that I do not want to have to rely on the airmass in which the first group deployed to have moved away from the point over the ground where the second group is deploying to achieve separation. Because if it does not, the two airmasses will be one and the same.
Canopy drift is fine for additional separation that is not counted in the time between groups. And as you point out, when jump run is opposite the opening altitude winds, the opportunity for skydivers from different groups to occupy the same space (no matter how it is defined) is increased.
Brent
Brent
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bmcd308 0
>>As long as the plane has a positive airspeed, the groups leave at least more than zero seconds of separation in the door, and the uppers and lowers aren't going in opposite directions, there will be no collision. Because the groups will never occupy the same space at the same time RELATIVE TO THE AIRMASS! <<
That is a bold statement.
I have never been on a plane with a negative airspeed on jump run, and I have often been told to leave five, ten, or even twenty seconds between groups, even when the winds were always out of the same direction all the way up.
Are you absolutely sure that if my 4-way team and yours left 0.5 seconds (that is greater than zero, right?) apart, there would be no chance of a collision at opening time? That seems a difficult assertion for me to swallow.
As a coach, I get out one second ahead of Cat G students, and they typically catch me no problem. Some of their docks even resemble freefall collisions.
Brent
EDITED to ADD:
So let's go through this again:
A collision cannot occur as long as:
1. the plane has positive airspeed - this one is pretty much always satisfied if you are jumping from a plane
2. >0 separation at the door - even a speed star exit leaves >0 separation at the door, so this one is pretty much always satisfied
3. uppers and lowers not in opposite directions - happens sometimes, but not that often
It sounds to me like you believe a collision is pretty much impossible. I disagree.
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That is a bold statement.
I have never been on a plane with a negative airspeed on jump run, and I have often been told to leave five, ten, or even twenty seconds between groups, even when the winds were always out of the same direction all the way up.
Are you absolutely sure that if my 4-way team and yours left 0.5 seconds (that is greater than zero, right?) apart, there would be no chance of a collision at opening time? That seems a difficult assertion for me to swallow.
As a coach, I get out one second ahead of Cat G students, and they typically catch me no problem. Some of their docks even resemble freefall collisions.
Brent
EDITED to ADD:
So let's go through this again:
A collision cannot occur as long as:
1. the plane has positive airspeed - this one is pretty much always satisfied if you are jumping from a plane
2. >0 separation at the door - even a speed star exit leaves >0 separation at the door, so this one is pretty much always satisfied
3. uppers and lowers not in opposite directions - happens sometimes, but not that often
It sounds to me like you believe a collision is pretty much impossible. I disagree.
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eames 0
QuoteEames is saying that it is OK for two people to open at the exact same point over the ground at staggered times because by the time the second group gets to that point, the airmass in which the first group is flying will have moved (i.e. the first group will be over a different point on the ground).
My point is that I do not want to have to rely on the airmass in which the first group deployed to have moved away from the point over the ground where the second group is deploying to achieve separation. Because if it does not, the two airmasses will be one and the same.
It's the difference in speed between the airmass and the plane (and the difference between the uppers and opening altitude). I never said we rely just on the drift under canopy, in fact I specifically said we rely on the drift under canopy and the airspeed of the plane.
QuoteCanopy drift is fine for additional separation....
There is a hole in your argument larger than the scope of your view. The scenario never changes with groundspeed. Actual separation only changes with the disparity between the velocity of the upper and lower winds and time left between groups.
-Jason
billvon 3,085
.thumb.jpg.4bb795e2eaf21b8b300039a5e1ec7f92.jpg)
>Your model assumes falling straight down the column of air.
Sort of. It uses that as a baseline, then adds additional space in all directions for sliding around, tracking, and flying your canopy after opening.
>Mine (the only way someone could get above you given any real
>separation between groups) is considering sliding of either group.
Either group can slide in any direction with equal probability. The worst case reduces your separation, so that's the case I always consider.
Sort of. It uses that as a baseline, then adds additional space in all directions for sliding around, tracking, and flying your canopy after opening.
>Mine (the only way someone could get above you given any real
>separation between groups) is considering sliding of either group.
Either group can slide in any direction with equal probability. The worst case reduces your separation, so that's the case I always consider.
eames 0
QuoteAre you absolutely sure that if my 4-way team and yours left 0.5 seconds (that is greater than zero, right?) apart, there would be no chance of a collision at opening time? That seems a difficult assertion for me to swallow.
As long as you waited long enough that the airspeed of the plane carried you the width of both groups through the airmass (and the groups didn't move relative to the airmass except vertically, and you accounted for the disparity in the velocity of the upper and lower winds), then YES!
If you thought about it long enough you'd understand why.
-Jason
By the way, I don't recommend trying it... I hope you found that obvious.
billvon 3,085
>Are you absolutely sure that if my 4-way team and yours left 0.5
> seconds (that is greater than zero, right?) apart, there would be no
> chance of a collision at opening time? That seems a difficult
> assertion for me to swallow.
If you both fell straight down the pipe, didn't slide around, didn't track, and opened unmodified rounds at the end of the jump - that would be correct. To allow for things like tracking, speedy canopies, backslides etc you have to leave more time.
> seconds (that is greater than zero, right?) apart, there would be no
> chance of a collision at opening time? That seems a difficult
> assertion for me to swallow.
If you both fell straight down the pipe, didn't slide around, didn't track, and opened unmodified rounds at the end of the jump - that would be correct. To allow for things like tracking, speedy canopies, backslides etc you have to leave more time.
bmcd308 0
We are not worried about whether identical spaceballs pitched out of the plane will collide before they get to the ground. Everyone agrees that even if they land in the same hole, they will arrive there at different times.
That does not mean that there is adequate separation between groups.
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That does not mean that there is adequate separation between groups.
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eames 0
QuoteThat does not mean that there is adequate separation between groups.
My argument is that separation between groups does not depend on groundspeed.
Actual separation (in terms of the airmass, which is all that matters) depends on three things: Airspeed of the plane on jumprun, the difference between the velocity of the winds at jumprun altitude and opening altitude, and the time left between groups.
Nowhere does groundspeed factor into separation.
-Jason
billvon 3,085
>That does not mean that there is adequate separation between groups.
That's because real skydivers do things like slide around, track, and fly their parachutes away from the center after opening. If none of those things happened you would need far less separation; just about any timing would work.
Since those things _do_ happen you have to allow for them. The figure of merit is distance at opening altitude. For me, the minimum for your typical 4-way is about 1000 feet. That means working backwards and figuring out how much time to leave between groups.
That's because real skydivers do things like slide around, track, and fly their parachutes away from the center after opening. If none of those things happened you would need far less separation; just about any timing would work.
Since those things _do_ happen you have to allow for them. The figure of merit is distance at opening altitude. For me, the minimum for your typical 4-way is about 1000 feet. That means working backwards and figuring out how much time to leave between groups.
eames 0
Can you get away with using groundspeed as a reference? Yes, as was stated earlier. If that's all you're arguing then, fine, you're right.
But if you want to understand the physics of separation, you'll have to let go of the idea that the ground plays into the equation.
-Jason
But if you want to understand the physics of separation, you'll have to let go of the idea that the ground plays into the equation.
-Jason
bmcd308 0
I do understand the physics of separation, and I understand that until my feet touch it, the ground has nothing to do with anything.
However, if I were to follow your separation advice and leave half a second behind you, I might go whistling by as you were saddling out and then deploy right under you. I bet the discourse on the ground would not be as civil as the one we are having here in that case, and you'd be telling me that next time I damn well better wait until we were farther apart.
If all you're arguing is that identical spaceballs dropped from the plane under almost any circumstances will not collide on the way down, then fine, you're right, too.
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However, if I were to follow your separation advice and leave half a second behind you, I might go whistling by as you were saddling out and then deploy right under you. I bet the discourse on the ground would not be as civil as the one we are having here in that case, and you'd be telling me that next time I damn well better wait until we were farther apart.
If all you're arguing is that identical spaceballs dropped from the plane under almost any circumstances will not collide on the way down, then fine, you're right, too.
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Nobody is talking about spotting here... we're talking about separation.
Of course the ground is relevant for spotting: You're trying to land on a specific "spot" on the ground.
-Jason
Edit: Spelling
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