Para Foils and Steerable Parachutes - Term Paper Example

Para foils and steerable parachutes Introduction Parachutes are devices that are used to slow down the motion of an object via the atmosphere through creating a drag effect, or in a situation of a ram air parachutes, aerodynamic lift. Often, parachutes are made out of silk, light and strong cloth, most commonly referred to as nylon (Tancredi, 2006). Parachutes slow the terminal vertical speed of an object by a minimum of 75 percent so as to be classified as a parachute. With respect to the circumstances, parachutes are used with numerous loads including food, people, bombs, space capsules, and equipment. Para foils and steerable parachutes are non rigid airfoils with aerodynamic cell structure inflated by wind. Para foils are commonly made out of rip-stop nylon. Ram air inflation forces the Para foil to a cross section of a classic wing. The immediate acceptance of Para foil as a parachute was prevented initially by deployment of shock. However, after the addition of drag canopy on riser lines that led to the slowed motion, the Para foil was accepted as a suitable parachute (Tancredi, 2006). This paper seeks to discuss the characteristics of aerodynamic parachutes including personnel parachutes and Joint Precision Airdrop parachutes used by the military. In comparison to the round simple canopy, Para foil parachutes have much greater steer-ability and allows greater control of the descent rate; the format of the Para foil parachute is mechanically a hang glider of a type of kite that is in free flight, this aspect spawned the use as a Para glider. The flowing air into the Para foil normally comes more from below than what might be suggested by the path of the flight, so ropes at the front most tow against the flow of air. However, when gliding, the attack angle is lowered and the flowing air meets the Para foil head on. This result into much difficulty in achieving an optimum angle of gliding without the Para foils deflating. The invention of Para foil parachutes led to the Gold Parachuting Medal award by Fédération Aéronautique Internationale (FAI) to Jalbert in 1984. There has been a wide use since then of Para foils in different widn sports including powered parachutes, kite flying, wing suit flying, paragliding, skydiving, and speed flying. In fact the largest kite in the world is a Para foil variant (U.S. Department of the Army, 2003). For numerous reasons, the analysis of aerodynamic ram air parachutes is a very crucial and intricate task. The textile (flexible) wing structure interacts with the distributed pressure originating from the surrounding internal and external flow field, and the reverse is also true. The unsteady processes that range from deployment of canopy, process of filling, spreading to dynamic flight maneuvers such as rapid turn initiation or landing flare are not within reach for the current standards of aerodynamic tools of analysis. The unique features and characteristics of the Para foil payload system such as free velocity stream originating from the actual path of flight, the complex arrangement of suspension lines large numbers and the close coupling between the mechanics of the flight, and the changing geometrical canopy shape; do not at all ease the aerodynamic analysis process (Tancredi, 2006). However, the modern parachutes of today are classified into categories such as descending canopies and ascending canopies. All the ascending canopies include Para gliders designed particularly to ascend and stay in the air as long as possible. Other types of parachutes such as ram air non elliptical are categorized under descending canopies by their respective manufacturers. In addition, some manufacturers have classified modern parachutes as semi rigid wings which can make a controlled and directed descend, and maneuverable to break on impact with the ground (United States Works Progress Administration, 2003). Round parachutes are primarily a drag device. In contrary to ram air types, round parachutes do not provide lifts. They are majorly used in emergency, military and cargo applications. Round parachutes have large dome shaped canopies that are made from one layer of triangular gores of a cloth. They have been referred to as “jellyfish chutes” by some skydivers due to the resemblance. However, the modern parachutists in sports seldom use this type of parachutes. The very first round parachutes were simple and flat circulars. They originally suffered from instability that was as a result of oscillations. A hole in the apex of the parachutes helped to reduce number of oscillations and to vent some air. However, many military applications used conical or parabolic shapes, for example the T-10 static line parachute used by the United States Army. It is important to note that a round parachute with no holes at all is prone to oscillate, and therefore not steerable. In order to achieve a large forward speed, 3 to 8 mph, and steering, cuts should be introduced in various sections across the back, gores, or through cutting four lines in the back and modifying the canopy in order to allow air to escape through the back of the canopy and provide to limited speed for forward motion. Some other modifications used normally are cuts in several sections, gores, in order to cause the bow out by some of the skirt. Turning is achieved through forming the modification edges resulting into more speed to the parachute from one side of the modification than the other side. This allows the jumpers to steer the parachute, allowing them to escape obstacles and to turn to the wind to reduce horizontal speed at jumping (Dan and Mike, 2003). The cruciform or square type of parachutes have special design features and characteristics that minimizes oscillation leading to its user swinging forth and back, with violent turns in descend. This has replaced the T-10 parachutes used by the United States Army under the program referred to as the Advanced Tactical Parachute System. The Advanced Tactical Parachute System canopy is a uniquely modified version of a cruciform platform and appears square in its appearance. The Advanced Tactical Parachute System (T-11) reduces the descent rate by about 30 percent from 21 feet per second to 15.75 feet per second (6.4 m/s to 4.80 m/s) respectively. The Advanced Tactical Parachute System (T-11) is made to have an average descent rate of 14 percent slower that the T-10D, hence leading to lower rated of landing injury for jumpers. The decline in the descent rate reduces the impact energy by about 25 percent to reduce the injury potential (United States Works Progress Administration, 2003). The annular and pull down apex parachute types varies from round parachute with the pull down apex. This type of parachute is referred to as a Para Commander canopy after the first model of the kind in some circles. It is just like a round parachute with lines of suspension to the apex of the canopy that applies the load and pulls closer the apex to the load leading to distortion of the round shape into a flattened kind of or lenticular shape. Some annular and pull down apex parachutes are designed with fabrics removed from the apex in order to open a hole where the air can exit, giving an annular geometry to the canopy. Such parachutes also have a decreased horizontal drag because of their flat shape and can have considerable forward speed when combined with vents that face its rear (Dan and Mike, 2003). Rogallo wing parachutes have been experimented by sport parachuting among other shapes and forms. This type of parachute was designed to reduce the speed of landing and to increase the forward speed offered by the other types of parachutes at the time. The development of ram air parachutes and the subsequent invention of the sail slider were aimed at slowing the deployment in order to minimize the level of soprt parachuting experimentation in the community. This type of parachute is also very difficult to build. Ring and ribbon parachutes have common similarities with the annular designs discussed earlier. They are designed frequently to deploy at very high speeds. Any other conventional parachute would burst instantly upon opening at that kind of speed. The ribbon parachutes have a canopy that is ring in shape, usually with a large hole in the center in order to release pressure. Sometimes the rng is broken into ribbons that are connected by ropes in order to have more air leaked out. This massive air leaks reduce the stress on parachute so that it may not shred or burst when opened. Ribbon parachutes are often made out of Kevlar and are normally used on nuclear bombs like the B83 and the B61 (White, 2007). As discussed earlier, the most modern parachutes are Ram air parachutes. They do self inflate and offer control over direction and speed similar to the Para gliders. Although Para gliders have greater range and lift, parachutes are designed to handle, mitigate and spread the stresses of deployment at their terminal velocity. All the Ram air Para foils have 2 layers of fabrics; bottom and top and are connected by fabric ribs in an air foil shape in order to form cells. These cells are filled with air of high pressure from the vents facing forward on the leading edge of the airfoil. This fabric is shaped and the lines of parachute are trimmed under the load in order that the ballooning fabric inflates into the shape of an air foil. Sometimes this air foil is maintained by the use of one way valve fabric known as the airlocks. The very first Ram air test jump was designed by Joe Crotwell, the Navy test jumper (U.S. Department of the Army, 2003). The Joint Precision Airdrop System (JPADS) parachute is a system of an American military airdrop that applies the steerable parachutes, the GPS, and an onboard computer in order to steer loads to the chosen drop zone at the designated point. This type of parachute system integrates the Precision and Extended Glide Airdrop System (PEGASYS) of the United States Army and the Precision Airdrop System (PADS) program of the Air Force. Precision and Extended Glide Airdrop System (PEGASYS) entails various systems of precision airdrop ranging between extra heavy and extra light payloads, however, the Precision Airdrop System (PADS) program is based on a laptop that is used to compute the points of release for the systems of non-steerable parachute by means of software that are capable of planning mission, current measurements, and weather forecasting of wind velocity, air pressure, altitude, and temperature. It is also able to en route mission changes and receives weather updates through links of the satellite (United States Works Progress Administration, 2003). The Para foil or steerable parachutes are referred to as the decelerator. It gives the Joint Precision Airdrop System (JPADS) control over direction throughout its descent through the mechanism of steering decelerator lines that are attached to the Airborne Guidance Unit (AGU). This creates a drag on either of the decelerator sides and turns the parachute, hence accomplishing control over direction. The Airborne Guidance Unit (AGU) has a battery pack, a GPS, and the guidance, control and navigation (GN&C) package of software. It houses the required hardware for operation and steering lines. The Airborne Guidance Unit (AGU) achieves its position before exiting the aircraft, and calculates its position continuously through the GPS throughout descent. Although the accuracy of the Joint Precision Airdrop System (JPADS) is classified, it is good enough to reduce drastically the size of the requirements of the drop zone significantly increasing the location numbers that can be used as a drop zone. In addition, the sequential loads that can need a conventional drop zone in a half a mile may be dropped using Joint Precision Airdrop System (JPADS) into a much smaller area. Precision Airdrop System (PADS) provides numerous benefits including the quantity of the drop zones available and an increase in the precision of cargo that the user benefits from. The Joint Precision Airdrop System (JPADS) additionally increases the ability to survive by the delivery aircraft as well as its crew (Dan and Mike, 2003). The Joint Precision Airdrop System (JPADS) currently require quite large drop zones (600 yards or more) sequential load air dropping (multiple loads aboard one aircraft) demands a very long drop zones in the order of about a half a mile or even more, otherwise the aircraft has to make multiple passes within that one particular region, which is an unsound thing to do tactically. In addition, obtaining a high degree of accuracy requires that the aircraft flies at a very low altitude as much as possible ranging from 400 feet above the level of the ground to a high of 1500 feet above the ground level, relying on the drop zone altitude, the type and number of parachutes, and the weight of the load required. The Joint Precision Airdrop System (JPADS) can accomplish better or same accuracy from much greater heights enabling the aircraft to drop the load at unusually safer and much higher altitude. Since the Joint Precision Airdrop System (JPADS) allows the aircraft to drop loads at a high altitude, the aircraft is actually able to drop the load from a good distance away from the designated drop zone. This enables the aircraft crew to stay free of enemy threats that may of course be near the region where the load needs to be dropped. The Joint Precision Airdrop System (JPADS) airdrops are often performed in slow speeds (140 kts for cargo and 130 kts for paratroopers) for aircrafts. When this is combined with the required low altitude for the precision, the aircraft become vulnerable to the ground fire of the enemy. With the Joint Precision Airdrop System (JPADS), the aircrafts have a higher survivability because it is able to drop loads at a much higher altitude which is above most of the ground fires of the enemies (U.S. Department of the Army, 2003). The personnel parachute systems are designed and manufactured by the Airborne Systems for all kinds of mission. This can range from the T-11 troop parachutes used for reasons of mass assault to the steerable MC-6 used for missions of tight drop zone landings, as well as the Raider Hi-Glide and the Raider Intruder that are used for special operations. Personnel parachutes are specifically designed for high performance and outstanding quality. The personnel parachute is mainly used by all branches of the United States military and the international allied forces. This is because the personnel parachute systems usually and routinely exceed specifications of the military and perform beyond expectations. Some of the unique features and characteristics of personnel parachutes include: deployment Bag with Static Line; 35 feet diameter maneuverable canopy that is modified to 17 openings, 0-5 CFM cloth, with Anti-inversion Net; pack tray with static line stow bars; 30 feet fluted risers with fasteners on log pocket; and harness assembly with capewell cable type canopy release and triangle link and ejector snaps (White, 2007). In summary, parachutes are used by skydivers and are designed to softly open. Ram air designs had an initial problem of overly rapid deployment. However, innovations emerged to slow the deployment of a ram-air canopy such as the slider, a small piece of rectangular fabric with grommet located near every corner. During the deployment, the slider slides from the canopy down to above the risers. This slider is then slowed by the resistance from air as during descend and minimizes significantly the spreading rate of the lines. Ultimately, this minimizes the speed at which canopy opens and inflates. This paper has generally discussed the features and characteristics of aerodynamic parachutes including personnel parachutes and Joint Precision Airdrop parachutes used by the military. References Dan Poynter and Mike Turoff (2003). Parachuting: The Skydiver's Handbook, New York: Para Publishing. Tancredi, S. (2006). Parachute technology, Chicago: University of Chicago United States Works Progress Administration, (2003). Aeronautics: Parachutes. New York: University of Michigan U.S. Department of the Army, (2003). Field Manual No. 3-21.220 (57-220) Static Line Parachuting Techniques and Tactics, Washington, DC: U.S. Department of the Army White, Lynn (2007), The Invention of the Parachute, Technology and Culture 9 (3): 462–467 Read More