separation question

[wilcox 0]

One of the main things behind the discussion about separation, is that airplanes, skydivers in freefall and canopies is affected by the wind the same way:

Ex:The wind is 10 m/s.
The freeflier is falling through the air. The freeflier is moving 10 m/s over the ground.
A canopy has a forward speed of 9 m/s and is flying into the wind. The canopy is moving 1 m/s backwards compared to the ground.

This is what I have learned to be true. But why is it true? Why is a canopy affected as much as a freeflyer and a airplane?

Is it really true that all objects instantly takes the same horizontal speed as the wind, doesn't it take different time to accelerate objects to the same wind speed as the air mass, depending on the characteristics of the object?

Why is a freeflyer and a belly flyer is affected the same way by the wind, if they are in freefall for the same amount of time?

I hope someone understand what specific physic knowledge that I need to learn to understand this

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[tso-d_chris 0]

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One of the main things behind the discussion about separation, is that airplanes, skydivers in freefall and canopies is affected by the wind the same way:

Ex:The wind is 10 m/s.
The freeflier is falling through the air. The freeflier is moving 10 m/s over the ground.
A canopy has a forward speed of 9 m/s and is flying into the wind. The canopy is moving 1 m/s backwards compared to the ground.

This is what I have learned to be true. But why is it true? Why is a canopy affected as much as a freeflyer and a airplane?

The free flyer, the airplane, and the canopy are all flying in a moving air mass. the air speed and the ground speed will differ by the speed of the air mass.

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Is it really true that all objects instantly takes the same horizontal speed as the wind, doesn't it take different time to accelerate objects to the same wind speed as the air mass, depending on the characteristics of the object?

It does take different amounts of time for different objects to accelerate to match the speed of the air mass. It does not happen instantaneously.

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Why is a freeflyer and a belly flyer is affected the same way by the wind, if they are in freefall for the same amount of time?

They are not in freefall for the same amount of time. Also, since free flyers and RW flyers present differing amounts of surface area to the horizontally moving wind, they will not accelerate to match the speed of the air mass at the same rate.

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I hope someone understand what specific physic knowledge that I need to learn to understand this

Kallend has some outstanding Powerpoint files that deal with the Physics of skydiving. Hopefully someone will link to them.

For Great Deals on Gear

[Orange1 0]

http://www.iit.edu/~kallend/skydive/ and click on the "physics of skydiving" ppt (or any other link that looks interesting)

Skydiving: wasting fossil fuels just for fun.

[pilotdave 0]

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Why is a freeflyer and a belly flyer is affected the same way by the wind, if they are in freefall for the same amount of time?

I like the analogy of 3 different aircraft, pointing north while the wind is coming from the west. Assume the same steady wind for each of them. One is a Cessna, one's an F-16, and the 3rd is a hot air balloon.

At any given time, they will each have drifted the exact same distance to the east. But given any particular distance to the north, they'll each have drifted different amounts. 10 MINUTES into their flights, they'll all be lined up perfectly on a North-south line. But after 10 MILES of travel (due east for the balloon of course), they'll each have drifted a different amount.

Same thing happens to skydivers. Freeflyers and belly flyers drift the same amount (not accounting for differences at exit) over a given period of time. But over a given distance, like say over a 10,000 foot freefall, they won't drift as far because they cover that 10,000 feet in less time.

Dave

[RossDagley 0]

Why? The freeflyers present more surface area to the (directional) wind than flat flyers. So although the flat flyers are subjected to the (directional) wind longer than the freeflyers, their wind resistance is less.

Our DZO / S&TA strongly believes in putting out freeflyers before flatflyers on normal jumprun (into the wind). When we discussed the old dz.com exit-order debate, and kallends graph etc, he countered my 'belief' with the surface area retort.

Anyone want to comment on it?

[popsjumper 2]

Ever looked down and found yourself over the top of the freeflyers?

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[rasmack 0]

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Why? The freeflyers present more surface area to the (directional) wind than flat flyers. So although the flat flyers are subjected to the (directional) wind longer than the freeflyers, their wind resistance is less.

Huh? I am a noob, but one thing I have had instructors trying to beat into my fat skull is that when exiting in a sit, you try to exit sitting with respect to the relative wind. Otherwise you are just back flying.

On the other hand, when you pull of an RW formation you try to have your chest presented to the wind. It takes a while to actually "go flat".

In other words I find what your DZO is saying to be rather counter intuitive.

HF #682, Team Dirty Sanchez #227
“I simply hate, detest, loathe, despise, and abhor redundancy.”
- Not quite Oscar Wilde...

[pilotdave 0]

Freeflyers present more surface area to a wind GUST, not to "the wind." They present no surface area to a steady wind, just like belly flyers. By "wind" I assume you mean the horizontally moving airmass, not "relative wind" that we feel in freefall that comes up from the ground, right?

We're all moving with the airmass... not instantly, but after a few seconds we accelerate to the same speed as the wind. It is no longer producing any force on our bodies, otherwise we'd continue to accelerate in the direction of the wind.

It makes absolutely no difference how much surface area you have, in the steady state. Picture a different scenario... 2 balloons. One is a HUGE hot air balloon, the other is a little helium filled party balloon. When subject to a steady wind and given some time to reach a steady speed, both balloons will travel at the same speed. Their surface area doesn't matter.

Think of wind like a treadmill. Freeflyers and belly flyers are both standing on the treadmill. Both will drift at whatever speed it's running at.

Now freeflyers do get more "throw" from the plane because they present less surface area to the relative wind on exit. That's a good reason to make em go out last. Putting them out first reduces separation at exit time between the last freefly group and the first belly group. Then freefall drift will just move them closer. It's an accident waiting to happen unless you guys plan for it and leave extra space between freeflyers and belly flyers.

Not understanding physics isn't an excuse for putting jumpers lives at risk.

Dave

[tso-d_chris 0]

Thanks, I didn't have the link handy.

For Great Deals on Gear

[tso-d_chris 0]

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Why? The freeflyers present more surface area to the (directional) wind than flat flyers. So although the flat flyers are subjected to the (directional) wind longer than the freeflyers, their wind resistance is less.

Our DZO / S&TA strongly believes in putting out freeflyers before flatflyers on normal jumprun (into the wind). When we discussed the old dz.com exit-order debate, and kallends graph etc, he countered my 'belief' with the surface area retort.

Anyone want to comment on it?

The difference in surface area presented to the directional wind only really matters when the direction or speed of the wind changes. Only then will the increased surface are (exposed to the horizontal component of the accelerating air mass) make a difference. As soon as the jumpers are drifting at the same speed as the air mass, the surface area is irrelevant.

This effect should not be given greater consideration when determining exit order than freefall drift, which will happen even when the air mass is not accelerating (changing speed or direction).

With jump run into the wind, RW jumpers should exit before FF jumpers.

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[RossDagley 0]

Gotcha - luckly you knew what I was talking about

Yep, that makes sense. In the discussion with the CCI (Think I said DZO earlier, sorry, hes not) I was putting forward the arguement for flat-flyers out first after reading quite a bit into it. It was his retort about surface area (against the side of the body, ie horizontally moving air) that stopped me.

I'm not a physics type person, so couldn't argue it then. But now I see where you're coming from - like when people argue they can "feel" when they're flying into the wind under canopy actually being nonsense as you're moving relative to the 'block' of air you're flying in - albeit moving over the ground at a different speed.

I might mention it again when I'm next at the DZ, although little me with 70 jumps and something i gleamed of dz.com doesn't carry much weight in a discussion with someone with 4000+ jumps and 25 years in the sports personal belief. I respect my CCI greatly, and dont want to speak out of line. Tricky!

Edited for speeling

[RossDagley 0]

Thanks for clarifying that Chris - my typing is slower than yours and I was formulating my previous post!

[kelpdiver 2]

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Yep, that makes sense. In the discussion with the CCI (Think I said DZO earlier, sorry, hes not) I was putting forward the arguement for flat-flyers out first after reading quite a bit into it. It was his retort about surface area (against the side of the body, ie horizontally moving air) that stopped me.

Surface area matters for sailing, as the drag of the water resists the push of the wind. But in the air, there's no opposing force.

[rehmwa 2]

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Gotcha - luckly you knew what I was talking about

Yep, that makes sense.

Hit kallend's model. Throw is pretty minor compared to the drift affect. Drift is what makes putting faster fallers out first dangerous (on a typical wind pattern type day).

Cross wind jump runs are more popular nowadays and seems to mitigate drift issues quite a bit. Then you just have to worry about landing patterns when everyone is moving quite a bit slower.

Edit: Your CCI is either a FFer or a swooper and justifies this in order to get the best spot for himself. Send him to Kallend's site - the laws of physics still apply, even in his world. His position is dangerous. If he starts to bring up vertical separation, work it through with the DZO, it will get worse at that point. Never rely on verical separation for anything, it's too uncontrollable in every day operations.

...
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

[rehmwa 2]

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Ever looked down and found yourself over the top of the freeflyers?

Maybe twice. Now if they are being unreasonable, we just wait longer and let the tandems bitch at them when they get to the ground. Because the tandems understand why the RW have to wait, so the delay is the FFers fault.

The worst excuses have to do with who has what canopies decides who gets out first. Like that will avoid collisions due to "freefall" drift. Ever hear those? some are just precious.

...
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

[marks 0]

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Ever looked down and found yourself over the top of the freeflyers?

Maybe twice. Now if they are being unreasonable, we just wait longer and let the tandems bitch at them when they get to the ground. Because the tandems understand why the RW have to wait, so the delay is the FFers fault.

The worst excuses have to do with who has what canopies decides who gets out first. Like that will avoid collisions due to "freefall" drift. Ever hear those? some are just precious.

I hate that excuse, It just shows ignorance.

If freeflyers leave the plane 10 seconds after belly fliers from 14000 ft. they will still deploy before you.

[rehmwa 2]

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I hate that excuse, It just shows ignorance.

If freeflyers leave the plane 10 seconds after belly fliers from 14000 ft. they will still deploy before you.

The problem is, people spend too much time worrying about canopy traffic (which is controllable) and not enough about freefall horizontal separation (which can be FATAL). It's a matter of complacency and personal comfort at that point. On this subject, Prof Kallend is my hero for making it abundantly clear on the right thing to do. www.omniskore.com also has a simulator that shows John's discussion in a graphical format.

Edit: and freefallers don't have exclusive rights to fast canopies and swooping. That's absolute nuts. In fact, with newbies freeflying earlier in their careers, lots of FFers have slow/beginner canopies. Just like every other discipline.

If feel bad for Ross - it's hard when the safety guy is wrong.

...
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

[JohnMitchell 16]

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1- Is it really true that all objects instantly takes the same horizontal speed as the wind, doesn't it take different time to accelerate objects to the same wind speed as the air mass, depending on the characteristics of the object?

2- Why is a freeflyer and a belly flyer is affected the same way by the wind, if they are in freefall for the same amount of time?

1- No, it takes time for the wind to accelerate the object. It depends on the sectional density as to how quickly the object accelerated. Freeflyers, either on their heads or sitting, have a higher sectional density than belly fliers. Therefore they get more forward throw and less wind drift than belly fliers.

2- They don't. Freeflyers fall faster, have shorter freefalls, and consequently less wind drift. That's why they get out last. See Kallend's feefall simulator.

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[Danne 0]

If your CCI believes that, I will never jump at Hinton!

/D

[RossDagley 0]

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If your CCI believes that, I will never jump at Hinton!

/D

Well, he's an extremely safety afare guy, and certainly not the type of person who would compromise anyones situation for his own gain. I've not met someone whos experience and advice I trust more. This is the only discussion I've encoundered where his beliefs are different to 'the norm' (and by that, I only mean dz.com, as I've only jumped there really), and whilst its certainly enlightening to review information posted by a whole host of people from all over the world, this is a belief thats held at my DZ - maybe thats because thats what the instructors believe is best, or maybe its just because thats the way its always been done.

Its certainly nothing to do with personal gain (better spot etc) and I dont recall seeing either the CCI or the DZO(s) fun-jumping - they normally do tandems or AFF instruction etc, so its not that.

I'll tentatively bring it up again when I can. I'm in the situation where I'm obviously on a steep learning curve, the DZ in question is very well organised and run, and the people I respect there the most (specifically the CCI) have a particular view on particular subjects!

[wilcox 0]

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At any given time, they will each have drifted the exact same distance to the east. But given any particular distance to the north, they'll each have drifted different amounts.

I understand this, and I've heard so many people tell me this that I think it's very likely to be true. So I do use these facts when I calculate separation distance.

Maybe most people think it's obvious and don't need an physical explanation, but I would like to know WHY it is like this.

[wilcox 0]

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Surface area matters for sailing, as the drag of the water resists the push of the wind. But in the air, there's no opposing force.

Something like that is what I thought might be part of the explanation too.

But if there is a force, isn't there always an opposing force?

And if an object didn't have any mass that the wind could catch to accelerate it, wouldn't it just stand still over the ground? So why don't objects with different shapes and size get affected different by the wind?

[kallend 2,146]

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Surface area matters for sailing, as the drag of the water resists the push of the wind. But in the air, there's no opposing force.

Something like that is what I thought might be part of the explanation too.

But if there is a force, isn't there always an opposing force?

Yes, but that is a different debate. The air pushes on you, you push on the air. Earth's gravity pulls on you, your gravity pulls on the Earth.

What he is referring to is that in a sailboat two different fluids exert forces on you - the air on the sails and the water on the keel or centerboard and the boat moves in response to the vector sum of those forces.

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And if an object didn't have any mass that the wind could catch to accelerate it, wouldn't it just stand still over the ground? So why don't objects with different shapes and size get affected different by the wind?

Wind doesn't "catch" mass. You do not seem to have a good grasp of basic physics.

...

The only sure way to survive a canopy collision is not to have one.

[pilotdave 0]

Well, different shaped objects or objects with different masses do react differently when exposed to a change in wind speed or direction. They accelerate in the direction of the wind at different rates. But after a while, they reach the speed of the wind. They can't go any faster than the wind, because once they reach the speed of the wind, there's no longer anything pushing them. It's like standing on a moving sidewalk... Once you're on it, your speed matches its speed. You don't feel like you're being pushed along... you can move just as freely within the moving sidewalk as you can on nonmoving ground. Thats like a plane flying in wind. It's flight characteristics are totally unaffected by (steady) wind. Only it's path over the "fixed" ground changes.

I think the balloon example is simplest... when the party balloon is released from someone standing on the ground into a steady wind, for a split second it's not moving and the wind moves around it. The wind produces a force on the balloon, causing it to accelerate. That acceleration continues until the speed of the balloon matches the speed of the wind. Then there's no longer any wind pushing on the balloon, so there's no more force on the balloon from the wind. The balloon doesn't stop... it continues to move at a steady speed. To stop would require a force in the opposite direction.

Dave

[Danne 0]

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Well, he's an extremely safety afare guy, and certainly not the type of person who would compromise anyones situation for his own gain. I've not met someone whos experience and advice I trust more. This is the only discussion I've encoundered where his beliefs are different to 'the norm' (and by that, I only mean dz.com, as I've only jumped there really), and whilst its certainly enlightening to review information posted by a whole host of people from all over the world, this is a belief thats held at my DZ - maybe thats because thats what the instructors believe is best, or maybe its just because thats the way its always been done.

Its certainly nothing to do with personal gain (better spot etc) and I dont recall seeing either the CCI or the DZO(s) fun-jumping - they normally do tandems or AFF instruction etc, so its not that.

I'll tentatively bring it up again when I can. I'm in the situation where I'm obviously on a steep learning curve, the DZ in question is very well organised and run, and the people I respect there the most (specifically the CCI) have a particular view on particular subjects!

I'm sorry if I came out a bit harsh... But, a CCI that doesn't understand the safety problems with putting freeflyers out first really needs to get educated about this, or stop being a CCI. This is not something to be taken lightly. The "dz.com norm" as you put it is not just dz.com´s, but 99% of all dropzones in the world.

It´s a good thing that you don´t take everything from the internet for granted, but listen to Kallend, he is a physics professor (I think). Think about it. Never stop learning.

Good luck,
/D