when being operated with higher payloads and flown at lower speeds. A Fenestron is normally paired with a larger vertical stabiliser unit that also performs the role of compensating for torque; this configuration has the effect of reducing wear on the Fenestron blades and transmission system, which in turn leads to maintenance savings. Furthermore, the adoption of larger diameter units, while posing some engineering challenges, normally increases their efficiency and decreases their power requirements. Advanced implementations of the Fenestron are provisioned with stators and adjustable weights in order to optimise the blades for a reduction in power required and pitch control loads imposed. During the 2010s, Airbus Helicopters stated that it expected the design of the Fenestron to continue to be refined, in order to suit rotorcraft of increasing tonnages and to enable additional innovations to be made in the field. Through multiple mergers from Sud Aviation to Airbus Helicopters, a considerable number of light, intermediate, and medium weight helicopters have used the Fenestron as an anti-torque tail rotor. Such implementations can be found on many of Eurocopter's helicopter range, such as the Eurocopter EC120 Colibri, EC130 ECO Star, EC135 (and EC635, the military version of the EC135), EC145, the AS365 N/N3 Dauphin (also built as the HH-65 Dolphin, a dedicated variant used by the United States Coast Guard, and the license-built Harbin Z-9), and the enlarged EC155 (a wider, heavier and more advanced version of the AS365 N/N3 series). Other than Airbus Helicopters and its predecessors, other companies have also made use of Fenestron anti-torque arrangements. One such rotorcraft was the American Boeing/Sikorsky RAH-66 Comanche, a stealthy aerial reconnaissance helicopter which was canceled in 2004. Another example is the Sikorsky S-67 Blackhawk, which, in 1974, had a Fenestron for testing purposes used for 29 flight hours. It was removed in August the same year. Ducted fan tail rotors have also been used in the Russian Kamov Ka-60 medium-lift helicopter, and also on the Japanese military's Kawasaki OH-1 Ninja reconnaissance rotorcraft. French light helicopter manufacturer Hélicoptères_Guimbal has also used a Fenestron for their Guimbal Cabri G2, a compact reciprocating engine-powered rotorcraft. Chinese Harbin Aircraft Industry Group use Fenestron in Z-19 reconnaissance/attack helicopter. American Bell Textron in Bell 360 Invictus proposed helicopter design intended to meet the United States Army requirement for a Future Attack Reconnaissance Aircraft. Advantages Increased safety for people on the ground because the enclosure provides peripheral protection.