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 requirements. 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.

Table 10-1.-Tensile Strengths

Elongation

The term elongation describes deformation, lengthening, or stretching caused by a tensile force. It's what you do when you stretch a rubber band. The ability to elongate gives stretch to a fabric. Elongation is expressed as the percentage of stretch over the original length. For instance, if a tape has 10 percent elongation, a 10-inch piece will stretch to 11 inches before it breaks. Parachute specifications call for 20-25 percent elongation.