Sub 100ft S/L??

[greeny]

This is just a theory I'm looking for some feedback on (compleatly untested).

The way I understand it when a canopy opens the first thing that happens is bottom skin inflation.

1) Does this mean that at this stage it is acting like a round ( only the drag of material is slowing you down, no lift being produced )?

2) Can we PLF a canopy with bottom skin inflation only, or are we going to break legs?

Next the canopy pressurises, the top skin comes into play and the canopy starts to act as a wing to produce lift.

3) Vents and ZP top skin aim to speed up top skin inflation and there by give you a pressurised flying canopy quicker. But am I slowing down or decresing my bottom skin inflation by letting some of this air throught to my top skin?

4) Could we push S/L lower with ZP bottom skins and not worrying about top skin inflation?

Burn this as much as you like what I'm looking for is some more informed feed back on all of the above before I go throw myself off some low stuff.

[JJ]

Go for the "tard" grenny...saw one performed this weekend and it inflates faster than a d-bag.

JJ
:-)

[space]

The way I understand it when a canopy opens the first thing that happens is bottom skin inflation.

1) Does this mean that at this stage it is acting like a round ( only the drag of material is slowing you down, no lift being produced )?
>Yes

2) Can we PLF a canopy with bottom skin inflation only, or are we going to break legs?
> what are you landing on? i think in most cases that you will break at least one appendage with a turf type LZ.

3) Vents and ZP top skin aim to speed up top skin inflation and there by give you a pressurised flying canopy quicker. But am I slowing down or decresing my bottom skin inflation by letting some of this air throught to my top skin?
>Zptopskin is not designed to speedup secondary inflation, only to give the same performance thru-out it´s life span. Meaning not degrading in performance..
4) Could we push S/L lower with ZP bottom skins and not worrying about top skin inflation?
> Sub 100ft jumps, but on the high side have been done, My lowest was 85 feet, and I had a solid 2.5 sec flight with a RavenIII PCA, the DBaggers only had 2 secs flite from the same site.
Low SL or PCA´rs must be aware that wind over 3mph has a freakin huge effect on heading perf. as usually you are opening at the end of one second
and going only 11mph and the wind even as low 5mph is almost half of your downward accelleration. then push it to a downwind exit, and your canopy has not the time to pulll out of a dive vectored by your downward fall and winds. hope this helps,
take care
space

[crwper]

Greeny,

That's what your earlier post has me thinking about... A canopy inflates like a balloon.

Have you ever seen high-speed photos of a round inflating? It inflates from the top down. You see a bulge at the apex, which works its way downward like when you blow up a long balloon. The air inside is travelling the same speed as the parachute itself, so it exerts more force on the fabric than the air outside the parachute, which is moving. Down at the skirt, this pressure mostly escapes out the open end of the parachute. But at the top, it has nothing to do but force the fabric outward.

It's the same sort of thing with a ram-air parachute, but the ram-air will inflate from the tail first. That's why lineovers are common without a tailgate -- because the tail is expanding first, flipping brake lines over the rest of the (uninflated) canopy. The tailgate helps to retard the inflation of the tail, and encourage the front of the canopy to inflate first.

After the initial balloon-like expansion, the bottom skin experiences most of the effect of the moving air, so it feels the greatest amount of pressure holding it open. The top skin sits in the wake of the falling canopy, so it isn't so well pressurized. This is clearly visible as ripples in the fabric.

Because the bottom surface of the canopy is tilted somewhat forward, the upward-moving air now starts to impart forward motion on the canopy. This pressurizes the cells, the top skin becomes more rigid, and the whole canopy acts as an airfoil.

Vents take some of the upward-moving air and redirect it into the cells from the bottom skin. This further increases the pressure inside the cells and helps keep the canopy open once it's there.

There's a trade-off, though. The vents decrease the effective area of the bottom skin, so it won't inflate quite as fast as before.

Also... The vents are located partway back on the bottom skin of the canopy. In the initial stages of inflation, when the bottom skin is not yet exposed to the airflow, this might encourage some of the high-pressure air contained inside the canopy to move out through the vents, instead of contributing to inflation.

My guess is that the vents will slow down the initial inflation, but speed it somewhat once the bottom skin is exposed to the moving air.

A zero-p bottom skin would help with the initial inflation, but would also stop the upward moving air from pressurizing the topskin before the canopy has begun moving forward.

But... My intuition tells me the F-111 fabric isn't letting much air through, anyway. Look at the inflation of an F-111 canopy. The top skin is still pretty wobbly in the initial stages of inflation, and that indicates to me that the bottom skin isn't letting much air through. I don't think porosity of the fabric is your greatest concern.

I think what would be a greater effect is the weight of the fabric. With zero-p treated fabric, you could potentially use a ligher fabric to achieve the same performance levels. This means less weight for the expanding air to push. Also, we all know that zero-p likes to expand on its own, even without the rushing air. That might help.

I know the weight of zero-p fabric varies a lot -- Chute Shop's zero-p seems a lot heavier than PD's. Does anyone have the numbers handy? I'm wondering what's the difference between the weight of a square meter of F-111 type fabric versus a square meter of zero-p.

Space -- I'm curious about your 85-foot jump. You say you got a solid 3-second flight. Jumping from 111-feet with a vented canopy, it was 5 seconds from when my feet left the bridge until I touched down. I was under an open canopy for approximately 3 seconds of that, but I wouldn't refer to any of it as "flight", as the canopy was acting almost entirely as a decelerator. Are you saying you had 3 seconds of forward flight, or just that your canopy was open for 3 seconds? Was this over land or water? How hard was the landing? Thanks!

Michael

[crwper]

I'm not a packed-canopy Nazi... But to me there's something beautiful about leaving the object with a packed canopy on my back. There's something beautiful about an unpacked canopy, too, but it's a different kind of beauty.

Michael

[guest]

I agree with JJ.
Serioulsy, just do a Tard or better yet do a Quad. Quads open slightly faster than a Tard unless the Tarder gets a really good upward throw. You will need to practice these jumps first though to get good at them.

[d-dog]

I saw this jump, as well, and I'd estimate that the tard opening had the canopy fully inflated and flying at least 20 feet higher than a properly-executed PCA from the same object on the same day. Given that we've seen pretty consistent PCA openings and landings from 135 feet or so, that'd mean that the tard could have handled a 110-ish jump with not only pressurization, but actual canopy flight and flare.

That said, I've no idea how consistent those tard openings are in terms of height of pressurization and flight. The data set just isn't there yet, and I'd sure not want to be the one to collect the data on a jump from 115 feet or so where a 20+% variability in performance would result in broken legs or worse!

Still, I'd not be surprised to find that in due time the tard (or something similar) ends up being the gold standard for super-low freefall. Remember, the tard got flying at least 20 feet higher than a PCA - and, technically, the tard IS a freefall.

Could this be the future of sub-150 foot freefalls? Could Spence be the crash test dummy that will figure it out? Tune in next year for the exciting season premier of BASE Today!

Peace,

D-d0g
ddog@wrinko.com
www.wrinko.com

[base698]

What's a tard?