Scary reserve Dbag hand extraction

[RiggerLee 61]

There's nothing fundomentally wrong with an inclosed pilot chute. Where you run into a problem is when people lose sight of functionality in favor of estetics. A lot of the problems I see come from an obsesion with the apearance of the rig. Honestly we don't actually need 70% of those stiffeners. Look at how heavy and complex some of the construction on the flaps has become. And why do we really need fucking stripes? Do we really need such a wide angle of overlap on the flaps? And do the rigs really need to be that small? How exactly do they figgure that it's allright to put the same size pilot chute into a rig that's half the size? Let me help you here, the hole it has to punch through is half as big. WTF? And let's not even get started on compatibility. So I don't buy the whole inclosed pilot chute=bad thing. I see a lot of other problems. Mostly with the skydiving public. Looking cool takes president over survival. That's the simple truth. And I hate to say it, I'm really not a racer fan, but if they were happy with a functionaly designed container you'd be selling a hell of a lot more racers. You and a couple of outhers would be doing a lot more bussines but no, people have to have the stripes to look cool. Other wise it just wouldn't be FREEFLY. End of rant.

More interesting question. How much drag do you think we need? How much is appropreit? based on bag weight? How high of a drag coeficent/surface area do we actually want? I know you're focased on bag extraction but there's also the other end of the spectrum, the extension to line streatch at high speed. And although most skydiving deployments are low speed cutaways there is more and more the potential for high speed, superterminal, deployments perticularly in head down orentations. I seem to recal an old study done of time to line streatch vs malfunction rates but I think that was focased on rounds. What are your thoughts on acaptible accelerations for bags on deployment, ratios between snatch force and bag weight?

I've been thinking a bit about this because I'm dealling with the oppisit problem. We're useing a drogue suspending about a thousand pounds and the way we have it set up right now it's creating 1000 pounds of snatch force on the 65 lb bag. So far we're getting by but we've had to go to tape hesitator loops for the locking stows. 80 lb break cord for each of the line stows, so 160 lb to release each stow rather then rubber bands. And a bag where every 160 lb stow is a locking stow ala speed bag but with a cover flap simular to a strong ALS bag. That's for a nominal deployment but on early burn out we could be looking at 3000 lb of snatch force. So in any case it's been on my mind.

I'm not a big fan of it for sport jumpers but this is a good argument for mard systems. I think they should be maditory on tandoms again do to the extream speed range that they could see and the weight of the bag. But I just think it's overly complicated for a sport rig. Any thought on where a compermise might lie? Ratios of drag to bag weight? Any real equiv studies done?

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[JohnSherman 1]

Oh, Lee you ask so many questions. I will try to sort them out and answer them one by one.

BTW: I am happy with a functionally designed container and I am the only one who has to be pleased. I don't make gear to look good, I make it to WORK!

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How much drag do you think we need? How much is appropriate? based on bag weight? How high of a drag coefficient/surface area do we actually want?

( Drag Coefficient * Surface Area = Effective Square Feet)

Of course this depends on what drag is available for the scenario encountered. If you are talking about cutaways we must assume a 20FPS ROD (minimum ROD for Mal'ed main) at about 2000 ft. The Dynamic pressure in that situation is 0.45 pounds per sq. ft. ( See:http://www.jumpshack.com/Q.htm Fortunately we have a bag, bridle & pilot chute which are about 16 feet in total length. The distance you fall in the first second after cutaway is 16 feet plus the distanced generated from the ROD of the mal’ed main (16+20=36FPS) the Dynamic pressure at that point has increased to about 1.45 pounds per sq. ft.

OK, so you have a pound and a half per square foot of drag at the point you pilot chute grabs air. If you bag and canopy (the components to be extracted) weigh 15 pounds you will need a pilot chute with an effective square footage of 10. This assumes no container retention (Certainly some retention is necessary to maintain sequence). In all cases there is a "Snatch "component when the pilot chute takes air. This is a relationship between the mass and forces.

If you were at terminal (33PSF) and your PC has 5.9 effective sq. footage, like mine, you would get just under 200 pounds of drag. With a 10 pound bag the "Snatch would be 20 G's (200 pounds drag /10 pound bag = 20 times the weight of the bag or 20 G's. At cutaway speeds, shown above, that snatch would be about 8.6G’s. A lower drag pilot chute will produce much lower snatch.

When you apply this process to your situation you can begin to realize why you need such reinforcement. Remember the Solid rocket boosters of the space shuttle recovery parachutes.The drogue had a reefing on it which held the canopy in a "Squid" (Skirt held together) configuration until it slowed enough to open fully.

A MARD on Tandem is the worst idea I have ever heard of. There is no scientific logic anywhere in that scenario. A MARD had a a variable drag component which can not be depended upon. Collapsed drogue, bag lock, on your back, RSL won't peel, you’re dead.

My tandem has 20 years experience with a calculated 5 million jumps and 2 fatal incidents both of which were proven to be rigger/operator error.

Everybody should be "Doing the Math"

[riggerrob 643]

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... Low drag is due to the small size of the pilot chute, due to it having to fit into the rig? Is that right?

"

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Not really.

The spring is by far the largest component in a pilot chute. About 20 years ago, a few manufacturers (Jump Shack, Mirage, Rigging Innovations, etc.) started offering smaller diameter pilot-chute springs to reduce the risk of pilot-chutes hesitating to leave the container as containers (Flexon, Power Racer, etc.) got narrower.

The second part of the equation is the amount of fabric in a pilot-chute. Since fabric weights almost nothing (compared to to the spring and cap), it is still easy to pack a large (fabric) pilot-chute into any container.
With modern free-bags, you can get stupid large with pilot-chute fabric before it pulls too hard.

IOW pilot-chute spring diameter is affected by the width of the container, while fabric diameter is affected by the weight of the canopy.

[JohnSherman 1]

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About 20 years ago, a few manufacturers (Jump Shack, Mirage, Rigging Innovations, etc.) started offering smaller diameter pilot-chute springs to reduce the risk of pilot-chutes hesitating to leave the container as containers (Flexon, Power Racer, etc.) got narrower.

I know you didn't mean for your statement to soud like this but people could read it and think that Power Racers mght have a risk of the pilot chute hesitating as it leaves the container.
As I know you know and that the world knows this is not the case because the pilot chute is external and the only thing that causes "Burble " on it is air flow or lack of it.
Just for the record.

[riggerrob 643]

Agreed!
Pop-Top diameter makes no difference in how fast the pilot-chute leaves the container.

My perception was that the smaller diameter on the Power Racer pilot-chute was to bring the esthetics into line with the narrower container, reducing the risk of the Pop-Top edge catching lines.

To bad most Power Racer buyers insisted on stuffing 15 pounds into a 10 pound Power Racer, making them tighter than $#@! and a %$##@! to close.

For the record, I also hate over-stuffed: Atom, Bullets, Centarus, Dolphin, Dual Hawk, Eclipse, Genesis, Icon, Javelin, Mirage, Naro, Reflex, Sidewinder, Talon, Vector, Xerox, etc.

[RiggerLee 61]

A couple of things stand out in you're description.

At the start of the first second you are decending at 20 fps. At the end of the first second you have accelerated 32.2 fps. So over the course of the first second you fall 16+20 ft aprox. So yes you do fall 36 ft in the first second but at the end of that second you are moveing 52.2 fps not 36 fps. Now you seem to be interested in this point because you have a 16 foot bridle. And if the pilot chute which was moveing at 20 fps alond with you when it was fired continued to move at 20 fps as you accelerated away at 32 fpss you would in dead reach bridle streatch at the end of that first second. But that assumes that the pc will not be accelerated by gravity and that there will be no extraction force for the bridle as it is extracted from the container. In truth it's a lot more complicated. What is the Cd for the PC as it is there before it hits true bridle streatch? It's probable falling on it's side fluttering (lets not turn this into a debate about booths sidewase jumping pilotchute). the bottom line is that it's a relivively low drag with a heavy spring in it. So when you reach bridle streatch is really more a question of the terminal velocity of the pc in this state relitive to you. How much force does it take to pull the bridle out? Don't laugh I think that's actually important. I think when the pc actually turns right side up and inflates is quite important in this. We see it as a factor in base in really low speed deployments. So I don't think part of the bridle being retained under the flaps is a bad thing. I think it can turn the pc bottom down sooner and leads to earlier inflation which makes up for any "towing" of the PC. The lighter and dragier the pc, the more the relitive velocities when it hits bridle streatch. Again some thing we see in base getting that first pin pulled. So light weight, fully inflated, high drag pc=fast to bridle streatch. The snatch force would seem to be a product of the kinetic energy stored over that distance which should be a product of the drag the pc is makeing over that distance. This doesn't have to be trivial. We actually broke a 5000 lb drogue release from that snatch force when a 100 ft bridle extended. Long story but it was at like mach 2.8. This is basicly nitpicking over math and I apologise for it.

Has any one ever done a study of malfunction rates/damage relitive to acceleration to line streatch? Not simple in terms of air speed or weight but in terms of actual snatch force as you hit line streatch? Looking at pilot chute drag vs bag weight. It's a slightly diffrent question. It's about that first spike rather then the bell curve that follows it. In a sence that's what that mill study was about and how excessive or inadiquit extraction force affected the deployment of rounds. And there was a sweet spot. I was wondering if you or others had seen any thing simular in your drop testing.

My cureosity in this relates to the probblems we're faceing. We've got quite a bit of line length on this 1200 sqft canopy. We're building up a good bit of kinetic energy on the way to line streatch and in the event of a heavy deployment or as we move to heavier air frames this will only get worse. I've been thinking about useing a long screamer or secondary pilot chute to eleaveate some of this spike rather then dragging the thing out by a drogue produicing 1000 lb or more of force. The down side being the trap door allowing the fusalage to fall onto it's side and crack like a whip on opening. And we have seen that. In fact we tore the aluminum on the side of the recovery section open like a beer can with our risers on just such a deployment.

Mards. I really do have mixed feeling on them but i do like them for tandom rigs. Here's why. Most malls are cutaways and very low air speeds. The bag is heavy. This would imply that you need a big ass pc. I'll use strong as an example. Fairly big high drag pc and even then I don't think it's big enough. too much time to line streatch, too much time to flail or have any thing else to go wrong. But there's also the potental for a opening pushing 200 mph or more. any pilot chute that would be adiquit for the first senario would be way, way too big for the second. At the oposit end of the spectrom is a Vector useing that same low drag pilot chute that he has on his sport rigs. WTF? Every cross traind tandom master I've ever spoken to has said the same thing. From a lowspeed cutaway on a vector there is enough time to have not one heart attack but two before the reserve hits line streatch. That's a dirrect quote. I've seen tandom masters with a thousands of jumps and a dozen cutaways on strong rigs scream like a little girl on there first vector cutaway with that v2 pilotchute. Now haveing said that it's probably exactly what you want at 200 mph. It's perfect. Mards, they seem to be the ideal solution to this. I'm not wild about the skyhook because I've seen it come loose so I think it's a flawed design but that failure mode at least doesn't leave you any worse off then an ordinary RSL. The various systems are still evolving and we're still figuring out all the new ways that they can kill us but for tandoms at least I think it's worth the complexity. I say that in spite of the way that sky hook killed the fuck out of those two. I'm calling that part of the learning curve and a fixable problem.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[JohnSherman 1]

Lee,

You are correct about the rate at bridle stretch. My bad! That rate brings the "Q" up to about 3 pounds per sq. ft. However, film study of cutaways (see the one on my home page) shows the pilot chute on its side while the bridle plays out. It takes almost exactly 1 second from launch to load (when the PC turns and grabs air). I attribute this to the total length and find it interestingly coincidental. However, my interest here is because this is the critical time when the bag should be extracted. Additionally there is a "Snatch" involved which assists extraction. I ignore the snatch in my calculations and consider it a bonus toward extraction. My bridles are folded under the side flaps and I see no retention occurring. In the film it seems to meter out into the air stream. There is no tension on it.
My focus on all this is to provide metrics for designers to predict their bag extraction requirements. It is also to provide metrics with which riggers may test and certify that the bag will indeed extract without having to pull it out by hand. We ARE on the same page.
As an aside: The pilot chute in the subject incident looks a lot like the Sigma from the bottom. Is It? If it is and if it has the same effective sq. ft. then it is dragging about 7 pounds, about the weight of most reserves, so any container hang up would inhibit it from extracting.

Can't comment on paragraph 2 as I have never seen it- only calculated it. I suggest you reef your drogue with a cutter as NASA does.

MARDS: well we agree on the Effective Square footage of the Vector/Sigma pilot chute. NASA measured it at 2.33 Square feet. That’s about half of a normal pilot chute.
However, because of the unknown/variable drag value of a malfunctioned main and collapsed drogue no engineer would ever depend on it. Additionally, I say the pilot chute alone will handle the job if it is of competent design. See: http://www.youtube.com/watch?v=FtNKXDW0Ixo
This was a tandem pair weighing over 550 pounds and an actual malfunction on a commercial jump.
Tandem is no different than sport, just a little heavier but within the same realm. Remember a drogue has less drag than a pilot chute, considerably less, I figure (measured) my drogue to drag about 90 pounds at terminal with a load of 375 pounds. My pilot chute drags about twice that. Strong's drogue is sure big but it doesn't drag much (about like mine). It's not supposed to. It's there to help compensate for the additional unexposed weight of the tandem pair.
I don't know how you are going to get a tandem pair to speeds of 200 or more but all of the tests I have seen or done are where a tandem pair (two very heavy guys) could not get much over 170 no matter how hard they tried. When you strap two guys together their surface area is about the same from any angle. Certainly a solo jumper could go much faster. I can, and do, deploy a tandem reserve faster and more consistently with just a pilot chute than anyone can with any MARD, and do it at any speed.
BTW: Have you, or anyone else for that matter, seen any MARD tests where a Tandem with a main bag lock and a collapsed drogue have been successfully deployed with a MARD? If you have, please forward them to me.

John

[skydiverek 63]

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BTW: Have you, or anyone else for that matter, seen any MARD tests where a Tandem with a main bag lock and a collapsed drogue have been successfully deployed with a MARD? If you have, please forward them to me.

Here are some real videos of similar situations:

http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=4166182;search_string=sigma%20lock%20;#4166182

[JohnSherman 1]

Thanks for the links. They are simular and interesting but non of them look like the MARD is doing the deploying. They look like a reserve pilot chute deployment after a cutaway. One was especially interesting as the pair were on their backs after the bag lock fully developed. This is the position I believe is the most dangerous for the RSL function.

[skydiverek 63]

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However, film study of cutaways (see the one on my home page) shows the pilot chute on its side while the bridle plays out.

Can you post a link?

[TMPattersonJr 0]

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However, film study of cutaways (see the one on my home page) shows the pilot chute on its side while the bridle plays out.

Can you post a link?

http://www.jumpshack.com/default.asp?CategoryID=Video_Gallery

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[riggerrob 643]

"... My curiosity in this relates to the problems we're facing. We've got quite a bit of line length on this 1200 sqft canopy. We're building up a good bit of kinetic energy on the way to line stretch and in the event of a heavy deployment or as we move to heavier air frames this will only get worse. ... The down side being the trap door allowing the fusalage to fall onto it's side and crack like a whip on opening. ...

Lee

[RiggerLee 61]

In climbing we call them screamers. Yates built the first ones. We're already useing them in a couple of places, like on the nose cone to dampen the force when it blows off and hits bridle streatch so that it wont snap back into the fusalage. It's one of the solutions we're looking at for the bag extraction although it might take a rather long one to control the force all the way to line streatch. You could be looking at a 60 ft long screamer if you tore at 500 lb and you were lifting a 64 lb bag to a 40 ft line streatch. It'd doable and it's our current fall back plan if we start seeing problems.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com