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# My first chop

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Shorter lines make it worse because you can get higher rpm.

That's not true when it comes to g-force. Yes you will have a higher rpm, but less g-force.
*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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>That's not true when it comes to g-force. Yes you will have a higher rpm, but less g-force.

History has shown that, when it comes to canopies, that is not true. You get much higher G forces with smaller canopies - sufficient to cause unconsciousness very rapidly.

From a post from a few years ago:

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At the American Boogie this year, a sub-100 foot VX jumper experienced a canopy collapse while doing CRW work. The canopy then violently spun up so hard that the jumper could not lift the weight of his arms up to reach his cutaway and reserve handles
===========

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>That's not true when it comes to g-force. Yes you will have a higher rpm, but less g-force.

History has shown that, when it comes to canopies, that is not true. You get much higher G forces with smaller canopies - sufficient to cause unconsciousness very rapidly.

From a post from a few years ago:

==========
At the American Boogie this year, a sub-100 foot VX jumper experienced a canopy collapse while doing CRW work. The canopy then violently spun up so hard that the jumper could not lift the weight of his arms up to reach his cutaway and reserve handles
===========

Imagine a baseball at the end of a 1 ft string. Now twirl the baseball directly over your head building up max RPMs... then let go of the string. How far will the ball go? 15' maybe 20'?

Now take the same baseball and put it on a 6' string, and repeat. How far will it go? 60' - 80'.

While the shorter string will have a much higher RPM it will build up less g-force.
*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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so a student 280 in a spin will put more g's on you than say, a velo 96?

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so a student 280 in a spin will put more g's on you than say, a velo 96?

Start with my first statement, I said: "If canopy lines were perhaps 3x longer I would have an easier time believing it to be plausible."

I'm saying longer lines will create a greater g-force. Take your Velo 96 and lengthen the lines x3, when in a flat spin will the shorter lines provide a greater g-force or the longer lines?

We should clarify that Bill changed the subject to high performance or smaller canopies. I think we all agree that high performance canopies will/can generate higher g-forces than a larger low performance canopies.

Now, if you didn't understand the baseball on the string scenario... then I can't help you.
*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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why would you tie a baseball to a string?

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I was thinking the same thing, a brick would work better.

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I was thinking the same thing, a brick would work better.

You want to tie a baseball to a brick?
"The ground does not care who you are. It will always be tougher than the human behind the controls."

~ CanuckInUSA

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>While the shorter string will have a much higher RPM it will build up less
>g-force.

1 foot string, 30 RPM: .3G acceleration
6 inch string, 60 RPM: .6G acceleration

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>While the shorter string will have a much higher RPM it will build up less
>g-force.

1 foot string, 30 RPM: .3G acceleration
6 inch string, 60 RPM: .6G acceleration

Ok, but the problem is, you are using simple set numbers there.

Which canopy set up would require a greater g-force to put a jumper in a flat spin:

1. A 150lb jumper under a 100 sf canopy with 10' lines.

or

2. A 150lb jumper under a 100 sf canopy with 30' lines.

I think we're both right, we're just thinking about it differently.

I might be wrong and if you can show me the math I will accept it.
*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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>While the shorter string will have a much higher RPM it will build up less
>g-force.

1 foot string, 30 RPM: .3G acceleration
6 inch string, 60 RPM: .6G acceleration

Also the math you are using is calculating the g-force of forward acceleration... NOT the direct outward g-force from the center of the axis, which is what we're really talking about.

Another simple way to think about is:

If you tape one nickle near the center of a bike wheel and another nickle on the outer rim of the wheel and then start to spin it... which one would fly off first? The one furthest from the axis.
*I am not afraid of dying... I am afraid of missing life.*
----Disclaimer: I don't know shit about skydiving.----

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I think it is possible for the right sized canopy and situation to arise where the g's are so strong you can't reach your handles. But to get to that point you would probably have to be incapacitated in some way to let yourself get to that point. Chances are any "proof" of a situation like this will never exist, the person would have died.

Wingsuiters say that with a violent enough spin there arms are not movable, but this likely has to do with the "rigid" wing not colapsing but still similar and in addition the "axis" is small compared to suspension lines.
Moriuntur omnes, sed non omnes vixerunt.

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>While the shorter string will have a much higher RPM it will build up less
>g-force.

1 foot string, 30 RPM: .3G acceleration
6 inch string, 60 RPM: .6G acceleration

Also the math you are using is calculating the g-force of forward acceleration... NOT the direct outward g-force from the center of the axis, which is what we're really talking about.

Another simple way to think about is:

If you tape one nickle near the center of a bike wheel and another nickle on the outer rim of the wheel and then start to spin it... which one would fly off first? The one furthest from the axis.

if you're turning this into an engineering problem, then you have to look into moment of inertia to understand why a shorter lines would create a lot more g-force, the moment of inertia grows exponentially with the distance away from the center, so a parachute with longer lines has a much harder time spinning someone around it's axis then a one with shorter lines. So smaller parachutes are capable of spinning someone at much higher rotational speed which translates into higher g-forces

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It seems you're holding different things equal. I think what he's trying to get at is that all things held equal, longer lines will generate more G-force. But all things are not equal of course.

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I can't prove anything, but get the impression that in practice, longer lines give lower g forces.

Basically, the canopy has a lot further to fly around a wide spiral than a tight one.

Say a small canopy with 10' can fly 60 mph easily enough in a diving spiral. A lot of its speed will be in a downwards component, and some will be in a horizontal component, circling around that 20 ft plus diameter cylinder.

(Of course line length doesn't actually equal radius of the helix or spiral. This isn't quite a baseball on a string. The jumper at the center is getting pulled away from the center by the canopy, so the jumper isn't just purely spinning, and the radius of the spiral is greater than the 10'.)

To make the example easy to picture in one's mind, now put 100' long lines on the canopy.

How will it get around its big cylinder fast enough? During its spiral, it may still be diving fast towards the ground, but how much speed can it put into flying around the cylinder in the horizontal direction? In degrees per second, it just can't get around that big spiral nearly as fast.

Will the speed change? Sure, maybe the canopy now dives faster than 60 mph, but it certainly won't increase to 600 mph either.

It doesn't need the same degrees per second to maintain the same G's, since for a given rotation rate, a longer radius increases the G's. To maintain the same G's at 100' not 10' radius, it would have to fly 316% faster -- and I don't think the canopy will now fly at 190 mph!

While the concept of a baseball swung around on a string doesn't really capture what is happening for a canopy flying a spiral, I think it was sufficient to understand the basic situation.

OPTIONAL DETAILS:
Why 316%?
The basic formula for the simplified baseball on a string idea gives the force over the mass (which is a measure of G loading) equalling the radius times the square of the angular velocity. i.e., F = m*r*omega^2 Angular velocity being the radians per second or degrees per second or whatever, that the object moves around its circular path.

So if you double the length of the string, and keep the same number of revolutions per second, the G load does double. If you double the length of the string, but only keep the physical speed of the baseball the same (same linear speed), the angular velocity is halved (same speed but twice as far to fly around the circle), and the net effect is a reduction in G loading. To get the same G loading with twice the line length, the answer will be somewhere between those two cases, needing 70.7% of the original angular velocity. Or, for 10 times the radius, G loading is maintained with .316 of the angular velocity, but with 10 times the distance to fly around the circle, that's 316% more speed.

Let me know anyone, if I buggered up the math.

(As a practical consequence of all this, when I turned a 97 square foot canopy into a 37 sq. ft cutaway-only canopy, I kept the original line length to try to reduce any potential G load issues if the thing spiralled hard.)

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It seems you're holding different things equal. I think what he's trying to get at is that all things held equal, longer lines will generate more G-force. But all things are not equal of course.

Right.

If the spin was at the same rpm, the G load would be higher for longer lines.

But the longer lines would keep the rpms down.
Shorter lines allow the spin to reach much, much higher rpms and generate a much higher g-load.
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

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Did anyone figure it out?

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....the spin could have developed enough force that the jumper couldn't have lifted his arms to reach the cutaway handle.

I've heard a few people say that. I have a hard time believing that g-forces can be accelerated high enough under canopy, as to not allow the average jumper to lift (or especially) flex their arms enough to reach their cutaway.

Until I see or read proof... I call BS.

Read between the lines on some fatality reports, some jumpers are blacking out before they can cutaway !!! That's what I think.... maybe it is not proven. Ask Ted Strong or Brian Germaine, though.. they have been there !!!

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....the spin could have developed enough force that the jumper couldn't have lifted his arms to reach the cutaway handle.

I've heard a few people say that. I have a hard time believing that g-forces can be accelerated high enough under canopy, as to not allow the average jumper to lift (or especially) flex their arms enough to reach their cutaway.

Until I see or read proof... I call BS.

For those who knew the gentleman please forgive me, but i cant rememebr his name.

A guy was jumping one of the ultra small canopies a few years ago (46ft i think), direct deployed off the door, He had line twists, he spun a couple of times, within about 2 or 3 revolutions he had blacked out. Unfortunately he spiralled in and died
You are not now, nor will you ever be, good enough to not die in this sport (Sparky)
My Life ROCKS!
How's yours doing?

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Did anyone figure it out?

The physics behind it is centrifugal force:

Fc = m*v^2/r
Fc= centrifugal force (can be used to figure Gs)
m = mass (kgs)
v = velocity (m/s) -- need to convert from RPS to m/s

Cheat calculator at : http://www.calctool.org/CALC/phys/newtonian/centrifugal

Shorter lines say, 1m, 1 RPS
60kg diver

Fc = 60 kg * (6.28 m/s)^2 / 1m
Fc = 2368 Newtons
Gs = 4.02

Longer lines, say 4m, 1 RPS
60kg diver
Fc = 60 kg * (25.12 m/s)^2 / 4 m
Fc = 9474 Newtons
Gs = 16.1

The short of it is, take the difference in the lines and multiply by 4 and that will be how many extra Gs the longer set will have, given same RPMs.
You stop breathing for a few minutes and everyone jumps to conclusions.

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Did anyone figure it out?

The physics behind it is centrifugal force:

Fc = m*v^2/r
Fc= centrifugal force (can be used to figure Gs)
m = mass (kgs)
v = velocity (m/s) -- need to convert from RPMs to m/s

Cheat calculator at : http://www.calctool.org/CALC/phys/newtonian/centrifugal

Shorter lines say, 1m, 1 RPM
60kg diver

Fc = 60 kg * (6.28 m/s)^2 / 1m
Fc = 2368 Newtons
Gs = 4.02

Longer lines, say 4m, 1 RPM
60kg diver
Fc = 60 kg * (25.12 m/s)^2 / 4 m
Fc = 9474 Newtons
Gs = 16.1

The short of it is, take the difference in the lines and multiply by 4 and that will be how many extra Gs the longer set will have, given same RPMs.

how did you come to:
v=6.28 m/s?
v=25.12 m/s?
r=1m?
r=4m?

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Did anyone figure it out?

The physics behind it is centrifugal force:

Fc = m*v^2/r
Fc= centrifugal force (can be used to figure Gs)
m = mass (kgs)
v = velocity (m/s) -- need to convert from RPMs to m/s

Cheat calculator at : http://www.calctool.org/CALC/phys/newtonian/centrifugal

Shorter lines say, 1m, 1 RPM
60kg diver

Fc = 60 kg * (6.28 m/s)^2 / 1m
Fc = 2368 Newtons
Gs = 4.02

Longer lines, say 4m, 1 RPM
60kg diver
Fc = 60 kg * (25.12 m/s)^2 / 4 m
Fc = 9474 Newtons
Gs = 16.1

The short of it is, take the difference in the lines and multiply by 4 and that will be how many extra Gs the longer set will have, given same RPMs.

You have calculated your velocities at 60 RPM (1 rev/second), not 1 RPM as you stated in the example.

At 1 RPM the forces are negligible!

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if you need 3 seconds for one full rotation your angle speed of 120 degrees per second. With 4m radius centrifugal acceleration is 1.79g

if you need 2 seconds per rotation for other parameters the same it's 4.02g

1 second per rotation (360 degrees) it's 16.1g

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