mildewproof, and is not affected by most ordinary oils, greases, or cleaning fluids. It is mothproof, and, because it is not an animal fiber like wool or silk, does not offer food to hungry insects. However, if insect larvae develop from eggs laid inside the folds of stored fabrics, they may eat their way out. Soiled or greasy spots in a fabric attract insects. Soot and certain chemical fumes are highly injurious to nylon, and direct heat and exposure to the sun’s rays seriously weaken it. Rayon has many of the characteristics of nylon. It is more easily damaged by direct heat or the sun’s rays and is more combustible than nylon. Rayon fabrics “take a set” (form a crease) more easily than other fabrics, and if left stored in folds for too long, they will form permanent creases. Cotton fabrics, webbings, and tapes, unless treated, absorb water readily. They dry more slowly than synthetic fabrics and are more susceptible to mildew and fungus growth. Mildew should never be ignored because it seriously weakens cotton or other fabrics. Heat is less damaging to cotton than to the synthetics. Bugs or their larvae will eat cotton or use it to make cocoons or nests. In all cases, fire is a constant threat to fabrics. Smoking should not be permitted where fabrics are handled or stored. The rayons are almost explosive when set afire. Nylon, although harder to ignite, will bum, but does not explode in the process. You should be careful to learn the storage problems peculiar to any specific locality or climatic conditions to ensure safe storage of these materials. The construction and characteristics of various fabric products has been explained to give you some basis for the intelligent use and storage of these materials. Besides textile materials, you are required to use dopes, cements, and solvents in the daily performance of your duties as a fabrication and parachute specialist. ENGINEERING REQUIREMENTS FOR FABRICS If a parachute is to serve its purpose, it must be reliable. To be reliable, parachutes must meet certain engineering requirements. At this point you may be wondering why you should be concerned with engineering re- quirements. After all, you are not designing parachutes. You service parachutes. Here is where the difference shows up between just a parachute packer and a good parachute rigger. Almost anyone can learn to pack a parachute. But a good parachute rigger needs to understand the’ ‘why” that determines maintenance procedures. When you have learned the engineering and aerodynamic principles that affect parachute reliability, you will know why it is so important to be a conscientious and precise worker. And, you will see to it that those who work for you do their job exactly right. First, the engineering requirements for parachutes are listed and explained below. Then you will learn why the textile most often used in parachute construction is nylon. Air Permeability The term air permeability refers to the measured volume of air in cubic feet that flows through 1 square foot of cloth in 1 minute at a given pressure. If a material gets wet and shrinks, it has less air permeability y, because the weave draws together and less air gets through. This is the reason for that very important rule: DO NOT, FOR ANY REASON, PACK A WET PARACHUTE. Also, a wet parachute assembly can freeze at high altitudes. Air permeability y affects the reliability, opening time, opening force, canopy drag, and stability of the parachute assembly. The proper ratio of air entering a parachute canopy to air passing over the canopy gives a parachute good performance. The greater the airflow through a canopy, the slower the opening time. This is why canopy designs differ. A quick opening time is required for personnel parachutes, but a slower opening time is desired for deceleration and cargo parachutes. The braking force in deceleration and cargo parachutes is built up over a longer period of time, which enables the parachute assembly to withstand and decelerate greater loads. Strength The term strength refers to a fabric’s ability to resist strain or rupture by external forces. Strength is expressed as tensile strength (a term you already know) and is measured in pounds per square inch. The strength of the fabric determines the strength of the parachute. Remember the old saying about the chain being only as strong as its weakest link. Strength is a very important requirement for a safe, reliable parachute. Refer to table 10-1 for tensile strengths of fabrics, webbings and tapes. 10-6