Forums: Skydiving: Safety and Training:
Re: [Martini] Opening High for Bad Spots

 

First page Previous page 1 2 3 4 5 6 7 8 9 10 Next page Last page  View All

Hooknswoop  (D License)

Feb 18, 2004, 6:42 AM
Post #1 of 246 (2480 views)
Shortcut
Re: [Martini] Opening High for Bad Spots Can't Post

Quote:
Derek, I have to disagree with you on the separation/groundspeed thing. The aircraft and the jumpers aren't influenced by the ground or it's relative speed, the ground doesn't even exist for them (unless ya get too close). Aircraft and it's spawn, that's you and me, only know the air.

Airspeed for a particular jump ship tends to be constant for each jump run. If the jump ship always flies into the upper winds for jump run, then as the upper wind speed increases, the jump ship’s ground speed decreases and more time must be left between exits in order to achieve the same amount of separation. The pilot could increase the aircraft’s jump run speed equal to the upper winds (if the aircraft had the performance to do so) and the amount of time between exits could remain constant, but this isn’t done. Therefore, exit separation is a function of the aircraft’s ground speed on jump run. In order to ensure separation between groups the following groups must look at the ground and allow sufficient ground distance to be covered before exiting. This is why the 45-degree rule for exit separation does not work.

So, as a jump ship’s ground speed decreases, more time must be left between exits to ensure the same amount of separation. This is why I said that separation is a function of ground speed. Ground speed (for a particular jump ship) is the variable, since airspeed is constant. Ground speed is used to adjust the length of time between groups.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 6:45 AM)


velo90

Feb 18, 2004, 6:52 AM
Post #2 of 246 (2444 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Hooknswoop, what you have just said roughly works for the normal case but....
Why do we keep having to redo the problem? What's even worse is there are a number people taking part in this thread that took part in all the other threads about exit seperation and they have not learned anything!
It's obvious numerous people never even bothered to look at Kallend's web site, or if they did, they never understood what was presented there.

Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.


Hooknswoop  (D License)

Feb 18, 2004, 7:14 AM
Post #3 of 246 (2433 views)
Shortcut
Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.

If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground?

Quote:
Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

Quote:
That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Derek


velo90

Feb 18, 2004, 7:29 AM
Post #4 of 246 (2424 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
Quote:
Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.


If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground?
I do neither

Quote:
Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

You will not have zero seperation at opening time! I am assuming the upper winds at at ~80kts, the same speed as the aircraft

Quote:
That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Derek

As I said, it's all been done before and although the resources are out there, some people just don't seem to use them.


eames  (D 23844)

Feb 18, 2004, 7:37 AM
Post #5 of 246 (2421 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

This was all figured out many, many years ago....

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 7:45 AM
Post #6 of 246 (2411 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

This was all figured out many, many years ago....

So if the jump ship has zero ground speed (130 fps-130fps), then it might a well be a 13,500 tower. If someone jumps off a 13,500 foot tower and then someone else jumps off the same tower 15 second later, they will open at the same exact point, as they will drift the exact same amount. The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.

And there would be no forward throw, since the jumper has no forward momentum.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 7:46 AM)


eames  (D 23844)

Feb 18, 2004, 7:48 AM
Post #7 of 246 (2408 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

-Jason


velo90

Feb 18, 2004, 7:50 AM
Post #8 of 246 (2405 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Quote:
The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.
That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.


Hooknswoop  (D License)

Feb 18, 2004, 7:51 AM
Post #9 of 246 (2404 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

Um, no, it will drift down wind, just like the jumpers exiting from the 13,500 foot tower would. If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.

Derek


Hooknswoop  (D License)

Feb 18, 2004, 7:56 AM
Post #10 of 246 (2400 views)
Shortcut
Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

Quote:
That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.

That means they will open in the exact same airspace with only the 15 seconds of canopy speed for separation. If a group of 4 exits first, then another group of 4, then the second group of four will be opening within the first group’s airspace. That is zero separation. We do not rely on a canopy’s airspeed for separation. What if the first jumper has line twists and flies into the wind and the second jumper has line twists and flies downwind? They will have a high chance of eating up that 15 seconds and having a collision.

Separation means opening points are separated not the same opening point with canopy speed used for separation. The same opening point means there is zero separation.

Derek


Premier PhreeZone  (D License)
Moderator
Feb 18, 2004, 7:58 AM
Post #11 of 246 (2400 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

No, but if you drop a second ball out of the plane that has 0 ground speed it will land at the same spot as the first.


velo90

Feb 18, 2004, 7:58 AM
Post #12 of 246 (2400 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Quote:
If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.
Thankfully we are parachutists and we don't hit the groundWink That is why ground speed is irrelevant. We need to look at what the plane/tower is doing relevant to point (air mass) where we open our canopies.


eames  (D 23844)

Feb 18, 2004, 7:59 AM
Post #13 of 246 (2397 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Are you serious?

If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Distance = Velocity * Time

Distance = 130 ft/sec * 15 sec = 1950 ft

-Jason


eames  (D 23844)

Feb 18, 2004, 8:04 AM
Post #14 of 246 (2393 views)
Shortcut
Re: [PhreeZone] Opening High for Bad Spots [In reply to] Can't Post

Good observation, but we don't hit the ground when we open our parachutes, do we? We continue moving in the airmass with a relatively slow descent. Think about it.

-Jason


velo90

Feb 18, 2004, 8:05 AM
Post #15 of 246 (2526 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

Try to think about the canopy speed as the speed of the canopy due to wind drift and not it's own speed.

After I got put down by Ron & Kallend I invested a lot of time reading the stuff Kallend put on the web. I believe at least for some people (me included) there are some ways in which we think that are fundamentally wrong. I finally figured out what was going on after I stopped thinking about the ground. The ground is irrelevant!


Hooknswoop  (D License)

Feb 18, 2004, 8:09 AM
Post #16 of 246 (2521 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Are you serious?

Very serious.

Quote:
If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Distance = Velocity * Time

Distance = 130 ft/sec * 15 sec = 1950 ft

If jumper one opens with line twists and flies upwind (the canopy’s airspeed minus the 130 fps wind speed gives jumper one’s ground speed) at time t and jumper 2 opens in the exact same airspace (which is the definition of no separation) at t+15 with line twists and flies down wind (for a ground speed of 130 fps plus the canopy’s airspeed). They have very little separation and may have a collision since they are flying towards each other at their combined airspeeds.

Canopies should have opening points some distance apart.

If canopy speed was sufficient for separation, why track at break off? Why not just turn away and pull and let the canopy’s airspeed provide the separation.

Derek


eames  (D 23844)

Feb 18, 2004, 8:11 AM
Post #17 of 246 (2518 views)
Shortcut
Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
The ground is irrelevant!

Just to clarify, the ground is irrelevant only until after opening. Exit separation must be increased in higher winds due to the increased speed and reduced time it takes under canopy to get to the target, i.e. due to congestion (assuming an upwind opening point).

-Jason


velo90

Feb 18, 2004, 8:17 AM
Post #18 of 246 (2515 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

When I think about exit seperation on a jump run I am not normally thinking about the problems of congestion when landing the canopy. I am mainly concerned that each jumper has his own column of air to use.


eames  (D 23844)

Feb 18, 2004, 8:18 AM
Post #19 of 246 (2515 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

The distance between the two canopies would be the same no matter what the groundspeed is, assuming a constant airspeed. The closing time for the two canopies would be the same as well (yes, regardless of groundspeed).

I give up.

-Jason


eames  (D 23844)

Feb 18, 2004, 8:21 AM
Post #20 of 246 (2511 views)
Shortcut
Re: [velo90] Opening High for Bad Spots [In reply to] Can't Post

Right, I agree. I should have said "Just to supplement what you said...."

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 8:34 AM
Post #21 of 246 (2500 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

I am curious, what canopy are you jumping and opening in 88.6 mph (130 fps) winds?

20 mph (29.3 fps) gives only 440 feet of drift after opening (and excluding the foward speed of the canopy) in 15 seconds, a long space betweeen exits.

Derek


(This post was edited by Hooknswoop on Feb 18, 2004, 8:34 AM)


eames  (D 23844)

Feb 18, 2004, 8:40 AM
Post #22 of 246 (2494 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

I'm not jumping any canopy in 88.6 mph (130 fps, 77.07 knot) winds. I used that number because it's a reasonable airspeed for a plane on jumprun. For the 15 seconds you wait on the plane, you are moving away from the person who got out before you at approximately the airspeed of the plane. Not some arbitrary groundspeed.

Separation is a function of airspeed and time between exits. Period.

-Jason


eames  (D 23844)

Feb 18, 2004, 8:42 AM
Post #23 of 246 (2492 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

In reply to:
20 mph (29.3 fps) gives only 440 feet of drift ....

Name any jumpship with an airspeed of only 17.36 knots (29.3 fps)....

-Jason


Hooknswoop  (D License)

Feb 18, 2004, 9:17 AM
Post #24 of 246 (2470 views)
Shortcut
Re: [eames] Opening High for Bad Spots [In reply to] Can't Post

Quote:
Name any jumpship with an airspeed of only 17.36 knots (29.3 fps)....

Here's 2: Bell 206 BIII Jet ranger, UH-60 Blackhawk.

Let’s look at 2 more realistic scenarios:

1) 70 knots of wind at altitude. 80 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, the opening altitude (I have jumped in these winds).

2) 80 knots of wind at altitude. 90 knot airspeed on jump run. 15 knots of wind speed at 3,000 feet, again, the opening altitude.

Scenario 2 has a 10-knot increase in airspeed over scenario 1. By your reasoning, less time needs to be left between groups in scenario 2 because of the 10 knot increase in airspeed. Both scenarios have a 10 knot ground speed.

I disagree.

Derek


nicknitro71  (D 26704)

Feb 18, 2004, 9:47 AM
Post #25 of 246 (2459 views)
Shortcut
Re: [Hooknswoop] Opening High for Bad Spots [In reply to] Can't Post

True story:

The dude's me!
Attachments: jdallgood.jpg (48.0 KB)


First page Previous page 1 2 3 4 5 6 7 8 9 10 Next page Last page  View All

Forums : Skydiving : Safety and Training

 


Search for (options)