Parachute

A parachute is a soft fabric device used to slow the motion of an object through an atmosphere by creating drag. Parachutes are generally used to slow the descent of a person or object to Earth or another celestial body with an atmosphere. Parachutes are also sometimes used to aid horizontal deceleration of a vehicle (an airplane or space shuttle after touchdown, or a drag racer). The word parachute comes from the French words para, protect or shield, and chute, to fall. Therefore parachute actually means to protect from a fall. Many types of modern parachute are quite maneuverable, and can be flown like gliders.

Design

A parachute is made from thin, lightweight fabric, support tapes and suspension lines. The lines are usually gathered through loops or rings at several strong straps called risers. The risers directly strap the item or person being supported, called the "load." Parachutes are pulled out of their packages by a smaller parachute called a pilot chute. Reserve pilot chutes usually have a large spring that pushes them into the air-stream, but most modern main parachutes use a form of hand deployed pilot chute with no spring. Hand deployed pilot chutes are usually pulled from a pouch located on the parachute container and they in turn pull a pin opening the main parachute container. Spring loaded pilot chutes are released by a cable called a "rip cord." Usually the rip cord pulls a metal pin that releases fabric flaps that hold the pilot chute and canopy a compact package. Pilot chutes and containers may be released by a "static line." The static line is a length of bridle attached to an airplane or platform. In most sport reserve and emergency parachutes the rip cord is a manually operated by pulling a "D" (pronounced "dee") handle attached to the harness although there are many variations. Cargo parachutes are always released by static lines. Paratroop parachutes are also usually deployed by static lines which release the parachute yet retain the bag which contains the parachute without relying on a pilot chute for deployment, making their deployment rapid, consistent and reliable.

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Paratroopers and sport skydivers carry two parachutes. The primary parachute is called a main parachute, the second, a reserve parachute. The reserve is quicker-opening with a manual rip cord but other secondary deployment systems are usually used. The jumper uses the emergency chute if the primary parachute fails to operate correctly. Reserve parachutes were introduced in World War II by the US Airborne Unit, and are now universal. The picture of the paratroopers canopy is not ripped or torn but has a T-U cut provided to allow airflow to aid in steering the chute in a direction to aid in landing.

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There are several types of parachutes in common use. Ribbon and ring parachutes can be designed to open at speeds as high as Mach 2 (two times the speed of sound). These have a ring-shaped canopy, often with a large hole in the center to release the pressure. Sometimes the ring is broken into ribbons connected by ropes to leak air even more. The large leaks lower the stress on the parachute so it does not burst when it opens.

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Often a high speed parachute slows a load down and then pulls out a lower speed parachute. The mechanism to sequence the parachutes is called a "delayed release" or "pressure detent release" depending on whether it releases based on time, or the reduction in pressure as the load slows down.

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Emergency parachutes and cargo parachutes designed to go straight down are pure drag devices. These have large dome-shaped canopies made from a single layer of cloth. Some skydivers call them "jellyfish 'chutes" because they look like dome-shaped jellyfish. Some dome parachutes can be steered by flaps. They usually have a small hole in the center of the dome to spill air, so that the parachute does not have to swing to spill air from its edges.

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Most modern parachutes are self-inflating "ram-air" airfoils known as a parafoil that provide control of speed and direction similar to the related paragliders.

Related Topics:
Airfoils - Paraglider

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Paragliders have much greater lift and range, but parachutes are designed to handle, spread and mitigate the stresses of deployment at terminal velocity. All ram-air parafoils have two layers of fabric; top and bottom, connected by shaped fabric I-beams and/or gores. The space between the two fabric layers fills with high pressure air from vents that face forward on the leading edge of the airfoil. The fabric is shaped and the parachute lines trimmed under load such that the ballooning fabric inflates into an airfoil shape.

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Ram-air parachutes loosely divided into two varieties. High performance ram-air parachutes have a slightly elliptical shape to their leading and trailing edges when viewed in plan form and are known as ellipticals. Usually they have smaller, more numerous fabric cells and are shallower in profile. Lower performance parachutes look more like square inflatable air-mattresses with open front ends. Smaller parachutes tend to fly faster for the same load and ellipticals respond faster to control input, small elliptical designs are therefore often chosen by experienced canopy pilots for the thrill of the flying they provide. This requires much more skill and experience to pilot and is considerably more dangerous to land. With high performance elliptical canopy designs nuisance malfunctions can be much more serious than with a square design and may quickly escalate into emergencies. All reserve ram-air parachutes are of the square variety because of the reliability and handling characteristics.

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Sport parachutes used by skydivers today are designed to open softly, rapid deployment was an early problem with ram-air designs. The primary innovation that slows the deployment of a ram-air canopy is the slider; a small rectangular piece of fabric with a grommet near each corner through which four collections of lines are routed to the risers. During deployment the slider slides down from the canopy to just above the risers, the slider is slowed by air resistance as it descends and reduces the rate at which the lines can spread and therefore the speed at which the canopy can open and inflate. The overall design of a parachute still has a significant influence on the deployment speed. Modern sport parachutes deployment speed varies considerably between designs but most modern parachutes open comfortably with individual skydivers preferring different deployment speeds. The deployment process is inherently a chaotic one and rapid deployments can still occur even with well behaved canopies, on rare occasions deployment can even be so rapid that the jumper suffers bruising or even injury. Emergency and reserve parachutes by design tend to deploy more rapidly than sports main canopies, they still have sliders but the sliders are designed to descend rapidly with for example a partial mesh construction to catch less air resistance than a fully fabric slider design.

Related Topics:
Slider - Grommet

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