Recommended Posts
billvon 3,006
.thumb.jpg.4bb795e2eaf21b8b300039a5e1ec7f92.jpg)
The same formula always works. Your separation is:
Separation = (aircraft's ground speed + wind at opening altitude) * number of seconds between groups.
So if you are in an Otter going 80kts into a 20kt headwind, and you want 1000 feet of separation, and there are light winds at 3000 feet, you need 8 seconds flying upwind and 5 seconds flying downwind. Since it's rare that any group can check the spot, get into position, give the count and go in under 5 seconds, you can probably just go as fast as possible.
Separation = (aircraft's ground speed + wind at opening altitude) * number of seconds between groups.
So if you are in an Otter going 80kts into a 20kt headwind, and you want 1000 feet of separation, and there are light winds at 3000 feet, you need 8 seconds flying upwind and 5 seconds flying downwind. Since it's rare that any group can check the spot, get into position, give the count and go in under 5 seconds, you can probably just go as fast as possible.
kallend 2,027
Actually, you are trying to space out the group of jumpers along the ground track of the jump aircraft, so ground speed has everything to do with it. Why do you think they use more time between groups when the winds aloft are cranking? It's because the groundspeed of the air craft is reduced.Quote***
A) What your ground speed measures has precious little to do with separation in the air. Minimum time between groups is still minimum time between groups; a function of air speed. This is especially true for a downwind jump run.
B) The temperature has even less to do with separation than does ground speed.
Blue skies,
Winsor
As far as the temperature (45 degrees), that was funny.
Groundspeed only works if taken in conjunction with the windspeed at opening altitude. That is what Winsor was saying in his own inimitable way. He is quite correct. If your opening altitude is zero feet agl, then groundspeed is the magic number. Personally I prefer to open higher.
...
The only sure way to survive a canopy collision is not to have one.
The only sure way to survive a canopy collision is not to have one.
kallend 2,027
QuoteOkay even though I said I was done (which I will be soon), you don't seem to quote me out of context so here's my take.
Yes I agree that the 45 degree rule alone isn't right. So one is in the door and they count ... whatever it is they were counting to ... then do they blindly just jump or do they look and if they look what happens if the group ahead of them isn't really moving all that much ... well they counting so according to some of the people here it's okay to jump right? I say no!!! If time and the group ahead of you hasn't tracked off (say 45 say something else) you don't jump, you continue to wait. Vertical seperation doesn't really mean much if I have a reserve ride and fall in on top of the group in front of me. Horizontal separation is everything and this is a factor of time in the door and using our eyes to ensure that we aren't jumping before we should because we didn't really properly understand how much the aircraft might be tracking across jump run.
People remember what the original poster was asking in this thread. How much seperation is needed on a downwind jump run. The answer is dependent upon the winds aloft and generally it's not as much time.
I realize that some of you are just interested in making another "this guy doesn't know shit" argument because he mentioned the god-aweful 45 degree rule. But go back to the original responses and read what was said. Not as much time is needed when the aircraft is hauling ass due to low winds aloft and extra time is needed when the winds aloft are strong.
You are missing the point. The angle made with the plane by the previous group has as much correlation with the separation you will get as does the number of fleas on my dog. This is absolutely provable using Newton's laws, and for those that don't think physics has relevance in the "real world", it is also provable by experiment.
So using the "45 degree rule" in combination with time is as sensible as using the number of fleas on my dog in combination with time to ensure separation.
Get it?
...
The only sure way to survive a canopy collision is not to have one.
The only sure way to survive a canopy collision is not to have one.
billvon 3,006
>If time and the group ahead of you hasn't tracked off (say 45 say
> something else) you don't jump, you continue to wait.
Then each group would need their own pass. Out of an Otter flown at normal speeds, the group never passes the 45 degree line.
> something else) you don't jump, you continue to wait.
Then each group would need their own pass. Out of an Otter flown at normal speeds, the group never passes the 45 degree line.
QuoteWould the second problem with a downwind jump run, in your opinion, be that faster-falling groups have to exit before slower-falling groups (i.e. the order is reversed), to avoid having the slower fallers drift under the faster fallers?
That's what I originally thought. But after playing with it in kallend's simulator, I found that it didn't work as I expected. Problem is that when the fast fallers go out first, they open significantly earlier than the slower fallers, giving them more time to drift under canopy underneath the slow fallers still in freefall. Having fast fallers go out last does bring them closer up high, but with any realistic delay, they still open first, but begin to drift out of the way, increasing separation.
In reality, I'd prefer to just avoid a downwind jumprun (in significant winds) because I wouldn't trust anyone on the plane to give me a safe exit separation. While it seems everyone knows the 45 degree rule doesn't work, I still do hear "watch the angle the previous group makes with the plane, and don't go till they're far enough back." I've never heard anyone compare upper winds with winds at opening altitude to judge exit separation. But at least everyone's experienced with upwind jump runs. I just wouldn't trust their judgement for a downwind jumprun.
Dave
winsor 236
Actually, you are trying to space out the group of jumpers along the ground track of the jump aircraft, so ground speed has everything to do with it. Why do you think they use more time between groups when the winds aloft are cranking? It's because the groundspeed of the air craft is reduced.Quote***
A) What your ground speed measures has precious little to do with separation in the air. Minimum time between groups is still minimum time between groups; a function of air speed. This is especially true for a downwind jump run.
B) The temperature has even less to do with separation than does ground speed.
Blue skies,
Winsor
Actually, I couldn't care less about how jumpers are spaced over the ground. My primary concern with regard to spacing between groups is keeping people apart in the air.
People confuse spotting with separation. They are both considerations, but hardly interchangeable.
Blue skies,
Winsor
QuotePeople confuse spotting with separation. They are both considerations, but hardly interchangeable.
I think it would help if you explained yourself a little more. You've confused enough people with your joke about the 45 degree rule.
You know very well that the wind speed plays a role in necessary exit timing to give appropriate separation in the air. The ground may not matter, but airspeed and wind speed do. For simplicity it's usually pretty accurate to say groundspeed matters.
If the airspeed is 80 kts and wind speed is 80 kts in the opposite direction, ground speed is zero and there is no possible exit timing to produce safe horizontal separation. On the other hand, a hot air balloon has an airspeed of 0, but with 80 kts of wind it has a groundspeed of 80 kts. Assuming constant wind, there's still no possible safe exit timing. So ground speed isn't exactly the number we're looking for. But for a typical plane dropping jumpers in typical conditions, ground speed at altitude is a good indicator.
Dave
bmcd308 0
>>If the airspeed is 80 kts and wind speed is 80 kts in the opposite direction, ground speed is zero and there is no possible exit timing to produce safe horizontal separation. On the other hand, a hot air balloon has an airspeed of 0, but with 80 kts of wind it has a groundspeed of 80 kts. Assuming constant wind, there's still no possible safe exit timing. So ground speed isn't exactly the number we're looking for. But for a typical plane dropping jumpers in typical conditions, ground speed at altitude is a good indicator. <<
If that 80 kts windspeed holds up all the way to the ground, then there can be sufficient separation at opening altitude despite the fact that everyone will open at the same point above the ground. A premature deployment will not result in a collision in that case. If no one pulls, they will all land in a pile in the same spot on the ground, but as long as everyone pulls, it is possible to space them 1,000 horizontal feet apart at opening altitude.
While I agree that groundspeed is a useful tool for determining exit separation most of the time, it does not work when the winds get very strong, or when the lower winds are opposite the uppers. I used to make an argument similar to yours all the time, but I have grown weary of defending a position that "sort of works."
Brent
----------------------------------
www.jumpelvis.com
If that 80 kts windspeed holds up all the way to the ground, then there can be sufficient separation at opening altitude despite the fact that everyone will open at the same point above the ground. A premature deployment will not result in a collision in that case. If no one pulls, they will all land in a pile in the same spot on the ground, but as long as everyone pulls, it is possible to space them 1,000 horizontal feet apart at opening altitude.
While I agree that groundspeed is a useful tool for determining exit separation most of the time, it does not work when the winds get very strong, or when the lower winds are opposite the uppers. I used to make an argument similar to yours all the time, but I have grown weary of defending a position that "sort of works."
Brent
----------------------------------
www.jumpelvis.com
bmcd308 0
>>B) The temperature has even less to do with separation than does ground speed. <<
LMAO.
----------------------------------
www.jumpelvis.com
LMAO.
----------------------------------
www.jumpelvis.com
billvon 3,006
>If the airspeed is 80 kts and wind speed is 80 kts in the opposite
>direction, ground speed is zero and there is no possible exit timing to
>produce safe horizontal separation.
Not true at all. Exit separation = (groundspeed of aircraft + winds at opening alt) * seconds. In your scenario, the plane will be hovering over one spot on the ground - yet if people get out and do clear and pulls, with 5 seconds between groups, they will end up with 530 feet of separation between them. And 530 feet is plenty of room for solo clear and pulls. Yet the plane isn't moving! How can this be? Because the winds are 80kts at altitude, and no parachute in the world can keep up with that. It's the separation in the _air_, not over the ground, that matters.
If, in the same scenario, they do a freefall and open in 40 kt winds, the same separation will give them 260 feet of separation. Waiting 20 seconds between groups would give them 1060 feet, which should be plenty.
However, if the winds at opening altitude are zero (unusual with 80kts of uppers) then you are correct - they will pile into each other.
>On the other hand, a hot air balloon has an airspeed of 0, but with 80 kts
>of wind it has a groundspeed of 80 kts. Assuming constant wind, there's
>still no possible safe exit timing.
Again, not true. If winds at opening altitude are 40kts they will get a separation of 53 feet for every second they wait.
>So ground speed isn't exactly the number we're looking for. But for a
>typical plane dropping jumpers in typical conditions, ground speed at
>altitude is a good indicator.
Agreed; it's a good approximation most of the time, since winds at opening altitude are generally lighter and from the same direction. Under those conditions, more wind at opening altitude only helps you, so if you ignore it you get more separation, not less.
>direction, ground speed is zero and there is no possible exit timing to
>produce safe horizontal separation.
Not true at all. Exit separation = (groundspeed of aircraft + winds at opening alt) * seconds. In your scenario, the plane will be hovering over one spot on the ground - yet if people get out and do clear and pulls, with 5 seconds between groups, they will end up with 530 feet of separation between them. And 530 feet is plenty of room for solo clear and pulls. Yet the plane isn't moving! How can this be? Because the winds are 80kts at altitude, and no parachute in the world can keep up with that. It's the separation in the _air_, not over the ground, that matters.
If, in the same scenario, they do a freefall and open in 40 kt winds, the same separation will give them 260 feet of separation. Waiting 20 seconds between groups would give them 1060 feet, which should be plenty.
However, if the winds at opening altitude are zero (unusual with 80kts of uppers) then you are correct - they will pile into each other.
>On the other hand, a hot air balloon has an airspeed of 0, but with 80 kts
>of wind it has a groundspeed of 80 kts. Assuming constant wind, there's
>still no possible safe exit timing.
Again, not true. If winds at opening altitude are 40kts they will get a separation of 53 feet for every second they wait.
>So ground speed isn't exactly the number we're looking for. But for a
>typical plane dropping jumpers in typical conditions, ground speed at
>altitude is a good indicator.
Agreed; it's a good approximation most of the time, since winds at opening altitude are generally lighter and from the same direction. Under those conditions, more wind at opening altitude only helps you, so if you ignore it you get more separation, not less.
winsor 236
QuoteQuotePeople confuse spotting with separation. They are both considerations, but hardly interchangeable.
I think it would help if you explained yourself a little more.
How's this grab you?
Oh, and Bill's algorithm - Exit separation = (groundspeed of aircraft + winds at opening alt) * seconds - is, in fact, the punch line.
Blue skies,
Winsor
QuoteExit separation = (groundspeed of aircraft + winds at opening alt) * seconds
That's all I wanted to hear.
Dave
winsor 236
QuoteQuoteExit separation = (groundspeed of aircraft + winds at opening alt) * seconds
That's all I wanted to hear.
Just so you know that the parameter in consideration here is the speed of the aircraft with regard to the air mass at opening altitude. The expression in parenthesis provides that value.
Uncorrected for the winds at opening altitude, groundspeed can give you an approximation. It does not, however, provide "the answer," which can be demonstrated both theoretically and experimentally.
In the case of a downwind jump run, effective speed is significantly less when corrected for winds at opening altitude than it would appear if groundspeed alone is taken into consideration.
As a matter of practice, a fixed minimum time between groups should likely be related to the true airspeed of the aircraft. If you want to increase the time between groups above that minimum, doing so by means of some systematic approach is advised. Going below the fixed minimum time between groups has all the appeal of low-pull contests. Getting away with it repeatedly does not make it a good idea.
Blue skies,
Winsor
I used to think that too, and it's true, if your not gonna pull. If you're gonna pull, then pull altitude is where you need the seperation.
Now in most wind conditions, they are real close, but on a day with the wind at 2 different altitudes moving opposite of each other, the ground as a reference goes out the window (or the door).
Johnny
--"This ain't no book club, we're all gonna die!"
Mike Rome
Share this post
Link to post
Share on other sites