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Re: [Martini] Opening High for Bad Spots

 

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bmcd308  (D 27472)

Feb 19, 2004, 2:12 PM
Post #126 of 246 (659 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 (658 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 (655 views)
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Re: [bmcd308] Opening High for Bad Spots [In reply to] Can't Post

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 (651 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 (649 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 (649 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 doesnt 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 (645 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 (645 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 (644 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 dont 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 (641 views)
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Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

>That doesnt 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 (632 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 (628 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)
Moderator
Feb 19, 2004, 2:42 PM
Post #138 of 246 (623 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 (622 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)
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Feb 19, 2004, 3:07 PM
Post #140 of 246 (610 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 (610 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 (608 views)
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Re: [billvon] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
>That doesnt 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)
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Feb 19, 2004, 3:13 PM
Post #143 of 246 (606 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 (602 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 (602 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 (595 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 (585 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 (574 views)
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Re: [diverdriver] Not listening. [In reply to] Can't Post

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 (572 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 (571 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 doesnt 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.


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