Unmanned Systems Technology 007 | UMEX 2016 report | Navya ARMA | Launch & recovery systems | AIE 225CS | AUVs | Electric motors | Lethal autonomous weapons

42 than the UAV to minimise the parafoil’s tendency to jump on release. Inexpensive to buy and operate, the equipment takes up little space when stowed and is adaptable to a range of UAVs. When deployed, however, it occupies considerable deck space and prevents helicopter operations. It also requires the ship to head into the wind at speed. Parachutes, nets and arresting lines If they are light and tough enough, UAVs can make perfectly acceptable landings on any reasonably flat surface without the encumbrance of undercarriage, perhaps with an easily replaced skid or a crush structure to absorb any residual impact, but heavier craft need more help. Parachutes have been used as both routine and emergency recovery systems for UAVs for many years, and the growing popularity and cost of hobbyist and commercial multicopters, which may carry expensive sensors, is expanding the market for the latter. Although they look very simple, parachute systems must nevertheless be carefully engineered to balance several parameters. For example, selecting for the softest possible landing means that a large parachute is needed to minimise the descent rate; size is inversely proportional to descent speed. This not only increases weight and cost but brings with it other undesirables, such as a greater tendency to drift with the wind and to drag along the ground after landing, causing damage that the very slow descent was intended to avoid. Sometimes, adding an airbag is a better solution than a larger parachute. The performance rating of a parachute divides its load capability by its static weight and expresses it as a ratio. For example, a parachute weighing 1 lb and able to support an 8 lb UAV would have a performance rating of 8:1. The load capability is always for a specified descent rate, although the rate doesn’t play a part in the calculation. A high drag coefficient (Cd) of around 2 is a desirable characteristic in a parachute because it allows for a small and light system with a good retardation capability. Stability, defined as the tendency to stay vertically above the load, is also desirable. Several shapes are available that provide different combinations of these qualities at different costs. The simplest and cheapest is the flat sheet type, with stability that is adequate at low speed but poor at high speed, a performance rating of 8:1 and a Cd of about 0.7. At the other end of the scale, in both performance and cost terms, is the annular type. Resembling half a bagel, these have good low-speed stability, a Cd of 2.2 and a performance rating of 32:1. Between about 50% and 80% of a parachute system’s weight is in the canopy, with the rest in the shroud lines, shock cords and other rigging. Most canopies are made from rip-stop nylon fabric, which weighs about 45 g per square metre, and is either coated or calendered (a process that involves passing the cloth through heated rollers to flatten the weave for better water resistance). Coatings add weight and can stick folds together if left packed for long periods, while calendering neither adds nor risks stickiness. Calendered chutes can be left for years and, if kept dry, will still open reliably. Shroud lines are typically made from nylon, polyester, Vectran or Spectra. As strong as Kevlar and very light, Spectra resists tangling and abrasion, and is said to reduce overall parachute weight by about 35%. Deployment is the next consideration, and can be passive or ballistic. With passive deployment, the wind extracts the parachute from its compartment and opens it when the compartment door opens. In ballistic deployment, the parachute is pushed out by a spring, compressed gas or a pyrotechnic device. Passive deployment is good for UAVs that always have some forward speed, not so for multicopters, which need ballistic deployment, which is also more successful in low-altitude deployments. While parachutes provide a gentle descent, the opening shock can be anything but, as the force applied to the load is proportional to the square of the speed, so these forces can be as high as 15 g . Therefore the parachute is attached to the UAV with a length of shock cord of about 1.5 to 2 times the parachute’s diameter and made of nylon or Kevlar. The latter has less ‘give’ to absorb shock, but sewn-in tearaway sections can compensate. April/May 2016 | Unmanned Systems Technology Focus | Launch and recovery systems The increasing value of multicopter UAVs and their sensor payloads has created a market for emergency recovery parachutes in lightweight packages with powered deployment mechanisms (Courtesy of Skycat Drone Recovery via Fruity Chutes)