Forums: Skydiving: General Skydiving Discussions:
Why Groundspeed?

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oldwomanc6

Apr 15, 2012, 8:54 PM
Post #76 of 87 (507 views)
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 Re: [hub1100] Why Groundspeed? [In reply to] Can't Post
In reply to:
With respect to this thread:

Isn't equilibrium defined as: the condition of a system in which all competing influences are balanced, in a wide variety of contexts. (balance)

Aren't we all taking about Terminal velocity? : A free-falling object achieves its terminal velocity when the downward force of gravity (FG) equals the upward force of drag (Fd). This causes the net force on the object to be zero, resulting in an acceleration of zero.

I would agree Anvil's, bantam weights and free fliers all reach a different terminal velocity. Is this factor the ultimate part of this discussion, not a flat timed fall rate?

Thx -hubs-

Skydivers never reach equilibrium. They keep moving around, changing stuff. However, they do reach terminal velocity.

popsjumper  (D 999999999)

Apr 16, 2012, 3:17 AM
Post #77 of 87 (501 views)
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 Re: [hub1100] Why Groundspeed? [In reply to] Can't Post
In reply to:
With respect to this thread:

Isn't equilibrium defined as: the condition of a system in which all competing influences are balanced, in a wide variety of contexts. (balance)

Aren't we all taking about Terminal velocity? : A free-falling object achieves its terminal velocity when the downward force of gravity (FG) equals the upward force of drag (Fd). This causes the net force on the object to be zero, resulting in an acceleration of zero.

I would agree Anvil's, bantam weights and free fliers all reach a different terminal velocity. Is this factor the ultimate part of this discussion, not a flat timed fall rate?

Thx -hubs-

We are talking about the effects of wind speed on the lateral movement of freefalling objects both with and without a canopy being involved. Terminal velocity is something different but you are on the money about terminal velocity.

I have a general question....isn't that lateral movement also considered to have a "terminal velocity"? Say, a lateral terminal velocity vs a vertical terminal velocity?

Off topic but related?:
What do they call it say, when planes reach maximum speed at a given amount of thrust? Cruise speed? Peanuts and soft drink time?

wolfriverjoe  (A 50013)

Apr 16, 2012, 5:15 AM
Post #78 of 87 (493 views)
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 Re: [popsjumper] Why Groundspeed? [In reply to] Can't Post
In reply to:
Off topic but related?:
What do they call it say, when planes reach maximum speed at a given amount of thrust? Cruise speed? Peanuts and soft drink time?

I would think it would also be called "equilibrium" because thrust has equalled drag and lift has equalled drag. I always call it "stable flight' because stuff has stopped changing.

MariusM

Apr 16, 2012, 5:20 AM
Post #79 of 87 (493 views)
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 Re: [hub1100] Why Groundspeed? [In reply to] Can't Post
Well, you are talking about vertical movement. Most of the thread is about horizontal movement, where gravity has no influence, but horizontal wind has.

rehmwa  (D 12816)

Apr 16, 2012, 7:34 AM
Post #80 of 87 (480 views)
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 Re: [popsjumper] Why Groundspeed? [In reply to] Can't Post
In reply to:
I have a general question....isn't that lateral movement also considered to have a "terminal velocity"? Say, a lateral terminal velocity vs a vertical terminal velocity?

the horizontal 'terminal' is just the end of the throw portion (that point where the asymptotic approach to velocity body completes - the difference in the body to air mass is negligible).

so it's either zero (relative to the air mass) or it equals the uppers (relative to the ground)

everything will complete the throw portion - given enough alititude - for skydivers, even Anvil - it's short enough lived but worth understanding. (I know as a late diver, it matters if the base tumbles (I'll have a steeper dive since they had more throw) than when it sails off nice - for example)

I have no idea what the one poster was arguing with Kallend about - it was a complete fail - maybe we'll hear an amusing rationalization involving coriolis effects or something

skr  (D 981)

Apr 24, 2012, 8:38 PM
Post #81 of 87 (431 views)
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 Re: [popsjumper] Why Groundspeed? [In reply to] Can't Post
I got so involved in drawing that ascii diagram
that I forgot about this part:

> Why is this not true in your example:
> When J1 passes through, it will take him horizontally farther away from J2 and when J2 pass through it will take him horizontally closer to J1, back to the original horizontal separation at exit.
>
> J1 will see J2 first moving horizontally away from him and then horizontally closer to him.

Except that I woke up a couple mornings later
thinking "No, he's right. That apparent extra
separation disappears right there in the forward
throw layer."

So then I started trying to think where I got off
the track, thinking I would come back here and say
something, but I can't seem to concentrate on this
right now.

This is especially annoying because I used to think
what you just said, and even posted about it.

rec.skydiving Sept 1999:

> One way I retrain my intuition is to practice standing in the door
> with my primary focus being my relation to the ground and my motion
> across it. With high uppers I can see that I am not moving very much
> and the previous group is being swept away by the upper wind. When
> I step out I too will be swept away by that same wind and will end
> up pretty close on top of them.
>
> With no uppers, I am covering distance across the ground and basically
> leaving the previous group where they got out (except for forward throw)
> and moving away from them.

Bryan Burke has a number of pithy sayings, one of which is

"Minds are like parachutes.
"Sometimes they just don't work.

So this effort foundered on poorly formulated physics,
but I'll be back sometime later and start a new thread.

I know that 98% of the attention these days needs to be
developing customs that allow different kinds of canopy
flying to coexist, but I'd just like to have a nice, clean
exposition that takes new jumpers from their initial intuition
of looking out the door and leaving room, to why that doesn't
work with uppers, to when groundspeed is a good technique
and when conditions are in unsolved territory.

Skr

physicsman  (A 58554)

Apr 25, 2012, 5:57 PM
Post #82 of 87 (385 views)
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 Re: [skr] Why Groundspeed? [In reply to] Can't Post
Great post. I've been trying to explain this as well. With some difficulty at my local DZ

physicsman  (A 58554)

Apr 25, 2012, 6:40 PM
Post #83 of 87 (374 views)
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 Re: [skr] Why Groundspeed? [In reply to] Can't Post
http://flic.kr/p/bBLy8m

I have a fully functional skydiving code written up in MATLAB. It would be a bit time consuming but if it would help elucidate matters I can create some simple movies that show what happens to separation for any given arrangement of conditions.

I think the main problem stems from the fact that most jumpers intuit skydiving by imagining the path during free-fall. The free-fall path is always misleading because you can change its shape by altering your frame of reference and it only tells you about one skydiver. The important picture that is reference frame invariant and harder to grasp is the time evolution of the line connecting any pair of skydivers. This picture gives you all of the relevant safety information and it doesn't change with respect to your vantage point.

(This post was edited by physicsman on Apr 25, 2012, 8:28 PM)

Squeak  (E 1313)

Apr 25, 2012, 9:35 PM
Post #84 of 87 (341 views)
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 Re: [physicsman] Why Groundspeed? [In reply to] Can't Post
In reply to:
http://flic.kr/p/bBLy8m

I have a fully functional skydiving code written up in MATLAB. It would be a bit time consuming but if it would help elucidate matters I can create some simple movies that show what happens to separation for any given arrangement of conditions.

I think the main problem stems from the fact that most jumpers intuit skydiving by imagining the path during free-fall. The free-fall path is always misleading because you can change its shape by altering your frame of reference and it only tells you about one skydiver. The important picture that is reference frame invariant and harder to grasp is the time evolution of the line connecting any pair of skydivers. This picture gives you all of the relevant safety information and it doesn't change with respect to your vantage point.
a similar thing has already been done by another skydiving physics dude. Dr John Kallend.
it's been on here a few times

physicsman  (A 58554)

Apr 25, 2012, 10:01 PM
Post #85 of 87 (333 views)
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 Re: [Squeak] Why Groundspeed? [In reply to] Can't Post
I see. He does have a decent simulator.

Another interesting thing that I'd like to look at in simulation is the fact that while a freeflyer will reach terminal quicker they will also have a shorter forward throw since the reduction of vertical drag usually means they are presenting a larger portion of their body, on average, to the relative winds and should come to rest relative to them quicker then a belly flier.

Squeak  (E 1313)

Apr 25, 2012, 11:36 PM
Post #86 of 87 (323 views)
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 Re: [physicsman] Why Groundspeed? [In reply to] Can't Post
In reply to:
I see. He does have a decent simulator.

Another interesting thing that I'd like to look at in simulation is the fact that while a freeflyer will reach terminal quicker they will also have a shorter forward throw since the reduction of vertical drag usually means they are presenting a larger portion of their body, on average, to the relative winds and should come to rest relative to them quicker then a belly flier.

Not really, when belly flyers and freeflier present PROPERLY, they are both presenting to the relative wind.
Ergo a belly flyer should be presenting hips and chest to the wind, and free flyers presenting feet,head or seat to the relative wind.

physicsman  (A 58554)

Apr 25, 2012, 11:45 PM
Post #87 of 87 (320 views)
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 Re: [Squeak] Why Groundspeed? [In reply to] Can't Post
Yup, your right.

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