Parachute cluster; paratrooper; ram-air parachute
Parachutes are devices utilized to decelerate or stabilize objects, people, or vehicles in their travel through an atmosphere. They are most commonly constructed of high-strength fabric. These lightweight and low-volume instruments develop a large surface area and significant drag upon deployment.
Parachutes (often abbreviated to "chutes") are used to decelerate and stabilize various types of payload. The direction of travel is largely dependent on the design of the parachute. Purely ballistic parachutes solely decelerate the payload's vertical velocity, and those with vents along one side of the canopy propel the chute with steerable, lateral motion. Parafoil-type canopies develop both lift and drag by using ram-air pressure to inflate a fabric-composed, cellular-structured, low-aspect-ratio wing; these varieties are capable of significant horizontal speed.
Most commonly, objects or humans are discharged from an aircraft at an altitude high above the intended landing zone, with the purpose of delivering them safely to the ground. Occasionally, parachutes are used as emergency landing systems for lightweight aircraft as well. Typically, the payload is in free-fall under gravitational force but is buoyed by its own aerodynamic qualities. To slow the descent the parachute must be deployed, either manually, mechanically, or electronically. Manual deployment requires a human to pull a 'ripcord', which is a handle attached to a pin that releases a spring-loaded pilot chute. Mechanical deployment utilizes a static line which connects the pin to the aircraft, and once the line has reached its length it pulls the pin to release the pilot chute. Electronic deployment requires the use of an automatic activation device (AAD), which is set to deploy the pilot chute when preset time, dynamic pressure, or altitude parameters are met.
The pilot chute is sized to extract the deployment bag containing the chute and suspension lines. When the suspension lines are taut the pilot chute strips off the deployment bag and inflates the main canopy. Note that many cargo parachute systems are multistage. Some chutes deploy multiple sequential parachutes, while others participate in 'reefing', where the canopy mouth is temporarily restricted in size by a rope. These techniques are used to obtain the appropriate descent rate or to control the opening load to an acceptable level. The main parachute must unfold reliably and consistently without twisting the fabric or lines. If the parachute deploys too quickly it can damage the payload. Heat generated by friction may also damage a parachute system, and is typically a consequence of poor parachute packing.
A drogue parachute is often directly inserted into the airstream behind the aircraft from the cargo door. This extracts the payload from the aircraft's opened cargo hold and ensures the correct orientation of the payload during its descent. It may also extract a larger drogue chute in the case of very heavy cargo. Complex, multi-parachute systems have been devised for descent of military equipment and to recover components used on spacecraft, such as launch boosters and capsules.
Sometimes parachutes are attached to munitions to retard a bomb during its descent. This allows low-flying delivery aircraft to leave the vicinity before detonation. Parachutes are also used to deliver sonar equipment to sea locations.
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Load-extraction parachutes must be highly stable upon deployment. Levels of canopy porosity determine the oscillation experienced by the payload. Stability is especially important for low-altitude-parachute extraction systems (LAPES). Examples of highly-stable parachutes include ribbon, ring-slot, and cruciform chutes.
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Ejection and Emergency Landing
Personnel which must eject from military aircraft face starkly different circumstances than those who jump from fuselage or cargo doors. In this instance, aircraft operators are located in a cockpit located in the front of the jet, and the crew only ejects from an inoperable vehicle. Significant gravitational forces, air resistance, and inertia prevent pilots from manually removing the cockpit hatch and jumping from the aircraft. Instead, the cockpit canopy and ejection seat are jettisoned above the plane by explosive charges or rockets. Crew members are strapped into the ejection seat, which contains the parachute assembly. Due to the excessive speeds of the crew, highly-stable parachutes are required.
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Ballistic recovery parachutes are available for lightweight aircraft. These parachutes are typically deployed within seconds via solid-fuel rocket. The impaired aircraft's descent is slowed and passengers have an increased rate of survival.
High-speed land vehicles, aircraft, and spacecraft utilize parachutes to aid deceleration. These parachutes are designed per application specifications and must be highly stable as any significant oscillation will compromise safety. In the case of vehicles and aircraft, the pilot or driver will deploy the chute which is strictly used for supplementary braking, as its effectiveness is defined by the inverse square of velocity.
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Lifting and Gliding
A parafoil (or specifically, a ram-air parachute) resembles a low-aspect-ratio wing. It is constructed entirely of fabric, which allows it to be stored and implemented in a manner similar to regular parachutes. The wing has an aerofoil cross-section which is sewn in a way to maintain the shape of a wing. Vents are cut to help maintain stability between cells and stabilizer panels add steering to the canopy design. The aspect ratio of the canopy is generally limited to a maximum of about 3, and the lift-to-drag ratio is also limited to about 3. As such, the parafoil is a gliding device with a gentle stall. Parafoils are used by most often in a sporting context, but are finding increased employment for military personnel and cargo delivery applications. Long range (15 miles) cargo delivery systems are controlled by auto pilot and can deliver a 10,000 lb.-plus load with great accuracy.
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Early parachute designs predominantly-used silk in parachute production due to its light weight and strength, but silk has largely been replaced by modern materials. Fabric and geometrical permeability are carefully considered during the design phases of the parachute. Dome-type parachutes are constructed from a series of triangularly-shaped sections of fabric called gores. Ram-air parachutes utilize two layers of rectangular fabric which are attached via nylon ribs to create individual cells that capture high-pressure air.
The following materials make up the majority of a parachute assembly, each with their own benefits and drawbacks.
|Nylon||Most common material; exceptionally strong, fabric has good elastic properties (elongation)||Sensitive to UV exposure; melts at high temperatures|
|Dacron||Temperature resistant; less weight and volume than comparable fabrics; low porosity||Requires chemical treatment for fabric stability; less elongation than nylon|
|PTFE||Very low friction; used primarily in deployment bags and as a cotton replacement||Expensive to produce; not as strong as nylon or dacron|
|Nomex||Thermal/UV resistant; moderate-to-high tensile strength||Low elongation|
|Kevlar||Very strong and durable; does not melt or burn; abrasion resistant||Very little fiber elongation; sensitive to UV; abrasive fabric; difficult to manufacture|
|Spectra||Second-strongest parachute material; low friction; commonly used for suspension lines; low density||Almost no fiber elongation; loses tensile strength at temperatures over 200°F; difficult to sew during parachute production|
|Vectran-LCP||Very similar to Spectra; specific gravity of less than 1.0; immune to acids and UV||Similar to Spectra; rather new, so designs/variants are limited|
|Zylon||Strongest parachute material; moderately low friction||Low elongation; very vulnerable to all spectrums of light|
Nominal area: the total surface area of the canopy. Generally speaking, there should be one sq. ft. of canopy for every pound of payload.
Tensile strength: the valued resistance of the fabric against tearing while deployed.
Trim angle: the angle of attack of the leading edge of the canopy while gliding.
Oscillation angle: the range of the angle of attack of the canopy after deployment.
Opening load: the inertial force that is imparted to the payload by the deployment and inflation of the canopy.
Drag coefficient: the ratio of the steady drag force to the product of dynamic pressure and reference area while inflated. This depends on the parachute shape, porosity, as well as the number and length of the suspension lines, and the size and shape of the payload.
Porosity: The fabric of the parachute may allow some air to permeate the material, while ventilation patterns and stabilizers are a method of geometric porosity. Increasing the porosity of the chute increases stability, while reducing drag and the opening load value.
There are several ways to differentiate between parachute types, but it is easiest to identify them by deployment sequence.
As previously mentioned, pilot chutes only serve as the deployment mechanism for the main parachute. As such, they must deploy quickly, reliably, and with stability. They must exert enough force to withdraw the main canopy from its package, but not do so in a manner that negatively affects the main parachute. Specialist designs have been created to optimize the deployment of the main parachute and are depicted above, but guide-surface, ribbon, and ringslot parachutes (explained in depth in the section below) have also been used as pilot chutes. Pilot chutes are spring-loaded, charge-loaded, or manually inserted into the airstream during descent, where they rapidly inflate and begins to pull the main canopy out of the deployment bag.
There are essentially two types of main parachutes once classified by their aerodynamic principles: ballistic parachutes, which are solely designed to produce a retarding force for payloads, and steerable parachutes, which are any type of parachute capable of piloted directionality due to vents strategically placed in the canopy. Ballistic parachutes are the preferred design for emergency situations and cargo delivery; small charges deploy the canopy in an extremely rapid manner and allow them to be opened at very low altitudes. Steerable parachutes are utilized in most other instances.
Some parachute designs and their specifications are listed below. A more thorough evaluation of each type is provided by ESDU document 09012, which is available in the Standards section below.
|Round parachute||The original and simplest form of parachute; resembles a hemisphere when inflated, or when unfolded the sum of the gore angles equals 360°; suspension lines are attached along the circumference and run to the central vent; various ventilation outlets may be placed in the canopy for steering; a high drag coefficient; a central vent aids manufacture and improves stability; significant oscillation.||
|Extended skirt parachute||Variation of round parachute, but an additional section of fabric extends downward and inward along the suspension lines; extension deflects some air around the canopy, which reduces drag and increases stability.||
|Shaped parachute||Modern variation of a round parachute where the sum of the gore angles is less than 360°; resembles a semisphere or cone when inflated.||
|Guide-surface parachute||A high-speed deployment parachute primarily used for munitions and aircraft ejections; resembles a bulbous, inflated disc when deployed; effective at high and low dynamic pressures; capable at supersonic speeds; rapid deployment, good reliability, and the greatest stability of any parachute; difficult to manufacture.||
|Ribbon parachute||Supersonic parachute, but more common in subsonic applications; composed of a series of high-strength fabric ribbons placed over a frame of lines with gaps of 1—3 cm; very stable, but with comparatively low drag; solely used as drogue, braking, or extraction chutes; a Kevlar-based ribbon parachute is used on the B83, the largest nuclear bomb in the American arsenal.||
|Ringslot parachute||A less expensive type of ribbon parachute, where the ribbons and gaps are wider apart and the fabric is not as strong; useful at moderately-high speeds for vehicle and cargo deceleration.||
|Ringsail parachute||An improved variant of a ringslot parachute that contains hundreds of small ventilation outlets formed along the seams of ribbon; very stable and has the highest drag of any ribboned (high-porosity) parachute; expensive to produce; the preferred parachute of space capsule recovery for early NASA missions.||
|Cruciform parachute||A canopy which resembles a cross when unfolded; upon deployment, the intersection of the canopy resembles a square and the 'legs' of the parachute are threaded through the suspension lines to direct air into the canopy; easily manufactured; suitable for high-speed deployment on munitions and land-based vehicles; prone to rotating when inflated thus requiring a rotating connector; commonly used on racing autos.||
|Disc-gap parachute||Resembles a hemisphere with a substantial void which runs around the canopy; gap can be varied to balance drag and stability; useful at supersonic speeds with low dynamic pressures; developed by NASA for extraterrestrial use.||
|Ram-air parachute||The non-rigid structure is inflated by ram-air which sustains the layered cell structure of the canopy; a slider is required to slow the deployment of the canopy; possesses excellent steering and gliding capabilities; primarily used for personnel, but also for precision cargo delivery.||
|Ballute||The most notable design of what are technically balloon/parachute hybrids; fitted around the widest point of the canopy, a burble fence with ram-air inlets inflate a canopy to precisely control drag; burble fence also ensures flow separation between chute components; inlets may be replaced with gas generators to provide lift; high weight and volume, but effective at supersonic speeds and extreme altitudes; used on unguided bombs.||
Deploying one or more of the same type of main parachute is an effective way of slowing the descent of objects through the atmosphere. Parachutes that are clustered typically exhibit better stability than if deployed individually, while maintaining a high degree of drag. The redundancy better ensures safety if one parachute fails, and clusters of up to 20 parachutes have been successfully deployed.
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Rarely, parachutes suffer malfunctions, but it does happen for a variety of reasons. Some estimates place main parachute failure at once for every 1,000 activations. For this reason, all free-fall parachutes are provided with a reserve chute which can be utilized if the main parachute fails. Typically, the main parachute and the original pilot chute are released from the payload to prevent them from interfering with the reserve, though it is possible to land with two separate, deployed canopies. Payloads and parachutes are typically connected by the 3-ring release system, animated below, which allows them to be disconnected rather easily. Reserve chutes are activated by an auxiliary ripcord, which frees the pilot chute from its pouch, and in turn pulls and deploys the reserve chute from a separate section of the deployment bag. While a main canopy will retain its pilot chute and bag, reserve chutes do not. Furthermore, main canopies are frequently folded and packed by the user, but reserve chutes are periodically repacked by certified riggers.
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Unequivocally, parachute design and evaluation is best completed under reference from IHS ESDU document 09012 — Aerodynamics of Parachutes, assembled from the knowledge and experience of the ESDU aerodynamics technical committee. Many states have passed legislation which requires parachutes to be monitored for their mechanical proficiency and for their deployment to be supervised by skilled professionals. Perhaps the most notable government standard in regards to parachutes is the United States FAA's CFR 14, Part 105: Parachute Operations.
Further industrial standards regarding the use of parachute have been developed, in large part due to the extensive range of applications in which the reliable performance of the chute is critical; a selection of these standards follows.
A-A-52197 (Parachute packing based on packed weight)
A-A-55183 (Parachute skirt extensions)
AD 80-13-01 (Parachute ripcord handles)
ASTM F2244 (Performance of powered ram-air parachute for aircraft)
ASTM F2316 (Emergency parachutes for entire aircraft assemblies)
FAA 8610.5 (Parachute rigger examiner handbook)
MIL-A-85097/16 (Rocket-powered aircraft ejection systems)
MIL-DTL-7620 (Parachutes for cargo; extraction from aircraft and deceleration)
PIA-TB-252 (Reserve parachute maintenance standards)
SAE-01-3928 (Parachute systems for UAVs)