ericber 0 #1 March 14, 2004 Article: Symmetry vs. Asymmetry I recently heard the following statement while on a drop zone and decided to do some investigation into the validity of the comment. "The nose of the canopy should periodically be flipped on the other side to prevent asymmetrical line stretch which causes a built-in turn." I contacted Mr. John Le Blanc, Vice President of Performance Designs parachutes, who helped me with some of the following technical information. In the very near future an informative article addressing this concern will be available. Clarification on how a pack job affects line stretch on a canopy is more complex than one might think. A symmetrically packed parachute inflates from the center and works its way to the end cells. This would result in a relatively symmetrical linestretch. An example of an asymmetric pack job is one that starts to inflate at one end cell and works its way towards the other end cell. In an asymmetric opening the lines can stretch more on one side than the other. Microline is an example of line that will stretch. Dacron will not stretch nearly as much. The method of packing can determine whether or not lines will stretch. Parachutes tend to open harder when opening asymmetrically. During opening the parachute overloads on one side. This overloading can lead to line failures, usually one cell in from the end cell. The standard stack pack method consists of placing the B lines over and on top of the A lines by pulling taut the B lines from the top of the canopy and folding towards the A lines. This is followed by pulling taut the C lines over and on top of the B lines. Then the D lines are pulled taut on top of the C lines. If you are stack packing your canopy this method should be quite familiar. Using the stack pack method places the canopies lines in a relatively uniform symmetrical pattern decreasing asymmetry. NOTE: THE LINES MAY HAVE SYMMETRY, HOWEVER, CANOPY MATERIAL SHOULD NOT BE OVERLOOKED. SYMMETRY APPLIES TO THE ENTIRE PARACHUTE AND NOT JUST THE LINES. A NEAT, PROPERLY ORGANIZED PRO-PACK IS THE MOST SYMMETRICAL PACK JOB OF ALL PROVIDED THE PARACHUTE IS PACKED SYMMETRICALLY. A PARACHUTE IS DESIGNED TO BE PACKED SYMMETRICALLY. The problem seems to occur when canopies are packed with the lines and fabric in an asymmetric pattern. One example of lines being packed in an asymmetric pattern is the method of folding the A lines over to the B lines followed by folding the D lines to the C lines. Folding these two groups in towards each other followed by a rolling technique is one example of packing an asymmetric canopy. This method is referred to as a "Briefcase Method" or "Roll Type Method." Another example of an asymmetric opening is created by rolling the entire nose from the A lines towards the B lines. If you must roll the nose to slow opening, roll each half of the nose towards the center cell. Rolling the nose to the point of twisting the A lines is all right ONLY if each half is rolled towards the center cell. Is it necessary to place the nose on the other side to even out the stretched lines? The answer is no providing you are packing the parachute symmetrically. What constitutes a symmetric pack job? Lines organized neatly rather than chaotic. Parachute material folded with uniformity. OTHER FACTORS INVOLVED: Packing a symmetrical canopy is the objective, however, over a period of time an uneven line group deployment can cause the same effect. Looking over one shoulder or the other during deployment and observing the bag coming off of your back through the point of line stretch is placing your body in an attitude that will cause uneven line stretch. A general rule of thumb is this. Throw or pull it out, lay square in the harness until deployment sits you up. Doing so will unstow the lines in a symmetric pattern. It is not necessary to look over your shoulder until you know a problem has occurred and emergency procedures take over. Paying attention to the deployment sequence does not necessarily mean watch it happen. What is primarily meant by this is to notice how the deployment feels while it is progressing. How does the container feel after pin extraction? Can you feel the lines unstowing in an orderly manner? Does the force at line stretch feel normal? Take the time to become aware of how you are packing your parachute. The ultimate responsibility for your own life is up to you and nobody else. Feel the deployment sequence. Know how it feels. Consistently packing a symmetric ` parachute and knowing how the deployment sequence feels will lend itself to many safe and 'full of fun' skydives.Blue Skies Eric Bernstein D-9298 ericber@oz.net Quote Share this post Link to post Share on other sites
PhreeZone 20 #2 March 14, 2004 Quote Microline is an example of line that will stretch. Dacron will not stretch nearly as much. This is backwards. Dacron lines will strech more on opening and return to their original size, but microline has a very low strech ability and tend to shrink due to wear. Some camera flyers have their canopies lined with Dacron since it streches on opening and leads to softer openings. Simple solution to the procieved problem? Propack.Yesterday is history And tomorrow is a mystery Parachutemanuals.com Quote Share this post Link to post Share on other sites
ericber 0 #3 March 14, 2004 Thank you so much for your comment. EricBlue Skies Eric Bernstein D-9298 ericber@oz.net Quote Share this post Link to post Share on other sites
towerrat 0 #4 March 15, 2004 QuoteSimple solution to the procieved problem? Propack ------------------ I agree. I have never had any problems with a standard Propack.Play stupid games, win stupid prizes! Quote Share this post Link to post Share on other sites yoink 321 #5 March 15, 2004 Quote An example of an asymmetric pack job is one that starts to inflate at one end cell and works its way towards the other end cell. I would have thought that this would lead to half the canopy turning whilst the other half is still inflating... wouldn't this lead to twists at the very least? I am guessing that rolling the entire nose in one direction could cause this sort of opening. Quote Share this post Link to post Share on other sites seal_S49 0 #6 March 15, 2004 This lengthy post could have been titled "why pro packing is better." However, I thinks these disadvantages of flat packing are overstated. If flat packing was going to put substantially higher stress on lines and fabric, why would manufacturers still recommend it, especially for reserves and tandems? I've been to DZs that flat pack tandems & student rigs exclusively and they don't seem to have a problem with equipment damage & line twist. When canopies go out of trim, it's because of line shrinkage, not line stretch. There is a reason to alternate L & R sides of canopy on packing mat for every other flat pack job. This practice will distribute floor abrasion to both sides of the canopy so the fabric of one end cell doesn't wear faster than the other, according to at least one manufacturer. Just because flat packing is less popular doesn't make it black death. I think pro packing is better for many reasons and that's how I pack all mains. S49 Quote Share this post Link to post Share on other sites riggerrob 643 #7 March 15, 2004 Dacron lines will strech more on opening and return to their original size, but microline has a very low strech ability and tend to shrink due to wear. Some camera flyers have their canopies lined with Dacron since it streches on opening and leads to softer openings. Simple solution to the procieved problem? Propack. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> I suspect that Dacron softens openings because it is bulkier and creates more friction with the slider grommets. Quote Share this post Link to post Share on other sites
yoink 321 #5 March 15, 2004 Quote An example of an asymmetric pack job is one that starts to inflate at one end cell and works its way towards the other end cell. I would have thought that this would lead to half the canopy turning whilst the other half is still inflating... wouldn't this lead to twists at the very least? I am guessing that rolling the entire nose in one direction could cause this sort of opening. Quote Share this post Link to post Share on other sites
seal_S49 0 #6 March 15, 2004 This lengthy post could have been titled "why pro packing is better." However, I thinks these disadvantages of flat packing are overstated. If flat packing was going to put substantially higher stress on lines and fabric, why would manufacturers still recommend it, especially for reserves and tandems? I've been to DZs that flat pack tandems & student rigs exclusively and they don't seem to have a problem with equipment damage & line twist. When canopies go out of trim, it's because of line shrinkage, not line stretch. There is a reason to alternate L & R sides of canopy on packing mat for every other flat pack job. This practice will distribute floor abrasion to both sides of the canopy so the fabric of one end cell doesn't wear faster than the other, according to at least one manufacturer. Just because flat packing is less popular doesn't make it black death. I think pro packing is better for many reasons and that's how I pack all mains. S49 Quote Share this post Link to post Share on other sites
riggerrob 643 #7 March 15, 2004 Dacron lines will strech more on opening and return to their original size, but microline has a very low strech ability and tend to shrink due to wear. Some camera flyers have their canopies lined with Dacron since it streches on opening and leads to softer openings. Simple solution to the procieved problem? Propack. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> I suspect that Dacron softens openings because it is bulkier and creates more friction with the slider grommets. Quote Share this post Link to post Share on other sites
riggerrob 643 #8 March 15, 2004 QuoteQuote An example of an asymmetric pack job is one that starts to inflate at one end cell and works its way towards the other end cell. I would have thought that this would lead to half the canopy turning whilst the other half is still inflating... wouldn't this lead to twists at the very least? I am guessing that rolling the entire nose in one direction could cause this sort of opening. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Yes. It also forces one shoulder to take most of the opening shock. Ouch! And they wonder why few old tandem masters (F-111 era) have intact rotator cuffs or hip sockets ????? Quote Share this post Link to post Share on other sites
riggerrob 643 #9 March 15, 2004 If flat packing was going to put substantially higher stress on lines and fabric, why would manufacturers still recommend it, especially for reserves and tandems? >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Actually most manufacturers recommend PRO packing, they are just too lazy to update their manuals. I PRO-stack most reserves (except for Strong tandems) and I have been PRO packing all tandem mains since the late 1990s. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Just because flat packing is less popular doesn't make it black death. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Agreed. A neat side pack is always better than a sloppy PRO pack. Quote Share this post Link to post Share on other sites
