0
mailin

G-Force question

Recommended Posts

Quote

In addition to the unequal attachments, you also have to consider the material themselves.

An extreme example would be Silly Putty.

It can withstand quite a bit of stretching if the force is applied slowly, yet if the force is applied rapidly, it fractures.

Ketchup in a bottle is similar.

Do a quick google for "thixotropic".

(Actualy bill, I assume you know this . . . just continueing on for the masses.)



That's called "strain rate sensitivity", and is another matter entirely.
...

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

Share this post


Link to post
Share on other sites
Quote

That's called "strain rate sensitivity", and is another matter entirely.



I'm just sort of guessing here, but I'm thinking it's pretty darn relevant when you consider all the different parts involved in the human/parachute deloyment system and which parts might fail first -- especially when you consider specific injuries or maladies that a person might be worrying about.

The original reason for this topic.
quade -
The World's Most Boring Skydiver

Share this post


Link to post
Share on other sites
Quote

That's called "strain rate sensitivity", and is another matter entirely.



Actually, strain rate is pretty damn analagous to jerk, I think. Strain is directly related to stress, which is related to force, which is related to acceleration. Rate of change of acceleration is jerk (as we've discussed), and I think rate of change of strain in a solid would be totally affected by the magnitude of the jerk (rate of change of acceleration).

Succinctly, the more quickly the acceleration (and force) changes, the more quickly the strain changes. The importance of jerk has to do with the material (as billvon pointed out earlier, electronic components or more "solid" objects), and the importance of strain rate also has to do with the material (as quade pointed out with silly putty and the human body).
www.WingsuitPhotos.com

Share this post


Link to post
Share on other sites
Quote

Quote

That's called "strain rate sensitivity", and is another matter entirely.



Actually, strain rate is pretty damn analagous to jerk, I think. Strain is directly related to stress, which is related to force, which is related to acceleration. Rate of change of acceleration is jerk (as we've discussed), and I think rate of change of strain in a solid would be totally affected by the magnitude of the jerk (rate of change of acceleration).

Succinctly, the more quickly the acceleration (and force) changes, the more quickly the strain changes. The importance of jerk has to do with the material (as billvon pointed out earlier, electronic components or more "solid" objects), and the importance of strain rate also has to do with the material (as quade pointed out with silly putty and the human body).



A substance that is strain rate sensitive has a relationship between stress and the time derivative of strain. sigma = f(epsilon-dot). It has a very specific meaning which is not what you referred to above. Silly putty is an extreme example of a strain rate sensitive material. Common structural materials (metals and alloys, ceramics) do not usually exhibit much strain rate sensitivity although they can be processed to be strain rate sensitive (e.g. superplastic alloys). Metals typically show sigma = f(epsilon) rather than sigma = f(epsilon-dot).
...

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

Share this post


Link to post
Share on other sites
Quote

A substance that is strain rate sensitive has a relationship between stress and the time derivative of strain. sigma = f(epsilon-dot). It has a very specific meaning which is not what you referred to above. Silly putty is an extreme example of a strain rate sensitive material. Common structural materials (metals and alloys, ceramics) do not usually exhibit much strain rate sensitivity although they can be processed to be strain rate sensitive (e.g. superplastic alloys). Metals typically show sigma = f(epsilon) rather than sigma = f(epsilon-dot).



Actually, I didn't refer to a relationship between stress and strain rate (I am aware of the relationship). I tried to explain that force rate (jerk) is related to strain rate, and therefore would affect stress... as I said in my first post stating that jerk (not just G force) affects the stress the jumper feels.

Even with your "silly putty" type materials where there is a relationship between stress and strain rate, there is still a relationship between stress and strain (like any solid). And even with stress in metals (which as you pointed at is generally looked at as a function of strain), strain rate is also a factor, just not nearly as large of a factor as in more flexible materials. With any material, both of your relationships are true.

(1) stress = f(strain)
(2) stress = f(strain rate)

Both are true! (for anything) However in most cases it may be more pertinent to focus on one relationship.

The reason you see certain relationships focused on in texts is because of the normal loading conditions of certain materials, and the relative importance of strain and strain rate for those materials. For instance, metal structures are normally designed to hold constant (or slowly varying, i.e. low-jerk) strains, and strain rate doesn't have too much of an effect on these types of solids, so you generally see equations relating stress to strain for rigid materials. Flexible structures, on the other hand, are normally designed to hold time-variant (quickly varying, i.e. high jerk) strains, which also have a more pronounced effect on these types of materials (as opposed to rigid ones), so you might see equations in a book relating stress to strain rate for flexible materials.

Now what (1) and (2) above really say, are what I said a very long time ago, way back when we were still talking about skydiving.

(1) The ability of a hard opening to hurt someone is dependent on the acceleration involved (G force).
(2) The ability of a hard opening to hurt someone is dependent on the jerk involved (G force rate).

Both are true!

The ability it has to hurt you, is how much stress it generates in the vital areas of your body. Both strain and strain rate affect stress produced in a material, and both force (acceleration) and force rate (jerk) affect stress produced in a jumper on opening.
www.WingsuitPhotos.com

Share this post


Link to post
Share on other sites
Quote

Quote

A substance that is strain rate sensitive has a relationship between stress and the time derivative of strain. sigma = f(epsilon-dot). It has a very specific meaning which is not what you referred to above. Silly putty is an extreme example of a strain rate sensitive material. Common structural materials (metals and alloys, ceramics) do not usually exhibit much strain rate sensitivity although they can be processed to be strain rate sensitive (e.g. superplastic alloys). Metals typically show sigma = f(epsilon) rather than sigma = f(epsilon-dot).



Actually, I didn't refer to a relationship between stress and strain rate (I am aware of the relationship). I tried to explain that force rate (jerk) is related to strain rate, and therefore would affect stress... as I said in my first post stating that jerk (not just G force) affects the stress the jumper feels.

Even with your "silly putty" type materials where there is a relationship between stress and strain rate, there is still a relationship between stress and strain (like any solid).
Quote



Silly Putty isn't a solid.




And even with stress in metals (which as you pointed at is generally looked at as a function of strain), strain rate is also a factor, just not nearly as large of a factor as in more flexible materials. With any material, both of your relationships are true.

(1) stress = f(strain)
(2) stress = f(strain rate)

Both are true! (for anything) However in most cases it may be more pertinent to focus on one relationship.



I didn't say it wasn't true. I focussed on the pertinent relationship.

That strain rate depends on stress rate doesn't require a strain-rate sensitive material, it will be true of a linear elastic material or a rigid/plastic material.

Once again, strain-rate sensitivity is a very specific materials property, not just a response to varying stress - "jerk" has nothing to do with it.


...

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

Share this post


Link to post
Share on other sites
Quote

>What is really causing the type of problem you describe is that the
>forces are applied in a non-uniform manner to the non-rigid system . . .

Well, actually I think the problem is applying a uniform field to a body which is not uniformly restrained. A common injury in impact, for example, is the aorta separating from the heart, because the two structures (aorta and heart) are not equally attached to the skeleton (the 'rigid' part of the system.) A very short high acceleration pulse may cause the heart and aorta to translate, but not over enough distance to cause them to separate. A longer pulse might.

In the elevator example the issue is onset, or jerk. A rapid onset pulse would cause the person to be rapidly accelerated towards the wall (well, in relation to the elevator at least) and then decelerated very, very rapidly when he hit the wall. A slow onset pulse would cause him to fall relatively gently against the wall and stop there; while on the wall he can better deal with the acceleration.



I don't see any fundamental relationship involving jerk that corresponds to Newton's 2nd law with acceleration. I haven't come across any constitutive relations for any material that involve jerk either. Surely you can analyze a non rigid system (modeling it with masses, springs and dashpots if necessary) using only the constitutive laws of the elements, the forcing function which is usually defined in terms of forces or displacements, and Newton? How does the use of jerk help the analysis? Does it simplify the system of equations or lead to some new insight in some way?
...

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

Share this post


Link to post
Share on other sites
Quote

Once again, strain-rate sensitivity is a very specific materials property, not just a response to varying stress - "jerk" has nothing to do with it.



Agreed. Jerk has nothing to do (in the sense that it does not affect them) with a material's properties, including its sensitivity to strain rate. However, I stand by my position that jerk is directly related to strain rate. So a material which, by definition, is strain-rate sensitive, I would also consider "jerk-sensitive".

Quote

I don't see any fundamental relationship involving jerk that corresponds to Newton's 2nd law with acceleration. I haven't come across any constitutive relations for any material that involve jerk either. Surely you can analyze a non rigid system (modeling it with masses, springs and dashpots if necessary) using only the constitutive laws of the elements, the forcing function which is usually defined in terms of forces or displacements, and Newton? How does the use of jerk help the analysis? Does it simplify the system of equations or lead to some new insight in some way?



Honestly, a "jerk-inclusive" mathematical analysis of a real-life situation is probably slightly over my head at the moment. But one reason I've always loved physics is because it's so intuitive (parts of it, at least). I'll use my simple car driving example again. In a car which is being decelerated at a very large magnitude, my torso will be pressed forward against the seatbelt and my head may even be hanging forward with my neck bent, if the "G-forces" are too strong for my neck to hold my head upright. But as long as the deceleration is applied smoothly and then held constant, I don't think my neck would break. But a very rapid onset of deceleration (jerk), while my torso was constrained by the seatbelt, would give me whiplash, or if severe enough, break my neck.

You are right that the fundamental rules of force-acceleration dependence (Newton) are not altered by jerk. The surface force exerted on your torso by the seatbelt is determined ONLY by the deceleration and doesn't care about the jerk. And if your body was truly rigid (which nothing is, especially not a body), the whole system would not care about jerk. But intuitively I can see a less than rigid body getting whiplash as I described above. My guess is that a detailed analysis of the situation would need to describe the time-domain motion of every single part of the body. The overall difference in translational response (due to inertia) of the head and torso would be such that it would generate certain forces in the neck, and these forces would be higher in the high-jerk case, to give you whiplash. That's what I suspect... as much as I love discussing this I'm not gonna even attempt that analysis right now...

If you want to analyze the situation without considering jerk, and it is a situation where acceleration is dynamically changing, then you will only be able to analyze one moment of the entire situation at a time (a=constant), and I don't think that would be sufficient to see the overall picture.

Keep in mind it's not actually the force of the seatbelt on the torso that's causing the guy's neck to break (that seatbelt force isn't dependent on jerk)... it's the tensile force in his neck that his own head and torso are respectively exerting on each other... and this force comes from the dynamic response of each member to the jerking and deceleration of the car. This dynamic response will be different in a situation with or without jerk.

I think that's the center point of this whole debate. The actual simple forces transmitted from car to seatbelt to human are independent of jerk, as you and Newton would both agree. :) But the flexible body would respond inertially to its "one-point constraint"... and it would respond different based on different jerks (a phenomenon of motion, which would help describe the subsequent motion of the not-entirely-constrained body). These different motions of the different parts of the body would generate different internal impulse forces from momentum. Just like billvon pointed out with the heart and aorta separating.

And since it is impossible to constrain any system entirely, I think jerk will always come into play in determining the interal forces which build up in the system. But fundamentally, you are right... the jerk does not affect the forces transmitted to the constraint points between the "force giver" and the "force receiver". But if the force receiver is flexible and only connected to the giver at one point, the internal forces which arise will be dependent on the subsequent motion of all particles in the system, and inherently, dependent on jerk.
www.WingsuitPhotos.com

Share this post


Link to post
Share on other sites
Quote

Quote



If you want to analyze the situation without considering jerk, and it is a situation where acceleration is dynamically changing, then you will only be able to analyze one moment of the entire situation at a time (a=constant), and I don't think that would be sufficient to see the overall picture.

Keep in mind it's not actually the force of the seatbelt on the torso that's causing the guy's neck to break (that seatbelt force isn't dependent on jerk)... it's the tensile force in his neck that his own head and torso are respectively exerting on each other... and this force comes from the dynamic response of each member to the jerking and deceleration of the car. This dynamic response will be different in a situation with or without jerk.



I don't dispute that, but it just seems like giving a name to something for the sake of naming it.

I disagree that the analysis you describe would be limited to one instant in time, all you need to do is set up the time-dependent system of diffeqs describing the system in terms of Newtonian mechanics and behavior of the elements describing the body, and solve as f(t). That's how we model crystal lattice vibrational dynamics, and they are as complex as anything. Prolly need a numerical solution for a complex non-linear body.
...

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

Share this post


Link to post
Share on other sites
I'm very edit-happy in case you hadn't noticed. I edited my post about 298 times and was hoping I would finish before you read it. Make sure you read the last part. :$ I'll reply more later, I have to go to bed. I enjoy this. :)
www.WingsuitPhotos.com

Share this post


Link to post
Share on other sites
Hi Jennifer!

To much to read it all so maybe someone beat me to it but why don't you try contacting Larsen&Brusgaard (Dytter, ProTrack) or Airtech (Cypres)?

They should have done at least some testing of their devises (at least I hope so) and have a good grip on the amount and type of forces that occur.

My .02 anyway.

/M

Share this post


Link to post
Share on other sites
Quote

I don't dispute that, but it just seems like giving a name to something for the sake of naming it.

I disagree that the analysis you describe would be limited to one instant in time, all you need to do is set up the time-dependent system of diffeqs describing the system in terms of Newtonian mechanics and behavior of the elements describing the body, and solve as f(t). That's how we model crystal lattice vibrational dynamics, and they are as complex as anything. Prolly need a numerical solution for a complex non-linear body.



Yeh, my comment about having to analyze one instant in time was kind of a half-joke... because if you consider acceleration as a function of time (which as you pointed out would be necessary), you are admitting that jerk matters. :) But I see where you're coming from when you say that it seems unnecessary to name jerk. You're right that the whole situation could be analyzed without explicitly naming jerk, but it would be present in the variation of a(t), and I think that an in-depth anaylsis of situations where flexible bodies are subjected to low and high da/dt at a single constraint point, the results would show what my intuition feels... that the high da/dt situation results in higher stresses in the weak points of the flexible body.

That, to me, is why jerk is worthy of a name. Also, because it is something that you can physically "feel" pretty well. Admittedly there are 4th, 5th, and 6th time derivatives of position that have been jokingly called snap, crackle, and pop... this is obviously ridiculous because it is very hard (for me at least) to physically imagine the "real-life" results of one of these things. But jerk is very real to me... I get motion sick in cars when someone else is driving (depending on how good or bad they drive)... and it is not the acceleration at all which makes me nauseous, but the jerk (which is very well-named, since it causes the car to visibly "jerk" around!). I, on the other hand, drive like a jerk... but I think that's from your other definition. :D

One last thing, I think I will rescind my previous statement that there is a direct relationship between jerk and stress. As I stated above, the high da/dt situation results in higher stresses in the weak points of the flexible body, i.e. jerk can result in higher stress. But this is really a result of the complex dynamics across the body and its different particles reacting to each other's motions. In a direct, single-particle situation, jerk will have no affect on force applied to the system, and therefore I can't really say there is a relationship stress=f(jerk). Though I do still think jerk affects stress, it's more of a thing that happens unique to each body's construction (the stresses occurring at different locations than the jerk is applied) and there's not some missing law of physics that considers jerk. You are right about that. :) Hopefully this paragraph made sense... I was thinking about the seatbelt/whiplash example for most of the things I tried to explain...

I guess a simple analog to my mistake (when I said stress=f(jerk)) would be saying (incorrectly) that stress=f(velocity), because it was experimentally observed that the faster you drive a car into a wall, the higher stresses you create in the car's structure. While the observation is very true, it doesn't imply a relationship between the two. Velocity on its own creates no stress, it's the specifics of this situation which are allowing stress to be generated, and all of those specifics can be described by a Newtonian system of DiffEQ's. :)
www.WingsuitPhotos.com

Share this post


Link to post
Share on other sites
LOL!
Glad Malin can Jump!
Malin I think the biggest thing you need to be aware of is the placement of the generator. Most of them are placed in the sub-q space below the left clavicle and that could be uncomfortable during the initial phase of deployment as you get "jerked" outta the belly to earth position. Padding to spread that force across the area of the generator, maybe in a donut shape, could be helpful.
I also heard that a bad opening most of the time, as well as most malfunctions, can be attributed to bad body position at deployment. So be aware at pull time and make sure your slow and stable.
Good luck and keep us posted to your progress!

Blue Skies, Soft Openings, and Sweet Landings



Natural Born FlyerZ.com

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

0