Hypothetical Question regarding exit order

[Piece 0]

The observed distance between the two groups still did not change, and will not change regardless of whether we view it from the first otter, the second, a canopy, freefall, the ground, a balloon, etc.

sqrt(((x_1 - p_x) - (x_2 - p_x))^2 + ((y_1 - p_y) - (y_2 - p_y))^2 + ((z_1 - p_z) - (z_2 - p_z))^2) = sqrt((x_1 - x_2)^2 + (y_1 - y_2)^2 + (z_1 - z_2)^2). This is still true even if p is a function of time.

The coordinate system is a red herring. You cannot make distance between two points change by translating the coordinate system.

In your example you have a moving coordinate system and you notice that this makes the distance to the origin change. But this is not relevant (unless the origin is one of the jumpers, then the picture will be very different and the distance between them will still be the same).

http://icanhascheezburger.com/2008/02/28/funny-pictures-i-come-with-sarcasm/
Proudly uncool since 1982.

[Fast 0]

Your calculations are wrong. Bill is right.

Everyone keeps thinking about this in terms of the ground. The ground doesn't matter. The ground speed of the airplane never makes a difference in air-to-air separation of groups of skydivers. The only effect on group separation is exposure time to the different wind speeds at different altitudes.

The part that you missed in your calculations is that, even if the plane is moving backwards relative to the ground, the airmass that the plane is flying in (this is almost like the argument of, can a plane take off on a treadmill....) is moving relative to the plane.

So, while in freefall, if the FS group drifts 1 mile backwards, then the FF group gets out and drifts .5 miles in freefall, the distance between the groups is equal to:

Airspeed/second * seconds between groups + (First group drift) - (second group drift)

Please take note that it is airspeed, not ground speed. So if you have a pilot flying a 80kt jumprun your seperation is as follows:

Feet Seperation	

Seconds	FS First	FF First

1	2775.024789	-2504.975211

2	2910.049578	-2369.950422

3	3045.074367	-2234.925633

4	3180.099156	-2099.900844

5	3315.123945	-1964.876055

6	3450.148734	-1829.851266

7	3585.173523	-1694.826477

8	3720.198312	-1559.801688

9	3855.223101	-1424.776899

10	3990.24789	-1289.75211

11	4125.272679	-1154.727321

12	4260.297468	-1019.702532

13	4395.322257	-884.677743

14	4530.347046	-749.652954

15	4665.371835	-614.628165

16	4800.396624	-479.603376

17	4935.421413	-344.578587

18	5070.446202	-209.553798

19	5205.470991	-74.529009

20	5340.49578	60.49578

At somewhere between 19 and 20 seconds they would collide, before that, the FS group would pass over the top of the FF group and open well on the other side of them. Assuming 1 mile and .5 miles of drift for the groups. (theorhetical numbers)

It's physics, it just works this way. It's not a subjective thing.

Edit to add:

I understand where you guys are getting hung up. But it still doesn't have anything to do with ground speed. Only relative air masses. Sometimes it is easier to think about it using the "ground" but in practice, stuff just doesn't happen like you are thinking. I mean, I haven't been jumping that long, but I haven't ever seen us go up in a situation where at some altitude the windspeed changes so drastically from very very high to nothing. Typically the wind direction is pretty consistant on days when it is that high, and if it isn't, it's because of thunder storms and you shouldn't be jumping anyways.

I think the situation that some are hung up on here is that, if 10 feet below the plane,(and this is the most drastic of circumstances) the wind went from exactly equal to airspeed, to nothing, then the groups would drop out on top of one another into the 0 windspeed air. But still, that has nothing to do with ground speed, it still relates exactly to exposure time to different wind speeds/directions at different altitudes.

~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

[billvon 2,400]

>The observed distance between the two groups still did not change . . .

Sure it did. It was zero.

The reason that was OK is that we do not care about the distance between the opening points of the groups to any observer. We care about the distance from group 1 to group 2 at the time that group 2's parachutes are opening.

>You cannot make distance between two points change by translating the
>coordinate system.

You absolutely can - unless you choose one fixed point in time.

[SethInMI 145]

Fast,

You can't talk about windspeed without referencing the ground. You say "The ground speed of the airplane never makes a difference in air-to-air separation of groups of skydivers." But windspeed is referenced to the ground. It is ground speed that matters.

Bill says the same thing in his 1st post in this thread:
Distance between groups will be given by aircraft ground speed plus wind speed at opening altitude, times seconds between groups.

He is exactly right for groups with the same fall rate.

I think you basically on the right track though. An FS group will drift more than a FF group. If the ground speed of the AC is negative, then put the FS out 2nd will help, it just adds the movement of the plane to the FS drift. The caveat is the canopy time. Here the windspeed at opening altitude can push the 1st opening FF group back toward the FS group.

Simple physics.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[billvon 2,400]

>It is ground speed that matters.

Ground speed doesn't matter at all, actually. The only reason we care about it is:

1) wind speed is referenced to the ground,
2) it affects where we choose to put the spot and
3) we can't land in winds that are too high.

[Piece 0]

Quote

Sure it did. It was zero.

This is playing tricks with a moving coordinate system. Say the first jumper opened at time t=0 and coordinates (0,0,1). The second jumper opened at time t=1 and coordinates (0,0,1). It is an illusion to say that the distance is zero because the position is the same. At time t=1 the position of the first jumper is (-65,0,1) or whatever.

I am talking about fixing the space, fixing two points in it, and only moving the coordinate system. As time flows the origin moves and the xyz values of the two points will change but the distance between them will not.

Quote

The reason that was OK is that we do not care about the distance between the opening points of the groups to any observer. We care about the distance from group 1 to group 2 at the time that group 2's parachutes are opening.

The reason why the two jumpers in your example did not collide is because they opened at different times. We only care about the fixed point in time when second jumper opened (this the point you are making). At this point in time the distance between the two jumpers (which is not zero) would have been the same measured from the plane or from the ground (this is the point I am making). But this means that it doesn't matter if we view the situation from the air at 3000' or 13,500'. Thus, Kallends program accurately computes separation even though the trajectory it plots isn't that meaningful. In particular, freeflyers should exit first in the situation under discussion.

Quote

You absolutely can - unless you choose one fixed point in time.

Only if the two points themselves are moving. If the two points do not move then the distance between them will stay the same regardless of the coordinate system you measure it in, even a moving one.

http://icanhascheezburger.com/2008/02/28/funny-pictures-i-come-with-sarcasm/
Proudly uncool since 1982.

[billvon 2,400]

>This is playing tricks with a moving coordinate system.

You are exactly correct! Which is why many people are confused by the issue when they try to think it through based on a moving coordinate system.

>It is an illusion to say that the distance is zero because the position is the same.

That's sorta the definition of "zero distance." But again, we don't care about that distance. We care about distance from the canopies in group 1 when group 2 is opening.

>Only if the two points themselves are moving. . . .

. . . relative to the observer.

I'm not disagreeing with you. I am pointing out that a lot of people's "common sense" thinking goes out the window when you are dealing with moving frames of reference - which is why this is such an oft-debated topic.

[SethInMI 145]

Quote

>It is ground speed that matters.

Ground speed doesn't matter at all, actually. The only reason we care about it is:

1) wind speed is referenced to the ground,
2) it affects where we choose to put the spot and
3) we can't land in winds that are too high.

Did you read my post Bill? You appear to be disagreeing with me, and then restate my point. I only refer to your 1) above in my post with Fast.

Ground speed is just a handy way to refer to the vector sum of plane airspeed and windspeed. It is this vector sum * exit delay that determines the group seperation for groups with same fallrate (plus winds at opening alt). You said so yourself in your 1st post in this thread.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[billvon 2,400]

>Did you read my post Bill? You appear to be disagreeing with me

Not really. I think others may have taken the statement "It is ground speed that matters" to mean that ground speed is really the issue, instead of just a handy reference frame.

>Ground speed is just a handy way to refer to the vector sum of plane
>airspeed and windspeed.

I agree!

(Edited to correct the mistake of replying to the wrong person!)

[Piece 0]

Quote

. . . relative to the observer.

And relative to each other. Only moving the observer won't change the distance observed between the two points at a fixed time, as before. We seem to be using different definitions of the word distance, which seems to be causing the confusion. I am sure that you understand how everything works.

Quote

I'm not disagreeing with you. I am pointing out that a lot of people's "common sense" thinking goes out the window when you are dealing with moving frames of reference - which is why this is such an oft-debated topic.

Cool. You're definitely right about moving frames being very confusing. Maybe this conversation will help someone understand.

http://icanhascheezburger.com/2008/02/28/funny-pictures-i-come-with-sarcasm/
Proudly uncool since 1982.

[Piece 0]

Quote

The caveat is the canopy time. Here the windspeed at opening altitude can push the 1st opening FF group back toward the FS group.

Only if the wind speed changes drastically right above deployment altitude. Otherwise the jumpers are moving in essentially the same airmass and so only the airspeed of the canopy matters. Thus, even though you appear to be moving backwards by looking at the ground, you are still moving away from the group behind you (if your heading is away from them). The group behind you will see the ground moving backwards also but at a faster rate than you will see it.

http://icanhascheezburger.com/2008/02/28/funny-pictures-i-come-with-sarcasm/
Proudly uncool since 1982.

[kallend 1,623]

Quote

John,

I thought you were agreeing with Bill, but I just wanted to make sure. I know moving the Earth around does not affect what is happening in the air, but the is the reference frame we are using. The plane speed is 80 knots w/respect to the air. The airspeed is 100 knots w/ respect to the ground.

NOPE. Airspeed is w.r.t. the air. GROUNDspeed is w.r.t. the GROUND.

...

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

[SethInMI 145]

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NOPE. Airspeed is w.r.t. the air. GROUNDspeed is w.r.t. the GROUND.

I said airspeed; I meant windspeed. The uppers as it is usually called. The speed of the wind is made with respect to the ground.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[kallend 1,623]

Quote

Quote

NOPE. Airspeed is w.r.t. the air. GROUNDspeed is w.r.t. the GROUND.

I said airspeed; I meant windspeed. The uppers as it is usually called. The speed of the wind is made with respect to the ground.

Successful physics requires a common vocabulary that we all agree on.

Regardless, the relative movement and positions of the skydivers at exit altitude and at opening altitude (and everywhere in between) depend only on what happens between those altitudes, not what is happening on the ground 3000' below opening altitude. There's no need for any math to reach that conclusion.

...

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

[Fast 0]

Quote

Fast,

You can't talk about windspeed without referencing the ground. You say "The ground speed of the airplane never makes a difference in air-to-air separation of groups of skydivers." But windspeed is referenced to the ground. It is ground speed that matters.

Bill says the same thing in his 1st post in this thread:
Distance between groups will be given by aircraft ground speed plus wind speed at opening altitude, times seconds between groups.

He is exactly right for groups with the same fall rate.

I think you basically on the right track though. An FS group will drift more than a FF group. If the ground speed of the AC is negative, then put the FS out 2nd will help, it just adds the movement of the plane to the FS drift. The caveat is the canopy time. Here the windspeed at opening altitude can push the 1st opening FF group back toward the FS group.

Simple physics.

Yes, you can talk about wind speed without referencing the ground. The reasons we do billvon posted above. I could however reference windspeed to the aircraft it's a whole lot more work and unnecessary though.

I am on the right track, in fact, everything I am saying is true. I know this from practical experience.

In all circumstances where the wind direction from exit to deployment is the same, and jumprun is flown into the wind, slower falling groups should get out first. If jumprun is flown downwind (which doesn't happen often, and is really a bad idea) or if there are mixed opposing wind directions, that might change. Luckily, the first for sure, and the second most often don't happen. So, FF always gets out after belly.

The airmass that the airplane is flying in, and that you jump out into is a system. When you transition from one altitude/windspeed condition to another, you are changing systems. That relative number is the one that you have to take into account. The only way that the groundspeed (and it's relative nature to what the plane is doing at altitude) makes a difference is if the opening point is exactly the ground (or if you were dropping pumpkins of different sizes out of the plane or something)

Opening point is never equal to the ground unless you are bouncing (let's hope that never happens) so the ground "system" doesn't matter. Only the airmmass systems and their relative speeds to one another.

Slower fallers spend more time in each of the systems they pass through on the way down, so as long as the direction between groups (i.e. the line of jumprun) remains in an upwind direction throughout as the jumpers pass through the systems going down, the slower jumpers will drift more. None of this has anything to do with the ground. That drift distance is often refered to by the ground, only because it is easier to do. You could refer all of this to where the airplane is, but then you also have to take into account the speed/direction of the airplane AND the airmass that it is flying in. The ground is used to help simplify things as a constant frame of reference.

~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