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murps2000 86
Groundspeed, in and of itself, has nothing whatsoever to do with separation in the air.***
I certainly won't argue that point given the lessons in both physics and humility provided me by you, Prof. Kallend, and Billvon, but I do have a question.
Since GPS does nothing to provide infromation about jumprun speed relative to the airmass at opening altitude, is there a reasonably accurate way for the avg. jumper on the DZ to calculate required separation without having access to Prof. Kallend's program?
I certainly won't argue that point given the lessons in both physics and humility provided me by you, Prof. Kallend, and Billvon, but I do have a question.
Since GPS does nothing to provide infromation about jumprun speed relative to the airmass at opening altitude, is there a reasonably accurate way for the avg. jumper on the DZ to calculate required separation without having access to Prof. Kallend's program?
winsor 236
Groundspeed, in and of itself, has nothing whatsoever to do with separation in the air.***
I certainly won't argue that point given the lessons in both physics and humility provided me by you, Prof. Kallend, and Billvon, but I do have a question.
Since GPS does nothing to provide infromation about jumprun speed relative to the airmass at opening altitude, is there a reasonably accurate way for the avg. jumper on the DZ to calculate required separation without having access to Prof. Kallend's program?
Reasonably accurate? Sure thing.
Average jumper? I guess that's the rub.
A veteran Physics Professor at my alma mater, Dr. Ralph Heller, once turned to a Freshman class who had just blown an exam and said, in frustration, "I've been teaching this course for thirty five years. You should know this by now!"
Thus, something that seems blindingly obvious to an engineer or physicist might take a bit of pondering for someone who has not lived, eaten and breathed it for decades. Please bear with me, since I do not remember what it was like not to know this material (no kidding).
I suggest that you break it down into two parts. Come up with a basic minimum delay between groups for your dropzone's jumprun style (all pilots and aircraft are not created equal), and then come up with a simple fudge-factor to account for uppers.
A thousand feet between group centers is a good place to start. It is a nice, round number, and allows everyone 400 feet of latitude with 100 feet of opening surge or line twist to avoid someone from another group. To put it into perspective using an airport, if the first group leaves when you are just over the threshold of a 3,000 foot runway, you can put out three more groups by the time you are over the end of the runway.
At 100 knots (true), it takes 6 seconds to cover 1,000 feet. Thus, for groups that might slide around at all and may be expected to track, you should consider 6 REAL seconds to be your minimum time between EXITS. If you know your climbout is at least four seconds, you can count "one thousand, two thousand," begin your climbout, and expect MINIMUM separation with NO WIND.
Working out the uppers is no big deal if you live in a cockpit and watch the GPS readout for hours out of boredom, at various altitudes and weather conditions. Since there are a lot of variables in exit and freefall, being overly exact buys you nothing, so I will recommend an approach that is sure to be misconstrued. If you use the groundspeed readout of the GPS (the pilot will usually pass it back if you ask) for the basis of your timing, this will USUALLY give you separation greater than the fast calculations indicate.
The second part of my fast and dirty approach is this: take the bare minimum delay for separation that you have worked out (say 6 seconds at 100 knots) and INCREASE it on the basis of your speed over the ground.
If your ground speed is 75 knots, for instance, you are only going 3/4 as fast as your basis speed so you have to wait 4/3 as long between groups to have the same nominal separation. So, for 6 seconds at 100 knots you would delay 4/3 * 6 = 8 seconds when you have a 75 knot readout on the GPS. Similarly, 66 knots is 2/3 of 100 knots, so the appropriate delay would be 3/2 * 6 = 9 seconds between groups.
Since you are counting seconds in a very inexact way, you only need to work out a few benchmark speeds that you use to up the count.
For example:
At 100 knots (dead air - very rare) - 6 seconds delay
Anything less than 100 knots - 7 second delay
Less than 85 knots - 8 seconds delay
less than 75 knots 9 seconds
Less than 65 knots - 10 seconds
Less than 60 knots - 11 seconds
Less than 55 knots - 12 seconds
Less than 50 knots - 13 seconds
and so forth.
50 knot uppers are getting moderately serious, and it up to you to decide how much to commit to memory before you would not jump anyway.
While ground speed is not related directly to separation in the air, it can still be used to come up with a MINIMUM delay between groups that will result in workable separation.
I make no guarantees, since someone on the first group that has the bright idea of having an impromptu tracking dive after funneling can be under the LAST group at opening altitude (it has happened).
Thus, if you work out some basic numbers in advance, pick a MINIMUM delay between groups with which everyone agrees, and stay HEADS-UP regardless, this approach is as good as any.
Blue skies,
Winsor
When you have high winds at jump run altitude, but lower wind speeds at pull altitude, wouldn't that tend to compress the jump groups?
bmcd308 0
>>When you have high winds at jump run altitude, but lower wind speeds at pull altitude, wouldn't that tend to compress the jump groups? <<
Sort of. Think of an example of a jump run at 12K feet with uppers almost equal to the plane's minimum airspeed, then no wind from 11,999 feet all the way to the ground. So the plane is almost standing still over the ground.
You will never have LESS horizontal separation than you did when you exited (assuming everyone falls straight down), so no "compression," but unless you leave lots of time, you will never have much at all.
Note that in this example, since there is no wind from 11,999 all the way down, the "airmass at opening altitude" is not moving over the ground.
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www.jumpelvis.com
Sort of. Think of an example of a jump run at 12K feet with uppers almost equal to the plane's minimum airspeed, then no wind from 11,999 feet all the way to the ground. So the plane is almost standing still over the ground.
You will never have LESS horizontal separation than you did when you exited (assuming everyone falls straight down), so no "compression," but unless you leave lots of time, you will never have much at all.
Note that in this example, since there is no wind from 11,999 all the way down, the "airmass at opening altitude" is not moving over the ground.
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www.jumpelvis.com
winsor 236
When you have high winds at jump run altitude, but lower wind speeds at pull altitude, wouldn't that tend to compress the jump groups?
Yes.
The key here is the speed of the aircraft with regard to the air mass at opening altitude.
Using my tethered balloon example (the DEA has some that hang out at 14,000 feet) in significant winds that are twice as high at exit altitude as at opening altitude, successive jumpers with a nominal separation of 1,000 feet at exit would have only 500 on opening - and ZERO on landing.
Frames of Reference problems cause more than their share of conceptual heartache among students of the subject. Once these problems make sense, they are no big deal.
Blue skies,
Winsor
billvon 3,049
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>When you have high winds at jump run altitude, but lower wind speeds
>at pull altitude, wouldn't that tend to compress the jump groups?
Yes. The formula is: separation = (groundspeed of aircraft + windspeed at opening) * number of seconds, as long as jump run is into the winds and the winds don't change direction on the way down. So if your groundspeed is 100 feet per second, and the airspeed at opening altitude is zero, then ten seconds will give you 1000 feet, which is likely plenty of room. If groundspeed is 100 feet per second and the windspeed at opening altitude is 20kts, then you get even more separation.
The danger area is when the winds at opening altitude are OPPOSITE direction than the winds on jump run, because then you have to subtract the windspeed from the groundspeed. This happens here on occasion because the lower winds are almost always out of the west, but uppers can get to be out of the east. When this happens, it can be very hard to leave enough separation. In the above example, if groundspeed is 100 feet per second and winds are 20 kts (34fps) then ten seconds gives you only 660 feet, which is getting pretty tight for medium sized RW.
Fortunately at most DZ's this is an unusual occurrence, so just relying on groundspeed _usually_ works OK.
>at pull altitude, wouldn't that tend to compress the jump groups?
Yes. The formula is: separation = (groundspeed of aircraft + windspeed at opening) * number of seconds, as long as jump run is into the winds and the winds don't change direction on the way down. So if your groundspeed is 100 feet per second, and the airspeed at opening altitude is zero, then ten seconds will give you 1000 feet, which is likely plenty of room. If groundspeed is 100 feet per second and the windspeed at opening altitude is 20kts, then you get even more separation.
The danger area is when the winds at opening altitude are OPPOSITE direction than the winds on jump run, because then you have to subtract the windspeed from the groundspeed. This happens here on occasion because the lower winds are almost always out of the west, but uppers can get to be out of the east. When this happens, it can be very hard to leave enough separation. In the above example, if groundspeed is 100 feet per second and winds are 20 kts (34fps) then ten seconds gives you only 660 feet, which is getting pretty tight for medium sized RW.
Fortunately at most DZ's this is an unusual occurrence, so just relying on groundspeed _usually_ works OK.
Cloudi 0
You guys are making my brain hurt! 
This is a great thread with lots of usable info for those of us who are mathematically challenged...
Where else but in skydiving can "separation" be such a good thing?!
Kim
Watch as I attempt, with no slight of hand, to apply logic and reason.
This is a great thread with lots of usable info for those of us who are mathematically challenged...
Where else but in skydiving can "separation" be such a good thing?!
Kim
Watch as I attempt, with no slight of hand, to apply logic and reason.
no i was not chasing. as i stated i was parked falling straight down so she can tell what her movements were. thats what i meant by a human spaceball. she cant learn when shes falling true if im making any movements tword her. she would make a few attempts to drive tword me on her head then transistion to a sit and drive back to me transistion back to her head and repeat. my job on this skydive was to keep her facing north south, on a east west jumprun. as well as a point of reference so she can see if shes falling true, backsliding, or forward driving. thanks for the feed back but i think were on the same page. i just didnt give full details of our diveplan. with this dive plan theres no reason to get out last.
and i should have known someone would turn this into an exit separation debate!!!! not saying you are gg. god that arguement is so old people!!!! let it go!
and i should have known someone would turn this into an exit separation debate!!!! not saying you are gg. god that arguement is so old people!!!! let it go!
>
When someone is back sliding, don't just chase straight at them, chase to one side.
Often people who don't realize they are back sliding do have heading control. If you chase to one side, they will keep adjusting so they are facing you. This will keep them backsliding in a circle and prevent you from drifting into the previous or next group if enough seperation was given.
If they are backsliding so hard that you can't get to them fast enough to make them turn, well you probably shouldn't be jumping together yet, but if you find yourdelf in this situation, simply stop chasing them.
Continuing to stay with them just convinces them everything is ok, and it puts 2 people where they shouldn't be instead of just one.
Methane Freefly - got stink?
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