A model aircraft is a small unmanned aircraft. Many are replicas of real aircraft. Model aircraft are divided into two basic groups: flying and non-flying. Non-flying models are also termed static, display, or shelf models. Aircraft manufacturers and researchers make wind tunnel models for testing aerodynamic properties, for basic research, or for the development of new designs. Sometimes only part of the aircraft is modelled. Static models range from mass-produced toys in white metal or plastic to highly accurate and detailed models produced for museum display and requiring thousands of hours of work. Many are available in kits, typically made of injection-moulded polystyrene or resin. Flying models range from simple toy gliders made of sheets of paper, balsa, card stock or foam polystyrene to powered scale models built up from balsa, bamboo sticks, plastic, (including both moulded or sheet polystyrene, and styrofoam), metal, synthetic resin, either alone or with carbon fibre or fibreglass, and skinned with either tissue paper, mylar and other materials. Some can be very large, especially when used to research the flight properties of a proposed full scale aircraft. Models are made for wind tunnel and free-flight research tests and may have components that can be swapped to compare various fittings and configurations, or have features such as controls that can be repositioned to reflect various in flight configurations. They are also often fitted with sensors for spot measurements and are usually mounted on a structure that ensures the correct alignment with the airflow, and which provides additional measurements. For wind tunnel research, it is sometimes only necessary to make part of the proposed aircraft. Full-scale static engineering models are also constructed for production development, often made of different materials from the proposed design. Again, often only part of the aircraft is modelled. Static model aircraft cannot fly, and are used for display, education and are used in wind tunnels to collect data for the design of full scale aircraft.