A parachute is an aerodynamic brake constructed of a low-porosity material, such as nylon, designed to decrease the speed of an object (person, race car, space shuttle) by increasing drag against the atmosphere.
Parachutes are often used by skydivers to slow their descent before colliding with the planet at which they have thrown themselves.
The ideal environs for such a device is a planet with a gravitational acceleration of 9.81 metres/second2 and an atmospheric pressure of approximately 14.7 PSI at mean sea level. As the parachute is an aerodynamic device, it has no effect in a vacuum, which would be closer to 0.0 PSI at mean sea level.
Parachute System Components
A parachute by itself is mostly useless. In order to function properly, all the pieces of the parachute system must function together and deploy in the proper sequence in order to achieve the desired effect of deceleration. The required components are as follows:
A smaller parachute (less then one metre in diameter) that the skydiver deploys either by placing it directly in the air-stream or by pulling a rip cord which launches a spring-loaded pilot chute from the skydiver's back. The pilot chute anchors itself in the air-stream, and lifts a bag containing a larger canopy out of the container, which, in turn, subsequently pulls the larger canopy out of the bag.
The bag holds the canopy and all the lines neatly in the container to help ensure an orderly deployment.
This is the actual parachute part. Modern canopies are usually rectangular or elliptical (older round canopies are still in use, though not as much because they do not provide nearly as much manoeuvrability as the rectangular variety). They are constructed of two skins (bottom and top) of a low or non-porous material that inflates to form a wing with the same properties as that of an airplane wing. In fact, a canopy can perform almost any manoeuvre an airplane can except go back up.
Made of nylon, Kevlar, or similar materials, the lines run through grommets1 in the slider and attach the canopy to the risers.
The slider is a small, square piece of nylon with grommets in each corner that a group of lines run through. The slider is stored towards the top of the lines when the parachute is packed in the container. Its purpose is to inhibit the canopy from opening too rapidly during inflation. Rapid canopy inflation is not only uncomfortable for the skydiver, it can cause the stress levels on the canopy to exceed their limitations. As the canopy inflates, it pushes the slider down along the lines until it comes to rest on top of the risers above the skydiver's head.
Nylon webbing that connects the harness with the lines of the parachute. A complete parachute system usually has two sets of risers. In the event of an emergency, the first set can be quickly detached, releasing the malfunctioning canopy so the backup or reserve can be deployed without entangling. The second set is an integral part of the harness and cannot be released.
The harness is the nylon webbing that holds the cargo (skydiver, jeep, etc) in place during the descent. The container is the backpack-looking part which usually carries two canopies (a main and a reserve).
Care and Feeding of a Parachute
Never smoke in the vicinity of a parachute. The cigarette ashes can quickly burn small pinholes in the fabric, which can cause annoying tears in the fabric during times of high stress, such as deployment.
Parachutes don’t like to get wet. It increases their porosity (the amount of air that can pass through the fabric), which decreases the canopy's efficiency and forward speed.
Different types of canopies have very different manoeuvring characteristics. Older, round parachutes descend with about a 1:1 glide ratio. That is, for every foot they go down, they can move about 1 foot forward. Rectangular canopies have a 3:1 glide ratio. Obviously, the horizontal distance covered will be affected by the prevailing head or tail winds for each individual jump but, in general, the higher the glide ratio, the bigger the margin of error the skydiver has in hitting his mark.
If, for example, a skydiver, jumping a round parachute, exits at 10,000 feet and he is 12,000 feet away from the target, he will end up 2,000 feet from his intended destination. Another skydiver with a rectangular parachute however, could fly past the target, circle around, come back and still hit dead centre.
Landing a round parachute is a bit like jumping from the roof of a single-storey house. Round parachutes have a constant rate of descent of about 10 feet/second, and the skydiver must know how to perform a proper parachute-landing fall (PLF) in order to avoid injury.
Rectangular parachutes, however can be slowed or 'flared' just prior to landing which brings the parachute's vertical speed to near zero and it's horizontal speed to just a few miles per hour, providing for a tip-toe landing.