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kallend

The "45 degree rule" for exit separation DOES NOT WORK

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I'm not suggesting anyone needs to dissect the wind conditions in great detail from top to bottom. What is important is for all jumpers to understand that the critical comparison for separation upon opening (discipline differences not withstanding) is the variance in wind conditions between exit and deployment.



The concept that winds at deployment altitude (combined with jump run ground speed) are sufficient to ensure separation is completely wrong. One can come up with a million of various wind profiles that will show this. For example, 0mph, or +50mph, or -50mph wind at 3000ft (deployment), and 100mph from 3100ft all the way up to exit altitude (with aircraft flying at 100mph, so 0 ground speed). Even though the difference between the ground speed and wind at deployment altitude is very different in each case, the result is virtually the same: bellies are all falling along the same trajectory, on top of each other; the final "bump" of shear layer in half a second it takes to fall through 100ft, will put their separation, probably, in the order of inches.

Using just wind conditions at exit and deployment is similar to calculating time it takes to drive from San Francisco to New York based on current traffic in these cities only. But it's conditions along the whole route that matter. A mountain slide over I-70 crossing the Rockies will throw your planning out the window when you're stuck in the car for days.

The only 100% sure way to ensure safe separation is for the plane to fly in circles and each jumper/group only jumps when the previous group has landed. A load of 23 solos will take a couple of hours. Until DZOs and jumpers are OK with this, NO RUSH is GE.

Anyway, happy fucking 10th anniversary, Wingsuit Equations!!! Beeeeeeeeeeeeeeeeeeeeeeeeeeeeer!!!!!!!
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chuckakers

***.. Later, as they become more experienced and learn more, they should be aware of the effects of the winds through the whole freefall. But when they're just starting, they should not be overwhelmed with a Ph.D.-level science of exit separation and bring a laptop running Java freefall simulation to the plane.



What is important is for all jumpers to understand that the critical comparison for separation upon opening (discipline differences not withstanding) is the variance in wind conditions between exit and deployment.


Any jumper falling through the air from 15000-0 during a load experiences the same variances in wind conditions and therefore the same drift. If everybody drifts the same, nobody does and separation will only be achieved by distance above the ground and not time or anything else given the same discipline.
Yuri gave a nearly perfect and easy tool and showed the math behind it for everybody to adjust for his/her foot size/altitude. Now you only need to understand and use it.

Of course the winds at exit and opening altitude are equally important. For spotting that is and not for separation.
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To absent friends

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Pobrause


Go troll somewhere else :P



I am not trolling. No offense, but I honestly believe that your understanding of basic physics is flawed.

Speed or location with regard to the ground means nothing in aerodynamics; this is because the only force acting between the earth and the jumper is gravity, which always points straight down and does not have any horizontal component.

If the earth can not cause a horizontal force, according to mr. Newton there also can not be an horizontal acceleration (F=m*a; a = F/m).
No acceleration means no change in speed. Or, in other words: no effect.

All forces in horizontal direction are a result of the interaction between the object (jumper/plane/parachute) and the surrounding air, and these are dependent of the speed difference between the two. Not on the speed w.r.t. an object that has no interaction with it (like the surface of the earth).

Groundspeed does not cause any aerodynamic forces, simply beacause there is no interaction between the two in the horizontal direction.

Still dont believe it? Drop a napkin on the floor while flying in an airliner. The groundspeed of the napkin is 500 mph, the airspeed is 0. Does the groundspeed cause any effect? No. The napkin drops straight down w.r.t. the plane. Or more precisely: straight down wrt it's surrounding air.
Looking from the ground, its trajectory will be curved, so for an observer it may seem that groundspeed causes something very complex - but is does not.


Now for your example:
If jumper number 2 waits an hour, by the time he opens, jumper no. 1 has been drifting with the wind for an hour, giving an enormous amount of separation between the two, instead of the zero you stated.
The distance between jumpers will be exactly the same in this case as when there would be no wind.


(again, differences in windspeed do matter, but if we cannot even agree on the simple case, let's not make it even more complex).

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Pobrause

Any jumper falling through the air from 15000-0 during a load experiences the same variances in wind conditions and therefore the same drift. If everybody drifts the same, nobody does ...



Bingo!

Pobrause

..and separation will only be achieved by distance above the ground and not time or anything else given the same discipline.


Separation in this case is simply the distance the plane flew during the period between exits. When jumper nr 2 exits, there is a certain distance, calculated by air speed x time. This distance will still be there during opening.

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yuri_base

The concept that winds at deployment altitude (combined with jump run ground speed) are sufficient to ensure separation is completely wrong. One can come up with a million of various wind profiles that will show this. For example, 0mph, or +50mph, or -50mph wind at 3000ft (deployment), and 100mph from 3100ft all the way up to exit altitude (with aircraft flying at 100mph, so 0 ground speed). Even though the difference between the ground speed and wind at deployment altitude is very different in each case, the result is virtually the same: bellies are all falling along the same trajectory, on top of each other; the final "bump" of shear layer in half a second it takes to fall through 100ft, will put their separation, probably, in the order of inches.



I wasn't saying that the ONLY winds that matter are those at deployment altitude and exit altitude. Obviously ANY wind variances between exit and deployment will affect trajectory. My point was that you said making complete calculations is too difficult for young jumpers and I disagree with that. My inclusion of the winds at deployment altitude was to demonstrate that ground speed on jump run alone does not do the job in many cases.

By the way your example is only partially correct. The wind at 3000 feet (deployment altitude in your example) does matter. If the wind is 0 mph at 3000 feet and the plane has a 0 mph ground speed at exit altitude you are correct that the jumpers would open on top of each other regardless of the delay between exits. However, if the wind at 3000 feet is 50 mph (another variable in your example) the jumpers would not remain in that spot upon opening, but rather would immediately move downwind at 50 mph (+/- canopy flight through the air). So yes, the groups would open in the same spot over the ground but would not be close to each other after deployment assuming each group gave adequate delay between exits.

This is exactly my point. The distance between groups of equal freefall profiles upon opening is absolutely dictated by the variance between ALL wind conditions between the exit altitude and deployment altitude - INCLUDING canopy drift during the delay time given between exits.

Beyond the math that has been repeatedly beaten to death in this thread, the foundation of my position is that too many jumpers are being taught that the ONLY calculation necessary for adequate separation is the ground speed of the aircraft on jump run versus time between exits, and that is a dangerous over-simplification of the concept.
Chuck Akers
D-10855
Houston, TX

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Hey Yuri, I'm understand what you're accomplishing by using a shoe to measure a quantity of distance across the ground but I'm having difficulty with a few real-life aspects of this. The first is that in many situations the time between groups is just about equal to the time it takes for that group to set up in the door. Next is that in order for your method to be accurate you really need that group's spotter's foot in the door the instant they leave and you also need the plane to be travelling very perfect and level in order for those bearings to be useful. I think we all know that there is a quantity of pitch and yaw as the plane goes through the sky. I'm an experienced navigator on the water and understand the difficulty in taking accurate bearings using instruments designed for this purpose, much less my eyeball and a shoe. Next is that at some point you need to stop your telemetry work and start the climb-out meaning you have to estimate that the time required for climb-out will put you at the correct location.

I don't see that as any improvement over the 45-degree rule since you're depending on many different people to be able to perform this in-air telemetry which is subject to all sorts of human flaws. A well thought-out exit plan starting on the ground and utilizing an exit separation chart in the back of the plane allows us to do the thing everyone can do, count slowly. 1-Mississippi, 2-Mississippi...etc.

Finally, if we're saying the issue is that students won't count to 30 accurately then we really need to think about the kinds of winds we're putting students out in. Maybe in some case there will be low ground winds but you have some honking uppers if you need 30 seconds between groups.
"I encourage all awesome dangerous behavior." - Jeffro Fincher

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>The concept that winds at deployment altitude (combined with jump run ground speed) are sufficient to
>ensure separation is completely wrong.

It's pretty simple.

The distance you will get between group centers is the ground speed of the aircraft plus the speed of the winds at opening altitude, multiplied by the time you leave between groups. (Assuming normal jump runs - into the wind - and assuming winds are the same direction at opening and at altitude.)

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yuri_base

Quote

I'm not suggesting anyone needs to dissect the wind conditions in great detail from top to bottom. What is important is for all jumpers to understand that the critical comparison for separation upon opening (discipline differences not withstanding) is the variance in wind conditions between exit and deployment.



The concept that winds at deployment altitude (combined with jump run ground speed) are sufficient to ensure separation is completely wrong.



Groundspeed is certainly important for those who choose to deploy on the ground.

Me, I prefer somewhere around 3k on most jumps, and that is where I want to be well clear of other jumpers.
...

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

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