Parachutes have been providing soft landings for people who have needed to leave airborne aircraft, either in an emergency or by design, for more than a century, enabling escape from damaged aircraft and the delivery of troops behind enemy lines during airborne assaults or special forces insertions.
Stronger, lighter and more durable materials coupled with advances in aerodynamic design have enabled parachutes to evolve from simple drag-producing devices designed to slow the airspeed of the descending person or object, to fully steerable parafoils (a non-rigid textile airfoil with a wind-inflated aerodynamic cell structure) that skilled parachutists can guide with precision to a chosen landing spot. Although simple in principal, there has always been what Manley Butler, the owner of Butler Parachutes based in California, describes as “wild variation” in the market, particularly in the United States, leading many customers to ask for individually-tailored parachutes.
As with any air vehicle, and a parachute must be considered as such, gross weight is a key determinant of the descent rate, which itself is a compromise between a highly desirable soft landing and the tendency of the parachute to drift: A simple rule–of–thumb is that the lower the descent rate, the greater the parachutes’ drift from its intended landing spot. Things are more complicated with steerable parachutes, as they are aerodynamic surfaces for which the lift-to-drag ratio is important, as is predictable behaviour when the shape of the parafoil is warped by the parachutist’s control lines.
Trade-Offs
As with any product, the principal trade-offs affecting parachute design are form, fit, function, cost, schedule and performance, Manley Butler emphasised, but because the company’s primary products are emergency parachutes it is very conservative in its recommendations of the most appropriate equipment for particular applications and conditions, when approached by a customer. In addition to gross weight, defined as the user’s body weight including their clothes, the parachute and harness system along with any other equipment that leaves the aircraft with that person, Mr. Butler factors in the wearer’s height to help determine the size of the pack and harness, alongside the aircraft type and its performance envelope to determine parachute strength requirements, plus the aircraft’s seating arrangements to help determine the configuration of the entire rig (the parachute, harness and pack) to ensure that it is safe and comfortable to wear inside the aircraft. The company’s conservatism generally manifests itself in recommendations for larger and more rugged equipment than can be expected from companies that specialise in skydiving equipment.
In the United States, the Federal Aviation Administration (FAA), which has overall responsibility for regulating aviation, regulates parachute equipment under its Technical Standard Order (TSO) process, specifically TSO C23 and C23B criteria, with performance standards based on the Society of Automotive Engineers (SAE) Standard 8015A. This latter document sets the minimum performance standards for personal parachutes. Most emergency parachutes are certificated under the FAA’s TSO C23B regulations, and FAA standards are widely recognised and emulated around the world.
The TSO C23B regulations cover standard and low speed parachute categories (see below), while the standard sets no legal limits on parachute gross weight or deployment speed, manufacturers specify reasonable physical limits, Mr. Butler says. He notes that even though standard category systems are often called ‘high-speed’ parachutes, that is not officially recognised nomenclature. He continues that low speed parachutes are limited to use at under 129 knots (238.9 kilometres-per-hour), but that there is still no limit on parachute gross weight.
Higher performance testing standards arrived in 1984 with the TSO C23C regulations, under which few traditional round parachutes, but many high-performance gliding parachutes, were approved. This standard introduced A, B and C strength categories in ascending order and mandatory labels showing weight and speed ratings. Mr. Butler emphasises that category B is the minimum for emergency use, with ratings of 150 knots (277.8km/h) Indicated Air Speed (KIAS) for deployment and a 254 pound/lb (115.5 kilogram/kg) gross weight, although some category B parachutes specify lower limits to minimise shock on opening and/or to moderate the descent rate.
Heavier Pilots
Responding to the fact that an unexpectedly large proportion of parachute users exceeded 254lb (115.5kg) in weight, in 1988 the FAA and SAE began revising their performance standards upward, eventually resulting in FAA’s publication of TSO C23D, which placed no limits on deployment speeds or gross weights, instead mandating that manufacturers specify those parameters on their products and test them with a safety margin, while parachute opening time and descent rate requirements are linked to gross weight. Mr. Butler argues that it is best if the parachute is the type the aircraft designer had in mind, with seat size, shape, angle and layout among the key factors along with the wearer’s size, which also affects canopy selection.
Most emergency parachutes have a round shape; a term that encompasses three dimensional ‘flat’, conical and tri-conical shapes; the latter generally having the highest drag coefficients (a result of calculating drag force, the size of the object, the object’s speed and the mass of the medium it is moving through). Most modern round parachutes are steer-able, according to Butler. Although originally made of silk, hence the Second World War aircrew slang of ‘hitting the silk’ as a reference to bailing out of an aircraft, most parachute canopies today are woven from artificial fibres, principally polyamide variant nylon-66, according to a 2005 presentation by parachute expert Dr Dean F. Wolf. Porosity to airflow affects several factors including the drag coefficient, opening shock (the jolt experienced by the user when the canopy fills with air), stability and filling time; the time taken by the canopy to fill with air after the parachute is opened, with standard military parachute cloth generally letting through between 2.2 cubic metre per minute (cmpm) to 3.3cmpm (80 cubic feet per minute/cfpm to 120cfpm), while military cloth classed as low porosity flows between 0.8cmpm to 1.4cmpm (30cfpm to 50cfpm), although zero porosity cloth is also available, according to Butler Parachutes.
Higher porosity increases filling time, reduces opening shock and improves stability but reduces drag, meaning the terminal descent rate is likely to be higher with more porous canopies. Stretchier fabrics are used in parachutes and are intended for use with ejection seats as they tend to be deployed at high speed and the fibre elongation lets more air pass through to reduce shock loads, Butler Parachutes adds. While canopy sizes are frequently expressed in terms of diameter, the most important measure of size is the drag area, which is the finished canopy area multiplied by the drag coefficient, according to the company.
Glide Ratio
On 3 October 2016, Airborne Systems introduced its new Hi-5, the latest in its line of high performance ram-air parachutes, emphasising its combination of high gliding performance and the ability to allow the parachutist to modulate the glide to descent quickly and to land accurately. Glide ratio is a flying object’s horizontal distance divided by change in altitude. Airborne Systems claims a glide ratio of 5.5:1 that can be reduced to 1:1 with the company’s Glide Modulation System (GMS) which equips the Hi-5. Unlike conventional methods of angle control such as front riser trim tabs outfitting the parafoil, GMS uses additional toggles on the front risers and does not increase the total speed of the canopy, thus providing a safe transition for the stages of flight discussed above from any altitude. Glide modulation can be performed with the steering toggles still in hand, says the company. Further, the company states that this eliminates the need for multiple spirals or ‘S-turns’ at low altitude for glide control, and allows for extremely accurate landings from safer straight-in approaches: “The process of designing the Hi-5 first came with trying to solve a problem,” said JC Berland, chief technology officer in one of the company’s videos discussing the new parachute: “We had a really high-performance canopy, which was the Hi-Glide, but this parachute was not for every jumper. On the other hand, we just came up with the Intruder, which was … selected by the US Army … which is a parachute that has fairly high performance, which is extremely docile and has very mild behaviour … Having learned from designing both parachutes, we thought we could bridge the gap between the two.”
Giorgio Piatti, Airborne Systems’ design manager, emphasised that the Hi-5 achieved a glide ratio very close to that of the Hi-Glide with a canopy that is both simpler to build and more intuitive for riggers to pack and jumpers to handle; the latter thanks in part to the GMS: “The glide modulation changes the airfoil section. You maintain the ability to control your heading, but instead of gliding with a five-and-a-half-to-one glide ratio, you are descending with a one-to-one glide ratio, so you are essentially falling out of the sky under a round parachute. That will allow you to really sink the canopy into an extremely tight spot with a very easy manoeuvre as you can steer and change the glide ratio at the same time.” Vincent Mignot, Airborne Systems’ international business manager added that the reserve parachute has the same characteristics as the main, which is rare in canopies with high glide ratios: “During a mission, even if the jumper cuts away and has to get rid of his main, under his reserve he will have the same performances and will be able to carry out his mission with his team mates.”
Clearer Communications
As with most other high risk operations, military parachuting puts a premium on clear communications while also making them difficult to achieve. In this case, the noise of engines and rushing air at an open door or ramp, and when in freefall, present obvious problems for voice communication. What is more the need to minimise the risk of equipment getting entangled in suspension lines or webbing means that anything that can eliminate bulky radio headsets and their associated wiring but still enable jumpers to talk to each other, the aircraft and to comrades on the ground is likely to be welcomed. This is what Sonitus Technologies and distributor PCMG found when it introduced its Advanced Two-way Acoustic Communicator (ATAC) system, which in October 2016 completed a trial with US Air Force (USAF) Pararescue operators, also known as Para Jumpers (PJs), from the USAF’s 131 Rescue Squadron at Moffet Federal Airfield in California. The heart of the system is a module that fits inside the user’s mouth and incorporates a microphone and a bone conduction transducer linked wirelessly to a standard personal radio, effectively isolating both the sensor that picks up the user’s voice and the path along which sound reaches the inner ear from ambient noise. Chatter occurring during the October 2016 tests illustrated the fidelity of the communications: “Radio check, we’re all freefalling.” The PJs started their freefall at around 10000 feet/ft (3048 metres/m): “Loud and clear. How me?” asked the controller on the ground. The PJs responded with “we copy.” “Awesome, dude,” the controller replied.
That reaction is typical from operators who have used the ATAC device in the field or anyone who has experienced bone conduction hearing, according to Alexander Konowka, business development manager at PCMG. The microphone, the bone conduction transducer, the wireless transceiver, a rechargeable battery and a USB (Universal Serial Bus) connector are all embedded in a custom-moulded module that fits around the rearmost two molars on one side of the wearer’s lower jaw. Vibrations from the transducer pass though the jaw and into the tiny stirrup, hammer and anvil bones in the ear. Intended for use in a wide variety of noisy environments, the ATAC needed no special modifications for use in parachuting, Mr. Konowka told Armada: “Every device is tailored to each user, so once the dental moulds are complete every device is the same whether it’s in the jump community or (issued to) joint terminal attack controllers who are on multiple radios calling in air strikes,” he said: “We also work with a lot of clandestine, low visibility operators that just need low visibility and can’t have wires over their ears.” The PJs in the above exercise did not use oxygen for the jump, so their mouths were exposed to the airflow but the audio quality was still very clear, Mr. Konowka emphasised.
In addition to the in-mouth device, the kit also contains a charger and a carrying case, a pair of 3M Peltor noise-cancelling earbuds, the neck loop wireless antenna and the antenna peripherals interface that connects with up to two radios or one radio and a mobile device. The PJs from 131 Rescue Squadron during the test discussed above used Harris AN/PRC-152 Very/Ultra High Frequency (30 megahertz to three gigahertz) handheld radios, but the ATAC system is also compatible with Motorola and Thales transceivers: “Currently we are selling into the United States special forces community and eventually, conventional forces could obviously benefit from this technology. However, the cost is too high for those user groups right now,” he continued. ATAC systems are sold as complete kits for around $14000 each, a high price that is expected to fall as sales volumes pick up, the largest order so far has been for just ten units, he continued. The fact that the in-mouth device is encapsulated in a personalised moulding that only fits one user is a major cost driver as it can only be used by the operator to whom it is issued. Sonitus, however, is developing a new generation in-mouth device for the ATAC that will fit into a custom-moulded Invisiline mouth guard from which it can be removed and passed on to another user who will have his or her own personal mouth guard; obviously meaning that the electronic components will have to be sterilised.
Future Directions
CIMSA classifies its personnel parachutes as tactical systems for special forces, systems for airborne troops and emergency parachutes and offers a wide range of system for use from low altitudes up to 25000ft (7620m) along with reserve parachutes. The Spanish company is also advancing parachute technology through a number of research and development efforts.
ARC, for example, is a project to improve control of upper and lower surface deformation in ram-air textile wings (ram-air parachutes have a self-inflating airfoil which the parachutist uses to control their speed and direction) under extreme manoeuvring loads. Instead of traditional cross-bracing designs that controls the upper surface through span (the side-to side-width of the airfoil) wise tension, which can deform aerodynamic surfaces, vary the local angle of attack and generate uneven chord (the distance between the leading and trailing edges of the airfoil) loads, the new system carries flight loads with an internal structural membrane that frees the outer surfaces to be shaped for aerodynamic requirements. This makes canopies more efficient and easier to deploy and control. The ARC design has been tested successfully in extreme performance skydiving canopies, says the company.
Meanwhile, CIMSA’s Synchronised Inflow Control System (SICS) is designed to synchronise the inflation sequence of highly elliptical ram-air textile wings to ensure constant deceleration and on-heading deployment. Introduced into high performance skydiving canopies, the SICS allows for very soft and straight canopy deployments even under very high wing loadings.
Beyond the SICS effort, the Ultra High Aspect Ratio Ram-Air Wing (UHARAW) project is investigating the feasibility of very high aspect ratio (which compares a parachutes’ span and its chord) ram-air textile wings capable of deployment at high wing loadings. Key technology areas include extreme wing planforms, critical parafoil sections and wing structures with emphasis on internal pressure control through standard manoeuvres. Finally, the firm’s Enhanced Ram Air Stabiliser (ERAS) effort is looking at wing tip vortex control by inflating the wing tip with pressurised air to eliminate the drag and noise caused by the stabilisers used in traditional parafoil designs. CIMSA says that the ERAS will give the wing tip a much more effective shape with better continuity, and increases the effective aspect ratio.
Dark arts
Mr. Butler said that core parachute technology has changed little over the last decade, following big improvements resulting in modern textiles that became available two decades ago, such of zero porosity nylon ripstop fabric, Dupont’s Dacron used in parachute lines, Dupont’s Kevlar in various areas including lines and webbing, and Honeywell’s Spectra in lines and deployment backs for high-density packs, although refinements to designs continue to improve the flight characteristics of gliding parachutes. While computer aided design has become important, he argued that computational fluid dynamics remains a dark art in parachute design because there are far too many variables to manage. For the time being at least traditional trial and error will remain the bread and butter of military parachute design.
