1 1
CoolBeans

Shock absorbing risers - with crumple zones - to soften hard openings

Recommended Posts

3 hours ago, fcajump said:

Solution concept:  Dual Collin's Lanyards

As far I understand (of course, possibly wrong) collin's lanyard is symmetric (one riser [one with rsl] releases the other one, but not the other way around).

When you say dual collin's lanyard you mean two lanyards so both riser release the other riser on separation? Is this something that's used in any rigs? Curious if it's already "a thing" as I've never heard of it.

Share this post


Link to post
Share on other sites
(edited)
4 hours ago, Binary93 said:

As far I understand (of course, possibly wrong) collin's lanyard is symmetric (one riser [one with rsl] releases the other one, but not the other way around).

When you say dual collin's lanyard you mean two lanyards so both riser release the other riser on separation? Is this something that's used in any rigs? Curious if it's already "a thing" as I've never heard of it.

I don't think that sort of system is in use currently.

With the existing skyhook sort of collins lanyard system, depending on which side of the main riser breaks, you either have both sides cut away and reserve activated, or a high speed streamer/one riser attached but no activation of the reserve, which is OK also.

Bill Booth I believe has previously written on this forum about the inherent weakness of mini risers possibly being the most practical way of limiting force. Maybe we'd be better off to not have the reinforcements on mini risers, but have to keep a close watch for signs of damage and replace them after a certain number of jumps or use some force test setup to confirm a minimum strength if in doubt...Risers would need to be made very consistently, maybe destructively tested samples confirming things. It would be a hassle, but could be effective.

 

Edited by sundevil777
  • Like 1

Share this post


Link to post
Share on other sites

This idea has come up once or twice. I remember one conversation about the value of dacron lines in mitigating hard openings. 

 

We use screamers the folded in half and zigzagged kind in some of our designs. It kind of comes down to the amount of kinetic energy absorbed. Force times distance. The bottom line is that the magnitudes are just to high. You just can't make a meaningful difference in the overall opening. But it can effect some things. Openings are not one smooth curve. The canopy is lifted to line stretch and you can feel a sharp jerk when all that momentum runs out of line. Depending on the snatch force size of pilot chute high speed that can be a painful high but brief spike. It can be the highest peak force in an opening. But it's generally followed by a much larger bell curve of some type. In a square it's mostly determined by the slider and at what speed it loses its dominance over the opening. The area of this second curve is just too large to be affected by a small amount of elongation. 

 

So the peak of that first spike is within the realm of something you could effect but the main opening just involves too much kinetic energy. Dacron lines play a different role in creating higher friction to slow the timing and decent of the slider. That is meaningful. More so then their elongation. That and improved staging from the rubber bands having a better grip on them is where their reputation really come from.

 

I've changed my mind over the years. I do think that there is value in risers breaking to control super high peak forces. It's not a common occurrence but I've seen fatalities that I attribute to opening shock. It's not a fun idea but there is something to be said for a fuse in the circuit. Maybe type 6 risers weren't such a crazy idea after all.

 

Lee

Share this post


Link to post
Share on other sites
On 6/11/2020 at 7:50 PM, Binary93 said:

As far I understand (of course, possibly wrong) collin's lanyard is symmetric (one riser [one with rsl] releases the other one, but not the other way around).

When you say dual collin's lanyard you mean two lanyards so both riser release the other riser on separation? Is this something that's used in any rigs? Curious if it's already "a thing" as I've never heard of it.

Not something I'm aware of currently in use, but my thinking is this:

 - With unreinforced Type 17, it would act as a weak link in the case of a very hard opening.

 - Currently, the Collin's lanyard is provided (mostly to my knowledge, in concert with Skyhooks) to ensure that, should the RSL side riser break, the other riser would be released prior to reserve activation.

- If we had a release system similar to that on each side, such that a break of either riser would result in the release of the other, having weak-link risers would not be as bad a thing.  (it would avoid the bad situation of single riser failure and the reserve being deployed prior to the second riser being released).

No, I don't know how this would be setup as I'm now saying the "long" side riser needs to pull the "short" side cable... but we've got smart people here...

 

JW

 

 

Share this post


Link to post
Share on other sites
On 6/12/2020 at 12:35 AM, sundevil777 said:

Bill Booth I believe has previously written on this forum about the inherent weakness of mini risers possibly being the most practical way of limiting force. Maybe we'd be better off to not have the reinforcements on mini risers, but have to keep a close watch for signs of damage and replace them after a certain number of jumps or use some force test setup to confirm a minimum strength if in doubt...Risers would need to be made very consistently, maybe destructively tested samples confirming things. It would be a hassle, but could be effective.

 

I know of at least one person who builds (built) risers to spec for an unnamed factory, but would build his own Type-17 without the reinforcing strip for exactly this reason...  he'd rather face the problems of a broken riser on opening than the overload situation.

Years ago we figured out how to build parachute systems where the weakest link is the body in the harness.
While I am grateful that we're not breaking/falling out of harnesses, as long as we are going to have the possibility of openings that overload the jumper's body, we need to figure out how to put a weaker link back in...  I agree with Bill that the risers seem to be the best place for it.

JW

 

Share this post


Link to post
Share on other sites

Someone posted something similar earlier this thread, but I thought it was still viable until I did some calculations.

Assuming that freefall speed is 120 mph (54 m/s), and canopy speed is 14 m/s (30 mph - still pretty fast), if the opening takes 1 second (pretty hard opening) you'll get about 4 Gs.

That doesn't sound very high until you realise the upright position is quite bad for this kind of acceleration.

Let's assume it's even worse, say your canopy opens in 0.5 s, giving an 8G acceleration.

If we were to completely absorb 0.5 seconds with an elastic stretching to bring it back to 4 G's, you'll need 10 metres of stretching since you're trying to get rid of 40 m/s (from 54 to 14 m/s).

Even if you're just trying to mitigate the very worst case, say 0.25-second opening which causes 16 G's, you'll need 5 metres of stretching to bring it down to 8 G's.

If your risers are stiffer and don't stretch 5 metres, they won't relieve the acceleration anyway. It would be like trying to use water to break a terminal freefall - it will feel hard as concrete when you get a hard opening.

So as much as I wanted this elastic stretching idea to work, unfortunately it probably won't. However, what might work is if the risers break completely at a certain load - if the RSL or a MARD is connected, then the reserve might give you a second chance of not opening so hard.

 

  • Like 1

Share this post


Link to post
Share on other sites
(edited)
21 minutes ago, olofscience said:

(...) However, what might work is if the risers break completely at a certain load - if the RSL or a MARD is connected, then the reserve might give you a second chance of not opening so hard.

 

What if I add 2 inch fold of the webbing around the breaking point, so once it breaks the risers are still usable?

That's exactly what I'm explaining from the beginning. Think of "Shock absorbing risers" just like regular risers that break at specified load, except, they can break multiple times at staggered loads and you can still land them afterwards. 

Edited by CoolBeans

Share this post


Link to post
Share on other sites

i would think that it would be ok if they just broke, they saved your life and you can get to your reserve now.  you may have a cutaway, and it may be spinning, but if you catch it fast, you should be good.  i was talking to a rigger about it and he was saying how adding things like the slinks i mentioned earlier would make them bulkier and harder to pack, so i think a fold would be better.  maybe you could use the stitching as the weak point.  make a fold and put a few lines of thin thread in there that would hold up to a point, but break if it got to a certain load. 

Share this post


Link to post
Share on other sites
15 minutes ago, CoolBeans said:

What if I add 2 inch fold of the webbing around the breaking point, so once it breaks the risers are still usable?

That's exactly what I'm explaining from the beginning. Think of "Shock absorbing risers" just like regular risers that break at specified load, except, they can break multiple times at staggered loads and you can still land them afterwards. 

The energy absorbed by linear elastic stretching is given by E = 0.5 * K * x^2

Where K is the elastic modulus, x is the distance stretched.

For anything breaking, the energy absorbed is equal to the area under the stress-strain curve. Meaning that, if something breaks without stretching, it hasn't absorbed much energy. If it hasn't absorbed much energy, it hasn't slowed you down very much.

(Kinetic energy is given by a similar equation: E = 0.5 * M * v^2 where M is your mass, v is your velocity)

So ideally, the risers should stretch as much as possible before breaking, then the reserve deploys from a slightly-slowed down skydiver.

If the risers simply break without stretching, then you're pretty much going as fast as you did before, and all you can hope for is that the reserve opens softer than your main. Which might still be a better gamble, but reserves aren't exactly designed to open slowly.

Share this post


Link to post
Share on other sites
1 minute ago, olofscience said:

For anything breaking, the energy absorbed is equal to the area under the stress-strain curve.

Just need to add - for anything breaking, the energy absorbed is equal to the area under the stress-strain curve, PLUS the surface energy used to create the extra fracture surface.

(theoretically, + the heat dissipated by the stretching + plastic deformation + fracturing, but we can probably assume it's negligible).

I was actually imagining if you had those hydraulic pistons/shocks (for car doors and stuff) as risers, but they might still not have enough stretch to actually mitigate a hard opening. But again this is all theory, still plenty of unknowns so testing would still teach us something ^_^

Share this post


Link to post
Share on other sites

i've got an old harness that i can't use.  what type of setup would one need to start testing something like this?  i could probably find out through searching, but if anyone has a starting point it helps.  i wanted to become a rigger anyway, may as well start with a project.  worst case scenario i fail and learn something in the process.

Share this post


Link to post
Share on other sites
1 hour ago, olofscience said:

Just need to add - for anything breaking, the energy absorbed is equal to the area under the stress-strain curve, PLUS the surface energy used to create the extra fracture surface.

(theoretically, + the heat dissipated by the stretching + plastic deformation + fracturing, but we can probably assume it's negligible).

I was actually imagining if you had those hydraulic pistons/shocks (for car doors and stuff) as risers, but they might still not have enough stretch to actually mitigate a hard opening. But again this is all theory, still plenty of unknowns so testing would still teach us something ^_^

Hi science,

I have no idea of how old you are or how long you've been in this skydiving 'thingy' of ours.  And, this is not to be disrespectful of you; I actually like your posts.

Pioneer built risers with shock absorbers ( hydraulic pistons/shocks ) back in the early days of ram-air canopies.  They were bulky & did not work consistently.  Then the slider came along and superseded everything that was being tried.

Jerry Baumchen

PS)  Please do ask me to tell you much more than this; the old memory is getting old.  Hmmm, maybe the VolPlane???????????

 

Share this post


Link to post
Share on other sites
1 hour ago, JerryBaumchen said:

Pioneer built risers with shock absorbers ( hydraulic pistons/shocks ) back in the early days of ram-air canopies. 

Oh wow, I did not know that. I was just throwing out a rather crazy idea (I thought) as yes I'd imagine plenty of practical problems for something like that.

Biggest of all is that those are only good in absorbing energy in just one direction. And they're practically the same as a high-stiffness spring except for the fact that they'll dissipate the energy as heat instead of springing back.

As you may have already guessed, I'm actually quite new and one of them 'millenials :E. I do think that the slider and variations thereof are the best we have at the moment.

I was hoping my calculations earlier in this thread would get to a good ballpark of what was needed for a good shock absorber, but unfortunately they're way too long. Even if they were half of what I calculated it still seems very impractical...:/

What's the VolPlane?

Share this post


Link to post
Share on other sites
34 minutes ago, olofscience said:

#1)  As you may have already guessed, I'm actually quite new and one of them 'millenials .

#2)  I do think that the slider and variations thereof are the best we have at the moment.

#3)  What's the VolPlane?

Hi science,

#1)  We were all 'quite new' once.  Nothing wrong about that.

#2)  As do I.  However, I'm a 'mostly' ret'd Mech Engr & always thinking about what could be.

#3)  A very early ram-air design by Pioneer Parachute Co.  I no longer have any documents related to parachutes, rigging, etc.  I think it was shown & discussed in Poynter's very first edition of the Parachute Manual.

Jerry Baumchen

PS)  So far in this thread, it does seem as though all of the calculations on the forces experienced during an opening(s) are using the idea of the forces being constant during the opening sequence.  That is simply not true of any parachute that I have ever known of.

 

  • Like 1

Share this post


Link to post
Share on other sites
(edited)

I think there’s three practical things people can do to help prevent hard openings

 

1. Tight locking stows, with high quality rubber bands or stows. If you’re using regular rubber bands, DOUBLE STOW THEM. If your locking stows release early, the opening can be very hard. I use fresh rubber bands for my middle two stows every jump, maybe excessive though.

 

2. Dacron lines, as everyone knows for several reasons.

 

3. A slightly domed slider. It will be a lot harder for the slider to come down the lines unevenly allowing wind to go past it, if it’s domed. I used a standard 23 x 31 size slider on my canopy, except with 2 inches of dome to it. The openings take the same amount of time as a regular 23 x 31 slider, but the way the slider inflates in the middle gives me peace of mind in knowing that it’s far less likely to slide down prematurely compared to one that’s just purely flat when it’s fully stretched out. Think about it, what catches air better, a bowl or a flat plate?

Edited by sheeks

Share this post


Link to post
Share on other sites

If you really want to fix hard openings, the solution that makes the most sense to me is to control the slider speed.

I'm convinced that a really hard opening is impossible if the slider goes down at a slow, controlled speed. If you can find a way to make it go down in a very controlled way, lets say constant speed in 3 seconds, I think you may have fixed the problem. 

In the current design, the slider is kept at the top end of the lines by air resistence, while it wants to come down due to the canopy wanting to spread out. Friction between the grommets and the lines also helps to keep the slider up, and once it starts moving, slows it down a bit.

This is a very delicate balance, especially since friction is super unpredictable and changes depending on linetype, wear, dirt, and probably temperature, humidity etc. Once the balance dictates that the slider goes down, this happens at an almost uncontrolled speed since the friction is nearly independent on the speed of the gommets over the line So what happens, once the slider starts moving it moves without much control of its speed. Designers can offcourse affect the velocity at which the slider starts moving; controlling the speed is almost impossible. 

So what we need, is a reliable  way to dampen the slider speed. Dry friction is not the perfect solution (altough very simple and cheap) because it is almost independent of speed; what we need is that the amount of friction depends on the velocity at which the slider moves over the lines. No friction at 0 speed, a lot of friction at high speed. This way you could make sure that the slider ALWAYS takes around 3 seconds to come down, at a constant speed. Problem solved! In theory at least... 

 

 

 

Share this post


Link to post
Share on other sites

You really need to document the the loads during opening. What I'm seeing here nobody has any Idea what they really are. At Para-Flite more than 40 years ago we used instrumentation on opening forces. You cannot use simple calculations as the opening characteristics are very complex. I can tell you from experience that a 3g opening can feel much harder than a 7g opening. The duration is a key point. I had several 14g plus openings documented.

 

Share this post


Link to post
Share on other sites
(edited)

i was talking to one of the riggers at my dz this weekend and have come up with a workable solution using off the shelf tested components and not requiring mods to the risers.  use a drogue system to slow the jumper down first.  when you pull, you would have a slightly larger pilot chute which would deploy like a drogue, then you wait 2-3 seconds and pull the drogue release.  if you have a slammer at 50-75 mph, it would dissipate much of the force vs. a 125 mph slammer.  or it seems like it should, i am not an engineer so don't know the calculations needed to figure it out.  this way, we are using tested items that have been used for years and we don't have to introduce a weakness into the system like break away risers.  there were discussions elsewhere about possibly using a tailgate like they do with base canopies but when i asked her about that, she said it wouldn't be a good idea putting that on a canopy already designed to open slowly just to avoid something that may not ever happen.  does this sound like it may be a decent idea?

Edited by sfzombie13
typo

Share this post


Link to post
Share on other sites
19 minutes ago, sfzombie13 said:

i was talking to one of the riggers at my dz this weekend and have come up with a workable solution 

"Workable" implies that it actually works, not that it might possibly work.  How far have you gotten in development?

Share this post


Link to post
Share on other sites

nowhere.  it is still a workable idea.  workable meaning that you can make it work.  it's called a tandem rig.  it is in use all around the world.  that is the best part, the only thing that  needs tested is the various size pilot chutes to slow you down to a specific speed, and i reckon that would be done depending on exit weight. 

Share this post


Link to post
Share on other sites

poor packing i would assume without any other information.

my design does not prevent a hard opening, that's why.  it simply slows the jumper to a tolerable speed so the resulting g force doesn't kill them.  it is an idea, not proven.  i think it is well worth testing, and i may just do that but it goes on the bottom of my list.  i hope that someone would look into it and run with it.  i would be willing to jump one. 

Share this post


Link to post
Share on other sites
33 minutes ago, sfzombie13 said:

poor packing i would assume without any other information.

my design does not prevent a hard opening, that's why.  it simply slows the jumper to a tolerable speed so the resulting g force doesn't kill them.  it is an idea, not proven.  i think it is well worth testing, and i may just do that but it goes on the bottom of my list.  i hope that someone would look into it and run with it.  i would be willing to jump one. 

Hi 13,

Keep thinking, that is how problems get solved.

Jerry Baumchen

Share this post


Link to post
Share on other sites

How about some magnets to hold the slider corners against the start of the line groups until the canopy inflates more? It could help solve sliders coming down too fast. Would also keep the canopy size constrained to the sliders until inflation overcomes the magnets.

Just throwing some thinking out there...

Share this post


Link to post
Share on other sites
(edited)

You guys are talking about completely different solutions to hard openings that are not related to adding shock absorbing component to risers or harness. They are all valuable but please keep them in separate topics. Otherwise it gets difficult to follow the discussion. 

Also, remember that multiple solutions can exist at once, they are not competing against each other.

Edited by CoolBeans

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.
Note: Your post will require moderator approval before it will be visible.

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.

1 1