Glide ratios

[gus 1]

Most of you will probably remember the discussion that was going on a while back (6 months?) about a French company called Nervures that is developing a paragliding canopy that can be deployed in freefall like a normal skydiving canopy.
Well I was back on their site, re-reading what's there and something got me thinking. They say they've got the glide ratio to 6:1, for every 6 units the canopy travels horizontally it will drop by 1. Fine. Then I found a site which compared paragliders, 8:1, with hang-gliders, 12:1, with sail planes, 30:1(!).
So my questions are: What's a typical glide ratio (or the maximum achieveable) for a skydiving canopy? Is 6:1 really that much of an improvement? And when we compare canopies and say that one has a much better glide than another, what magnitude of difference are we actually talking about.
Well, I thought it was interesting anyway Gus

[RemiAndKaren 0]

Thx for the link Gus... interesting reading (for us frogs anyways!).. How freaky would that be: RW jump, open at 2.5 and go up again!!! lol
Remi
Muff 914

[weid14 0]

I believe right now a normal glide ration is in the 3:1 range for a regular 9 cell.

[riggerrob 643]

I would guess that most skydiving canopies are trimmed to glide ratios between 2:1 and 3.5:1.
The same canopies could be re-lined to fly much flatter, but that is not what the market wants.
Remember that accuracy competitors want to go almost straight down.
CReW competitors don't care about glide angle, but they do want fast rotations. Fast rotations require trimming a canopy pretty steeply nose-down.
Most skydivers only want to slow down a little after they toss their pilotchutes. They also want canopies that will automatically recover from stalls when the operator throws his hands/controls up in dispair.
The higher performance para-gliders require special techniques to recover from stalls and re-inflate.

[AggieDave 6]

I'm probably wrong, but wouldn't a glide ration that gave you a longer glide give a better swoop? So would there be a market for a conpy that had a glide ratio bordering on something like was mentioned above (6:1)? Or would that just be really dangerous?
Once you're gone, you can't come back
When you're out of the blue
And into the black-NeilYoung

[markbaur 0]

We like to imagine the airflow over a wing as going straight from the leading edge to the trailing edge, but that's true only for the airflow in the center of the wing. At the wing tips, the air tends to flow toward the wingtip -- the shortest distance between between the high pressure area underneath and the low pressure area above. The air swirls off as a "wingtip vortex," a form of turbulence and a waste of energy, since we're interested in lift. For a given wing size (square feet) and airspeed, a greater span means less wingtip vortex, a more efficient airflow, and higher glide ratio. An infinite wing span would eliminate wingtip vortices, but is a significant design challenge.
We can increase the span of our skydiving parachutes by making the cells wider, but the individual cells are only approximations of the desired airfoil, and the wider they are, the less efficient they are. One design solution is cross-braces, if you are willing to pay extra and accept a higher pack volume.
We can increase the number of cells. We've gone from 5-cell Strato-Stars to 7-cell Strato-Clouds to 9-cell designs common today. Each cell is additional labor, and additional bulk in seams and lines, and we are near the limits of this approach. Not many 11-cell parachutes around, are there?
As the span increases, if the lines for each cell are the same length, the outer cells contribute less and less to vertical lift -- they're oriented outboard, not up. If you try to compensate by increasing the outboard line lengths, you decrease the relative pressurization in the outboard cells, and you increase the chances of opening problems as well as stability in turbulence. On the other hand, if you compensate by making all the lines longer (so the canopy occupies fewer degrees of arc), you increase line pack volume and line drag. Pack volume is not a problem for soaring paragliders, and their lines need be strong enough only for soaring, not opening shock at 120 mph.
Elliptical planforms offer a significant performance improvement over rectangular wings, but are more difficult to design and manufacture. It's interesting to note that for all the improvements in ram-air canopies since their wide-spread acceptance 20 years ago, the glide ratio hasn't changed much -- still around 3:1.
Mark

[markbaur 0]

Quote

So would there be a market for a canopy that had a glide ratio bordering on something like was mentioned above (6:1)? Or would that just be really dangerous?

I have seen a Jedei Knight make hook turns to downwind landings in winds up to 30 mph. He's getting a glide ratio over the ground of better than 6:1 before he begins his swoop, and the swoop itself is a magnificent display of skill.
For me, it would just be really dangerous. But Obi Wan practices, picks the conditions, and has nearly 2000 feet of smooth grass runway to work with, so for him it's safe (relatively).
Mark

[Frogger 0]

WOW!!!
Outstanding post Mark!
Thanks for that humorous and extremely informative discussion of canopy performance.
Thanks and Blue skies!
Frogger

[freeflyguy 0]

Yah, good post Mark
J

[zaidush 0]

thank you for a grate post

[cobaltdan 0]

rigger rob,
changing the lines to open up the angle of attack will give you a flatter glide.
but it will not increase the glide ratio of your canopy. it does in full flight but does not make the modified canopy capable of out gliding the unmodified canopy. a given airfoil/planform design is capable of a maximum glide, this will require pilot input, no line trim modification can increase this maximum glide for the particular design.
opening up the angle of attack to provide for maximum glide in full flight will translate to undesirable flight characteristics, ie. slow foward speed in full flight, very hard to extremely hard front riser pressure, greatly reduced performance flight envelope.
there is no point to doing this.
sincerely,
dan
atair

[markbaur 0]

I just got back from a visit to MIT, where one of the items on display is Robo-tuna, a mechanical fish used to investigate Gray's Paradox, which was a question of how dolphins were able to swim faster than the mathematics said they should be able to. That's hydrodynamics, but it got me back looking into low-speed aerodynamics. I'm happy to report that the field is still poorly understood, which means there is still hope for a major improvement in canopy technology and performance. For example, it may be possible to use something like vortex generators to better attach the airflow over the canopies. I'm eager to see what the next generation of improvements brings.
Mark

[nacmacfeegle 0]

"I'm eager to see what the next generation of improvements brings."
Like reliable cross brace manufacturing techniques?
Joking, just joking........