Elasticity (or Elastic Recovery)

The term elasticity describes the ability of a fabric to elongate (or stretch) when tension is applied, and to recover its original shape when the tension is released. If you stretch a rubber band and then let it go, it comes back to its original size. To test the elesticity of a material, stretch it 4 percent and then measure to see how closely it returns to its original length. A fabric that returns to within 75 to 95 percent of its original length after being stretched is said to have satisfactory elasticity. A parachute made from fabric with good elasticity is stronger and gives less opening shock.

Weight

Lightweight fabric is an absolute necessity for all parachute canopies. A canopy of lightweight material opens faster. Can you imagine a pilot walking around wearing anything as heavy as a canvas beach umbrella? Lightweight cargo and deceleration parachutes enable the aircraft to carry more weight in cargo and fuel.

Resistance to Abrasion

This refers to a fabric's ability to withstand wear and rubbing. In its lifetime, a parachute is subjected to a great deal of abrasion. When you pack a parachute, you pull the canopy down the table. A deceleration parachute slides along the runway. For this reason, deceleration parachute riser webbings and personnel parachute harnesses and risers are treated with Merlon (brand name) to make them more resistant to abrasion damage.

Resistance to Mildew and Insects

Moths and other insects love to feast on fabrics; mildew and other fungi thrive on them in warm, damp climates. Parachutes damaged by mildew or insects would be unsuitable for Navy use. Therefore, it is necessary that parachute fabric be as resistant as possible to this type of damage.

Moisture Regain

The term moisture regain refers to the percentage of moisture that a bone-dry fiber absorbs from the air under standard conditions of temperature and humidity (65 percent relative humidity and 70 "F). Less than 5 percent moisture regain means that the fibers build up static electric charges when rubbed. If static electricity builds up, the parachute assembly is more difficult to service. Static electricity also adversely affects the opening time of a parachute assembly.

The ability to take on dye (color) is another important consideration when selecting parachute fabrics. The percentage of moisture regain possible in a fabric determines whether it can be successfully dyed. Dying gives the fabric color, which is important for a parachute canopy. Rescue teams can easily spot multicolored canopies from the air. Pickup crews can quickly identify colored deceleration canopies on runways. Also, yellow dye in a canopy makes it more resistant to ultraviolet damage from sunlight, which relates to the next engineering requirement on this list.

Resistance to Sunlight

Ultraviolet light, which is found in sunlight, reduces the strength of fabrics. Ultraviolet rays give you a painful sunburn when you're out on the beach too long. All parachutes are exposed to some sunlight. Military specifications for parachute materials state fabrics should not lose more than 25 percent of their original strength after 50 hours exposure to sunlight. Investigations into causes of deceleration parachute failures have shown strength loss of more than 50 percent after 50 hours of exposure to sunlight.

Resistance to Heat

In addition to sunlight, heat and friction are natural enemies of a parachute. In case of fire on an aircraft, personnel and deceleration parachutes may be exposed to great amounts of heat. Friction and heat are generated when the deceleration parachute comes in contact with the runway. Lineovers cause friction and burn holes in parachute canopies. Line-overs happen when an improperly stowed suspension line is drawn over the canopy during deployment.

Resistance to Chemicals

Because parachute assemblies are exposed to various chemicals, it is important to know which chemicals are harmful and which are not. Most damaging are mineral-type acids, such as the type used in batteries.

You have studied a long list of engineering requirements that are important to know when you service parachutes. You already know that nylon is the most widely used fabric in the parachute shop. In the following text, we examine the good and bad characteristics of nylon in parachute construction.

There is no fabric known to man that measures up perfectly to all the engineering requirements for parachute construction. But nylon comes closer, by far, than any other fabric. Nylon, when properly handled by the parachute rigger, has more good than poor qualities. First, we will review the good characteristics of nylon:

1. Strength. Nylon is one of the strongest synthetics made.

2. Elongation. Nylon stretches from 18 to 40 percent, which is well above military specifications of 20 to 25 percent.

3. Elasticity. Nylon returns to 100 percent of its original length.

4. Weight. Nylon fibers are very strong for their weight; therefore, the fabrics manufactured of nylon fibers are lightweight fabrics.

5. Resistance to abrasion. Nylon doesn't have enough resistance to last forever under all the rugged use parachutes get, but it is better than any other material tested for parachute use.

6. Resistance to mildew and insects. Nylon has no food value. This makes it unappetizing to moths and other insects. It cannot support the growth of mildew. This isn't true, however, when a cup of coffee is accidentally spilled on a parachute assembly, or other edible foreign matter comes in contact with it. What we really mean is clean nylon has no food value.

We warned you that nylon is not the perfect fabric. Where possible, improvements have been made in manufacturing nylon. The limitations you must keep in mind when handling nylon are as follows:

1. Moisture regain. Remember, we said that if the percentage of moisture a fiber absorbs from the air is less than 5 percent, the fiber is difficult to dye and builds up static electricity when rubbed. The moisture regain of nylon is only 4.2 percent, so you can expect static electricity to develop as you service the assembly.

2. Resistance to sunlight. We mentioned earlier that yellow dye improves resistance to ultraviolet light damage. That is why yellow dye is added to deceleration canopy material. In addition, a chemical known as Chemstrand "R" has been developed, which, when added to nylon fiber as the yarns are manufactured, makes nylon more resistant to ultraviolet light.

3. Heat resistance. Nylon has a relatively low melting point, 482 F, which makes it very susceptible to damage from heat. This is why it is so important that suspension lines be stowed properly. In the rapid deployment sequence, lines crossing each other will break from the friction heat generated.

In short, there are several natural enemies to be aware of when you handle parachute textiles of any type. These are the hazards of sunlight, abrasion, heat, chemicals, insects, and fungi on parachute components.

Keep in mind the natural enemies of textiles you have learned. Then it is easy to see which elements are to be avoided when you store parachute fabrics.