sundevil777 93
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With no tracking or sliding, people (bellyfliers) are going down at 100 knots and horizontally at 40 knots - they are traveling along an incline. For 2,500 feet of vertical separation there is a 1,000 foot horizontal component.
It's like being on an escalator. Everyone gets on at the same point, and off at the same point, but nobody is directly over anyone else.
It's basic physics - freshman stuff (at least where I went to school).
If so basic, you're making a pretty weak argument using this analogy. As soon as the jumpers get off the escalator, their horizonal speeds drop, and their vectors are in all sorts of directions. And in the real world, the entry and exit points of escalators are a major bottle neck.
And with the amount of backsliding or tracking (you really can't assume that out of the equation) and the different pull altitudes, that 1000ft of separation seems more mythical.
MWGemini 0
I'm with SunDevil on this one. Let's refrain from personal attacks. Explain or provide links to your proof, but don't insult us. I just downloaded Kallend's freefall simulator, and punching in a lot of different numbers, every time I put the Indicated Airspeed and Upper winds the same, the horizontal separation is very low (a few hundred feet, nowhere near your 1000ft claim). With 80 and 80 in both, and 0 for the below 6k winds, there is only 97 feet between deployment with a 3 second exit. That becomes 162 at 5 seconds. Far from 1k. How did you get that number?
Thanks,
Mike
Thanks,
Mike
Why don't you sit down and just write one post that explains your argument in such a way that us laymen can understand? (I'd tell you I have a degree in aerospace engineering, but you'd probably suggest I should give it back 
).
I'm 100% sure you understand this stuff. But it is very clear that you are doing a very poor job of explaining your position. No amount of playing with kallend's simulator is going to change anyone's mind because it only proves the point everyone else is trying to make.
How about the extreme example of a downwind jumprun? Still doesn't have any effect on necessary exit timing? What about the fact that as canopies open, they fly toward the airport, up the jumprun?
So write out your complete thoughts, and give us some scenarios to try in the simulator that prove your point.
Dave
). I'm 100% sure you understand this stuff. But it is very clear that you are doing a very poor job of explaining your position. No amount of playing with kallend's simulator is going to change anyone's mind because it only proves the point everyone else is trying to make.
How about the extreme example of a downwind jumprun? Still doesn't have any effect on necessary exit timing? What about the fact that as canopies open, they fly toward the airport, up the jumprun?
So write out your complete thoughts, and give us some scenarios to try in the simulator that prove your point.
Dave
billvon 2,406
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> 1000 feet from what?
That's the $64,000 question.
Imagine a jumper looking out the door of an otter, looking at people doing hop and pops, jumping over solid clouds. They are all going to open at exactly the same point - from HIS point of view. But they will not collide if they leave enough separation, because his point of view is moving 80 knots away from them as they open and stop with respect to the (relative) wind.
This is true even if the plane is bucking 80 knot winds! In that case, they all open in the same place from his point of view, AND they open in the same point even from the point of view of someone on the ground. Yet they still can open with safe separation. Why? Because if they leave enough time, the wind is going to give them sufficient separation.
(Now, it's probably not a good idea to do hop and pops in 80 knot uppers, but that's another issue.)
>You already acknowledge that everyone would open at the same point,
>everyone is affected by the winds during frefall the same.
But they are affected for different amounts of time depending on their fallrate, which is where the 'RW out first' thing comes from.
That's the $64,000 question.
Imagine a jumper looking out the door of an otter, looking at people doing hop and pops, jumping over solid clouds. They are all going to open at exactly the same point - from HIS point of view. But they will not collide if they leave enough separation, because his point of view is moving 80 knots away from them as they open and stop with respect to the (relative) wind.
This is true even if the plane is bucking 80 knot winds! In that case, they all open in the same place from his point of view, AND they open in the same point even from the point of view of someone on the ground. Yet they still can open with safe separation. Why? Because if they leave enough time, the wind is going to give them sufficient separation.
(Now, it's probably not a good idea to do hop and pops in 80 knot uppers, but that's another issue.)
>You already acknowledge that everyone would open at the same point,
>everyone is affected by the winds during frefall the same.
But they are affected for different amounts of time depending on their fallrate, which is where the 'RW out first' thing comes from.
sundevil777 93
Thanks Kid 35 for the link to the free Powerpoint viewer.
I can't understand why the simulation shows the plane going forward when the airspeed and winds aloft are equal.
I can't understand why the simulation shows the plane going forward when the airspeed and winds aloft are equal.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am
sundevil777 93
Quote> 1000 feet from what?
That's the $64,000 question.
Imagine a jumper looking out the door of an otter, looking at people doing hop and pops, jumping over solid clouds. They are all going to open at exactly the same point - from HIS point of view. But they will not collide if they leave enough separation, because his point of view is moving 80 knots away from them as they open and stop with respect to the (relative) wind.
This is true even if the plane is bucking 80 knot winds! In that case, they all open in the same place from his point of view, AND they open in the same point even from the point of view of someone on the ground. Yet they still can open with safe separation. Why? Because if they leave enough time, the wind is going to give them sufficient separation.
(Now, it's probably not a good idea to do hop and pops in 80 knot uppers, but that's another issue.)
>You already acknowledge that everyone would open at the same point,
>everyone is affected by the winds during frefall the same.
But they are affected for different amounts of time depending on their fallrate, which is where the 'RW out first' thing comes from.
Of course we have been arguing over jumpers with the same fallrate.
So you acknowledge that the opening point is the same, and we have to rely on the drift due to wind after opening. In my opinion and experience, it is very hard to get jumpers to adequately adjust their time between groups when the winds are really high. And then you have people tracking away up and down jump run and flying up and down jump run after opening. 10 seconds of drift from opening altitude winds doesn't give me much confidence unless the winds are comparable to the speed of the aircraft.
I don't like jumping when the winds at 3K are 90kts.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am
MWGemini 0
I've been wondering the same thing, shouldn't the exit point (deployment separation) be 0 if the airspeed and headwind are equal? Then, assuming completely identical fall rates, freefall times, and winds, the two jumpers would open at the exact same point in space. The only separation there is the temporal separation between jumpers and whatever the first jumper did under canopy during the gap. Right?
Mike
Mike
sundevil777 93
Professor, I have a question!
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am
billvon 2,406
>So you acknowledge that the opening point is the same, and we have to
> rely on the drift due to wind after opening.
Yes, but that's like asking "so I have to rely on random wind directions to get enough wind over the canopy to land safely?" Technically yes, but it's pretty dang reliable.
>In my opinion and experience, it is very hard to get jumpers to
> adequately adjust their time between groups when the winds are really
> high.
Agreed here. But that's because the time when you have to leave a lot of time is high uppers and not-as-high lowers, which is what happens 90% of the time. If there were a time when uppers were the same as lowers, and they were really high, separation wouldn't be a problem - but then again, no one would be jumping.
And to answer a previous question, crosswind jump runs are a good way to deal with some separation problems, provided you have good spotters.
> rely on the drift due to wind after opening.
Yes, but that's like asking "so I have to rely on random wind directions to get enough wind over the canopy to land safely?" Technically yes, but it's pretty dang reliable.
>In my opinion and experience, it is very hard to get jumpers to
> adequately adjust their time between groups when the winds are really
> high.
Agreed here. But that's because the time when you have to leave a lot of time is high uppers and not-as-high lowers, which is what happens 90% of the time. If there were a time when uppers were the same as lowers, and they were really high, separation wouldn't be a problem - but then again, no one would be jumping.
And to answer a previous question, crosswind jump runs are a good way to deal with some separation problems, provided you have good spotters.
billvon 2,406
> I've been wondering the same thing, shouldn't the exit point
>(deployment separation) be 0 if the airspeed and headwind are equal?
Depending on your frame of reference, that could well be true.
> Then, assuming completely identical fall rates, freefall times, and winds,
> the two jumpers would open at the exact same point in space. The only
> separation there is the temporal separation between jumpers and
> whatever the first jumper did under canopy during the gap. Right?
Well, no. If winds at opening are 80 kts, the only direction you are going is downwind. If you're facing into the wind you might be going 70kts instead of 80kts downwind, but there is zero chance you will be able to stay there to cause problems for the next group.
>(deployment separation) be 0 if the airspeed and headwind are equal?
Depending on your frame of reference, that could well be true.
> Then, assuming completely identical fall rates, freefall times, and winds,
> the two jumpers would open at the exact same point in space. The only
> separation there is the temporal separation between jumpers and
> whatever the first jumper did under canopy during the gap. Right?
Well, no. If winds at opening are 80 kts, the only direction you are going is downwind. If you're facing into the wind you might be going 70kts instead of 80kts downwind, but there is zero chance you will be able to stay there to cause problems for the next group.
MWGemini 0
Not sure what you mean by that.
Mike
Mike
sundevil777 93
QuoteNot sure what you mean by that.
Mike
Just raising my hand in cyberspace to Prof. Kallend.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am
MWGemini 0
QuoteWell, no. If winds at opening are 80 kts, the only direction you are going is downwind. If you're facing into the wind you might be going 70kts instead of 80kts downwind, but there is zero chance you will be able to stay there to cause problems for the next group.
Right, by the time the second jumper gets there, the first is already gone, but the second jumper would open at the same point in space, or am I mistaken?
Assuming my understanding of this is correct, and given a 5 second separation with 60mph winds at all altitudes, the jumpers would be 440ft away horizontally if the first jumper was under a round canopy (60mph = 1mpm = 5,280fpm = 88fps = 440 feet in 5 seconds). That's not including any vertical separation, which I'm too lazy to do the math for right now.
Mike
MWGemini 0
Another thing- since we've been (for simplicity's sake) assuming headwind and airspeed are equal (thus making groundspeed zero or negligible), we could also look at it from the perspective of a tethered balloon or other fixed point in space, which should give the same result in the end.
Mike
Mike
Quote>In my opinion and experience, it is very hard to get jumpers to
> adequately adjust their time between groups when the winds are really
> high.
Agreed here. But that's because the time when you have to leave a lot of time is high uppers and not-as-high lowers, which is what happens 90% of the time. If there were a time when uppers were the same as lowers, and they were really high, separation wouldn't be a problem - but then again, no one would be jumping.
If that's agreeing with winsor, then winsor seems to be wrong "90% of the time." He says that we adjust exit timing on windy days for the same reason dogs lick their balls. You are saying that we adjust exit timing when the uppers are strong and the lowers are light to increase separation at opening. Yeah I noticed winsor threw in a comment about the differential between the winds at exit and the winds at opening, but he seemed to ignore his own statement with most of the rest of what he said.
It's easy to see on kallend's simulator. Set the uppers just higher than the speed of the plane and the lowers to 0. There will be no separation at opening. But winsor has been suggesting (from my point of view anyway) that wind has no effect on necessary exit separation. He may have used cunning wording to make sure what he said was technically correct, but he may have also confused many people.
So billvon and winsor don't seem to be in agreement here... at least the way I'm reading this. Kallend? You're up!
Dave
billvon 2,406
>Right, by the time the second jumper gets there, the first is already
> gone, but the second jumper would open at the same point in
> space, or am I mistaken?
Right. You don't much care about the distance between people at the time they open; you care about the distance between people when the second group opens (i.e. between the second group opening and the first group that's been open for ten seconds.)
> gone, but the second jumper would open at the same point in
> space, or am I mistaken?
Right. You don't much care about the distance between people at the time they open; you care about the distance between people when the second group opens (i.e. between the second group opening and the first group that's been open for ten seconds.)
winsor 186
QuoteWhy don't you sit down and just write one post that explains your argument in such a way that us laymen can understand?
The notes from my seminar at WFFC are here.
Tammy turned it into HTML from Word, so I simply used it as is.
QuoteI'd tell you I have a degree in aerospace engineering, but you'd probably suggest I should give it back.
No, I have often been impressed by how many people of my acquaintance have advanced degrees (MS & PhD) yet never got beyond the level of expertise at the basics that they achieved during Freshman year. They may be able to set up second-order partial differential equations just fine, but if faced with, say, rotational dynamics, through which they struggled to get a C, they would do well to show C level comprehension even now.
QuoteI'm 100% sure you understand this stuff. But it is very clear that you are doing a very poor job of explaining your position.
I'm not trying very hard. From where I sit it's pretty obvious, and it is often most difficult to convey something that seems inherently apparent.
QuoteNo amount of playing with kallend's simulator is going to change anyone's mind because it only proves the point everyone else is trying to make.
The point being?
QuoteHow about the extreme example of a downwind jumprun? Still doesn't have any effect on necessary exit timing? What about the fact that as canopies open, they fly toward the airport, up the jumprun?
That scenario has two parts - freeefall separation and window of opportunity.
Freefall separation is based on the minimum relative speed of the aircraft with regard to any stratum of air throught which the jumpers pass. The extremes are generally found at either exit altitude or opening altitude, and the delay necessary to maintain separation at exit airspeed it the very least delay you should consider taking.
Without wind direction reversal (more common than most think), the speed of the aircraft w.r.t. the air at opening altitude is the lowest pertinent value. With a tailwind, speed of the aircraft v.r.t. that air at exit altitude is the lowest.
As the last part of my presentation describes, the window of opportunity on a pass is a function of speed over the ground. With a tailwind, your time on target can be short enough that you can get only one group out per pass without someone being hosed. With a headwind that reduces groundspeed to zero, you can put out one group after another all day long and maintain adequate separation at all times.
QuoteSo write out your complete thoughts, and give us some scenarios to try in the simulator that prove your point.
I no longer teach Physics for a living. If you are an engineer, you should have the study tools to find the readily available resources and figure it out for yourself. It's pretty basic stuff.
John Kallend is a professor, and actually has the patience to convey the fundamentals to a never-ending stream of neophytes.
Bill von Novak is a skydiving instructor, engineer and pilot, and also has much more patience than do I.
Ask them.
Blue skies,
Winsor
winsor 186
So billvon and winsor don't seem to be in agreement here... at least the way I'm reading this.
We're talking two different things here. Regarding the speed of the aircraft over the airmass at opening altitude we're in agreement. The other factor is TOT - time on target.
If you have 7 groups getting out, and figure the minimum "safe" separation requires a 10 second delay, it takes 60 seconds to clear the plane.
If, however, the length of time during which you can exit and make it back to the DZ without problems is 90 seconds (your time on target), you may well use the full 15 seconds between groups instead of the nominal 10 seconds.
You take the extra time for the same reason as Fido - "because you can."
There is no disagreement regarding the fundamentals of which I am aware.
Blue skies,
Winsor
If you jump from a stationary platform up in the sky, you will end up at the same place as the person before you, given the same set of wind conditions and same fall rate. I think you agreed on this point. It is the only thing that matters on this particular part of the scenario we are discussing, I believe.
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