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Was: Recommended specs on reserve exit weight, do you go over max?

 

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mjosparky  (D 5476)

Jul 12, 2006, 1:00 AM
Post #26 of 134 (1449 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
In reply to:
Were you in the same geometry class as this blond?

Your insult is not a proof or not even an argument.
It could be a sign showing you are incapable to describe the your point using the tools of physics.

That was not meant as an insult but as an observation.

You are right; I do not know the finer points of applied physics. But I do know there are 3 things that affect the forces generated during the deployment of a canopy. They would be load, deployment speed and fill time.

I can not for the life of me understand how you can believe that weight is not a factor in deployment loads. How many test drops have you made using load links to come up with this concept?Crazy

Can you explain why all TSO testing requires that a canopy be tested using a minimum weight and a minimum airspeed if weight is not a factor.

As I posted before your opinion is wrong.


riggermick  (D 17071)

Jul 12, 2006, 1:57 AM
Post #27 of 134 (1441 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

 
A canopy does not "know" about the suspended weight on deployment, only the airspeed. So why would it blow up?

I'm aware that I might use it beyond its certification.


You haven't been included on many drop test progams have you?

In the "real world" Stuff like this happens all the time. That's how the "industry" works stuff out. What you are currently using is a DIRECT result of the drop testing done within that industry. Basic physics (in the real world) will hold true and theoretical science is still the underlying foundation to what happens during deploment, but in "real world " situations things become a little bit messier. In this realm nothing is going to be "text book" That's how it is, like it or not! Get used to it.



Mick.


980  (D 980)

Jul 12, 2006, 5:53 AM
Post #28 of 134 (1415 views)
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Re: [slotperfect] Was: Recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
Hey gang,

The original poster exercised their privilege of removing the original post. No need for me to know the reason, but since the conversation was so constructive, I clipped all of the replies and put them back in here in G&R with a new subject line.

except my reply? what gives?





From slotperfect: Clipping the original content is not an easy process, nor is it perfect. Here is your reply resurrected from the original thread . . .

Quote:
Why would you want to get a reserve that you will load above the manufacturer's recommended maximum?

When you can get a reserve that will fit the same size container that is within the manufacturer's recommended maximum loading?

and by all accounts has better flight characteristics and landings??


(This post was edited by slotperfect on Jul 12, 2006, 8:11 AM)


fcajump  (D 15598)

Jul 12, 2006, 7:02 AM
Post #29 of 134 (1409 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
In reply to:
F=MA

F= force
M=mass
A=acceleration

---------------------------------------
F=m * a is one side of the equation.

Here is the other side:

F = c * A * v^2

where A is the area of the canopy
V is the speed
c is a coefficient

m * a = c * A * v ^ 2

As you see lager acceleration/deceleration belongs to bigger mass,because forces in this system does not depend on the mass of the system.

c is deppending on the shape, air desity....
A the area of the canopy is constant
V is the deployment speed

---------------------------------

I will agree with your initial equations, however...

1. c and A will be constantly changing during the deployment
2. v (velocity at deployment) is a function of:
m (mass)
c2 (shape, air density of freefalling body prior to deployment), and
A2 (the freefalling body's area)

Finally, it is common practice in modern science to take the theoretical equations and place them in the lab for validation. As we have both test results and actual experiences that seem to show that (in some combinations) overloaded and/or overspeeded canopes (and harnesses) have/will fail due to excessive stress, it seems that the mathmatical model is itself flawed.

As a heavier jumpers, who is a self-proclaimed chicken-sh!t, I will heed the warnings of the manufacturers and testers who have more practical experience with this stuff than I. (Matter of fact, this is why I retired a reserve...) To me, failure of a reserve system (including harness) because someone wanted to second guess the test results is unacceptable. If you want to be a test jumper or designer, go for it, but understand that is what you are doing when you go outside the placarded limits.

Just my rant.
Jim


phoenixlpr  (D 3049)

Jul 12, 2006, 7:19 AM
Post #30 of 134 (1402 views)
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Re: [fcajump] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
1. c and A will be constantly changing during the deployment
In any case, but that is not depending on the mass
In reply to:
2. v (velocity at deployment) is a function of:
m (mass)
We assumed that we don't exceed maximum deployment speed.
In reply to:
c2 (shape, air density of freefalling body prior to deployment), and
A2 (the freefalling body's area)

In reply to:
If you want to be a test jumper or designer, go for it, but understand that is what you are doing when you go outside the placarded limits.
That was not a question.


SethInMI  (A 47765)

Jul 12, 2006, 11:06 AM
Post #31 of 134 (1374 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

[SEE LATER POST FOR BETTER DATA]

I did some more thinking about this, and ran some spread sheet numbers to try and make some sense of it.

In my spreadsheet I assumed a parachute was completely open (a round) and checked to see how fast it would stop with different weights.

I believe what happens is the heavier jumper does cause the parachute to open more slowly, but this subjects the lines and canopy to the high opening drag forces for a longer period of time which is what will cause the failure.

Now a slower opening will change this, as has been pointed out, but the fact remains, a heavier load causes a longer deceleration which subjects the canopy to high drag forces for a longer period of time, which is tougher on the lines and fabric.

Seth


(This post was edited by SethInMI on Jul 12, 2006, 2:03 PM)


phoenixlpr  (D 3049)

Jul 12, 2006, 11:47 AM
Post #32 of 134 (1359 views)
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Re: [SethInMI] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

Where has your data set coming from?

As you see the peak forces are the same.
AFAIK textiles are rate by the peak break force.


Premier billvon  (D 16479)
Moderator
Jul 12, 2006, 12:18 PM
Post #33 of 134 (1342 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

You're using ideal physics; the real world is not ideal.

Here's one effect:

A light test object is dropped at a high speed. The reserve opens. Due to the high speed, elements of the parachute begin to overstress and elongate. Meanwhile, the force on the test object (caused by drag) decelerates it very quickly. Lower speed = less force on the elements of the parachute under load; the parachute does not ultimately fail (although some parts of elongated/saw a lot of stress during the opening.)

A heavy test object is dropped at a high speed. The reserve opens. Due to the high speed, elements of the parachute begin to overstress and elongate. Meanwhile, the force on the test object (caused by drag) decelerates it more slowly, since the mass is higher. Higher speed = more force on the elements of the parachute under load; the parachute fails.


SethInMI  (A 47765)

Jul 12, 2006, 12:29 PM
Post #34 of 134 (1337 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

I got the data from my own calculations. (Warning: Math!)

As you know, Fd=Cv^2 (where C is area and drag coefficent and some other things). If a parachute is allowing a 220 lb person to descend at a constant rate of 10mph then 220 = C (10mph)^2. So C = 49 in SI units. Knowing C means that one can solve Fd=Cv^2 for any velocity (assuming the parachute has the same shape), and find the drag force for a given velocity (say 120mph).

Then I used A = Fd/M for the 220lb weight and got deceleration A, which slows the parachute to a new velocity, which creates a lower drag force, which creates a smaller A, etc. (Someone better at math please provide a formula, I just iterated a spreadsheet).

Now it takes time for the parachute to deploy, so C is changing during deployment, but I think the basic math is ok. My calculated deployment took less that 0.1 sec, and travelled a distance of about 8 feet, but that is fully open at terminal. If my math (excluding assumptions) is off, let me know.

Seth


SethInMI  (A 47765)

Jul 12, 2006, 2:01 PM
Post #35 of 134 (1320 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

OK. I feel much better about this spreadsheet. I even attached a picture. The distortion I got was from assuming a constant C in Fd=Cv^2. I knew C varied as the parachute opened, but I did not think how dramatically that would affect the opening. This spreadsheet contains a crude means for ramping up C over time to simulate the opening.

The difference is dramatic. There are two main mechanisms "fighting" eachother. As the parachute opens Drag increases, but that causes Speed to decrease which decreases Drag.

Now it is easy to see that increased weight causes increased peak forces on the canopy lines. If you download the sheet, change the values in:
J23 for exit weight
L23 for opening speed
Q20 for opening time in seconds

Watching the results on the chart, it is very evident that larger masses cause higher peak loads. Try a very large mass like 20000 lbs. The force just keeps ramping up, enough to blowup the chute like that cargo parachute that opened early.

Seth


(This post was edited by SethInMI on Jul 12, 2006, 2:25 PM)
Attachments: OpenForce1.JPG (30.7 KB)
  Parachute Drag3.xls (67.5 KB)


mjosparky  (D 5476)

Jul 12, 2006, 3:52 PM
Post #36 of 134 (1303 views)
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Re: [riggermick] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
A canopy does not "know" about the suspended weight on deployment, only the airspeed. So why would it blow up?

I'm aware that I might use it beyond its certification.



You haven't been included on many drop test progams have you?

In the "real world" Stuff like this happens all the time. That's how the "industry" works stuff out. What you are currently using is a DIRECT result of the drop testing done within that industry. Basic physics (in the real world) will hold true and theoretical science is still the underlying foundation to what happens during deploment, but in "real world " situations things become a little bit messier. In this realm nothing is going to be "text book" That's how it is, like it or not! Get used to it.



Mick.
Mick,

Looks like a lot of time was wasted. All those tests could have been done with just a couple of 10 pound shot bags.Tongue


SethInMI  (A 47765)

Jul 12, 2006, 5:04 PM
Post #37 of 134 (1289 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

OK!! I feel even better about this spreadsheet.

The only difference between this and previous one is how I ramped up C in Fd=Cv^2. In the previous spreadsheet, I increased C linearly. In this one, C increases at a quadratic rate. Since C is proportional to the area of the canopy, and area proportional to the square of the radius (for a round), I figured the radius of the canopy increases linearly during inflation (or close to it, and this would roughly apply to squares as well), so C increases quadratically.

The interesting thing is with this spreadsheet, the values in the NAB test that Sparky posted make sense. I can plug in the combinations and see peak forces in the 5000 and 3000 lb range for the various tests with a 2 second opening time. This gives me some confidence that I am in the neighborhood of reality with the model that I have come up with.

Now obviously this is just an approximation of the actual forces involved, but it further demonstrates the basic point I want to make: Heavier people cause higher forces on the canopy during opening.

Seth
Attachments: ParachuteDrag4.xls (97.5 KB)


sprtdth  (D 4730)

Jul 12, 2006, 6:05 PM
Post #38 of 134 (1278 views)
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Re: [mjosparky] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

 
You haven't been included on many drop test progams have you?

In the "real world" Stuff like this happens all the time. That's how the "industry" works stuff out. What you are currently using is a DIRECT result of the drop testing done within that industry. Basic physics (in the real world) will hold true and theoretical science is still the underlying foundation to what happens during deploment, but in "real world " situations things become a little bit messier. In this realm nothing is going to be "text book" That's how it is, like it or not! Get used to it.
__________________________________________________

Sparky, Mick
Obviously these guys have never had to cutaway an "experimental" only to hear (after landing) " But it worked on paper!".


mjosparky  (D 5476)

Jul 13, 2006, 1:05 AM
Post #39 of 134 (1250 views)
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Re: [SethInMI] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

I just reviewed you last spreadsheet and it seems your model is pretty close. It also seems that you have way to much free time. I am going to lay down my head hurts.Smile

And yes, heavier loads create higher forces.Wink A very basic concept.


phoenixlpr  (D 3049)

Jul 13, 2006, 1:46 AM
Post #40 of 134 (1245 views)
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Re: [SethInMI] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

Your calculations are incorrect opening time is depending on the deployment speed and suspended weight.

In reply to:
Now obviously this is just an approximation of the actual forces involved, but it further demonstrates the basic point I want to make: Heavier people cause higher forces on the canopy during opening.
It shows what you want to see.


riggermick  (D 17071)

Jul 13, 2006, 2:01 AM
Post #41 of 134 (1237 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
Your calculations are incorrect opening time is depending on the deployment speed and suspended weight.

In reply to:
Now obviously this is just an approximation of the actual forces involved, but it further demonstrates the basic point I want to make: Heavier people cause higher forces on the canopy during opening.
It shows what you want to see.



Basicly, small parachutes HATE fat people and (for the most part) vise versa. Not aholelota math in that one!!Sly


Mick.


phoenixlpr  (D 3049)

Jul 13, 2006, 2:25 AM
Post #42 of 134 (1233 views)
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Re: [riggermick] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
Basicly, small parachutes HATE fat people and (for the most part) vise versa. Not aholelota math in that one!!Sly
I'm sorry but I don't have love and hate relationship with my reserve canopy. Cool


riggermick  (D 17071)

Jul 13, 2006, 2:32 AM
Post #43 of 134 (1230 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
In reply to:
Basicly, small parachutes HATE fat people and (for the most part) vise versa. Not aholelota math in that one!!Sly
I'm sorry but I don't have love and hate relationship with my reserve canopy. Cool



What do you care? Youv'e already hung it up!! OR HAVE YOU?.............?


I'm betting NOT!!!! @ least for a while.Cool


MickLaugh


phoenixlpr  (D 3049)

Jul 13, 2006, 2:46 AM
Post #44 of 134 (1227 views)
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Re: [riggermick] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

I really don't know what you mean.

I've jumped it, check first post.


SethInMI  (A 47765)

Jul 13, 2006, 5:46 AM
Post #45 of 134 (1208 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
Your calculations are incorrect opening time is depending on the deployment speed and suspended weight.

...

It shows what you want to see.

Phoenix:

It is true I don't have a good way to model opening time, and that will affect the calculations. However, if increasing suspended weight makes the parachute open faster, as I think it would, then this would only increase the peak forces on the heavier jumper.

I think you are only seeing what you want to see.


Sparky:

I did take way too much time to build that spreadsheet. I am glad I got something out of it that seemed to make sense, but I wish it didn't take me so long.CrazySmile Stupid Math!


(This post was edited by SethInMI on Jul 13, 2006, 5:46 AM)


phoenixlpr  (D 3049)

Jul 13, 2006, 5:59 AM
Post #46 of 134 (1202 views)
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Re: [SethInMI] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
It is true I don't have a good way to model opening time, and that will affect the calculations. However, if increasing suspended weight makes the parachute open faster, as I think it would, then this would only increase the peak forces on the heavier jumper.

Heavier load had longer opening time and high average opening forces, but peak forces were the same in your previous simulation.


bch7773  (C License)

Jul 13, 2006, 6:28 AM
Post #47 of 134 (1198 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

even IF what you are saying is true, that heavier loads cause slower openings....
then a reserve that is supposed to open in 500 ft and save your life might take 1000 ft and you go splat.


phoenixlpr  (D 3049)

Jul 13, 2006, 6:42 AM
Post #48 of 134 (1195 views)
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Re: [bch7773] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

In reply to:
even IF what you are saying is true, that heavier loads cause slower openings....
then a reserve that is supposed to open in 500 ft and save your life might take 1000 ft and you go splat.

That can be true. I think the blow up is a myth.


SethInMI  (A 47765)

Jul 13, 2006, 7:54 AM
Post #49 of 134 (1179 views)
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Re: [phoenixlpr] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

Phoenix:

I will try once more to convince you, by trying something that is commonly done in engineering when testing a formula: try the extreme case to see if that makes sense. Here I ran a 15k lb load from 160mph, simulating the cargo deployment blowup that was mentioned earlier. The kink in the load graph is where the chute finishes deploying.

If you deploy a 15k lb skydiver shaped load at 160mph, it would start to accelerate, then slow as the chute opened. The new terminal velocity for such a load would be ~80 mph, which is one reason you would need more than one chute to stop the load. The other reason is the peak load forces for this opening are >50k lbs, even though peak g forces are only about 1 g. At 50k lbs the parachute will blow up.

You can modify the opening time a bit, but the fact remains, higher suspended loads cause higher loading forces on the chute during opening.

Other than getting a trusted parachute designer to personally explain things to you, I don't know what else to say.

Seth
Attachments: 15kOpening.JPG (54.5 KB)


phoenixlpr  (D 3049)

Jul 13, 2006, 8:31 AM
Post #50 of 134 (1172 views)
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Re: [SethInMI] recommended specs on reserve exit weight, do you go over max? [In reply to] Can't Post

I was not motivated enough to make my own model about the deployment of a canopy, but I have found this scientific publication:

Parks College Parachute Research Group
Outline of the presentation at the 2001 Parachute Industry Association Symposium


It is worth reading.

Quote:
Skydivers have asked us this question all the time: When does a jumper feel the greatest force from his/her risers? Does opening shock occur during the slider-descent phase or during the other two phases of parachute inflation? As mentioned before, the answer depends on the specifics of the parachute design being flown. In particular, it depends on:

1) The size of the pilot chute and whether it is of a collapsible type;
2) The size of the slider;
3) The size of the main parachute;

Other factors may also enter into the picture, but to a lesser extent, such as: the amount of steering line paid out by brake stowing, fabric porosity, suspended weight, etc.

So blowing up because of overload is just an unconfirmed a myth.


(This post was edited by phoenixlpr on Jul 13, 2006, 9:02 AM)


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