Reserve PC design: fabric to mesh ratio?

[RiggerLee 61]

It's in Q. Q=1/2*Ro*v^2

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[dpreguy 14]

Determining the ratio of mesh to fabric is in my opinion, a silly quest; and knowing this ratio is of no value, unless there are assumptions that having a higher ratio of one to the other is better than having a low ratio. And if one is to chase assumptions, then there would have to be data on which pilot chute ratio creates the most drag, and whether that drag result is due to the ratio or due to design.

No one has taken the bait of determining ratios, nor does anyone care about the ratios. What every following poster is discussing is the important stuff: design, size, comparing manufacturer's performance, etc. Maybe the original poster was actually trying to find about performance, but didn't know how to ask.

[dpreguy 14]

I reread my post and found it to be snarky. I apologize about the way I said it. I always find skydiverderek's posts to be on point and intelligent. Didn't mean to come off as such a smartass.

This discussion on pilot chutes is a good topic.

[Unstable 9]

John,

This thread is absolutely fascinating ~ with multiple topics going here, maybe you can clarify something that has caught my eye, and I'm sure the eye of other riggers lurking this thread.

Quote

No manufacturer can prevent you from installing a better pilot chute, in a reserve, no matter what they say in their manual. They don’t have the authority to override the FAA permission granted in AC105. As long a there is no degradation of performance you may make the change if it is functionally compatible.

Let me take Sunpath for example. Aug 2007 revision of their manual, page 6 states

Quote

*** Only the Javelin Reserve Pilot Chute may be used with the Javelin Harness/Container System. Do not substitute any other pilot chute

So then AC-105D, section 13.c gives us the direction (bold and italics added for the purpose of this post...

Quote

c. Assembly of Major Components.
The assembly or mating of approved parachute components from different manufacturers may be made by a certificated, appropriately rated parachute rigger in accordance with the parachute manufacturer’s instructions and without further authorization by the manufacturer or the FAA. Specifically, when various parachute components are interchanged, the parachute rigger should follow the canopy manufacturer’s instructions as well as the parachute container manufacturer’s instructions. However, the container manufacturer’s instructions take precedence when there is a conflict between the two.

So, Hypothetically if I decided to use a Dolphin Reserve PC on a Javelin Container, I would most assuridly receive flack from the Manufacturer. (I've actually seen this happen with JCO Metal reserve ripcords on Javelins). So then do we read the above section from AC105d to give precedence to the underlined section rather than the italicized section as the latter uses the qualifier should?

I'm not trying to put fourth an argument here, but not only would a rigger need to present data to justify the decision (perhaps in the format you presented). Have you seen this argued successfully before?

=========Shaun ==========

[RiggerLee 61]

I think he would like to see it happen but I think this is a case where wishing does not necessarily make it so. There is a skill to interpreting the meaning in these document, and I don't have it, but I think it hinges on the phrase "...in accordance with the parachute manufacturer’s instructions..." Which this would deffenintly violate. I once brought up the idea of getting an approval through the FSDO as an alteration. Which would be legal but no one really seemed to jump on that idea.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[pchapman 279]

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approved parachute components from different manufacturers may be made by a certificated, appropriately rated parachute rigger in accordance with the parachute manufacturer’s instructions and without further authorization by the manufacturer or the FAA.

You wondered how to argue things. If one were supporting free interchange of components I would argue it this way about AC 105:

(I'm not saying this is the truth, but just that this is a way one would argue it!)

a) In the part not quoted above, the FAA says you "should" follow manufacturers' instructions, not "shall". So it is wise to take them to heed, but not mandatory.

b) The assembly of a reserve pilot chute can indeed be done according to the manufacturer's instructions (as stated in the quote above): It may not be the type they want, but they may write in their manual, "Assembly: Lark's head the reserve pilot chute to the freebag bridle. Ensure knot is pulled tight." Voila, that's how you assemble a new pilot chute, you've followed their instructions!

[Southern_Man 0]

pchapman

You wondered how to argue things. If one were supporting free interchange of components I would argue it this way about AC 105:

(I'm not saying this is the truth, but just that this is a way one would argue it!)

a) In the part not quoted above, the FAA says you "should" follow manufacturers' instructions, not "shall". So it is wise to take them to heed, but not mandatory.

b) The assembly of a reserve pilot chute can indeed be done according to the manufacturer's instructions (as stated in the quote above): It may not be the type they want, but they may write in their manual, "Assembly: Lark's head the reserve pilot chute to the freebag bridle. Ensure knot is pulled tight." Voila, that's how you assemble a new pilot chute, you've followed their instructions!

In addition to your excellent points quoted above, An Advisory Circular is Advisory only and does not have the force of law.

It could, however have a powerful influence in defining "industry standard practices" however.

"What if there were no hypothetical questions?"

[Unstable 9]

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a) In the part not quoted above, the FAA says you "should" follow manufacturers' instructions, not "shall". So it is wise to take them to heed, but not mandatory.

I agree with that statement 100%. I think the line without further authorization by the manufacturer or the FAA takes precedence to the should of following manufacturers instructions.

Now let's consider the ramifications of actually taking this stand. If I swapped out a Javelin Reserve Pilot chute with a dolphin, and I took the position as above ~ I would pit myself against the manufacturer and the FAA investigator from the FSDO office. Even if I had the data from my own drop tests, that pitts a respected gear manufacturer against a a local Senior or even Master rigger.

=========Shaun ==========

[highspeeddirt 0]

John, can you explain the GK tragedy?

[Unstable 9]

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It could, however have a powerful influence in defining "industry standard practices" however.

YES! Good point. However, I would argue that in an audit situation, the term Industry standard practices quickly breaks down. Unless documented by a self-regulatory board such as PIA, industry standard practices really doesn't have any meaning. John Sherman uses a speedbag, everybody else uses a safety stow, so is he then producing nonconforming equipment per Industry Standard?

=========Shaun ==========

[KellyF 16]

JohnSherman

No pilot chute will have enough snatch/drag to strip a bag off of a canopy if the bag is properly designed. The “Safety Stow” is not such a design. Any bag design must retain the canopy until the lines are extended in ALL CASES no mater how big or powerful the pilot chute is.

This new design (the Safety Stow) provides no stiffener to hold the grommets in separation during line bight extraction and in practice is no better than the Buna-N “O” rings. This allows one stow to have an retention force of 12 pounds +/- and forces the other stow to have little or no retention force because the grommets are forced together during packing. Even though the stows are balanced they easily allow the loose stow to escape during bag extraction, especially at terminal. When one of two locking stows release on a “Safety Stow” bag it releases the canopy from the bag – “Line dump”. This is likely when using a Velcro pouch with only 12 square inches of Velcro is attempting to hold the mass of the entire length of lines in the pouch during extraction.

John, Newton's first law of motion applies to the parachute thats IN the freebag as well as the lines that are in the stow pocket and safety stow, which you seem to be ignoring. In fact, since the lines have less mass than the parachute itself, they are easier to accelerate (decelerate)to the speed of the pilot chute than the parachute itself is. What this means is that upon initial snatch from the pilot chute, the parachute is trying to fall out of the bag (it's at rest in the container). The only thing keeping it in the bag is the closing flap and safety stow, which will stretch and create a tighter grip on the bights of suspension lines. There will be no line dump from the locking stows unless the safety stow breaks.

Lets say that all of the lines DO dump out of the stow pocket. That could only happen after the bag has moved a minimum of 6" or so (the distance from having the lines inside the pocket vs. outside the pocket). This would mean that the parachute has also moved (or at least tried) 6" inside the bag, toward the open end, loading the closing flap and locking stow. Now, you're stating that the mass of the lines falling out of the pocket is going to pull a bight of line out of the safety stow, but unless all the lines from the pocket are stuck together, the only time the bight in the safety stow will have the full weight of those lines on it is at line stretch. Think about it- try to pick up an entire coil of rope while only holding one end of the rope...

I've packed for, and videoed drop tests with standard safety stow freebags with both "high drag" and "low drag" (to use your terms) pilot chutes at 240 knots (276 mph) with not a HINT of what you're implying is a common occurance in a terminal (120 mph) reserve deployment.

Thats all I've got to say about that. /Forrest Gump

BTW, the Infinity reserve pilot chute is about 75% fabric, 15% mesh, and 10% open at the base of the spring.

[pchapman 279]

Any comment on the design decisions behind the Infinity's reserve PC, Kelly?

For a while it almost seemed that a reduced mesh ratio was seen as something particularly modern, whether on the Vector, Wings, or Infinity. It seemed like a way to start deployment better, by perhaps better catching air if it happened to bounce around in a turbulent burble. But now people are thinking more about the overall drag of pilot chutes, and are wondering if the reduced mesh has now resulted in some pilot chutes that are too "streamlined", with a lower coefficient of drag than they could have.

It's easy for skydivers to be critical of manufacturers despite all that the companies accomplish, but respectful discussions on design questions can be interesting.


@ John Sherman:
At least with the velcro closing the freebag pouch, if one doesn't like the standard solution (and aren't going all the way to a speed bag), one can increase (maybe doubling) the holding effect by folding the velcro over such as on the Wings or Infinity.

I think your post also gets into the issue of confusing people by using "line dump" to mean stripping the bag off the canopy as well as dumping the lines, when I think more people now like "line dump" to mean the lines but not full bag strip.

[JerryBaumchen 1,377]

Hi highspeed,

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the GK tragedy

I can only tell you the story going around 'back in the day;' long before the internet.

It was a practice jump, prior to the Nationals, but at the Nationals. As was common in those days, he went into a high speed dive ( similar to today's lawn darts ) to build up speed. It was a Style jump and as he came out of the first back loop he ( apparently, as no one knows ) he saw that he was really low. He dumped his reserve and the pilot chute/bag departed, leaving the canopy sitting in the pack tray. He did not get the canopy off of his back in time to survive.

JerryBaumchen

[JohnSherman 1]

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Cd is the Drag coefficient of the device.
So is the Square footage of the device as shown in plan form.
Q is the Dynamic pressure or 1/2 Rho V^2
V is the velocity in Feet per second.

Where is V in the formula?

My Bad. Velocity (V) is part of Q.
I neglected to include the formula for Q (Dynamic pressure.)

Q = !/2 Rho * V^2
Rho is the Density of air in Slugs per Cubic foot and Velocity is expressed in Feet per second.
Rho at sea level is .002378

Good Catch

Sorry 'bout that

[JohnSherman 1]

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John, Newton's first law of motion applies to the parachute thats IN the freebag as well as the lines that are in the stow pocket and safety stow, which you seem to be ignoring.

I am certainly not ignoring Newton. In addition to embracing Newton I am applying micro timming to the process. What loads when.
The PC loads the bridle which loads the bag. The lines will load next as they have less mass than the canopy. The lines are now pushing against the velcro and the safety stow is trying to retain the bight but it can't because there is no tension on the bight on that critical side. The canopy loads after this because it is bigger and has further to go.
The resason the bight doesn't release every time is that the last stow of the 2 locking stows is not always on the side with no retention from the bungee.

The GK fatility is as explained earlier and was the first documentation of line dump. Additionally another GK had a bag strip in Key West. It was wittnessed by a Randy Mathews who followed him down to a grand where the GK (Capt Keller if I remember right) sat up and got an instant canopy. Those were with the "O" ring but the Safety Stow has the same problem.
Jay Engle had a Raven blow up in Colorado and it was attributed to "Over loading". It was in a Racer with a Safety Stow Free Bag. He was at terminal and the bag stripped. That Raven will take twice it's specified weight at terminal.
He is the reason we finally put the "SPEED" Bag on the Reserve. It's optional on the main but not on the reserve.
You may not have experienced it but Para-Flite did during testing of the Flyer.
Additionally, This phenomeon happens so fast that you can't see it even on stop frame standard video. I have caught an ocasional single frame which hints at this. The only way to know it is happening is analysis of the data and the results of the tests. The industry has built some really tough canopies because of this problem and the inability to solve it.

If I could guarantee that any canopy I built would always be used with a SPEED bag I could reduce the weight and bulk signifficantly.

[pchapman 279]

(Now I'm just musing since I of course don't have all those years of design experience:)

Say that somehow there is an issue with the safety stow getting a little unloaded during the packing process, pulling the grommets towards each other, or everything being compressed into the pack. So then as the bag gets extracted, for whatever reason one bight in the safety stow pops out. Whether it is some inertia thing as you suggest, or some drag of lines scraping in the pack tray, one bight pops out before the bag really starts accelerating away, which will cause the canopy mass to slump down and firmly tension the safety stow. I'm not sure about your scenario but let's assume something like that happens.

Then how about going to two safety stows instead of one?

Even if that very first bight pops out and unloads the other end of the safety stow, there's a second complete stow that will hold the canopy in the bag.

Go to two stows (and a stow pouch) instead of a dozen plus elastics for a full speedbag.

That maintains a system close to what the industry is used to but with more protection against accidentally losing that first bight, which with a "safety" stow, then compromises the whole system.

(And you avoid riggers cringing when pulling a reserve speedbag across the floor, and have the elastic bands hang up in their little slots, causing 60+ lbs of pressure yank the damn bag off the canopy, squeezing the canopy out of one corner of the bag while the other side remains hung up...
Even though your tests all show that at speed, the rapidity of motion causes everything to jiggle free, time after time. I know you keep saying that it always works, and you tested it a million times on main canopies, but it still makes one tend to cringe!
I also can't get used to the long stow bights right next to each other on speedbags -- I just think about bights catching each other and causing baglocks. At least on round canopy diapers all those stows next to each other are kept as short as possible.
Just saying how I feel, whether or not it reflects any real risks.)

[highspeeddirt 0]

thanks Jerry.

[JohnSherman 1]

If one stow releases on the subject free bag the canopy comes out and can allow the slider to drop as it inflates before reaching line stretch. Ouch!

There is no need to suppose any other scenerio as the SPEED bag exist and it was designed to speciffically address this problem.
Tests at the USFA over a 3 year period showed the ability of the SPEED to almost eliminate malfunctions. They reduced their total malfunctions in a 3 year period to 1. Previous 3 year spans were generating as hign as 51 mals.

[KellyF 16]

pchapman

Any comment on the design decisions behind the Infinity's reserve PC, Kelly?

Hi Peter, I wanted to create the most labor intensive reserve PC on the market Actually, the idea was to create a pilot chute that inflated quickly at any angle between upright and laying on it's side. In order for the pilot chute to be able to inflate (partially) on it's side, the canopy fabric needs to extend below the equator (where most pilot chutes have the seam connecting the mesh to the canopy fabric). This creates a pocket to capture the air and start inflating the pilot chute. The mesh portion allows more airflow into the canopy and aids in inflation at odd angles.

JohnSherman

Quote

John, Newton's first law of motion applies to the parachute thats IN the freebag as well as the lines that are in the stow pocket and safety stow, which you seem to be ignoring.

I am certainly not ignoring Newton. In addition to embracing Newton I am applying micro timming to the process. What loads when.
The PC loads the bridle which loads the bag. The lines will load next as they have less mass than the canopy. The lines are now pushing against the velcro and the safety stow is trying to retain the bight but it can't because there is no tension on the bight on that critical side. The canopy loads after this because it is bigger and has further to go.

Assuming we're talking about a belly to earth deployment (since thats the way these systems are intended to be deployed, and to eliminate infinite variables and scenarios), as the bridle loads the bag and tries to turn it 90°, the parachute will most certainly see an acceleration force before the lines in the stow pocket, since it occupies the full length of the bag, and the stow pocket only about the bottom 50%. As the bag bends, it immediately starts trying dump the canopy out of the mouth of the bag before the lines see ANY acceleration.

Again, the lines are easier to accelerate than the canopy because they have less mass. They are not going to load before the canopy because they can't- they're in the same bag. As explained above, the canopy can see the acceleration force before the lines, but not the other way around. The "worst case" scenario is the bag getting pulled parallel to the pack tray, in which case the canopy and lines will see an acceleration force at the same time- but the lines, having less mass than the canopy, will be easier to accelerate and more likely to move with the bag than the canopy.

JohnSherman

Additionally, This phenomeon happens so fast that you can't see it even on stop frame standard video. I have caught an ocasional single frame which hints at this..

I disagree with this comment. You may not be able to catch the actual moment the lines dump out of the pocket, but the evidence would be clearly visible IF it happened.

Look at it this way- lets say it takes 1/5 of a second (6 frames of video) for the bag to go from the3 container to line stretch. I think we agree that line dump is most likely to happen as the bag is getting snatched out of the container, right? So that leaves at least 5 frames of video to show either a messy wad of lines floating between the bag and container and/or the lines going directly from the locking stows into the container where the rest of the lines presumably are. I'm not even addressing the possibility of bag strip, since the result of that would be VERY apparent, even to the naked eye since there would be an empty freebag floating down and nothing else outside of the container.

[riggerrob 643]

... That rigger ... would be subject to severe pier ... review. ...

......................................................................................

Sounds like being beaten with a plank from a dock.
Sounds painful!
Ouch!

[JohnSherman 1]

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This creates a pocket to capture the air and start inflating the pilot chute. The mesh portion allows more airflow into the canopy and aids in inflation at odd angles.

It restricts the mouth and retards inflation. See T-10 parabolic shape extended skirt consideration for example. Additionally, after inflation it directs flow to wrap around the surface reducing the drag/burble/low pressure/ area on top. Therefore; the lower Drag capability.

Additional interesting fact: A pilot chute with a 50/50 design which has an “Effective Size” of 5.9 Sq. Ft and weighs 7 ounces will inflate and reach maximum drag capability in .667 second when free dropped by hand from any height.

Quote

Assuming we're talking about a belly to earth deployment (since thats the way these systems are intended to be deployed,

Bad assumption.

It should be deployable from the belly to earth position but any good Skydiver knows to "Sit up and Dump".
In fact most reserve deployments are from a more vertical position, after a cutaway for example.

Think of the bag as a table cloth with a canopy (in a lump as in the bag) and the lines as they would be in the bag. Now pull the cloth. The cloth will move from under the lines before the bag has a chance to load the closing flap which is a vertical extension of the cloth. It works the same way when vertical.

The fact is it has happened and will continue to happen until that design is improved. The other fact is the extreme difference in opening shock. Perhaps as much as 10 times has been my experience.

[masterrigger1 2]

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as a rigger has the authority granted in AC-105.

An AC does not grant authority as it is not a legal or regulatory document.
In fact, the current AC has several conflicts with the regulations that if you follow the guidelines ( which is what a AC is supposed to be).

We are getting very close to fixing most of the major conflicts in the AC by the way.


MEL

Skyworks Parachute Service, LLC
www.Skyworksparachuteservice.com

[masterrigger1 2]

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But the TSO standard is in no way a quality control standard either of manufacture, component assembly compatibility, packing, wear or any of the other things that can effect performance like body position (towing over the shoulder on your back). Its best described as a design confirmation program.

Terry, I hate to disagree with you but the TSO is in fact a standard to manufacture a specified actile to certain performance standard.

When testing an article to meet the TSO requirements, you are testing to meet at least the minimum standard.

If the article fails to meet the performance standard in the field, the FAA can and will step in and issue ADs or emergency directives to either remove the article from service or have a plan of repair/modification.

I can point to several instances with regard to seat belts, engines, and even parachutes.


MEL

Skyworks Parachute Service, LLC
www.Skyworksparachuteservice.com

[councilman24 37]

I'd agree with you if manufacturers made entire integrated systems, had to test all.sizes and only reserves tested in a specific container/size were allowed. We may want the testing and QC manual to guarantee meeting the standard but it doesn't.

How can anyone guarantee that canopy x size y( size y never tested ),never tested in container xx size yy (size yy never tested) will meet the opening performance standard? And given with the wide variation inherent in manufacturing with fabric (what other TSO item allows 10% variation) and wide variation in rigger performance along with items left to their discretion I think absolute expectation that any given rig would meet the.performance standards is a reasonable hope but not a certainty. Especially in untested orientations like back to earth. Anybody saying "It WILL open in three seconds, the TSO tells me so" or " the PC should have rolled.them over" is dreaming.

I'm not talking so much about the strength standards. As Lee pointed out material and manufacturing controls should handle that.

And we still need to get the FAA to issue AD's on parachutes again.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[masterrigger1 2]

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I'd agree with you if manufacturers made entire integrated systems...

Well actually there are a couple that do. Strong Enterprises is one example.

The FAA has already stated that the performance standard has to be maintained post production in order to be considered airworthy.

In the case of the Wings container, towing a pilot chute for several seconds after the ripcord is pulled is not maintaining the 3 second rule that is part of the 23b performance standard.

Quote

And we still need to get the FAA to issue AD's on parachutes again.

According to the FAA, it probably will happen pretty soon.

JFYI, the last AD (parachute related) that I know of was issued in 2001 IIRC.

MEL

Skyworks Parachute Service, LLC
www.Skyworksparachuteservice.com