eames
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Everything posted by eames
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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.
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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. 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
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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
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Thanks, that's very profound. It does, and so does the plane carrying the next ground to a different point in the airmass. -Jason
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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
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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
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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
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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
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Nobody is talking about spotting here... we're talking about separation. Of course the ground is relevant for spotting: You're trying to land on a specific "spot" on the ground. -Jason Edit: Spelling
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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
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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.
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Exacly.... I asked this same question earlier in the thread. He just hasn't thought it out enough.... -Jason
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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
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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
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Yeah, that's funny. -Jason
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This is true, but anywhere in between there will still be separation, will there not? 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. Same here. -Jason
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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
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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
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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
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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
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No, in actuality, your separation would be the same no matter what your ground speed. 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
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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
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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
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Relative to the moving airmass, yes. -Jason
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They would, if they were not subject to canopy opening (sudden shallow descent-rate) followed by controlled canopy flight. -Jason