billvon 2,395
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billvon 2,395
> So it sounds like the REALLY important things to know are:
>1) the AC heading
>2) Winds aloft at opening altitude
Well, I'd change that slightly to the airplane's ground speed and winds at opening altitude. Total separation = (ground speed + winds at opening altitude) * time in seconds. This is group center separation, so add whatever you wish for tracking/flying away from the center. 1000 feet is a pretty good number for group center sep; that gives you about 300 feet from one jumper to the next after you factor in tracking/canopy flight.
>1) the AC heading
>2) Winds aloft at opening altitude
Well, I'd change that slightly to the airplane's ground speed and winds at opening altitude. Total separation = (ground speed + winds at opening altitude) * time in seconds. This is group center separation, so add whatever you wish for tracking/flying away from the center. 1000 feet is a pretty good number for group center sep; that gives you about 300 feet from one jumper to the next after you factor in tracking/canopy flight.
tdog 0
Quote
True. But if you have 70kt uppers, you're not usually going to have zero winds at 3000 feet. If you do, you are in a very difficult separation situation. (Note that if you have 70kt uppers and 40kts at opening, you are really 'dealing with' a 30kt upper, at least when it comes to exit timing.)
This was indeed a weird day. The uppers were so strong we called the plane a helicopter - yet the ground winds were so light and variable from 3000 down that we could not determine a prevailing direction for the day...
hansv 0
QuoteShort answer:
The 45 degree angle does not work at all. I took a bunch of pictures of people exiting during both upwind (long delay required) and downwind (short delay required) jump runs, and there's no difference. Heck, they never even reached 45 degrees under any conditions.
The reason people THINK it works is it makes them stick their head outside the plane and try to figure out what 45 degrees looks like. After a few seconds they either:
1) decide they think the group is at 45 degrees, even though it isn't
2) give up because they don't want to look stupid
3) give up because everyone else is screaming "GO! GO!"
But in all 3 cases, they waste 5-10 seconds, and that's enough (usually) to give them safe separation.
Finally somebody who does the math.
The 45 degrees rule is bull because one NEVER reaches a 45 degree angle. 45 degrees means the vertical distance is equal to the horizontal distance. If the plane flies at 120mph you will have a hard time climbing out
I spent a lot of my money on booze, birds and fast cars. The rest I just squandered.
kallend 1,623
QuoteQuoteShort answer:
The 45 degree angle does not work at all. I took a bunch of pictures of people exiting during both upwind (long delay required) and downwind (short delay required) jump runs, and there's no difference. Heck, they never even reached 45 degrees under any conditions.
The reason people THINK it works is it makes them stick their head outside the plane and try to figure out what 45 degrees looks like. After a few seconds they either:
1) decide they think the group is at 45 degrees, even though it isn't
2) give up because they don't want to look stupid
3) give up because everyone else is screaming "GO! GO!"
But in all 3 cases, they waste 5-10 seconds, and that's enough (usually) to give them safe separation.
Finally somebody who does the math.
Finally? The math was done years ago, where have you been.
...
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.
winsor 186
BTW - I think our ground speed was 15 MPH (give or take) per the GPS. That is 45 seconds between groups for 1000 feet separation.
This is separation WITH REGARD TO THE GROUND! Since separation at opening altitude is the key issue here, your separation wrt the ground is only important if everyone bounces - and then you have other problems than the proximity of the nearest crater.
One problem people seem to have is that they leave out a key dimension in their analysis - time. If you draw the paths of two cars, and these paths intersect, does this mean they collided? Only if they were trying to occupy the same place at the same time; if one passes the point of intersection an hour, or even a minute, after the other has passed, you have no problem.
Thus, you can have people exiting from an airplane doing 12 mph over the ground at 150 foot intervals as measured from the ground, and maintain 900 feet separation between groups IN THE AIR. Why? Because the airplane is going six times the speed through the air that it is over the ground, and the frame of reference that matters in freefall is the air, not the ground.
If you have an airplane that can match the uppers for speed and remain stationary over the ground during jumprun (think AN-2), you can have zero ground speed and still maintain adequate separation between groups. In this case, everyone will open at the same point over the ground - but at different times. By the time one group opens, the previous group will have been blown 900 feet downwind and so forth.
Once you have it figured out, it's pretty obvious. If you do not have it figured out, it is obvious that you do not understand to someone who does.
Blue skies,
Winsor
apoil 0
Quote
The only distance that counts is the distance with respect to the air at opening altitude, and you can't see that!
Then why does the speed of the uppers matter?
Sounds to me like separation is based only on the planes airspeed indicator, and separation sould be the same regardless of the direction of jump run.
This has plagued me for a while.. intuition has told me that that distance covered over the ground shouldn't matter because separation needs to be achieved in the airmass. But using the ground as a reference IS what is taught - and ground speed is used to determine separation.
In the case of the balloon
Untethered: airspeed = 0. No separation achieved.
Tethered: airspeed = windspeed. Separation achieved.
billvon 2,395
>Then why does the speed of the uppers matter?
Because the "figure of merit" if you will is the difference in speed between the airplane and the winds at opening altitude. You need to know both with respect to any given reference frame to accurately calculate exit separation.
The easiest reference frame is probably the ground, because you can see that when you look out of the plane. In that case, separation (from group centers) is given by (groundspeed + winds at opening altitude) * time between groups. The groundspeed of an aircraft is, of course, dependent on upper winds.
You can also use the reference frame of the groups that have just opened. In that case, the important speed to know is the speed of the aircraft relative to the winds at opening altitude - or how fast an observer under a round canopy at 3000 feet would see the plane flying away. The math's a bit more involved in that case; now it's the difference between the winds at 3000 and 12,000 plus the speed of the plane that's the figure of merit. Which, if you do a bit of algebra, reduces to (groundspeed + winds at opening altitude) anyway.
This also assumes everything is directly into (or away from) the wind. If you have crosswinds, then vector math rears its ugly head - although for most purposes you can neglect everything but the jumprun-aligned component.
In most cases, assuming the winds at opening altitude are zero is close enough, and will not result in getting too close to another group. So assuming that aircraft ground speed is the only thing you care about will work 90% of the time, and isn't a bad simplification. The only times this will bite you is if the winds at opening altitude are in the opposite direction - and that's rare.
Because the "figure of merit" if you will is the difference in speed between the airplane and the winds at opening altitude. You need to know both with respect to any given reference frame to accurately calculate exit separation.
The easiest reference frame is probably the ground, because you can see that when you look out of the plane. In that case, separation (from group centers) is given by (groundspeed + winds at opening altitude) * time between groups. The groundspeed of an aircraft is, of course, dependent on upper winds.
You can also use the reference frame of the groups that have just opened. In that case, the important speed to know is the speed of the aircraft relative to the winds at opening altitude - or how fast an observer under a round canopy at 3000 feet would see the plane flying away. The math's a bit more involved in that case; now it's the difference between the winds at 3000 and 12,000 plus the speed of the plane that's the figure of merit. Which, if you do a bit of algebra, reduces to (groundspeed + winds at opening altitude) anyway.
This also assumes everything is directly into (or away from) the wind. If you have crosswinds, then vector math rears its ugly head - although for most purposes you can neglect everything but the jumprun-aligned component.
In most cases, assuming the winds at opening altitude are zero is close enough, and will not result in getting too close to another group. So assuming that aircraft ground speed is the only thing you care about will work 90% of the time, and isn't a bad simplification. The only times this will bite you is if the winds at opening altitude are in the opposite direction - and that's rare.
QuoteThen why does the speed of the uppers matter?
Think of it this way. If you're in an Otter doing 90mph into a 90mph headwind, the Otter is standing still relative to the ground. Assuming that everyone falls straight at the same fall rate and that the winds are not changing with time, every person jumping out of that plane will be following the exact same trajectory through the air. If they deploy at the same altitude, they'll be deploying at the same point in the sky,
relative to the ground.
This is the extreme case, of course, but hopefully helps illustrate the point. In reality, people will also have different fall rates, so their trajectories will be slightly different, but not by much and it's also not something I'd want to rely to givce me separation at opening.
By the same principle, if the uppers were 45mph, then your separation at opening will be half what it would be in a 0mph upper if your jump plane's airspeed is 90mph at exit.
So, while our movement up there is relative to the air mass around us, unless we're going to jump out of the plane and hover until we're all ready to start dropping, there will always be more to separation than just the plane's velocity with respect to the upper winds.
billvon 2,395
> every person jumping out of that plane will be following the exact same
>trajectory through the air. If they deploy at the same altitude, they'll be
>deploying at the same point in the sky, relative to the ground.
Right! However, it's worth noting that that may be fine from a separation point of view (if you give enough time) if the winds at 3000 feet are also strong. By the time the next group gets there everyone in the first group will be 1000 feet away.
Which is why you have to know both to be truly accurate.
>trajectory through the air. If they deploy at the same altitude, they'll be
>deploying at the same point in the sky, relative to the ground.
Right! However, it's worth noting that that may be fine from a separation point of view (if you give enough time) if the winds at 3000 feet are also strong. By the time the next group gets there everyone in the first group will be 1000 feet away.
Which is why you have to know both to be truly accurate.
kallend 1,623
Alex and Bob go for a balloon jump. They decide to do separate solo head down jumps. They are both good freeflyers and won't track or backslide inadvertantly. They have the same fallrate.
The winds are light on the ground, but at all altitudes above 4000ft are around 20kt. In fact, they are the same strength all the way up to 13,000ft where they will exit.
They decide they need 400ft separation at opening for safety.
The balloon takes them to 13k, and the pilot's GPS readout shows a groundspeed of 50 ft per second (just over 20kt). They calculate that a 8 second delay will give the required 400ft of separation (8 x 50 = 400).
Alex jumps. Bob accurately counts off 8 seconds and jumps. After 10 seconds Alex's round reserve prematurely deploys. Bob is looking at the horizon in the opposite direction and doesn't see this.
By what horizontal distance will Bob miss Alex?
The winds are light on the ground, but at all altitudes above 4000ft are around 20kt. In fact, they are the same strength all the way up to 13,000ft where they will exit.
They decide they need 400ft separation at opening for safety.
The balloon takes them to 13k, and the pilot's GPS readout shows a groundspeed of 50 ft per second (just over 20kt). They calculate that a 8 second delay will give the required 400ft of separation (8 x 50 = 400).
Alex jumps. Bob accurately counts off 8 seconds and jumps. After 10 seconds Alex's round reserve prematurely deploys. Bob is looking at the horizon in the opposite direction and doesn't see this.
By what horizontal distance will Bob miss Alex?
...
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.
jakee 1,254
QuoteBy what horizontal distance will Bill miss Alex?
Trick question, Bill wasn't on the load
Do you want to have an ideagasm?
kallend 1,623
QuoteQuoteBy what horizontal distance will Bill miss Alex?
Trick question, Bill wasn't on the load
Yes, it started as Bill but I didn't want the real Bill to take offense!
...
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.
Fast 0
QuoteAlex and Bob go for a balloon jump. They decide to do separate solo head down jumps. They are both good freeflyers and won't track or backslide inadvertantly. They have the same fallrate.
The winds are light on the ground, but at all altitudes above 4000ft are around 20kt. In fact, they are the same strength all the way up to 13,000ft where they will exit.
They decide they need 400ft separation at opening for safety.
The balloon takes them to 13k, and the pilot's GPS readout shows a groundspeed of 50 ft per second (just over 20kt). They calculate that a 8 second delay will give the required 400ft of separation (8 x 50 = 400).
Alex jumps. Bob accurately counts off 8 seconds and jumps. After 10 seconds Alex's round reserve prematurely deploys. Bob is looking at the horizon in the opposite direction and doesn't see this.
By what horizontal distance will Bob miss Alex?
He wont miss him. Bob will pound into Alex's reserve.
~D
Where troubles melt like lemon drops Away above the chimney tops That's where you'll find me.
Swooping is taking one last poke at the bear before escaping it's cave - davelepka
Where troubles melt like lemon drops Away above the chimney tops That's where you'll find me.
Swooping is taking one last poke at the bear before escaping it's cave - davelepka
winsor 186
QuoteQuoteAlex and Bob go for a balloon jump. They decide to do separate solo head down jumps. They are both good freeflyers and won't track or backslide inadvertantly. They have the same fallrate.
The winds are light on the ground, but at all altitudes above 4000ft are around 20kt. In fact, they are the same strength all the way up to 13,000ft where they will exit.
They decide they need 400ft separation at opening for safety.
The balloon takes them to 13k, and the pilot's GPS readout shows a groundspeed of 50 ft per second (just over 20kt). They calculate that a 8 second delay will give the required 400ft of separation (8 x 50 = 400).
Alex jumps. Bob accurately counts off 8 seconds and jumps. After 10 seconds Alex's round reserve prematurely deploys. Bob is looking at the horizon in the opposite direction and doesn't see this.
By what horizontal distance will Bob miss Alex?
He wont miss him. Bob will pound into Alex's reserve.
We have a winner!
Scenario 2:
Alex and Bob go for a balloon jump from a heliostat (tethered balloon). They decide to do separate solo head down jumps. They are both good freeflyers and won't track or backslide inadvertantly. They have the same fallrate.
The winds are light on the ground, but at all altitudes above 4000ft are around 20kt. In fact, they are the same strength all the way up to 13,000ft where they will exit.
They decide they need 400ft separation at opening for safety.
The balloon takes them to 13k and the pilot's GPS readout shows a groundspeed of zero (they're tethered), but the wind speed indicator shows 50 ft per second (just over 20kt). They calculate that a 8 second delay will give the required 400ft of separation (8 x 50 = 400).
Alex jumps. Bob accurately counts off 8 seconds and jumps. After 10 seconds Alex's round reserve prematurely deploys. Bob is looking at the horizon in the opposite direction and doesn't see this.
By what horizontal distance will Bob miss Alex?
rehmwa 2
static balloon
so 8 seconds at 20 knots
the rest is left as an exercise for the student.
thus you show that the moving aircraft is very important in the calcs
...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants
so 8 seconds at 20 knots
the rest is left as an exercise for the student.
thus you show that the moving aircraft is very important in the calcs
...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants
Fast 0
I know what it is but I am gonna let someone else answer.
I hope that this all is making it clear to people that the 45 degree rule is a bunch of garbage.
I hope that this all is making it clear to people that the 45 degree rule is a bunch of garbage.
~D
Where troubles melt like lemon drops Away above the chimney tops That's where you'll find me.
Swooping is taking one last poke at the bear before escaping it's cave - davelepka
Where troubles melt like lemon drops Away above the chimney tops That's where you'll find me.
Swooping is taking one last poke at the bear before escaping it's cave - davelepka
rehmwa 2
actually, i do need to change it, as it will still take some time for the jumpers to achieve zero relative horizontal speed with the uppers. these guys are jumping with a 20knot wind on their bodies...... just don't know how much time it takes before that cancels out - but they both leave in the same relative wind, so it might still not matter - they still get their 400 feet
the nice thing is the depoying reserve with go with the wind almost immediately
the best part of the example is the guy in headdown suddenly getting a reserve ride after ten seconds. I hope he didn't get jerk around too much.
the last answer is the deploying parachute will jump upwards on deployment (just like in the cartoons) and strike the balloon - thus leaving the jumper comfortable in the basket with his sandwich.
...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants
the nice thing is the depoying reserve with go with the wind almost immediately
the best part of the example is the guy in headdown suddenly getting a reserve ride after ten seconds. I hope he didn't get jerk around too much.
the last answer is the deploying parachute will jump upwards on deployment (just like in the cartoons) and strike the balloon - thus leaving the jumper comfortable in the basket with his sandwich.
...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants
kallend 1,623
Quoteactu
the last answer is the deploying parachute will jump upwards on deployment (just like in the cartoons) and strike the balloon - thus leaving the jumper comfortable in the basket with his sandwich.
It's great that someone around here understands basic physics.
...
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.
Yo!
Here's the angle to skydiver as seen from airplane for various jumprun airspeeds. The methodology of calculations can be found here.
As you can see, 45 degrees can only be seen at some special airspeed of 135mph. But variation of the angle is too small anyway to make it a practical indicator.
Next!
Yuri
Here's the angle to skydiver as seen from airplane for various jumprun airspeeds. The methodology of calculations can be found here.
As you can see, 45 degrees can only be seen at some special airspeed of 135mph. But variation of the angle is too small anyway to make it a practical indicator.
Next!
Yuri
Android+Wear/iOS/Windows apps:
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L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio
kallend 1,623
QuoteYo!
Here's the angle to skydiver as seen from airplane for various jumprun airspeeds. The methodology of calculations can be found here.
As you can see, 45 degrees can only be seen at some special airspeed of 135mph. But variation of the angle is too small anyway to make it a practical indicator.
Next!
Yuri
www.iit.edu/~kallend/skydive/ allows the same thing, with air density variation with altitude taken into account. Same conclusion (of course).
...
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.
>you are going to have to "invade" someone else's airspace eventually...
Right. All we really care about is that, at the point the next group opens, you are not under/near them. After that, of course, you no longer have your "own" space to land in - you have to start getting into a common pattern. But at that point, you're doing 20mph or so forward, and it's a lot easier to navigate.
>If the plane takes a minute to fight headwinds and cover 1000 horizontal
> feet on the ground/canopy opening altitude, and you like 1000 feet
>separation - then all of a sudden you need to take into consideration how
>far a canopy can fly in a minute - which is quite far.
Right. On a day like that, you may need to wait a long time, only put 2 groups out per pass, or even do 1 group per pass. A crosswind jump run can ameliorate those problems.
>That means the canopy at 3000 feet with no winds was actually flying
> faster over the ground up jump run than the plane fighting the super
>strong uppers!
True. But if you have 70kt uppers, you're not usually going to have zero winds at 3000 feet. If you do, you are in a very difficult separation situation. (Note that if you have 70kt uppers and 40kts at opening, you are really 'dealing with' a 30kt upper, at least when it comes to exit timing.)
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