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The opening canopy is pushing the slider down and the air on the slider is pushing it up. There is also friction on the grommets that is resisting the movement of the slider.

With a big mesh slider, is friction the main resisting force?

Kris.

In my opinion, yes.

You can see this if you take a large hole mesh slider and a sail slider for PCA's (off a nice safe span, preferably) and compare the opening altitude. The two different slider types produce remarkably similar openings. I haven't tried manufacturing a "no fabric" sider (just tape and grommets) but I suspect it would also be very close (on that zero airspeed PCA).

As your airspeed increases, the difference between the various slider types (large hole at the fast end, sail at the slow end) widens, which is, in my opinion, the result of the airflow resistance coming into play.

for terminal jumping, i would say that air drag/resistance is by far greater than grommet/line drag, at least until the canopy is mostly inflated.

why do say grommet drag is so important tom?

or was i confused? i agree, grommet drag is probably the strongest on slow Sliderups, but other than that...

I don't actually know that I have an opinion as to which (grommet drag or air resistance) is larger at terminal, with large hole mesh, so I believe I misstated that in my first post.

I do think that the air resistance force is almost negligible at zero airspeed.

I suppose that constructing a "tape only" slider and taking it to terminal would give some compelling data points.

i have one, and have used it once. ill bring it next time im up there if you want to use it.

it was a blown up tandem slider. (so its REALLY big on all the canopies, dangerous i think) i cut all the ZP out of it.

i actualy think that it would not be that bad. (i did it at potato, 1sec delay. fealt like i thought it would. fast fast slider up opening. fealt like a 4-5 sec slider up deployment, but at 1sec delay.

Don't forget the amount of force pushing the grommets down AND out will vary as a function of the distance of the slider down the lines. It's just simple angles. Also keep in mind that the frictional force will be directly related to the amount of the force perpendicular to the motion of the grommets. So the frictional force of the grommets against the lines will not be constant throughout opening.

The main idea of a mesh slider is not to provide resistance when sliding down, although this may take place to a small degree. The main idea is to "reef" the canopy. This is why a slider with an empty center will still work.

IMO dont think this would have the same result of a properly fitted "tape and grommet" for the canopy size. close but not accurate enough to make it fact, or even loose fact. A tandem is a COPMLETLY different best than a standard BASE jump. 330+ sq ft., almost always terminal, up to 550+ lbs. no worries about off headings, line length, etc.

the way I look at a slider is (as said above) is a reefing action. something to slow the canopy deployment. a tape and ring slider will slow down the deployment only so much based on airspeed. vs what a sail slider with a pocket on a sabre will do at terminal. each tool can and sould be used in an appropriate manner. proceed with caution if you decide sail is your best choice slider up.

I woke up 4 in the morning thinking about this half in my sleep... Weird, I must be a dork, huh? Well, the voices in my head were the voice of a friend telling me a few days ago that "I relined my (skydiving) canopy and it fixed the hard openings."

Well, suddenly I thought about line trim versus line condition. Everyone seems to assume the trim is the only factor in opening characteristics when discussing lines...

Clearly as lines get older, their texture, cleanliness and thickness change - all changing the coefficient of friction.

If slider grommet drag really is a large factor in the slider's slowness or quickness, do you think there is something to be said about the age of linesets and the increased or decreased friction of older lines, changing opening characteristics???

And the next step to that thought... I recently installed new control lines on a friend's canopy. The A-D lines were all factory original, but the control lines were new from the cascade to toggles... Well, the new lines had a lot of texture where the old lines were rubbed/worn rather smooth. In this situation, could one grommet rub a newer line, where the rest rub older lines - thus causing uneven deployment/slider movement? On a skydiving canopy, who cares... In BASE....

I've noticed the same thing -- I've got an old (skydiving) canopy with crusty Vectran lines. They are out of trim for sure and the canopy always wants to veer to the left, but I think it's the roughness of the lines that is causing the drunken openings, because they are at least on-heading -- the keeling over only starts to happen after opening.

[but wtf do I know...]

I think you may be correct that this is taking place, but the difference in the friction coefficients has to be nearly immeasurable. I'm not smart enough to figure it out, maybe someone who can will chime in.

It would be difficult and expensive to get a truly accurate measure of the friction coefficients. With Dacron lines, I doubt there would be much difference, but I've learned to never assume things in the the scientific world.

would it though?

get old canopy lines, along with new ones, build a test rig that can adjust the tesion of the line between 2 points. use pictures of skydive/base openings to get the angle of the lines relative to above or below the slider grommet, and a simple pull scale to test the friction.

it would take a while, but i doubt it would cost more than a few hundred bucks. I think that would be pretty accurate.

You would be dealing with small coefficients and small differences (though possibility relatively large). You would need to be very exact and have very sensitive measuring tools to get an accurate number, not to mention a fairly precise test rig. Something like a pin and disc tester might be able to be used, but you'd have to customize it for this use.

Unfortunately, the more physics classes you take, the more times you're shown that what you were taught previously was not applicable in the real world due to the assumptions made to keep things simple.

That's not true at all. If I'm doing a rock drop to see how high something is, I'm not going to use quantum mechanics to solve for the height of the object. I'll use Newtonian mechanics, maybe the speed of sound, and a healthy error margin, because I don't need to know exactly how high the thing is. Newtonian mechanics is very much applicable in the real world.

Similarly, I think you'd go a long way toward answering the question here if you just put a #8 brass grommet in a bench vise and dragged a few short sections of line through it by hand. If you can't feel the difference, it's probably not a major factor.

With significant airspeed, the aerodynamic effects of the slider are much more important than the drag of the grommets. If you're not sure about this, try jumping your skydiving canopy with a spider slider. I guarantee you'll be sure by the time you reach the ground.

Further, I would say we're not just talking about the airspeed of your body here. If you do a slider-up PCA from a stationary object, you may not have much airspeed yourself, but the slider will be forced down the lines very quickly by the outward pressure the canopy exerts on the lines. The speed of the slider is quite high, and once again the aerodynamic effect of the slider is dominant.

Michael

I was mostly referring to high school and college freshman level physics, which I believe is the level of physics most base jumpers have studied. And allowing for a healthy margin of error is a result of the assumptions made to keep it simple. You can't use Newtonian physics to figure out how long it will take you to impact from a decently high exit unless you consider air resistance which would have to (and has) been found experiementally. I'll concede that for some things you're right, assuming you use a healthy enough margin of error.


There's many variables drawing from some complex disciplines at play. I doubt you'd be able to completely figure out an answer to Kris's question without making some fairly large assumptions and invalidating your answer. But yeah, for high airspeed, the aerodynamics are much more important, simply because a smooth metal against Dacron will not have a significant friction coefficient.

Re-reading Kris's original post, I just realized I got a bit sidetracked by other issues brought up in the thread. Kris's specific question was how mesh sliders work, which I will concede is a slightly more delicate issue because the aerodynamic and frictional effects are better balanced.

With that in mind, I think Tom is on the right track with his suggestion of direct experimentation, and Tornolf is not so far off the mark as I thought he was. :) The best way to answer the question is likely with actual jumping, rather than lab testing.

Michael

The canopy has spectra lines. I have jumped it 15 time so far with different sliders and modifications to the factory slider (added more tape around the edges leaving the center mesh alone).

A sail slider with a hole is the only thing that slowed it down to a level which I would be comfortable to take a longer delay.

Doesn't dacron provide more friction than spectra? If the slider is made smaller...will it take longer to travel down the lines or will the inflating canopy push it down the lines quicker?

Kris.

smaller slider -> less surface area -> less aerodynamic forces pushing it up -> faster decent

smaller slider -> outer control lines have a more severe angle -> control lines pull acts more outward -> faster decent

I do not think it's that straightforward.

I also think the more severe angle actually slows the descent of the slider due to the vectors of the forces.

More of the force is directed outward (and counteracted by the other side of the canopy's lines pulling the opposite direction), leaving less available force to move the slider down the lines, while the slider has more friction due to the higher sideways forces on the lines throught the grommets.

More important than actual size is the length/width ratio of the slider in respect to that of the canopy.

For instance, the standard small hole mesh slider for my Blackjack260 is very wide and not too long. It is small mesh and has a large surface area, so by your reasoning it should result in a slow opening.

It does not, it results in hard openings at most any airspeed (especially terminal) and inconsistent openings too.

The large hole mesh slider for a Rockdragon266, by comparison to the small mesh CR slider, is much narrower and a little longer. It has a much smaller surface area, so by your reasoning it should open fast.

It does not. It provides a better staged opening than the CR small mesh slider and opens MUCH softer at terminal.

I believe the reason is that it is narrow enough to constrict the inflation of the centre cell and allow time for decelleration before the inflating canopy has built up enough force to start moving it down the lines. It cannot be doing this by aerodynamic drag, as it has much less than the small mesh slider.

Also, once it starts coming down the lines it moves at a slower speed than the smallmesh slider, which I think is due to the higher friction created by the more acute line angle thourgh the grommets and the lower airpseed due to initial decelleration before slider descent starts.

If your slider is too wide, I think it allows your canopy too much freedom to inflate initially and the canopy's inflation will 'overpower' the sliders aerodynamic and grommet drag and force it down the lines too quickly.

I have done a lot of comparitive jumps with the two sliders I have mentioned, as I had 2 standard Blackjack 260s before and would set one up which each slider and jump them back to back.

I now have two CUS Blackjack260s and a few dozen terminal jumps which has convinced me to put the standard CR small hole mesh slider away until I sell the canopies, as it opens harder and less consistently than the Rockdragon266 sliders do (on my Blackjacks).

that's my theory based on my experience

Your observation is not entirely correct.

There are some good info and also some bad info in this thread.

A slider is first and foremost a chocking device regardless the type of fabric used for its construction. Even if a slider had no fabric whatsoever, just the tape, its reefing effects will be dramatic.

The choice of fabric comes in play at higher airspeeds. At low airspeeds a fine mesh, large mesh, or even z-po will not make much of a difference. As the deployment speed increases, passes a certain threshold, then the more resistance the type of fabric offers the slower the slider descents, all other things being equal.

As for the size, it is not true that smaller = faster descent.

A smaller slider keeps the canopy constrained more reducing the time of inflation. It is true that a slider with shorter span, once the canopy inflates, will more likely decent faster but the overall opening rate might actually be slower than a large slider.

In most cases the span of the slider is dictated by the span of the canopy and the line length. The longer the lines the shorter the span, the longer the canopy span the longer the slider span, not always true either though.

Let me give you and example. In skydiving I jump Alphas 94s. When I reline my canopies switching from the original line set I build continuous HMA line set with a complete different trim. The biggest difference between the two line sets is the length, the new ones being over one foot longer. With the longer lines the angles of the lines, especially those most outboard is reduced significantly. If I keep the slider to the original size, the thing does not come down all the way. By getting rid of 3" span-wise, it comes down smoothly WITHOUT changing the opening characteristics: same snivel and same time to full inflation.

On the other hand, if a slider comes down way too fast then maybe its span is too short, maybe...

My wife skydives with a pilot 140. She has been complaining since day one that the slider does not come down until she pumps the brakes a few times. I looked at the slider and it measures 34" in span! Quite a lot for such a canopy. I got rid of 3" span-wise and now the pilot opens just as sweet but the slider does come down fully.

The cord of a slider is also dictated by the cord of the canopy and to a much lesser extent by the length of the lines.

The cord of the slider is what is used to slow down or speed up the inflation at TERMINAL, once you run out of fabric choice, if that is even a choice. A longer cord means more fabric over the nose of the canopy and to lesser extent more drag from the airspeed.

Another example; the Mojos came with two different sliders, a large holes and a fine holes. The fine mesh one had a longer span and shorter cord than the one with large mesh being more "square". According to both Adam and Marty, the large hole one worked slightly better. When I ordered my gone Ace they had two choices for slider, large and fine mesh ALA Mojo. Strangely on the Ace the fine mesh with longer span works better...

The bottom line is that there are guidance for building sliders such as the canopy span, cord, line length, line type, length of the cascades, etc. However only trial and error will produce the optimal slider...for that particular delay...that usually is terminal...there could be very well being a slider that works better at sub-terminal deployment, and I am not just talking about different fabrics.

Again large mesh does not mean faster opening, and shorter span does not mean necessarily faster opening.

I'd guess the Spectra lines are an important factor here, but not necessarily because of the interaction with the slider. One of the benefits of Dacron is that it absorbs a bit of the shock of opening, whereas Spectra will transmit much of that shock straight through to the jumper. This can make a huge difference, since the Dacron will tend to smooth out spikes in the opening shock. How are your openings slider down?

Michael

I can't imagine that there is a considerable force being created in the friction of the grommets on the lines. It seems that the brass is very smooth, and if there was friction becoming a factor, your lines would last 3-5 jumps before they would be frayed beyond belief.

I think I might have to try wrapping some lines lengthwise with masking tape, surely that would create more "friction" without causing damage.

Overall, the first internet post in a long time that we might share and learn something...

I don't really see friction being of much consequence, but I do see it varying widely throughout an opening. The friction might be much higher immediately after linestretch due to the angle of the lines pushing against the grommets. While I suspect the peak force applied to the lines would occur just after linestretch during full bottom skin pressurization, I don't think that it would be enough to compensate for the change in angle of the lines relative to the grommets.

Really, I only want to satisfy my own curiosity. I don't see this helping jumpers in any significant way.

WRONG !!!!!
don't forget it's a MESH slider.
ask all base manufacturers, they will answer the oposite!!!

kris : do you have any problem with a canopy ? best will be to ask the manufacturer, they make lot's of test jump

Yes, I contacted the manufacturer.

Just trying to understand how mesh sliders work. Is that something we should try to do? Or should we just be dumb users?

Micheal,

It is a slider up only canopy.

Kris.

dude, do you really think the manufacturers have exact numbers and know everything bout how a specific parachute works?

not a chance i think. not a chance.

trial and error methinks.

some jumpers would use vegetable grade silicone spray on their lines to make them very slick and dirt resistant. they would have much, much harder openings than normal. seems like a good idea.

another jumper i know used to use a mesh that was very large - holes with a size about that of a nickel - and he would have much quicker openings than i thought could be possible, and were much faster than his regular large mesh. i am unsure if the slider dimensions were different than the previous slider so i am uncertain about the concluding cause. i thought the mesh was a danger due to entanglement factors though.

i'm not sure i understand, you are saying they do get hard openings with the silicon? or you aren't sure.

also who came up with the mesh slider idea? like where did it start/how did it evolve might help answer questions.

Like many things in BASE the idea was "stolen" from CRW.

Same goes for the tailpocket and free pack.

Unique to BASE canopies are the tailgate, the differential stow, the LRM, the multi, and the vents/valves.

The skyhook though was "stolen" from a BASE rig.

In my CRW canopies I use a slider tailgate (stolen from BASE), and on ICARUS tandem canopies the brake lines are anchored to the D lines (because there is no brake setting)...I bet a real taigate would work even better...

All manufacturers have different opinions...and sometimes those opinions vary among individuals in the organization itself.

Fliks have large tailpockets, Rockdragons have small tailpockets.

Fliks have large sliders (dimensions not mesh), Rockdragons have smaller sliders.

Some say a primary stow is neccessary for all jumps, some say primary stows make no real difference.

Some manufacturers make velcro and pin rigs, some haven't made a velcro rig in years.

Some make 48" PC's, some only make 46's...except fo custom circumstances.

Etc etc etc etc .

the silicone method works for speeding up the opening since the friction between the grommets and the slider is slightly reduced, hence a quicker slider decent.

Mark Hewitt, BASE 46, came up with the mesh slider. He also came up with the direct bag, the Sorcerer rig, the line mod, and the static line method.

I am uncertain about how much the hole size of the mesh affects subterminal openings. there is some preliminary results that are not yet conclusive in my mind.

Any idea if the size of the mesh could create a turbulent boundary layer in front of the mesh holes which would basically cause the holes to act if they were smaller without a turbulent layer? If so, once you passed the critical point for the formation of the boundary layer, there would be a drastic change in the air resistance of the mesh slider.

Then again, I have slept through nearly every fluid mechanics class I've had this semester.

is it true that sail sliders are supposed to force the airstream around the nose so inflation is affected?

so finnaly how does it work ??????????????

Don't know yet. Have not jumped it since .

Kris.