There’s No Such Thing As Kite Fabric
by Steve Ferrel
Originally published in Kiting Magazine 2010 Vol 32 Issue 3
Kite fabric is the fabric that you use to build a kite. You can use anything to build a kite. I’ve been judging kites for twenty years and have seen many fabrics: bed sheets, blue tarps, tree leaves, plywood, raincoats, suit lining, that webbing in those old woven lawn chairs, various plastics and papers, potato chip bags, pizza boxes. I even saw a flying bra at one festival. Anything can be used. It’s just that some fabrics may work better than others. If you are making a large, high wind delta and you want it to be stable, a bed sheet makes an excellent kite fabric. The porosity actually increases stability.
It is just that these days, most people want a kite fabric that is lightweight, strong, durable, easy to work with, has a porosity close to zero, and availability of colors that look pretty in the sky. I know that was my goal when I started building kites. So ripstops fit the bill. When I first got into kiting I was fortunate enough to have Bill Tyrell as a neighbor. Back in the early 1980s, Bill was known as the “Fabric Lady,” and he was basically the only person selling ripstop nylon as a kite fabric. He’s to blame for getting me started on my career path. When my father and I began Kite Studio we said we would only sell the materials that we used on our own kites. Since that time, all the kites I’ve made have been made with ripstop nylon. Most often, ¾ ounce fabric. Although, I’ve never really been too concerned with weight. When I use ¾ ounce nylon, most often I layer two or three colors to get the desired effect. In my opinion, 1½ ounce ripstop is one of the most underused fabrics. Sure it’s heavier, and it probably needs to be hot cut if you have exposed edges, but in return you get great color: deep reds, crisp whites, and the blackest black. Color effects are what I’m most concerned with. There are many things to think about when you are making a ripstop nylon. (The following is borrowed from Kites, Fabrics, and Airfoils by Brian Doyle of North Cloth.)
¾ ounce would appear to refer to the weight per square yard, but it’s not that easy. These fabrics typically weigh 1.1 ounce per sailmaker’s yard, which is 36” long but only 28.5” wide. So, ¾ ounce really weights about 1.4 ounces per square yard. The weight will also vary a little from lot to lot and manufacturer to manufacturer.
Almost all fabrics used kites, sails, and parachutes have some sort of polymer finish which stabilizes the cloth and helps to resist diagonal stretch. These are applied as either coatings spread on one side of the fabric (the coating is usually a urethane polymer similar to urethane varnishes; Bainbridge fabric uses this), or impregnations which soak into the bulk of the cloth. Melamine is an impregnation with a polymer which is the same as that in Formica countertops. What are the differences between impregnated and coated fabrics? And how do you choose when to use one or the other?
Here are some rules of thumb:
- Melamine-finished fabrics are lower in stretch and higher in breaking strength than the same weight of coated fabric.
- Coated fabrics, with their softer “hand,” usually have higher tear strength than the firm melamine fabrics.
- Coated fabrics replace fiber content with coating weight. Coating is cheaper than fiber. Therefore, coated fabrics are less expensive to make.
There are three directions in the fabric: the 0° (warp, or long direction in a roll of cloth), 90° (fill or cross direction in fabric roll), and 45° (bias, halfway between the other two). An important thing to notice is that the 0° is about twice as resistant to stretch as the 90° and 45°. This is intentional. The fabric is intended to be used with the warp lined up with the highest loads in the sail. This is important to keep in mind when choosing the cloth orientation in a kite. There is also elastic stretch, which is elongation if the load is removed from the fabric and how it returns to its original length. Beyond this range the fabric “yields,” and plastic stretch takes place. Plastic stretch is permanent even after the load is removed. Why do some fabrics stretch more than others? Many factors in the design and manufacture of cloth can have an effect on stretch, but the most important are: • yarn tenacity and modulus • weave count and density • finish firmness. There is a wide diversity of yarns to choose from in designing ¾ ounce ripstop. Some are better for clothing. Some are less expensive. The lowest stretch yarns are produced for industrial fabrics and are known as “high tenacity.” As might be expected these are also the more expensive yarns. Fabrics with higher yarn counts and weaving density will have lower stretch. It is possible to weave a lower count, less expensive fabric and finish it with a coating for low porosity, but this is at the expense of higher stretch. Impregnations and coatings are available in a range of hardness. It is also possible to apply more or less finish to adjust the firmness of the fabric. In general the trade-off is between cloth stretch and tear resistance. Firm finishes, while achieving lower stretch, lock the yarns in place so they can not bunchup ahead of tears to stop them.
Resistance to tearing in a woven fabric is a complex business depending on many variables. Fortunately it is usually sufficient to look at only two variables: yarn tenacity (breaking strength) and finish firmness. It is easy to see that stronger yarns will give higher breaking strength, and all things being equal, premium fabrics woven with higher tenacity yarns will have higher tear strength. Finish firmness, either coating or impregnation, can make things complicated. A firm finish, bonded tightly to the yarns is often desirable to decrease stretch. But this type of finish also decreases tear strength. An important mechanism in resisting tears in fabrics is the movement of individual yarns so as to bunch at the leading edge of the tear and create a smoothed out reinforced area to stop the tear. This is why cheese cloth is so difficult to tear even though the yarns are relatively weak. What is sufficient tear strength? This will depend to some extent on the design of the kite and how it will be used. Usually firm, low stretch, ¾ ounce cloth should have tears of at least 3-4 pounds. Some soft, higher stretch ¾ ounce products can have tears as high as 10-20 pounds. In general, fabrics of high tenacity yarn and firm finishes in which the finish is tuned so as to be as firm as possible without exceeding some lower limit of tear strength make the best fabric airfoils. Some manufacturers will emphasize the high tear strength of their cloth. But this is usually very soft cloth with poor stretch performance and shape holding.
Why is porosity important? Is zero porosity necessary? Military specification parachute cloth has both a minimum and maximum porosity requirement. The minimum porosity allows a small amount of air to pass through the fabric. This turbulates the boundary layer, keeping the flow around the top of the chute attached, and stabilizes the parachute. Obviously very high porosity is not good because it can reduce the pressure differential across the airfoil and throw away lift. There are many opinions and no certain answers regarding the best levels of porosity. Experience in making a large number of kites and paying attention to the effects of porosity is probably just as good as most theories.
Nylon vs. Polyester
Nylon is hydroscopic (it attracts water) so on wet or humid days it gets heavier as the water bonds to the fibers. Nylon has more stretch than polyester so nylon will handle gusts better, but polyester holds the designed shape better. Both fabrics fade, but polyester generally fades at a slower rate. However, with both fabrics, different colors fade at different rates. Flourescent colors fade faster. Nylon is usually cheaper.
Most cloth vendors classify their fabric as either a “first” or a “second.” First quality means the fabric is 99% perfect, meaning almost no cosmetic flaws at all. North Cloth goes one step further and classifies their fabric in four grades: A, B, C, and D. In a nutshell, grade A is basically perfect, and D is the least perfect. However, on a roll of any grade it is quite possible that you will see no flaws at all. Your chances of seeing a flaw are just increased the further down the alphabet you go. In all cases, if there is a flaw it is cosmetic only, meaning creases, stains, varying dye, or even a smashed bug. All cloth is usable and most often you can cut around a blemish if present. We sell a lot of seconds, and I can count on one hand the number of times fabric has been returned. You’d be surprised at how many kites are manufactured with second quality fabrics.