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PrefaceUnmanned Aerial Vehicles (UAVs) or Unmanned Aircraft Systems (UAS), a term preferred by the U.S. Department of Defense, have seen unprecedented levels of growth over the last decade. Even though UAVs have been mainly used for military applications, there is a considerable and increasing interest for civilian applications. It is not an exaggeration to consider that as the technology matures, as small-scale UAVs become cost-effective with proven reliability and safety, and as the roadmap to integrating UAS into the National Airspace System (NAS) progresses, civilian applications will dominate the field. It is postulated that UAVs will be used in the future extensively for environmental monitoring, forest protection, wildfire detection, traffic monitoring, building, power line and bridge inspection, emergency response, crime prevention, search and rescue, mapping, surveillance, reconnaissance, border patrol, to name several applications.From all classes of UAVs, unmanned rotorcraft, and in particular unmanned helicopters, have advantages over fixed-wing UAVs because they take-off and land vertically, they do not require a runway, and they have the ability to hover and fly in (very) low altitudes. It is reasonable to assume that light-weight (<150 Kgr) and small-scale (<50 Kgr) helicopters will be the first ones to be allowed to fly in civilian airspace. Such helicopters, though, still retain the flight characteristics and physical principles of their full-scale counterparts. In addition, they are naturally more agile and dexterous compared to full-scale helicopters. Their flight capabilities, reduced size and cost have recently monopolized the attention of the UAV research community as they are preferred for a wide spectrum of applications. However, helicopters are highly unstable, nonlinear and coupled underactuated systems, and controller design for such systems is a rather challenging problem.The problem of designing autonomous flight controllers for small-scale helicopters is equally challenging, and the flight controller design problem is tightly connected with the helicopter modeling. Helicopter dynamics may be represented by both linear and nonlinear models of ordinary differential equations. Typically, the validity of the linear models is restricted in a certain region around a specific operating point, while nonlinear models provide a global description of the helicopter dynamics.