Published on June 5th, 2012 | by Adam0
So this past weekend was fairly rainy, so I didn’t get the opportunity to do any skydiving while I was in Victoriaville visiting my girlfriend, however I did get the chance to give her her birthday gift. I’m taking her Hang gliding.
I know it’s not skydive related, but she’s already a skydiver (with more jumps than I have I might add) so what do you get someone who already loves the sky? A completely new experience that will throw her off guard! =D
We’re planning on going within the next couple of weeks, so look out for an article about our experience with that.
I came across an article on the PD website about Pack Volumes that is quite interesting as most skydivers don’t seem to think about that when buying their canopies/containers. Different canopies pack differently and each type has a different total volume when packed, this article talks a bit about it, figured I’d share it with all of you.
Pack Volume: The Untold Story
by: Ian Bellis
“How big does it pack?”, “Will it fit in my container?”, “I’ll buy the one that packs smaller”.
How many times have you asked or heard someone ask these types of questions?
Pack volume has become a major advertising point for manufacturers and a decision making factor for purchases, but before you go out and buy the “smallest packing canopy” of a given size There are a few things you should know:
I. Canopy Size: “All 170’s are not created equal”
Q. When you see a canopy advertised as 170 square feet, how big do you think it really is? The answer may surprise you…
A. It all depends on the measuring method! Canopies can have their span (wingtip to wingtip) and chord (nose to tail) measured in different locations. Which locations are used can have a significant impact on the resulting numbers. P.I.A. attempted to devise a method which would standardize the measurement of canopy area. There were problems inherent in this method however, and based upon the resulting variance, which are due to factors in the construction of canopies, we cannot use these numbers to accurately compare one manufacturer=s canopy to another. This has resulted in the current situation where the different manufacturers have addressed the “area question” with their own measuring methods. Each method has a certain validity to it. There is no one correct method, however everyone should be aware of the following: Whether it is top surface or bottom surface, along the curve of the airfoil or along the chordline (see fig. 1), the resulting area can easily vary by 10%-20%! What this means is that unless you are comparing two canopies within a certain manufacturers measuring method, you cannot directly compare the advertised area of one to another! If one manufacturer measured another’s canopy, they would undoubtedly advertise it as a different size. Even if the sizes were identical, the airfoil size and shape alone could significantly affect the volume of the canopy, not to mention its flight and landing characteristics.
Another aspect is whether a canopy is measured while laying flat on the ground or if the measurements are based on the canopy’s configuration while inflated. Once a canopy inflates, its “actual” surface area is quite different (smaller) than if it was measured lying flat. The reason for this is the distortion of the cells when actually inflated and flying (see fig. 2). The important thing to remember is the size of the inflated and flying canopy is what you will have to land.
The bottom line here is, you might not have as much canopy over your head or reserve packed in your container as you think…
II. Construction Method
Q. If you have two canopies of the same size (measured the same way of course), then they should have the same pack volume?
A. Not necessarily. The type and amount of reinforcing tapes, types of seams, the number of ribs, air locks, etc. will all play a part in the pack volume. In the final analysis, the more fabric, tapes etc. in a given canopy, the larger the pack volume. This is something each individual needs to decide on their own in terms of its worth. While some people might accept a slightly larger pack volume for a reserve with more reinforcement, they might not feel the same way when it comes to their main. It is up to you, the important thing is to understand the difference in construction, their benefits and drawbacks, and then make an informed decision on which canopy to buy.
III. Fabric Type and Related Variances
Q. Why does one type of fabric have a larger pack volume than another?
A. Several reasons for this one:
Fabric thickness is one aspect. Different types and production runs of fabric have different
thicknesses. This affects the pack volume of the fabric itself.
Zero porosity vs. Low porosity (F111 type):
While most people assume 0P fabric packs larger than low porosity, in part due to its thickness, the more important factor is the person packing it. We have all seen individuals who can pack a 0P canopy of a given size into a container easily, while that same canopy container combination would give others “fits” trying to pack it. This is obviously not due to any change in the pack volume of the canopy. The truth is, it is easier to maintain control of low porosity fabric while packing, which allows the average person to pack it noticeably smaller. Zero porosity fabric has great advantages in longevity and consistency of performance though. One option for people who do not want to deal with the packing of zero porosity, but would still like some of the benefits in longevity and performance, are canopies constructed of a combination of zero porosity and low porosity fabrics. At least two major manufacturers offer canopies of this description.
Zero porosity vs. Zero porosity:
Some people believe that, all things being equal, (which we have already stated is almost never the case) one 0P canopy would pack the same as another. Not true. “Not all 0P fabric is created equal”. The different methods for producing the yarn, weaving the fabric and processing this woven fabric into low porosity or “zero porosity” can have a great impact on the pack volume. It can also have an effect on the longevity of the zero porosity aspect of the fabric. Not all 0P fabric remains 0P. While this change could result in easier packing, it may not be desirable in terms of opening, flight and landing characteristics (another subject for another article). Suffice it to say, if you are not sure about the fabric used in a particular canopy, ask the manufacturer about any long term porosity testing they have done.
Even with the same type of fabric from the same manufacturer, there can be variances of 30%-50% from one fabric lot to another! Amazing as this might seem, this is an area that no parachute manufacturer can control. Parachute manufacturers buy their fabric from textile manufacturers and much as we would like it to be otherwise, this variance is inherent in the fabric manufacturing process. While it does not affect the fabric structurally, it does have the stated impact on pack volume. Everything involved from the drawing of filaments, to weaving those filaments into fabric which is then processed into low porosity or zero porosity “canopy fabric” incorporate processes which can change the ultimate pack volume of a canopy. One aspect which is often overlooked is what is known as the “hand” of the fabric. This is the ease with which a fabric can be folded, pretty important when you consider that’s all packing is. A fabric with a “soft hand” will pack smaller than a fabric with a “stiff hand”. The best analogy used to explain this concept to me was the “wet paper” analogy. Take a piece of paper and wad it up into a ball as tightly as you can, now take another piece of paper (the same size) and wet it, now ball that piece up. Which one is smaller? The wet paper would be said to have a “soft hand” the dry paper a “firm hand”.
IV. Testing Methodology
Q. If I use the P.I.A. chart, won=t I be able to get an unbiased comparison of pack volumes?
A. This is a valid question that gets asked often. P.I.A. has attempted to provide an objective listing of pack volumes. The problem is that the current equipment and methodology for arriving at the P.I.A. pack volume numbers is not 100% repeatable. What I mean is, the same person can test the same canopy twice using the same technique and still obtain significantly different results. How can this be? As hard as P.I.A. has tried, the equipment and the test cannot alleviate many potential variables. It involves a volume chamber, which is basically a graduated cylinder manufactured to specific dimensions, and the application of weight to a canopy placed in this cylinder. The canopy can be placed in the chamber differently; the required weights can be placed on the canopy with or without applying pressure, etc. The current temperature and humidity can have an impact as well. Although P.I.A. has employed a procedure which arrives at an average value from multiple tests, the results are volumes that we cannot objectively compare. The P.I.A. chart itself shows the difficulty of the task. Although P.I.A. has attempted to eliminate as many variables as possible, the chart still shows a seven cell PD-235 (700 cubic inches) as having a larger volume than a nine cell PD-260 (650 cubic inches). This difference would lead one to believe the smaller 7 cell packs larger than the 9 cell, even though the 9 cell has similar construction methods, 25 square feet more area, 2 additional cells, and more suspension lines! This is not very likely. Several companies are currently trying to devise their own testing methods, but until a consistent method is developed that all parachute manufacturers utilize, we cannot compare one canopy to another in any meaningful way.
Q. So what does it all mean?
A. The statement “the numbers never lie” is not always true, as shown here. Luckily, most canopy manufacturers provide canopies to the container manufacturers for sizing purposes. The container manufacturers are therefore an excellent source to determine canopy-container compatibility. A few recommendations from the author regarding canopy pack volume:
1. Never base your purchasing decision on pack volume.
The numbers you are using may be deceiving.
2. Decide on canopy model and size before container size.
It is not the container you will have to fly and land for hundreds or thousands of jumps in all conditions (bad spots, high winds, turbulence, etc). While certainly your harness must fit properly, the container size itself should be based solely on your canopy choice. To do otherwise would be akin to finding a set of wheels that you like the looks of and then purchasing your car to fit them (doesn’t make much sense, does it?)
3. Test jump a canopy before purchasing.
Many companies offer demo programs on both mains and reserves, use them. Do not put a canopy in your container (especially a reserve) without giving yourself the opportunity to fly and land the same model and size under “controlled conditions”. You may find that a “small packing reserve” is not as big as you think. It may not be a canopy you would feel comfortable landing under all circumstances. The same is true of the main. Would you buy a car without a test drive?
4. Choose your canopy size based on the “worst case scenario”.
If you can “just barely” pull off a landing in the landing area, with a bit of wind, you probably should look at another (larger) inflated size and/or model.
5. Be careful accepting advice from the local “sky gods”.
In some cases they may have forgotten what it was like to be at your experience level. What seems normal or “conservative” to them now, would have been a canopy they wouldn’t even consider when they were at your level.
6. Utilize the experience of the people building your equipment.
If you have questions, get your dealer to ask the container or canopy manufacturers or call them yourself.
7. When choosing a canopy size and model, take into account several factors:
Your current canopy (frame of reference), your currency (number of jumps per year, layoffs due to season, etc.), your pilot profile (conservative, aggressive, etc.), your experience (how many jumps, what types of canopies), your home DZ (small or large landing area, quantity and location of alternate landing sites, etc.).
8. Choose your canopies wisely…
When you find yourself wishing you had more canopy over your head in order to make it back from a bad spot or land your reserve off field, it’s too late.
Too much emphasis has been placed on pack volumes. We need to remember that it is just a
“numbers game”. We can see that comparing the “pack volumes” of canopies may not have any corresponding impact on our ability to pack a given main or reserve into a given container. The first thing is to choose your canopies based on performance characteristics (opening, flight, and landing), then factor in the ease of packing (not the volume), 0P vs. F111 vs. combination. Then ask the container manufacturer of your choice what size container accepts your canopy choices. If you stick to this method you will have a system that is easy to pack and performs the way you would like.