Slow motion openings....

[lodestar 0]

[url]http://www.youtube.com/watch?feature=player_embedded&v=Hm8DW7CF7dQ

Some very interesting stuff on openings, the NASA video featured is a 100 foot chute used with two others for the shuttle boosters....

[pchapman 261]

clicky
http://www.youtube.com/watch?feature=player_embedded&v=Hm8DW7CF7dQ

One can just see that they used an anti inversion net on it. The opening is typical for a round without a slider: some parts of the canopy push outward as it pressurizes, while other whole "lobes" of the skirt & canopy are still being held inwards towards the centre, so the process is not symmetrical at all.

What is interesting is how much the fabric is fluttering when it isn't 'pressurized' from either side, when neither bulged outward by the slug of air filling the canopy from the top, nor bulged inward just below the inflated part. The area which flutters is pushed towards the skirt as the canopy does the usual thing where it inflated from the top.

Looks like a full speed video of a similar test, with the lights on is at:
http://www.youtube.com/watch?v=ola9cG3RgOM

There the canopy snivels a while but I think it is a low speed test, based on the speed of a few items flying by in the breeze.

[JohnRich 4]

When a round canopy is at line stretch and opening, I was told when I was new to the sport that the canopies don't really open because of air filling the inside and pushing outward. But rather that the high speed air flowing past the canopy all around it, with the ambient still air inside, causes it to be sucked open from the outside. Sort of like aerodynamic lift with high speed air on the top of a wing, and lower speed air on the underside, causing the wing to be sucked upwards into the lower pressure area. What's your knowledge on this?

[airtwardo 6]

Quote

When a round canopy is at line stretch and opening, I was told when I was new to the sport that the canopies don't really open because of air filling the inside and pushing outward. But rather that the high speed air flowing past the canopy all around it, with the ambient still air inside, causes it to be sucked open from the outside. Sort of like aerodynamic lift with high speed air on the top of a wing, and lower speed air on the underside, causing the wing to be sucked upwards into the lower pressure area. What's your knowledge on this?

I know for a fact some of MY round openings SUCKED!

~ If you choke a Smurf, what color does it turn? ~

[dks13827 2]

That is interesting video. The early manned spacecraft depended on reefed rounds of course for crew survival. I heard 2 Apollo guys, at least, mention that of course there was some concern about the chutes, but they didnt worry about it until it was time to do so. No sense in worrying for 12 days, you know. They always seemed to work, though on Apollo 15 they did get a burned and collapsed chute before splashdown. It seems to me that a reefed round is indeed reliable, but of course so are the current day square reserves.

[JerryBaumchen 1,046]

Hi John,

Wanna really find out:

http://en.wikipedia.org/wiki/Bernoulli's_principle

JerryBaumchen

[pchapman 261]

I think that a rough answer is that both forces create the opening -- extra pressure inside, and suction outside on parts of the curved, expanding parachute. I'm guessing that the suction is a lesser factor, but still an important one.

Inside, air eventually starts getting into the almost closed mouth of an initially streamered parachute, and as long as the canopy isn't too porous, fills up the apex of the canopy and starts to push the canopy outward, creating a bulge up there. The pressure inside can go above what the steady state wind pressure (dynamic pressure) would be, as the column of air comes to a sudden stop.

Meanwhile on the outside, as the air just outside the canopy first hits the bulge of the canopy, it can be pressing inwards there (positive pressure), trying to keep the bulge from expanding. (So it isn't suction everywhere.)

But as the air goes further, as it flows up and over the bulge it will create outwards lift (negative pressure) -- basic Bernoulli stuff. And in the turbulent flow behind the parachute, where the airflow isn't staying attached along the curve of the parachute, the air pressure will be a little bit lowered, also creating a little suction -- that would be "the burble" to skydivers.

Even in engineering sources describing parachute openings, the focus tends to be on the slug of air getting inside and filling the canopy from the apex down. That's true, but doesn't go into the source of the forces trying to move the canopy fabric outward, where it is both inside pressure and outside suction.

(E.g., http://accessscience.com/content/Parachute/486500 from Jean Potvin of the Parks College Parachute Research Group. This very brief overview of parachutes includes a description of parachute openings. It does mention the suction aspect, but tends to focus more on the internal pressure aspect.)

So it is important to remember that there are suction forces at work too.

I saw one old military study where the suction forces outside were similar in size to the positive pressures inside, when at a point say two thirds the way up the canopy, early in the inflation process, before the canopy had spread to nearly full diameter. While not knowing what contributed more to the inflation overall, it shows that at certain points and times, it can be close to a 50/50 split between the two complementary forces.

I haven't seen enough sources to really understand the exact mix of forces vs. time vs. location, whether overall it is 50/50 or 75/25, for the mix of what "caused" the opening to happen. Modern computational fluid dynamics work should have a good handle on what the pressures are, but I can't find any good reference from quick internet searches.