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
