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spinglebout

Parachute Performance Factors

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This guy is kind of working through a little home project where he's building his own canopies.

http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=4342216;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;;page=unread#unread

There's a lot of discussion where he's trying to understand some of these issues.

As to what makes a canopy the canopy that it is... The shortest answer by my guess...

1. trim angle
2. aspect ratio
3. airfoil thickness

There can be other factors but I think those are the driving characteristics. Zp allows you to have a higher internal pressure with out leaking too much air through the surfaces, higher wing loading. Cross bracing reduces the distortion in the wing that reduces the span and makes it less dependent on pressurization to maintain it's shape. Better low speed performance allowing you to land at higher wing loadings. It also allows you to get by with a thinner air foil. Changes to the nose reducing the size of the openings can reduce drag and improve air flow but they can also lead to issues the stagnation point on the front of the canopy. Line length can affect the dynamic characteristics of the canopy. So there are a lot of things that feed into it but I think they all feed back in to the ones above.

What are you really asking? Define your question better and we'll refine our answers.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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thanks for the reply RiggerLee!

I am doing a research project on canopies and I'm trying to do a comparative analysis between a canopy designed for a student and a canopy designed for an expert... in terms of performance. Low performance for the student and high performance for the expert. (correct me if my statement is wrong)

So using several hypothetical and quantitative input variables such as different exit weights(150lb and 200lb(, different inflation times(2 secs and 3 secs), different canopy opening altitudes (2000ft, 3000ft)etc. for both low performance and high performance canopies. Ill also be talking about the effect of varying density altitudes, canopy materials etc. to support the analysis

Now my difficulty lies in identifying a fixed set of performance factors that can 'influence' the way the canopy behaves for both low performance and high performance. This way I have a list of factors to work on analyzing in terms of different input variables.

(still in the process of figuring everything out.. pardon me if i sound vague in any way)

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Vague, yah a little vague. But more then that a little ambitious. You've listed half a dozen different things that you might do a study on any one of which would probable be enough to keep you busy. You've casually tossed around a number of questions any one of which would actually entail solving a number of non trivial problems. If you're looking for advice there are people here that can give it to you. If you're looking for information, there is some. A lot of it resides within companies that make their living off of it. And some of it just isn't around. A lot of people have tried to study and analyze parachutes with only limited success. They are not as... nice as, for example, an airplane. It's hard to do a model when you have trouble even saying what shape they are. I mean that's pretty basic and with the... Let's just call it aero elastic issues, even that is non trivial. Up till recently there wasn't even any "good" experimental data on them. There has been some work done in a few wind tunnels large enough to hold a scale model. Gyro did some of that. Recently there has been a lot of work done with data gathering putting instrumentation on to canopies. It's kind of the next big frontier that people are looking at. For the first time we are starting to get measurable quantifiable data but I don't think there is that much of it yet. All though there are people doing it most of them don't seem to be all that organized. There way a guy from Europe that seemed to be accumulating some pretty good data. I met him at PIA, he gave a lecture four five years ago. If he pursued it then he might have some good GPS date on flight paths, turn and sink rates, and speeds for you.

So my advice is that if you're going to try to do some thing... theoretical, like a model, then pick ONE problem or question and try to create some thing to examine it. If on the other hand you're looking to do an analysis of empirical data, start by cheating. Look around and find some data that you think you can work from, that will actually be the hardest part, then based on what is available to work with define what you want to try to study. I know that seem back ward but it's the only way you'll avoid running into a dead end.

Just some thoughts. Pick a question and we'll try to help.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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Haha! Yeah.. I'm doing my final semester undergrad. At moments like this, I truly feel like I have dug my own grave by choosing ram-air parachute analysis.
But no matter, your thoughts actually make me feel a little less crappy.B|
As of now, I'm going to go with whatever data I can acquire. Also, I'm going to pay a visit to my local DZ here in DUbai. Hopefully I will get more insight into the matter and come back with a better set of questions in the forum

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Have you contacted Gary Peek? He's on here I think under the name PEEK

http://www.dropzone.com/cgi-bin/forum/gforum.cgi?username=peek;

He has actually taught classes in parachute design. He was involved in the early efforts to put accelerometers and load cells on rigs. He has a lot of his data and reports on his web site. There's another group you should try. I'm brain locking, can't think of their name, but they were at the last PIA symposium giving lectures. They built a load cell/data acquisition system that is being used by a number of companies in their certification process and to prosue government contracts. They have the best equipment around. They are part of or associated with Performance Design. So if you talk to the right people there they can help you. See if they or any of their client's, like PD, would be willing to share some data.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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RiggerLee

So my advice is that if you're going to try to do some thing... theoretical, like a model, then pick ONE problem or question and try to create some thing to examine it.



I'm a new guy bugging John Sherman and many other notable people on questions relating to canopies too. But I certainly agree with RiggerLee here, where scientifically we've all been taught to change one variable and isolate the rest, in order to draw the link between change and effect. And I think this will be pertinent since I suspect the scope of spinglebout's question will eventually lead towards a full spectrum analysis utilizing computational fluid dynamics to derive the specific answers desired here.

P.S. Many thanks for the great article mjosparky; it certainly provides a great starting platform for me too!

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Interesting. You ask "What factor most influences canopy performance?" and get two very smart people (possibly 2 of the smartest and most experienced on here) answering you.

And nobody has said "SIZE" yet. Or wingloading.

Isn't that the biggest factor?
Take 2 canopies, identical in shape, construction, planform, materials, all that. The smaller one will be faster and more responsive. If identical weights are under it, the difference will be significant. If the weights are proportioned in respect to the size (equally loaded) then the smaller will still be faster and more responsive, just not as much.

That's a huge part of what makes a JVX a HP canopy when a Navigator isn't. The X-brace is a factor, so is the elliptical shape. But most of it is the fact that the JVX will be 1/3 the size of the Nav. And loaded more heavily.
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

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wolfriverjoe



That's a huge part of what makes a JVX a HP canopy when a Navigator isn't. The X-brace is a factor, so is the elliptical shape. But most of it is the fact that the JVX will be 1/3 the size of the Nav. And loaded more heavily.



I'd agree that it is a huge part, but the performance of said JVX is more holistic.
To riggerlee's point, the other design factors are what make the JVX landable/controllable at wingloadings over double what Navigators are designed for. That is, the cross bracing, nose design, more narrow draft of the wing (etc) allow the wing to still be usable at wingloadings that a lower performance wing design would struggle or fail. This is why you don't see Navigator 59's rocking the swoop course.

like building a race car, you have to have more than a big engine to get "high performance".

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Perhaps because wing loading is not a fixed parameter of the design. In fact you might be able to say that size, loading, and density altitude are all a juggling act that could in theory balance out in the same way at radically different numbers. It's kind of like Reynolds Numbers where you could get equivalent performance at a relative scale. It's just not the place where I would start to examine the fundamental characteristics of a design.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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Hello there,

I'm not a parachute designer but I am a commercial pilot with a background in physics, I'm a novice skydiver but here's my 2 cents if you want them.

Performance is a very relative term, when you're talking about performance it must always be done in context, IE, the discipline of choice where this canopy will be used.

A large 7 cell accuracy canopy will outperform a highly elliptical crossbraced 9 cell when it comes to accuracy landings.

On the other hand if what you're looking for is speed, rate of descent, stability in turbulence, etc. Then the small elliptical crossbraced swoop machine will outperform the accuracy 7 cell.

As to what affects performance?

Absolutely everything, the parachute is a wing and anything you do to change the shape of it will affect performance. Line trim will affect performance, brake length, aspect ratio, thickness, the way it's braced, whether the pilot opens his chest strap or not, how he starts the flare (with rear risers and then toggles or just toggles).

The complexity of the analysis is quite extensive so if I were you I would focus on one aspect of the performance itself. IE, explain why a 260 Navigator is better than the 120 katana for a given exit weight when it comes to a novice jumper. You can then put an expert jumper on the katana and compare why it will outperform the navigator when it comes to speed, swoop, rate of bank, rate of descent, etc...

A fun few things to consider, give the 260 nav to an expert and get him to extract as much performance as he can out of it, compare to the HP wing and then figure out what the safety margin is between the canopies, for example, try to spin the Nav and determine it's recovery arc from terminal velocity, then do the same for the 120 katana and show how much more time a novice might have with his huge nav to recover from the dive.

Compare terminal velocities on both canopies when pointed straight at the ground, you can use a pair of sensitive audibles to measure this for example.

Of course, do all this very very high and far away from other canopies.

Have fun and give us a copy of your work when you're done. Good luck!

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Polorutz

Hello there,

I'm not a parachute designer but I am a commercial pilot with a background in physics, I'm a novice skydiver but here's my 2 cents if you want them.

Performance is a very relative term, when you're talking about performance it must always be done in context, IE, the discipline of choice where this canopy will be used.

A large 7 cell accuracy canopy will outperform a highly elliptical crossbraced 9 cell when it comes to accuracy landings.

On the other hand if what you're looking for is speed, rate of descent, stability in turbulence, etc. Then the small elliptical crossbraced swoop machine will outperform the accuracy 7 cell.

As to what affects performance?

Absolutely everything, the parachute is a wing and anything you do to change the shape of it will affect performance. Line trim will affect performance, brake length, aspect ratio, thickness, the way it's braced, whether the pilot opens his chest strap or not, how he starts the flare (with rear risers and then toggles or just toggles).

The complexity of the analysis is quite extensive so if I were you I would focus on one aspect of the performance itself. IE, explain why a 260 Navigator is better than the 120 katana for a given exit weight when it comes to a novice jumper. You can then put an expert jumper on the katana and compare why it will outperform the navigator when it comes to speed, swoop, rate of bank, rate of descent, etc...

A fun few things to consider, give the 260 nav to an expert and get him to extract as much performance as he can out of it, compare to the HP wing and then figure out what the safety margin is between the canopies, for example, try to spin the Nav and determine it's recovery arc from terminal velocity, then do the same for the 120 katana and show how much more time a novice might have with his huge nav to recover from the dive.

Compare terminal velocities on both canopies when pointed straight at the ground, you can use a pair of sensitive audibles to measure this for example.

Of course, do all this very very high and far away from other canopies.

Have fun and give us a copy of your work when you're done. Good luck!



Who says you can’t learn anything from noobs. Your post shows a greater understanding of the various flight characteristics of the ram air parachute than most people heavily involved in the sport. Very well done.:)

Sparky
My idea of a fair fight is clubbing baby seals

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spinglebout

Well, that's what I'm trying to find out.
Like, what makes a JVX parachute deserve the title of 'high-performance' for example..



Poor girl, thought that was a simple question huh? Here's a straight answer without any if/then scenarios.

The highest performance canopy is the one which generates lift most efficiently. This would be a rigid wing such as a fixed wing aircraft. The metal is hard, smooth and never flexes. This generates lift efficiently and consistently across the top surface of the wing. Parachutes are no different.

The first thing we did was make the parachute square and give it ribs with an upper and lower surface area like an airplane wing. This was way before I started jumping though. Then we created material called Z-PO(which stands for zero porosity) to help increase negative air pressure on the top skin for more lift and better response. At the same time the design advanced from a 7 cell dual chamber design(giving us 14 individual arcs across the top of the canopy) to a 9 cell dual chamber design(giving us 18 individual arcs across the top of the canopy). I"ll get into the performance gain on this a bit later.

So now we're up to the days of Sabres in the early 1990s. From this point designers realized that if you tapered the leading and trailing edges of the canopy it was much more responsive. This gave us the era of 9 cell high performance ellipticals. Stilettos, Jedeis, Batwings, Turbo ZXs they would all respond to toggle, harness and riser input much more than the traditional square canopy and were hot shit back in the day!

By the mid to late 90s designers were looking for a way to make the wing even more stiff and rigid. The first step was stitching extra flaps in the nose of the parachute to create a one way valve kind of like a raccoon trap. The Jedei was the first to have them I believe and they were called "Airlocks." This kept the wing firmer even at slower air speeds and in turbulence and also was an improvement for generating lift at the tail end of the flare when the canopy slowed down.

Even with all this though the designers still recognized that the wing would be more efficient if it was flatter and generated lift more perpendicular to gravity instead of in a rounded arc above the jumper. This lead to cross bracing. If you have a box with four sides, it can easily be manipulated into a parallelogram. Now if you take that same box and attach the opposite corners to each other(like bracing on a bridge) that box is now extremely rigid and it's shape cannot change easily. Buy utilizing all the previous advancements and going to triple chambered cross braced cells, canopies like the JVX and Velocity are extremely rigid and create lift very efficiently. This allows a steeper angle of attack(angle the parachute flies at the ground in normal full flight) for more speed and longer diving turns because of the ability to generate lift more efficiently. Also by adding the cross bracing and third chamber to each cell not only is the overall arc of the canopy flattened out, but each individual chamber(27 on a JVX) has less arc creating a smoother top surface which increases overall lift.

So at this point you would think swoopers would be satisfied with having something capable of going 80+ MPH straight at the ground and generating enough lift to easily swoop an entire football field... you would be wrong :-p Being the speed freaks we are we always want MORE!!

Modifying the harness and deployment system can also increase the performance and efficiency of the wing. A traditional parachute will typically have the smaller pilot chute used for deployment and the bag which the canopy is packed inside of attached to the top of the parachute and trailing behind after opening. By attaching that pilot chute and D bag to the slider and putting detachment points on the slider, we can remove all that garbage after opening the parachute for a clean efficient wing. The big gain isn't just from the reduced parasitic drag. The canopy will generate much more lift, especially late in the flare, because the pilot chute and D bag are not creating a burble on the top skin of the canopy which is where all your lift comes from.

One other trick we use(other than just plain strapping on lead to increase wingloading) is to fold our risers over lengthwise and stitch the edges together to get a knife edge. This eliminates the 1" x 22" wide air brake attached to each of your shoulders. This is all about reducing parasitic drag on the overall system and is most effective at wingloadings above 2lbs/sq ft.

Enjoy :-)

Blue Skies!!!

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Here's a couple I snapped. No adverse affects on opening but it does make any canopy considerably more aggressive. The biggest thing that can be an issue is that it's easy to have your risers twisted when your packing. I also route my risers around the flaps and tuck tabs for hop n pops so there is less material flapping around creating drag and it's less wear and tear on the container. Just cleaner in my opinion but only for hop n pops.

Also I've heard people complain that the RDS rings on the Chupacabra being too small. With my risers stitched this way I have zero issues with the rings hanging up on the risers. The system works awesome!

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