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Re: [Martini] Opening High for Bad Spots

 


Hooknswoop  (D License)

Feb 18, 2004, 6:42 AM
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Derek, I have to disagree with you on the separation/groundspeed thing. The aircraft and the jumpers aren't influenced by the ground or it's relative speed, the ground doesn't even exist for them (unless ya get too close). Aircraft and it's spawn, that's you and me, only know the air.

Airspeed for a particular jump ship tends to be constant for each jump run. If the jump ship always flies into the upper winds for jump run, then as the upper wind speed increases, the jump ship’s ground speed decreases and more time must be left between exits in order to achieve the same amount of separation. The pilot could increase the aircraft’s jump run speed equal to the upper winds (if the aircraft had the performance to do so) and the amount of time between exits could remain constant, but this isn’t done. Therefore, exit separation is a function of the aircraft’s ground speed on jump run. In order to ensure separation between groups the following groups must look at the ground and allow sufficient ground distance to be covered before exiting. This is why the 45-degree rule for exit separation does not work.

So, as a jump ship’s ground speed decreases, more time must be left between exits to ensure the same amount of separation. This is why I said that separation is a function of ground speed. Ground speed (for a particular jump ship) is the variable, since airspeed is constant. Ground speed is used to adjust the length of time between groups.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 6:45 AM)


velo90

Feb 18, 2004, 6:52 AM
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Hooknswoop, what you have just said roughly works for the normal case but....
Why do we keep having to redo the problem? What's even worse is there are a number people taking part in this thread that took part in all the other threads about exit seperation and they have not learned anything!
It's obvious numerous people never even bothered to look at Kallend's web site, or if they did, they never understood what was presented there.

Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.


Hooknswoop  (D License)

Feb 18, 2004, 7:14 AM
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Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.

If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground?

Quote:
Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

Quote:
That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Derek


velo90

Feb 18, 2004, 7:29 AM
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In reply to:
Quote:
Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.


If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground?
I do neither

Quote:
Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

You will not have zero seperation at opening time! I am assuming the upper winds at at ~80kts, the same speed as the aircraft

Quote:
That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Derek

As I said, it's all been done before and although the resources are out there, some people just don't seem to use them.


eames  (D 23844)

Feb 18, 2004, 7:37 AM
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I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

This was all figured out many, many years ago....

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 7:45 AM
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I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

This was all figured out many, many years ago....

So if the jump ship has zero ground speed (130 fps-130fps), then it might a well be a 13,500 tower. If someone jumps off a 13,500 foot tower and then someone else jumps off the same tower 15 second later, they will open at the same exact point, as they will drift the exact same amount. The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.

And there would be no forward throw, since the jumper has no forward momentum.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 7:46 AM)


eames  (D 23844)

Feb 18, 2004, 7:48 AM
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Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

-Jason


velo90

Feb 18, 2004, 7:50 AM
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The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.
That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.


Hooknswoop  (D License)

Feb 18, 2004, 7:51 AM
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Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

Um, no, it will drift down wind, just like the jumpers exiting from the 13,500 foot tower would. If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.

Derek


Hooknswoop  (D License)

Feb 18, 2004, 7:56 AM
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That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.

That means they will open in the exact same airspace with only the 15 seconds of canopy speed for separation. If a group of 4 exits first, then another group of 4, then the second group of four will be opening within the first group’s airspace. That is zero separation. We do not rely on a canopy’s airspeed for separation. What if the first jumper has line twists and flies into the wind and the second jumper has line twists and flies downwind? They will have a high chance of eating up that 15 seconds and having a collision.

Separation means opening points are separated not the same opening point with canopy speed used for separation. The same opening point means there is zero separation.

Derek


Premier PhreeZone  (D License)
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Feb 18, 2004, 7:58 AM
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No, but if you drop a second ball out of the plane that has 0 ground speed it will land at the same spot as the first.


velo90

Feb 18, 2004, 7:58 AM
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If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.
Thankfully we are parachutists and we don't hit the groundWink That is why ground speed is irrelevant. We need to look at what the plane/tower is doing relevant to point (air mass) where we open our canopies.


eames  (D 23844)

Feb 18, 2004, 7:59 AM
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Are you serious?

If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Distance = Velocity * Time

Distance = 130 ft/sec * 15 sec = 1950 ft

-Jason


eames  (D 23844)

Feb 18, 2004, 8:04 AM
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Good observation, but we don't hit the ground when we open our parachutes, do we? We continue moving in the airmass with a relatively slow descent. Think about it.

-Jason


velo90

Feb 18, 2004, 8:05 AM
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Try to think about the canopy speed as the speed of the canopy due to wind drift and not it's own speed.

After I got put down by Ron & Kallend I invested a lot of time reading the stuff Kallend put on the web. I believe at least for some people (me included) there are some ways in which we think that are fundamentally wrong. I finally figured out what was going on after I stopped thinking about the ground. The ground is irrelevant!


Hooknswoop  (D License)

Feb 18, 2004, 8:09 AM
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Are you serious?

Very serious.

Quote:
If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Distance = Velocity * Time

Distance = 130 ft/sec * 15 sec = 1950 ft

If jumper one opens with line twists and flies upwind (the canopy’s airspeed minus the 130 fps wind speed gives jumper one’s ground speed) at time t and jumper 2 opens in the exact same airspace (which is the definition of no separation) at t+15 with line twists and flies down wind (for a ground speed of 130 fps plus the canopy’s airspeed). They have very little separation and may have a collision since they are flying towards each other at their combined airspeeds.

Canopies should have opening points some distance apart.

If canopy speed was sufficient for separation, why track at break off? Why not just turn away and pull and let the canopy’s airspeed provide the separation.

Derek


eames  (D 23844)

Feb 18, 2004, 8:11 AM
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In reply to:
The ground is irrelevant!

Just to clarify, the ground is irrelevant only until after opening. Exit separation must be increased in higher winds due to the increased speed and reduced time it takes under canopy to get to the target, i.e. due to congestion (assuming an upwind opening point).

-Jason


velo90

Feb 18, 2004, 8:17 AM
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When I think about exit seperation on a jump run I am not normally thinking about the problems of congestion when landing the canopy. I am mainly concerned that each jumper has his own column of air to use.


eames  (D 23844)

Feb 18, 2004, 8:18 AM
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The distance between the two canopies would be the same no matter what the groundspeed is, assuming a constant airspeed. The closing time for the two canopies would be the same as well (yes, regardless of groundspeed).

I give up.

-Jason


eames  (D 23844)

Feb 18, 2004, 8:21 AM
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Right, I agree. I should have said "Just to supplement what you said...."

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 8:34 AM
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I am curious, what canopy are you jumping and opening in 88.6 mph (130 fps) winds?

20 mph (29.3 fps) gives only 440 feet of drift after opening (and excluding the foward speed of the canopy) in 15 seconds, a long space betweeen exits.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 8:34 AM)


eames  (D 23844)

Feb 18, 2004, 8:40 AM
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I'm not jumping any canopy in 88.6 mph (130 fps, 77.07 knot) winds. I used that number because it's a reasonable airspeed for a plane on jumprun. For the 15 seconds you wait on the plane, you are moving away from the person who got out before you at approximately the airspeed of the plane. Not some arbitrary groundspeed.

Separation is a function of airspeed and time between exits. Period.

-Jason


eames  (D 23844)

Feb 18, 2004, 8:42 AM
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In reply to:
20 mph (29.3 fps) gives only 440 feet of drift ....

Name any jumpship with an airspeed of only 17.36 knots (29.3 fps)....

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 9:17 AM
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Name any jumpship with an airspeed of only 17.36 knots (29.3 fps)....

Here's 2: Bell 206 BIII Jet ranger, UH-60 Blackhawk.

Let’s look at 2 more realistic scenarios:

1) 70 knots of wind at altitude. 80 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, the opening altitude (I have jumped in these winds).

2) 80 knots of wind at altitude. 90 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, again, the opening altitude.

Scenario 2 has a 10-knot increase in airspeed over scenario 1. By your reasoning, less time needs to be left between groups in scenario 2 because of the 10 knot increase in airspeed. Both scenarios have a 10 knot ground speed.

I disagree.

Derek


nicknitro71  (D 26704)

Feb 18, 2004, 9:47 AM
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True story:

The dude's me!
Attachments: jdallgood.jpg (48.0 KB)


johnny1488  (D 25453)

Feb 18, 2004, 10:07 AM
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99% of the time at my dz we use an unpwind jumprun. If I needed to, without knowing the windsaloft, I could simply look at the ground speed and tell what kind of seperation I needed to give. The ground is the only thing that matters when giving seperation. Winds aloft really has no bearing because we are all subject to the same wind after we exit the plane. throw from the plane means nothing (with the exception of presentation to the relative wind, in which the effect is minimal). Look at Kallends simulator again. You could be going 500 mph through the air. If your groundspeed is zero, you will have zero seperation at pull time, period.


eames  (D 23844)

Feb 18, 2004, 10:10 AM
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In reply to:
Here's 2: Bell 206 BIII Jet ranger, UH-60 Blackhawk.

Right, in that case you'd need to take more time between exits.... because there would be less (or no) airspeed for separation.

Unless you have a function that describes the significant decrease in horizontal airspeed between the winds at altitude and 3000 ft for your scenarios, your scenarios are not objective. But I'll comment anyway:

Yes, if the airspeed is greater, less time is required between groups to achieve the same separation.

Okay, we disagree.

One more try: Let's say there's is NO wind. You throw a toy paratrooper (nearly strait-down trajectory) out of an otter flying at 80 knots (airspeed), and you keep flying strait until the paratrooper hits the ground. Once the paratrooper hits the ground you've covered a certain distance across the ground. Now perform the same experiment at 90 knots (airspeed)... let the paratrooper go at the exact same spot. When the paratrooper hits the ground this time the plane has traveled farther along the ground away from the paratrooper, right? Was this due to a change in groundspeed or airspeed?

I know, groundspeed.... Then how would you explain the fact that the difference in distance that the plane traveled away from the impact point of the paratrooper would remain constant regardless of the direction of flight, windspeed, or wind direction?

-Jason


johnny1488  (D 25453)

Feb 18, 2004, 10:14 AM
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If groundspeed were irrelevent, then on a jump run into 70mph headwind with 70mph airspeed, you would give the same amount of time as you would if the pilot then turned 180 degrees and did another jump run at 70 mph airspeed. The entire first pass would open on top of one another and the second pass would be miles apart. It is groundspeed that determines the amount of time left in between groups.


velo90

Feb 18, 2004, 10:15 AM
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Quote:
Let’s look at 2 more realistic scenarios:

I think that is the problem. I say ground speed is irrelevant, you say it is not.
I can provide a good example where ground speed is irrelevant but it does not fit into your real world of skydiving. Hence the minus 25 kts problem.

So, if you can accept what is more important than ground speed, is the speed of the aircraft relative to the air mass where the parachutes will be opened, we have made a 1st step.

Of course things get far more complicated when you have wind shear, but for the moment we will leave that aside.

Try reading the stuff from Kallend


johnny1488  (D 25453)

Feb 18, 2004, 10:19 AM
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In reply to:
Then how would you explain the fact that the difference in distance that the plane traveled away from the impact point of the paratrooper would remain constant regardless of the direction of flight, windspeed, or wind direction?

I dont think he is trying to say that. He is saying that it is ground speed that determines how long a group has to wait in order to have a predetermined distance between groups at opening. once you leave the plane, where it is when when you open is irrelevent. After you leave the plane, the plane now only matters to those still in it.


Hooknswoop  (D License)

Feb 18, 2004, 10:58 AM
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One more try: Let's say there's is NO wind. You throw a toy paratrooper (nearly strait-down trajectory) out of an otter flying at 80 knots (airspeed), and you keep flying strait until the paratrooper hits the ground. Once the paratrooper hits the ground you've covered a certain distance across the ground. Now perform the same experiment at 90 knots (airspeed)... let the paratrooper go at the exact same spot. When the paratrooper hits the ground this time the plane has traveled farther along the ground away from the paratrooper, right? Was this due to a change in groundspeed or airspeed?

Because of the 10 knot increase in ground speed.

Quote:
I know, groundspeed.... Then how would you explain the fact that the difference in distance that the plane traveled away from the impact point of the paratrooper would remain constant regardless of the direction of flight, windspeed, or wind direction?

Because there was zero wind and with zero wind ground speed = airspeed.

Derek


Hooknswoop  (D License)

Feb 18, 2004, 11:03 AM
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If airspeed is all that matters, why doesn't the 10 knot increase in airspeed create more separation in my examples below?;

1) 70 knots of wind at altitude. 80 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, the opening altitude (I have jumped in these winds).

2) 80 knots of wind at altitude. 90 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, again, the opening altitude.

The ground speed is constant and so is the amount of separation for the same delay between groups in both examples. An increase in airspeed didn’t result in an increase in separation with the same delay between groups.

3) 70 knots of wind at altitude. 90 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, again, the opening altitude.

With #3, and a 10 knot increase in ground speed, and no increase in airspeed, there would be more separation between groups for the same delay between groups.

Derek


eames  (D 23844)

Feb 18, 2004, 11:51 AM
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In reply to:
If airspeed is all that matters, why doesn't the 10 knot increase in airspeed create more separation in my examples below?

There would be more separation, but your examples are useless anyway because you're not specifying what's happening with wind velocity between altitude and 3000 ft.... as I said above. In your examples, is the change between jumprun wind speed and 15 knots at 3000 ft linear, or what? Put numbers in there and show me that there would be no more separation....

-Jason


Premier PhreeZone  (D License)
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Feb 18, 2004, 11:51 AM
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Here is one, a plane flies into a head wind, its Indicated air speed is 90 knots (Otter exit speed) the wind is 30 knots. Its effective ground speed is 60 knots. But if the same plane was flying the other way with the wind pushing it its airspeed would still be 90 knots but its ground speed would now be 120 knots.

Airspeed has not changed at all, but its ground speed has changed a lot. With the higher ground speed you could allow for a shorter delay between groups.

This is almost like the "Canopies will always open downwind" argument Laugh


eames  (D 23844)

Feb 18, 2004, 11:59 AM
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In reply to:
This is almost like the "Canopies will always open downwind" argument

Really, how is that exactly?

In reply to:
Airspeed has not changed at all, but its ground speed has changed a lot. With the higher ground speed you could allow for a shorter delay between groups.

Relative to the moving airmass, the separation will be exactly the same. Of course it won't be the same across the ground, you're going a different speed with respect to the ground! Why is that hard to understand?

-Jason


kallend  (D 23151)

Feb 18, 2004, 12:01 PM
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In reply to:
In reply to:
If airspeed is all that matters, why doesn't the 10 knot increase in airspeed create more separation in my examples below?

There would be more separation, but your examples are useless anyway because you're not specifying what's happening with wind velocity between altitude and 3000 ft.... as I said above. In your examples, is the change between jumprun wind speed and 15 knots at 3000 ft linear, or what? Put numbers in there and show me that there would be no more separation....

-Jason

You put the numbers in. I have a computer program that saves your doing any arithmetic at http://www.iit.edu/~kallend/skydive/

For separation at opening altitude, what the wind does in between exit and opening altitude doesn't matter at all as long as it doesn't change between one group and the next.


Hooknswoop  (D License)

Feb 18, 2004, 12:24 PM
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Quote:
There would be more separation

Exactly, more separation as a result of the increased ground speed with no change in the airspeed.

Quote:
but your examples are useless anyway because you're not specifying what's happening with wind velocity between altitude and 3000 ft.... as I said above. In your examples, is the change between jumprun wind speed and 15 knots at 3000 ft linear, or what? Put numbers in there and show me that there would be no more separation....

OK, call it linear, that doesn’t change anything. It is still ground speed that matters.

Derek


eames  (D 23844)

Feb 18, 2004, 12:42 PM
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Go check your math on kallend's website... your second example yields more separation than the first... and the third even more than the second... just as it should.

Check out the website, put your examples in there yourself, and then tell me you still disagree.

-Jason


velo90

Feb 18, 2004, 12:54 PM
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You put the numbers in. I have a computer program that saves your doing any arithmetic at http://www.iit.edu/~kallend/skydive/

At last, a voice of reason Smile

Not that I can do the arithmetic, or that I will blindly believe a computer program. However, after reading your power point presentation (and notes to the presentation) I finally understood Cool

I just wish that more people would take the time to try and understand. It was not easy for me, not being the brightest spark in the firework display, but if I can understand it, then I am sure other people will.

I think the biggest problem for most skydiver's is to remove the ground from the equation. After all that is their main reference point, that is where they are going to land. Here we are not talking about landing, we are talking about opening a parachute in an air mass, that may or may not be moving relative to the aircraft.

Thanks for the education John Smile


Premier PhreeZone  (D License)
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Feb 18, 2004, 12:55 PM
Post #40 of 246 (910 views)
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On Kallends program with both groups belly speed:

80 kias
20 k headwind
0 knot lowers
6000 feet wind change
5 second delay results in:

669 feet seperation.

80 kias
20 k tailwind
0 knot lowers
6000 feet wind change
5 second delay results in:

1007 feet seperation.

Air speed of the plane never changed, but its ground speed did.


(This post was edited by PhreeZone on Feb 18, 2004, 12:59 PM)


eames  (D 23844)

Feb 18, 2004, 12:55 PM
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In reply to:
If your groundspeed is zero, you will have zero seperation at pull time, period.

I've jumped out of several Cessnas with zero groundspeed before and I definitely didn't have to wait infinitely to exit after the person in front of me. No tracking, no high pulling, and we weren't even close at pull time. How would you explain that? Was it a miracle?

-Jason


eames  (D 23844)

Feb 18, 2004, 1:01 PM
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Indicated Airspeed (kts) 80
Upper winds (kts) 80 Headwind
Lower winds (kts) 15 Headwind
Altitude of wind change 6000
Exit delay (sec) 15

Both slow fallers

Separation between deployment points: 484 ft.

BUT WAIT A MINUTE, THERE'S NO GROUND SPEED!!!

-Jason


(This post was edited by eames on Feb 18, 2004, 1:04 PM)


johnny1488  (D 25453)

Feb 18, 2004, 1:54 PM
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Someone had to move out of the air. If you dont move you are basically a wind drift indicator. If you throw a wind drift indicator off a bridge (or plane in the air that doesnt move across the ground) and wait 2 minutes and throw another one, unless the winds have changed, they will follow the same path. I have exited an otter with almost zero airspeed after waiting 45 seconds (counted on video) and opened almost right were the group before me opened (the exact same space they occupied only a few seconds earlier). The distance the plane covers over the ground between groups is the distance the groups will have on opening barring differences in drift (rw, freefly, tracking ect)

edited to add: and I am sure its me, but I dont see a way to make both fallers on Kallends program slow, only to put the faster faller out 1st or 2nd.


(This post was edited by johnny1488 on Feb 18, 2004, 2:02 PM)


eames  (D 23844)

Feb 18, 2004, 2:00 PM
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In reply to:
I have exited an otter with almost zero airspeed after waiting 45 seconds (counted on video) and opened almost right were the group before me opened (the exact same space they occupied only a few seconds earlier).

The space they occupied only a few seconds earlier? I don't understand... why don't you explain it to me with your bridge analogy. So you throw a wind drift indicator off a bridge, wait 45 seconds, then throw another one, the second one will land within a few seconds of the first? That's quite a riddle....

-Jason


johnny1488  (D 25453)

Feb 18, 2004, 2:03 PM
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no it will land 45 seconds later in the same spot

edited to change my time cause i misread your post.

If someone has a slow moving canopy or turns back up the line of flight or opens off heading, its pretty damn close, especially when drift isnt static and things change in the air.


(This post was edited by johnny1488 on Feb 18, 2004, 2:06 PM)


eames  (D 23844)

Feb 18, 2004, 2:07 PM
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Have you considered that maybe you were just barely upwind of the dropzone and the people that went in front of you were holding into the wind under canopy for 30 seconds of your skydive?

-Jason


(This post was edited by eames on Feb 18, 2004, 2:07 PM)


Hooknswoop  (D License)

Feb 18, 2004, 2:07 PM
Post #47 of 246 (857 views)
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Quote:
BUT WAIT A MINUTE, THERE'S NO GROUND SPEED!!!

If there is no ground speed, then they should have the same exit point on the graph and the little red block that represents the aircraft shouldn't move. According to the graph, they 2 groups do not have the same exit point and the little red block is moving to the right.

I think something is wrong with the program.

Two bodies that fall at the same rate are tossed off a 13,500 tower and subject to the same winds. They will impact the ground at the same place. If you imagine ground impact as the opening point, then there is zero separation between opening points.

For canopy drift in the above scenario, I calculate 380 feet (6076*15/60/60*15). This is not enough separation. Imagine an 8-way followed by a 4-way. The groups would over-lap on deployment as the 8-way drifted away from the opening point.

For zero ground speed, you would have to wait for the previous group to clear the airspace that you will be in when you open. 45 seconds would have the 8-way drift 1139 feet, still not enough wait time. You are no longer relying on separation between opening points for clearance from other jumpers, you are sharing the same opening point and waiting for them to clear the airspace so you can use it.

For opening point separation, airspeed doesn’t matter, ground speed determines the amount of time that must be left between groups in order to have, for example, a constant 2,000 feet between opening points. Increasing airspeed does not increase the distance between opening points, increasing ground speed does (for a constant time between groups).

Airspeed for a particular jump ship is usually the same from jump run to jump run. As the upper winds increase, a longer time must be left between groups to account for the decreased ground speed.

If only airspeed mattered for separation, we wouldn’t have to increase the time between groups as the upper winds increased, since the airspeed of the jump ship hasn’t changed.

Derek


johnny1488  (D 25453)

Feb 18, 2004, 2:12 PM
Post #48 of 246 (853 views)
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In reply to:
Have you considered that maybe you were just barely upwind of the dropzone and the people that went in front of you were holding into the wind under canopy for 30 seconds of your skydive?

And if they were that would still give us the same amount of freefall drift and no (or not enough) opening separation.

Also on days where the otter would have zero airspeed on jumprun, we would usually wait a considerable amount of distance past the dz, a usual "there is no too long" day.


cliffharris  (D 25832)

Feb 18, 2004, 2:29 PM
Post #49 of 246 (834 views)
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I don't think I like your tone mister. Two demerits!


eames  (D 23844)

Feb 18, 2004, 2:34 PM
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I've made a diagram. I just want you to see what I see, even if you don't agree.

On the diagram, just for argument's sake, the thick black line represents a freefall trajectory and canopy flight. Just say for instance that it's a cylinder instead... and you can roll a marble down it. No tricks, only one tube, not to be moved. Can you see that if you roll one marble down it, then another in 10 seconds, they will still never be at the same place at the same time?

-Jason


(This post was edited by eames on Feb 18, 2004, 2:35 PM)
Attachments: Diagram.jpg (20.9 KB)


eames  (D 23844)

Feb 18, 2004, 2:40 PM
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What? But, I have no merit Colonel!


johnny1488  (D 25453)

Feb 18, 2004, 2:44 PM
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In reply to:
Can you see that if you roll one marble down it, then another in 10 seconds, they will still never be at the same place at the same time?

Never, except at the end of the tube/impact point of the ground/opening point if the canopies dont move.


Hooknswoop  (D License)

Feb 18, 2004, 2:44 PM
Post #53 of 246 (876 views)
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The alternative is not unthinkable. I caught that before you edited it.

You agree that the objects should hit the same point, and the aircraft should not be moving, but the program doesn’t show that, so there must be something wrong with it. Do you agree?

I agree that after opening, the first group will drift and eventually if the second group waits until they drift far enough away, they can exit and use the same airspace safely. They will share the exact same opening point because the aircraft has zero ground speed.

As the aircraft’s ground speed increases, for a constant airspeed, less time needs to left between groups.

As I already said:

“For canopy drift in the above scenario, I calculate 380 feet (6076*15/60/60*15). This is not enough separation. Imagine an 8-way followed by a 4-way. The groups would over-lap on deployment as the 8-way drifted away from the opening point.

For zero ground speed, you would have to wait for the previous group to clear the airspace that you will be in when you open. 45 seconds would have the 8-way drift 1139 feet, still not enough wait time. You are no longer relying on separation between opening points for clearance from other jumpers, you are sharing the same opening point and waiting for them to clear the airspace so you can use it.”

The bottom line is that separation of opening points is a function of ground speed and time between exits. This is the reason the free fall drift program requires the airspeed of the jump ship and the wind speed it is flying in. It then calculates the ground speed of the jump ship and uses that number in the calculation of the separation of opening points. Increase airspeed and wind speed 10 knots each and there is no change in the separation between opening points.

For your diagram, there is zero separation between opening points. The marbles pass through the same points in space. The only reason they do not collide is that the first marble is out of the way by the time the second marble comes by. For someone tracking up the line of flight, then holding, they may be in that way if only 15 seconds are left between the exits. The first group must clear the airspace in order for the second group to use the same airspace.

Your marbke example demonstrates that there is something wrong with the free-fall simulator program.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 2:48 PM)


eames  (D 23844)

Feb 18, 2004, 2:50 PM
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Yeah, I'll admit: You can still collide with someone if you try really hard.

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 2:50 PM
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Do you leave more time between the group in front of you if the upper winds increase, but the jump ship's airspeed is the same?

Derek


eames  (D 23844)

Feb 18, 2004, 2:56 PM
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In reply to:
The alternative is not unthinkable.

My bad on that point, I got a little too excited.... I hope you understand. I'm very cynical.

In reply to:
You agree that the objects should hit the same point, and the aircraft should not be moving, but the program doesn’t show that, so there must be something wrong with it. Do you agree?

Not necessarily. The program might be shifting the visual so that the trajectories can be seen more easily. True, the reference points don't move, but it may have been easier to program it with them fixed in place. Maybe we should ask kallend.

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 2:57 PM
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Just to clarify, you do not agree that to identical objects tossed of a tower and subjected to identical winds will hit the same point on the ground?

Derek


eames  (D 23844)

Feb 18, 2004, 3:01 PM
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In reply to:
Do you leave more time between the group in front of you if the upper winds increase, but the jump ship's airspeed is the same?

Yes, and I've explained why in this thread. Higher wind speeds promote canopy traffic congestion (assuming an upwind opening point), because it takes less time to get to the landing area going downwind. Ground speed comes into play after opening, when everyone is aiming for the same spot on the ground.

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 3:02 PM
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Quote:
Not necessarily. The program might be shifting the visual so that the trajectories can be seen more easily. True, the reference points don't move, but it may have been easier to program it with them fixed in place. Maybe we should ask kallend.

I think the error we are seeing is the difference between indicated airspeed and true airspeed. The opening separation should be zero.

Derek


Hooknswoop  (D License)

Feb 18, 2004, 3:05 PM
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So you think that regardless of ground speed, for a constant airspeed, 15 seconds between groups will result in the same amout of separation between opening points?

Derek


eames  (D 23844)

Feb 18, 2004, 3:05 PM
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In reply to:
Just to clarify, you do not agree that to identical objects tossed of a tower and subjected to identical winds will hit the same point on the ground?

They would, if they were not subject to canopy opening (sudden shallow descent-rate) followed by controlled canopy flight.

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 3:08 PM
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Quote:
They would, if they were not subject to canopy opening (sudden shallow descent-rate) followed by controlled canopy flight.

Then you agree that, for zero ground speed, group 1 and group2 will share the same exact opening point and that for a true airspeed of 80 instead of indicated, the separation would be zero for your example and not 484 feet?

Quote:
Indicated Airspeed (kts) 80
Upper winds (kts) 80 Headwind
Lower winds (kts) 15 Headwind
Altitude of wind change 6000
Exit delay (sec) 15

Both slow fallers

Separation between deployment points: 484 ft.

BUT WAIT A MINUTE, THERE'S NO GROUND SPEED!!!

Derek


eames  (D 23844)

Feb 18, 2004, 3:09 PM
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In reply to:
So you think that regardless of ground speed, for a constant airspeed, 15 seconds between groups will result in the same amout of separation between opening points?

Relative to the moving airmass, yes.

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 3:16 PM
Post #64 of 246 (930 views)
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Quote:
So you think that regardless of ground speed, for a constant airspeed, 15 seconds between groups will result in the same amout of separation between opening points?

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


Relative to the moving airmass, yes.

Let me clarify the question.

You think that regardless of ground speed, and assuming that 15 seconds will leave 2,000 feet of separation between opening points in zero wind, as the upper winds increase and the ground speed of the jump ship decreases, 15 seconds delay between groups will still create 2,000 feet of separation between groups?

Derek


Hooknswoop  (D License)

Feb 18, 2004, 3:32 PM
Post #65 of 246 (919 views)
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Quote:
Indicated Airspeed (kts) 80
Upper winds (kts) 80 Headwind
Lower winds (kts) 15 Headwind
Altitude of wind change 6000
Exit delay (sec) 15

Both slow fallers

Separation between deployment points: 484 ft.

BUT WAIT A MINUTE, THERE'S NO GROUND SPEED!!!

Actually there is a 18.5 knot ground speed.

The program is set up for stand ICAO atmosphere. The standard temperature at 15,000 ft MSL is 5.51 degree F. The program uses 14,000 ft MSL for the exit altitude. Allowing the 1,000 foot error, that gives a true airpseed of 98.5 knots.

65 knots indicated is equal to 80.1 knots true airspeed at 15,000 feet. Try these numbers to get zero (or almost zero ground speed):

Indicated Airspeed (kts) 65
Upper winds (kts) 80 Headwind
Lower winds (kts) 15 Headwind
Altitude of wind change 6000
Exit delay (sec) 15

Both slow fallers

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 3:36 PM)


eames  (D 23844)

Feb 18, 2004, 4:05 PM
Post #66 of 246 (911 views)
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In reply to:
Then you agree that, for zero ground speed, group 1 and group2 will share the same exact opening point and that for a true airspeed of 80 instead of indicated, the separation would be zero for your example and not 484 feet?

The opening point would be the same point in space, with respect to the ground, but by the time the second group got there, the first group would have continued downwind with an arrested descent rate at a velocity equal in magnitude and opposite in direction to that of the aircraft (aircraft speed = -windspeed here) for the same amount of time that the second group gave them in the door.

-Jason


johnny1488  (D 25453)

Feb 18, 2004, 5:46 PM
Post #67 of 246 (900 views)
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So the first jumper who opens facing up wind and colapses their slider and fumbles with their chest strap sees the second jumper scream past them when they pull 5 seconds late after they have a hard pull.

Thats why I believe horizontal seperation is the only thing that matters. I look out of the plane to see how much distance the plane has made since the last group exited.

I just think jumpers need to know the ralationship between the ground speed of the plane and the amount of time needed to be given between groups.


(This post was edited by johnny1488 on Feb 18, 2004, 5:57 PM)


Hooknswoop  (D License)

Feb 18, 2004, 5:52 PM
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Quote:
So the first jumper who opens facing up wind and colapses their slider and fumles with their chest strap sees the second jumper scream past them when they pull 5 seconds late after they have a hard pull.

Or the second group funnels and falls fast while the first, and larger group builds a slow, tight formation. The second group catches up to the first group and eats up the separation they would have had from canopy drift.

Derek


mnischalke  (D 26290)

Feb 18, 2004, 8:36 PM
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I think what some folks may not be realizing is that the aircraft, even traveling 100 kts in 100 kts headwind (thereby negating groundspeed), is still traveling through the air at 100kts. The first group may exit the aircraft at exactly the same spot above the ground as the second group, but the first group will be in a totally separate column of air than the second group, given a reasonable delay between exits. Therefore, the space in which the first group will deploy will be continuing to move downwind, and the position the second group's deployment will be further upwind (even though it may be over the same place on the ground). The reason to increase time between groups on jumpruns with high winds at altitude is because generally, the winds decrease by opening altitudes and thereby causing a stacking effect. Nonetheless, the groups will be opening in different columns of air, no matter their position over the ground. The only case in which this whole thing is negated is when the wind at 3000 is zero or 180 degrees from the winds at altitude.

Something else to consider when looking at comparing zero groundspeed jumpruns to jumping from fixed objects is bridge day. They huck how many jumpers through the same column of air in how many hours with how much separation? Think about it.

I guess the thing is with groups falling through columns of air (irrespective of position over the ground), if groups are falling faster or slower, with columns which are straight down, or askew from sliding, the closer to the ground you get, the higher the likelyhood of converging columns of air. If you leave an aircraft after me with a reasonable delay and you have a backslide which is causing you to directly intersect my column of air at some point, if I dump at 6k, you have a lesser chance of hitting my canopy than you do if I open at 3k. That chance increases further by 2k (unless your backsliding so hard you bisect my column at 3k and get past me. got me?


Nullified  (C 32259)

Feb 18, 2004, 9:35 PM
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Quote:
I think what some folks may not be realizing is that the aircraft, even traveling 100 kts in 100 kts headwind (thereby negating groundspeed), is still traveling through the air at 100kts. The first group may exit the aircraft at exactly the same spot above the ground as the second group, but the first group will be in a totally separate column of air than the second group, given a reasonable delay between exits. Therefore, the space in which the first group will deploy will be continuing to move downwind, and the position the second group's deployment will be further upwind (even though it may be over the same place on the ground). The reason to increase time between groups on jumpruns with high winds at altitude is because generally, the winds decrease by opening altitudes and thereby causing a stacking effect. Nonetheless, the groups will be opening in different columns of air, no matter their position over the ground. The only case in which this whole thing is negated is when the wind at 3000 is zero or 180 degrees from the winds at altitude.
Given a ground speed of 0mph at exit, and assuming that 1st and 2nd exiting groups fall at the same rate and experience the same winds at various altitudes and open at the same altitude, groups 1 and 2 will have opened at the exact same place.
With 0 groundspeed, the aircraft becomes a fixed object. Assuming all variables remain equal for both groups, they will open in the identical position, whether the 2nd group gave 3 seconds of separation or 125 years.


mnischalke  (D 26290)

Feb 18, 2004, 9:48 PM
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It remains a fixed object relative to the ground.

The ground doesn't matter until you land.

The aircraft is still traveling through the air. The air is moving, thus causing separation horizontally and vertically based upon delay between groups. No matter where above the ground the groups open, the air continues to move the first group away from the spot of deployment. Each group has it's own column of air and unless the second group is sliding, the two groups will never share the same air.


Nullified  (C 32259)

Feb 18, 2004, 10:38 PM
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Quote:
It remains a fixed object relative to the ground.

The ground doesn't matter until you land.

The aircraft is still traveling through the air. The air is moving, thus causing separation horizontally and vertically based upon delay between groups. No matter where above the ground the groups open, the air continues to move the first group away from the spot of deployment. Each group has it's own column of air and unless the second group is sliding, the two groups will never share the same air.

If you shoot a bullet from a rifle in to 100 mph headwind, wait 5 minutes and shoot another bullet from that rifle, providing that the wind hasn't changed and that you are standing in the same place, aiming the same and from the same height, both bullets will land in the exact same place.

Exiting a plane is no different. At 0mph groundspeed, both groups will exit at the exact same point over the Earth. 2 seconds or 20 minutes makes no difference...again, providing that all other variables remain equal.

The aircraft is the gun, the skydivers are the bullets and the ground awaits.

Another...
If at 1000' I'm under canopy at point A, and I fly with no input until I land, I will land at point B. If the next person is under the same canopy and loading and at the same point A at 1000' that I was at 1000', given the same winds, flying without input will put that person exactly at my landing point B.

The ground DOES matter before you land. You're using it as a fixed reference point to spot your exit. Given the same fixed reference point and fixed variables, 50,000 skydivers exiting at the same point over the course of the life of our universe will open at the exact same point.

As for sharing the same air, you're right...providing that all variables remain equal, they will not occupy the same space/time. But, the opening coordinates will be identical with regards to space.

Stay safe,
Mike


mnischalke  (D 26290)

Feb 18, 2004, 10:48 PM
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I understand your analogy, but it's inconsistent with this discussion. Bullets are affected by wind, but they don't have parachutes.

Nonetheless, no matter how fast you fire your gun, those bullets will not run into each other in flight.

Remember, it's what happens at the deployment altitude that matters. The ground has nothing to do with this situation other than it's where you're going.


Nullified  (C 32259)

Feb 18, 2004, 11:27 PM
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Quote:
Nonetheless, no matter how fast you fire your gun, those bullets will not run into each other in flight.
If this is the point that you've been trying to make, then I agree with you.
It's possible that we've been on not exactly the same page during this discussion.
I'm not arguing that the two groups will deploy at the same time and position...just at the same position.

If this clears it up between us, then either I have misinterpreted your earlier posts or you weren't quite clear...or a combination of both.

Stay safe,
Mike


velo90

Feb 19, 2004, 12:50 AM
Post #75 of 246 (838 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Have you read Kallend's power point presentation yet?

All your thoughts about separation are correct IF you would forget about the ground and think about where you deploy the parachute. We are not interested in the ground, we are interested in the air mass where the parachutes open.

If you have a 15 kts wind at 3000 ft and jumper a deploys at a point A above the ground. Then 10 seconds later the second jumper deploys into the first jumper. They have deployed at different points above the ground, but unfortunately they have no seperation.


If the wind speed lower to the ground is blowing in the same direction as the uppers then .....
Your method (using ground speed) to calculate the time between exits for good separation at pull time will give a time that is (most often) longer than necessary. Not a bad thing, more separation is better, but on big planes you might need a go around more often.

Now if we have wind shear and the lowers are blowing 180° to the uppers, your method (ground speed) will give you a time that is too short. This problem may not be of interest to you, but for me it is very important. At my dropzone we have this situation occasionally.

I don't think anyone here is ever going to convince you that ground speed is irrelevant. Most of the time the way you think about this problem is not going to cause any harm. BUT, if we do have wind shear, and the lower's are 180° to the uppers, then please do what you put in your sig line , THINK.


(This post was edited by velo90 on Feb 19, 2004, 12:50 AM)


Hooknswoop  (D License)

Feb 19, 2004, 1:47 AM
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Something else to consider when looking at comparing zero groundspeed jumpruns to jumping from fixed objects is bridge day. They huck how many jumpers through the same column of air in how many hours with how much separation? Think about it.

They have to wait until the previous jumper(s) are below where the next jumpers will be opening and that when the next jumper opens, there will be separation between the already open canopy and the opening canopy. You can’t have someone take a 1 second delay followed immediately by a 3 second delay from the same exit point. They do not do that a bridge day.

Derek


Hooknswoop  (D License)

Feb 19, 2004, 1:58 AM
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Have you read Kallend's power point presentation yet?

All your thoughts about separation are correct IF you would forget about the ground and think about where you deploy the parachute. We are not interested in the ground, we are interested in the air mass where the parachutes open.

If you have a 15 kts wind at 3000 ft and jumper a deploys at a point A above the ground. Then 10 seconds later the second jumper deploys into the first jumper. They have deployed at different points above the ground, but unfortunately they have no seperation.


If the wind speed lower to the ground is blowing in the same direction as the uppers then .....
Your method (using ground speed) to calculate the time between exits for good separation at pull time will give a time that is (most often) longer than necessary. Not a bad thing, more separation is better, but on big planes you might need a go around more often.

Now if we have wind shear and the lowers are blowing 180° to the uppers, your method (ground speed) will give you a time that is too short. This problem may not be of interest to you, but for me it is very important. At my dropzone we have this situation occasionally.

I don't think anyone here is ever going to convince you that ground speed is irrelevant. Most of the time the way you think about this problem is not going to cause any harm. BUT, if we do have wind shear, and the lower's are 180° to the uppers, then please do what you put in your sig line , THINK.

Ground speed is the only way to determine distance between opening points.

*With 5 seconds between exits and 80 knots of airspeed and 80 knots of uppers for zero grounds speed, the groups will open at the same point in space, 5 seconds apart. The only thing that will prevent a collision is the first group drifting downwind once they open.

*With 5 seconds between exits and 80 knots of airspeed and 0 knots of uppers for 80 knots grounds speed, the groups will have opening points 674 feet apart. Same airspeed, different ground speed, increased separation of opening points.

No one has contradicted this.

Same airspeed, change in ground speed caused change in separation between opening points.

*With 5 seconds between exits and 180 knots of airspeed and 180 knots of uppers for zero grounds speed, the groups will open at the same point in space, 5 seconds apart.

100-knot increase in airspeed, no increase in separation between opening points. Again, the only thing that will prevent a collision is the first group drifting downwind once they open.

Derek


Hooknswoop  (D License)

Feb 19, 2004, 2:19 AM
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*Let’s take the high wind example again. Let’s say there is 80 knot uppers and the jump ship has an airspeed of 80 knots and a ground speed of zero knots. Let’s also say these winds taper, linearly, to zero knots at 3,500 feet.

Group 2 exits 10 seconds after group 1. Group 1 pulls at 3,500 feet, group 2 pulls at 3,000 feet. Do see a reason to be concerned here?

*Let’s take it another direction. Let’s say the uppers are 90 knots and the jump ship has an airspeed of 80 knots. The jump ship has a ground speed of –10 knots. Let’s again say these winds taper, linearly, to 10 knots at 3,500 feet.

Group 2 exits 10 seconds after group 1. Group 1 pulls at 3,500 feet, group 2 pulls at 3,000 feet. Do see a reason to be concerned here?

In zero wind, at 80 knots, the jump ship covers 674 feet across the ground. In both of theses examples, airspeed has not changed, only ground speed.

I would like for Kallend to weigh in on what is more important for separation, airspeed or ground speed.

Derek


velo90

Feb 19, 2004, 2:32 AM
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Can we go back to your tower?

13,500 ft high and 60 mph winds at the top. No ground speed cos it is a tower. We are both on top of the tower and I jump first and pull after 3 seconds.

You are also going to pull after three seconds. According to your theory you will have to wait ages before you jump because we have no ground speed.

What do you think is going to happen to me on my canopy in 60 mph winds? I am certainly not going hang around very long. What would be the separation if you wait 5 seconds before jumping assuming my canopy is capable of 30 mph and I try to fly back to the tower?

I know this is extreme but it's explains the principle that ground speed does not matter. In reality we only use ground speed as a good indicator for calculating separation.


Hooknswoop  (D License)

Feb 19, 2004, 5:10 AM
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Quote:
Can we go back to your tower?

Sure, but let's use an example that is not extreme. Let's say the winds are 40 knots at the top and zero at 4,000 feet, my deployment altitude. You plan on deploying at 3,000 feet. How long would you wait after I exited to follow?

Jumping in winds of 60 knots at deployment altitude is not realistic. If you did, yes you wouldn't have to wait very long between groups because of canopy drift. Usually winds at deployment altitudes are a lot less than 60 knots and therefore a lot of time is required to alloy the previous group to drift out of the airspace for a zero ground speed jump run.

If the winds are equal to the canopy's airspeed at deployment altitude and the jumper holds into the wind, they are slowly descending down the same airspace the next group will occupy shortly. IF the first canopy pulled a but high and the second canopy pulls a bit low, there is no separation and risk of collision.

Derek


(This post was edited by Hooknswoop on Feb 19, 2004, 5:30 AM)


johnny1488  (D 25453)

Feb 19, 2004, 5:14 AM
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Again I dont think derek is arguing that jumpers dont move once under canopy. If you pull after 3 seconds and then hold into the wind, can he exit the tower? WE have been discussing opening points, not what happens after that. WE exit over a point on the ground, we open over a point on the ground, we land at a point on the ground, it is the reference by which we measure everything. You dont say "I opened over that mass of air over there" Yes jumpers move under canopy, but that would mean we would have to wait for there freefall to be over to be sure they would not be in the same point as we were planning to open.

This horse is getting pretty damn tired of being beat. I think it was dead 3 pages ago, but I think we actually agree on most points, we just dont know it.


velo90

Feb 19, 2004, 5:30 AM
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Quote:
--------------------------------------------------------------------------------

Can we go back to your tower?
--------------------------------------------------------------------------------


Sure, but let's use an example that is not extreme.

That's the problem, you are always using examples where the speed over ground is not much different to the speed of the aircraft relative to the air mass where you open.

I have already agreed that generally ground speed works. You keep proving to me that ground speed generally works. But it works by chance!

Go back to my tower example, ZERO ground speed, and still separation. Ground speed is irrelevant!


(This post was edited by velo90 on Feb 19, 2004, 5:31 AM)


velo90

Feb 19, 2004, 5:38 AM
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Quote:
WE exit over a point on the ground, we open over a point on the ground, we land at a point on the ground, it is the reference by which we measure everything. You dont say "I opened over that mass of air over there"

And that is the problem, because we are in an air mass.
You need the separation in that air mass. We are not on the ground until we land. That means we have to study what happens to objects that move through the air mass. The ground does not matter.

This horse has been beaten many times on DZ.com and it will continue to be flogged as long as people say ground speed is what matters for exit seperation.

Oh, BTW the way, I don't think we agree at all. Wink


Hooknswoop  (D License)

Feb 19, 2004, 5:53 AM
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That's the problem, you are always using examples where the speed over ground is not much different to the speed of the aircraft relative to the air mass where you open.

Isn't that usually the case, or do you usually jump in winds where the uppers are 80 knots and 80 knots at 2,000 feet?

You are relying on canopy drift for separation, but there isn't much wind at deployment altitude to give you that drift/separation. Hence the longer amount of time required to wait for that drift to provide separation.

With a low ground speed, more time must be left between groups than with a high ground speed.

I didn't get a straight answer before, so I'll ask you;

Do you think that for a ground spped that provides 2,000 feet of separation between opening points for 10 seconds between groups provides that same 2,000 feet with 10 seconds between groups as the ground speed decreases from increased upper winds?

Quote:
Go back to my tower example, ZERO ground speed, and still separation. Ground speed is irrelevant!

Go back to my tower example, zero ground speed and zero separation because of zero wind at deployment altitude.

Now change it to 80 knots of ground speed and zero knots at deployment altitude with time left between groups, you get separation.

What is the difference between my tower example and this one? Ground speed providing separation. You cannot rely on canopy drift to provide separation.

Derek


Premier SkymonkeyONE  (D 12501)

Feb 19, 2004, 6:03 AM
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If you shoot a bullet from a rifle in to 100 mph headwind, wait 5 minutes and shoot another bullet from that rifle, providing that the wind hasn't changed and that you are standing in the same place, aiming the same and from the same height, both bullets will land in the exact same place.

That is a brilliant analogy. I must admit that what few working brain cells I have left have been scrambled by this back and forth banter, but I will now conclude that both people can be/are right (assuming their positions on their respective arguments).


I am going to try my best to split this thread in two and move this part of the discussion over to Safety and Training. Any mixing of the "chuck/alan/brian" part is simply a product of my inability to unscramble the thread to that degree.

Chuck


velo90

Feb 19, 2004, 6:09 AM
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Please read this

Actually read the whole thread.

I done.


Hooknswoop  (D License)

Feb 19, 2004, 6:20 AM
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I read the thread and it seems that the majority agree that referencing groundspeed to ensure separation is the way to go........

Derek


mnischalke  (D 26290)

Feb 19, 2004, 6:23 AM
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Quote:
If you pull after 3 seconds and then hold into the wind, can he exit the tower?

Simply, yes. If we are basing this on the 60 mph model, if one canopy opens at 3 seconds, he's facing wind which is 60 mph, relative to the ground. Even facing into the wind, there is no canopy that I know of--which someone will actually land--that will go 60+ mph and therefore cannot get into the airspace of the next jumper.

The danger of hitting a canopy which was out before you comes into play only when the differential between windspeed at jump altitude and the windspeed at deployment altitude differs greatly. If it's 100kts at jump altitude, and 0 at deployment altitude, the canopys can be open in the same airspace, if the second group delays deployment for a distance equal to the first group's opening altitude plus the descent rate of the open canopy of the first group. Still, this is all based on speed in relation to the ground, but irrelevant to the ground because you're not on the ground.

Do we agree that the canopy arrests the descent of a jumper and prolongs the exposure to the wind? Then--and this is the important part--if we use the sliding rule of windspeed reduction with altitude loss, a canopy will move even more quickly away from the next group's path (column of air) the higher the deployment (since the winds only decrease with loss of altitude in this model).


velo90

Feb 19, 2004, 6:42 AM
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Quote:
I read the thread and it seems that the majority agree that referencing groundspeed to ensure separation is the way to go........

Also from that thread.....

Kallend - Strictly speaking, you want to cover distance relative to the air at opening altitude. Usually this is pretty much the same as distance over the ground, but if the uppers are in the opposite direction to the lowers then you can find yourself with reduced separation.

Winsor - If you wish to pick a critical speed, it is that of the aircraft with regard to the airmass at opening altitude.


Winsor - the ground winds and ground speed are entirely immaterial, and have no effect whatsoever on the path of bodies THROUGH THE AIR.

Kallend - and the ground is irrelevant until you get there. It does serve as an approximate guide to what is happening at opening altitude, which is why groundspeed based methods usually work OK.

Winsor - Groundspeed is related only by coincidence.


eames  (D 23844)

Feb 19, 2004, 8:19 AM
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In reply to:
I read the thread and it seems that the majority agree that referencing groundspeed to ensure separation is the way to go........

Dude, don't pull some kind of dz.com rank here. It's split pretty evenly.

Just for argument's sake let's say there's a layer of clouds at 200 ft and you can't see the ground at all. You have no idea which way you are flying with respect to the ground (no instruments except your airspeed indicator). Your airspeed remains constant and positive.

So according to you, PhreeZone, johnny1488, Nullified, and the rest of the "majority," you wouldn't be able to tell how far each group would be separated? So you're saying that depending on which cardinal direction you're facing your separation would change?

Hook, give me a relationship, give me a set of equations, that relates ground position and separation in that scenario, or in any scenario for that matter (ground speed is a function of ground position). I know that kallend could give us one that relates airspeed and separation.

If you can, you should be give the Nobel prize for proving that relativity as we know it is inconsistent.

-Jason


(This post was edited by eames on Feb 19, 2004, 8:26 AM)


Hooknswoop  (D License)

Feb 19, 2004, 8:29 AM
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Dude, don't pull some kind of dz.com rank here.

Rank? Me? Ha!Wink

Quote:
Just for argument's sake let's say there's a layer of clouds at 200 ft and you can't see the ground at all. You have no idea which way you are flying with respect to the ground (no instruments except your airspeed indicator). Your airspeed remains constant and positive.

So according to you, PhreeZone, johnny1488, Nullified, and the rest of the "majority," you wouldn't be able to tell how far each group would be separated? So you're saying that depending on which cardinal direction you're facing your separation would change?

Hook, give me a relationship, give me an equation, that relates ground position and separation in that scenario (ground speed is a function of ground position).

If you can, you should be give the Nobel prize for proving that relativity as we know it is inconsistent.

Correct, if you did not know your ground speed, you couldn't know your separation.

Again the tower analogy;

*14,000 tower. Winds are 40 knots at the top, tapering to zero, linearly, at 3,000 feet, the deployment altitude. Two jumpers, jumping rounds, exit 5 seconds apart. The second jumper pulls a little late. He will hit the first jumper.

*Same tower except the winds taper off to 10 knots, linearly. The second jumper will miss the first jumper as the first jumper drifts for 5 seconds at 10 knots. They won't miss by much.

No one seems willingy to answer my question:

Do you believe that at a given ground speed, 10 seconds delay produces 1,000 feet of separation. As the uppers increase, and the airspeed remains constant, does the same 10 second delay produce the same 1,000 foot separation between opening points? The only change being ground speed.

Derek


velo90

Feb 19, 2004, 8:33 AM
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Do you believe that at a given ground speed, 10 seconds delay produces 1,000 feet of separation. As the uppers increase, and the airspeed remains constant, does the same 10 second delay produce the same 1,000 foot separation between opening points? The only change being ground speed.

NO, there I have answered.

The only change being ground speed
I'll give you a tip here, the ground speed is not the only thing that has been changed Shocked


velo90

Feb 19, 2004, 8:44 AM
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Quote:
Do you believe that at a given ground speed, 10 seconds delay produces 1,000 feet of separation. As the uppers increase, and the airspeed remains constant, does the same 10 second delay produce the same 1,000 foot separation between opening points? The only change being ground speed.

YES, there I have answered (again)

I am assuming that the wind speed from exit height all the way to the ground has increased by the same amount and the wind is blowing in the same direction from top to bottom.


(This post was edited by velo90 on Feb 19, 2004, 8:45 AM)


winsor  (D 13715)

Feb 19, 2004, 9:15 AM
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In reply to:
I read the thread and it seems that the majority agree that referencing groundspeed to ensure separation is the way to go........

Derek

Yeah, and if you polled one population or another you could support any number of demonstrably incorrect conclusions.

This is a definitive case of the classical fallacy of relevance "Argumentum ad Populum ."

Physics is not amenable to solution by quorum.


Blue skies,

Winsor


JFC3  (C 31790)

Feb 19, 2004, 9:40 AM
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What caused me to read this incredibly long thread is the very first post I see which states that the 45 degree rule does not work. Yet no one has shown me why it doesn't.

Whether I'm supposed to peak into the cockpit and calculate airspeed vs. wind speed or look 3 miles down at the ground and determine how fast I'm moving is it just not easier and much more reliable to look out at the last group, wait until they're at 45 degrees from the door and then jump and have some fun? Please tell me how that does not work. They're on a different column of air. They have an appropriate time-relationship distance. Half of one 6 dozen of another how will I end up on top of them (unless you back slide like an SOB like me).Crazy


eames  (D 23844)

Feb 19, 2004, 9:41 AM
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In reply to:
Correct, if you did not know your ground speed, you couldn't know your separation.

No, in actuality, your separation would be the same no matter what your ground speed.

In reply to:
14,000 tower. Winds are 40 knots at the top, tapering to zero, linearly, at 3,000 feet, the deployment altitude. Two jumpers, jumping rounds, exit 5 seconds apart. The second jumper pulls a little late. He will hit the first jumper.

No, he wouldn't. The first jumper would have drifted downwind of the opening point (with very little vertical speed because he's under a parachute) for 5 seconds at 40 knots. Did you not catch my cylinder/marble analogy. Should I draw you another diagram?

This is silly anyway, because most jump planes travel 80+ knots, and that is what determines separation. The only time I've agreed that the speed of the uppers has anything to do with separation is in the special case that the speed of the wind equals the speed of the plane. The airspeed of the plane is all that matters... until after opening.

I'm quite happy to just disagree with you at this point.

-Jason


pccoder  (A 43773)

Feb 19, 2004, 9:44 AM
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Re: [JFC3] Opening High for Bad Spots [In reply to] Can't Post

I hear ya. Smile

My DZ has two planes, a Grand Caravan and a King Air.

Basically the rule is something like > 7 secs separation on the Caravan and > 5 secs on the King Air.

I guess there will be the time where the winds, etc. can cause that to be a problem. But, I don't see myself busting out the scientific calculator at the door to compute all that. Smile


Premier PhreeZone  (D License)
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Feb 19, 2004, 9:47 AM
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Go to Kallends calculator and see when the groups hit 45 degrees. Hint, they never will. Kallends entire website with the powerpoint will teach you about why the 45 degree thing is wrong.


JFC3  (C 31790)

Feb 19, 2004, 9:51 AM
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I went to it when it was mentioned in the thread. But if it's going to tell me the group before me never reaches 45 degrees it's wrong. It's time vertical and more importantly horizontal. 45 degrees from where I am when I exit. Granted in different conditions that time to get to 45 is longer and they're much smaller but they will get there.

Edit to say: I am no genius at this and I won't present any calculations to prove my point I'd rather have someone else (and there seems to be a bunch out there) who has got their minds wrapped around this better show me plainly why common sense just doesn't work in this situation.


(This post was edited by JFC3 on Feb 19, 2004, 9:54 AM)


Hooknswoop  (D License)

Feb 19, 2004, 9:55 AM
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Quote:
14,000 tower. Winds are 40 knots at the top, tapering to zero, linearly, at 3,000 feet, the deployment altitude. Two jumpers, jumping rounds, exit 5 seconds apart. The second jumper pulls a little late. He will hit the first jumper.

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


No, he wouldn't. The first jumper would have drifted downwind of the opening point (with very little vertical speed because he's under a parachute) for 5 seconds at 40 knots.

Why would he drift at 40 knots from the opening point, when there is zero wind at the deployment altitude of 3,000 feet?

Derek


freakydiver  (D 26421)

Feb 19, 2004, 10:01 AM
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That's a pretty balsy assumption - one I sure as hell wouldn't want to make.


freakydiver  (D 26421)

Feb 19, 2004, 10:05 AM
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"Basically the rule is something like > 7 secs separation on the Caravan and > 5 secs on the King Air."

People - there is no hard a fast BASIC rule for seperation. Every single year I see more and more close calls because of the 45 degree friggin MYTH and people never adjusting their exits based on uppers. If your DZ told you 5 seconds on the King Air and never ever varied from that 5 seconds, find another DZ with experienced LOs. pccoder, you really should put a great deal of thought into this because it really affects not only you, but the group exiting before you. There really should be an on-board calculator becuase as I've stated, I've seen more and more close calls every single year in this sport due to shitty generalizations on how to achieve proper seperation. THIS IS NOT A TOPIC TO GET LAZY ON.


johnny1488  (D 25453)

Feb 19, 2004, 10:18 AM
Post #103 of 246 (756 views)
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In reply to:
No, in actuality, your separation would be the same no matter what your ground speed.

Same uppers/ same direction on the same day
Get out of an otter facing into a 70kt headwind 10 seconds after another jumper. Then get out of an otter with a 70kt tailwind 10 seconds after another jumper.(180 degrees from the first jump run) The first you could very well have a close call with. The second you propably will never see.


johnny1488  (D 25453)

Feb 19, 2004, 10:22 AM
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In reply to:
Just for argument's sake let's say there's a layer of clouds at 200 ft and you can't see the ground at all. You have no idea which way you are flying with respect to the ground (no instruments except your airspeed indicator). Your airspeed remains constant and positive.

If you trust the winds aloft you will know the seperation using the airspeed indicator. All this can be worked out on the ground and the exit seperation time can be worked out knowing the direction of the winds and jumprun.


johnny1488  (D 25453)

Feb 19, 2004, 10:37 AM
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I think Tom Buchanan (tombuch here) wrote a pretty good article about this subject. it can be read here:
http://www.ranchskydive.com/safety/tb_article15.htm

He brings the useable length of the jumprun into the equation.


Premier billvon  (D 16479)
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Feb 19, 2004, 10:48 AM
Post #106 of 246 (742 views)
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> wait until they're at 45 degrees from the door and then jump and have some fun?

Cause colliding with an open canopy is often not that fun, and you risk that if you rely on the 45 degree rule. They never reach 45 degrees, which means you either wait forever or you just guess at what _you_ think 45 degrees is. And everyone has a different opinion on what 45 degrees is.

>Half of one 6 dozen of another how will I end up on top of them

From your point of view:

The upper winds are strong, but they drop off with altitude. You follow a freefly group out. They have a perfect head-down exit, so they get a lot of throw and reach the angle you like quickly. You get out 4 seconds later.

You are now in freefall about 1000 feet above them and about 600 feet from them laterally. Since the winds drop off with altitude, that means they are always in _less_ wind than you are (cause they're 1000 feet lower.) They get less push than you do. Since you are upwind of them, the stronger winds at your altitude pushes you slowly towards them.

At deployment altitude, they have slid almost directly beneath you. One guy deploys a little high, his canopy has a hard opening and surges directly beneath you. Result - you collide with a canopy.

How to avoid that? Leave more time between groups when the plane is flying into strong winds and the winds at opening altitude are lighter (which is usually the case.)

"Why can't I just jump and have fun?" is a foolish thing to say. It's a bad excuse no matter what you're neglecting, whether it's gear checks, dirt dives or exit separation.


kallend  (D 23151)

Feb 19, 2004, 12:10 PM
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In reply to:
What caused me to read this incredibly long thread is the very first post I see which states that the 45 degree rule does not work. Yet no one has shown me why it doesn't.

Whether I'm supposed to peak into the cockpit and calculate airspeed vs. wind speed or look 3 miles down at the ground and determine how fast I'm moving is it just not easier and much more reliable to look out at the last group, wait until they're at 45 degrees from the door and then jump and have some fun? Please tell me how that does not work. They're on a different column of air. They have an appropriate time-relationship distance. Half of one 6 dozen of another how will I end up on top of them (unless you back slide like an SOB like me).Crazy

Do a forum search on the 45 degree rule, and you'll find so many explanations about why it doesn't work you'll be up til midnight reading them. It doesn't work. It is dangerous to rely on it.


kallend  (D 23151)

Feb 19, 2004, 12:12 PM
Post #108 of 246 (709 views)
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In reply to:
I went to it when it was mentioned in the thread. But if it's going to tell me the group before me never reaches 45 degrees it's wrong. It's time vertical and more importantly horizontal. 45 degrees from where I am when I exit. Granted in different conditions that time to get to 45 is longer and they're much smaller but they will get there.

Edit to say: I am no genius at this and I won't present any calculations to prove my point I'd rather have someone else (and there seems to be a bunch out there) who has got their minds wrapped around this better show me plainly why common sense just doesn't work in this situation.

I always find it amusing when people argue physics with physicists after claiming no talent for the subject.


kallend  (D 23151)

Feb 19, 2004, 12:18 PM
Post #109 of 246 (708 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Quote:
Have you read Kallend's power point presentation yet?

All your thoughts about separation are correct IF you would forget about the ground and think about where you deploy the parachute. We are not interested in the ground, we are interested in the air mass where the parachutes open.

If you have a 15 kts wind at 3000 ft and jumper a deploys at a point A above the ground. Then 10 seconds later the second jumper deploys into the first jumper. They have deployed at different points above the ground, but unfortunately they have no seperation.


If the wind speed lower to the ground is blowing in the same direction as the uppers then .....
Your method (using ground speed) to calculate the time between exits for good separation at pull time will give a time that is (most often) longer than necessary. Not a bad thing, more separation is better, but on big planes you might need a go around more often.

Now if we have wind shear and the lowers are blowing 180° to the uppers, your method (ground speed) will give you a time that is too short. This problem may not be of interest to you, but for me it is very important. At my dropzone we have this situation occasionally.

I don't think anyone here is ever going to convince you that ground speed is irrelevant. Most of the time the way you think about this problem is not going to cause any harm. BUT, if we do have wind shear, and the lower's are 180° to the uppers, then please do what you put in your sig line , THINK.

Ground speed is the only way to determine distance between opening points.

*With 5 seconds between exits and 80 knots of airspeed and 80 knots of uppers for zero grounds speed, the groups will open at the same point in space, 5 seconds apart. The only thing that will prevent a collision is the first group drifting downwind once they open.

*With 5 seconds between exits and 80 knots of airspeed and 0 knots of uppers for 80 knots grounds speed, the groups will have opening points 674 feet apart. Same airspeed, different ground speed, increased separation of opening points.

No one has contradicted this.

Same airspeed, change in ground speed caused change in separation between opening points.

*With 5 seconds between exits and 180 knots of airspeed and 180 knots of uppers for zero grounds speed, the groups will open at the same point in space, 5 seconds apart.

100-knot increase in airspeed, no increase in separation between opening points. Again, the only thing that will prevent a collision is the first group drifting downwind once they open.

Derek

Scenario: 80kt true airspeed into 80kt headwind at 13k. Groundspeed = 0. Headwind at 12k also 80kt.

Jumper 1 exits.

Jumper 2 waits 10 seconds and exits. Follows exactly same path down as jumper 1 relative to the ground (agree?).

Unfortunately, 0.001 second after jumper 2 commits, jumper 1 has a premature deployment. He is at 12k in 80kt winds. Jumper 1 is looking the other way and does not see.

Do they collide?


velo90

Feb 19, 2004, 12:19 PM
Post #110 of 246 (708 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
The airspeed of the plane is all that matters... until after opening.

No eames Crazy The air speed is not all that matters. The speed of the aircraft relative to the air mass where you open is what matters.

By the way, you would reach 45° if the aircraft is flying fast enough. But we don't generally exit at those sort of speeds.

I feel sorry for Kallend. He's put so much effort in trying to educate people and has not really changed much. Or is it just in these forums?


kallend  (D 23151)

Feb 19, 2004, 12:20 PM
Post #111 of 246 (708 views)
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In reply to:
Quote:
Not necessarily. The program might be shifting the visual so that the trajectories can be seen more easily. True, the reference points don't move, but it may have been easier to program it with them fixed in place. Maybe we should ask kallend.

I think the error we are seeing is the difference between indicated airspeed and true airspeed. The opening separation should be zero.

Derek

Depends which version you use. The online (java) version does it differently from the downloadable version. You are likely to see the true/indicated problem on one of them (forget which right now).


kallend  (D 23151)

Feb 19, 2004, 12:25 PM
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In reply to:
Quote:
The airspeed of the plane is all that matters... until after opening.

No eames Crazy The air speed is not all that matters. The speed of the aircraft relative to the air mass where you open is what matters.

By the way, you would reach 45° if the aircraft is flying fast enough. But we don't generally exit at those sort of speeds.

I feel sorry for Kallend. He's put so much effort in trying to educate people and has not really changed much. Or is it just in these forums?

Anyone that wants to use my Powerpoint presentation and calculator for Safety Day is very welcome so to do.


JFC3  (C 31790)

Feb 19, 2004, 12:35 PM
Post #113 of 246 (695 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

Thanks billvon and I think I see what you're saying but to the situation you propose:

If a group gets out and hits a perfect head down in high winds they're not going to get to my "45 degree rule" (which I have no stronger basis for other than hearing it from people I trust and seeing good results from it so please don't think it's my bible I just want to understand this as it's always been a concern of mine) not in 4 seconds at all. In fact while they will become very small before they are there they would get there. So no matter if I caught the same gust they did leaving the plane so much time has gone by that we've all but eliminated the possability of collision. They would have been sitting in that gust for the last 15 seconds + (please don't say you'd have left in 4 - although I've been to DZ's that insisted on it - I don't go to there anymore) No? Hell, they'll be packing up before I open!Sly

To the point that 45 degrees is different to everyone I just gotta say, huh? I mean my 45 may be your 46 but I think any simpleton could identify 45 degrees. Right???

And I am not foolisd (a foolish thing to accuse me of). If I were I doubt this subject would interest me enough to ask. And you had me believing in the value of what you had to say until then. Thanks anyway.

Anyone who can make a point without making an insult?Unsure


JFC3  (C 31790)

Feb 19, 2004, 12:42 PM
Post #114 of 246 (694 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

Geez. Touchy much?

I wasn't saying it was faulty and I've been given some interesting leads to read up on the faulty-ness of the 45 degree rule. And will. But please relax. I'm asking only because with such a discussion on this subject and no discussion on the claim the 45 degree doesn't work I just wanted to ask. And still while being insulted no one has shown my why it does not work.

And I tell people skydivers are the best nicest people aroung.Crazy


diverdriver  (D 19012)

Feb 19, 2004, 1:13 PM
Post #115 of 246 (686 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

Ok, I'm coming into this thread late here. Man did it take me a long time to read through all that AGAIN. Anyhow, I realise that we always have new people here and they must be educated. This education will continue until the day we no longer skydive.

I don't know if I'm going to add anything to this thread or just muddy the waters. When I talk to people about exit seperation I talk about indicated grounspeed. And you will have the physicists like Kallend, and Winsor roll their eyes at me. And yes, guys, I know that it is the differential between exit winds and deployment winds that will get you the seperation. But I think you will agree that during normal operations that looking at the groundspeed will give you a basic idea of what that differential will be? You certainly won't have 80 knots winds at 3K will you? So, we do know there is some differential and grounspeed is an indirect indicator (read easier to understand) way of looking at seperation.

John knows I'm a hell of a spotter with my GPS and he lets me roll on with my grounspeed explanations like a loving pappa who knows his kid is still wrong but close enough.

The things we need to remember when choosing exit seperation is:

1. As the upper winds increase we DO need to give more time in the door for proper seperation.
2. The 45 degree rule DOES NOT WORK and can get you killed potentially.
3. Running jumprun when the winds are in OPPOSITE directions requires special care be given to seperation.

People need to stop arguing who's physics is better. I've seen a lot of useless bantering on this thread. The work has already been done by some very bright minds. There are some highly educated (on the subject) people trying to teach something here and the newer jumpers might be mindful of listening to them a bit.


mnischalke  (D 26290)

Feb 19, 2004, 1:15 PM
Post #116 of 246 (685 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Do they collide?

no, not even close.


eames  (D 23844)

Feb 19, 2004, 1:33 PM
Post #117 of 246 (679 views)
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In reply to:
No eames The air speed is not all that matters. The speed of the aircraft relative to the air mass where you open is what matters.

Okay, there are other factors, and my explanation is by no means comprehensive, but I will maintain that the only thing the ground has to do with exit separation is that it adds a bit of dynamics to the fluid flowing over it.

-Jason


eames  (D 23844)

Feb 19, 2004, 1:37 PM
Post #118 of 246 (676 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Why would he drift at 40 knots from the opening point, when there is zero wind at the deployment altitude of 3,000 feet?

Okay, I understand what you're saying, but this would only happen in the very special case that the speed of the uppers matches the airspeed of the plane and there is no wind at opening altitude. Do you ever see this happening?

-Jason


eames  (D 23844)

Feb 19, 2004, 1:48 PM
Post #119 of 246 (666 views)
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In reply to:
14,000 tower. Winds are 40 knots at the top, tapering to zero, linearly, at 3,000 feet, the deployment altitude. Two jumpers, jumping rounds, exit 5 seconds apart. The second jumper pulls a little late. He will hit the first jumper.

I totally see where you're coming from with this now. But the only time that skydiving is analogous to being on a 14,000 ft tower is when the uppers match the AIRSPEED OF THE PLANE. Separation is mostly due to the airspeed of the plane.

Yes, if the change in velocity of the mass of air is so drastic that it goes from a magnitude equal to the airspeed of the plane at 14,000 ft all the way down to zero at 3000 ft, there is a very high collision possibility. But I can't say I remember a single day when this condition existed.

-Jason


(This post was edited by eames on Feb 19, 2004, 1:49 PM)


Hooknswoop  (D License)

Feb 19, 2004, 1:57 PM
Post #120 of 246 (656 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Okay, I understand what you're saying, but this would only happen in the very special case that the speed of the uppers matches the airspeed of the plane and there is no wind at opening altitude. Do you ever see this happening?

Not any more likely than exiting with 80 knot winds from exit to opening.

The second jumper would hit the first jumper, even though there was 40 knots of wind at the exit point. No ground speed resulted in zero separation of opening points.

It illustrates my point though. As you increase the wind speed at the opening altitude in my example, separation is created by canopy drift. Say the wind is 15 knots at opening altitude and 10 seconds is left between groups. That means the canopy of jumper 1 will drift 253 feet down wind in 10 seconds. This is not enough room between jumpers, as that distance can be easily tracked and flown under canopy, eating up the separation created by the drift at opening altitude.

Now let’s make the tower an aircraft and give it a ground speed of 15 knots instead of the zero for the tower. Same 10 seconds between exits. The aircraft will cover 253 feet over he ground in that 10 seconds and jumper 1’s canopy will drift for another 253 feet. More separation due to the increased ground speed of the aircraft.

More room must between exits as the upper winds increase in speed.

Some examples from Kallend’s simulator (the only variable I changed for each simulation is the upper winds):

Indicated airspeed: 80
Upper winds: 20
Lower winds: 0
Altitude of wind change: 3000
Exit delay: 10
Both slow fallers

Separation: 1337


Upper winds: 30
Separation: 1168

Upper winds: 40
Separation: 999

Upper winds: 50
Separation: 830

Upper winds: 60
Separation: 661

Upper winds: 70
Separation: 492

Upper winds: 80
Separation: 323

Upper winds: 90
Separation: 154

Upper winds: 100
Separation: 15

As the upper winds increase, the separation between opening points decreases.

What am I missing? (Honestly, if I am wrong, and that is a real possiblility, I will admit itSmile I am either correct or not seeing something)

Derek


eames  (D 23844)

Feb 19, 2004, 1:59 PM
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Here's the conclusion I've come to: Separation is due to the airspeed of the plane and the changes in velocity of the airmass relative to itself.

The changes in velocity of the airmass relative to itself are inevitably due to the interaction of the fluid with the ground.

But separation in skydiving has more to do with the airmass itself, and very little to do with the ground (and only very indirectly at that).

-Jason


bmcd308  (D 27472)

Feb 19, 2004, 2:06 PM
Post #122 of 246 (648 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>I've jumped out of several Cessnas with zero groundspeed before and I definitely didn't have to wait infinitely to exit after the person in front of me. No tracking, no high pulling, and we weren't even close at pull time. How would you explain that? Was it a miracle? <<

One of you backslides.


eames  (D 23844)

Feb 19, 2004, 2:06 PM
Post #123 of 246 (648 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Not any more likely than exiting with 80 knot winds from exit to opening.

This is true, but anywhere in between there will still be separation, will there not?

In reply to:
It illustrates my point though. As you increase the wind speed at the opening altitude in my example, separation is created by canopy drift.

It's not due to canopy drift, it's due to a greater difference between upper and lower winds (like I said before, the airmass with respect to itself). Try increasing the lower winds proportionally.

In reply to:
What am I missing? (Honestly, if I am wrong, and that is a real possiblility, I will admit it I am either correct or not seeing something)

Same here.

-Jason


eames  (D 23844)

Feb 19, 2004, 2:07 PM
Post #124 of 246 (646 views)
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In reply to:
One of you backslides.

Yeah, that's funny.

-Jason


bmcd308  (D 27472)

Feb 19, 2004, 2:09 PM
Post #125 of 246 (644 views)
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Re: [johnny1488] Opening High for Bad Spots [In reply to] Can't Post

>>Also on days where the otter would have zero airspeed on jumprun, we would usually wait a considerable amount of distance past the dz, a usual "there is no too long" day. <<

Were you able to turn points inside the otter that had zero airspeed on jumprun, or did you all just hug each other and wait for the pilot to to sort it out?


bmcd308  (D 27472)

Feb 19, 2004, 2:12 PM
Post #126 of 246 (656 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>Can you see that if you roll one marble down it, then another in 10 seconds, they will still never be at the same place at the same time? <<

Can you see that if the first marble is in the same spot over the ground when the second marble passes through it, both marbles will be killed?


mnischalke  (D 26290)

Feb 19, 2004, 2:15 PM
Post #127 of 246 (655 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Maybe you're not missing it, but it's simply the differential between jump altitude winds and deployment altitude winds. If the differential is large, you need more time between groups. If the differential is less, you can give less time.

Still, this may all relate to speed above ground, but it has nothing to do with the ground itself, nor should the ground be used as your true indicator for separation. The spot (canopy flight back to your landing area) has to do with ground. Separation has to do only with the air between 13.5k(+/-) and 3k (+/-). Why else do you think the pilots post those little predicted wind numbers on the dry-erase board near manifest? Nothing else better to do?Sly


eames  (D 23844)

Feb 19, 2004, 2:17 PM
Post #128 of 246 (652 views)
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In reply to:
Can you see that if the first marble is in the same spot over the ground when the second marble passes through it, both marbles will be killed?

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?

-Jason


diverdriver  (D 19012)

Feb 19, 2004, 2:19 PM
Post #129 of 246 (648 views)
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Not listening. [In reply to] Can't Post

Yal didn't read my post at all did you? Everyone, take a breath for a moment.


eames  (D 23844)

Feb 19, 2004, 2:22 PM
Post #130 of 246 (646 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

There is something very critical that the program is missing: An opening point.

At the opening point, the vertical speed is reduced drastically and the horizontal speed remains the same. This is actually what explains the difference in "opening point" difference (except in your extreme case where the winds at 3000 are always zero). Jumper 1 will continue away from his opening point nearly horizontally for the period of time that jumper 2 gives him in the door.

I'll bet that if you account for this difference (assuming round parachutes), the separation distances will remain the same.

-Jason


(This post was edited by eames on Feb 19, 2004, 6:41 PM)


Hooknswoop  (D License)

Feb 19, 2004, 2:23 PM
Post #131 of 246 (646 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

Quote:
If the differential is large, you need more time between groups. If the differential is less, you can give less time.
Quote:

That doesn’t work though. 80 knot uppers and 80 knots at deployment is zero difference and means there will be zero separation between opening points.

Derek


bmcd308  (D 27472)

Feb 19, 2004, 2:25 PM
Post #132 of 246 (642 views)
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Re: [JFC3] Opening High for Bad Spots [In reply to] Can't Post

>>I went to it when it was mentioned in the thread. But if it's going to tell me the group before me never reaches 45 degrees it's wrong. It's time vertical and more importantly horizontal. 45 degrees from where I am when I exit. <<

It will be rare for a group to move further in the horizontal than in the vertical. The moment they go out, they have some forward speed which they start losing immediately. So they appear to accelerate backward from inside the plane. However, they are accelerating down at 9.8 m/s/s. If their terminal velocity in the vertical is greater than the plane's airspeed on jump run, they will never get to 45 degrees.


Premier billvon  (D 16479)
Moderator
Feb 19, 2004, 2:25 PM
Post #133 of 246 (642 views)
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Re: [JFC3] Opening High for Bad Spots [In reply to] Can't Post

>To the point that 45 degrees is different to everyone I just gotta say,
> huh? I mean my 45 may be your 46 but I think any simpleton could
> identify 45 degrees. Right???

Well, no. People can't even identify straight down; if they could, spotting would be trivial. You really think they can identify 45 degrees with any accuracy?

In any case - Say the group before you gets to 35 degrees, max. The guy ahead of you sees that and exits after 6 seconds. What angle did he wait until? 20? 25? 30 degrees? If he uses 20 degrees and you use 30 degrees, will you both leave enough separation?

That's not even counting that whether they get to 20, 25, 30 or 35 degrees after exit has nothing to do with how far away they will be when you open your parachute.

The 45 degree thing simply does not work. The group before you doesn't ever get to 45 degrees, and the angle they get to has nothing to do with how far away you will be at opening time. If you want to use a plan that's that simple, count to 20 before you exit. It has as much to do with winds, exit separation and opening separation as the 45 degree rule, but is much less subjective, and will almost always result in sufficient separation.

>And I am not foolish (a foolish thing to accuse me of).

Didn't mean to insult you, and I didn't say that you are foolish. But "Why can't I just jump and have fun?" is just a foolish thing to say. One guy at our DZ used to say that all the time. "I just want to jump and have fun, and you guys are coming down on me all the time!" He will never walk without crutches again after his (inevitable) accident; I don't think he's having much fun now. To have fun skydiving you have to take care of all that safety stuff first. Sounds like you are trying to understand separation, which is a good thing from a safety viewpoint.

Exit separation is a complex problem that many people have problems with. There are a lot of close calls under canopy as a result. The best way to solve this problem for the average skydiver is to ask someone knowledgeable how much time to leave between groups; an S+TA or a pilot might be good people to ask.

You can also do the math yourself if you choose. Figure out the ground speed of the plane in feet per second; then figure out the true airspeed of the winds at opening altitude in feet per second. If jump run is into the wind and wind at opening altitude is the same direction but not as strong (which is usually the case) then add the opening airspeed and the plane's airspeed. Multiply that by the number of seconds you want to wait, and you get your separation at opening time. Example:

Plane is doing 80mph on jump run per its GPS. That's 120 feet per second. Winds at opening elevation are same direction as jump run, 20 knots. That's 35 feet per second. Add them; that's 155 feet per second. If you wait five seconds, that's 775 feet of separation. You probably want a little more than that. Eight seconds would be 1240 feet, which should be OK for small formations. If winds at opening are zero, that's only 960 feet, which is on the hairy edge.

If winds at opening altitude are _opposite_ subtract wind speed from aircraft ground speed. That's worst-case; you may have to wait a lot longer between groups.

(Note to John Kallend - yes, I'm neglecting wind vector calculations, but the last thing anyone needs in this discussion are a bunch of cosines.)

Note that that's the separation between the _centers_ of the groups assuming no backsliding or tracking. If both people track towards each other for 400 feet, and you have 1000 feet of separation, you still have 200 feet of separation when you open, which is a bare minumum. If you're going to be backsliding (i.e. you're learning to freefly) then leave more time.

Another option is just to wait the "standard" amount of time (5-10 seconds) and then adjust if you open too near to someone else. The problems with this are pretty obvious; if you collide on your first jump, it doesn't help you. However, it is the system many DZ's use.


Hooknswoop  (D License)

Feb 19, 2004, 2:25 PM
Post #134 of 246 (641 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Kallend-

This is your forte. We have thoroughly discussed this and don’t seem to be convincing anyone. Please tell me if I am wrong and explain why.

Thank you.

Derek


Premier billvon  (D 16479)
Moderator
Feb 19, 2004, 2:29 PM
Post #135 of 246 (638 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

>That doesn’t work though. 80 knot uppers and 80 knots at
> deployment is zero difference and means there will be zero
> separation between opening points.

Correct. But in an 80 knot wind, open parachutes will rapidly vacate that "fixed opening point" and make room for the next jumpers. After five seconds, there will be 675 feet of separation for the next jumpers, minus the distance both groups track and the distance their parachutes glide in five seconds.


mnischalke  (D 26290)

Feb 19, 2004, 2:36 PM
Post #136 of 246 (629 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

totally false. (unless you are talking about point in space in relation to the ground, which is useless to talk about).

Think about it. the two groups may share the same coordinates in space, but they do not share the same airspace. Each group has it's own column of air. If an aircraft is doing 80kts through the air, regardless of groundspeed, one jumper may exit an aircraft and deploy immediately. The next jumper can give 10 seconds, jump and deploy immediately. Using your theory, the second jumper would hit the first jumper. In fact, the jumper is x (math folks with time on your hands, help a brother out) distance away from the second. If the windspeed is the same at deployment as it is at jump altitude, the columns of air are perfectly parallel down to deployment altitude and basically transpose the hop-n-pop separation as above.


(This post was edited by mnischalke on Feb 19, 2004, 2:38 PM)


bmcd308  (D 27472)

Feb 19, 2004, 2:37 PM
Post #137 of 246 (625 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

I am a little surprised that you are an advocate of the "it's OK for everyone to open at exactly the same point over the ground and at exactly the same altitude as long as they are five seconds apart" side of things.

While the wind will blow the canopies downwind for five seconds before the next group gets to that fixed point over the ground, I would sure prefer to open at a point over the ground different from (upwind of) the previous group's.

But maybe that is just me.


Premier billvon  (D 16479)
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Feb 19, 2004, 2:42 PM
Post #138 of 246 (620 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>While the wind will blow the canopies downwind for five seconds
>before the next group gets to that fixed point over the ground, I
> would sure prefer to open at a point over the ground different from
> (upwind of) the previous group's.

No problem; just wait longer or take another pass. (Or stay on the ground when the uppers are so absurdly high.) If you open five seconds after someone in an 80kt wind, they will be 675 feet away from you when you open, so you won't collide. (and no parachute on the planet will do even 40kts in brakes.) But I can understand how opening over the same piece of land could be unnerving.

This isn't much of a realistic example anyway. If you open in an 80kt wind, you're going to have _way_ more problems than exit separation. And if the uppers are so strong that the plane's not moving, the plane is going to have some issues getting there to begin with.


bmcd308  (D 27472)

Feb 19, 2004, 2:46 PM
Post #139 of 246 (619 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

My problem is with the tendency to generalize it down - Would you want the group behind you to open at the same point over the ground as you 5 seconds later if there was a 50 kt wind? 20 kt? 10 kt? No wind?

A quick read of the post to which you responded and your post might make a reader believe that you would be OK with those scenarios, as well.


Premier billvon  (D 16479)
Moderator
Feb 19, 2004, 3:07 PM
Post #140 of 246 (607 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>Would you want the group behind you to open at the same point
> over the ground as you 5 seconds later if there was a 50 kt wind? 20
> kt? 10 kt? No wind?

Real world? I wouldn't be jumping in a 50kt wind. 20kts? I'd be OK if they opened at a point 800 feet from my opening point, because if they did, the winds would give me another 165 feet of separation before I opened. 0 kts? I'd want them to be 1000 feet away.

Put another way, if I had to use the same opening point as someone in 30kt winds, I'd wait 20 seconds, because they'd be 1000 feet from me by the time I opened at the same spot over the ground (which better be upwind of the DZ if anyone has any hope of making it back.)


bmcd308  (D 27472)

Feb 19, 2004, 3:07 PM
Post #141 of 246 (607 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>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? <<

You are, of course, correct. But many, myself included, are disinclined to rely on canopy drift to create separation. If my planned opening point is at the same spot over the ground as yours, I have to rely on you to be out of the way or I hit you. If my planned opening point is at a spot over the ground upwind (at the opening altitude) of yours, you will have to work very hard to be in my way.

Brent


diverdriver  (D 19012)

Feb 19, 2004, 3:08 PM
Post #142 of 246 (605 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>That doesn’t work though. 80 knot uppers and 80 knots at
> deployment is zero difference and means there will be zero
> separation between opening points.

Correct. But in an 80 knot wind, open parachutes will rapidly vacate that "fixed opening point" and make room for the next jumpers. After five seconds, there will be 675 feet of separation for the next jumpers, minus the distance both groups track and the distance their parachutes glide in five seconds.

But, to ensure that we do not have a premature deployment collision they must wait about 50 seconds (exit from 13k) before the next group can exit.

I flew the otter one time with 17 knots of grounspeed ( I know, I know!) and we used 45 seconds seperation. Worked great. Made one pass. Everyone landed on. ATC didn't know what to make of it (5 minute jumprun) but hey, it doesn't concern them really unless someone is flying right at me.


Premier billvon  (D 16479)
Moderator
Feb 19, 2004, 3:13 PM
Post #143 of 246 (603 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>If my planned opening point is at the same spot over the ground as
>yours, I have to rely on you to be out of the way or I hit you.

Not really. In 30kts of wind, you cannot stay in the same place. You don't have to get out of the way; the wind gets you out of the way. And there is no difference in someone trying their level best to fly under you whether the winds are calm or 30kts. The physics work the same in both cases.

Take it to an extreme. The winds are insanely high. All you can see of the ground is a blur, but you can see the next group very well. Do you want them to open over the greenish part of the blur instead of the greenish-gray part of the blur, or do you want them to be 1000 feet from you in the air when they open?


Premier billvon  (D 16479)
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Feb 19, 2004, 3:17 PM
Post #144 of 246 (599 views)
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Re: [diverdriver] Opening High for Bad Spots [In reply to] Can't Post

>But, to ensure that we do not have a premature deployment collision
> they must wait about 50 seconds (exit from 13k) before the next
> group can exit.

You don't need that much time, strictly speaking, but I'm all for leaving more time between exits, especially when low groundspeeds allow people to take more time without landing out. No one ever had a canopy collision because they left too _much_ time between exits.


bmcd308  (D 27472)

Feb 19, 2004, 3:18 PM
Post #145 of 246 (599 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

>>You don't have to get out of the way; the wind gets you out of the way.<<

I guess I should have said, "I have to rely on the wind getting you out of the way." I agree that what keeps you and I from killing each other is never occupying the same point as defined by x,y,z, and t, and how we achieve that does not really matter.

We leave the least to chance and the vagaries of the weather, however, when we have adequate separation over the ground at pull time. If canopy drift helps, then so be it.


mnischalke  (D 26290)

Feb 19, 2004, 3:22 PM
Post #146 of 246 (592 views)
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Re: [diverdriver] Opening High for Bad Spots [In reply to] Can't Post

Quote:
But, to ensure that we do not have a premature deployment collision they must wait about 50 seconds (exit from 13k) before the next group can exit.

Please explain why you believe this.

From what I see, you are saying that the open canopy will follow the same descent path as a freefalling jumper. In fact, the jumper under canopy will be blown off the freefall trajectory as soon as the canopy opens. Further, the second jumper is NOT falling through the same air as the first. He is falling through a column of air several hundred feet upwind of the first jumper, depending upon the speed of the aircraft through the airmass and delay between jumpers.


diverdriver  (D 19012)

Feb 19, 2004, 3:36 PM
Post #147 of 246 (582 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

What I'm saying is that in my situation we didn't have 80 knots at deployment altitude. We had 30 knots which is a big difference. Yes, the wind would drift them away but a canopy could have almost the same airspeed as the wind speed so they would stay relatively stationary to where they opened up. Give another group exiting after them that fall slower and they will drift farther than the first group. (an example would be a lighter tandem group exiting after a heavier tandem group). You would not want that first tandem group to open up, turn into the wind to hold position or minimize backing up.

So, it follows, that if you are approaching headwind = KTAS then you could have a close call. Yes, 45 seconds between groups IS resonable and is what I will tell anyone jumping my plane on a day like that.

Does this all really matter? In the most of extreme jumping days yes. But over all, the lesson people need to take away from this thread is that if the upper winds increase and you are running into the winds directly you will need to increase the amount of time between exits. That's fact.


(This post was edited by diverdriver on Feb 19, 2004, 3:37 PM)


kallend  (D 23151)

Feb 19, 2004, 4:26 PM
Post #148 of 246 (571 views)
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In reply to:
Yal didn't read my post at all did you? Everyone, take a breath for a moment.

I didn't read a word of it, Chris. Especially I didn't read the part about how I know how good you are at spotting with a GPS.


kallend  (D 23151)

Feb 19, 2004, 4:31 PM
Post #149 of 246 (569 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
I am a little surprised that you are an advocate of the "it's OK for everyone to open at exactly the same point over the ground and at exactly the same altitude as long as they are five seconds apart" side of things.

While the wind will blow the canopies downwind for five seconds before the next group gets to that fixed point over the ground, I would sure prefer to open at a point over the ground different from (upwind of) the previous group's.

But maybe that is just me.

So would I under normal circumstances. But 80kt winds at 3000ft are not normal circumstances. I am prepared to accept the separation that 5 seconds gives me, 'cos I shall certainly have a bunch of more serious problems to worry about.


kallend  (D 23151)

Feb 19, 2004, 4:37 PM
Post #150 of 246 (568 views)
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Re: [diverdriver] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
In reply to:
>That doesn’t work though. 80 knot uppers and 80 knots at
> deployment is zero difference and means there will be zero
> separation between opening points.

Correct. But in an 80 knot wind, open parachutes will rapidly vacate that "fixed opening point" and make room for the next jumpers. After five seconds, there will be 675 feet of separation for the next jumpers, minus the distance both groups track and the distance their parachutes glide in five seconds.

But, to ensure that we do not have a premature deployment collision they must wait about 50 seconds (exit from 13k) before the next group can exit.

In reply to:

Not if the premature deployment altitude has winds of 80kt too. Say they left 10 seconds apart, AND the premature canopy then flies directly into the wind (up jumprun) at 30kt. It is still going downwind from the path of the next jumper at 80-30 = 50kt, and will cover about 750ft in this time. So the second jumper will pass 750ft upwind of the open canopy even though it's flying towards him.

The bigger problem is the premature deployment at an altitude where the wind is << 80kt.


kallend  (D 23151)

Feb 19, 2004, 4:42 PM
Post #151 of 246 (726 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

 

You know, I'm not sure why I'm participating here. Everything I believe has been laid out for anyone to see on my web site. I update it as my ideas evolve, so there's nothing I can say here that isn't already there.

If you think I got it wrong, let me know and if I agree I will fix it.


diverdriver  (D 19012)

Feb 19, 2004, 4:58 PM
Post #152 of 246 (722 views)
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In reply to:
Not if the premature deployment altitude has winds of 80kt too. Say they left 10 seconds apart, AND the premature canopy then flies directly into the wind (up jumprun) at 30kt. It is still going downwind from the path of the next jumper at 80-30 = 50kt, and will cover about 750ft in this time. So the second jumper will pass 750ft upwind of the open canopy even though it's flying towards him.

The bigger problem is the premature deployment at an altitude where the wind is << 80kt.


Yah, I know. And in my later post I talked about real world circumstance and the winds at lower altitudes WERE less than 80 knots. (Actually, the winds aloft at exit were 85 knots and my KTAS was 105).

But I do understand what you are getting at.


johnny1488  (D 25453)

Feb 19, 2004, 5:10 PM
Post #153 of 246 (715 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Separation has to do only with the air between 13.5k(+/-) and 3k (+/-). Why else do you think the pilots post those little predicted wind numbers on the dry-erase board near manifest?

With out the ground, those little numbers are completely meaningless. Actually, without the ground, those numbers dont exhist. I believe that is why the ground is a factor.

I might be totally wrong but I dont think I am stupid. I have been arguing over opening points, not seperation after a canopy has drifted away. For my point, I visualize throwing ballons filled with paint out of an aircraft over a ground that will show the impact points. Winds aloft or airspeed win not change the IMPACT POINTS. Only groundspeed of the aircraft will. Once each balloon leaves the aircraft, they are all affected by the same winds, be they in the same direction, every diferent direction, 100kts or 0 kts.


kallend  (D 23151)

Feb 19, 2004, 6:25 PM
Post #154 of 246 (708 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

There's a whole lot of talking at cross purposes here. People are talking at each other rather than to each other.

1. Physics. There is absolutely no doubt whatsoever that from the point of view of physics, the ground is completely irrelevant. The factors that control horizontal separation AT ANY ALTITUDE are (a) the difference between airplane speed at exit altitude and the windspeed at the opening altitude and (b) the time between exits. Since it is a DIFFERENCE in windspeeds, adding or substracting any arbitrary number corresponding to groundspeed to each number makes no difference whatsoever to that difference.

EXAMPLE: (all winds assumed headwinds)

Case A. airspeed 80 kt, wind at altitude 40kt, wind at 3000ft = 20kt., groundspeed 80-40 = 40kt, plane speed with respect to 3000ft air: 80 - 40 + 20 = 60kt.


Now reduce winds at both altitudes by 10kt: (same as changing the Earth's rotation rate by 10kt and keeping the airmass exactly the same)

Case B airpseed 80 kt, wind at altitude 30kt, wind at 3000ft = 10kt, groundspeed 80-30 = 50kt, plane speed with respect to 3000 ft = 80 - 30 + 10 = 60kt, same as Case A. So the groundspeeds are different but the speeds over the 3000ft airmass are the same and the horizontal separation at 3000ft will be the same.

Arguing that impact points on the ground have any relevance is only meaningful if you choose to deploy your canopy at zero agl. I for my part do not like that idea.

2. From practical point of view. Will you get in trouble if you use groundspeed as a reference? Probably not. Not because the physics is incorrect (it isn't) but because the windspeed at opening altitude is USUALLY pretty close to the windspeed on the ground. Hence the error created by using groundspeed is USUALLY negligibly small, and USUALLY acts in a direction that increases separation (so it's kind-of "fail safe".

The only time that using groundspeed is likely to give less than desirable separation is when the low altitude winds are in the opposite direction to the uppers, and wind drift blows the canopies of the first group in the direction of the next group instead of away from it.


If you took college physics, it's all about Galilean relativity.


Can we go home now?


(This post was edited by kallend on Feb 19, 2004, 6:31 PM)


Hooknswoop  (D License)

Feb 19, 2004, 8:17 PM
Post #155 of 246 (694 views)
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Quote:
Can we go home now?

Yes, I was wrongBlush. Thank you for explaining it to me and for everyone else's patience with me.

Derek


velo90

Feb 20, 2004, 1:11 AM
Post #156 of 246 (681 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

I thought I said that about 50 odd posts ago Wink
But there again, I am not a physics professor so who is going to believe me Frown


kallend  (D 23151)

Feb 20, 2004, 7:40 AM
Post #157 of 246 (670 views)
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In reply to:
I thought I said that about 50 odd posts ago Wink
But there again, I am not a physics professor so who is going to believe me Frown

I'm just more persuasive. It must be something to do with your hat (in the avatar).Cool


bmcd308  (D 27472)

Feb 20, 2004, 8:09 AM
Post #158 of 246 (659 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

>>Yes, I was wrong<<

No, you weren't. Here is the quote that ties it all together.

>>Separation means opening points are separated not the same opening point with canopy speed used for separation. The same opening point means there is zero separation. <<


You think about separation the way I do - I do not want to open in the same x,y,z coordinates over the ground as the previous group.

Velo90, Eames et. al. point out that while you are freefalling into the x,y,z coordinates of the previous group, the previous group is under canopy, being moved in the x and y axes.

You (and I) define that as zero separation. They do not. A million posts will not resolve that.

Brent


velo90

Feb 20, 2004, 9:19 AM
Post #159 of 246 (648 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Firstly, I am not sure what x, y, and z are.
Secondly, Idefine separation as the horizontal distance between myself and other jumpers. I would like this separation when I deploy my parachute. Also I would like this separation if my parachute prematurely deploys or I have a mal and need to deploy my reserve.
Of course there are cases where I won't have this seperation, like if my parachute prematurely deploys while in the middle of a relative jump Unsure By default there are people next to me. However, I do not want people on top of me, from the following group for instance.

Your example (and maybe mine, I forget) of people deploying at the same point above the ground and then making space due to wind drift for the next person is something that does not happen in reality. The lower winds would have to be so strong that no sane person would be jumping.


velo90

Feb 20, 2004, 9:20 AM
Post #160 of 246 (646 views)
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Quote:
I'm just more persuasive. It must be something to do with your hat (in the avatar). Cool
Ok, I have changed my avatar (you might need to refresh your browser). Do you think I will be taken seriously now? Wink


kallend  (D 23151)

Feb 20, 2004, 9:25 AM
Post #161 of 246 (650 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>>Yes, I was wrong<<

No, you weren't. Here is the quote that ties it all together.

>>Separation means opening points are separated not the same opening point with canopy speed used for separation. The same opening point means there is zero separation. <<


You think about separation the way I do - I do not want to open in the same x,y,z coordinates over the ground as the previous group.

And with respect to WHAT ORIGIN are you measuring x, y, z? Then justify your choice.

And what if the previous group has already landed? Would that make a difference?


mnischalke  (D 26290)

Feb 20, 2004, 9:37 AM
Post #162 of 246 (644 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

Quote:
However, I do not want people on top of me, from the following group for instance.

If they are above you at, say 7k, they're still gonna be above you at 3k.


bmcd308  (D 27472)

Feb 20, 2004, 9:38 AM
Post #163 of 246 (642 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>And with respect to WHAT ORIGIN are you measuring x, y, z? Then justify your choice. <<

The origin could be any point on the ground. Let's say it's the center of the peas.

So x,y, and z define an opening point. In this thread, at least a portion of the argument (when there is an actual argument) concerns whether it is acceptable for two groups to share the same opening point. By postulating unrealistic winds at opening altitude, there are those who can demonstrate that separation can be achieved as a result of canopy drift by the first group before the second group reaches that same opening point over the ground. Using their vocabulary, the first group will be in a different airmass than the second. Using mine, the first group will have moved away from the opening point, so the values of x, y, and z will have changed.



>>And what if the previous group has already landed? Would that make a difference? <<

Of course. The goals of exit separation are to avoid freefall collisions and to minimize the opportunity for canopy collisions at pulltime between members of different groups. If the first group has landed, then by definition there will not be a freefall collision or a collision at pulltime.


velo90

Feb 20, 2004, 9:42 AM
Post #164 of 246 (637 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

Quote:
If they are above you at, say 7k, they're still gonna be above you at 3k.

Maybe, but I don't want people on top of during any portion of the dive. I am not sure what you are trying say.


kallend  (D 23151)

Feb 20, 2004, 9:43 AM
Post #165 of 246 (636 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>>And with respect to WHAT ORIGIN are you measuring x, y, z? Then justify your choice. <<

The origin could be any point on the ground. Let's say it's the center of the peas.

So x,y, and z define an opening point. In this thread, at least a portion of the argument (when there is an actual argument) concerns whether it is acceptable for two groups to share the same opening point. By postulating unrealistic winds at opening altitude, there are those who can demonstrate that separation can be achieved as a result of canopy drift by the first group before the second group reaches that same opening point over the ground. Using their vocabulary, the first group will be in a different airmass than the second. Using mine, the first group will have moved away from the opening point, so the values of x, y, and z will have changed.



>>And what if the previous group has already landed? Would that make a difference? <<

Of course. The goals of exit separation are to avoid freefall collisions and to minimize the opportunity for canopy collisions at pulltime between members of different groups. If the first group has landed, then by definition there will not be a freefall collision or a collision at pulltime.

Well, I disagree with you. The only origin that makes sense is an origin in the airmass you are in. Canopies aren't drifting in that airmass, the ground is, but the people you are trying to avoid aren't on the ground.


bmcd308  (D 27472)

Feb 20, 2004, 9:49 AM
Post #166 of 246 (630 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>Well, I disagree with you. The only origin that makes sense is an origin in the airmass you are in. Canopies aren't drifting in that airmass, the ground is, but the people you are trying to avoid aren't on the ground. <<

I'm not sure where we have room to disagree. If we choose a different origin, one of the two of us will simply have to transalate into the other's coordinate system. At any time, t, neither of us wants to have the same x,y, and z coordinates as the other. At any time, t, it is possible to translate one system of defining a point in space to the other. We could just as easily translate it to polar coordinates with an origin in the peas, or polar coordinates with an origin that stays with the plane.

Brent


mnischalke  (D 26290)

Feb 20, 2004, 9:52 AM
Post #167 of 246 (629 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

What I am saying is that if they converged on your column of air and end up above you at any time during the jump, they're gonna be there when you deploy too. I just tried to explain the same thing in this thread.


kallend  (D 23151)

Feb 20, 2004, 9:55 AM
Post #168 of 246 (627 views)
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In reply to:
>>Well, I disagree with you. The only origin that makes sense is an origin in the airmass you are in. Canopies aren't drifting in that airmass, the ground is, but the people you are trying to avoid aren't on the ground. <<

I'm not sure where we have room to disagree. If we choose a different origin, one of the two of us will simply have to transalate into the other's coordinate system. At any time, t, neither of us wants to have the same x,y, and z coordinates as the other. At any time, t, it is possible to translate one system of defining a point in space to the other. We could just as easily translate it to polar coordinates with an origin in the peas, or polar coordinates with an origin that stays with the plane.

Brent

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.

What if a layer of light industrial haze obscured the ground? Whast relevance does your origin have now?


Premier billvon  (D 16479)
Moderator
Feb 20, 2004, 10:01 AM
Post #169 of 246 (625 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

>What I am saying is that if they converged on your column of air and
> end up above you at any time during the jump, they're gonna be
> there when you deploy too.

No. Winds change with altitude. If the winds drop 10kts for every 1000 feet, and you are 1000 feet directly above another group, they will move away from you at a speed of 10kts (17 feet per second.) After a 60 second freefall they would be 1020 feet from you. (Again, you wouldn't be jumping in wind conditions that bad, but it is still a factor to consider in moderate winds.)


eames  (D 23844)

Feb 20, 2004, 10:02 AM
Post #170 of 246 (625 views)
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In reply to:
What if a layer of light industrial haze obscured the ground? Whast relevance does your origin have now?

Exacly.... I asked this same question earlier in the thread. He just hasn't thought it out enough....

-Jason


mnischalke  (D 26290)

Feb 20, 2004, 10:11 AM
Post #171 of 246 (622 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

Your model assumes falling straight down the column of air. Mine (the only way someone could get above you given any real separation between groups) is considering sliding of either group. The farther through space the two groups get, considering a slide, the closer they get until the converge. The angle of the sliding group decides the point of convergance. So, the higher you open, the less the likelyhood of convergance.


eames  (D 23844)

Feb 20, 2004, 10:14 AM
Post #172 of 246 (618 views)
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In reply to:
At any time, t, it is possible to translate one system of defining a point in space to the other.

Prove it. Show me a change of basis for these two systems. It's impossible because there is no relationship between the two systems. Q.E.D.


kallend  (D 23151)

Feb 20, 2004, 10:26 AM
Post #173 of 246 (613 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
In reply to:
At any time, t, it is possible to translate one system of defining a point in space to the other.

Prove it. Show me a change of basis for these two systems. It's impossible because there is no relationship between the two systems. Q.E.D.

Of course there's a relationship.

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. 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.

The only thing I can possibly collide with is something at the same altitude I am.


velo90

Feb 20, 2004, 10:28 AM
Post #174 of 246 (612 views)
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In reply to:
What I am saying is that if they converged on your column of air and end up above you at any time during the jump, they're gonna be there when you deploy too. I just tried to explain the same thing in this thread.

I presume you are refering to your idea that pulling higher can give more seperation than going lower.
If this is the case, during normal wind conditions I would entirely agree. I am of the opinion, and I think you are, if the exit timing is right, there is no harm in pulling high. Pulling low may make thing worse.

Pulling low is bad news anyway, I have an AAD.


eames  (D 23844)

Feb 20, 2004, 10:35 AM
Post #175 of 246 (610 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Of course there's a relationship.

Once again, I've oversimplified. 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?

-Jason


(This post was edited by eames on Feb 20, 2004, 11:07 AM)


VectorBoy  (F 321)

Feb 20, 2004, 10:40 AM
Post #176 of 246 (710 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

  I finally figured out what was going on after I stopped thinking about the ground. The ground is irrelevant!
If the ground is irrelevant you could never have a bad spot = you can land anywhere with no problems. Some DZs have acres of landing outs. Others have swamps, windmill farms, angry livestock, angry livestock owners, shooting ranges, mental health and penal correctional facilities to contend with, oh and obstacles.
The fact of the matter is that the upper winds, the winds along your descent and the winds under canopy all matter, as does the ground speed of the aircraft ( GPS data for calculating jump run ) and whether you are on a floaty 20-way RW formation or a weight loaded solo speed dive.
On 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! we had a JM for this reason alone ( which we don't typically do ).On jump run the aircraft was flying much faster than a standard lighter wind day those that chose to float were told. The spot looked to be about 4 or more miles out on a jump run that resembled flying a radial up wind and away from the DZ in a completely foreign direction.
Dam if all this didn't provide for a perfect spot right over the landing area. Those that opened a little higher than planned, for lack of complete faith in the plan figured out for us, actually had a little more horizontal separation than the rest. The ones that opened lower as planned had a good spot but it was tighter.
Twenty seconds is a long time when the aircraft is miles from and getting farther from the DZ and you are watching scattered clouds ZIP by you.
I know better today and would never jump in conditions like that but it was hugely educational.

The fact is the optimal group sizing and sequencing has been figured out by the DZs that have vastly different disciplines and group sizes regularly. The bigger ones. Its not a small core group that hold to these beliefs. Its what happens to work the best.
Its what we teach here if you are hosed on the spot, pull higher. If we are running multiple aircraft its probably on an offset jump run of up to a quarter mile. Even if its not you have a few minutes of separation between jumpcraft under a canopy that will descend you a few thousand feet in those minutes.
Pulling higher will not work as well if other jumpers are following the 45 degree concept or the come out right on top of you concept, to be seen in your teams video after the camera man deploys, or a mixture of concepts that don't work together like a the WFFC which is really conceptless and dangerous.


eames  (D 23844)

Feb 20, 2004, 10:42 AM
Post #177 of 246 (707 views)
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Re: [VectorBoy] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
If the ground is irrelevant you could never have a bad spot

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


(This post was edited by eames on Feb 20, 2004, 10:44 AM)


bmcd308  (D 27472)

Feb 20, 2004, 11:03 AM
Post #178 of 246 (699 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>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


eames  (D 23844)

Feb 20, 2004, 11:08 AM
Post #179 of 246 (695 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

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


(This post was edited by eames on Feb 20, 2004, 11:21 AM)


eames  (D 23844)

Feb 20, 2004, 11:14 AM
Post #180 of 246 (693 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
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.

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  (D 27472)

Feb 20, 2004, 11:23 AM
Post #181 of 246 (690 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

>>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.


(This post was edited by bmcd308 on Feb 20, 2004, 11:55 AM)


eames  (D 23844)

Feb 20, 2004, 11:32 AM
Post #182 of 246 (685 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
What 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  (D 27472)

Feb 20, 2004, 11:48 AM
Post #183 of 246 (676 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>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


velo90

Feb 20, 2004, 11:51 AM
Post #184 of 246 (673 views)
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Re: [VectorBoy] Opening High for Bad Spots [In reply to] Can't Post

I think eames has pointed it out already. You are correct. The ground is very relevant when you consider where you want to land.


Quote:
On 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 Smile


eames  (D 23844)

Feb 20, 2004, 12:00 PM
Post #185 of 246 (667 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
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.

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


(This post was edited by eames on Feb 20, 2004, 12:04 PM)


velo90

Feb 20, 2004, 12:02 PM
Post #186 of 246 (666 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Quote:
I 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  (D 23844)

Feb 20, 2004, 12:04 PM
Post #187 of 246 (664 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
A collision occurs when two people attempt to occupy the same point in space at the same time.

Thanks, that's very profound.

In reply to:
I 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  (D 27472)

Feb 20, 2004, 12:06 PM
Post #188 of 246 (661 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>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


eames  (D 23844)

Feb 20, 2004, 12:15 PM
Post #189 of 246 (655 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>>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


(This post was edited by eames on Feb 20, 2004, 12:16 PM)


bmcd308  (D 27472)

Feb 20, 2004, 12:22 PM
Post #190 of 246 (653 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

>>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


bmcd308  (D 27472)

Feb 20, 2004, 12:31 PM
Post #191 of 246 (649 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>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.


(This post was edited by bmcd308 on Feb 20, 2004, 12:41 PM)


eames  (D 23844)

Feb 20, 2004, 12:32 PM
Post #192 of 246 (648 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
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.

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.

In reply to:
Canopy 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


(This post was edited by eames on Feb 20, 2004, 12:37 PM)


Premier billvon  (D 16479)
Moderator
Feb 20, 2004, 12:34 PM
Post #193 of 246 (645 views)
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Re: [mnischalke] Opening High for Bad Spots [In reply to] Can't Post

>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.


eames  (D 23844)

Feb 20, 2004, 12:36 PM
Post #194 of 246 (644 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
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 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.


Premier billvon  (D 16479)
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Feb 20, 2004, 12:37 PM
Post #195 of 246 (643 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>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.


bmcd308  (D 27472)

Feb 20, 2004, 12:45 PM
Post #196 of 246 (635 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

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.


eames  (D 23844)

Feb 20, 2004, 12:48 PM
Post #197 of 246 (632 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
That 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


(This post was edited by eames on Feb 20, 2004, 12:49 PM)


Premier billvon  (D 16479)
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Feb 20, 2004, 12:52 PM
Post #198 of 246 (630 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

>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.


eames  (D 23844)

Feb 20, 2004, 12:52 PM
Post #199 of 246 (629 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

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


bmcd308  (D 27472)

Feb 20, 2004, 1:13 PM
Post #200 of 246 (619 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

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.


velo90

Feb 20, 2004, 1:14 PM
Post #201 of 246 (751 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Quote:
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).

And Eames is correct in that assumption.

Quote:
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.

You don't have to rely on it, the way a number of people work these days they ignore it, in most cases to their advantage. When you understand why it makes a difference, you can save your ass on unusual days.

Quote:
Canopy drift is fine for additional separation

But sometimes it can give reduced separation.

And unless you can see that, you are out of touch with reality. Sorry, but the information has been provided in these forums often enough. Read, study and then you will understand why canopy drift is (sometimes) a factor that should not be ignored.


velo90

Feb 20, 2004, 1:20 PM
Post #202 of 246 (746 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Quote:
and leave half a second behind you, I might go whistling by as you were saddling out and then deploy right under you.
I doubt that very much. You would never catch him up. You'd be close, that true, but you would not be right under him.

Have you ever done tandem videos?


eames  (D 23844)

Feb 20, 2004, 1:23 PM
Post #203 of 246 (746 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
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.

That was not advice, I wrote, "By the way, I don't recommend trying it... I hope you found that obvious." Apparently you didn't.

I also never said "half a second." You said, "0.5 seconds."

Enough with semantics.

Quote:
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.

Do you dispute this?

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 1:24 PM
Post #204 of 246 (745 views)
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Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

Quote:
--------------------------------------------------------------------------------
Quote
--------------------------------------------------------------------------------


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).

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


And Eames is correct in that assumption.

Not necessarily. If that airmass is not moving (winds died at opening altitude), it will not have moved.

Quote:
But sometimes it can give reduced separation.

And unless you can see that, you are out of touch with reality.

Agreed.

Lower winds that push the skydivers back up the line of flight represent a special case that needs to be understood.

This whole most recent "argument" has been the result of my pointing out that the original argument was one that resulted from a difference in the definition of "separation" between the two sides.

I do not think I am actually arguing with anyone.

Brent


bmcd308  (D 27472)

Feb 20, 2004, 1:29 PM
Post #205 of 246 (742 views)
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Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

>>Do you dispute this? <<

Only to the extent that some were defining separation as the distance between opening points relative to the ground. If separation is defined as the distance between opening points relative to the ground, then groundspeed on jumprun matters rather than airspeed on jumprun.

Brent


velo90

Feb 20, 2004, 1:34 PM
Post #206 of 246 (738 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Not necessarily. If that airmass is not moving (winds died at opening altitude), it will not have moved.

can't argue with you there.
Quote:
This whole most recent "argument" discussion has been the result of my pointing out that the original argument was one that resulted from a difference in the definition of "separation" between the two sides.

Well actually I thought it started due to the idea that ground speed determines separation.
It doesn't.


eames  (D 23844)

Feb 20, 2004, 1:37 PM
Post #207 of 246 (736 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
If separation is defined as the distance between opening points relative to the ground, then groundspeed on jumprun matters rather than airspeed on jumprun.

Are the jumpers on the ground? Then why would you define separation in terms of the ground?

Actual separation (the distance between two groups when the second group opens, with respect to the ground or the airmass) does not change with groundspeed. It only changes with the 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. That's it. No matter what the groundspeed is.

-Jason


(This post was edited by eames on Feb 20, 2004, 1:42 PM)


eames  (D 23844)

Feb 20, 2004, 1:39 PM
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In reply to:
...definition of "separation" between the two sides

By the way, I'm not sure who is left on your side. I believe I can safely say that Hook now agrees with what we're saying.

-Jason


(This post was edited by eames on Feb 20, 2004, 1:49 PM)


bmcd308  (D 27472)

Feb 20, 2004, 1:50 PM
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>>By the way, I'm not sure who is left on your side. <<

I don't have a side. But if I did, Isaac Newton would be on it.


velo90

Feb 20, 2004, 1:57 PM
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Quote:
I don't have a side. But if I did, Isaac Newton would be on it.

Errr , Kallend is as near as we get to Isaac Newton here, trust him.


eames  (D 23844)

Feb 20, 2004, 1:57 PM
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In reply to:
I don't have a side. But if I did, Isaac Newton would be on it.

I award you zero points, we are all dumber for reading that, and may god have mercy on your soul. Wink

[That was a joke.]

I think we're done here.

-Jason


bmcd308  (D 27472)

Feb 20, 2004, 2:02 PM
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Let me run a quote from one of my very first posts on this thread by you again:

>>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. <<

I think you will see that I have not been specifically arguing with your choice of inputs into your separation model.

I will admit that as a practical matter I very much like the idea of opening at a different point over the ground than the one at which the group before me opened five seconds earlier in every case except the one where their airmass is moving toward my different point over the ground. And in that case, I would want a different jump run.

Brent


eames  (D 23844)

Feb 20, 2004, 2:08 PM
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In reply to:
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.

Depends on the windspeed at altitude (difference between the velocity of wind at jumprun altitude and opening altitude). If the difference is a large, yes, there is something to worry about. This all fits perfectly into the model that has been described.

In reply to:
I will admit that as a practical matter I very much like the idea of opening at a different point over the ground than the one at which the group before me opened five seconds earlier in every case except the one where their airmass is moving toward my different point over the ground. And in that case, I would want a different jump run.

That's fine, I'm not trying to tell you what to do.

-Jason


(This post was edited by eames on Feb 20, 2004, 2:24 PM)


kallend  (D 23151)

Feb 20, 2004, 3:21 PM
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In reply to:
>>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

So you are willing to bet your life on the reliability of the air at exit altitude and on the ground for landing, but at opening altitude you consider it unreliable. Interesting concept.


kallend  (D 23151)

Feb 20, 2004, 3:24 PM
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In reply to:
Quote:
I don't have a side. But if I did, Isaac Newton would be on it.

Errr , Kallend is as near as we get to Isaac Newton here, trust him.

I was in college with Isaac Newton. About 300 years later than him.


kallend  (D 23151)

Feb 20, 2004, 4:03 PM
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Quiz time.

1 kt is very close to 1.5 ft/sec, and using ft/sec is easier.

A balloonist and his girlfriend are floating along at 2500ft agl. He looks at his chart and sees there is a DZ below. Not that he can do anything about it. The DZ is near the ocean and an easterly wind is blowing him due west at 40ft/sec. He looks down and sees the DZ drift by below him. He feels not a breath of wind, because he is the wind. He lights a cigar.

He looks up and sees an Otter way above him, going west. He recalls the weather briefing, that at altitude the winds are prevailing westerlies, but down low the ocean effect has them in the opposite direction. He gets out his laser rangefinder and determines that the Otter is moving at 50 ft/sec relative to his balloon. A quick calculation tells him that the Otter's groundspeed is 50+40 = 90ft/sec. He knows an Otter flies jumprun at 140ft/sec, so he deduces the uppers are 50ft/sec from the west.

A 4-way team leaves the Otter (they saw the balloon but mistook it for a water tower). They turn 22 points. At 4000ft they are directly above the balloon, breakoff and track. Each tracks 300 ft before deploying. They are all open at 2500ft. One has line twists and is in a slow turn, another opened off heading, a third is stowing his slider while the other is messing with his booties. Each skydiver is 300ft from the balloon, in various directions. It is a cool sight.

The balloon pilot sees a second 4-way team leave the Otter after 11 seconds, (they figured 990ft should be ample separation from the first group based on the 90ft/sec groundspeed; that leaves 300ft for their tracks, 300ft for the other teams' tracks, and 330 ft as a buffer between the groups). They turn 26 points.

He sees the second team also break and track at 4000ft, each tracking 300ft. in equally spaced directions.

The girlfriend coughs because the cigar smoke is hanging around, there is no wind to disperse it.

How far from the balloon is the center of the second group, and is there a risk of collision between groups?

Note - It is not at all uncommon for upper and lower winds to be in opposite directions if you are near the Great Lakes, the ocean, or mountains.


(This post was edited by kallend on Feb 20, 2004, 6:15 PM)


mnischalke  (D 26290)

Feb 20, 2004, 4:12 PM
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Damn,

That was awesome! I especially liked the bit about the girlfriend coughing from the cigar smoke!

Very illustrative...Laugh


eames  (D 23844)

Feb 20, 2004, 4:46 PM
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In reply to:
How far from the balloon is the center of the second group, and is there a risk of collision between groups?

I figure the center of the second group will be about 1100 ft from the balloon. And No, there's no chance of collision because even if a member of each group tracked right at each other there would still be 500 ft between them. How'd I do?

Edit: They may have less than 500' between them if the member of the first group that was heading toward a member of the second group while tracking continued toward him under canopy. But, he couldn't travel 500' in 11 seconds under canopy in brakes, so there's still no change of a collision.

-Jason


(This post was edited by eames on Feb 20, 2004, 5:06 PM)


johnny1488  (D 25453)

Feb 20, 2004, 4:49 PM
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Damn I was gonna say 110 feet. My brain hurts, but I like itCool

And as for collision If they didnt already they are lucky because the second group overtook the first.


(This post was edited by johnny1488 on Feb 20, 2004, 4:51 PM)


kallend  (D 23151)

Feb 20, 2004, 6:18 PM
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In reply to:
In reply to:
How far from the balloon is the center of the second group, and is there a risk of collision between groups?

I figure the center of the second group will be about 1100 ft from the balloon. And No, there's no chance of collision because even if a member of each group tracked right at each other there would still be 500 ft between them. How'd I do?

Edit: They may have less than 500' between them if the member of the first group that was heading toward a member of the second group while tracking continued toward him under canopy. But, he couldn't travel 500' in 11 seconds under canopy in brakes, so there's still no change of a collision.

-Jason

No.


eames  (D 23844)

Feb 20, 2004, 6:43 PM
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In reply to:
No.

Okay, I think I see where I went wrong. The center of the second group is only 110 ft from the balloon and yes, there is a risk of collision between groups (and the balloon).

I think johnny1488 would have been right.

-Jason


mnischalke  (D 26290)

Feb 20, 2004, 7:47 PM
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horizontally, 550 ft. yes, if the closest jumpers on each group went directly along the line of flight.

(edit, I am an idiot. I reversed the groups in my first try. oops)


(This post was edited by mnischalke on Feb 20, 2004, 8:11 PM)


kallend  (D 23151)

Feb 20, 2004, 8:37 PM
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In reply to:
horizontally, 550 ft. yes, if the closest jumpers on each group went directly along the line of flight.

(edit, I am an idiot. I reversed the groups in my first try. oops)

You win.Smile

And the collision hazard doesn't depend on the presence of an observer in the balloon. He just provided a convenient origin.

The ground is irrelevant except as an approximate indicator of what is going on at opening altitude. All you need to know is that the plane is going 50ft/sec faster than the balloon, and multiply that by the exit delay 50 x 11 = 550ft. The groundspeed doesn't enter the equation, and it never does.

And real circumstances exist where blindly relying on groundspeed might kill you.


johnny1488  (D 25453)

Feb 20, 2004, 8:59 PM
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I see where I made my mistake also. This is starting to make a lot more sense to me now. Thanks for puttin up with me and others like me Blush


eames  (D 23844)

Feb 20, 2004, 9:04 PM
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In reply to:
50 x 11 = 550ft.

Don't you have to subtract the 40 x 11 = 440 ft that group one drifts while under canopy since the uppers and the lowers are heading in opposite directions?

-Jason


mnischalke  (D 26290)

Feb 20, 2004, 9:06 PM
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I do things the hard way, I guess. I used the 990 you gave me minus the 440 from the 11 seconds x 40 fps. Oh well

Btw, you certainly are the professor, with all that distracting bs mixed in there.

I guess, I could have pared it down further, but this is what I used:
Quote:
at 2500ft agl. an easterly wind is 40ft/sec.

at altitude the winds are westerlies 50ft/sec

A team leaves the Otter. open at 2500ft.

a second team leave after 11 seconds.

How far from (the center of the first group) is the center of the second group, and is there a risk of collision between groups?

Thanks for the fun!Tongue


kallend  (D 23151)

Feb 20, 2004, 9:18 PM
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In reply to:
In reply to:
50 x 11 = 550ft.

Don't you have to subtract the 40 x 11 = 440 ft that group one drifts while under canopy since the uppers and the lowers are heading in opposite directions?

-Jason

How can they drift when there's no wind (remember the cigar smoke).


eames  (D 23844)

Feb 20, 2004, 9:41 PM
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In reply to:
How can they drift when there's no wind (remember the cigar smoke).

They are the wind. Uppers and lowers are pointing in opposite directions.

11 sec in the door at 50 ft/sec in one direction, then 11 sec under canopy at 40 ft/sec in the other direction.

I guess I'll have to think about it more.

-Jason


mnischalke  (D 26290)

Feb 20, 2004, 9:47 PM
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Methinks kallend is fucking with us...Crazy

My head hurts already after second guessing myself earlier...

On another note: Don't we got a freshly-painted otter to play with tomorrow? hehe


velo90

Feb 20, 2004, 11:37 PM
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Excellent quiz Kallend Cool
You tell a nice stroy as well, I really liked the guy messing with his booties.
It's rather sad that 550 ft did not pop into peoples head straight away. But there again, unless you forget about the ground you will let all the references to the ground in your story affect how you answer. What's more, unless you ignore the ground, you never see all the other useful hints in your story.


eames  (D 23844)

Feb 21, 2004, 12:13 AM
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In reply to:
It's rather sad that 550 ft did not pop into peoples head straight away. But there again, unless you forget about the ground you will let all the references to the ground in your story affect how you answer.

I don't think I'm considering the ground at all. In fact, if the guy in the balloon saw another balloon in the upper winds, and used his handy laser rangefinder, he would see it moving 90 ft/sec. He would still see the Otter moving at 50 ft/sec in the other direction. The Otter and the upper balloon would be moving away from each other at (90 + 50) 140 ft/sec.

So the groups gain 550 ft of separtion from the upper winds and the plane, then lose 440 ft due the first group reversing direction in the lower winds for the 11 seconds before the second group opens.

Would you explain to me why I'm wrong?

-Jason


eames  (D 23844)

Feb 21, 2004, 12:13 AM
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Can you explain to me why I'm wrong?


eames  (D 23844)

Feb 21, 2004, 4:18 AM
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I hear bells ringing....

Now that I think of it, who cares about 50 ft/sec x 11 sec? That's not how fast the plane is flying through the upper airmass.

140 ft/sec x 11 sec = 1540 ft in the upper airmass.

-90 ft/sec x 11 sec = -990 ft (because of the difference in speeds of the air at exit altitude and opening altitude).

1540 ft + (-990 ft) = 550 ft

Ding, ding, ding. Man, if I had only plugged the numbers in according to how I've been preaching the whole time! Sorry for being slow.

Yay, we all agree!

-Jason


(This post was edited by eames on Feb 21, 2004, 4:39 AM)


eames  (D 23844)

Feb 21, 2004, 4:38 AM
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In reply to:
All you need to know is that the plane is going 50ft/sec faster than the balloon, and multiply that by the exit delay 50 x 11 = 550ft.

This is (of course) a better way to do it. But it took doing it with the real numbers to understand why.

Now I really have it down... conceptually and practically. Thanks for the quiz.

-Jason


diverdriver  (D 19012)

Feb 21, 2004, 7:05 AM
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Quote:
And real circumstances exist where blindly relying on groundspeed might kill you.

And what do you see me doing on days we have uppers out of the west and lowers out of the east?

I fly south to north or north to south offset along the river (Fox River next to SDC). Seperation.....no problem. (and then I can go back to relying on my groundspeed is god warm blankee Wink)


kallend  (D 23151)

Feb 21, 2004, 7:33 AM
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In reply to:
Quote:
And real circumstances exist where blindly relying on groundspeed might kill you.

And what do you see me doing on days we have uppers out of the west and lowers out of the east?

Wink)

In another thread I said I spent a lot of time watching a master.Tongue

Did I ever mention that all but one out-landings I've had were when a certain DZO was spotting?


kallend  (D 23151)

Feb 21, 2004, 7:35 AM
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In reply to:
In reply to:
All you need to know is that the plane is going 50ft/sec faster than the balloon, and multiply that by the exit delay 50 x 11 = 550ft.

This is (of course) a better way to do it. But it took doing it with the real numbers to understand why.

Now I really have it down... conceptually and practically. Thanks for the quiz.

-Jason

Now you're pleased that I didn't tell you, 'cos when you figure it out for yourself you're more likely to believe it.


diverdriver  (D 19012)

Feb 21, 2004, 7:38 AM
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In reply to:
Did I ever mention that all but one out-landings I've had were when a certain DZO was spotting?


You know, if he couldn't see the ground he was right on! But man, if it was a clear day look out!

LOL...I had him climb out early on me so many times. God Bless him. That sure was funny.


freeflybella  (D 21113)

Feb 21, 2004, 8:14 AM
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Sorry for coming into this late - especially when everyone now seems to agree . . .a question from a right-brained thinker.

On a normal jump day (normal conditions, ie, uppers same direction as lowers) what are the 'experts' agreeing to?

That groundspeed should or should not be used to determine time between exits? If not, please explain?

Please explain in a very practical way (look at ground winds, winds aloft, jump run direction, number/size of groups, etc.) instead of numbers and equations.


diverdriver  (D 19012)

Feb 21, 2004, 8:56 AM
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In reply to:
Sorry for coming into this late - especially when everyone now seems to agree . . .a question from a right-brained thinker.

On a normal jump day (normal conditions, ie, uppers same direction as lowers) what are the 'experts' agreeing to?

That groundspeed should or should not be used to determine time between exits? If not, please explain?

Please explain in a very practical way (look at ground winds, winds aloft, jump run direction, number/size of groups, etc.) instead of numbers and equations.


In the true form of talking about exit seperation the ground plays no part. However, the error caused by using groundspeed to reference seperation adds in an error that potentially increases actual seperation at opening altitude. So, that is why even after all this discussion it is still ok to use groundspeed AS LONG AS the upper winds are blowing in the same general direction (not opposite of) as the lower winds.

Does that help?


(This post was edited by diverdriver on Feb 21, 2004, 8:58 AM)


VectorBoy  (F 321)

Feb 21, 2004, 8:16 PM
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In reply to:
In reply to:
If the ground is irrelevant you could never have a bad spot

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
In reply to:


But people use the ground as a reference for their separation from the previous group.
What do you do when it apears that you are far from the DZ. Follow the procedure and allow what seems to be ridiculous interval between groups, wait till the previous group reaches the 45 degree point or just come out right on top of the group before you, say 2 seconds of separation? This is why I bring it up.


kallend  (D 23151)

Feb 21, 2004, 8:19 PM
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In reply to:
In reply to:
In reply to:
If the ground is irrelevant you could never have a bad spot

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
In reply to:




But people use the ground as a reference for their separation from the previous group.
What do you do when it apears that you are far from the DZ. Follow the procedure and allow what seems to be ridiculous interval between groups, wait till the previous group reaches the 45 degree point or just come out right on top of the group before you, say 2 seconds of separation? This is why I bring it up.


Ask for a go-around?


(This post was edited by kallend on Feb 21, 2004, 8:20 PM)


VectorBoy  (F 321)

Feb 21, 2004, 8:59 PM
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Ask for a go-around?
Sometimes you can, sometimes you can't as in the extreme jumprun example that I describe in my first post to this tread relating to Extreme winds aloft.

Try asking for a go around at WFFC or some major boogies. Some places talking to the pilot is verbotten ( to explain why you want a go around ). If the spot is good and the pilot can proove it there may be no go- around, you always have the option to ride down.


bmcd308  (D 27472)

Feb 23, 2004, 9:09 AM
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>>So you are willing to bet your life on the reliability of the air at exit altitude and on the ground for landing, but at opening altitude you consider it unreliable. Interesting concept. <<

What is worse, an off landing or a collision?

When I "rely" on the wind at exit altitude and on the ground for landing, all I'm doing is making sure that I land on the dz. If the wind dies at exit altitude, I wind up long. If the wind dies at 0 AGL, I might have to run out my landing.

While an off landing might be a contributor to an incident, an off landing is less likely to be an incident than a collision between a jumper in freefall and an open canopy.

I'm not sure the concept of being wary of allowing canopy drift to create separation is all that odd.

I don't advocate basing the entire exit separation decision on groundspeed, blindly assuming that canopy drift will not create problems. I am not even sure that anyone has advocated that position in this thread.

However, I do not advocate planning to have everyone open at the same spot over the ground, blindly assuming that the wind at 3 grand will push the previous group out of the way, either.

The policy implication of my position is that on days when the lower winds are roughly consistent in direction with the uppers, you use groundspeed to maximize separation between the groups, stringing them out along the available jump run. On days where the winds at opening altitude are roughly opposite the winds aloft, you run jump run crosswind or in an arc (like both DD and Mullins do).



Brent


kallend  (D 23151)

Feb 23, 2004, 11:29 AM
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In reply to:
>>So you are willing to bet your life on the reliability of the air at exit altitude and on the ground for landing, but at opening altitude you consider it unreliable. Interesting concept. <<

What is worse, an off landing or a collision?

When I "rely" on the wind at exit altitude and on the ground for landing, all I'm doing is making sure that I land on the dz. If the wind dies at exit altitude, I wind up long. If the wind dies at 0 AGL, I might have to run out my landing.

While an off landing might be a contributor to an incident, an off landing is less likely to be an incident than a collision between a jumper in freefall and an open canopy.

I'm not sure the concept of being wary of allowing canopy drift to create separation is all that odd.

I don't advocate basing the entire exit separation decision on groundspeed, blindly assuming that canopy drift will not create problems. I am not even sure that anyone has advocated that position in this thread.

However, I do not advocate planning to have everyone open at the same spot over the ground, blindly assuming that the wind at 3 grand will push the previous group out of the way, either.

The policy implication of my position is that on days when the lower winds are roughly consistent in direction with the uppers, you use groundspeed to maximize separation between the groups, stringing them out along the available jump run. On days where the winds at opening altitude are roughly opposite the winds aloft, you run jump run crosswind or in an arc (like both DD and Mullins do).



Brent

Hasn't this discussion left any impression? In the context of separation between skydivers at canopy opening time the ground is irrelevant. A fixed point over the ground is irrelevant. Canopy "drift" relative to the ground is irrelevant. The only thing having any relevance is the behavior of the air between exit altitude and opening altitude, inclusive.


bmcd308  (D 27472)

Feb 23, 2004, 12:26 PM
Post #246 of 246 (540 views)
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Re: [kallend] Opening High for Bad Spots [In reply to] Can't Post

OK. I'll give up trying to explain myself.



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