Pack Volume: The Untold Storyby: Ian Bellis

AAHow big does it pack?@@, AAWill it fit in my container?@@, AAI==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 factorfor purchases, but before you go out and buy the Asmallest packing canopy@ of a given size there area few things you should know:

I. Canopy Size: AAAll 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? Theanswer may surprise you...

A. It all depends on the measuring method! Canopies can have their span (wingtip to wingtip) andchord (nose to tail) measured in different locations. Which locations are used can have a significantimpact on the resulting numbers. P.I.A. attempted to devise a method which would standardize themeasurement of canopy area. There were problems inherent in this method however, and based uponthe resulting variance, which are due to factors in the construction of canopies, we cannot use thesenumbers to accurately compare one manufacturer=s canopy to another. This has resulted in thecurrent situation where the different manufacturers have addressed the Aarea question@ with their ownmeasuring 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 by10%-20%! What this means is that unless you are comparing two canopies within a certainmanufacturers measuring method, you cannot directly compare the advertised area of one toanother! If one manufacturer measured another=s canopy, they would undoubtedly advertise it as adifferent size. Even if the sizes were identical, the airfoil size and shape alone could significantlyaffect the volume of the canopy, no to mention its flight and landing characteristics.

Another aspect is whether a canopy is measured while laying flat on the ground or if themeasurements are based on the canopy=s configuration while inflated. Once a canopy inflates, itsAAactual@@ surface area is quite different (smaller) than if it was measured lying flat. The reasonfor this is the distortion of the cells when actually inflated and flying (see fig. 2). The important thingto 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 packedin 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 shouldhave the same pack volume?

A. Not necessarily. The type and amount of reinforcing tapes, types of seams, the number of ribs, airlocks, etc. will all play a part in the pack volume. In the final analysis, the more fabric, tapes etc. ina given canopy, the larger the pack volume. This is something each individual needs to decide ontheir own in terms of its worth. While some people might accept a slightly larger pack volume fora 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 anddrawbacks, 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 differentthicknesses. 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, themore important factor is the person packing it. We have all seen individuals who can pack a 0Pcanopy of a given size into a container easily, while that same canopy container combination wouldgive others Afits@ trying to pack it. This is obviously not due to any change in the pack volume of thecanopy. The truth is, it is easier to maintain control of low porosity fabric while packing, whichallows the average person to pack it noticeably smaller. Zero porosity fabric has great advantagesin longevity and consistency of performance though. One option for people who do not want to dealwith the packing of zero porosity, but would still like some of the benefits in longevity andperformance, 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 thecase) one 0P canopy would pack the same as another. Not true. AANot all 0P fabric is created equal@@.The different methods for producing the yarn, weaving the fabric and processing this woven fabricinto low porosity or Azero porosity@ can have a great impact on the pack volume. It can also have aneffect on the longevity of the zero porosity aspect of the fabric. Not all 0P fabric remains 0P. Whilethis change could result in easier packing, it may not be desirable in terms of opening, flight andlanding characteristics (another subject for another article). Suffice it to say, if you are not sure aboutthe fabric used in a particular canopy, ask the manufacturer about any long term porosity testing theyhave done.

Fabric lots: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 parachutemanufacturer can control. Parachute manufacturers buy their fabric from textile manufacturers andmuch as we would like it to be otherwise, this variance is inherent in the fabric manufacturingprocess. 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 fabricwhich is then processed into low porosity or zero porosity Acanopy fabric@, incorporate processeswhich can change the ultimate pack volume of a canopy. One aspect which is often overlooked iswhat is known as the Ahand@ of the fabric. This is the ease with which a fabric can be folded, prettyimportant when you consider that=s all packing is. A fabric with a Asoft hand@ will pack smaller thana fabric with a Astiff hand@. The best analogy used to explain this concept to me was the Awet paper@analogy. Take a piece of paper and wad it up into a ball as tightly as you can, now take another pieceof paper (the same size) and wet it, now ball that piece up. Which one is smaller? The wet paperwould be said to have a Asoft hand@ the dry paper a Afirm 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 listingof pack volumes. The problem is that the current equipment and methodology for arriving at theP.I.A. pack volume numbers is not 100% repeatable. What I mean is, the same person can test thesame canopy twice using the same technique and still obtain significantly different results. Howcan this be? As hard as P.I.A. has tried, the equipment and the test cannot alleviate many potentialvariables. It involves a volume chamber, which is basically a graduated cylinder manufactured tospecific dimensions, and the application of weight to a canopy placed in this cylinder. The canopycan be placed in the chamber differently, the required weights can be placed on the canopy with orwithout 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, theresults are volumes that we cannot objectively compare. The P.I.A. chart itself shows the difficultyof the task. Although P.I.A. has attempted to eliminate as many variables as possible, the chart stillshows a seven cell PD-235 (700 in3) as having a larger volume than a nine cell PD-260 (650 in3). Thisdifference would lead one to believe the smaller 7 cell packs larger than the 9 cell, even though the9 cell has similar construction methods, 25 ft2 more area, 2 additional cells, and more suspensionlines! This is not very likely. Several companies are currently trying to devise their own testingmethods, but until a consistent method is developed that all parachute manufacturers utilize, wecannot compare one canopy to another in any meaningful way.

V. Summary

Q. So what does it all mean?

A. The statement Athe numbers never lie@ is not always true, as shown here. Luckily, most canopymanufacturers provide canopies to the container manufacturers for sizing purposes. The containermanufacturers 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 allconditions (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 akinto finding a set of wheels that you like the looks of and then purchasing your car to fit them (doesn=tmake 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 canopyin your container (especially a reserve) without giving yourself the opportunity to fly and land thesame model and size under Acontrolled conditions@. You may find that a Asmall packing reserve@ isnot as big as you think. It may not be a canopy you would feel comfortable landing under allcircumstances. The same is true of the main. Would you buy a car without a test drive?

4. Choose your canopy size based on the AAworst case scenario@@.If you can Ajust barely@ pull off a landing in the landing area, with a bit of wind, you probably shouldlook at another (larger) inflated size and/or model.

5. Be careful accepting advice from the local AAsky gods@@.In some cases they may have forgotten what it was like to be at your experience level. What seemsnormal or Aconservative@ to them now, would have been a canopy they wouldn=t even consider whenthey 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 themyourself.

7. When choosing a canopy size and model, take into account several factors: Your currentcanopy (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 ofcanopies), 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 froma bad spot or land your reserve off field, it=s too late.

VI. Conclusions

Too much emphasis has been placed on pack volumes. We need to remember that it is just aAnumbers game@. We can see that comparing the Apack volumes@ of canopies may not have anycorresponding impact on our ability to pack a given main or reserve into a given container. The firstthing 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 thecontainer manufacturer of your choice what size container accepts your canopy choices. If you stickto this method you will have a system that is easy to pack and performs the way you would like.